R. Adur, Shoreham-by-Sea to Beachy Head
1. Introduction - References Map
The arcuate planform of the East Sussex coastline, between Shoreham-By-Sea and Beachy Head, is dominated by erosional landforms east of Brighton Marina, except at Newhaven Harbour, Seaford Beach and Cuckmere Haven. Cliffs and shore platforms are developed on a variety of Chalk formations (Photo 1), with other rock types making a contribution locally. West of Brighton Marina, the Chalk outcrop strikes inland, and unresistant Eocene strata and overlying Quaternary drift sediments form a low elevation coastline that forms the eastern extremity of the West Sussex coastal plain.
The lithology and structural characteristics of the Chalk are spatially variable, with the disposition of parting planes - especially master joints - accounting for the nearly vertical form of the cliffs between Roedean and Beachy Head. The more complex morphology and higher elevation of the east-facing cliffs of Beachy Head (Photo 2) reflect the fact that the coastline cuts across the chalk cuesta. Much of the cliffed shoreline east to Seaford Head is protected by seawalls and groynes; defences have been progressively extended and upgraded since the early to mid nineteenth century to give security to property and infrastructure. The coastal hinterland between Shoreham-By-Sea and Seaford forms a major element of the south coast conurbation, much of which has been developed over the past 150 years.
Wave energy is high in comparison to much of the rest of the coastline of south-east and central southern England. This is the result of its orientation with respect to both dominant and prevalent waves. Maximum wave energy is experienced along the shoreline between Seaford and Beachy Heads; for this sector, the influence of rock structure and resistance is probably subsidiary to wave forces in determining rates of recession and planform evolution.
Beaches are dominantly made up of a gravel backshore and a sandy foreshore, but the latter is often absent or suppressed where shoreline platforms occupy most of the inter-tidal zone. Littoral transport of coarse sediment follows a west to east pathway, but with local and short period reversals. Actual rates of longshore movement are almost everywhere below potential rates, primarily due to insufficient supply of gravel from input sources. Consequently, many beaches have a history of volume loss, with cross-profile steepening also diagnostic of supply deficit. Throughout the nineteenth and much of the twentieth century this problem has been addressed through the use of control structures, particularly sets of long and short groynes. Since the mid 1980s, emphasis has moved to replenishment and recycling as means of long-term beach management, exemplified by sites such as Seaford Beach and Rottingdean.
The mouths of the Adur, Ouse and Cuckmere rivers interrupt the continuity of this coastline. Well into the historical period each was a natural estuary, albeit gradually modified by mid to late Holocene sedimentation. Strong west to east longshore transport created shingle spits that caused the eastwards migration of each of these river exits. It is probable that they are barrier spits, fed in part from offshore, with a history of growth, breaching, retreat and regrowth over the past two or three millenia. Starting in early medieval times, the entrances to the Adur and Ouse have been utilised as major regional ports. Since the late nineteenth century, the ports of Shoreham and Newhaven have had their access and entrance channels protected by massive breakwaters (to the west) and piers/training walls (to the east). Their navigation channels have also been progressively deepened and routinely dredged.
The effect of breakwater and piers at both sites has been to impede the longshore transfer of coarse sediment, with very substantial updrift acccretion. However, it is apparent that some by-passing occurs, thus they represent partial, rather than absolute, barriers to transport. Brighton Marina (Photo 3) is another artificial structure with a potentially similar effect, but the evidence suggests that it occupies the site of a natural transport discontinuity. Sand and finer sediments (silt, clay) move within an inshore zone wider than the seaward projection of any of these structures, and they are therefore less affected than coarse (gravel-sized) sediment. The principal transport sub-cells of the western and central section of this coastline are therefore delimited by these artificial constraints on littoral sediment drift, viz: (i) Shoreham-By-Sea to Brighton Marina (West); and (ii) Brighton Marina (East) to Newhaven Western Breakwater. Further east, Seaford Bay is a well-defined sub-cell, with its eastern boundary determined by its large terminal groyne and by Seaford Head (Photo 4). A further sub-cell boundary may exist immediately downdrift of the mouth of the Cuckmere River, but the evidence for this is uncertain.
During the last decade, knowledge and understanding of the coastline of East Sussex has been considerably advanced. The regional Shoreline Management Plan (Gifford Associated Consultants, 1997) presented a comprehensive and detailed synthesis of previous, but hitherto fragmented, research. It also included original work on littoral sediment transport rates and provided elements of sub-cell sediment budgets. Beach morphodynamic behaviour was clarified by examining trends of beach volume, inter-tidal width and steepness over the past 100 - 120 years. This work has been followed up with the publication of four Coastal Defence Strategy Studies (Halcrow Maritime, 2001a and 2001b; Mouchel Consulting Ltd, 2001a; Posford Duvivier, 2001). Each contains a more specific analysis of coastal geomorphology in the particular context of defence history and future strategic approaches to shoreline management. Academic research based at the Universities of Brighton, Sussex, Bournemouth, Portsmouth and North London has contributed new insights into cliff and platform erosion processes, beach behaviour and shoreline evolution. Two recently completed European Union projects have examined (a) cliff geology and morphology in the context of risk analysis (ROCC, University of Brighton); and (b) beach sedimentology and budgets (BERM, University of Sussex).
This report attempts a fully up to date and comprehensive review of both historical and contemporary knowledge of sediment inputs, throughputs, stores and outputs. Its main objective is to collate and integrate analytical research previously published, concluding with suggestions of topics where further work is needed.
1.1 Coastline Evolution
The Open Coast
The ancestral coastline of East Sussex may be traced back to the interglacial period following the creation of the Strait of Dover by 'ponded' glacial meltwater. The current consensus is that this "catastrophic" geomorphological event occurred towards the end of the Anglian glacial of the Middle Pleistocene, or Oxygen Isotope Stage 12 (Keen, 1995). Ice sheet expansion during this phase is likely to have created a glacio-isostatic forebuldge in the present region of the English Channel, but subsequent subsidence cannot be determined from any surviving evidence. The earliest firm evidence of relative sea-level change along the present East Sussex coastline is the presence of an interglacial raised beach and adjacent buried cliffline west of Roedean. This is clearly exposed at Black Rock (Brighton Marina), but extends westwards to Portslade and Shoreham-By-Sea beneath more recent sediments (Redman, 1852; White, 1924, Young and Lake, 1988). The basal rock platform at the type locality has a maximum elevation of +8.5 m OD, but descends to +6.0 m OD at Durrington and coincides with mean sea-level at Lancing. Numerous authors have described the sequence of basal erratics; rounded chalk, flint and some sandstone clasts, and overlying solifluction/colluvial sediments resting against an obliquely-exposed fossil cliffline (Smith, 1936; White, 1924; Hutchinson and Millar, 1998 and ROCC, 2001). Although the mean elevation of this displaced shoreline has increased with progressive marine erosion, amino acid ratios obtained from raised beach molluscan shells give a last (Ipswichian or oxygen Isotope stage 7) interglacial age). This has been independently confirmed by Parfitt et al (1998) using palaeoecological indicators. The soliflucted material is evidently a product of cold climate conditions during the succeeding Devensian glacial stage. The presence of an interbedded palaeosol at the Black Rock section has been provisionally dated to Oxygen Isotope stage 5 (Keen, 1995).
The westwards decline in elevation of the Ipswichian raised beach might be interpreted as evidence of subsequent glacio-isostatic and/or tectonic downward displacement of this sector of coastline. However, there are complications arising from the probable creation of an isobaric forebulge during the late Devensian and its subsequent, and probably ongoing, relaxation. Although modern tide gauge records indicate relative sea-level change, it is currently difficult to quantify neotectonic movement from the fragmentary survival of evidence of earlier sea-levels (Keen, 1995). The tectonic setting of the adjacent West Sussex coastal plain and Solent terrace system is distinct from that of East Sussex and does not, therefore, provide a simple or direct analogy for longer-term coastline displacement in this region.
During the last glacial (Devensian) stage of the Pleistocene, sea-level fell by at least 100 m. Robinson and Williams (1983) ascribe a distinct break of slope at this datum, some 14 km south of Beachy Head, to cliffline formation during a period of low sea-level stability. Jennings and Smyth (1987; 1990) briefly describe a submerged cliff some 6km seawards of Beachy Head, which they ascribe to erosion during one or more stages of shelf emergence in the Devensian. Jones (1971; 1981) has identified the basal Chalk surface below the Ouse Valley to be -29.6 m OD at Newhaven and -12.2 m OD at Lewes. Smith (1985) has traced a buried channel extension of the River Cuckmere, tributary to his "Northern Palaeovalley", some 5 to 7 km southwards of the modern East Sussex coastline. Unlike most other palaeovalley segments of the eastern English channel, this is only partially sediment-filled and is therefore readily discernible from sea bed surveys. Bellamy (1995) reports evidence for a fluvially-eroded, but now infilled, buried channel that connects the mouth of the Adur with the same palaeovalley system. The confluence is inferred to have been some 30 km south of Shoreham-by-Sea, suggesting a contemporary mean sea-level 50 to 60 m below its present position. By analogy with his detailed description and analysis of the sediment stratigraphy of the buried channel of the Arun further west, Bellamy (1995) proposes that much of the gravel infill was fluvially supplied under a periglacial regime. Castleden (1996) has recorded a bedrock floor at the mouth of the Cuckmere at about -25 m.O.D., declining upstream to -13.1m O.D. at Exceat Bridge. Young and Lake (1988) state that a buried channel graded to -23m O.D., now infilled with Holocene alluvial deposits is present beneath New Shoreham. Bell (1976) has determined a buried channel at -25.8m O.D. close to Newhaven, below an excavated Romano-British site. These various reports therefore suggest that, allowing for seaward extension of valleys at gradients of between 2 and 4 m/km (Robinson and Williams, 1983), sea-level was at approximately -35m O.D. some 10 to 10.5 ka B.P. Jennings and Smyth (1987; 1990) state that the extrapolation of rates of cliff recession for the past 200 years would locate the coastline at this time some 6-12km seawards of its modern position. White (1924) suggests it was approximately 3-4km seawards 4-5000 years B.P.
Devensian cold climate conditions promoted severe freeze-thaw weathering and active solifluctional mass movement over pre-existing Chalk valley slopes. The accumulation of soliflucted colluvial infill ("Coombe Rock") in pre-existing valleys is clearly visible along several coastline sections, where it has been exposed by cliffline retreat. All of these are now dry valleys, due to the loss of surface run-off on chalk slopes and absence of discharge in chalk streams, following the replacement of permafrost conditions by temperate climate during the modern Holocene interglacial. The permafrost table was within a few metres of the ground surface, thus preventing normally rapid rates of infiltration and percolation. Most dry valleys are also "perched" or "hanging", as their former seaward extensions have been removed by rapid coastline recession during the Holocene. Although several Coombe Rock sections between Roedean and Saltdean have been placed behind robust coastal defences, at locations such as Birling Gap this mechanically weak material is fully exposed to wave erosion. In addition, Devensian cold climate weathering has also reduced the coherence and stability of sub-surface Chalk and added other structural properties, which induce cliff failure along most parts of this coastline east of Brighton Marina (ROCC, 2001). A low angle soliflucted fan of Coombe Rock occupies the low coastline west of Kemp Town, Brighton. It is difficult to reconstruct its former seaward extent, but it is likely to have been sufficiently extensive to provide a substantial source of flint debris for offshore to onshore shingle transport during the Holocene marine transgression (Jennings and Smyth, 1990; 2001). Release of flint may have been assisted by decalcification of this deposit during its exposure to earlier sub-aerial weathering.
Rapid recovery of sea-level rise during the early and mid Holocene is recorded in complex sequences of minerogenic and organogenic sediments infilling the valleys of the Adur, Ouse and Cuckmere. Sea-level stood at approximately -27m at 9 Ka B.P., -15m at 7.5 ka B.P. and -5m at 5.5 ka B.P. It was during this period that brackish, and then marine, conditions penetrated into what are now the floodplain sections of the three main river valleys. Between about 5ka and 3.2 to 3.3ka B.P. inundation created a strongly embayed coastal plan, with well-developed ria-like inlets. Major headlands, especially Beachy Head, were probably well defined by about 5ka B.P., with consequent reduction of longshore sediment movement around them. Jones (1971) has dated the earliest entry of brackish conditions at the present day mouth of the Ouse to approximately 6.3 ka B.P., using radiocarbon dating of organic sediments. Sediment stratigraphy indicates that marine conditions replaced brackish and that the tidal limit extended upstream of Lewes by approximately 3.2 ka B.P., when sea-level was -2m O.D. After 3 ka B.P., terrestrial sedimentation became dominant, and steadily converted an estuarine embayment into a river floodplain. Human-induced acceleration of alluvial sedimentation due to forest clearance and land cultivation in Neolithic and later times is a significant cause of this relatively abrupt change in environmental conditions. Research undertaken at sites east of Beachy Head, such as Coombe Haven; Willingdon Levels (Jennings and Smyth, 1997; 1990; Jennings, 1990), Pevensey and Pett Levels and Romney Marsh has provided more detailed and securely-dated evidence for a complex suite of Holocene environment changes. However, these areas may not provide a precise analogy for the lower Ouse, as sedimentation of formerly open embayments was probably controlled by locally complex relationships between the growth and periodic breaching of gravel barriers increases in river discharge due to reduced vegetation cover, and the hydraulic response of estuary shape to accommodate fluctuations in tidal prism.
The lower Cuckmere demonstrates local variations in the sequence, and timing, of Holocene events. Castleden (1996) argues that it was a narrow estuarine inlet by about 5 ka B.P., with tidal conditions penetrating upstream to Alfriston. He points to perhaps ambivalent evidence of marine "trimming" of the 5 m high cliffed slope on the eastern side of the valley below Exceat Barn. Estuarine sediments below modern alluvium suggest that tidal flat formation may have commenced as early as 4.5 ka B.P., and that brackish conditions persisted until about 1.5 ka B.P.
It is therefore probable that the East Sussex coastline was well embayed in Romano-British times, and that the lowermost sections of the Adur, Ouse and Cuckmere valleys persisted as natural tidal basins as late as the twelfth century A.D. (Bell, 1976; Robinson and Williams, 1983). Drainage and land claim does not appear to have commenced earlier than the tenth century in the case of the lower Adur, and it did not become an organised process in the Ouse Valley until the early sixteenth century (Brandon, 1971). Some minor river re-entrants, eg. at the original sites of Brighton and Seaford, were not removed until the seventeenth and eighteenth centuries. Thus, the broad arcuate planform of the modern coastline between Shoreham-by-Sea and Beachy Head is largely a feature of the modern historical period.
Estuarine sedimentation was accelerated by longshore drift-fed growth of shingle spits eastwards across the entrances of each of the main inlets. This may have been accomplished during earlier stages of inlet definition following the deceleration of the rate of relative sea-level rise after approximately 5.5 to 5 ka B.P. It is considered probable that spit elongation was accompanied by net accretion of gravel due to onshore barrier migration. Phases of rapid landwards barrier movement during recent centuries may be inferred from various historical sources (Gifford Associated Consultants, 1997). Breakdown, and backbarrier inundation, of the original (Norman) site of the fishing community of Brighthelmstone (Howell, 1870; White, 1924) is consistent with this interpretation. Nicholls (1991) considers that more or less continuous beach depletion occurred here between 1545 and 1713, with subsequent short episodes of erosion. Howell (1870) describes brackish estuarine deposits beneath the Pool Valley, which may therefore have been created by an earlier breach of a gravel barrier. The 'Old Town' of Brighthelmstone was accommodated beneath cliffs either side of a narrow alluvial valley, but retreated to higher ground between the thirteenth and early eighteenth centuries as a result of storm-induced erosion. The 'Great Storm' of 1705 destroyed what remained of the Old Town, and up until the mid eighteenth century the 'New Town' was an open coast site exposed to erosion (Redman, 1852; Howell, 1870; White, 1924; Castleden, 1996). However, from approximately 1750 there was an abrupt change in morphodynamic behaviour, with net accumulation of beach material as part of the creation (or re-creation) of a barrier structure. This continued to move landwards across the site of the Pool Valley, until finally restrained by the urbanisation of the hinterland and the introduction of control structures after about 1790.
Indirect evidence in favour of barrier morphogenesis may be inferred from the evolutionary changes, especially eastward elongation, crest elevation and foreshore progradation, of the spits at the entrances of the Adur and Ouse. Barrrier emplacement is also a plausible explanation for the periodic closure of the mouth of the Cuckmere.
Jennings and Smyth (1987; 1990) provide detailed evidence from bio- and lithostratigraphic analysis of valley infill sediment at Coombe Haven, near Hastings, for coarse clastic barrier growth in the late Holocene. The original sediment feed may have been via eastwards longshore drift, with subsequent supply derived from offshore sources. Further west, Willingdon Levels and the Crumbles cuspate gravel complex provide further evidence of a complex sequence of events involving cyclic erosion and deposition linked to barrier growth and breakdown.
Willingdon Levels may provide a framework for understanding the late Holocene coastal evolution of the lower valleys of the Ouse and Cuckmere. During the period 5 to perhaps 3 ka B.P. it was a low energy tidal embayment that interrupted what is now unidirectional longshore drift. Once sea-level stabilised, barrier spit growth extended across the entrance; this condition may have persisted for at least two millenia, and was not disrupted until the twelfth century (Jennings and Smyth, 1987; 1990). The relative importance of sources of supply of gravel to feed barrier growth is uncertain, but derived from either updrift sources, bypassing the then less prominent headlands of Seaford and Beachy Heads; or from offshore sources. The late Holocene growth of the Crumbles cuspate foreland at Eastbourne is strongly suggestive that the gravel supply, rather than sand, was predominant, though there may have been an earlier phase during which much of the sediment input was sand. It is thus conceivable that the first generation barrier here was composed of sand and that the structure broke down as this supply became exhausted, and/or the entrance of the tidal pass widened and deepened. Much of this offshore supply could have been derived from Pleistocene raised beach and Coombe Rock deposits, the latter rich in weathered flint. The Arun, Adur, Ouse and Cuckmere would have contributed additional coarse sediment when they drained a tundra environment. All of this material would have been reworked and redistributed during the complex sequence of Quaternary climate and sea-level changes. Substantial input would also derive from the earlier denundation of Chalk and Eocene bedrock, a long-term process which would have bevelled some 12 to 15km of Chalk outcrop, as well as the Portsdown to Littlehampton Chalk pericline further west. More speculatively, sand and gravel is likely to have been introduced into the eastern English Channel once the Strait of Dover had been created.
There was, then, an abundant supply of flint to build gravel barrier structures along the entire length of the shoreline of East Sussex at this time. Nicholls (1991) considers that the major period of barrier growth was 3 to 2,300 years B.P., known from sites elsewhere (e.g. Dungeness) to have been a period of exceptional storminess.
The analogy with better-developed barriers along the south and southwest coasts of England (e.g. Chesil Beach; Slapton Sands; Dungeness; Sandwich Bay) is close. However, all of these examples are now eroding, or are in a condition of chronic disequilibrium. A main cause of their degeneration is the exhaustion or substantial decline of offshore "reservoirs" of gravel and sand. In East Sussex, the erosion of beaches and accretion structures such as The Crumbles and Dungeness, since the eighteenth century is consistent with this. Barrier breakdown has also been accelerated by a switch from swash to drift alignment, which had the effect of promoting strong spit growth across the previously wider tidal passes of the Adur, Ouse and Cuckmere. This, in its turn, caused a previously more segmented shoreline, characterised by several discrete cells or sub-cells, to become dominated by uni-directional (eastwards) drift between major headlands (Nicholls, 1991).
Jennings and Smyth (1990), after documenting the history of the Crumbles cuspate foreland hypothesise that there have been several short period (up to 50 year?) "pulses" of offshore to onshore gravel transport superimposed on a longer-term history of diminishing feed. This is considered to result from a change from a dissipative to a reflective domain. The last such "pulse" coincided with a major phase of growth some 300 years ago. It is tempting to adapt this concept to the coastline west of Beachy Head, but there is no specific evidence to support it. One important implication, however, is that the morphodynamic behaviour of regional beaches is subject to cyclical fluctuation, with rapid phases of sediment gain separating longer periods of loss. Seen in this context, coastal defences are not necessarily a fundamental cause of beach deterioration (e.g. Seaford Beach) as both current and historical losses are diagnostic of a tendency for natural erosion. However, in the case of the coastline between Shoreham and Newhaven, modern beach budgets have been greatly modified by the insertion of orthogonal structures such as massive breakwaters and groynes.
Nicholls (1991), in discussion of the hypotheses of Jennings and Smyth (1990) places strong emphasis on the fact that headlands (especially Beachy Head) were less prominent features of the coastline in the mid to late Holocene. They were therefore less effective barriers to longshore transport, thus linking West and East Sussex in a more unified littoral drift system. This view would allow potentially large quantities of gravel moving onshore at, and east of, Selsey Bill to feed beaches up to some 50km eastwards. Rates of long-shore transport during the late Holocene would have been a function of prevailing coastal planform, mean wave height, wave approach, the magnitude/frequency of large storms and offshore bathymetry. By-passing of estuary mouths would have been achieved under storm conditions, when remobilisation of offshore stores might also have occurred. This would create conditions for "pulsed" longshore transport, in contrast to episodic offshore to onshore gravel inputs preferred by Jennings and Smyth. Nicholls (1991) also calculates that an annual drift rate of approximately 80,000m3a-1 along the West and East Sussex coasts would have been needed to create the gravel store of the Crumbles, Eastbourne over the 500 year period (800-300 years B.P.) during which he argues it was formed. However, it is possible that there were precursors to the modern Crumbles foreland, and that gravel from as far west as Selsey Bill might have moved longshore to supply Dungeness and the Camber ness. If this were the case, the drift rate along the East Sussex coast prior to approximately 2000 years B.P. must have been substantially more than 120,000m3a-1. This is some five to six times prevailing rates, and is considered by Jennings and Smyth (1991, in reply to Nicholls) to be unlikely. They therefore prefer a sediment budget dominated by offshore supply.
Evolution of the mouth of the Adur and adjacent coastline
Although it is likely that the river Adur had a wide estuary mouth in Romano-British times, its exit was deflected progressively eastwards by the sustained growth of a gravel barrier spit during the ensuing 1000 years. Although Morris (1931a and b) noted substantial erosion at the site of Old Shoreham in the fourteenth and fifteenth centuries, this should not be taken as evidence of the absence of a spit enclosing the harbour at that time. It is probable that Shoreham spit is a barrier structure that has experienced periodic breaching, a hypothesis originally proposed by Ward (1922). Residual salt marsh and mudflats at Lancing (reclaimed in the sixteenth century) and a chain of marshy lagoons between Goring and Hove surviving up to the mid eighteenth century (Brookfield, 1952) have been interpreted as formerly contiguous barrier-confined features. Widewater lagoon is the most well defined surviving component of what may have been a nearly continuous back-barrier wetland. Brookfield (1949; 1952) made the indirect suggestion that Shoreham spit was substantially the result of onshore barrier migration, rather than simply linear growth fed by longshore drift. Breaching would have occurred on several occasions, almost certainly during the several "super" storms that occurred during the thirteenth and fourteenth centuries. If this hypothesis is accepted, Shoreham spit is thus one part of a regional coarse clastic barrier system that has experienced progressive landwards translation over the past several centuries. The erosion and loss of the former port of Pende, to the west (Ballard, 1910; Ward, 1922); and rapid storm-induced onshore beach migration at Brighton in the mid seventeenth century and Worthing in the 1820s (Gifford Associated Consultants, 1997) are consistent with this explanation. The latter event involved almost 100m of shoreward movement in a few days, eliminating the pre-existing lagoon or marsh at East Worthing. Young and Lake (1988) report borehole evidence for well rounded storm beach gravels overlying alluvium beneath the A259 at Shoreham, as further support for barrier translation.
Net eastwards longshore transport west of the modern entrance to Shoreham Harbour is a significant process, and was undoubtedly responsible for the eastwards migration of the mouth of the Adur prior to the construction of breakwaters and training walls (Martin, 1939). It was the likely cause of the infill of the 'new cut' of 1698, which temporarily provided New Shoreham with a direct outlet to the sea, possibly through a breach site. However, the effectiveness of longshore drift in closing this artificial entrance within 20 to 30 years was aided by contemporary land claim of the alluvial marshes of the lower Adur. This occurred over a relatively short period (Ward, 1922; Martin, 1939) and reduced river mouth current velocities due to a reduction in the estuarine tidal prism. Between 1700 and 1760 there was almost 2 km of eastward spit migration, leaving New Shoreham - built in the eleventh century because of channel siltation up-estuary - some 1.3 km from the entrance to the Adur. A new channel, opposite Kingston-by-Sea (Southwick), was cut in 1762, piers were constructed and the former river mouth closed off. This proved ineffective, requiring a new channel some 0.6 km eastwards in 1783. Poor maintenance quickly rendered this entrance un-navigable and the spit migrated 30 to 40 m a-1 eastwards during the subsequent 35 years. Prompted by concerns of economic decline, the 1762 channel was re-opened in 1818 and provided with substantial training walls (Brookfield, 1949). Dredging was thereafter undertaken regularly to avoid shoaling, and the former river channel east of Kingston was converted to a canalised harbour basin in the 1850s. The modern western breakwater and eastern training wall continue to maintain this entrance, with a routine dredging operation to avoid natural sedimentation in areas of weak tidal currents and/or limited wave penetration. The western breakwater, in particular, intercepts a substantial proportion of potential longshore by-passing of the harbour mouth by gravel. This has necessitated deliberate management of the local sediment budget, removing a proportion of the updrift surplus and transferring it to downdrift beaches, which have a long history of sediment deficit. This operation, and the complex flux of both onshore and offshore - directed sediment flows in the entrance channel is addressed in Section 2.4. and Section 4.2, respectively.
Historical evolution of the mouth of the Ouse
The medieval exit of the Ouse was southeast of the original port of Seaford, as a consequence of the progressive eastwards downdrift growth and onshore migration of a gravel barrier spit across an original shallow tidal embayment. This phase of growth presumably occupied at least several earlier centuries and promoted estuary mud flat accumulation. The Mill Creek represents a surviving portion of the early to mid sixteenth century estuary, whose mouth was abandoned before 1540 by continuing mobility of the distal end of the spit and by siltation of the tortuous access channel to the moribund port of Seaford (Morris, 1931; Carr-Gregg, 1952). Alluvial sedimentation was promoted by the reduction of the tidal prism due to lower estuary land claim or "inning" (Brandon, 1971; Farrant, 1972). The latter process, however, increased flood hazard from high discharges of the Ouse (Robinson and Williams, 1983). It is uncertain if inundation of medieval properties, dating from the mid fourteenth century at Seaford (Bell, 1976) was due to this cause or to storm overtopping of the barrier spit. Morris (1931) stated that a new exit was created in 1565, between Tide Mills and the site of the Buckle Inn, as a direct result of a storm surge induced breach. However, Brandon (1971) used documentary evidence to confirm that a channel was deliberately cut, and a port facility established, at New Haven in 1539. This was partially blocked by eastward longshore transport over the next 150 years, despite periodic attempts to clear and re-open it. This probably explains, in part, the progressive deterioration of the drainage of Lewes and Laughton Levels during the seventeenth century (Brandon, 1971). The Ouse is known to have overtopped, and broken through, the spit at Tide Mills, in 1676 and 1698. The latter event created a new exit, but the "cut" of 1539 was re-opened in 1731 with the addition of protective piers. However, the problem of accretion was apparently accelerated by their presence, and eastwards drift was not effectively constrained. In response, the first substantial rock breakwater was constructed, west of the harbour entrance, in 1791; the river channel was straightened and Newhaven re-established as a port. To ensure access, the approach channel was given additional protection through the extension of the length of this breakwater to 150 m (1847) and 800 m (1878-1890). The latter, constructed of rock and concrete, is effectively the modern structure. The first breakwater, or pier, on the eastern side of the Ouse entrance dates to 1664, and has been periodically reconstructed, strengthened and relocated to widen the river mouth over subsequent centuries.
Although breakwaters, together with channel deepening and dredging, have removed the hazard of the accumulation of a bar at the outer harbour entrance since the mid nineteenth century, other impacts have been experienced. These include the accumulation of gravel immediately updrift of the western breakwater; the accretion of finer sediment on the eastern sides of both breakwaters and the progressive erosion of Seaford Beach. The latter effect, however, may not be linked directly to the impedance of longshore transport by the presence of the harbour breakwaters. Other causes, including the gradual depletion of the finite sand and gravel store in Seaford Bay, may be implicated. Substantial losses of beach sediment can be correlated with individual storm surge events, eg. 1825 and 1877 (Morris, 1931); 1897 (Robinson and Williams, 1983) and 1943 (Hydraulics Research, 1986c). These issues are examined in more detail in Sections 3 and 4.
Cuckmere Haven: Historical Change
The original ria-like estuarine inlet was progressively infilled by alluvial sediments during the later Holocene, a process that was accelerated by the eastwards growth of a coarse clastic barrier spit fed by both longshore and onshore gravel transport. This caused the lower channel and mouth of the Cuckmere River to be deflected eastwards, with periodic closure due to a combination of "pulses" of increased onshore or longshore sediment supply and reduced tidal prism due to land claim in the lower floodplain. Fluctuations in the position of the mouth between the late eighteenth and early twentieth centuries are recorded by Gifford Associated Consultants (1997). Isaac (1915) has described short-term blockage of the river entrance by surge-induced beach sedimentation in July 1912, which caused extensive floodplain inundation. The river mouth (Photo 5) is now in its approximate late eighteenth century position (Castleden, 1996) and is confined by training walls originally introduced in the late nineteenth century, but subsequently upgraded following periodic failure (Clifton and Cecil, 1999). This has induced the formation of a small ebb delta with a continuously shifting pattern of distributary channels. Net accretion at this point is probably maintained by local wave refraction caused by the set back of the coastline, creating a shallow embayment.
1.2 Wave and Tidal Conditions
The mean (spring) tidal range for this coastline, increases from west to east, viz: Shoreham-By-Sea 5.6m; Brighton, 6.01m; Newhaven, 6.3m; Beachy Head, 6.4m. Tidal streams, derived from Associated British Ports Research and Consultancy Hydrodynamic Model of the English Channel, are given in the regional Shoreline Management Plan (Gifford Associated Consultants, 1997). Flow is eastwards on the flood tide, westwards on the ebb. A strong tidal current residual (headland vortex) exists off Beachy Head, characterised by rectilinear currents with peak (spring) velocities of 1.3m.s-1. Velocities are much lower elsewhere, e.g. 0.8m.s-1 (springs) and 0.4m.s-1 (neaps) in the entrance channels to the port of Shoreham and Newhaven Harbour. These decline to less than 0.3m.s-1 within 200m of mean low water. Ebb tidal current speeds are faster than on the flood at the mouths of the Adur, Ouse and Cuckmere.
Knowledge of the regional wave climate is limited to a number of relatively short term studies undertaken by consulting companies for specific projects. Most have involved hindcasting using regional data sets on wind speeds.
Both observational (instrumental) and hindcast data on significant wave heights (Hs) are available for Shoreham-By-Sea from several studies. Hydraulics Research (1984) recorded maximum Hs of 5.25m during a short-term monitoring period of offshore waves. A later study (Hydraulics Research, 1989; Jelliman, et al, 1991) used the MULTIGRID wave model to create an inshore wave climate, derived from a hindcast offshore wave climate using data for a one year period. This approach utilised three-dimensional bathymetry to compute the effects of refraction and diffraction set up by both seabed relief and - more importantly - harbour breakwaters. The latter can increase wave height by a factor of 1.6 to 1.8. This study concluded that waves up to 6.0m in height can be generated during south and south west approaching storm surge waves with a recurrence of 1 in 20 years. The equivalent height for waves coming from the east or south-east is 4.7m. Sir William Halcrow (1990) measured Hs values between 1.45m and 4.3m at the outer entrance channel over a 6 month period between October and April. These values were re-calculated at 2.30 and 4.87m, respectively, for a 1 in 5 year storm recurrence. Hydraulics Research (1986b), specifically examining long period waves off Shoreham seawards of the approach channel, give Hs values of between 4.0m (annual recurrence) and 4.7m (1 in 10 year frequency). Posford Duvivier (1993) reported that Hs exceeded 3.0m for 1.6% of a three month summer season monitoring period, within the main entrance channel. This apparently related to diffracted waves "caught" between the western breakwater and eastern pier. The longest hindcast data series is that assembled by Duane (1998) covering the period 1971 to 1998. The wave climate is described in detail by means of "wave roses" giving the frequencies of occurrence of different combinations of wave height and direction. Reasonable correspondence was achieved between the hindcast waves and a comparison made with several months of direct field measurements.
Further east, Bailey (1999) notes a maximum wave height of 7.23m off the West Pier, Brighton; however, the basis for this calculation is not clear. Hydraulics Research (1986a) state that there is a 1 in 10 year frequency of Hs of 6.24m off Brighton. Halcrow Maritime (2001b) state Hs to be 4.79m for a 1 in 5 year recurrence at Brighton, but this appears to have been derived directly from earlier analyses for Shoreham. Mouchel Consulting (2001) propose a maximum Hs of 4.4m (annual recurrence) for waves offshore Newhaven. Hydraulics Research (1985) undertook wave monitoring, over a 12 month period, to construct a wave climate for Seaford Bay. Wave rider buoy data was fed into the OUTRAY wave refraction model to derive Hs of 1 m and 3m for 7.2% and 0.9% respectively, of the measurement period during which south-westerly waves were operating. A maximum Hs of 1.8m was achieved by waves approaching Newhaven Harbour from the south-east.
Posford Duvivier (1993), using wave data for Shoreham and Pevensey Bay calculate a once per year frequency of Hs of 4.56m at Birling Gap. This increases to 5.48m for a one in 20 year recurrence. There is no quantitative data for wave height or energy impacting the shoreline east and west of Birling Gap, but it is generally acknowledged that regional maxima are experienced between Seaford and Beachy Heads. Diffraction set up by the ebb delta offshore Cuckmere Haven would be expected to locally reduce Hs values. Conversely, they would be increased by refractive convergence of incident waves around the several salients of Beachy Head.
All the above studies are in agreement that waves from the west and south west are both dominant and prevalent (May, 2000) along most parts of this coastline. Hydraulics Research (1989a) calculated that 48% of all waves approaching outer Shoreham Harbour propagated from between south and west. Waves moving in from between east and south-east directions represented 28% of the total during the measurement period. The remaining 16% were represented by waves from the south, with Hs below 0.8m. These figures remove the complicating effect of re/diffraction set up by breakwaters and training walls, which causes changes in approach as waves move from offshore to inshore. Although only three studies (Hydraulics Research, 1985 and 1989a and Jelliman, et al, 1991) were conducted over most, or all, of a one year period, it is well established that wave height and energy is greatest during the winter months. Most surge events have occurred between October and February, though documented records are almost non-existent for some sectors of this shoreline. One small sector where waves from the south-east may be locally dominant is between Newhaven Harbour East Pier and Tide Mills (Seaford Bay). This is due to a combination of coastline orientation and protection afforded by the western breakwater (Hydraulics Research, 1985). Local diffraction, and reduced incident energy, has been noted for the sector between Brighton Marina and some 2-300m further east (Posford Duvivier, 1994b; 1995; 2001).
Seaford Bay was one of the locations for which wave modelling excercises were undertaken as part of the DEFRA Futurecoast Project (Halcrow, 2002). An offshore wave climate was synthesised based on 1991-2000 data from the Met Office Wave Model and then transformed inshore to a prediction point in Bracklesham Bay at -5.5m O.D. Potential sensitivities to likely climate change scenarios were then tested by examining the extent to which the total and net longshore energy for each scenario varied with respect to the present situation. Results suggested that a one to two degree variation in wave climate direction could result in a 6-12% variation in longshore energy and confirmed that the Bay was significantly more sensitive to this factor than most other south coast locations, as might be expected of a swash aligned coastline.
Sea-LevelSeveral studies (Gifford Associated Consultants, 1977; Posford Duvivier, 1995 and 2001; Mouchel Consulting, 2001; Halcrow Maritime, 2001; Hydraulics Research, 1985 and 1986a; HR Wallingford, 1997) calculate extreme water levels and their recurrence intervals. All approaches require an estimate of contemporary relative sea-level rise, but at present there is a wide range of estimates. This reflects, above all, uncertainty over prevailing rates of land movement, including possible co-adjacent seabed displacement. Shennan (1989) has proposed isostatic/isobaric subsidence to be 0.1 to 1.4mma-1, whereas Bray, Carter and Hooke (1994) calculate the range to be 0.5 to 1.5mma-1. The latter authors therefore suggest a future relative sea-level rise of 5-8mma-1. Bailey (1999) gives the much lower figure of 1.70mma-1 for Brighton, but this would seem to be a form of extrapolation of recent sea-level data for Dover. Thompson (unpublished, 1977) estimates 4 to 5mma-1 for Newhaven (and the coastline west to Portsmouth), whilst Blackman and Graff (1978) deduce from a part of the tidal gauge data for Newhaven that sea level is currently rising at a rate of 4.1mma-1 (plus/minus 1.65mma-1).
2. SEDIMENT INPUTS - References Map
2.1 Marine inputs
F1 Wave-Powered Onshore Shingle "Creep" between R. Adur and Peacehaven
Experiments conducted between 4km and 10km offshore of Worthing and Shoreham by Crickmore et al. (1972) indicated a landward drift of gravel, which was small in volume and decreased inshore. At the 9m water depth onshore feed was measured at 1,000-1,500m3a-1 km-1 whilst at 12m depth it reduced to <500m3a-1 km-1. No transport was recorded in excess of 18m depth. If these studies are representative of the 48km of coast between Brighton and Selsey then this feed might total a potential of 48,000-72,000m3a-1 over this whole frontage, but this rate of feed will only occur where potentially mobile shingle exists on the seabed. There is a potential for this process to occur over the nearshore bed between Brighton and Peacehaven, but it is uncertain whether material can actually pass across exposed shore platforms to be supplied to beaches. Furthermore, it would be expected that the operation of the process over time could have already depleted potentially mobile inshore gravel reserves since re-supply would be unlikely from further offshore due to lack of gravel mobility in greater water depths. The knowledge of these processes was derived from experiments with radioactive tracers carried out over 2 years and correlated with wave data. As the study period was representative of storm conditions, the results are therefore reliable. HR Wallingford (1993) subsequently confirmed patchy distribution of inshore gravels together with the lack of shingle mobility beneath water depths greater than 15m to18m.
Joliffe (1978) also undertook tracing of coarse particles at a number of experimental sites, overlying Chalk surfaces and "patchy" gravels, 4.8 to 9.0km offshore Shoreham Harbour. Although observation was limited to three weeks, shoreward migration below water depths of 6 to 15m was observed, but none in depths greater than 15m. Smallest clasts moved furthest, with disc shaped particles the most mobile. Angular/sub-angular shapes were more readily entrained than rounded forms. Highest mobilities occurred across bedrock surfaces, with almost no movement recorded over silt and sand. There was no apparent correlation between clast transport and the velocity and direction of residual tidal streams, thus all movement was imparted by wave-induced stresses. In an earlier, experiment, Joliffe (1972) introduced tagged gravel clasts into water depths up to 4m below maximum low water 90-180m south of Hove. Rapid shoreward dispersal occurred wherever bedrock or packed cobble surfaces were present, with some reaching the beach within 2-3 weeks. Tidal currents (<0.75m.sec-1) were again considered an ineffective transportation agency. Wave heights during the experimental period varied between 0.5 and 1.0m. Sand ripples and fine gravel ribbons inhibited the movement of larger clasts.
Although patchy seaweed growth was observed, there was no sighting of kelp-rafting; Joliffe (1972; 1978) dismissed kelp-rafting as a possible auxiliary mechanism for moving gravel onshore in his other experiments. However, Gifford Associated Consultants (1997) incorporate assumptions of kelp-rafting of gravel sized clasts into the estimations of net onshore gravel transport. These vary from 2,500m3a-1, between Shoreham Harbour Entrance and Brighton Marina, to 6,000m3a-1 along the nearshore sector between Cuckmere Haven and Beachy Head. As kelp-rafting has not been directly observed along any part of the East Sussex coast, its relative importance in relation to total onshore "creep" cannot be assessed. The volumes quoted by Gifford Associated Consultants (1997) are very small components of the total sediment budget (Section 6). For the entire beach environment between Shoreham-by-Sea and Beachy Head, total inputs by offshore to onshore processes are unlikely to exceed 30,000m3a-1
F2 Wave-Powered Onshore Shingle "Creep" between Newhaven and Cuckmere
Experimental work and diving observations by Joliffe (1972) off Newhaven Harbour indicated that gravel particles were mobile over bedrock and other smooth surfaces free of silt sand and gravel. However, they lost mobility once they encountered areas of fine-grained deposition. Similar work carried out in Seaford Bay (Joliffe, 1964; 1972; 1978), using fluorescent-coated indigenous rounded gravel clasts, did demonstrate that material could move shorewards from modest (<5m) depths. As almost all of this material was originally derived from beach foreshores, it probably represents recycling rather than fresh input. Similar processes would be likely to operate within the Cuckmere Haven embayment, although they have yet to be studied directly.
2.2 Fluvial input
FL1 R. Adur, FL2 R. Ouse, FL3 R. CuckmereRendel Geotechnics and the University of Portsmouth (1996) estimated suspended and bedload delivery at the mouths of all rivers discharging at the coast of central-southern England. Their figures are based on an assumption that more than 90% of load is diverted into storage before reaching the open sea, much of it in alluvial plains, estuarine channels and inter-tidal flats. On this basis, suspended sediment delivery by the Cuckmere is calculated at 1,320 tonnes a-1, and by both the Ouse and Adur at 2,682 tonnes, a-1. Gifford Associated Consultants (1997) give figures of 800 m3a-1(Cuckmere); 3,700 m3a-1 (Ouse) and 2,800 m3a-1 (Adur) based on rating curves. In all three cases, coarse bedload is considered negligible, as only very limited exposures of river terrace gravels provide suitable input within catchments mostly underlain by Chalk and a variety of clays and fine grained sandstones. A figure of 131 tonnes a-1 of bedload is estimated for the Cuckmere (Gifford Associated Consultants, 1997), but no data is given for either the Ouse or Adur.
It is probable that input of river-transported coarse sediment into the nearshore zone occurs episodically, during high discharge events. Suspended load is removed seawards almost immediately, but the presence of a small ebb delta at Cuckmere Haven and dredging of navigation channels giving access to Shoreham and Newhaven Harbours indicate some potential retention of fluvially-derived coarse sand in their estuarine and nearshore environments.
Any further evaluation is hindered by the absence of gauging and monitoring data, but it is clear that the quantitative significance of fluvial input into the regional sediment budget is very low.
Davies (1973) located several large zones of freshwater discharges from the Chalk aquifer of the South Downs in the nearshore area between Hove and the Seven Sisters. Some are linked to higher rates of groundwater transmissivity within major joint and fissure systems of dry valleys. They discharge across inner shore platforms and beneath overlying beach sediments. The evidence was derived from airborne thermal line-scan imagery, which was insufficient to resolve either quantities of discharge or any suspended sediment output.
2.3 Coast Erosion - E1 E2 E3 E4 References Map
2.3.1 Cliff Erosion
There is a considerable literature providing estimations of rates of cliffline recession, both before and after the construction of coastal defences. Most data derives from analysis of the position of the cliff top on successive Ordnance Survey maps and plans, of which the most detailed is presented in Dornbusch (2002). This report presents a review of previous work and calculates a mean retreat rate for each 50m sector of Chalk cliffline between Black Rock, Brighton and Beachy Head between 1873 and 1995. This research, and other sources (e.g. Halcrow Maritime, 2000a; Posford Duvivier, 1993; 1997; 1999; 2001, Mouchel Consulting Ltd 2001a and b attempt estimations of the input of coarse (gravel sized) sediment from the release of flint from eroded Chalk. With the exceptions of Moses, et al, (2001) and Dornbusch (2002), additional input from shore platform abrasion are excluded from these calculations (see Section 2.3.2 on shore platform erosion).
Contemporary erosion rates are less than those calculated for the past 100-130 years where basal protection, cliff re-profiling, drainage and other management measures have been introduced. Most of these date to the early years of the twentieth century, with major works undertaken in the early 1930s and the period 1975-1985. The BERM project (Moses et al, 2001) concluded that 36% of former (now potential) yield of gravel from cliff erosion between Rottingdean and Beachy Head has been lost due to shoreline defence and protection.
Most of the cliffed coastline consists of near-vertical Chalk slopes, whose morphology is controlled by toe erosion exterted by breaking waves and various sub-aerial weathering processes (Photo 1 and Photo 2). Spatial variations of details of morphology reflect complex inter-relations between rock lithology, structural planes, and coastline orientation with respect to wave climate. Dip angles are low, but there are numerous steeply inclined joint, fault and other fracture surfaces. Two local morphological variations occur at (i) Newhaven Heights, where Palaeogene sands overlie the Chalk; and (ii) eastern Beachy Head, where slope failure has created an Undercliff and offshore reefs.
Details of Chalk stratigraphy, lithology and structure are given in the ROCC (2001) report. Most of the protected cliff frontage is developed in the Newhaven and Culver Formations of the Upper Chalk, which has an estimated flint content of 1.5 (+ 0.5)% of total volume. Between Seaford and Beachy Heads, the Seaford Formation, also Upper Chalk, dominates. In this case, the flint content is 2.5 (+ 0.5)%, but increases to 3.5 (+ 0.5)% at Seaford Head itself and 4.5% east of Bel Tout. These volume estimations, given by Dornbusch (2002) are lower than the average of 5% used in several previous studies.
Cliff top fissures, parallel to the cliff face, are frequent and usually joint-controlled. They promote block detachment via shear failure, occasionally creating substantial falls in excess of 50-70,000 tonnes (e.g. Seven Sisters, 1925; Seaford Head, 1986; Beachy Head, 1813 and 1999 - see Photo 6). Most falls and topples are small scale, involving the detachment of wedge-shaped units of less than 0.5m3 at a frequency of 8-10 years at any site. There are several weathering processes involved in reducing the coherence and stability of Chalk cliff slopes, notably salt crystallisation; freeze-thaw and wetting and drying cycles; and repeated heating and cooling. The most potent process is probably frost weathering (Cleeve and Williams, 1988), which is also highly effective in the denudation of chalk shore platforms (see following section). However, there is thought to be a complex link between frost disintegration and salt crystallisation induced by spray. Wetting and drying tends to have most effect on the frequent thin seams and bands of clay and silty clay within the Chalk. Groundwater saturation, affecting pore pressures and capillarity, are fundamental to the triggering of large falls. In places, e.g. eastern Beachy Head, groundwater seepage at the cliff base assists mechanical toe erosion. Most researchers (e.g. May, 1971; Posford Duvivier, 1993) have reported that the majority of losses due to both weathering and erosion occur during the harsher climatic conditions of winter. As an example, over 200,000 tonnes of chalk were detached in several failures behind the fully protected cliffs west of Peacehaven Steps during the winter of 2000/2001 (ROCC, 2001). However, heavy rainfall and storm surge events do occur in summer, when there may also be shrinkage of clays following drought conditions. All of these processes can open up fissures to critical widths and depths. Solution is also likely to be involved in joint and fissure opening, but its role has not been investigated. Overall, there is little direct and convincing evidence for the present day effectiveness of basal erosion. Notches and occasional shallow caves may not be contemporary features.
May and Heaps (1985) conclude that subaerial processes of weathering and mass movement operating on the Chalk cliffs of East Sussex are more effective than basal abrasion by waves. Much wave energy is absorbed by cliff foot debris created by falls, slides, topples, etc. which is also broken down and eventually removed by wave action. In comparison to most other rock types outcropping on the south-east and central southern coastlines of England, Chalk talus has a relatively short residence time, but is frequently replenished. May and Heaps (1985) observe that there is no evident positive correlation between rates of cliff top retreat and the presence/absence of persistent basal talus. The same is also the case for cliff retreat rates and beach width. Cliff height also fails to correlate with mean retreat rates, but there is a strong positive statistical relationship between platform width and cliff top retreat. This latter correlation is counter to what would be intuitively expected, i.e. highest retreat rates where platform widths are narrowest, thereby least modifying wave energy. These statements require further more detailed research before their validity can be fully accepted, but point towards the importance of climatic and geohydrological conditions.
Robinson and Williams (1983) calculate that the mean retreat rate for the 22km of Chalk cliffs between Black Rock and Beachy Head is 0.3 to 0.5m.a-1 (1873-1975). This refers to cliff top recession and uses an average cliff height of 45m. Locally, rates may be up to 1.5m.a-1 where single large volume and/or frequent smaller falls have occurred. Posford Duvivier (1997) estimate a mean rate of retreat for the sector between Seaford and Beachy Head, over the last 100 years, to be 0.4m.a-1. A retreat rate of 0.5m.a-1 over the past three to five millenia, previous to cliff protection, would yield approximately 25,000m3a-1 of gravel (Jennings and Smyth, 1990). This, however, is based on the assumption of 5% flint content, now considered to be an over-estimate (Moses, et al, 2001; ROCC, 2001). It also does not factor in the progressive increase in cliff height as the coastline has retreated. Nevertheless, 25,000m3a-1 is close to the average potential littoral drift rate under contemporary conditions (Section 3), suggesting that there would be a close correspondence between input and throughput without human modification of process regimes. Any difference might be accounted for by offshore to onshore transfer.
Several studies have made independent estimates of coarse sediment yield from calculations of retreat rates for specific sectors of this coastline. Most of these assume an average flint content of 5%. For the length of coastline between Brighton Marina and Peacehaven, Posford Duvivier (2001) calculated a yield of 2,200m3a-1 for the period 1870-1990. This, however, has now reduced considerably to 640m3a-1, where there are defences. Posford Duvivier (1999) considered that the current yield for the relatively short lengths (total 1.5km) of unprotected cliffs (Photo 7) was a little more than 1,900m3a-1. For the cliffline between Seaford and Beachy Head, Posford Duvivier (1999) propose that approximately 5,000m3 of flint gravel has been released annually over the past century. Of this total, 1,400m3 is contributed by the cliffline between Seaford Head and The Seven Sisters; 1,300m3 derives from Birling Gap (Posford Duvivier, 1993) and 2,000 to 2,500m3 is introduced by the denudation of the cliffs between Bel Tout and Low Gap, Beachy Head. Halcrow Maritime (2000a) suggest a yield of 5,400m3a-1 for the dynamic and complex cliff environment between Beachy Head and Eastbourne. All of these figures are derived from basic formulae using cliff retreat rates (back to 1870) of 0.3 to 0.7ma-1 and an average cliff height of 60m.
The BERM project (Moses, et al, 2001, Dornbusch, 2002) has made a detailed re-assessment of available data on the retreat rate of currently unprotected cliffline at the present time, between Rottingdean and Beachy Head. This produces a cumulative flint gravel delivery of 4610+ 890m3a-1, assuming a flint content of the Chalk of 3%. The equivalent volume proposed by Posford Duvivier (1999; 2001) is 5640m3a-1 though subject to a 7% error margin. It is clear that there has been an apparently substantial reduction in gravel input from cliff erosion over the past two centuries, in comparison to earlier millenia (Jennings and Smyth, 1990). The protection of over one half of the cliffed coastline is clearly the major explanation. Additional input from shore platform abrasion is a relatively negligible quantity.
E1 Brighton Marina to Peacehaven (see introduction to coast erosion)
With the exception of the 1.5km length of frontage that remains unprotected between Saltdean and Peacehaven (Photo 7), the Chalk cliff toe has been removed from the influence of wave-induced erosion since the completion of seawall construction. This occurred between 1928 and 1936 along the shoreline linking Roedean to Saltdean, but the present bullnose seawall, and walkway, at Peacehaven (Photo 8) was built between 1975 and 1982 over a frontage length of 2.2km (Stammers, 1985). Reconstruction of the defences between the Marina and Saltdean has been on-going since the early 1990s. The principle behind the new defences is to build a strong wall with (in the case of the latest and last phase) some attempt at breaking wave energy at the foot of the wall using a boulder 'fillet'. This approach was derived from the view that the coast in this area had insufficient shingle feed for groynes to accrete beaches of any useful size (Eade – personal communication). The absence of protection at Telscombe Cliffs is interrupted by a headwall around the Portobello wastewater discharge facility.
Before protection, the 30-44m high cliffs were nearly vertical, and the cliff top was retreating at rates of between 0.76ma-1 (Roedean, 1826-1897, May and Heaps, 1985) and 0.35m.a-1 (Western Peacehaven, 1873-1962, May, 1966). Various authors have calculated retreat rates for specific locations between the mid nineteenth century and the 1940s; these average at about 0.40ma-1 (Thorburn, 1977; Mott MacDonald, 1997; Gifford Associated Consultants, 1977; Mouchel Consulting, 2001).
In addition to seawall construction, several sections of the former cliffline were regraded, by hand, to angles of between 70( and 80( in the early to mid 1930s. This was undertaken to reduce the hazard of rock falls and slides originating from continuing sub-aerial weathering and mass movement. The Chalk to the west of the Portobello outfall is mechanically weaker than it is at, and east of, Peacehaven (ROCC, 2001; Posford Duvivier, 2001). It is also intensely fractured and fissured in places, especially close to the cliff top and adjacent to truncated shallow dry valleys infilled with Coombe Rock. East of Peacehaven, chalky loessic silt has infilled fissures, reducing mechanical strength. Planar and wedge failures are the most usual form of cliff face degradation that continue to operate; their size and shape is controlled by joint set orientation and their intersection by faults (Mouchel Consulting, 2001; ROCC, 2001). Several falls and topples have occurred in recent years, induced by high pore pressures following heavy or prolonged rainfall. It has been estimated that weathering, especially due to freeze-thaw, became active 15-20 years after cliff trimming (ROCC, 2001). An example, at Black Rock, Brighton Marina, occurred in 1971 (Corbett, 1990) and several during the winter of 2000/2001. Most failures involve relatively small quantities of material, but a few larger scale events have removed 1-2 m of the cliff top. There is evident potential for larger magnitude failures along reactivated relict slip planes, especially where basal protection modifies groundwater movement (Corbett, 1990; ROCC, 2001). Current rates of cliff recession are difficult to quantify, as failure events are apparently spatially random and a temporally-integrated periodicity has not been established. Moses, et al (2001) reports that BERM project researchers estimate an approximate rate of 0.26ma-1, lowest east of Telscombe Cliffs.
Posford Duvivier (2001) calculated that cliff and platform erosion between Brighton Marina and Peacehaven yields 2,200 m3a-1 of gravel, derived from the release of flints from the Chalk. Input of coarse clastic material is thus no more than 0.5 m3 per metre (Posford Duvivier, 2001).
E2 Peacehaven Heights to Newhaven (Western Breakwater) (see introduction to coast erosion)
Thorburn (1977) calculated that the 30-60 m high cliffs of this sector, which are unprotected along the length of Friars' Bay, receded at a mean rate of 0.46 m a-1, 1925-1955. Castleden (1996) gives a similar figure, and Mouchel Consulting (2001) propose a maximum rate of 0.6 m a-1 for the period 1875 to 1975. Gifford Associated Consultants (1997) state that cliff top retreat between 1875 and 1980 was 0.4 to 0.5 m a-1. Moses, et al (2001) calculate a contemporary rate of 0.25 m a-1. Conjugate joint sets control cliff morphology, giving a near vertical slope and promoting occasional plane failures.
From Harbour Heights to Castle Hill, Newhaven, the Chalk is overlain by Palaeogene sands and clays, preserved in a shallow syncline. Although the cliff base at Castle Hill is fully protected by the wide gravel beach that has accumulated against the western breakwater, the Palaeogene sediments have produced a dynamic 15-20 degree hummocky upper slope. Shallow translational sliding, gullying and mudflows are all active processes, particularly in winter, with a major slip occurring in 1943 (Castleden, 1996). Debris aprons, partly vegetated, have accumulated at the cliff base, concealing parts of the Chalk outcrop. This has created a distinct undercliff feature, subject to advance when sub-aerial processes are especially active (as during the exceptionally wet winter of 2000/2001). Prior to the construction of the first breakwater and groynes in the early nineteenth century, Palaeogene sediment was rapidly removed by wave action. The wide beach now largely prevents this, and material delivered to the cliff base by mudflows, gullying, etc is now artificially managed. Where Palaeogene sediment does reach the unprotected cliff base, at Harbour Heights, it is very quickly dispersed offshore via suspension transport.
Solution pipes in the chalk below superincumbent Palaeogenes direct the downward movement of groundwater, and may provide potential sites for cliff slope failure (ROCC, 2001).
E3 Seaford Head to Cuckmere Haven ( see introduction to coast erosion)
Defence structures are limited to tetrapod blocks at Splash Point and a short sea wall below the east-facing cliffs at Cuckmere Haven. Thus cliff erosion and retreat is largely unrestrained, producing near vertical faces up to 85 m in height. Palaeogene sediments overlie the Chalk to an average depth of 9 m, thickening eastwards. They have collapsed into vertical solutionally-widened joint planes, or pipes, which in places descend to the cliff base. Wedge and plane failures are a regular occurrence, with shear planes defined by inclined fractures; flow slides are more characteristic where the Palaeogene overburden is thickest. Solid Chalk is replaced by Coombe Rock at Hope Gap, an infilled truncated dry valley, thus locally enhancing erosion rates. Caves have developed along joints and other primary fractures, and may have contributed to occasional block falls.
This sector of cliffline is more exposed to wave energy than those to the west, a fact apparently confirmed by calculation of retreat rates since the 1870s. Thornburn (1977) calculated maximum rates of 1.26 m a-1, 1925-1955, though both Castleden (1996) and Gifford Associated Consultants (1997) consider cliff top recession at Seaford Head (Photo 4) to be 0.3 m a-1, 1875-1980. Moses, et al (2001) state that contemporary retreat is approximately 0.26 m a-1 taking the cliffs of this unit as a whole and factoring in the protective role of basal debris from cliff falls. Bedwin (1986) considered that less than half of the original Iron Age hillfort at the top of Seaford Head now survives, due to cliff erosion. Making some allowance for the probable location of this defensive structure in relation to the position of the cliffline at the time of its construction, based on its surviving plan, he deduced a retreat rate of between 0.25 and 0.35 m a-1 over the past 2,500 years.
E4 Cuckmere Haven to Beachy Head ( see introduction to coast erosion)
Cliff height increases progressively eastwards, with greater morphological complexity in the same direction.
The cliffline of the Seven Sisters (Photo 1) is developed at right angles to the dip of the Chalk, and truncates six backslope dry valleys. The original coastal planform would have been more indented, but it now has a straight north-west to south-east orientation. This is due to a complex combination of the trend of primary joint planes; a low elevation shoreline platform and full exposure to only moderately refracted south-west approaching waves (May and Heaps, 1985; May, 2000). This sector of the East Sussex cliffline may owe its character to the subservience of structural controls to wave energy at the cliff base (May, 2000). Small volume falls and topples are frequent, with large-scale events occurring once every 50 to 60 years. Basal debris has a short residence time as it is broken down and redistributed quickly. Cartographic and photographic evidence for the past 100 years indicates that the Seven Sisters have maintained a near vertical face over that period; this suggests equilibrium between cliff top and cliff base recession. Although some authorities state that the retreat rate is lower than for the immediately adjacent cliffline sectors, May (1966) calculates it to be 0.51 m a-1 for 1873-1962. Gifford Associated Consultants provide a figure of 0.6 m a-1 for the 1 km stretch of shoreline west of Birling Gap, 1875-1962; May (1971) considered that cliff top retreat may have been over 0.9 m a-1 following failure events, and Thorburn reports 1.25 m a-1 recession between 1973 and 1975. Moses, et al (2001) give a range of between 0.11 and 0.57 m a-1 at the present time for the sector between Cuckmere Haven and Beachy Head; a rate specific to the Seven Sisters has not been computed. The consensus view is that this is an actively eroding sector of cliffline, at rates equal to or above the regional average. There are, however, no specific calculations of coarse clastic sediment yield from the release of flints. Posford Duvivier (1999) calculate shoreface erosion to be a mean of 5 mm a-1.
The cliffs at, and immediately adjacent to, Birling Gap (Photo 9) have been subject to detailed inspection by several authorities, not least because of the threat posed by their retreat to the survival of clifftop properties. The recently concluded public enquiry generated several independent analyses, all of which observed that recession rates here are faster than to the east or west. This is partly the result of the presence of Coombe Rock in-filling a sectioned dry valley, with intensely shattered Chalk bedrock beneath that descends below normal beach crest level (Robinson and Williams, 1998). In addition, the Chalk to the east of Birling Gap lacks marl seams and has a lower density of major joints. Erosion may also be facilitated by the lower elevation of the shore platform at Birling Gap. However, as observed by Robinson and Williams (1998), it might have been expected that the low mechanical strength of cliff material exposed here would have created a well defined re-entrant in the coastal plan. Coombe Rock infill fails readily when saturated, but only yields small amounts (1-10 m3) per event. In contrast, joint set controlled failures in the Chalk to either side create detached rock masses of up to 10,000 m3 in volume (ROCC, 2001). The latter occur with much less frequency than the former, which may partly explain the nearly uniform retreat rate.
Wealden District Council, and its predecessor authorities, have conducted annual monitoring of cliff top retreat since 1951. The average rate for the period 1951-1999 is 0.73 m a-1 Robinson and Williams (1998) calculate recession to have been 0.68 m a-1 since 1873, based on analysis of successive Ordnance Survey maps. Posford Duvivier (1997) propose 0.77ma-1 for the same period. Cleve and Williams (1987) give a similar figure of 0.7 m a-1 for the period 1873-1976. May (1971), in a more detailed analysis, proposed a rate of 0.91 m a-1 of clifftop retreat, 1875 to 1961, whilst May and Heaps (1985) noted considerable temporal variability - eg between 0.28 and 0.98 m a-1, 1951-1962. This is emphasised by Moore, Collins and King (2001), who integrated cliff foot and cliff top retreat, 1874-1975, to reveal a range between 0.64 and 1.41 m a-1. Rates for recent decades (post 1960) are higher than longer-term averages. May (1971) was able to elucidate magnitude-frequency characteristics of slope instability events at Birling Gap. He reported considerable year on year variability, with 42% of cumulative losses from slips and falls, 1950-1962, occurring in just two winter seasons. 70% of recorded failure events each detached less than 11 m3, which together were only 12% of total loss. Lower frequency, higher magnitude events, all of them wedge or planar failures, were therefore dominant. Although most erosion took place during winter months, due to a combination of frost weathering, prolonged rainfall and storm waves, summer losses also occurred. These were induced either by late summer storm surges effecting basal notching or by drought-induced shrinkage, followed by intense rainfall. This was especially effective in Coombe Rock and overlying silty drift sediments.
Beachy Head (Photo 6) is a morphologically complex cliffed headland, whose south-facing slopes are cut into the backslope, and east and south-east-facing slopes are sectioned through the scarp slope, of the South Downs cuesta. May (2000), Castleden (1996) and ROCC (2001) distinguish several subdivisions based on cliff height, morphological properties, structural attributes and lithological variations. Between Birling Gap and Beachy Head Lighthouse, there are pronounced fault-bounded failures that generate falls and slides. Each large-scale event yields 50-100,000 m3 of Chalk, with consequent large debris accumulations running out over the inner part of a wide shoreline platform. These are relatively rapidly broken down, but produce only small quantities of gravel sized sediment stake on local beaches. Arcuate shaped ridges of Chalk boulders mark the limit of earlier falls and slides, and increase the surface roughness of the platform. This, in turn, absorbs wave energy that might otherwise have impacted at the cliff base. A major flow slide occurred in January 1999, probably triggered by critical pore water pressure in the controlling joint(s) and /or fault(s). This event released some 75 to 80,000 tonnes, burying the cliff base to a height of up to 10 m. Basal notching is active, creating comparatively short-lived caves. One example, Parson Darby's Hole, was artificially enlarged in the eighteenth century to provide sanctuary for shipwrecked crews and a base for smuggling; it had totally disappeared by 1930 (The Times, 8 November, 1958).
Gifford Associated Consultants (1997) calculated a cliff top retreat rate of 0.3 to 0.5 m a-1, 1875-1979, for this sector, with a mean of 0.35 m a-1. Some parts, however, are remarkably stable, receding at a rate of less than 0.1 m a-1 (Moses, et al, 2000). Here, and also to the east, the highest cliffs are eroding at the lowest rates. As cliff failure is episodic, controlled by the rate of opening up and downward penetration of tension fissures, mean rates of recession are purely notional. Larger events are separated by intervals of 5-15 years (Robinson and Williams, 1998).
Between Beachy Head lighthouse and Falling Sands, cliff height varies between 55 and 125 m, with periodic large falls dominating the erosion system. These are controlled by near vertical primary joints. "Bottom up" failures; flow slides; and plane, wedge and slab failures also occur, creating both spatially and temporally discontinuous debris stores (ROCC, 2001). The fronting shore platform is of variable width, but provides significant protection from the full force of wave impact against the cliff base. The upper free face is broken by tensional fissures, which created a set of pinnacles ("The Seven Charles") in the early eighteenth century. These fell, one by one, over some 150 years up to 1870. May (2000) states that basal wave erosion selectively picks out lines of weakness to create emphemeral caves, and has created a "fretted" outline. Basal debris stores have a characteristic threshold angle of 30 degrees; fine material is progressively flushed out, leaving boulders as the dominant particle size. These can constitute minor headlands, confining small "pocket" beaches of flint gravel. Some parts of the cliff base appear fresh and lack evidence of basal notching. These have probably been recently revealed following the clearance of large amounts of debris, which may have a residence time of anything between 5 and 50 years. There are few estimates of rates of cliff erosion; Gifford Associated Consultants (1997) state 1.1 m a-1, 1875-1979.
Between Falling Sands and Holywell (Eastbourne), the east-southeast-facing cliffs have been developed in Chalk overlying Upper Greensand and Gault Clay. The geological strike runs approximately parallel to the coastline trend, with both older formations outcropping over the shore platform. The Upper Greensand forms a set of ridges, or reefs, due to its outcrop being repeated by slip faulting (Castleden, 1996). The largest example of this is Head Ledge. The effect of their presence is to reduce the potential of wave erosion at the cliff base. The stratigraphical succession here has created conditions for rotational slope failure, with marine erosion of the Gault Clay undermining the overlying Upper Greensand and Marly Chalk. Cliff morphology is thus characterised by an upper backscar and a sequence of semi-rotated blocks making up an undercliff. An aquiclude at the lower cliff base probably assists hydraulic erosion by breaking waves (Castleden, 1996). Whitebread Hole and Cow Gap are both truncated chalk scarp face coombes infilled with relatively weak colluvial sediments. There are no reported research results on current landslipping, although ROCC (2001) mentions occasional small-scale failures. Cliff recession rates have not been calculated for this sector. The geomorphology of the shore platform is briefly described by Castleden (1996), who establishes that it is partly cut across landslide blocks. Rates of vertical platform abrasion and horizontal expansion are not known.
Halcrow (2001) calculated an annual loss of 540,000 m3 of material from erosion of the cliffs and platforms composing the entire Beachy Head promontory. However, almost all of this is fine sediment that is removed offshore. Only 5,400 m3 is coarse clastic (gravel-sized) material that contributes to local beaches and can potentially be moved downdrift. This assumes that the average flint content of the various chalk divisions outcropping at Beach Head is 1%. ROCC (2001) indicates a content closer to 2%. Moses et al (2001) do not provide data on flint yield for the entire length of Beachy Head, but a 1-2% content for the south-facing cliffs may be a realistic estimate. Posford Duvivier (1999) propose that 2,500 m3a-1 of flint is released from the erosion of the cliffs, and reworking of debris stores, between Birling Gap and Beachy Head Lighthouse. This quantity is based on the assumption that the flint content of the in situ chalk is 5% of volume, which is probably an over-estimate.
2.3.2 Shore Platform Erosion
The Chalk cliffline is extended seawards by a platform that varies in width, at mean low tide, of between 100 and 200m. Overall gradient is mostly below 5 degrees, (maximum of 7 degrees, minimum of 0.5 degree) but micromorphology is provided by sequences of near-horizontal benches separated by "risers" between 0.1 and 0.5m in height. The backing cliffs slopes are commonly between 60-80 degrees, but the cliff: platform junction is often obscured by the presence of a narrow gravel beach and/or groynes and seawalls (Rottingdean to Peacehaven), and rip-rap (east of Splash Point, Seaford). Gradient is usually steepest adjacent to the backing cliff, and declines seawards between High and Low Water marks. Seawards of the upper beach, platforms are characteristically cut into exposed Chalk bedrock. Scattered patches of fine to medium gravel clasts, forming a locally thin veneer, sometimes temporarily conceal the rock substrate. Common microtopographic features are gutters and runnels, some 0.5 to 4.0 m apart, 0.25 to 0.5m in width, orientated perpendicular to the low tide shoreline. Their density of dissection increases seawards (down gradient), probably as a function of the progressively longer duration during tidal cycles that these features channel flowing water. At their terminal point, some runnels can be 0.5m deep and 1.0m in width; a few appear to extend below maximum low water (Robinson and Jerwood, 1987). Ridges between gullies are frequently developed on bands of flints.
The lower part of many platforms are often partially, and occasionally wholly, covered by seaweed. Their gently sloping surfaces can be traced below low water, where in some cases they appear to be replaced by submergent reef-like forms.
Platform elevation with respect to mean sea-level is regulated either by position within the local tidal range or by inequalities in the relative erodibility of the substrate. Wright (1967) has noted that the cliff: platform junction between Cuckmere Haven and eastern Beachy Head coincides with the level of mean high water at spring tides, but descends to that of mean high water during neap cycles in the vicinity of Birling Gap. In the latter case, fractured, deeply weathered Chalk infilling the dry, truncated valley, exposed in the modern cliff face (and presumed to extend seawards) accounts for its low elevation close to mean low water springs (Robinson and Williams, 1998). A lower than average elevation is also apparent adjacent to Seaford Head, where there are several small but permanent pocket beaches. It has been suggested that locally enhanced sub-beach lowering of the upper platform surface may be taking place (May, 2000). This might be promoted by processes such as solution, though given that there is a higher than average rate of supply of flint debris at this site, abrasional lowering is a plausible mechanism. Wright (1970) has made the general point, in relation to various Chalk platform sites along the East Sussex and Kent coastlines, that platform elevations below the regional mean may be a direct function of relative exposure to wave energy. None of these competing hypotheses have been examined in sufficient detail to provide safe generalisations on cause: effect relationships.
A local variant of platform microtopography occurs near Hope Gap Beachy Head, where there are a set of circular depressions with raised rims, in addition to slightly sinuous gutters developed approximately along the principal joints. These have been interpreted as truncated solution pipes (Castleden, 1996) developing examples of which are visible in the face of the cliffs at Seaford Head.
Weathering and Erosional Processes
Jones (1981) provisionally identified three "activity zones" on the Chalk platform in the vicinity of Brighton Marina, viz:
In recent years, several researchers have undertaken quantitative research on the efficacy and rates of operation of some of these processes. To test the widely held, but previously unsubstantiated view that there is little effective movement of gravel clasts across the middle platform zone Ellis (1986) monitored paint-sprayed flint particles at sites between Rottingdean and Friars' Bay over a seven-month period. He was able to detect small net eastwards movements of both gravel and sand, but the quantities involved were very small. Concentrations of gravel in runnels were apparent, with movements of up to "several" metres during single tidal cycles. Thin spreads of coarse sand and fine gravel were observed covering relatively small areas of middle and lower platforms following storms, possibly implying net offshore to onshore transport. These did not persist, with the exception of the runnels. Ellis (1986) therefore concluded from this evidence that wave-induced abrasional erosion is currently relatively insignificant. This is even more likely where there is a persistent cover of seaweed. However, he did concede that abrasion, particularly during winter months, might be more important when he valuated the results of repetitive microerosion meter surveys of a total of 44 sites between Black Rock (Brighton) and Peacehaven. These cover a range of elevations and incorporate upper and lower, as well as middle, platform zones. Andrews and Williams (2000) observe that many of Ellis's sites have high limpet population densities, thus implying bioerosion as a potentially significant contributory process to platform reduction. Indeed, Ellis (1986) makes a similar point, but his research approach did not include the collection of appropriate data.
Both Ellis (1983, 1986) and Andrews (1996, unpublished) present micro-erosion data indicating that rates of platform reduction are highest in the upper zone, adjacent to beaches, as there is strong circumstantial evidence in favour of the dominance of abraisonal scour. It is also an area where limpet colonisation does not occur.
It is thus apparent that the concept of the 'wave-cut' or wave abrasion surface can only be applied to a small area of the Chalk shoreline platforms of East Sussex. This is reinforced by the lack of any statistical correlation between platform orientation with respect to (i) wave fetch and mean wave height; and (ii) mean rates of downwearing (Ellis, 1986). The implication must be that other geomorphological processes are more significant. Three that have received some attention are frost weathering; salt weathering (haloclasty) and bioerosion.
Robinson and Jerwood (1987a and b) have examined the evidence for frost weathering and salt crystallisation, using three groups of sites, viz: (i) Saltdean to Friars' Bay (west of Newhaven); (ii) Seaford Head, and (iii) Cuckmere Haven to Beachy Head. The impact of freeze-thaw cycles was evaluated using field data collected during periods of exceptionally cold temperatures in January 1985 and February 1986. They describe extensive cracking and spalling of coherent Chalk, especially where it is adjacent to thin, irregular intercalated bands of clay. Detached fragments were observed to fall, or be washed, into runnels, where they either broke up after a few tidal cycles or suffered volume loss (presumed to be due to abrasion). The percentage frequency of spalling per unit area decreased down-platform, from 36% close to the limit of the upper beach to less than 1% at the seaweed encrusted low tide platform edge. In addition to spalling, mostly of small projecting surfaces, intense sub-vertical cracking by repeated freezing and thawing was evident. This particularly affected detached rock masses, including those of boulder size. In comparison to the Chalk, very little spalling, flaking or cracking of flint nodules was observed. However, the long-term role of frost weathering in releasing flints (which occur in bands, and make up 2-5% of the Upper Chalk areas studied) may be significant.
Disintegration of Chalk by repeated freeze-thaw cycles takes place once air temperatures fall below freezing point but seawater remains unfrozen. Because of its salt content and the dissipation of latent heat released by liquid: solid phase change, seawater has to cool to -2.5oC before freezing. In practice, as laboratory experimentation demonstrated, sub-surface temperatures in seawater-saturated Chalk may need to decline to between -4.5oC and -6.0oC before freezing commences. Thus, upper platform areas are more susceptible to freeze-thaw effects because of their longer exposure to cooling effects during the tidal cycle. The high thermal conductivity of seawater ensures rapid thawing during the rising tide, therefore minimising the cooling period over platform areas adjacent to mean low water. Mid and upper platforms exhibited substantially more evidence of frost weathering, especially the corners and edges of runnels and detached boulders with a larger ratio of surface area to mass and relatively greater exposure to wind chill. Robinson and Jerwood (1987a) considered that although their field data relate to two exceptionally cold periods, the processes described are likely to occur during most winters. They observe that a single frost cycle may be sufficient to initiate Chalk surface spalling, thus its cumulative effects as a factor in the sculpting of shoreline platform relief must not be underestimated. Whether, or not, frost weathering reduces microrelief is an open question, though the evidence is in favour of its effect in widening runnels and wearing back micro-scarps.
It is probable that frost weathering does not operate alone, but combines in a complex way with salt crystallisation that occurs in the micropores and cracks of Chalk surfaces. The laboratory experiments undertaken by Robinson and Jerwood (1987b) did demonstrate that the two processes when working together are potentially highly destructive; however, the dynamics of their inter-relationship are not yet clear. Most damage is likely to occur in winter, when warming is least and near surface saturation will persist even over the highest parts of platforms. Water will also collect in pools, thus potentially promoting salt crystallisation during periods of tidal emergence.
The role of bio-erosion has recently been evaluated by Andrews and Williams (2000), whose quantified approach provides some objective evidence for processes previously implied by others (e.g. Jones, 1981; Ellis, 1986). Their work focuses on the effects of the limpet Patella vulgata, which colonises large areas of the entire platform surface except for sites close to the upper beach. They are most numerous around mid-tide levels, and co-habit with other fauna, e.g. the polychaete worm Polydora ciliata and piddocks such as Pholas dactylus, that also contribute to bioerosion through drilling and burrowing lower platform surfaces. Patella vulgaris removes Chalk through grazing (i.e. ingestion) and by excavating hollows ("homescars") for shelter after feeding. Through analysis of the calcium carbonate content of captive adult specimens, it was calculated that each consumes approximately 5.0ga-1 of Chalk. Based on estimations of limpet population densities, this converts to a mean rate of platform lowering of 0.15mma-1. Rates may be as high as 0.5mma-1 in those areas of highest concentration. Other researchers (e.g. Ellis, 1986) have deduced mean rates of platform lowering, due to all processes combined, of approximately 2.3mma-1; however, this figure appears to decline to close to 1.3mma-1 where limpets are present in significant numbers. Thus the question arising is whether they provide a protective role or simply colonise more stable areas where erosion and weathering processes are less active.
In an earlier study, restricted to one site (Hope Gap, Beachy Head), Day (1990) calculated a rate of platform lowering of 0.28 mma-1 based on the carbonate content of limpet faecal debris. Andrews and Williams (2000) comment that the results from their study are probably conservative. This is because they confined measurements to ingestion and radula rasping, omitting the effects of shell edge attrition and dissolution within homescars. Other questions remain, in particular the ecological relationships between limpets and barnacles. The latter also contribute bioerosion, but it is uncertain how or if their population densities and productivities are affected by the presence of limpets. Solutional weathering and erosion by algae is another source of biological reduction of platform surfaces that awaits research.
Rates of platform lowering and sediment yield
Ellis (1983; 1986) reported rates of vertical erosion of between 2mm and 1cma-1, based on repetitive micro-erosion meter measurements of 44 sites between Brighton Marina and Peacehaven. These sites combined a wide range of platform elevations, microtopography and positions within the tidal frame. The majority produced rates between 1.2 and 8.5mma-1, with a mean of 2.32mma-1. Lowest rates were associated with sites lacking abrasional material, where covered with seaweed or with little or no development of microrelief. Highest rates correlated with localised areas of active abrasion, such as adjacent to beaches or in gutters and runnels transporting debris. For almost all sites, some 50-70% of erosional loss occurred between December and March/April, thus implying the significance of frost weathering, low evaporation rates and higher energy waves (in areas of abrasion).
Assuming that the Upper Chalk underlying almost the entire shoreline platform of East Sussex has a 4 to 5% average volume content of flint, a platform reduction rate of 2.3mma-1 would yield between 300 and 400m3a-1 (Moses, et al, 2001; Dornbusch, 2002). This is in the order of only 5-10% of flint release from Chalk cliff erosion, averaged for the past 100-140 years. This is considerably less than the potential yield derived from a flint content of 5-10% assumed in some earlier studies, based on simplistic calculations, (see below) and therefore represents a very small input to regional beaches. As coastal protection since the 1930s has excluded a calculated 36% of potential input of flint clastic material from cliff erosion (Moses, et al, 2001), the implications for the present and future littoral sediment budget are clear.
Rates of lowering due to limpet grazing and burrowing summarised in the previous section, are, on average, below those ascribed to the combined effects of non-biotic erosion. In general, bioerosion contributes some 12-15% of total losses, rising to nearly 35% at sites of exceptionally high faunal densities (Andrews and Williams, 2000). Research into frost and salt weathering has been unable to quantify rates of platform denudation specific to these two processes.
Posford Duvivier and British Geological Survey (1999) have calculated mean rates of shoreface erosion for all sectors of the East Sussex coastline. For the sector between Rottingdean and Seaford, a rate of between 1.3 and 4.0mma-1 is deduced from the application of a set of assumptions to limited detailed knowledge of nearshore/offshore conditions. The figures for sectors to the east (5mma-1, Seaford Head-Birling Gap; 15mma-1, Birling Gap to the eastern end of Beachy Head) are evidently well in excess of those proposed by Ellis (1986) and others. Using the figures above, and an assumed flint content of 5% for the entire length of platform, annual input of coarse clastic debris would be approximately 5,500 to 5,700m2.
Jennings (1990) has stated that the level of the Chalk platform seawards of the eastern part of Seaford Beach fell some 3-5m between 1900 and 1985 (4.1mma-1). Posford Duvivier (2001) report a rate of lowering of nearly 2cma-1 along parts of the Brighton Marina to Saltdean frontage. These very high rates were calculated by reference to protection structures of known construction date. Mouchel Consulting (2001) suggest similarly high rates (1.0-1.5cma-1) for platform surfaces adjacent to the seawall between Saltdean and Peacehaven. Further confirmation is provided by Robinson and Williams (1998) who describe 'ribs' of Chalk beneath concrete groynes built in the early 1970s at Peacehaven standing almost 0.5m above the adjacent inter-groyne platform. This gives a rate of lowering of 1.6cma-1. These high rates of platform reduction do not necessarily imply an acceleration in recent years, as all are located in areas of wave turbulence and therefore localised very high abrasion losses. They give an indication of maximum rates of lowering in the upper zones of platforms, where abrasion is the likely dominant process.
Several authors (Robinson and Williams, 1983; Ellis, 1986; Cleve and Williams, 1987; Andrews and Williams, 2000; Moses, et al, 2001 and Dornbusch, 2002) have stated that there has been no increase in width of platforms over the last 150 years of reliable map evidence. As cliff recession has been ongoing, except for those sectors where seawalls have been built since the 1920s and 1930s, it must be concluded that they have maintained an equilibrium profile. In broad terms, this might be the result of the reduction of wave energy at the cliff: platform junction as the platform widens, wave energy reduces and less debris is removed. However, the evidence suggests that Chalk debris has a short residence time and fresh inputs of flint material are small. Equilibrium, therefore, must be maintained by an approximate balance between cliff retreat and sea-level rise - a relationship that can be assumed to be nearly constant for the last 5,000 years. The shore platform of East Sussex is therefore a contemporary, not a relict, landform. Ellis (1986) has made the point that if assumed average historical rates of cliff recession of 0.5ma-1 have operated over the past 2,000 years, a platform 1 km in width would have been created. However, to maintain a constant equilibrium gradient over this time period, a rate of lowering of 11mma-1 would be required. As contemporary rates are much lower, perhaps platform evolution takes place episodically rather than progressively. The alternative is to assume that measurements of modern platform lowering rates are gross under-estimates, but this seems unlikely. Ellis (1986) has suggested a development model characterised by a regime where frost weathering is dominant over bioerosion and wave abrasion, with short periods of intense erosion, for example during very cold and/or stormy winters, separated by phases of very modest geomorphological activity. This could give rise to a sequence of evolutionary changes, starting with a flat surface, which is thereafter diversified by runnel/gutter initiation, frost weathering and bioerosion. Storm-induced wave quarrying creates block removal and micro-scarping between discrete platform levels. This model accommodates both vertical erosion and horizontal retreat, and can progress at different rates at specific locations. If various "stages" were to be defined, the morphological expression of each would be expected at the present time - indeed, they might be co-adjacent. Field mapping supports this, but Ellis's model is far from definitive - there are several alternative sequences, as well as arresting factors.
Given (i) rising sea-level, possibly up to 4mma-1 on this coastline by 2030-2040; and (ii) continuation of protection at the cliff: upper platform junction along more than 40% of the length of the platform-fronted shoreline, there are some evident implications for future management. Landward expansion will mostly be excluded, but present rates of lowering will continue - thus the period of submergence will increase and beaches will be more vulnerable to wave erosion. This will enhance the effects of abrasion, but increased water depth and a longer submergence time might suppress sub-aerial and biological weathering and erosion. Whatever the outcome, there will be a reduced input, to sub-cell littoral sediment budgets.
Several aspects of platform development remain subject to uncertainty, and require further research. These include:
There have been no direct measurements of erosional losses by either down- or back-wearing due exclusively to abrasion by gravel and coarse sand sediment. A comment made by Chadwick, et al (1986) that the irregular microtopography of lower and mid-platform surfaces may offer high hydraulic resistance to wave-induced currents, and thus reduce shallow water sea bed shear stress to below thresholds required for bedload transport, deserves further attention. This effect may suppress potential surface abrasion.
(iii) Solution weathering
Several authors (e.g. Ellis, 1986; Robinson and Jerwood, 1987a) have either stated, or implied, that solution weathering is insignificant in comparison to other processes responsible for platform reduction. However, this has not been demonstrated quantitatively. It is probable that solution loss is linked in complex ways to processes such as salt crystallisation.
The work undertaken by Andrews and Williams (2000) strongly deserves to be extended, including the study of the effects of algae and intra-littoral fauna other than limpets.
2.4 Beach Replenishment and Recycling - N1 N2 N3 N4 References Map
Several beach systems along this coastline have experienced long-term net loss of volume and narrowing of their inter-tidal widths. Indices of retreat (and, in a few locations, advance) are given in Gifford Associated Consultants (1997), and are discussed further in Section 5. Operating authorities have responded in several cases with programmes of beach management involving substantial inputs of recharge materials. Most involve further periodic renourishments and/or routine recycling of sediment from areas of recurring accretion to those of persistent deficit.
N1 East Beach, Shoreham-by-Sea - ( see introduction to beach replenishment)
Since the completion of the breakwaters defining the entrance to Shoreham Harbour in the late 1880s, beaches immediately eastwards (downdrift) have suffered sediment starvation. This is confirmed by apparently 'one-off' replenishments in 1886 and 1902 at Hove (Joliffe, 1972). Sir William Halcrow (1988; 1990) calculated a net reduction of volume of almost 150,000m3 between 1962-1988 subject to considerable year on year variability due to changes in incident waves. This was approximately 25% of the "equilibrium capacity" of this beach, calculated theoretically on the basis of average wave climate, mean particle size and unimpeded longshore transport. Loss would have been greater but for periodic dumping of gravel and rubble (from urban waste), estimated at 35,000m3 between about 1970 and 1985. Sir William Halcrow (1967), in a report on the condition of Brighton beach, revealed that some 400,000m3 of spoil dredged from the entrance to Shoreham Harbour was placed on, or close to, the toe of East Beach between 1954 and 1957. Smaller quantities may have been introduced subsequently. As much of this material is likely to have been silt and fine sand, only a small proportion would have been retained. Ward (1922) reported that in the first decade of the twentieth century approximately 90,000 tons of gravel was removed annually from between and west of, the harbour breakwaters. Some was placed on East Beach, though only when seriously depleted. Most was exported as aggregate, much of it overseas.
When Shoreham Port Authority assumed responsibility for East Beach in 1987, it was in a depleted condition. The Beach Management Plan proposed by Sir William Halcrow (1990) proposed a biannual transfer of gravel from the beach immediately updrift of the western breakwater to East Beach. It was suggested that up to 15,000m3a-1 might be artificially by-passed, without impacting on the geomorphological or ecological integrity of West Beach (i.e. an amount equal to the calculated drift rate). In practice, the annual average transfer, 1993-2000 has been 8,500m3 (Vaughan, 2001), though there have been substantial inter-annual variations. For example, no material was moved in 1996, but over 22,000m3 was transferred in 2000. All recharge between 1988 and 1992 used spoil from reclamations for harbour development for groyne bay infilling; similar opportunities are likely to arise in the future. Analysis of Environment Agency ABMS profiles for East Beach, 1988-2000 reveals a substantial increase in volume up to 1993, but some marginal reduction since (Vaughan, 2001). It is therefore evident that managed by-passing of gravel has stabilised beaches up to some 3km downdrift of the harbour entrance, but this condition can only be maintained if this practice is continued.
N2 Ovingdean and Rottingdean to Saltdean - ( see introduction to beach replenishment)
Replenishment of beaches has been undertaken at Rottingdean and Saltdean with maintenance of existing beaches in a recent scheme at Ovingdean and against the eastern arm of Brighton Marina. Sites in question are flint gravel beaches with a history of depletion long pre-dating the construction of Brighton Marina. Only where beaches have been seen as necessary for amenity purposes have they been artificially created, elsewhere sea walls and rock toe revetments have been constructed to dissipate wave action. A total of 200,000m3 of gravel, dredged from the Owers bank south of Littlehampton, was added between 1992 and 1996. This is approximately 200m3 per metre frontage within the main schemes. Posford Duvivier (1997) state that the recharge material is close to the mean particle size of indigenous clasts, i.e. 13mm long axis. Beach profile modelling was carried out in conjunction with replenishment, with foreshore slope and crest height designed to maximise capacity to absorb and re-distribute wave energy. Rottingdean, Saltdean and Ovingdean beaches are also able to adjust their orientation, between 11 and 25 degrees, in response to changes in vectors of wave approach (Posford Duvivier, 1997). Future recycling may be practised, using any foreshore accumulation downdrift, east of Saltdean.
Rock groynes have been constructed to encourage the retention of recharge material, but it is recognised (Posford Duvivier, 2001) that progressive future loss of volume is likely despite recycling.
N3 Seaford Beach - ( see introduction to beach replenishment)
Joliffe (1972) provides documentation of the history of Seaford beach since the early nineteenth century, though details of specific levels and volumes are difficult to reconstruct. Erosion and depletion were the dominant features, indicating that gravel was difficult to retain and tended to move rapidly alongshore, or offshore. The position of mean Low Water moved 107m landwards, 1879-1961 (60m between 1910 and 1945). The first groynes were introduced at Tide Mills in 1836 (Large, 1989), and a seawall between the Buckle Inn and Tide Mills was built in the 1880s. Joliffe (1972) states that free gravel by-passing of Newhaven Harbour occurred up to breakwater construction 1844. However, the pathway of supply between Newhaven and Seaford Beach may have been indirectly via Seaford Bay. After breakwater and pier construction by-passing appears to have been reduced; especially following the lengthening of the western Breakwater, completed in 1890. Significant erosion of the central sector of Seaford Beach occurred within a few years, requiring the insertion of some 80 closely-spaced groynes and the upgrading/extension of the seawall. The new breakwater also introduced a drift reversal west of Tide Mills because of localised change in wave climate (Large, 1981; Shave, 1989). Although there was some net periodic accretion at either end of the beach, between the Buckle Inn and Seaford Head, e.g. 160,000m3, 1898-1927; and 112,000m3, 1927-1961 near East Pier (Joliffe, 1972), progressive lowering and steepening was experienced up to the late 1970s. This was despite recharge with gravel, taken from Dungeness, in 1936 and 1958; the construction of a new set of alternating long and short groynes in the 1950s, and attempts at sediment recycling. The latter utilised surplus accretion against the eastern pier of Newhaven Harbour (Shave, 1989).
The causes of depletion were the subject of considerable debate and controversy by the early 1960s (Hydraulics Research Station, 1963). The conventional view was that the prime cause was the effect of the western breakwater of Newhaven Harbour on substantially intercepting potential longshore drift. This argument was largely based on the historical evidence for eastwards barrier spit migration (see Section 1.1), but ignored the fact that there had been several events of beach drawdown and breaching (the most recent in 1875). It was becoming apparent that the seawall and groynes promoted both drawdown and scour (May, 1966) whilst erosion of the Chalk platform providing the beach foundation was seen as one other possible cause. The latter was lowered by 3m between 1900 and 1950 (Joliffe, 1972).
Experimental studies carried out by the Hydraulics Research Station in 1961-62 (Joliffe, 1964) used physical models, fluorescent tracers and diving inspection to determine movement of gravel on and offshore Seaford Beach. This work demonstrated that gravel clasts had a tendency to move offshore when wave heights exceeded 4m. They did not return if moved into deep water or if they were incorporated into the silt-clay-fine sand layer occupying much of the seabed (see Section 3 and Section 4 for further detail). This work added a further dimension to the debate on the causes of beach depletion, emphasising that net onshore to offshore transport may be a significant reason why coarse clastic sediment is not inherently stable on Seaford Beach.
Subsequent research on regional shoreline evolution (Jennings, 1990; Jennings and Smyth, 1990; Nicholls, 1991) has contributed the concept that the gravel beaches of the East Sussex shoreline are, in part, relict barrier or barrier spit forms. This could be relevant to Seaford Beach, which partly originated from barrier elongation across the mouth of the Ouse. If its sources of supply from offshore have diminished, or have been exhausted, then it is a degenerate accretion structure that would be expected to suffer overtopping, breaching and net loss of volume. This effect would have been accentuated by west to east longshore drift impedance caused by the insertion of training walls, piers and breakwaters at the entrance to Newhaven Harbour. Thus, the latter factor is not seen as the fundamental cause of beach erosion along the Seaford frontage (Jennings, 1990). The idea of "pulses" of gravel supply from offshore may also be relevant, with erosion over the past 200 years progressively removing earlier inputs (Jennings and Smyth, 1990). However, Seaford Bay is deeper than the area offshore of the Crumbles, Eastbourne to which this argument was specifically applied. The analogy may therefore be only superficially attractive. The work of the former Hydraulics Research Station (Joliffe, 1964) would appear to discount any significant net onshore transfer of gravel, despite the fact that peak wave energy for the East Sussex coast occurs close to Seaford Bay.
The Southern Water Authority took management responsibility for Seaford Beach in 1981, and commissioned Hydraulics Research to carry out a range of hydro- and morphodynamic studies (Hydraulics Research, 1985; 1986c) to determine appropriate measures to secure future beach integrity (Holmes, 1989). This work concluded with a recommendation to renourish with 1,450,000m3 of gravel, over a frontage length of 2,500m, together with a large terminal groyne to contain eastwards littoral drift. This work was carried out in 1987 and also involved beach geometry specifications such as a 1 in 7 seaward slope; crest height of 6.5m O.D. and crest width of 25m (Stone, 1990).
It was assumed that the previous eastwards littoral drift rate of between 20,000 to 38,000m3a-1 would operate after renourishment and re-profiling were complete. In the absence of intermediate groynes, this would create net accretion of 50,000-70,000m3a-1 updrift of the terminal groyne, therefore necessitating routine recycling within the beach system. Losses of some 15% of volume (approximately 200,000m3) were anticipated within 6 months of scheme completion, mostly due to removal of fines. Monitoring studies carried out after final remodelling of the beach profile in March 1988, using 58 transects (Hydraulics Research, 1988b; 1989b; 1991; Brampton and Millard, 1996) revealed close correspondence between theoretical expectations and actual performance. Losses were less than anticipated at first, but storms in 1989/90, and 1992 generated strong mid/backshore scarping following the consolidation of the matrix between gravel clasts. This assisted wave reflection, creating significant losses from foreshore scour.
Between 1987 and 1994, monitoring revealed a spatial pattern of net losses and grains over different sectors of the beach, with gains recorded at both eastern and western ends (Brampton and Millard, 1996). These were equalised through periodic recycling, with a small net gain of 5,000m3 for the beach as a whole over this period. Since 1995, this practice of recycling has been maintained, with quantities determined by analysis of ABMS data for preceding years. ABMS data, 1991-2000 indicates an average annual gain of 13,800m3 (maximum accretion, 1992, of 65,000m3, maximum depletion, 2000, of 75,000m3). Management has thus sustained the equilibrium of this restored and modelled beach. The central section of the beach is illustrated by Photo 10.
N4 Cuckmere Haven - ( see introduction to beach replenishment)
Recycling of gravel making up the beach face of the barrier spit across the mouth of the Cuckmere River (is practised by the Environment Agency. The main objective is to counteract the effect of net west to east longshore drift of partially blocking the river mouth or reducing the effectiveness of the training walls (Clifton and Cecil, 1999). No data on quantities involved has been made available.
3. LITTORAL TRANSPORT (BEACH DRIFT) - LT1 LT2 LT3 LT4 References Map
All research conducted on longshore littoral transport, from early qualitative/ observational studies to recent numerical modelling, concludes that the dominant pathway is west to east. Local net directional reversal, however, occurs between the western end of the replenished beach at Seaford and the East Pier of Newhaven Harbour. Short-term reversals have also been documented at several locations, most often in association with south-easterly waves during the winter. The dominant process inducing longshore movement is that of breaking waves, with gravel-sized clasts being the principal sediment type. Tidal currents alone are ineffective in moving sediment in the breaker/nearshore zone, but may combine with waves in transporting fine sediment, mostly sand, in the nearshore and offshore zones and across and into estuary/harbour entrances.
Gifford Associated Consultants (1997) propose a conceptual model with estimated rates of longshore transport derived from the LITPACK numerical deterministic process model. Rates are calculated for a sequence of cross-sections through the transport corridor, using EA ABMS profiles (1973-1995); local bathymetry; bottom sediment samples; tidal levels and hindcast wave climate. In addition, measures for the "spread" of bed material sizes; bed roughness and transport velocities for the size-range of sediments potentially involved in longshore transport are used to refine model predictions. The SANDS database provided an important input source for much of this information, and helped to produce adjustments to predictions in areas of more complex offshore bathymetry (eg. bedrock outcrops) or where obstructions to movement occur such as marinas; breakwaters; long groynes and other local salients.
Calculated transport rates (quoted below) are for potential movement and are adjusted for groyne spacing and dimensions as far as possible. Because the latter are impediments to uninterrupted transport, actual drift rates are likely to be lower than predicted. Lack of sediment supply is however, the principal reason why potential rates are rarely attained.
LT1 R. Adur to Brighton Marina (West) - ( see introduction to littoral transport)
Halcrow (1988) calculated that some 24,000m3 of both sand (mostly) and gravel (small quantities) annually by-passed the breakwater protecting the western side of the harbour entrance channel. The gross rate of longshore transport of gravel declines markedly immediately eastwards of the eastern training wall, not only because of the presence of both structures impeding downdrift movement, but also as a result of a reduced angle of approach to the shoreline of shoaling and breaking waves (Chadwick, 1990). Gifford Associated Consultants (1997) calculate from LITPACK that the drift rate along East Beach is 17,400m3a-1; Scott, Wilson, Kirkpatrick (1994) propose the somewhat lower rate of approximately 15,000m3a-1. Almost all of this is gravel. Halcrow (1990) computed an eastwards transfer of 14,200m3a-1 based on an adjustment factor taking into account the role of groyne storage. It is generally agreed that net transport rates are highest in winter, at least between East Beach and Hove. However, Morphett (1989) observed a period of westward longshore movement on West Hove beach during winter, when waves approaching from the east/south-east were prevalent. If this were experienced routinely, it would reduce the net annual rate of eastward movement. There is uncertainty if higher winter season rates prevail along Brighton beach; this may again be due to drift reversals suppressing the total amount of coarse material moved eastwards during this period of the year (principally November-February).
Between central Hove and Kemp Town, Brighton, the potential eastwards drift rate (all sediments) is close to 50,000m3a-1 (Halcrow Maritime, 2001). However, insufficient sediment feed, coupled with the storage effect imposed by an efficient groyne system, reduces this to an actual rate of some 18,000m3a-1. In the zone 400m updrift from the Brighton Marina breakwater (Photo 3) this declines to 4,000m3a-1, a feature of the longshore transport pathway that apparently pre-dates the building of the Marina (Scott, Wilson, Kirkpatrick, 1994). Halcrow Maritime (2001) express the view that net longshore drift is lowest for this sector along central Brighton beach (Photo 11). This is due to the fact that it has a better developed swash orientation than adjacent beach lengths.
The Beach Plan Shape Model applied to East Beach, Shoreham and extended eastwards to Portslade (Halcrow, 1988; 1990) indicated some of the difficulties involved in calculating net rates of littoral drift of coarse sediment for the shoreline between Shoreham Harbour and Brighton. These involve inadequate quantitative modelling, or qualitative estimation, of: (i) wave refraction induced by offshore and nearshore seabed relief and roughness; (ii) complex diffraction and refraction set up by breakwaters and piers; (iii) frequent, usually short-term, reversals of drift direction, but with high year-to-year variability of periods of high wave energy; (iv) retention of non-indigenous material introduced via renourishment (e.g. rock rubble), and (v) the time-dependent efficiency of sediment trapping by groynes.
Although not numerically estimated, nearshore and foreshore transport of finer sediment, mostly sand, occurs in substantial quantities. This is probably not less than 25-30,000m3a-1, and has a much stronger uni-directional (eastwards) component than gravel. Peak tidal current velocities are capable of sand entrainment, but are probably only significant around the entrance to Shoreham Harbour. This is because of the tridal exchange of the estuary and the "channelling" effect of breakwaters and their generation of localised turbulent eddies. Optimum conditions for sand transport occur when waves approach from the south-west (Halcrow Maritime, 2001b).
LT2 Brighton Marina to Newhaven, West Breakwater - ( see introduction to littoral transport)
Chadwick, Morfett and Rees (1986) derived a theoretical gross potential rate of longshore transport of gravel of 34,300m3a-1 for the sector between the Marina and Saltdean. This was based on the application of a longshore power energy equation, and was acknowledged to be higher than the actual rate because of the role of groynes and seawalls. They proposed an adjusted figure of 28,530m3a-1. The rate computed by LITPACK (Gifford Associated Consultants, 1997) is 29,000m3a-1, which is also based on an allowance for groyne storage. Actual drift rates immediately downshore from Brighton Marina are very low, but this does not necessarily implicate this structure in the interruption of throughput from further west. The site of the Marina is Black Rock Ledge, a drift barrier and a site of near zero transport for several decades prior to its creation, in 1973-5 (Posford Duvivier, 2001; Halcrow Maritime, 2001b). The presence of the Marina does now create a minor wave diffraction effect, but this is not thought to be sufficient to have any impact on downdrift transport. Ellis (1986) carried out a short experiment on a beach site 400m east of the Marina wall. He tracked the movement of several gravel sized clasts, and found that they moved a short distance westwards in January but remained static in July. This apparent evidence for counter drift was considered to be due to a "tidal vortex" set up by the Marina wall augmenting the wave diffraction effect. Posford Duvivier (1997; 2001) give evidence of sustained drift reversal, during winter, over the Marina-Saltdean frontage, particularly in the vicinity of Ovingdean. Nonetheless, the overall, net component of movement is eastwards. Drift reversals are the most probable cause of fluctuations in beach volumes (Posford Duvivier, 2001). Ellis (1986) observed a site at Rottingdean that exemplifies this, and also demonstrated more or less uninterrupted eastwards transport. Both here, and at Bastion Steps, Peacehaven, Ellis (1986) reported almost no measurable lateral movement of gravel clasts on the chalk shoreline platform surface. This is probably due to the effects of microrelief. It needs to be understood also that that beach volumes and profiles have changed radically in places over the last 20 to 30 years as a result of the Council's inability to fund groyne maintenance in advance of the reconstruction schemes which in general have removed the old groyne field and left most of the coast open to the free movement of shingle (Eade – personal communication).
There are no definitive estimates of the gross or net longshore drift rate for the sector between Peacehaven and Newhaven Western Breakwater. Joliffe (1972) stated that 60,000m3 of shingle accumulated against the updrift side of the breaker within three years of its reconstruction in 1890. This would suggest a drift rate of 20,000m3a-1 if there was no bypassing, however this is probably not representative for such high accretion rates have not been sustained since. Given that some coarse material does move in the shallow water (nearshore and breaker zones) along the breakwater perimeter, then a higher drift rate of up to 30,000m3a-1 is feasible. Joliffe (1964) considers coarse bedload transport around Newhaven breakwater to be probable, thus accounting for some supply of gravel deposits in outer Seaford Bay. There is no direct evidence for this, although Ridehalgh (1959) does mention that some "exotic pebbles" were placed on the beach between Friars' Bay and the terminal stub of the breakwater. These were subsequently recovered on Seaford Beach. As there is no mention of quantities involved, length of experimental period and prevailing hydrodynamic conditions, this observation has low reliability and cannot be verified.
Unidirectional (eastwards) littoral transport has been assumed for the sector east of Peacehaven. Ellis (1986) undertook tracing of gravel-sized clasts at a site on Friars' Bay over a six month period. This revealed net movement westwards during the winter months, and the reverse during spring and summer. Overall, somewhat more sediment moved eastwards. As with other results reported by Ellis (1986), the representativeness of hydrodynamic conditions during the experimental period cannot be ascertained.
LT3 Newhaven Harbour (East Pier) to Cuckmere Haven - ( see introduction to littoral transport)
Between the East Pier and Tide Mills, the net direction of longshore transport is east to west; east of Tide Mills, it resumes the regional norm of west to east. A well-defined drift divergence therefore exists in this vicinity. Counter-drift west of Tide Mills apparently operated towards the end of the nineteenth century. Morris (1931b) located the divergence boundary at Bishopstone, and suggested that east to west movement of both sand and gravel towards Newhaven East Pier was due to the fact that the latter structure, and the western breakwater, attenuated waves approaching from the south west. Waves coming from the south-east were therefore dominant over this short sector. This has been partly confirmed by later studies (e.g. Hydraulics Research Ltd, 1985; 1986c), although the ineffectiveness of modified south-westerly waves is also related to wave refraction set up by the breakwaters, piers and training walls of Newhaven Harbour. East Pier may also create an anticlockwise tidal circulation, although this would only account for the transport of sand (Robinson and Williams, 1983). LITPACK (Gifford Associated Consultants, 1997) calculates a drift rate of 25,000m3a-1 for this sector, a figure that is presumed to include both sand and gravel.
For Seaford Beach east of Tide Mills, Hydraulics Research (1986c) calculated net eastwards drift to be between 20 and 38,000m3a-1. For the period October 1988 to September 1989, Large (1989) computed a rate not in excess of 17,000m3a-1, following comprehensive renourishment and profile remodelling in 1987. LITPACK (Gifford Associated Consultants, 1997) gives a rate of 21,500m3a-1, with a probable acceleration downdrift. Joliffe (1964), reporting on the results of a fluorescent tracer study of gravel transport on the foreshore of Seaford beach, tentatively identified an impersistent transport divide near the central section of the beach. Thirty years later, and following massive beach morphological modification in 1987, this was still apparent from detailed analysis of beach profiles recorded during intensive post-nourishment monitoring (Brampton and Millard, 1996). However, it was considered possible that the transport divergence at Tide Mills/Bishopstone might temporarily migrate eastwards under specific wave energy conditions.
Between Seaford Head and Cuckmere Haven, the potential longshore drift rate of gravel calculated using LITPACK is 25,000m3a-1, but shortage of input sources reduces this to a much lower figure. Because of the change in coastline orientation, rates are probably lower east of Seaford Head.
Experimental work on onshore to offshore, and reverse, movement of shingle was undertaken by Hydraulics Research Station (Joliffe, 1964; 1972) for Seaford Beach. Results were derived from recovery of fluorescent tracers. The specific objective of this work was to assess the relative effectiveness of extant long and short groynes in reducing prevailing littoral drift rates. It was reported that gravel mobility in the surf zone was inhibited where the seabed substrate was overlain by silt, mud or muddy/silty sand. Where gravel sized particles by-passed groynes, there was a tendency for a proportion of clasts to move offshore and thereafter return to inter-groyne bays where the nearshore beach gradient was relatively shallow and where the gradient exceeded 1:6. Where the substrate was muddy or silty, there was no return onshore movement. This imbalance between net off and onshore cross-transport, in favour of offshore loss, was especially evident in winter. This was considered to be due to higher energy breaking waves moving some gravel sized particles into deeper water (>10m), in Seaford Bay. Although the beach envelope at Seaford Beach has since been significantly modified by renourishment/reprofiling, this tendency towards removal of coarse foreshore sediment offshore may still prevail, and may account for a proportion of the temporary loss of beach volume during certain years since 1987 (see Section 5).
LT4 Cuckmere Haven to Beachy Head ( see introduction to littoral transport)
LITPACK (Gifford Associated Consultants, 1997) determined a potential drift rate of 25,000m3a-1, but again actual rates are only a fraction of this as input of sediment is relatively small. Some, perhaps most, gravel in transit is subject to temporary storage in 'pocket' beaches between minor Chalk and landslide debris salients. The latter breakdown eventually, providing occasional "pulses" of longshore moving gravel. Posford Duvivier (1999) suggest that 5,000m3a-1 arrives on Eastbourne beach from Beachy Head, but it is not clear if this figure is based on long-term averaging of episodic throughput. If so, it might equate with the prevailing drift rate, though it is a probable over-estimate. Offshore to onshore gravel input is also thought to be subject to "pulses" of supply, though Nicholls (1991) considers this component to have been inactive since at least the eighteenth century. The place name of Beachy Head is unrelated to any former presence of well-developed beaches. Most authorities accept that there is some by-passing of this headland by gravel, but that the quantity is difficult to estimate. Sand is transported more freely and in greater quantity in the nearshore zone, with a probable superimposed component of reversing on to offshore movement. The strong tidal eddy (vortex) current system augments the role of wave-induced sand movement (Halcrow Maritime, 2001a), both on and offshore.
4. SEDIMENT OUTPUTS - O1 References Map
4.1 Nearshore and Offshore Transport
O1 Nearshore and Offshore Transport
By comparison with research and monitoring work undertaken in the inter-tidal zone, knowledge and understanding of sediment transport dynamics in the adjacent nearshore and offshore areas is both limited and tentative. Posford Duvivier and the British Geological Survey (1999) describe the seabed, out to 20km from the coastline and to a depth of 50m, as a "broad, open shelf". Gradients are gentle between Shoreham-by-Sea and Brighton, with the -20m isobath some 3,000m offshore Shoreham. This reduces to 1,500m between Newhaven and Birling Gap, and less than 800m seawards off Beachy Head.
Bedrock slopes exist in the extreme east of the area, with occasional steep gradients representing a possible former Quaternary low sea-level cliffline. Much of the seabed is nearly featureless, with Quaternary palaeovalleys now infilled by subsequent sedimentation. The one exception is a discernible channel that trends south-east of Cuckmere Haven.
Out to about 1000m seawards of mean low water, sediments are predominantly fine to medium sands; sandy silts and silty clays. There is also shell debris, occurring in discrete patches and mixed with minerogenic sediment. However, local diversity is provided by outcrops of exposed bedrock (usually Chalk) and areas of well-packed rounded flint cobbles. These have been described by Joliffe (1972), at a site adjacent to the seaward terminus of the western breakwater protecting Newhaven Harbour, and are interpreted by him as the result of exposure by combined wave and tidal current scour. It is probable that in several areas of sand waves and 'ribbons', the thin sand layer (less than 0.5m) overlies consolidated gravels from which fine sediment has been removed by winnowing. Much of this gravel deposit is composed of flint and is probably relict. (White, 1924; Bellamy, 1995; Posford Duvivier and British Geological Survey, 1999). Its ultimate provenance is the Chalk, but a significant proportion has been transported from the catchments of the Adur, Ouse and Cuckmere rivers under earlier morphogenetic conditions (refer to Section 1). Some flints may derive from Palaeogene outcrops further west. Most flints with a polygenetic history are rounded or sub-rounded; those more recently removed from submarine Chalk outcrops, or from coastal sites, are angular or sub-angular in shape (Joliffe, 1972; Posford Duvivier and British Geological Survey, 1999; May, 2000).
Beyond approximately 1000m from the coastline, sediments become coarser, with sandy gravels dominating. This pattern is particularly evident in the extreme west of the region, and also in outer Seaford Bay. Most of inner Seaford Bay is occupied by a relatively thick layer of muddy and silty sand.
Offshore sediment mobility is indicated by ripples, patches, waves and ribbons of sand and possibly fine gravel moving across apparently immobile, weed-covered cobbles (Joliffe, 1972). At Jenny Ground, 700m seawards of Portslade, sand patches give some indication of net eastwards movement (Halcrow Maritime, 2001b). Indeed, eastward movement is suggested by the majority of the features observed, but there has been no monitoring of changes in bedform morphology and position, and some may be fossil features (Gifford Associated Consultants, 1997). An anti-clockwise circuit of sand movement may be a semi-continuous feature offshore and in the lee of Beachy Head, set up by the headland-induced tidal vortex there. Halcrow Maritime (2000b) inferred from beach foreshore change that both on and offshore pathways of sand transport operate. However, modelling using offshore wave climate statistics; wave refraction co-efficients and bathymetry did not provide a definitive picture of how and when sand is mobilised. It is probable that coarse sand moves around Beachy Head as bedload, as well as in suspension.
Although the mineralogy of the sediments of Seaford Bay is not well known, the predominant silty and muddy sands do not appear to be well sorted, thus implying only limited mobility. Joliffe (1964) described how gravel sized clasts, moving offshore from Seaford Beach, became trapped in a silt-dominated surficial layer. Between Cuckmere Haven and Beachy Head, sandy gravels in the near-shore zone are apparently immobile. They grade into well-sorted sands at a water depth of 30m, which possibly form a source for periodic onshore sand transport.
4.2 Estuarine Sediment Exchange - EO1 Brighton Marina EO2 References Map
EO1 R. Adur Inlet (Shoreham Harbour) - ( see introduction to Estuarine Sediment Exchange)
Following the stabilisation of the entrance channel to the port of Shoreham in 1820, dredging has been a continuous activity (see also Section 4.3). However, there are no reliable records of quantities removed prior to 1960. Between 1954 and 1957, over 1 million m3 of material was removed as part of a programme of channel and berth widening and deepening to allow collier access to the newly-constructed power station on East Beach. It is reported (Ridehalgh, 1958; 1959) that a "substantial" quantity of dredged spoil had been dumped offshore, close to maximum low water in previous decades. Loss of detailed dredging records for the immediately following years makes it impossible to assess whether this material had an impact on the local sediment budget. Ridenhalgh (1958) states that the tendency for a bar to form near the end of the eastern breakwater, prior to 1955, was reduced by breakwater extension and reconfiguration; this created a scouring effect that was reinforced by a local "breakaway" tidal eddy during the ebb. The entrance channel has been stable in position for many decades, as it has been cut into iron-cemented Eocene sandstones and lignites (Mortimer, 1997).
Posford Pavry (1984) used a wide variety of secondary data sources, including daily wind speed and direction for 1980-1984; Admiralty and Port of Shoreham routine quarterly hydrographic surveys between 300 m offshore and 250 m inshore of the harbour mouth, December 1975-1984; dredging records, and seabed sediment samples in a study reporting on the feasibility of increasing the depth and extending the length of the entrance channel. To this was added primary data on tidal current velocities and monthly hydrographic surveys between October 1983 and October 1984. Analysis revealed a persistent tendency for two bars to form, one just inside the head of the western breakwater and another between 120 and 170 m to the south. The inner bar, which was shallower, had a quasi-cyclic behaviour, accreting each winter and eroding during the summer. There was no convincing evidence of net loss or gain of sediment on either bar. The rate of accretion during the winter of 1983/84 proved to be exceptional, and was the product of a higher than average frequency of high wind speeds and storm waves from the south-east. During the period of close monitoring of hydrographic change, sediment was lost from the outer, and appeared to be gained by the inner, bar. Dredging records for 1960-1973 indicated removal of between 12 to 23,000 m3a-1, but only 8-13,000 m3a-1, 1974 to 1984, from the outer approach channel. Over the period 1974 to 1983, much larger quantities were dredged from the eastern and western arms of the entrance channel (means of 55,600 m3a-1 and 18,000 m3a-1, respectively). This suggested that for silt, silty sand and fine sand at least, there was a positive sediment budget. Maximum ebb tide velocities of 0.8-1.0 m.sec-1 in the channel between the breakwaters ensured net seaward transport of medium and coarse sand, which was then deposited over the seabed beyond the terminal points of both breakwaters in response to reduced bedload transport threshold velocities. Seabed samples indicated that fine gravel was present in this area, and occasionally also further up the entrance channel. The latter could not be transported by tidal currents, and must therefore be introduced by wave-induced currents, estimated at a mean of 1.0 m.sec-1 (compare mean tidal velocities of 0.3-0.4 m.sec-1).
Port of Shoreham dredging volume data for spring 1995 to Autumn 2001 indicates removal of an average of 19,670 m3a-1 from the entrance channel between the harbour breakwaters; and 13,100 m3a-1 from the outer approach channel. Both figures conceal considerable inter-annual variation. Without detailed analysis of hydrographic charts for this period, providing data on the volumetric equivalent of seabed topographic change, it is difficult to know if dredging removes much of the inferred net input of sediment from nearshore and offshore sources. Hydraulics Research (1984), working with dredging data for the period 1975-1983, and using theoretically calculated values for inshore wave climate and tidal current velocities, concluded that net accretion for the entrance channel as a whole was 1,630 m3a-1. Gifford and Partners (1997), however, estimate a net input from marine sources of 8,000 m3a-1, but it is uncertain if this figure is adjusted for long-term average losses induced by dredging.
Hydraulics Research (1984) also used data on (i) inshore wave heights (derived from wave refraction calculations); (ii) local bathymetry and current metering; (iii) seabed samples, and (iv) formulae relating to nearshore sediment transport to estimate that approximately 200,000 m3a-1 of suspended sand moves across the outer entrance channel. There is uncertainty as to how much of this sediment may be moved up-channel by tidal currents, and/or might be diverted into short-term storage within the inner and outer bars, but it is within a range between 30 and 100,000 m3a-1. It was suggested that significant quantities might be temporarily driven into the outer entrance channel under high energy wave conditions, occurring approximately once every 10 years. As much as 24-25,000 m3 in a week of sustained south-easterly approaching waves might be a reasonable estimate; much of this would be dispersed offshore and longshore shortly thereafter. Sea bed sampling indicates some cross-entrance bedload transport of gravel, estimated to be a net volume of approximately 14-20,000 m3a-1 (Hydraulics Research, 1984; Sir William Halcrow, 1990), with an eastwards component. Most of this transfer occurs at or around high water irrespective of the type of incident waves. The presence of a stable, armoured sea bed surface seawards of the outer channel approach has been taken as evidence of the absence of gravel transport in the offshore zone (Joliffe, 1972; 1978; Posford Pavry, 1984). This, however, cannot be regarded as a reliable conclusion.
Brighton Marina - ( see introduction to Estuarine Sediment Exchange
It is known that siltation occurs within the Marina basin and that dredging operations periodically remove between 1 and 50,000 m3 . There is currently no publicly available data that allows an average annual rate of output to be calculated. All of this sediment must be derived from marine sources. Dredged spoil is deposited in the offshore zone and is presumed not to re-enter the littoral sediment budget in any significant quantity.
EO2 R. Ouse (Newhaven Harbour) - ( see introduction to Estuarine Sediment Exchange)
The present configuration of the harbour dates from the completion of the Western Breakwater in 1890 and the extension of the East Pier in the early twentieth century. The approach channel is approximately 125 m wide and 500 m in length with a nominal depth of -6.0 m LAT maintained by routine dredging. Between 1878 and 1974 depth above the outer bar was -3.7 m LAT, but deepening was undertaken in the late 1970s and mid 1980s to provide access for larger draught vessels. Mean spring tidal range is 6.1 m, and 3.0 m during neaps, with peak (ebb) current velocities of 0.8 m sec-1 occurring some 200 m offshore the western breakwater. On the flood tide, water enters the main channel from the west and creates a tidal eddy in the lee of the Western Breakwater. This stream then takes a nearly circular route across the shallow water area to the east of the dredged channel, which it re-enters nearer to the channel head. The ebb tide is confined to the dredged channel seawards of the breakwater, but then flows westwards in a more diffuse current (Hydraulics Research, 1988a).
The local wave climate is complex, and has been modelled (Hydraulics Research, 1988a) using data from the Seaford wave study (Hydraulics Research, 1985). The latter employed the OUTRAY wave refraction model, calibrated using hydrographic knowledge of Seaford Bay. The area to the immediate east of the dredged channel is exposed to wave approach from the south-east, but is protected from south-westerly waves by the 900 m length of the Western Breakwater. Complex local refraction and reflection causes an increase in wave height to the east of the channel. Waves moving directly into the channel, from the south or south-south-east are also refracted by its well defined (dredged) boundaries. As both Beachy Head and Selsey Bill provide a protective influence, Newhaven Harbour is principally exposed to waves approaching between 90o and 270o(Posford Duvivier Environment, 1997).
Hydraulics Research (1988a) observe that south-westerly waves have the capability of entraining and suspending sand in Seaford Bay, which is then carried into the dredged channel on the flood tide. In an earlier study on Seaford beach, Hydraulics Research (1985) concluded, from wave energy and refraction modelling, that south-westerly approaching waves moving at peak velocities were able to disturb sand in water depths of up to at least 6.0 m. Deposition of suspended sediment occurs due to the rapid reduction of wave energy in the landward sector of the approach channel, induced by depth shallowing and refraction. This effect is greatest when peak tidal current velocities operate in the area to the immediate east of the dredged channel, apparent from carefull and detailed analysis of hydrographic chart data. Net westwards littoral drift between Tide Mills (Seaford) and the East Pier may also contribute a net transfer of fine sand into the inner approach channel. Hydraulics Research (1988a) quote an example of approximately 14,000 m3 of sand and silt being deposited over a one month period in early 1988 during which high energy south-westerly waves were dominant. This accretion took the form of a shoal spreading across the channel, in a north-westerly direction from the end of the East Pier. Simultaneously, a presumed scour hole developed seawards of the eastern end of the dredged channel, possibly caused by a strong tidal eddy set up by the Western Breakwater. This event was probably characteristic of a long-sustained pattern of net winter siltation (Posford Duvivier Environment, 1997). Optimum conditions are the coincidence of maximum ebb tidal current velocities and large waves immediately following low water.
It is thus apparent that the dredged channel is effective in trapping fine sediment, only a relatively small proportion of which can be re-distributed or removed seawards by tidal currents. Once deposited, sediment is less likely to be remobilised by diminished wave power, except under specific conditions involving incident waves from the south-east. However, some re-mobilisation may be induced by the deepest draught vessels that can use the navigation channel.
The mouth of the Ouse, in particular Newhaven Harbour, is therefore a natural sediment sink, though specific input does depend on how much material is moved out by the ebb current, thereby feeding shorewards moving waves and the incoming flood stream. Sediment sampling from the inner entrance (Hydraulics Research, 1988a) indicated a well sorted sand with a median diameter of 0.12 mm, increasing to a coarser deposit further offshore. It would appear from this evidence that very little, if any, gravel-sized material is retained in the approach channel, although it is probable that material of this grade moves eastwards from the end of the Western Breakwater. Presumably, it is transported seawards into and across Seaford Bay (see Sections 3 and 4.1 for further discussion). Longshore transport is therefore subordinate to onshore movement in determining the local sediment budget.
Gifford Associated Consultants (1997) suggest that Newhaven Harbour experiences a net annual input of about 7,000 m3. However, it is not clear if this figure subtracts losses from dredging. Data provided on dredged quantities for both the inner and outer harbours, 1982-2002 (Newhaven Port and Properties Ltd, 2002) reveal an annual average of 55,400 m3 and 137,100 m3 respectively. There have been substantial inter-annual variations, reflecting changes in vessel sizes, port operating conditions and specific infrastructure projects.
4.3 Dredging - References Map
Dredging for navigational access occurs at Shoreham and Newhaven Harbours, and involves removal of mostly fine sediment from both the approach channels and inner harbours. Excavation for larger berths has been periodically undertaken in recent years in response to increasing vessel sizes and traffic expansion. Details of quantities removed over the past 20 years are given in Section 4.2.
Sediment is also dredged from the outer basin of Brighton Marina, estimated at 5,000 m3a-1 (detailed figures have not been made available). As there is no longshore supply, this quantity must represent input from marine (nearshore and offshore) sources. Most is fine sand and silt, and is therefore introduced as suspended load by both waves and tidal currents. Dredged sediment is dumped to the southeast of the Marina site; it is not known if it makes a contribution to the downdrift sediment budget, but if so it is likely to be very small.
4.4 Beach Mining- References Map
Past sites include:
4.5 Beach Shingle Abrasion Loss - References Map
An original component of the University of Sussex-led BERM project (Dorbusch, 2002; Moses, et al, 2001) has been experimental research on the durability of flint gravel sized clasts on East Sussex beaches. Data has been obtained from (i) laboratory mill tumbling of flint clasts taken from sites between Telescombe and Friars' Bay; and (ii) field monitoring of abrasion loss of 'exotic' quartzite and limestone clasts introduced onto two East Sussex beaches composed of contrasting types of flint particles. The exotics had size, shape and other characteristics that were closely comparable to indigenous flints and were used to facilitate ease of recovery and re-measurement (Dornbusch, et al, 2002 a and b).
Laboratory experiments used both well rounded and freshly-broken flints, the latter derived from recent cliff falls. Abrasion rates were observed to increase in a linear form with increasing clast size and weight, at approximately 0.06% of weight per hour. However, precise abrasion rates were a partial function of water:mass particle ratios in tumbling mills, and it was noted that under these controlled conditions chatter marks, characteristics of abrasion in the field, were not created. Abrasion rates declined over time, but losses were consistently greater for newly-fractured, broken, angular flints than for those which had a previous history of natural wear. Abrasion debris was predominantly silt sized, but some sand was also produced. It is the provisional conclusion that experimental tumbling cannot achieve the rates that prevail under natural hydrodynamic conditions.
Field monitoring of marked exotic tracers was carried out over a one year period (approximately 700 tidal cycles). Liassic limestone clasts suffered an 18.6% weight loss whilst for Budleigh Salterton metaquartzite the weight reduction was lower, at 5.7%. These figures indicate the important control of lithology, but mean wave height also provided some correlation with spatial variations of abrasion loss. Losses were, not surprisingly, greatest after storm events, but less expected was the observation that well rounded clasts continued to experience abrasion at a "significant" rate under normal conditions following storms. Initially angular particles became sub rounded or rounded within a few months. No direct correlation between weight loss and shape change with distance travelled was determined.
Tracer re-weighing suggests an average weight loss of 0.0078% per tidal cycle (maximum of 1.23%). At this rate, destruction (to sand) would occur within about 20 years. Relating these results to flint, via tumbling mill experiments, suggests that it is some ten times more durable than quartzite, giving an estimated weight loss of some 2% per annum. Thus, existing beach clasts might be removed by abrasion over the next 200 years (a 25% loss every 50 years). Moses, et al (2001) apply this conclusion to Telscombe Beach, which has a surface area of 1500 m2 and an estimated volume of 31,000 m3. On the assumption that 50% of the beach area is subject to wave action during an average tidal cycle, and that the mean depth of disturbance is 0.5 m, some 12% of the total clast population is exposed to active abrasion. If the mean annual weight loss is 2%, then 75 m3 of volume is lost annually as a result of this process. This needs to be compensated by fresh input, principally from eroding cliffs. In this case, this is achieved, as annual natural erosion yield is calculated to be 96 m3.
Notwithstanding that some of these figures are rather crude estimates, the wider implications for beach management are clear. Although a positive budget can be established for Telscombe Beach, it is one of only a few localities between Brighton and Seaford Head which are unprotected. At all other sites on this sector, abrasion loss cannot be naturally counterbalanced by sediment yield from cliff erosion. Recharge is therefore a longer term inevitability, but replenishment supplies will also suffer in situ reduction due to abrasional wear.
5. BEACH MORPHODYNAMICS - References Map
Gravel beaches are the dominant form along this shoreline, although many are built on coarse to fine sand foundations. Sandy foreshores are a feature of some sectors, but tend to be absent or of limited development where shoreline platforms front a narrow coarse clastic backshore beach. Gravel beaches are everywhere characterised by a narrow, relatively steep, cross-sectional profile often with one or more berm ridges. Seasonal changes in elevation, width and profile morphology are in response to variations in incident wave energy and approach.
There are no systematic and comprehensive reviews of gravel beach sedimentology. Halcrow Maritime (2001b), describing beach form along the Southwick to Kemp Town (Brighton) frontage, state that medium to coarse gravel dominates the backshore, but is replaced by fine gravel where its toe is adjacent to the sandy foreshore. A good example of this also occurs between Saltdean and Telscombe Cliffs (Mott McDonald, 1999). In places, for example between Southwick and Hove, well-sorted medium to fine gravel extends below mean low water (Young and Lake, 1988). Here, and at beach replenishment locations such as Rottingdean and Seaford, the introduction of large quantities of gravel taken from offshore sources has modified indigenous beach sedimentology. Following a few weeks of varied wave conditions, the mid-profiles of wider beaches, e.g. Brighton and Seaford, often exhibit a mix of gravel and sand. Thereafter, sand particles move down through the voids between larger particles and add to a well compacted sandy substratum. This process is characteristic of the winter period, when storms may temporarily remove some of the gravel fraction into the nearshore zone. Under these conditions, when more of the sand basement is exposed it proves relatively resistant to wave-induced scour due to its low porosity. However, under extreme high energy conditions most sand is removed, revealing the underlying Chalk substrate.
Gifford Associated Consultants (1997) state that the mean grain size of gravel on regional beaches increases progressively eastwards. This is particularly evident on the lower part of the backshore, where the mean for the principal particle axis increases from 12 to 15mm between Hove and Seaford. This is taken to be in response to increasing wave energy in the same direction.
The majority of gravel clasts are angular to sub-angular in shape, but experience progressive "smoothing" and rounding of corners as a function of abrasional wear. This increases with residence time (Moses et al, 2001), but it also related to provenance (Sections 1, 2 and 4) discuss the origins of beach gravel in more detail). Sand is introduced from nearshore and offshore sources, examined further in Section 4.1. At a few locations where the longshore transport of gravel is impeded, clasts exhibit a higher than average degree of rounding. May (2000) notes that there is a larger proportion of "rolled" flint particles on the beach at Cuckmere Haven, where fluvial input via the adjacent ebb delta is a possibility.
The presence of control structures, mostly groynes, introduces local variation to beach morphology by promoting updrift accretion and inducing downdrift sediment depletion. Beach profiles are sometimes exceptionally steep where long or widely-spaced groynes are effective in trapping a high proportion of gravel moving within groyne compartments, especially where drift rates are low (Cleeve and Williams, 1988). In some examples of recent groyne re-construction, beaches have been designed to re-orientate or "rotate" in response to changes in wave approach, height and period. An example occurs between Brighton Marina and Saltdean (Posford Duvivier, 1994b; 1995 and 1997). Where beaches are backed by seawalls, reflective scour is also a widely acknowledged factor influencing beach morphodynamic behaviour, especially in the immediate vicinity of breakwaters and long groynes. Several examples, closely tied to the history of sea defence construction, are documented by Joliffe (1972).
The regional Shoreline Management Plan (Gifford Associated Consultants, 1997) undertook detailed analysis of historical change, between 1870-1992, of beach profile form and inter-tidal width. Successive Ordnance Survey map editions, supplemented by post-1973 ABMS and other aerial photographic cover, were used in the LITPACK numerical model to quantify net movements of the relative positions of mean High and Low Water for eleven discrete sections of shoreline. This was then converted to a Retreat/Advance classification. The LITPACK model was also able to calculate net volume changes 1972-1992 between adjacent profiles to a depth of 10 to 12m below ODN. As ABMS survey data only extends to mean low water, extrapolation seawards was based on bathymetric information. The salient points of this, and independent calculation of beach volume change, 1973-1999 (Harlow, 2001b) are:
1. Shoreham Harbour entrance to West HoveHistorical long-term retreat of Mean Low Water (0 to 1.5m.a-1) is evident with recent net accretion due principally to beach recharge. Overall trend is for profile steepening with morphodynamic behaviour linked to recharge periodicity and wave reflection/refraction caused by the training wall. Rapid drawdown occurred following western breakwater extension in 1874, leading to "emergency" recharge at Hove in 1886 (Ward, 1922). The sector between Southwick and Aldrington has been maintained subsequently through a combination of groyning (dating from c.1870), deposition of dredged spoil, rubble dumping and recharge by of excess gravel accretion updrift of the western breakwater.
2. West Hove to Brighton MarinaLong-term trend has been Low Water retreat (up to 1.25m.a-1), but seawalls have fixed the position of mean High Water since the mid -1850s. However, since at least the late 1960s, net accretion and mean low water advance has prevailed at a mean rate of 18,200m3a-1, 1973-99 (Halcrow 2001b). This beach now exhibits a progressive increase in width between the Palace Pier and east Kemp Town (Photo 11), and is efficiently managed by groyning. The role of the west breakwater of the Marina in inhibiting longshore drift (Photo 3), and thus being a cause of steady beach volume increase, since its construction in 1973, is strongly implicated (Jezzard 2003; Halcrow, 2001b). However, this has not been proven definitively. Thornburn (1977) implied that equilibrium has characterised the eastern sector of this beach since approximately 1930. The first generation of groynes were built in 1790, and have been maintained/upgraded continuously since. Halcrow Maritime (2001b) regard Kemp Town beach as a self-regulating, equilibrium dissipative system. This conclusion is based on detailed analysis of volume changes over six groyne-limited sectors for a 12 year period, using ABMS data. Jezard (2003) has confirmed net volume gain and advance of the toe of the upper gravel beach using ABMS profiles for 1990-1995. However, there was no discernable increase in beach volume in the beach sector immediately updrift of the Marina wall. No significant changes in profile gradient and morphology are apparent, with only modest losses - rapidly recovered - during storms. Halcrow Maritime (2001b) present photographic and anecdotal evidence of inter-annual changes in beach volumes, but state that there were no discernible effects on longer-term condition. A tentative conclusion is that the Kemp Town beaches were accreting for many years prior to the construction of Brighton Marina, possibly due to changing bathymetry in the Black Rock area and the local effect of the Black Rock ledges (Eade – personal communication). Crest flattening and some artificial reprofiling undertaken during the spring of each year modify the beach between the piers along the central Brighton frontage. This is undertaken for tourist amenity purposes.
3. Brighton Marina (Black Rock)The site of the Marina previous to its construction between 1973 and 1975 was characterised by sediment dispersal and no beach accumulation. It was ascribed by Joliffe (1972) to the effects of scour/outflanking induced by the large terminal ("Banjo") groyne at Kemp Town. He was able to correlate loss of beach volume and width with the history of seawall construction, commencing in 1928. Other authorities have ascribed the absence of beach accumulation to a zero littoral transport rate (Posford Duvivier, 1997; 2001).
4. Brighton Marina to SaltdeanBoth the long and short-term (post 1980) trends are those of profile steepening and Low Water Mark retreat at rates of 2.0 to 3.0m.a-1. This is confirmed by Dornbusch (2002), who identifies both strong control structures and apparent loss of offshore to inshore input as probable causes. Groyning commenced in 1887, but the seawall was completed in stages between the 1920s and the mid 1980s. Chadwick, Moffett and Rees (1986) used ABMS data for 1972-1979 to demonstrate that this beach system, as a whole, was slowly reducing in volume. Ellis (1986), however, regarded the upper gravel beach at Rottingdean as stable. Posford Duvivier (1997; 2001) state that the beaches of this sector have a history of depletion, although there is comparative stability at Ovingdean. This was ascribed to a local, subtle change in shoreline orientation. There is uncertainty over the morphodynamic behaviour of the beach immediately downdrift of the Marina. Posford Duvivier (2001) state that wave diffraction set up by the breakwater creates reduced wave energy and provides conditions for modest sediment accretion. Jezard (2003), concludes that ABMS data for 1990-1995 indicates loss of volume, landward migration of the single toe of the beach and mean low water retreat but beach stability apparently prevailed between 1974 and 1985. It is this uncertain if the Marina has had any specific impact on beach drawdown, though Jezzard (2003) considers trends for 1990-1995 to demonstrate its effect on interrupting updrift supply.
Two substantial replenishment schemes in the 1990s (Section 2.3) have been constructed in response to the history of depletion of this gravel beach. Wide platform development has inhibited the extension of the narrow sandy foreshore. The long-term history of groyne control has been relaxed since the mid 1990s to reduce the effects of differential accretion/depletion. This should also facilitate more flexible response to seasonal profile changes, especially where enhanced by reflective scour from massive backing seawalls (Posford Duvivier, 1997; 2001). Short-term littoral drift reversals are frequent, but beach recovery has been checked, in the past, by closely-spaced groynes. Beach management since 1995 has involved the design of optimum crest heights and profile gradients to minimise volume fluctuations (Posford Duvivier, 1997).
5. Telescombe Cliffs to Peacehaven StepsThe historical trend is for Low Water Mark retreat (0.6 - 1.4m.a-1, accelerating to 3.0m.a-1 in recent decades), and profile steepening. The western section of this length of shoreline remains unprotected except around headworks of the Portobello outfall. At Peacehaven, the seawall dates from the 1930s, though built in sections through to the 1970s see Photo 8 (Stammers, 1982). Ellis (1986) estimated some 5,000m3 of volume loss from the beach at Bastion Steps, Peacehaven, 1979-1984. Ellis (1986) also describes substantial profile fluctuations affecting the backshore beach at a site opposite Bayview Road. He ascribed losses to the role of inefficient groynes trapping an excess of potential sediment input.
Mouchel Consulting (2001a) note that beach sediments tend to be "patchy" in distribution and profiles variable. Groynes are decisive controls on beach width, with progressive downdrift widening within each groyne bay. Where beaches are widest their profiles are steepest, possibly a function of groyne orientation with respect to dominant wave approach. Recent groyne reconstruction/redesign will address these problems, providing more opportunity for natural beach morphodynamic response to forcing conditions.
6. Peacehaven Heights to Harbour Heights, Newhaven and Western BreakwaterBetween Friars' Bay and Harbour Heights, longer-term, but modest, depletion has been reversed by net accretion since the late 1960s. Profile steepening has characterised both phases, but the upper beach frequently exhibits a well defined storm ridge and multiple berms. The wide (1800-2000m) beach trapped by the western breakwater protecting Newhaven Harbour shows net accretion (approximately 5,000m3a-1 since the 1840s), beach profile steepening and crest elevation. The accumulation zone commences some 800m to the west of the breakwater, having migrated slowly in this direction since the 1940s (Robinson and Williams, 1983). Jezzard (2203) reports a strong statistical relationship between distance updrift from the Western breakwater and beach volume change; as distance increases, the accretion gain diminishes. This conclusion is based on analysis of EA ABMS profiles, 1973-2000.
7. Newhaven (East Pier) to Tide MillsProfile form has steepened, but this short sector exhibits seawards advance of both Mean High and Low Water of between 1.0 to 3.0m.a-1 over the last 100 years (Jezzard, 2003). This is ascribed by several authorities to net east to west littoral drift and protection from the direct impact of south-westerly approaching waves. Local wave refraction due to the pier may also contribute to low erosion potential.
8. Tide Mills to Seaford HeadThis frontage was renourished and its profile reconstructed in 1987, with subsequent monitoring and maintenance. Full details, together with the previous history of severe depletion, retreat of mean High and Low Water and profile steepening are given in Section 2.4. Since 1987, recycling of surplus sediment accumulating in Sector 7 and against the terminal rock groyne at the eastern end has maintained morphodynamic equilibrium and achieved a small net accretion gain.
9. Seaford Head to Cuckmere HavenMost of the beaches of this sector have been retreating (a mean of 0.5m.a-1 since the 1880s), although the barrier spit beach that has grown across the mouth of the former Cuckmere tidal inlet (Photo 12) has advanced over the longer-term, at 5.0 to 6.0m.a-1. This trend has been accompanied by profile flattening. Since the late 1970s, however, beach behaviour has been fluctuating, perhaps because of revetment and seawall building.
10. Cuckmere Haven to Birling GapThis sector receives high wave energy and is characterised by a narrow, discontinuous, upper gravel beach that has retreated and steepened since the late nineteenth century. The beach at Birling Gap (Photo 9) occupies a small embayment and is up to 15m in width. Robinson and Williams (1998) observe that volume losses are greatest when waves approach from the south-east. They state that the entire beach has been removed, for brief periods, at least three times during winter storms since 1930; the most recent occasion was in 1996. Posford Duvivier (1993) describe a residual beach here in the winter of 1992, when wave notching of the cliff base took place; there was no record of this having occurred during the previous 35 years.
Moore, Collins and King (2001) state that ABMS data, 1973-2000, indicates ongoing beach narrowing; Robinson and Williams (1998) calculate that this beach was some 100m wider in the 1890s, giving an annual retreat rate of about 1.0m over the past century. However, there is no direct evidence that narrowing has been accompanied by any significant loss of storage volume, as it has also experienced crest elevation and has steepened.
11. Birling Gap to Beachy Head (Holywell)Halcrow Maritime (2001a) have examined foreshore changes for a selection of pocket beaches around Beachy Head, using post -1980 ABMS data. This work confirms Gifford Associated Consultants' (1997) conclusion that beach profile steepening and Low Water Mark retreat has been ongoing since at least the mid twentieth century at a rate of 1.0 to 1.5m.a-1.
6. SUMMARY: SEDIMENT TRANSPORT SUB_CELLS AND BUDGETS - References Map
Based on current knowledge of regional pathways of transport of gravel and sand, four sub-cells are delimited. From west to east these are:
Gifford Associated Consultants (1997) identified a similar set of sub-cells from application of their regional conceptual sediment transport model (RCTM), but did not recognise the Shoreham Harbour breakwaters as a subdivision within a cell extending from Selsey Bill to Brighton Marina.
There is inadequate quantitative data to calculate full sediment budgets for any of the above sub-cells, but sufficient knowledge is available to present a conceptual understanding of each. One component that is shared by all is that contemporary stores of gravel are considered to be substantially 'fossil'. They represent now partially redistributed and eroded former coarse clastic barrier beaches created during the late Holocene. The offshore supplies of gravel from which they were built appear to have either diminished or are no longer moved shoreward by waves in sufficient quantities to offset continuing losses due to offshore transfers, abrasion, etc. This issue has been discussed in further detail in Section 1, drawing in particular on the arguments of Jennings and Smyth (1990) and Nicholls (1991). As sediment yield from eroding Chalk cliffs produces insufficient coarse sediment to compensate for losses, the reduction of beach volumes (detailed in Section 5) over the past 100 years is inevitable. However, it is important to emphasise that this fundamental budget deficit prevailed before the construction of shoreline defences. The latter have served to accentuate an existing problem, necessitating measures such as large scale renourishment along frontages experiencing unrecoverable losses.
A notional annual budgetary statement for each sub-cell is presented below, with added explanatory or qualifying comments. Quantities quoted are given in earlier sections; all figures are approximations.
R. Adur to Brighton Marina (West)
Brighton Marina is used as the eastern sub-cell boundary as there is no evidence for any significant by-passing by coarse sediment. Several studies have concluded that the platform site on which the Marina was constructed historically received negligible input from updrift beaches. Halcrow Maritime (2001b) conclude that the gravel budget for this sub-cell is strongly positive, as there inputs, but no clear observational or experimental evidence for losses. Inputs are not understood fully for the actual amount of natural bypassing of the Shoreham breakwaters is uncertain. As accretion immediately up-drift of the Marina breakwater is slow, most storage occurs on the beach fronting Kemp Town (see Section 5). Posford Duvivier (2001) state that this quantity was approximately 12,000m3a-1, 1980-1994, whereas Halcrow Maritime (2001) calculate this quantity to be slightly in excess of 18,000m3a-1. The latter estimate derives from more detailed modelling of forcing conditions. However, it is impossible at present to provide any reliable quantification of outputs - for example, the proportion of gravel in dredge spoil removed from Shoreham Harbour approaches is not recorded. Gravel input via bedload "creep" from offshore; onshore kelp-rafting and delivery of coarse sediment from the river Adur are also unquantified, but are thought to be collectively a small quantity.
A proportion of the gravel stored in the beach system between Aldrington/West Hove and Kemp Town, Brighton is 'fossil', inherited from: (i) updrift littoral input pre-dating the construction of breakwaters at Shoreham Harbour, feeding eastwards spit migration; (ii) onshore barrier migration (see Section 1).
The sand budget is difficult to assess, and cannot presently be quantified. Halcrow Maritime (2001b) consider that it is dominated by longshore and both nearshore and offshore suspended transport and gains and losses would appear to be in balance. The Marina breakwater is not considered to impede downdrift movement of sand, and finer sediment grades. Data for quantities of sand and finer sediments deposited in the approach and entrance channels, and removed by dredging, are given in Section 4.2.
In summary, this is a nearly closed transport cell for gravel, but its boundaries are less restricting on sand movement.
Brighton Marina (East) to Newhaven Harbour Western Breakwater
With the exception of cliff inputs, most of the above budgetary elements are only crudely quantified, whilst some are little more than notional. The imbalance between losses of beach volume between Rottingdean and Saltdean (see Section 5) and storage induced by the Western Breakwater indicate that the overall gravel budget is strongly negative. This is demonstrated by the requirements for renourishment of beaches in the 1990s. Further east, however, there may be an approximate balance between gains and losses, with a net positive balance of up to 10,000m3a-1 between Friars' Bay and Newhaven Harbour Heights. If this calculation is reliable, gravel by-passing of the breakwater might be in the order of 5,000m3a-1.
The sand budget is wholly unquantified, with no proof that dredged sand removed periodically from the Marina is actually retained in the littoral transport zone. It is more probable that much of this material is silt and clay, and is dispersed offshore as suspended load.
Newhaven Harbour to Cuckmere Haven
Joliffe (1972) considered that Seaford Bay (East Pier, Newhaven Harbour to Seaford Head) functioned as a virtually closed transport cell even before intensive beach management commenced. This view has been endorsed by Posford Duvivier (1993). Work carried out using fluorescent tracers, diving observation and profiling (Joliffe, 1964; 1972) indicated that wave-induced net on and offshore movements of gravel were imbalanced, producing slow but progressive loss to deep water areas of Seaford Bay. However, his work also implied that inputs of both gravel and sand from sources updrift and seawards of Newhaven Western Breakwater, across Seaford Bay, may occur. Despite the robust construction of the Seaford Beach terminal groyne, it is by-passed under high wave energy conditions (Joliffe, 1978, Posford Duvivier, 1993). It is therefore more appropriate to locate the eastern limit of this sub-cell at the the beginning of the Seven Sisters cliffs that form an eastern margin to the Cuckmere Haven embayment. Its role as a partial transport boundary has not been directly demonstrated, but May (2000) notes that the significantly higher proportion of rolled flint clasts on Cuckmere Haven beach compared to the regional average could indicate their retention there. There may also be a more complex circulation, whereby a proportion of gravel from the beach enters the Cuckmere inlet and is flushed seaward to be stored in the ebb-tidal delta before being driven back to the beach by wave action. The lower foreshore morphology is strongly indicative of such processes, but specific investigative studies have yet to be undertaken.
The massive replenishment of Seaford Beach, south-east of Tide Mills in 1987 (Section 2.4) is a clear indication that its previous sediment budget was strongly negative. Since then, however, various techniques of beach management have been applied to maintain volume stability, though annual changes have varied from +65,000 to -75,000m3 (1991-2000).
Progressive loss of sediment from re-entrant beaches between Seaford Head and Beachy Head (Gifford Associated Consultants, 1997) is probably the direct outcome of the conservation of Seaford Beach long before, as well as since, its replenishment. Periodic recycling of coarse sediment on both Seaford and Cuckmere Haven beaches has no impact on the overall budget. Recharge at Seaford does not appear to have had any downdrift benefits beyond Seaford Head.
Cuckmere Haven to Beachy Head (Holywell)
Although Beachy Head is a substantial, well defined headland that occupies nearly 5km of frontage, it is not an absolute boundary to longshore sediment transport of either gravel or sand. Posford Duvivier (1999) suggest that there is a potential for 2,500 to 5,000m3a-1 supply of gravel to Eastbourne beaches from east of Cuckmere Haven, but it is unclear how much of this derives from cliff falls and how much is in transit from updrift beaches. Storage is likely within the several re-entrant beaches and/or when large cliff falls create temporary barriers to the lateral movement of flint debris. However, net storage is small, indicating that most erosion products must be removed seaward in suspension (fines) or laterally by drift. Indeed, Nicholls (1991) states that there is no evidence for any net accumulation of gravel on the west facing beaches below Beachy Head over the past 300 years. Halcrow Maritime (2001a) consider that much more sand than gravel by-passes Beachy Head, moving within a wide littoral zone. This observation is based on numerical modelling of wave climate, using refraction co-efficients, and analysis of recent (1990s) ABMS data. As the latter revealed little fluctuation in the extent and volume of sandy foreshores of headland beaches since 1985, net onshore and offshore movements appear to be balanced. Rectilinear tidal currents probably assist the longshore flux of sand.
Posford Duvivier (1993) use unspecified documentary evidence to support their conclusion that the longshore pathway of gravel transport around Beachy Head has become much weaker since the late eighteenth century. The progressive erosion of the Crumbles gravel ness, Eastbourne, since the mid eighteenth century may be indirect evidence that helps to corroborate this argument, although groynes at Eastbourne may have intercepted its gravel supply. However, Jennings and Smyth (1990) interpret the erosion and retreat of the Crumbles to be a response to diminished feed from offshore sources. This factor may also apply to Beachy Head and the coastline further west.
The major gravel store comprises the pocket beach occupying the Birling Gap embayment. Moore, Collins and King (2001) attempted to calculate a gravel budget for the beach and argued that longshore transport of gravel had declined over the past 200 years due to the building of breakwaters and sea defence works along the updrift coast as far as west as Brighton. The longshore extent of Birling Gap beach has reduced by some 800m since 1874, involving a volume loss of 48,600m3 (386m3a-1). A figure close to this is also calculated by Posford Duvivier (1996). Reasons for this include reduced input of flint gravel from the erosion of the dry valley sediment infill (Coombe Rock) exposed in the cliff line behind the beach. This in turn is due to a reduction in the exposed width of this section, from 230m in 1909 to 170m in 1999. Some 315m3a-1 of fresh flints are derived from current cliff degradation. To this can be added approximately 1300m3a-1 from the erosion of updrift cliffs, platforms and beaches along the Seven Sisters shoreline. Moore et al (2001) estimate that losses are at or about 2,000m3a-1 at this site, thus giving a net deficit approaching 400m3a-1. As the beach at Birling Gap is slightly lower than those to the immediate east and west, losses may be at a maximum here. However, a persistent budget deficit is likely to characterise all beaches on this sector.
Note on Cliff Erosion Input:Estimates of the yield of flint debris from cliff and platform erosion between Rottingdean Head and Beachy Head depend, in particular, on assumptions of % flint content of in situ chalk. This is itself spatially variable (ROCC, 2001). Most calculations use a minimum approximation of 5% (e.g. Posford Duvivier and British Geological Survey, 1999; Posford Duvivier, 2001), but 2% is a more justifiable estimate for locations between Cuckmere Haven and Holywell, Beachy Head. The BERM project (Dornbusch, et al, 2000) calculates a contemporary yield of 4610+ 890m3a-1 for all undefended sectors combined between Rottingdean and Beachy Head based on an average flint content of 3-5%. This is lower than estimations used in the above tables (and substantially below the 25,000m3a-1 calculated by Jennings and Smyth (1990) for mid to late Holocene period).
7. KEY COASTAL DEFENCE AND HABITATS ISSUES - References Map
SSSI and European conservation designations apply to large areas of this coastline, notably along the sectors of Chalk cliff and shoreline platform. These relate especially to geological and geomorphological interests. (Gifford Associated Consultants, 1997; May, 2000, ROCC report, 2001).
With the exception of short lengths of the coastline between Saltdean and Peacehaven (Photo 7), the entire frontage between Shoreham Harbour and Seaford Head is protected by groynes, seawalls and revetments. Newly replenished beaches (e.g. Seaford; Rottingdean) rely more on the creation of morphodynamic analogues, and less on control structures. East of Seaford Head, defences are limited to the vicinity of Cuckmere Haven. Recent proposals to construct defences at Birling Gap have been rejected as being incompatible with conservation criteria.
Long-term and substantial investment in coastal defences and estuarine reclamation has either destroyed or degraded many pre-existing habitat interests. Many beaches are of little ecological interest and re-profiling has modified many cliff faces between Roedean and Newhaven. Exceptions, however, occur where important shingle vegetation communities adapted to stable backshore gravel or gravelly sand environments have become well established at sites of accretion and beach progradation such as Seaford west beach and the gravel barrier to the east of the river mouth at Cuckmere Haven (Photo 12). Embanked tidal rivers with some adjoining saline habitats occur within the Ouse and Cuckmere Valleys. Given the presence of high density residential, commercial and infrastructure development, immediately adjacent to most of the coastline between Seaford and Shoreham-by-Sea, there is relatively little scope for major changes in shoreline management approaches. However, past or current defences have not significantly impacted the sequence of inter-tidal and sub-littoral habitats represented by the wide shoreline platform east of Brighton Marina. It is essential that this should continue, as this environment is nationally and internationally scarce.
Opportunities for the creation, or re-creation, of coastal habitats through the application of principles of sustainable development are limited to a few sites. The most important is at Cuckmere Haven (a href="photos/photo5.htm">Photo 5), where managed retreat could be undertaken in the interests of a modest expansion of salt marsh and intertidal habitat. Binnie, Black and Veatch (1998) have undertaken a preliminary evaluation of opportunities, with further more detailed evaluation work in progress. The small area of residual salt marsh, brackish lagoons and ponds behind the beach and embankment between Tide Mills and Newhaven has been given a preliminary assessment by Posford Duvivier Environment (1997). There are prospects for habitat restoration here, but managed retreat is less feasible because of flood risk to adjacent commercial development. Proposals for land claim, to extend port infrastructure and allow industrial expansion, add to pressures here. Brackish water at Seaford Green provides a further local opportunity for habitat improvement.
Gravel beach habitats could perhaps be enhanced on the upper backshore confined by Newhaven Western Breakwater and the accreting beach fronting Kemp Town and the Marina, Brighton (Packham et al., 1995; Packham and Spiers, 2001). Proposals to reclaim an area behind East Beach, Shoreham-by-Sea might also include deliberate encouragement of vegetation over the backshore. The existing shingle habitat at Cuckmere Haven will need to be carefully conserved if managed retreat incorporates natural beach "roll back". Future beach restoration/renourishment projects should also address opportunities for habitat creation as explained by guidance contained within (Doody and Randall, 2003). In this context it would be extremely valuable to extend eastward the West Sussex Vegetated Shingle Project (2003). It has mapped the habitats to the west of the R. Adur and has sought to increase general awareness of the local resource, it has provided guidance for contractors working on vegetated shingle (potentially highly relevant to Cuckmere Haven) and further guidance has been produced for residents with shingle gardens.
Weathering and falls/topples/slips periodically affect the protected chalk cliffs between Brighton Marina and Peacehaven. Re-profiling occurs where there is potential risk to beach and promenade users; this work might include some provision for bird nesting and roosting as a supplement to the contribution of natural weathering and erosion.
8. OPPORTUNITIES FOR CALCULATION AND TESTING OF LITTORAL DRIFT VOLUMES - References Map
Estimates of gross and net littoral drift derived from numerical modelling based upon wave hindcasting are available at several points along the shoreline due to previous studies in support of four Coastal Defence Strategy Plans (Halcrow Maritime, 2001a and 2001b; Mouchel Consulting Ltd, 2001a; Posford Duvivier, 2001). and the SMP (Gifford Associated Consultants, 1997). The overall patterns and volumes of drift have been established with medium reliability with variable correspondence between modelled and observed processes (moderate to good agreement between R. Adur and Brighton and on Seaford Beach). Uncertainties encountered in applying these models included: (i) the problem of selecting a representative sediment gain size on the mixed sand and gravel beaches (sediment mobility is highly sensitive to grain size), (ii) the need to estimate (or ignore) the extent to which groynes on the upper beaches intercepted any potential drift, (iii) uncertainty relating to bypassing of the Shoreham and Newhaven breakwaters, (iv) limitations upon transport imposed by poor sediment availability, (v) uncertainties in the manner in which shore platforms could affect transport processes and (vi) high uncertainty in estimates of exchanges of gravel between beaches and the nearshore.
Some opportunities are available for calculation and testing of littoral drift volumes at Brighton (Kemp Town) Beach, West Newhaven (to W of breakwater) and Seaford Beach where beaches are open to inputs from the west, but drift is intercepted by substantial artificial structures or headlands. At such sites it may be possible to infer minimum rates of net drift based on analyses of beach volume change determined from Environment ABMS data. Such estimates could then be compared with corresponding data derived from transport modelling and beach plan shape modelling studies. Seaford may be the most problematic site for beach management affects it most, and there are complexities of nearshore bathymetry and sediment budget introduced by the R. Ouse entrance and breakwaters. A difficulty at all sites except Brighton is that the terminal structures are thought to be partially bypassed by sediment so that drift estimations based upon observed beach volume changes could significantly underestimate the amount of material actually in transit. An analysis is required of drift within the Cuckmere Haven embayment, but there is presently insufficient information assembled for this to be feasible. Further studies as outlined below would be required.
A potentially useful approach with which to investigate some of the uncertainties might be to undertake detailed sediment budget analysis of (i) Seaford Beach (Newhaven west breakwater to Seaford Head) and (ii) Cuckmere Haven (Seaford Head to Seven Sisters). In each case, a gravel barrier type beach occupies a partially enclosed embayment. It may be that by testing alternative input and output scenarios the recorded patterns of historical beach behaviour can be accounted for. Provision of realistic estimates of sediment inputs and losses would assist interpretations of drift using volume changes and aid the setting up of numerical models of drift. It could reduce reliance upon non-measured assumptions of offshore loss/gain previously employed as a means of explaining the lack of fit between modelled and observed changes in beach morphology. With such studies in mind, it is important that beach volume changes continue to be monitored with the Environment Agency ABMS and good records should be maintained of all beach management activities undertaken.
9. RESEARCH AND MONITORING REQUIREMENTS - References Map
There has been a significant increase in both the quality and quantity of knowledge and understanding of the coastal sediment transport process system on this frontage over the most recent 6 years. The SMP (Gifford Associated Partners, 1997) and four Coastal Defence Strategy Plans (Halcrow Maritime, 2001a and 2001b; Mouchel Consulting Ltd, 2001a; Posford Duvivier, 2001) have reviewed, synthesised and contributed to this much of this information. Furthermore, many of their recommendations are in the process of implementation by the Strategic Regional Coastal Monitoring Programme, a consortium of coastal groups working together to improve the breadth, quality and consistency of coastal monitoring in South and South East England (Bradbury, 2001). A Channel Coastal Observatory has been established at the Southampton Oceanography Centre to serve as the regional co-ordination and data management centre. Its website at www.channelcoast.org provides details of project progress (via monthly newsletters), descriptions of the monitoring being undertaken and the arrangements made for archiving and dissemination of data. Monitoring includes directional wave recording, provision of quality survey ground control and baseline beach profiles, high resolution aerial photography and production of orthophotos, review and continuation of Environment Agency ABMS to incorporate new ground control, LIDAR imagery and nearshore hydrographic survey. Data is archived within the Halcrow SANDS database system and the aim is to make data freely available via the website.
On this basis, the recommendations for future research and monitoring here attempt to emphasise issues specific to the reviews undertaken for this Sediment Transport Study and do not attempt to cover the full range of coastal monitoring and further research that might be required to inform management as follows:
2. Studies of beach planform and volumes, especially volume changes, provide valuable insights into the rates of operation of littoral transport and the effectiveness of beach management. These have been facilitated by routine Environment Agency ABMS aerial photography since 1973, with subsequent photogrammetric measurement of profiles. There are have been some uncertainties in the past relating to the reliability of parts of the profile data so that it is important both to validate the historical data and to introduce robust methods for future profile data collection. It is understood that the Environment Agency initiated such work in 2002 and intend to incorporate the new ground control provided by the Strategic Regional Coastal Monitoring Programme into their profile measurement procedures.
3. Supplementary to the ABMS it would be advantageous to undertake post-storm profile surveys at critical locations e.g. Seaford Beach, where there is a tendency for major cross-shore exchanges of beach material. ABMS profiles only extend to low water, and do not therefore reveal changes in seabed topography occurring over the full active profile "envelope." Periodic bathymetric surveys are therefore needed for sites such as Seaford and west of Brighton Marina, ideally seawards to water depths where there is no demonstrated sediment mobility. The approach of the authorities in Poole and Christchurch Bays provides a model. It could also be integrated with routine bathymetric surveys undertaken by the Shoreham and Newhaven port authorities.
4. Once the quality and consistency of the ABMS profile data sets have been assured it will be important to consider how the profiles should best be analysed. It will be important to identify indicators of beach health such as sediment volume, crest height and crest position. Volume is especially important, but can be difficult to monitor reliably using widely spaced profiles on groyned coasts. Furthermore, an error analysis should also be undertaken so as to identify the minimum volumetric change that can be resolved with the techniques. Past trends in these indicator parameters (decadal, annual and seasonal) need to be established and a system of routine analysis instituted that would provide early warning of "unusual" trends. It may be that local engineers could identify critical thresholds, or minimum values of these parameters that could be applied to trigger specific warnings. To effectively interpret the trends recorded, it will also be vitally important to maintain good records of all beach management activities undertaken.
5. To understand beach profile changes it is important to have knowledge of the beach sedimentology (gain size and sorting). Sediment size and sorting can alter significantly along this frontage due to beach management, especially the practices of recharge and recycling. Ideally, a one-off field-sampling programme is required to provide baseline quantitative information along this shoreline together with a provision for a more limited periodic re-sampling to determine longer-term variability. Such data would also be of great value for future modelling of sediment transport, for uncertainty relating to grain size is often a key constraint in undertaking modelling.
6. It is apparent that littoral transport is subject to short-term reverals of dominant (net) directions of movement, particularly during the winter months. Routine inspection of defence structures might include observations, and simple quantitative measures e.g. recording of beach levels against updrift and downdrift sides of intercepting structures, when this condition prevails. Ideally, this would be linked to a record of preceding wave height, period and direction. Such information would assist the interpretation of ABMS profile data, possibly explaining some otherwise spurious records.
7. Using data now available on beach volumes, cliff and platform erosion yield; clast abrasion and littoral drift rates, it would be feasible to produce initial sediment budget estimates for discrete sectors of shoreline. Major uncertainties requiring specific research could then be identified and prioritised. For example, major difficulties exist in deriving accurate budget analyses exist for harbour/estuary entrances, where tracer techniques might be deployed to gain more confident estimations of sediment by-passing of artificial structures, such as breakwaters, terminal groynes, piers, outfalls and marinas. This type of investigation would need to be repeated for different types of incident waves and (arguably) for various stages in the tidal cycle to give representative data. Results could be tested against known changes in bathymetry and possibly against pathways identified from analyses of grain size trends. Recently published research on rates of flint clast abrasion also needs verification, as suggested rates may be overestimations that reflect conditions at only a few sample sites where beaches may be unusually thin and susceptible to marine working of a large proportion of their sediment, thus exaggerating apparent rates of wear. This work has significant long to medium-term management implications, and deserves critical evaluation.
8. Wave-driven offshore to onshore gravel transport is poorly understood. It is not known, for example, if estimated quantities include an element of kelp rafting of coarse clastic debris. Although difficult to justify on economic grounds, every opportunity should be taken to make visual observations of beaches for evidence of pebbles cast ashore with kelp holdfasts attached following storms. Gravel "creep" has been inferred from a limited number of tracer studies carried out up to 40 years ago in water depths up to 18 m. Mobility was studied further by a series of modelling studies that produced valuable results for the wider offshore area between St Catherine's Point and Brighton HR Wallingford, 1993). It is, perhaps, timely to objectively re-examine the relative importance of the potential pathway of beach supply closer inshore from the -15m bathymetric contour right in to MLW. Perhaps this might be integrated into research on offshore seabed sedimentology and morphology and also the bathymetric measurement of key ABMS profiles.
9. Considerable improvement in quantitative estimation of cliff erosion yield has occurred in recent years, although only a few unstable sites are routinely monitored. Further basic research on rates and scales of rock platform downwearing would be appropriate, including some estimation of losses due to solution weathering.
10. Most of the mentioned above studies would greatly benefit from additional measured data on inshore wave characteristics, so that a more precise local wave climates might be determined. The existing network of wave recorders operated/managed by the Channel Coastal Observatory presently has a major gap within this sector with the nearest recorders being at Rustington in the west and Pevensey in the east. Consideration could be given towards re-establishing wave recording at sites occupied in the past such as Seaford Bay and possibly Shoreham. Seaford is especially important in view of its exposure to the SW and the records of sudden beach losses.
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MMIV © SCOPAC Sediment Transport Study - Shoreham to Beachy Head