1. Introduction
The coastal zone of the Selsey peninsula is an exceptionally complex environment, not least because the well-defined headland of Selsey Bill separates shorelines with different orientations and process regimes (Bray, Carter and Hooke, 1995) (Photo 1). Because of spatial variation in wave climate, and the effects of both plan shape and submerged relief on the local tidal current system, the apex of the peninsula functions as a regionally significant boundary between adjacent sediment transport cells. The presence of offshore and nearshore banks, bars, shoals and reefs adds unusual complications to the sediment budgets of each of the several distinct littoral transport sub-systems. Exceptionally rapid erosion over at least the last five millennia has resulted in the submergence of both natural and human-modified coastal landscapes. This legacy has not been fully explored, but has generated considerable speculation over the sequence of coastal evolutionary changes as the archaeological evidence has not been systematically documented and historic records are ambiguous.
At the shoreline a partially swash aligned shingle barrier storm ridge and sandy lower foreshore extends the length of Bracklesham Bay to the Chichester Harbour inlet. The eastern side of the Selsey peninsula is fronted by a drift aligned gravel beach. The hinterland is low-lying, but elevated slightly at East Wittering and Selsey. At Medmerry, the hinterland is close to or below mean sea level and is formed of soft alluvial deposits comprising a reclaimed but now partially re-opened (realigned) estuary channel. A weak to moderate net shoreline drift transports sediments from the south-east to north- west, although actual volume of drift of coarse sediment is presently very low due to the widespread controlling effects of groynes.
It is only within the last 50 years that the majority of this coastline has been fully protected by formal defences and regulated by other shoreline management practices. Artificial control of beach volumes and sediment transport pathways has not succeeded in achieving conditions of shoreline stability at all points; indeed, there are several critical locations where natural shoreline behavioural tendencies have been either conceded and implemented or examined critically for their feasibility and relax management controls (as suggested by Posford Duvivier, 2001; HR Wallingford, 1995, 1997; Cobbold and Santema, 2001). The Medmerry managed realignment scheme, completed in 2013, is a major example of the recognition of the unsustainability of defending the southern and central sectors of the west facing shoreline of the Manhood peninsula.
A major new source of coastal data is from the Defra-funded National Network of Regional Coastal Monitoring Programmes. The Programmes consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping. Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.channelcoast.org.
The Southeast Regional Coastal Monitoring Programme commenced in 2002. The Lead Authority is New Forest District Council, with data collection, analysis and reporting led by specialist teams at the Channel Coastal Observatory (CCO), Canterbury City Council and Adur and Worthing Councils. (See CCO Annual Survey Reports for further details).
1.1 Coastal Evolution
The Selsey coastline is developed in Eocene (principally Bracklesham Group) sandstones and clays, overlain by Quaternary drift deposits. The former provide the substrate beneath the inter-tidal foreshore and are highly erodible; prior to the construction of comprehensive "hard" defences, coastline recession rates were up to 8m per year in places. Last Interglacial (Ipswichian stage) Raised Beach deposits (Photo 2) overlie earlier Quaternary deposits (Reid, 1892; West and Sparks, 1960; West, et al., 1984 and Bates, 1998 and 2000), but their formerly extensive exposure at the shoreline is now restricted to a few localities. Late Devensian or early Holocene loamy silt ('Brickearth') overlies the Raised Beach and provides the substrate to modern soil profiles. These deposits overlie the most recent of a sequence of marine erosional platforms that extend 25km inland. They have been interpreted as the product of successive Middle Pleistocene sea-level transgressions, punctuated by regressive stages and subsequently displaced by neotectonic movements (Bates, 1998, 2000; Hodgson, 1964; Bates, Parfitt and Roberts, 1997). Further detail is contained within the separate section on the Quaternary History of the Solent.
During the last (Devensian) cold stage, sea level was at least -50 to -60mOD, and the regional shoreline some 5-7km seawards of its present position. Subsequent Holocene sea- level rise has therefore released a substantial quantity of sediment, including gravels derived from the ancestral Coastal Plain composed of Raised Beach, Coombe Rock (periglacial) and River Solent fluviatile (terrace) materials. Some of this continues to be available as scattered deposits on the seabed, but it is thought that much lies stranded offshore within submerged barrier beaches built during several stages of mid to late Holocene sea-level transgression. Wallace (1990 and 1996) has tentatively identified the foundations of several indurated barrier structures, some of which may have originally been independent barrier islands, separated by tidal passes. Others may have become "anchored" to the predecessors of modern reefs, such as the Mixon, and once extended as far eastwards as Bognor. Strongly indurated, stable cobble "pavements" and large boulders of local Eocene and Oligocene rocks located in water depths in excess of 8m offshore the west and south-west coastlines might be the foundations of barriers that were submerged during one or more stages of rapid sea-level rise. Others continued to be driven landwards, probably by storms, to eventually produce modern barrier forms, as at Medmerry and Church Norton beach and spit. Overstepping of relict ebb deltas, banks and shoals, adjusted to earlier sea-level stillstands, is also likely to have occurred. Bone (1996) and Wallace (1990) offer several speculative dates for barrier breaching and reformation. Part of the substantial sediment resource of earlier gravel barriers has been redistributed to modern stores such as the Kirk Arrow spit; the Inner Owers; and the Pagham Harbour spits and tidal delta. Wallace (1990 and 1996) has attempted to fit a chronology of stages of sea-level rise to the apparent evidence of barrier breaching, breakdown and submergence. Evidence of brackish and estuarine sediments offshore Medmerry does suggest the former existence of a back-barrier lagoon that formed during a phase of sea-level stability; however, knowledge of the precise distribution and age of these sediments is insufficient to provide a more specific timeframe for barrier evolution.
Archaeological and sedimentological evidence supports the reconstruction of a continuous tidal creek linking Pagham Harbour with Bracklesham Bay (Heron-Allen, 1911; Millward and Robinson, 1973; Hinchcliffe, 1988; Wallace, 1990 and 1996; Castleden, 1998; Bone, 1996; Thomas, 1998). This may date back at least 2,000 years, perhaps resulting from a major breach of an earlier Bracklesham Bay barrier beach at Medmerry (Wallace, 1990). The Medmerry barrier is believed to have reformed and breached several times during subsequent centuries; at times isolating the Selsey peninsula as an island. Archaeological evidence demonstrates that the coastline was some 2 to 3km seawards of where it is now at about 5,000 years Before the Present (Cavis-Brown, 1910; White, 1934; Wallace, 1967, 1968 and 1996; Aldsworth, 1987; Goodburn, 1987; Thomas, 1998). Coastal erosion over this period must have occurred at a rate at least as fast as that recorded for the nineteenth and first half of the twentieth centuries (May, 1966). Documentary evidence for the medieval period (Bone, 1996) also indicates rapid coastline recession, especially during major storms. The latter probably caused the Medmerry barrier to repeatedly breach and break down, although there is evidence that it was in place in the mid-sixteenth century. Stratigraphy from shallow boreholes into sediments infilling the former tidal creek isolating Selsey (Hinchcliffe, 1988; Wallace, 1990) clearly indicate oscillations between lagoon and brackish water conditions. A barrier spit may have connected Selsey Island with the mainland in the sixth century AD, but was permanently removed by a storm surge of exceptional magnitude in 1048.
Reclamation of some 120 hectares of saltmarsh occupying the tidal channel between Pagham Harbour and Medmerry was achieved when the Broad Rife sluice was built in 1884 (Photo 3). This was undertaken in response to back barrier flooding resulting from a large pulse of gravel drift that blocked the Medmerry exit of this stream in 1880 (Bone, 1996). Further temporary blockages occurred in 1918, 1920 and 1924 before stabilisation of its present mouth in 1930.
The approximately triangular shape of the Selsey (or Manhood) peninsula results from the protective presence of the Mixon reef some 2.5km seawards of Selsey Bill. This feature is composed of a relatively resistant Eocene calcareous Alveolina limestone cap rock overlying Bracklesham sands and clays. Wallace (1967, 1968, 1990 and 1996) has described a well-defined valley, up to 25m in depth and scoured by tidal currents, to the immediate south of the Mixon. The Outer and Malt Owers and The Streets are smaller bedrock reefs, but other offshore banks within 3km of the modern coastline appear to be sediment accumulations. They may be relict parts of a multistage barrier structure that was progressively segmented and submerged between 2,500 and 800 years before the present (Wallace, 1990; 1996). A remnant area of lagoonal and colluvial sediment that accumulated behind this structure survives inland of East Beach. Very fast erosion of this weak material occurred in the 50 years prior to the completion of coastal defences in 1960.
Wallace (1990; 1996) has speculated that the Mixon reef formed a part of the coastline in early Romano-British times. It may have "anchored" the contemporary position of the barrier beach mentioned above. A 17m deep sediment-infilled v-shaped gap between the Mixon and Malt Owers mark the course of the ancestral River Lavant. The latter is likely to have discharged via what is now Pagham Harbour prior to its diversion to Chichester and Fishbourne by Roman engineers in the second century A.D. Wallace (1990) also suggests that the proto-Lavant followed the line of the buried channel that runs roughly parallel to the modern East Beach coastline some 300-400m offshore. This feature has been largely infilled with late Holocene sediments, but continues to act as a local trap of mobile gravel during winter months. Some of this material is stabilised by weed growth during summer months, and may subsequently be transported by rafting to supply the Inner Owers and Kirk Arrow gravel accumulations. The strike-directed east to west valley south of the Mixon may also have been part of the course of an ancestral Lavant river.
Barrier breaching and shoreline recession associated with rising sea-level and storm events caused The Mixon to become an offshore bank, or shoal, probably at about 950-1050 AD (Wallace, 1990). It would have been emergent during mean low water, whilst the Inner Owers would, by this time, have been fully submerged. The Mixon therefore acquired its reef-like form and function from early medieval times onwards as sea-level rose further and both tide and wave-induced currents caused bedrock scour.
1.2 Hydrodynamics
The tidal range is 4.9m (springs) and 2.7m (neaps) at Pagham Harbour mouth and at the entrance to Chichester Harbour, with the ebb phase shorter than the flood. The early ebb stage, gives rise to rectilinear, nearshore parallel, residual currents off the east-facing coastline. This stream moves towards the banks and reefs south of the Bill, where it is confined and movement is determined by their alignment. During the peak ebb flow, movement is north/north-eastwards. Maximum surface currents offshore the apex of the peninsula are between 1.4m per second (springs) and 0.7m per second (neaps), reducing slightly at the seabed. Tidal currents adjacent to the west/south-west facing coastline flow predominantly eastwards/south-eastwards, as indicated by both float tracking and the morphology of patches of sand waves on the seabed (HR Wallingford, 1995, 1997, 2000). The protrusion of the Selsey peninsula into this net eastwards moving tidal stream creates an anticlockwise circulating gyre (or "back-eddy") to the north-east, where residual current speeds are between 0.3 to 0.4m per second at the peak of the flood stage. A smaller, clockwise moving eddy between The Streets reef and Kirk Arrow spit is set up when the ebb tidal flow is east to west (Wallace, 1990).
Within Pagham Harbour the mean Spring tidal range is 5.1m, that for Neap tides is 2.5m with a tidal prism of 3.3 million m³ during springs and 0.9 million m³ during neaps. The tidal cycle is asymmetric, with a shorter, higher velocity flood current and a longer and weaker ebb. Peak flood current velocities are higher than peak ebb flows, thus the harbour is flood dominant- confirmed by studies of sedimentation rates and patterns (see section 5.3), which indicate greatest potential for fine sediment transport on spring tides. A depth-integrated two dimensional modelling of harbour hydrodynamics was undertaken to predict flood and ebb current velocities, with bathymetry derived from Lidar imagery and surveyed profiles in addition to Environment Agency monitored water level gauge data - refer to Environment Agency (2012) for details, including data on the probability occurrence of extreme water levels and velocity vectors at several harbour locations). This study found only minor differences between modelled and measured peak tidal levels and current speeds.
The Coastal Monitoring Programme measures nearshore waves using a network of Datawell Directional Waverider buoys. The buoy deployed at Hayling Island in 10mCD water depth, confirmed the prevailing wave direction is from south-by-west, and an average 10% significant wave height exceedance of 1.26m. The buoy deployed at Bracklesham Bay in 10mCD water depth, from 2008 to 2012, confirmed the prevailing wave direction is from southwest-by-south, and an average 10% significant wave height exceedance of 1.47m (CCO, 2012).
The offshore wave climate is dominated by waves from the south and south-west with periodic episodes of less energetic waves from the south-east. However, the shoreline wave climates are complex, as the east and west facing coastlines have contrasting orientations and western parts of Bracklesham Bay are partially sheltered by the Isle of Wight. Selsey Bill and East Beach are directly exposed to waves approaching from the south and east, but they also receive highly oblique refracted and diffracted swell waves that propagate from the south-west (HR Wallingford, 1992; 1995; 1997; 1998). West and north-west of Selsey Bill, dominant wave approach is from the south-west and wave crests are frequently parallel to the nearshore contours and shoreline. Bracklesham Bay is therefore a swash aligned shoreline, whereas Selsey Bill to Pagham Harbour is a classic drift aligned shoreline.
Wave shoaling and refraction is complicated by the presence of the submerged offshore reefs, shoals, banks, scarps and troughs. The Mixon, in particular, protects the southernmost shoreline from waves from the west and south-west, but the high incident angle of their approach is least modified immediately west of Selsey Bill. Wave climate for any one location on this coastline is a result of complex relationships between offshore to inshore transformation as a function of shoreface width and water depth; seabed relief; approach angles and interaction between wave and tidally induced currents in the breaker zone. Generally, waves steepen where tidal currents flow in opposition to dominant wind wave direction of approach. Overfalls at specific tidal states add further complications.
Bracklesham Bay was one of the locations for which wave modelling exercises were undertaken as part of the DEFRA Futurecoast Project (Halcrow, 2002). An offshore wave climate was synthesised based on 1991-2000 data from the Meteorological Office Wave Model and then transformed inshore to a prediction point in Bracklesham Bay at -4.34mOD. 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 2-4% 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.
2. Sediment Inputs
Two potential sources of sediment are identified for this coastline comprising offshore to onshore transport and shore erosion. These have been supplemented in recent decades by beach replenishment at several sites.
2.1 Offshore to Onshore Transport
A high resolution, 100% coverage swath bathymetry survey was commissioned by the Southeast Regional Coastal Monitoring Programme. This extensive survey, covering 194km², extended between Lee-on-the-Solent and Selsey Bill and offshore from MLW to abut with the northeast Isle of Wight survey boundary, between Cowes and Bembridge, was completed in July 2013. The seabed to the west of Selsey Bill is generally shallow sloping and dominated with sand with coarse-grained offshore banks and shoals, such as the Medmerry Bank. Rocky outcrops and reefs are interspersed throughout such as the larger Streets Reef and a number of smaller reefs inshore. Despite the relatively extensive areas of sediment, which are of sufficient thickness to mask the underlying bedrock, there are few localised areas of bedforms; these are generally in association with steeper slopes, such as the banks of the Chichester harbour channel, and adjacent to or within the intertidal zone, on the flanks of the sand bars in Chichester harbour entrance. The seabed from Selsey Bill eastwards comprises coarse-grained sediment and gravel, and is generally a constant depth or gently sloping with small, localised shoals and mobile bars.
Wave-transported sediment supply to the beaches of this coastline derives from several discrete sources, as detailed below. Tidal currents are not considered to be an independent mechanism of sustained onshore transport, but wave and tidal stream interaction creates complex patterns of turbulence that can entrain sediment.
The Kirk Arrow Spit is a mobile gravel bank with a mean volume of 20-40,000m³ exposed at low water some 300-500m offshore Selsey Bill. The bank comprises mostly flint clasts deposited as a result of turbulence generated by interaction of waves and tidal currents off the apex of Selsey Bill (Jolliffe and Wallace, 1973). The source of this material is gravel that "carpets" the sea floor south and east of the Selsey peninsula (see 1.1 Coastal Evolution). Some of this is so well compacted as to be immobile at bed stresses experienced here. It is believed that shingle is periodically transported onshore from the spit to feed adjacent beaches when waves approach from the south or south-west (Lewis and Duvivier, 1977; Wallace, 1990; Posford Duvivier, 2001). Evidence for this is mainly circumstantial and comprises reported observations and air photos. These sources suggest that beach levels opposite the spit are maintained by sudden onshore-directed influxes of shingle induced by high energy (storm) waves. A good example occurred between January and March 1999, when the beach to the west of the drift divide at Selsey benefited from a strong pulse of onshore gravel feed.
Aerial photos indicate gravel influxes from the bank to the shore during the periods 1959-60, 1971-72, 1986-92 and 1997-99 and corresponding extensions of the bank shoreward such that they temporarily attach to the shore. This suggests that gravel is transported onshore from the inshore end of the bank by shoaling waves approaching from the south. Lewis and Duvivier (1977) concluded that this feed occurs in pulses, separated by intervening periods of erosion, but averaged 5,000m³ per year. This estimate is based on the quantity of feed necessary to maintain beach levels over longer-term periods, in spite of output by littoral drift and beach drawdown under high-energy wave conditions. Gifford Associated Consultants (1997) propose the higher figure of approximately 10,000m³ per year, but this also includes an estimate of the quantity gravel that is moved eastwards of the Selsey peninsula. HR Wallingford (1995) and Posford Duvivier (2001), using both recent and historical data, estimate that 80-85% of onshore deposition is subsequently moved rapidly eastwards, whilst the remaining 15-20% either remains in place or slowly drifts westwards.
It can be concluded that strong circumstantial evidence exists indicating significant but intermittent onshore transport of gravel from the Kirk Arrow Spit. The quantitative estimates of this feed are of medium reliability because it apparently occurs as high magnitude, low frequency pulses that are not easily measured. Additional information is required on the frequency, volume and duration of typical pulses, as well as on the pattern of changes in the shape and volume of the spit itself. Better knowledge is required of how it came to expand rapidly in the late 1980s to become habitually exposed during low water spring tides, thus creating a wide inter-tidal foreshore. However, during the previous three decades, it was detached and only rarely emergent. Ultimately, over a long timescale that cannot be determined at present, the Kirk Arrow Spit represents a finite source of supply, as there is a probability that its sources of replenishment will decline over time and eventually become exhausted.
Analysis of Coastal Monitoring Programme bathymetry data and sediment sampling provide no conclusive evidence of onshore transport between the limestone outcrops of Streets Reef or Malt Owers and the Selsey-Medmerry shore. Therefore, the 2004 arrows indicating onshore transport and speculative weed rafted gravel transport have been removed.
Analysis of Coastal Monitoring Programme bathymetry and aerial photography data indicates that the Inner Owers are a series of mobile nearshore gravel banks, situated between East Beach, Selsey and Pagham Harbour inlet, which periodically migrate onshore. Due to the relatively low rates of littoral drift (LT2) that have been monitored and calculated, the onshore transport of gravel from Inner Owers and other shoals offshore of Church Norton spit is regarded as the dominate source and feed of gravel material to the spit, with an estimated onshore drift rate of more than 20,000m³ per year. This rate is an increase from the estimated rate of 10-20,000m³ in 2004. The shape and form of these mobile shoal features are determined by wave diffraction and refraction, are characterised by a gentle offshore slope and a steeper inshore slope. Based on estimations of the reasonably constant shape and volume of these gravel banks, apparent during their migration phases, a total input of 10,000m³ was calculated for the period 1970-75; a longer term average input of 3,000-5,000m³ per year is quoted by Lewis and Duvivier (1977). This process has been observed directly via diver surveys, and as its contribution to beach levels is evident, this information is regarded as of medium to high reliability. Since 2003 there has been progressive erosion of the Inner Owers, estimated at 60,000m³ up to mid-2011 (Environment Agency, 2012).
Analysis of Coastal Monitoring Programme bathymetry and aerial photography data indicates that the ebb delta south of Chichester Harbour entrance comprises a lobe of coarse material extending seawards approximately 2km, surrounded by a sandy seabed. The seabed to the east of Chichester harbour is dominated by sand, and a mixture of coarse and fine sediment between the West Pole Sands and Eastoke Point.
At Chichester Harbour Entrance, the ebb tidal current is of shorter duration, but significantly greater velocity, than the flood current. The latter transports sand into the harbour, where a proportion accumulates as the flood tide delta of Pilsey Sands. Net transport of coarser bedload sediment moving into the channel is therefore offshore, thereby creating an ebb tidal delta comprising a major sediment accumulation with its terminal lobe extending 4km offshore (Harlow, 1980; Wallace, 1988; ABP Research and Consultancy, 2000; GeoSea Consulting, 2000; Fitzgerald, 2012). Between 1 km and 2 km offshore, the ebb tidal current diminishes and is increasingly opposed by wave action, so that shingle cannot be transported beyond this area. Sand, however, is transported further before deposition on the outer bank or shoal, some 3.0-3.5 km offshore (Webber, 1979; GeoSea Consulting, 2000). The sediment volume of the ebb tidal delta was estimated as being approximately 25 million cubic metres by Webber (1979). Water depths over the delta are relatively shallow, particularly over the outer and inner bars (Webber, 1979; Harlow, 1980; Wallace, 1988; Geosea Consulting, 2000). Transport of sediments occurs on the delta by combined action of waves and tides with clear patterns of sorting. Sedimentological analysis indicate that there is net transport of gravel from the inner tidal delta westward, resulting in accumulation in banks seaward of and closely adjacent to West Pole Sands, Hayling Island (Harlow, 1980; GeoSea Consulting, 2000). By contrast, sand is more widely distributed both eastward and westward forming the outer bank deposits and an episodic north-eastwards pathway of sand (but apparently not gravel) transport that operates towards Cakenham and West Wittering beaches and East Head has been identified(Webber, 1979; Harlow, 1980; GeoSea Consulting, 2000; Fitzgerald, 2012).
F4 Sand Feed from the Chichester Ebb Tidal Delta to West Wittering and Cakesham Beaches and East Head (see introduction to sediment inputs)
Analysis of Coastal Monitoring Programme bathymetry and aerial photography data indicates that the ebb delta south of Chichester Harbour entrance comprises a lobe of coarse material extending seawards approximately 2km, surrounded by a sandy seabed. Numerous dynamic and mobile bars and shoals and bedforms on East Pole Sands may indicate current driven onshore transport between the eastern flank of the delta and the East Head foreshore. The seabed to the east of Chichester harbour is dominated by sand, a mixture of coarse and fine sediment between the West Pole Sands and Eastoke Point.
On indirect evidence it was suggested (Webber, 1979); Harlow (1980) that sand deposited on the furthermost lobe of the Chichester Harbour ebb tidal delta and East Pole Sands had the potential to be transported onshore by wave action to Cakenham and West Wittering beaches and thereafter downdrift to East Head. Evidence for this was based upon (i) particle size variations within and between the component banks and shoals of the tidal delta, which may involve a contribution from tidal currents during spring cycles (GeoSea Consulting, 2000); (ii) retrospective hydrographic chart evidence of up to 3m of erosion on the western margins of East Pole Sands, and 2-3m of accretion further east (ABP Research and Consultancy, 2000). This had occurred periodically since 1923, and appears to have involved a progressive change from a previously mobile gravel and sand surface to one, which at that time was stable. The latter feature was first reported by Webber (1979) and may be due to bevelling of the underlying clay substrate and its subsequent armouring by gravels. The research of Fitzgerald (2012) and Bray (2010) has provided fresh insight into the morphodynamics and sediment circulation pattern within the tidal delta. Their work is primarily based on the detailed analysis of repetitively surveyed closely spaced nearshore, foreshore and mid/upper beach profiles between Cakeham and East Head between 2003 and 2011. Fitzgerald (2012) draws analogies with comparable tidal estuarine re-entrants characterised by complex sediment stores and transfers, observing that it is effectively a closed system with a well-defined counter-clockwise pattern of sand transport. The north to south directed ebb tidal stream on the western margin of the delta feeds the ebb shoal of Middle Pole Sands, which functions as a store that supplies sediment to the nearshore by-passing bank of East Pole Sand. The latter, during phases of excess storage, is the source for the construction of shore-parallel attachment or swash bars that weld to the foreshores of Cakeham and West Wittering beaches. Between 2005 and 2010, individual swash bars emerged on average at intervals of 1.4 years (between 1 and 3 years according to specific profiles) and took 2 to 4 years to arrive at the upper beach. In the four years 2005 to 2009, total foreshore accretion was in the order of 240,000m³, although over a slightly longer period gross nearshore deposition may have been as high as 600,000m³. Net foreshore sediment gain was nearer to 150,000m³, as there were losses due to northwards littoral drift (feeding the post-2004 recovery of the foreshore and beach of the length of East Head spit) and to scour by the flood tidal channel. Mean bar amplitude was 2.4m, with a wavelength of 400m. Nearshore, i.e. by-passing, bars were substantially larger than those that subsequently attached to the foreshore, commencing their shoreward migration at-4mOD at a rate of between 100 to 200m per year. Swash bars moved across the lower foreshore at approximately 100m per year, reducing to 20m per year on the upper foreshore and low/mid beach. This progressively slower migration rate is considered to be due to their shortening periods of submergence as they progress up-profile.
The recent phase of foreshore and beach accretion, immediately following some ten years of erosion and beach drawdown implies a possible cyclic periodicity of sediment excess and deficit within the tidal delta complex. Fitzgerald (2012) suggests several factors are responsible, both acting singly and in combination. The most important is likely to be fluctuations in sediment availability, with the major stores releasing sediment when their capacities are exceeded and thus creating the primary condition needed for swash bar welding. Variation in incident wave climate may also be involved, with more energetic constructive (storm) waves more capable of propelling bars landwards. A further very plausible influence is the tidal nodal cycle of 18.6 years, during which the position of maximum high water moves through a vertical range of 24cm (13mm per year). Swash bar migration is more likely to occur when the tidal range is at its minimum, as when it is higher the increased tidal prism (11% greater at the peak of the nodal cycle) the main ebb “jet” will transport sediment further seawards than the by-passing and attachment banks. Furthermore, at this stage potential wave-induced shoreward sand movement will be diminished. During the accretion phase of 2005-10, these latter two factors were both favourable. The question arises as to whether there have been previous alternating episodes of sustained erosion and deposition that would help to confirm the hypothesis outlined above. The historical record of bathymetric changes at the harbour entrance is both incomplete and lacking in adequate detail, but there is an indication that there were previous phases of accretion in 1898-1911, 1933-1960 and 1967-1978. The latter followed a breach near the neck of East Head in 1963, thus the rapid recovery of the beach fronting the spit following the overwash event of 2004 is an apparently similar response. However, it has so far proved difficult to quantify the periodicity of accretion/swash bar welding events, an achievement that would provide insight for the strategic management of this highly dynamic shoreline.
2.2 Coastal Erosion
Selsey Bill, East Beach and the coastline of Bracklesham Bay have a history of rapid erosion of low cliffs and the beach with several km being lost in historical times and a maximum rate of 7.6m per year, being recorded at East Beach, Selsey for the period 1932-51 (Duvivier, 1961; Millward and Robinson, 1973). The peninsula includes some superficial raised beach deposits to the west and east of the Bill around Selsey. It is likely that these were formerly much more extensive, but have been reduced greatly in extent as the headland has diminished. Erosion at such rapid rates necessitated installation of extensive and robust coast protection and defence structures, undertaken in the period 1945-61, subsequently upgraded and partly reconstructed, and added to recently (2012) along the frontage of the Bunn Leisure caravan complex. These include groynes, revetments and seawalls which have either halted cliff erosion or reduced beach losses. The only unprotected areas where natural recession of the shoreline continues are:
- East Head;
- A 300m long cliff frontage from Hillfield Road to Medmerry, Selsey;
- Between the proximal end of Church Norton spit and Pagham Harbour inlet.
These sites are detailed below. In addition, erosion takes place across the approximately 2000m wide shoreface between Selsey Bill and East Head. The material released by shoreface erosion is believed to constitute fine sediments that are likely to be removed seaward in suspension.
The morphological development of the East Head spit has been fully documented by Searle (1975), May (1975), Lewis and Duvivier (1977), Harlow (1980), ABP Research and Consultancy (2000), Baily et al., (2002) and Carter (2006) (see section 5). These studies have reported a clockwise rotation of the spit accomplished by very rapid recession of its seaward face, at 6.8m per year during the period 1875-1896 and 2.3m per year during the period 1896-1909. This rate had slowed by 1926 and by 1963 the spit was in approximately its present position. Although East Head as a whole has retreated very little since 1963 it cannot be regarded as stable for it was breached along its neck immediately north of "The Hinge" by a storm in 1963, was overtopped in 1987 and has experienced rapid thinning since the early 1990s (Photo 5). Its apparent stability has only been achieved by extensive use of artificial structures to stimulate dune growth (Searle, 1975; Baily et al. 2002) and more recent structural and sediment recycling at the vulnerable proximal end. Most authorities agree that a combination of a spring tide and a severe storm could again breach the neck of the spit resulting in further recession and possibly its permanent breaching and ultimate destruction. Several alternative breach scenarios have been modelled and evaluated (HR Wallingford, 1995, 2000; ABP Research and Consultancy, 2000). The cause of the phase of erosion at The Hinge between the mid-1990s and 2005, (refer to section 5.3) involved reductions in natural sand supply from updrift longshore and nearshore sources. An additional factor, then and in the future, may be a continuing adjustment of cross-section of the Chichester inlet mouth to the changing tidal prism of the harbour (ABP Research and Consultancy, 2000). It has involved lowering since 1923 of the Winner sand and gravel bank by up to three metres allowing increased wave exposure and reducing the intertidal foreshore width in front of East Head.
The undefended 300m long section of low cliffs between Hillfield Road and the eastern boundary of Bunn Leisure, which cut into raised beach pebble and sand deposits, had eroded relatively steadily at 1.09 to 1.25m per year between 1875 and 1972 (Harlow, 1980; Posford Duvivier, 1997). This rate was calculated from examination of successive OS 1:2500 map editions. Earlier maps suggest that erosion was more rapid in the period 1840-1880 (Harlow, 1980). Harlow (1979, 1980) attempted a calculation of sediment yield based on an appreciation of (i) the lithology of eroding raised beach and drift sediments, (ii) calculation of cliff height variation between successive periods over nearly one hundred years and (iii) an allowance for losses via suspended transport. He suggested that up to 1,480m³ per year of gravel and 3,820m³ per year of fine gravel and sand could be released and potentially contribute to the upper beach and foreshore. Posford Duvivier (1997) calculated a total supply of 1,000m³ per year divided equally between gravel, sand and clay. Use of historical map sources coupled with Ordnance Survey maps enabled Lewis and Duvivier (1950) to calculate mean retreat at 1.34-1.8m per year over the period 1778-1953, with a sediment yield of approximately 10,000m³ per year. This higher rate probably reflects the more rapid erosion in the 18th and 19th centuries along a rather longer essentially undefended coastline so that the figures calculated by Harlow (1980) and Posford Duvivier (1997) are more representative of the present-day inputs. Unpublished local authority records (Lewis and Duvivier, 1950) report that between 1930 and 1952 annual rates of erosion between Medmerry and East Beach were as high as 8m. This was the highest rate recorded for any location in southern England during the twentieth century.
However, analysis of Coastal Monitoring Programme 2008 and 2013 aerial photography, lidar and baseline topographic data indicates an average recession rate of 0.13m per year. This released very low volumes of sand and gravel, and far less than 1,000m³ per year; this is a reduction from the estimated 2004 rate of 3-10,000m³ per year. Erosion at West Sands (Bunn Leisure frontage), to the immediate south of the Medmerry barrier, has now ceased (2012) in consequence of the insertion of massive rock armour fronted by a substantial renourished beach - refer to section N1 for further details.
Prior to the construction of comprehensive 'hard' sea defences between 1962 and 1969 (Photo 1), much of the tip of the Selsey peninsula provided inputs of easily eroded sediment from wave-induced cliff and shoreface erosion. This has been the case for over 1300 years, thus accounting for over 2km of coastline retreat since the second or third centuries AD (Ballard, 1910; Heron-Allen, 1911; White, 1934; Aldsworth, 1987; Wallace, 1990 and 1996; Castleden, 1996; Bone, 1996; Thomas, 1998;). Shoreface erosion has accelerated over this period due to ongoing sea level rise and the lowering of protective off-shore rock outcrops. There is a rich, only partially explored, offshore archaeological legacy of submerged Romano-British, Saxon and early medieval landscape features, partially recorded in documentary and archival records (Heron-Allen, 1911; Wallace, 1990). Sediment yield derives not only from (former) rapid retreat of low cliffs, but abrasional scour of the complementary, expanding, shoreface platform. Hydraulics Research (1995) estimated that rapid erosion between 1850 and 1950 could have released as much as 2 million m³ of gravel from raised beach deposits. Much of this resource has now become exhausted as the headland has diminished. Lewis and Duvivier (1977) calculate an annual volume of 7,500m³ of shingle for 1909-1962, feeding East Beach, Selsey. The quantity of sand was, and may continue to be, substantially greater, but is too fine to be retained on local beaches.
Posford Duvivier (2001) calculated that approximately 150m of recession of mean low water occurred between 1900 and 1950, with substantial beach drawdown and erosional loss along this south-east orientated shoreline. Losses could not be compensated by updrift littoral or nearshore transport, although some fresh supply of shingle may have derived from erosion of Raised Beach deposits periodically exposed in the back and mid shore areas. A series of successively landward relocations of the Lifeboat Station, 1909-1960, are described by Wallace (1990).
Several specific sites have recorded beach erosion since the early 1980s, considered (Gifford Associated Consultants, 1997; Posford Duvivier, 2001) to be between 4,000 and 8,000m³ per year. These constitute changes in the beach sediment store and are not specifically regarded as inputs. Routine nourishment of the beach fronting Church Norton spit took place between the early 1980s and 2005 to offset this loss and thus maintain the stability of Pagham Harbour inlet. Since 2005, Church Norton spit has expanded in volume (refer to entries in section 5).
The following table presents saltmarsh extent loss for Pagham Harbour, mapped from historical aerial photography interpretation with ground control (Baily and Pearson, 2007; Cope et al., 2008). The findings, which employed both panchromatic and false colour infrared imagery at a scale of 1:10,000 obtained between 1947 and 2001 are presented in The Solent Dynamic Coast Project (Cope et al., 2008). This research study adds to the datasets mapped by Baily et al., (2000) at the University of Portsmouth as part of the Solent Coastal Habitat Management Plan (Bray and Cottle, 2003).
Table 1 presents the earliest (Year 1) to most recent (Year 2) saltmarsh extent mapping for Pagham Harbour. All of the loss occurred between 1947 and approximately 1965, with marginal gain between then and 2001. The % saltmarsh loss per year can be used to compare the differences in loss compared with that in other units such as the West Solent and Portsmouth, Langstone and Chichester Harbours. Variability in % saltmarsh loss per year is attributed to historical land reclamation, sediment availability (promoting accretion) as a result of the inundation of the harbour in 1910, exposure to wave attack, elevation and consolidation of the marsh, the presence of salt pan formation, sea level rise leading to coastal squeeze and Spartina dieback (Cundy et al., 2002; Baily and Pearson, 2007; Cope et al., 2008). Further discussion of mudflat erosion and accretion, and of the response of saltmarsh, is given later in this text.
2.3 Beach Nourishment and Recycling
Medmerry
The Environment Agency and its predecessors have conducted a long-term nourishment and re-profiling scheme at the critical Medmerry barrier beach site (Environment Agency, 1998a). This structure has a long history of intermittent landward migration. Its present form dates to approximately 1600, when it was reported to have blocked a former tidal inlet channel. Subsequently, it breached and reformed on several occasions (Bone, 1996). However, in the period 1985 to 2008 it exhibited increasing instability, and has experienced regular cutbacks and overwashing. This necessitated increasingly urgent nourishment and profile reconstruction in order to maintain the defence line. This sector of beach is in a condition of chronic disequilibrium, unable to adjust to natural sediment losses and foreshore lowering. HR Wallingford (1995) demonstrate that it is located at a focal point for wave erosion due to refraction induced by complex offshore relief. The initial nourishment was in 1976 and comprised a pilot scheme involving deposition of 14,500m³ shingle and extension of groynes to LWM. The main phase was conducted between 1976 and 1980, with the addition of 225,000m³ of gravel obtained from inland gravel pits and delivered by truck to build the berm. This was preferred to dredged marine gravels that could be deposited on the lower foreshore because Hydraulics Research (1974) were unable to confirm that this material would move onshore. The nourishment material comprised nodular flint gravels significantly coarser and more angular than the indigenous beach material. The scheme also involved insertion of 38 groynes along a 3.8 km frontage and their extension to MLWST. The artificial beach was re-profiled to a slope of 1:10, with gravel that accumulated at the most westerly downdrift groyne subsequently being recycled updrift. Further replenishment and crest elevation was completed between 1989 and 1996, involving some 90,000m³ to compensate for losses of 102,000m³ over the period 1974-1992 (HR Wallingford, 1995; Environment Agency, 1998a). Several major storm surges during the winters of 1998- 9, 2000-1, 2001-2 caused overwashing, crest lowering, beach drawdown, with a 300m breach in 1999. This necessitated the emergency dumping of over 500,000m³ of gravel (again taken from inland sources), together with profile reconstruction. By the mid-1990s the groyne field had deteriorated and was virtually redundant as a mechanism for beach sediment conservation.
These events led to the proposal, in successive sub-regional strategic defence management plans, to abandon the then current line of defence and construct the largest managed realignment open coast scheme yet undertaken in the UK. The flood embankments and breach of the Medmerry gravel barrier was implemented in September 2013. As later than the scope of this update, the influence and transport rates have not been included in this review.
N1 East Head
Following the Pagham to East Head Coastal Defence Strategy (2009), developed by Chichester and Arun District Councils and the Environment Agency, the preferred management policy for East Head is adaptive management. This reflects that the existing sea defences that had been in place for almost 50 years and elements such as breastworks and gabions were reaching the end of their effective and design life. The alignment of the breastworks at the Hinge represented a discontinuity in the coastal frontage, leading to instability and erosion of the lower foreshore and adjoin frontages. In 2005, 12,000 tonnes of shingle and sand was recycled from the northern end of the spit to reconstruct an area of the neck, which had been overwashed. In 2009, 9,000 tonnes of sand and shingle was recycled from the northern end of the spit and placed immediately behind the Hinge to reduce risk of a tidal breach. Damaged gabions were removed between groynes C23 and C24 allowing a more natural beach profile to develop. Groyne C22 was modified to allow onward movement of shingle and raised groynes C24 to retain shingle (East Head Coastal Issues Advisory Group, 2009).
N2 Bunn Leisure (Medmerry)
Emplacement of rock armour at Bunn Leisure, coupled with a large beach recharge occurred in preparation for the Medmerry Managed Realignment scheme in July 2012, and was completed in March 2013 totalling 360,000m³ of sand and gravel (shingle), with a top up in May 2013 of 11,000m³. This regraded beach has been constructed between two offshore rock breakwaters 610m apart, designed to protect from all but extreme wave conditions. It varies from 25 and 100m in width and has a profiled crest height of 25m above mean sea-level. Further details of all aspects of this privately-funded protection scheme can be found at: www.beautifulbeach.bunn-leisure.co.uk. This includes a time lapse video of the project from initial vulnerable eroding beach condition to its completed state.
N3 Church Norton Spit
The beach fronting the southern (Church Norton) spit, protecting the entrance to Pagham Harbour, has been replenished by routine artificial cycling of gravel since 1990, taken from the adjacent nearshore banks of the Pagham tidal delta, south-west of the inlet from the Inner Owers and surplus sediment that accumulated on Pagham spit. Between the early 1990s and 2004 this averaged 15,000m³ per year, but reliable figures for the previous decade are not available. Profile reconstruction was also carried out as part of this practice. In November 2009, sediment was recycled from the distal end of the spit, and placed at Pagham Beach (at the east end of West Front road and the west end of East Front road), totalling 10,000m³. In March 2010, a further 20,000m³ was extracted from licenced offshore dredged areas and placed in the same location at Pagham Beach as in 2009. Areas of Pagham Beach undergoing erosion due to the extension of the Spit in 2012/13 were recharged with locally recycled materials from west of ‘Groyne 4’ totalling 5,000m³.
3. Littoral Transport and Beach Drift
The longshore drift system involves both gravel and sand, but a significant quantity of fine sand is probably removed in suspension directly offshore (Posford Duvivier, 1999). Tidal gyres either side of Selsey Bill created by the intrusion of the headland into the pattern of rectilinear eastwards moving residual currents result in locally complex transport of fines by tidal streams towards (i) East Beach and (ii) Medmerry Bank (Paphitis, et al., 2000; HR Wallingford, 1995; Gifford Associated Consultants, 1997). The littoral transport system is dominated by the influence of shoaling and breaking waves. Although there are well defined net transport pathways, short-term reversals occur, especially along the swash-aligned shoreline between Selsey Bill and East Head (HR Wallingford, 1995). Monitoring of beach levels has revealed significant fluctuations in annual drift rates since the early 1970s (Posford Duvivier, 2001; HR Wallingford, 1995, 1998; Bray, 2007; 2010).
Analysis of Coastal Monitoring Programme baseline topography and aerial photography data demonstrates net drift of gravel is north-westward from West Street, Selsey to East Head, although the rates are variable Between Hillfield Road and Bunn Leisure the rate has been calculated as 1-3,000m³ per year, increasing to 10-20,000m³ to Medmerry. The rate of more than 20,000m³ per year between Medmerry and East Wittering is higher than the estimated 2004 rate (10-20,000m³ per year). Further westward along the Cakeham frontage the rate decreases to 3-10,000m³ for both gravel transport on the upper foreshore and sand on the lower foreshore. The rate further decreases towards West Wittering with 1-3,000m³ per year transported north-westwards (as compared to the estimated 2004 rate of 10-20,000m³ per year). These reduced rates reflect the increased deposition and onshore storage of sand at Cakeham and East Head, removing material from alongshore drift.
Hydraulics Research (1974) showed north-westward drift from shingle tracer experiments undertaken over a 6 month period, and Harlow (1980) and Lewis and Duvivier (1977) estimated drift on the basis of beach volume and shoreline changes since 1868. However, detailed observations and transport modelling studies based on hindcast wave climates have revealed short-term and short distance reversal of the net drift direction as a result of varying incident wave approach (Posford Duvivier, 1992, 2001). Sensitivity to wave approach direction and the capacity for drift reversals are high because Bracklesham Bay is a swash-aligned shoreline (Halcrow, 2002).
Quantitative analysis has been attempted by assuming littoral drift to be the minimum sediment volume required to explain observed beach volume trends. Using this technique Harlow (1980) calculated a drift rate (all sediment grades) of 35-40,000m³ per year for the period 1965-1973; 40-50,0000m³ per year for 1933-65, but only 1,000-8,000m³ per year for the period 1973-77. Between 1846 and 1896 it is estimated to have been of the order of 70,000m³ per year. A present day mean actual drift volume of between 2,800 and 7,000m³ per year is suggested for this unit as a whole (Posford Duvivier, 2001; ABP Research and Consultancy, 2000). The potential drift rate at West Beach, Selsey is 15,000-16,000m³ per year, declining to 2-6,000m³ per year at Bracklesham (Photo 8), because of the reduction in wave approach angle north-westwards (HR Wallingford, 1995, 1997). These, however, are net values, so that given the frequency of drift reversal, gross values are probably considerably higher. It should be noted that the lower rates of drift associated with more recent decades mostly reflect the role of defence structures in reducing fresh sediment inputs and intercepting transport. Lack of sediment availability associated with the dynamics of the Chichester Harbour ebb tidal delta may also be implicated.
Erosion of the proximal end of East Head spit between the mid-1990s and 2005 suggests that during this period the input of sediment via littoral drift updrift of the terminal groyne was virtually zero, although a net westwards drift here of 7,000m³ per year was suggested by Webber (1979), revised downwards by HR Wallingford (1995) to 2,600m³ per year. Through the second half of the twentieth century, the drift rate steadily fell, a feature attributed both to fluctuations in the volume of onshore feed and the effects of progressively more robust and comprehensive coastal protection structures in reducing supply from coast erosion and arresting beach drift. Lewis and Duvivier (1977), HR Wallingford (1995) and Posford Duvivier (1992, 2001) calculated a prevailing natural drift for the upper gravel beach of 2,000-5,000m³ per year downdrift of Medmerry, but added that this figure was only valid for the ungroyned coast; thus a negligible actual net drift rate of 300-500m³ per year was considered more realistic for the heavily groyned East Wittering to Bracklesham frontage (Photo 9) and 1,000-2,000m³ per year at West Beach, Selsey. In general, drift rates on much of this shoreline are determined by sediment availability and the condition of groynes, as deteriorating or overflowing structures can locally increase throughput for limited periods. Significant interruption to upper beach transport occurs at outfall sites, particularly Broad Rife (Photo 3), leading to immediate downdrift starvation. Thus, the downdrift benefits of the substantial gravel recharges of Medmerry beach have been surprisingly modest. Cross-shore rather than long-shore fluctuations have been dominant at Medmerry, varying between annual gains of up to 40,000m³ to annual losses of over 60,000m³. Volumetric assessments of littoral transport are based on minimum net drift values derived from assessing inter-tidal beach changes. Throughput, which causes no discernible beach volume change, cannot be detected so that absolute actual drift rates could be significantly greater. Modelling studies more effectively identify the natural littoral drift potential, but such rates cannot be achieved due to the effects of groynes diverting sediments into storage.
Analysis of Coastal Monitoring Programme baseline topography and aerial photography data demonstrates that littoral drift of gravel is from Selsey Bill north-eastwards to the entrance to Pagham Harbour (Photo 1), (as also observed by Lewis and Duvivier, 1955; Duvivier, 1960; Wallace, 1990; HR Wallingford, 1995; Gifford Associated Consultants, 1997; Posford Duvivier, 2001) (see section 5). Detailed analysis of maps, aerial photos, Coastal Monitoring Programme beach level measurements and observations in groyne compartments have confirmed the location of the regional significant littoral drift divergence pathways (Lewis and Duvivier, 1976; Harlow, 1980; HR Wallingford, 1995, 1997, and Posford Duvivier, 2001) as between Warner Road (net westward drift) and Hillfield Road (net eastward drift). At this point, groynes and a seawall have been constructed, either side of which erosion has occurred, thus forming an artificial headland.
Between Hillfield Road and Selsey Bill the beach is aligned closely to the dominant wave approach (Lewis and Duvivier 1977; Posford Duvivier, 2001) and the eastward drift rate along this highly defended and groyned frontage has been calculated as 3-10,000m³ per year from analysis of Coastal Monitoring Programme baseline topographic and aerial photography data.
Analysis of Coastal Monitoring Programme baseline topography and aerial photography data demonstrates that the north-eastward drift rate is variable and lower than between Selsey and Cakeham. Eastward drift around the Bill has been calculated as 3-10,000m³ per year, and reduces to north-eastwards transport of 1-3,000m³ per year along the Selsey east frontage and less than 1,000m³ towards Park Lane. Further north to the proximal end of Church Norton spit the drift rate increases to 1-3,000m³ per year. These variable rates are lower than the 2004 estimated rates of 10-20,000m³ per year, as analysis of the Coastal Monitoring Programme data demonstrates that alongshore transport is not the dominant pathway.
Posford Duvivier (2001) estimated a drift rate of 13,700m³ per year at Selsey, reducing to 5,500m³ per year once groyne performance is factored in. Drift potential was found to increase north-eastwards on East Beach, where it is between 15,000 and 25,000m³ per year. Much of this beach frontage is managed, so that actual drift depends on groyne performance and sediment availability. HR Wallingford (1995, 1997) state that a mean quantity of 5,000m³ per year is supplied from the Kirk Arrow Spit and is augmented by a further 3 to 5,000m³ per year from the Inner Owers, giving a total drift potential of 25-30,000m³ per year. This figure is based on the fact that on formerly unprotected stretches erosion of up to 10,000m³ per year occurred to maintain drift rates; it corresponds closely with the estimated gravel input from erosion behind East Beach before coast protection. The sediment supply and transport system had previously achieved an equilibrium so that no net beach erosion occurred between East Beach and Church Norton. Terminal scour and thus some downdrift starvation has been a significant past problem at the north-east end of this protected frontage.
HR Wallingford (1995) calculated a drift rate of 33,000m³ per year for East Beach, reducing to 8,000m³ per year when adjusted for assumed groyne efficiency in their model studies. For Selsey Bill, their equivalent figures are 13,700 and 5,500m³ per year. Barcock and Collins (1991) re-calculated the prevailing drift rate between East Beach and Pagham Harbour entrance to be between 24,000 and 42,000m³ per year, based on data on the frequency distribution of wave heights and approach directions and considers sediment exchanges with Pagham tidal delta. Using HR Wallingford's (1995) DRCALC model, updated by later wave climate information, Posford Duvivier (2001) proposed a potential drift rate of 32,000m³ per year without control structures. Actual rates are considered to be at least 25-30% of the above volumes (i.e. 8 to 10,000m³ per year) because of the role of groynes.
Overall, there is variation in rates of processes along this complex frontage and numerous alternative estimates of drift have been produced. There is some consensus that potential drift in recent decades of all sediments approximates to around 30,000m³ per year (range of estimations between 10,000 and 40,000m³ per year with actual drift in the range 7,000 to 11,000m³ per year being controlled by beach management practices (Environment Agency, 2012). The mean rates reported above make assumptions on the magnitude and frequency of pulses of supply and the overtopping or by-passing of groynes. Despite this, the overall pattern and volume of drift has been established at medium to high reliability with moderate to good correspondence between modelled and estimated rates of transport.
The effect of sustained long-term unidirectional north-east drift along this sector, over at least the last half millennia, has been to deliver much sediment to build the southern spit that helps to define the entrance to Pagham Harbour (Photo 10). It has a history of fluctuation, thus indicating temporal variation in littoral and offshore sediment supply (see section 5). It was in a phase of depletion from the late 1980s to 2005 (following slow growth between 1972 and 1984), with erosion losses offset by deliberate cycling of gravel from co-extensive nearshore bars, at a rate of approximately 15,000m³ per year from the early 1990s to 2005. The northern (Pagham) spit is the product of a local 'counter' drift, resulting from a transport divide north-east of the harbour entrance. This itself is the outcome of interaction between tidal currents generated by the inlet and complex wave refraction over the Pagham tidal delta (Geodata Institute, 1994). With a maximum south-westerly directed drift rate of 5,000m³ per year (Barcock and Collins, 1991; Collins, et al., 1995), this northern spit has had less capacity for growth and change than its southern counterpart. The latter has substantially increased in length and volume since 2005 as it has migrated north-eastwards and deflected the inlet mouth of Pagham Harbour (Environment Agency, 2012.) Section 5.3 provides further details.
Offshore tidal current transport of sand, inferred from sample surveys of bedforms and numerical modelling (Barcock and Collins, 1991) is considered to be towards the south-west, or west, in water depths of less than 15m.
Analysis of Coastal Monitoring Programme baseline topography and aerial photography data demonstrates that significant volumes of gravel are transported onshore from the mobile nearshore banks and shoal, which is evident by the increased littoral drift rate of 3-10,000m³ per year along the spit.
HR Wallingford (1995) used a modelling approach informed by measured beach level changes to calculate potential drift to be approximately 32,000m³ per year along the Church Norton spit, although the rate is less than 17,000m³ per year for the upper gravel beach alone.
Experiments employing fluorescent sand and shingle tracers at Medmerry (Hydraulics Research, 1974) have indicated that sand transport may be reversed on the lower foreshore, seaward of groynes, due to strong eastward residual tidal currents. Any reversal was, however, considered to be local to the study site because the westward tidal current increases in velocity west from Medmerry so inducing net sand transport westward to East Wittering; a transport divide may therefore exist in the vicinity of Bracklesham. The tracer experiments were only conducted over a 6 month period so it is possible they were unrepresentative of typical conditions. The experiments offer the only direct information on this component of sediment transport, although HR Wallingford (1995) state that an anticlockwise circulating tidal eddy exists outside the entrance to Chichester Harbour that transports sand from the foreshore between East Wittering onto East Pole Sands. Other studies fail to acknowledge such a net drift reversal and Lewis and Duvivier (1977) argued that all sand transport on the lower foreshore was, on balance, westward (i.e. in the same direction as the shingle of the upper foreshore). Their evidence was sand distribution observed within groyne compartments, analysis of residual offshore currents, and the lack of sand at Selsey contrasted with its abundance at East Pole Sands and East Head. Modelling by HR Wallingford (1995) computed the transport rate for sand to be some 25,000m³ per year from east to west.
Analysis of Coastal Monitoring Programme baseline topographic and aerial photography data provided no conclusive evidence for drift reversal on the lower foreshore, therefore the 2004 indicative arrows have been removed.
Analysis of Coastal Monitoring Programme baseline topography and aerial photography data demonstrates that a local reversal of drift occurs at the northern boundary of Pagham Harbour entrance channel. Westward drift rate has been calculated as less than 1,000m³ per year. This reduction from the 2004 estimated rate of 3-10,000m³ per year reflects the significant accretion and extension of the Church Norton Spit, which has limited the exposure of the frontage and altered the tidal currents interacting with the shoals and spit on the Pagham frontage. This drift divide has migrated some 500m eastwards between 2003 and 2012 as the length of the Church Norton spit has extended, introducing changes in the offshore pattern of wave convergence due to refraction either side of the Pagham Harbour inlet. Its current position (2013) is immediately south of the beach groyne field (Environment Agency, 2012). The Pagham Harbour ebb tidal delta and wide, accreting foreshore sets up complex local wave refraction and provides protection against the dominant south-westerly waves enabling a very local dominance of south-easterly waves (Barcock and Collins, 1991; Gifford Associated Consultants, 1997; Posford Duvivier, 2001). Atkins (2010) reported ongoing erosion between the location of the drift divide and the distal point of the eastern spit enclosing Pagham Harbour up to 2008; this intensified in subsequent years due to lack of supply to this short westwards littoral drift pathway caused by the rapid north-eastwards extension and distal recurvature of the Church Norton Spit that commenced in 2003. This not only diverted potential shingle feed into storage, but resulted in the migration of the entrance channel of Pagham Harbour. Consequently, fast-flowing flood and ebb tidal currents constantly scour the foreshore of Pagham Beach and result in the recession of the beach backshore.
4. Sediment Outputs
4.1 Estuarine Outputs
EO1 Chichester Harbour Entrance
Eastward shoreline drift from Hayling Island and westward drift from West Wittering and East Head converge at the harbour entrance (Photo 11). Sand and gravels entering the Chichester tidal channel are flushed offshore by the strong ebb residual tidal current and deposited at varying distances from the entrance depending upon sediment size, wave conditions and water depth. This is confirmed by the orientation and shape of bar and bank topography (HR Wallingford, 2000) and sediment trend analysis (Geosea Consulting, 2000). Gravel can be transported a maximum of 2km offshore and sand a maximum of 3.5km offshore (Webber, 1979). The result of this offshore flushing of sediments has been the accumulation of some 25 million cubic metres of sediment within a major ebb tidal delta (Webber, 1979). Sediment sampling by Harlow (1980) and GeoSea Consulting (2000) revealed a series of sedimentary zones and potential transport pathways, and suggested that wave action can mobilise sediments on the tidal delta and drive them back shoreward towards Eastoke, Hayling, Cakeham and West Wittering beaches. The significant accretion of the nearshore and foreshore areas of Cakeham and West Wittering, and subsequent mid to backshore beach elevation, between 2005 and 2010 has provided conclusive proof of this transfer (Bray, 2010; Fitzgerald, 2012). The net result of these processes is an anticlockwise circulation of mobile sediments within the ebb tidal complex of Chichester Harbour entrance, as first deduced by ABP Research and Consultancy (2000) and subsequently demonstrated by Fitzgerald (2012). It is, however, subject to fluctuations in magnitude and intensity, which may be cyclic.
The volume of sediment transported and deposited offshore by tidal currents has not been calculated, but fresh supply to the tidal delta could be approximately (and perhaps rather crudely) estimated from littoral drift inputs at the harbour entrance. Drift inputs to the tidal channel were undoubtedly greater in the past (over 70,000m³ per year), but have substantially declined over the past 100 years as coast protection has intercepted and reduced drift along the shoreline of Bracklesham Bay (Harlow, 1980; Posford Duvivier, 2001). Not only have inputs to the delta thus reduced, but losses due to dredging of Chichester bar have increased since 1973 (see section 4.2). In fact, analyses of bathymetric data have suggested that the ebb tidal delta suffered a net loss of 1.4 million cubic metres from 1974 to 2000 (Posford Duvivier, 2001).
The rotation and recession of East Head during the nineteenth century (section 5.3) caused the formerly deeper entrance channel close to the east Shore of Hayling Island to widen eastwards along a north to south axis, thus reducing the size and changing the dimensions of Winner Bank. This may continue a long-term trend, as the absence of drift deposits beneath Sandy Point (Hayling Island) suggests that the harbour channel was further west at least two or three millennia before the present, and perhaps as recently as the sixteenth century.
Analysis of the hydraulic regime at the entrance channel, via hydrodynamic modelling (ABP Research and Consultancy, 2000; HR Wallingford, 1998) has shown that there has been a rather variable, fluctuating pattern of channel narrowing and deepening; widening and shallowing since the mid-nineteenth century. The channel initially decreased in size, as the Winner bank accreted, 1887-1923; but overall, given the rotation and retreat of East Head and subsequent lowering of the Winner, the cross-sectional area of the channel has increased over the last 150 years. This suggests that it is adjusting towards a new equilibrium condition, but is below its optimum cross-sectional area given the tidal prism of Chichester Harbour. It stimulates the suggestion (ABP Research and Consultancy, 2000) that the harbour mouth has adjusted, or is adjusting, to a change from a wave-dominated littoral transport fed sediment budget to one which is controlled more strongly by tidal currents.
EO2 Pagham Harbour Entrance
Currents generated by tidal exchange at the Pagham Harbour entrance are effective in interrupting littoral drift (Photo 10).The spring tidal prism is approximately 5,300,000m³. As at Chichester Harbour entrance, the ebb current (1.0-1.5m per second) is more powerful than the flood (0.4ms per second) so sediment movement into the entrance channel by littoral drift is mostly flushed seaward to a significant ebb tidal delta (Barcock and Collins, 1991). A sediment quantity between 30-75,000m³ per year is potentially available at the entrance to the harbour delivered by convergent longshore transport from the south-west and north-east. A proportion of this, which is primarily coarse sand and gravel, becomes stored in the beaches that make up the twin spits, potentially leaving around 24-40,000m³ per year to enter the entrance channel (Gifford Associated Consultants, 1997; Environment Agency, 2012). Based on an assumption of bedload transport rate at the harbour entrance and calculation of the tidal prism, potential output to the delta by ebb current flushing is likely to be between 16-40,000m³ per year (Gifford Associated Consultants, 1997). However, a significant proportion of this quantity represents material introduced into the entrance channel by wave transport of gravel and coarse sand from landward migrating offshore banks that are components of the tidal delta (Geodata Institute, 1994; Barcock and Collins, 1991). Actual quantities are less than those given above, for example input at the harbour mouth, prior to diversion to storage, is more likely to be within the range of 15 to 40,000m³ per year). A net loss of gravel storage in the ebb delta, estimated at 70,000m³, occurred between 2003 and 2011 (Environment Agency, 2012), all of which contributed to the expansion of Church Norton spit beach as it migrated north/north-east, diverted the inlet channel and occupied part of the former delta. Erosion was therefore the result of loss of a principal supply source, converting it into a partially remnant feature that was re-worked by wave action (i.e. during this eight year period the delta switched from functioning as an accumulation to a donor store (Royal Haskoning, 2009; Environment Agency, 2012).
Ebb tidal currents are moderate and their influence does not extend very far seaward compared to those generated by the much larger tidal prism of Chichester Harbour. Wave action is modified by local refraction induced by complex bathymetry, but HR Wallingford (1993) calculated a mean significant wave height of 1m, and a maximum of 4.5m, close to Pagham Harbour entrance. Wave induced currents oppose seaward transport and tend to drive material back landward when ebb tidal currents are weak. A consequence is that the ebb tidal delta store is located close to the inlet re-entrant. Sediment therefore has a short residence time within the delta and is liable to being driven back ashore within swash bars to the west and east of the inlet.
Net seaward sediment transport that generates accretion of the ebb tidal delta at Pagham entrance is in the order of 16,000m³ per year representing the balance estimated between landwards (flood tide and wave-driven) input of 18,000m³ per year, and seawards removal of 34,000m³ per year (Geodata Institute, 1994).
Flushing processes and the ebb delta sediment budget have undoubtedly changed over the past four centuries due to reduction of the tidal prism of the harbour as a consequence of land claim. The earliest record of this process is in 1580, with the first major flood embankment constructed in 1637 (Brown, 1981; Cavis-Brown, 1910; Graves, 1981; Environment Agency, 1998; 2012). Approximately 1.26km² were reclaimed between 1672 and 1809 and most of the remainder in 1877. Breaching in 1910 led to rapid inundation, to create the present harbour area of 2.83km². It can be postulated that the ebb tidal delta would have reduced in size and area as the tidal prism was reduced and it is likely that much sediment was driven ashore from 1876 to 1910 when there was a very small active inlet.
4.2 Dredging
Dredging across Chichester Bar, on the ebb tidal delta, is carried out routinely to maintain a channel for navigation; some of the material is used for beach renourishment on south-east Hayling Island. Dredging for aggregates, on a small scale on The Winner, was carried out between the early nineteenth century and about 1920. Dredging for navigation access, however, did not become significant until 1973.
A total of 60,000m³ of gravel was thus removed over the period 1974-1982 (Harlow, 1985) and between 1988-1996 dredging was permitted up to an annual limit of 20,000 tonnes (12,500m³ per year). Analysis of hydrographic surveys indicated that water depths increased over the dredged area in the 1970s (Webber, 1979). Despite this, it was difficult to attribute this change solely to dredging because the Chichester tidal delta is a large sediment reservoir (25 million m³) characterised by major natural internal fluctuations and sediment redistribution (ABP Research and Consultancy, 2000). From bathymetric information (Posford Duvivier, 2001) it can be concluded that the Chichester tidal delta has more recently lost material due to reduction of littoral supply from Bracklesham Bay in combination with outputs by dredging. For the period 1974 to 2000 this can be estimated at 1.4 million cubic metres. This quantity is quite significant in comparison to the total estimated volume of the delta, particularly when it is considered that the bulk of output comprises gravel from the inner bar whilst much of the estimated stored volume comprises sand on the outer bar. Continued dredging and onshore feed might therefore be expected to deplete reserves, such that natural onshore feed could reduce in the near future (or may already have done so).
4.3. Aeolian Transport and Deposition
A1 East Head
In addition to sand recycling and other beach management operations for the adaptive management of the Hinge and East Head, analysis of Coastal Monitoring Programme 2006/07 and 2012/13 lidar and 2008/13 aerial photography datasets indicated a trend of dune accretion at East Head at a rate of 3-10,000m³ per year. Sand that accumulates on the wide foreshore becomes entrained by southerly and south-westerly winds and is blown landward and deposited at the leading seawards margin of the dune system (Photo 4 and Photo 5). As sand continues to accumulate specialise dune vegetation have colonised, thus further increasing the "roughness" of the ground surface and encouraging further deposition of sand. Aeolian transport reduces as areas of bare sand diminish and higher vegetation intercepts airflows. Some areas suffer temporary loss of ground cover due to trampling of the vegetation by the large number of recreational summer visitors to the dunes.
A2 Cakeham
Analysis of Coastal Monitoring Programme 2006/07 and 2012/13 lidar and 2008/13 aerial photography datasets indicated a trend of dune accretion at Cakeham at a rate of 1-3,000m³ per year. Sand that accumulates on the wide foreshore becomes entrained by southerly and south-westerly winds and is blown landward and deposited at the leading seawards margin of the dune system that have continued to develop at the rear of the beach slope (Photo 9).
5. Sediment Stores
Sediments are stored along this shoreline within its beaches, spits and within ebb tidal deltas associated with harbour inlets. Further information may be found at http://www.scopac.org.uk/sediment-sinks.html.
5.1 Beach Morphology and Sedimentology
Along much of this coast the upper beaches are steep and composed of flint gravel, whilst the lower foreshore has a relatively shallow slope and is composed of medium and fine sand (Hydraulics Research, 1974; Lewis and Duvivier, 1977; Harlow, 1980; Posford Duvivier, 2001; Environment Agency, 2012). Patchy gravel frequently overlies foreshore sand, which in turn normally conceals the sandy clays of the Eocene Bracklesham Series. The main beach morphodynamic and sedimentary features are distinctive along the several discrete sectors of this coastline, namely:
- East Head: The seaward side is composed of a wide gently sloping fine sandy foreshore (ABP Research and Consultancy, 2000), which narrows abruptly at the point of distal curvature. Here, a steep convex shingle beach has become a more established feature, and may represent the re-exposure of the original shingle platform on which the spit has been developed. A very narrow shingle backshore beach at The Hinge and spit neck was eliminated by erosion between 1999 and 2005, but has been retained immediately updrift by groynes. Further details of the recent morphodynamics of this beach are contained in Bray (2007; 2010) and Fitzgerald (2012), summarised in the relevant part of section 5.3.
- Bracklesham Bay: Beach crest levels are typically maintained at around 5.4mOD. An exception to this is along and immediately west of the Medmerry frontage (Photo 6) where maximum levels exceed 6mOD due to artificial profile reconstruction or "beach scraping" (Harlow, 1980; Hydraulics Research, 1974; Posford Duvivier, 2001). Beach width decreases south-eastwards whilst the slope of both the upper and lower foreshore increases north-westwards. Beach sediment sampling between 1km west of Selsey Bill and 0.5km east of Bracklesham indicated that backshore pebbles were generally larger than those on the foreshore and at Bracklesham (Photo 8) mean size was larger than at Selsey (Cole, 1980). No statistically significant sorting trends were detected across the profiles other than for size. Although the measurements and analyses were carried out carefully, the spatial extent of sampling was limited and the temporal variability of the beach sediments was not considered in this study. Harlow (1980) sampled beach sediments from 124 sites between Gilkicker Point (Gosport) and Selsey Bill. Surface sediment samples were collected after a storm affecting the shoreline between East Wittering and Selsey in an attempt to describe the form to which the beach was tending in response to dominant waves. He noted that: (a) gravels on both the upper storm beach and the mixed midbeach increased in size from east to west; (b) Medium sand on the midbeach showed limited coarsening westward from Selsey Bill and (c) Fine to medium sand on the lower foreshore showed no sorting trend. It should be noted that the sedimentology of this beach would have been affected considerably by the numerous beach replenishments undertaken for coastal defence.
- West Beach, Selsey and Selsey Bill: The beach is generally narrower and steeper than the Bracklesham Bay frontage and is backed by hard defences. It is usually fairly empty of gravel in the vicinity of Hillfield Road (the littoral drift divide), but gravel accumulations increase both eastward and westward from this point (Lewis and Duvivier, 1977). Sand is only of significance at the lower beach fronting Selsey Bill, though it is not a constant feature (HR Wallingford, 1997).
- East Beach, Selsey: The beach has been frequently described as dominantly gravel, and steeper and narrower than West Beach (Duvivier, 1961; Hydraulics Research, 1974; Posford Duvivier, 2001). Immediately east of Selsey Bill, Lewis and Duvivier (1977) reported that the beach was composed of loose gravel sloping steeply down to a gravely pavement near or below LWMST. Further north-east, a variable proportion of sand was an impersistent feature of the lower foreshore. In this vicinity there were exposures of clay and other in-situ strata after winter storms, which tended to be concealed in summer by renewed beach accretion. Mean grain size is reported to increase rapidly northwards for the sandy-gravels of the foreshore, suggesting selective longshore transport, but is fairly constant for the upper beach (Gifford Associated Consultants, 1997). Quantitative analysis of beach sediments has not been undertaken, although Duvivier (1964) noted that Selsey Beach consisted predominantly of rounded flint clasts. Attrition tests were carried out by placing weighed samples in a revolving steel drum and re-weighing and re-sieving the residue at intervals. This technique established that attrition rate was inversely related to pebble size.
- Church Norton and the Pagham Entrance Spits: The beach at Church Norton is composed mostly of flint shingle (Lewis and Duvivier, 1977; Wallace, 1990; Posford Duvivier, 2001). Barcock and Collins (1991) report that Pagham Beach (the north eastern shingle spit) consists of a steep upper beach, a shallower mid-beach, and an extensive low gradient foreshore. This beach is mostly flint gravel, but grades to coarse sand and granules on the lower foreshore. The series of closely-spaced ridges that make up much of the Church Norton and Pagham spit beaches may result from the bifurcation of the ebb channel immediately seaward of the harbour entrance. The abandoned channel then fills with gravel, forming a linear bank that subsequently migrates on shore. Repetition of this process has created the pattern of multiple ridges (Barcock and Collins, 1991) over a cyclic timescale of 4 to 5 years during at least the second half of the twentieth century. Beach morphology along this sector has been considerably modified by nourishment, control structures and profile reconstruction (refer to Section 5.3 for details of the fluctuations of the size and orientation of Church Norton spit, especially its growth since 2005 as it migrated north and east across the previous tidal inlet of Pagham Harbour, expanding in volume by approximately 150,000m³ up to 2011). This increase consisted of shingle derived from updrift beaches, the Inner Owers shoals, the tidal delta and offshore, the latter amounting to at least 80,000m³ (Environment Agency, 2012). However, the proportionate share of each of these sources has not been determined.
5.2 Beach Volumes
Volumetric information has been collected by a variety of sources and covers most of the area:
- East Head. The total volume of material (sand and gravel) comprising the inter-tidal beaches of East Head has been calculated for a variety of dates using MHW and MLW on successive OS maps (Harlow, 1980). Its volume was 499,400m³ in 1846, increased to 894,000m³ by 1965, and remained stable up to 1975. The total volume of sand comprising the entire spit and dune system exceeds 2.2 million m³ (ABP Research and Consultancy, 2000) the former volume of gravel on East Head beach was calculated independently by Webber (1978) and Jarosz (1979). Webber measured 7 profiles and estimated volume at 22,000m³, while Jarosz measured 3 profiles and estimated volume at a nearly identical 21,183m³. Direct measurement of sediment thickness (depth) was not employed in either of these investigations thus the volumes quoted are probably underestimates, though representative. Harlow (1980) indicated that inter-tidal sediment volume can vary greatly at East Head according to incident winds and waves. The oscillations of foreshore and beach erosion and accretion between the mid-1990s and 2010 are described in detail, and interpreted, in Bray (2005; 2010) and Fitzgerald (2012) - see section 5.3 for a summary. The foreshore of the Winner in front of East Head has eroded and lowered by up to 3m since 1923 (ABP Research and Consultancy, 2000) as a result of tidal channel migration.
- West Wittering and Cakeham Beaches to West Beach, Selsey. A series of profiles were measured at monthly intervals along this frontage by Hydraulics Research (1974). Beach volume was calculated above 0.16mOD, a level equivalent to the toe of Medmerry Beach. Calculated beach volumes revealed seasonal variations, with significantly larger volumes retained in summer. Mean quantities (i.e. inter-annual values) were:
West Wittering: 135,000m³
East Wittering: 167,000m³
Medmerry: 450,000m³
West Beach, Selsey: 30,000m³
Distinction was not made between gravel and sand, for the two were frequently intermixed on the foreshore and their relative proportions at depth were unknown. The temporal representativeness of these volumes was moderate to low because profiles were only measured during a 9 month period, insufficient to include exposure to the full range of wave energy states. Severe winter storms were observed to remove all foreshore sediment and expose the underlying substrate to wave abrasion (Lewis and Duvivier, 1977). However, much of this material was recovered during the following summer so that the operation of seasonal "cut" and "fill" cycles is likely to be an inherent feature of this sector of coastline.
Analysis of beach levels, 1973-1995 (HR Wallingford, 1995 and Gifford Associated Consultants, 1997) using Environment Agency ABMS profiles revealed that the upper gravel beach was narrow at Bracklesham (Photo 8), and did not appear to derive much benefit from up-drift replenishment. Further north-west it was more substantial, although there were site-specific variations in width and height reflecting groyne trapping efficiency. The lower foreshore appeared to have maintained its form, but levels dropped progressively throughout the above period, especially at West Wittering, Bracklesham and Medmerry. High rates of volume loss and profile flattening along the Medmerry frontage, associated with storm waves, were offset by intensive renourishment and reprofiling (see section 2.3). This was especially marked in the late 1990s (Posford Duvivier, 2001) and the unsustainability of this management practice was the major reason for the implementation of the Medmerry managed realignment scheme, implemented in 2013.
Between 2003 and 2005, West Wittering beach experienced variable cross-profile gains and losses, but with overall erosion. However, in the eastern sector of this beach from 2004 through to 2007, the foreshore was strongly characterised by an onshore migrating swash bar and intervening troughs (Bray, 2007; 2010; Fitzgerald, 2012). In 2005-6, this dominant bar moved landwards 100m, coalescing with earlier bars on the upper foreshore and mid-beach by May 2009. Upper backshore gains during this four year period were evident from significant dune growth in front of properties at West Strand and Cakeham.) The western sector experienced less swash bar accretion and foreshore erosion due to the landward movement of the nearshore channel delimiting the lower foreshore. Overall, the eastern sector gained nearly 21,000m³ of sand, whilst there was a net loss of 50,000m³ over the area of the western sector (Bray, 2010). Adjacent central Cakeham beach witnessed substantial mid and upper foreshore accretion between 2004 and 2009 resulting from the sustained onshore migration of swash bars at an average rate of approximately 75m each year. These bars were spaced at roughly 100m intervals, with amplitudes between 0.4 to 1.2m, and added some 150,000m³ of sand over this five year period. In the western area of this beach, migrating bars “welded” with the mid and upper beach over four years (2004 to 2008), providing blown sand for backshore dunes up to 7m in height. (Bray, 2010; Fitzgerald, 2012). Some of this sediment was excavated and used to nourish the foreshore in front of the neck and Hinge of East Head.
- Selsey Bill. HR Wallingford (1995), using ABMS extending data back to 1973, identified a steady lowering of foreshore levels, particularly west of Hillfield Road. At this point, and north-west to West Beach, there is no direct feed from the Kirk Arrow Spit. The narrow depleted strip of upper gravel beach is also a result of wave reflection from backing seawalls. Further east, towards and at Selsey Bill, the shoreline is less exposed to waves from a wide approach sector. This is apparent from the comparatively wide mixed shingle and sand beach, though substantial drawdown occurs when there are incident waves from the south-east. The foreshore close to the Bill (Photo 1) has tended to resist the trend towards depletion, showing some modest accretion in recent years probably due to receipt of influxes of gravel from the Kirk Arrow Spit since at least 1997 (Posford Duvivier, 2001). HR Wallingford (1995) calculated an overall upper beach gravel loss of 1,000m³ for the period 1973 to 1992 prior to the recent gravel influxes. Hydraulics Research (1974) calculated that the beach at Selsey Bill (Hillfield Road) stored some 65,000m³ of sediment; this is a mean value that accounted for seasonal fluctuation.
- East Beach, Selsey. Lewis and Duvivier (1977) note that beach volume is variable spatially and temporally, but estimate that 50,000-55,000m³ of material is permanently retained on the beach. This calculation was undertaken using aerial photographs (1972- 75) and beach morphology observations. Analysis is complicated by the fact that there are significant differences in beach levels north and south of the lifeboat station. Gifford Associated Consultants (1997) calculated a net loss of 40,000m³ per year, 1972-1992, for the entire sector between Selsey Bill and Pagham Harbour entrance, based on application of the LITPAK numerical model. For East Beach alone, a loss of 6,000m³ per year was derived from analysis of ABMS beach profiles for the same period (HR Wallingford, 1995). Taking into account the complexities of loss and gain on this beach, especially nearshore and inshore exchanges within and between the Inner Owers shoals, and the retention of gravel by closely-spaced groynes, annual depletion over this period is considered to be close to 4,000m³ per year. This, however, is substantially less than the rate of loss between 1900 and 1950, when recession at about 3m per year occurred prior to the construction of defences. During this period, underlying Raised Beach deposits were intermittently exposed, thus adding to sediment supply and helping to offset losses (Posford Duvivier, 2001). Under present hold the line policies this source is no longer available.
A number of specific locations on East Beach, Selsey, involving both upper and lower beaches, have exhibited net depletion and lowering over the period approximately 1965 to 1990 (HR Wallingford, 1995). However, profiles at Inner Owers and Church Norton have been characterised since the early 1900s by net periodic accretion at rates of up to 4,800m³ per year (100,000m³ between 1970 and 1994), probably due to sequential "welding" of onshore moving bars (Barcock and Collins, 1991; HR Wallingford, 1995). Depletion of East Beach and on the Pagham spit, was apparent between 1910 and 1940 and by the mid-1980s the latter was losing material at a minimum rate of 2,000m³ per year, necessitating subsequent gravel recharge, recycling and re-profiling in an attempt to compensate for beach losses. These trends were a continuation of those measured by Lewis and Duvivier (1977), using aerial photography, for the period 1967-1975. However, small scale spatial variability of net accretion/depletion patterns are apparent, and appear to be most directly related to groyne re-construction, the inshore movement of gravel banks and bars and spatial variation in exposure to wave energy. Terminal scour at the easternmost groyne was evident before the latter was buried following replenishment in 1990. Variable losses and gains between 2003 and 2011 are calculated and tabulated in Environment Agency (2012). Net accretion of 40,000m³ occurred on the beaches between Selsey and the Inner Owers, principally in the form of several shore-perpendicular bars. Beaches fed by the Inner Owers shoals lost approximately the same quantity of sediment, whilst there was a net gain of 63,000m³ on the beach between the Inner Owers and the distal connection of the Church Norton spit. Overall, the shingle budget for this sequence of beaches during this eight year period is positive - a loss of 146,000m³ counterbalanced by a gain of 271,000m³. The sources of this “surplus” are presumed to be drawn from the erosion of the remnant part of the ebb delta and from unspecified offshore stores.
The erosion losses from East Beach need to be placed in an historical context. Excepting locations subject to irregular nourishment, LWM has retreated at a rate of between 0.3 and 3.0 m per year since 1875 (Gifford Associated Consultants, 1997). As HWM is held static by seawalls and embankments along much of this frontage, this has also led to a long-term trend of profile steepening. Foreshore width diminished by up to 650m over the past 150 years at the most rapidly receding locations. The fastest rates of retreat at specific points occurred within a few years of the completion of seawalls, which was undertaken in piecemeal fashion between 1910 and 1969.
Taking the sector from Selsey Bill to Pagham Harbour entrance as a whole, beach budgets are negative, with groyne-assisted accretion and gravel recycling along the sector north of East Beach only partly counteracting overall losses of beach volume further south. Details of site-specific gains and losses, 1973-1992, are given in HR Wallingford (1995). Between 2003 and 2011 erosion has been an ongoing trend, with the Inner Owers and adjacent beaches losing 60,000m³ (Environment Agency, 2012). Details are provided in a sequence of Digital Terrain Models in Bovington (2011), reproduced with additional commentary and interpretation in Environment Agency (2012).
5.3 Stores: spits and estuarine sediments
The principal stores are the spits at either side of the entrance to Pagham Harbour; East Head Spit, the ebb tidal delta offshore of Pagham Harbour, and the estuarine sediments within Pagham Harbour.
Pagham Harbour Spits
Pagham Harbour (2.83km²) is a product of Holocene sea-level submergence of the former mouth of the river Lavant prior to its diversion in Roman times (Wallace, 1990). However, its present extent is the result of storm surge inundation on 6th December 1910 following complete enclosure and land claim commencing in 1876 (Environment Agency, 1998b; 2012). Siltation of the original larger estuary and progressive, piecemeal, reclamation took place earlier, the latter commencing in the seventeenth century (Graves, 1981; Brown, 1981; Environment Agency, 2012).
The convergent gravel spits that define the Pagham Harbour entrance channel have behaved in a highly dynamic fashion over at least the past seven centuries (Robinson, 1955; Robinson and Williams, 1983; Barcock and Collins, 1991; Geodata Institute, 1994; Gifford Associated Consultants, 1997; Environment Agency, 1998b; Posford Duvivier, 2001; Environment Agency, 2012). The earliest reasonably reliable map evidence from 1587 suggests that the southern (Church Norton) spit had a configuration similar to the present, possibly in response to one or more breaches dating back to 1340-1410. Between 1672 - when it was 1km. in length - and 1724, it extended some 90 m north-eastwards, with a further rapid acceleration in this extension of almost 900 m between 1774 and 1885. Episodes of breaching interrupted the spit growth in 1820, 1829 and 1840. As the spit extended and thinned during this 100 year period, there may not have been a significant supplementary supply from inshore sources (i.e. erosion of low clay cliffs exposed near Church Norton as well as migratory bars associated with the tidal delta). The main source of sediment feed was probably delivered by littoral drift along the shoreline from the south west. Rapid shore erosion occurring around the Selsey peninsula at this time would have provided a local source of sediment. Over this same period, the northern (Pagham) spit, the product of 'counter drift' determined by a littoral transport divide north-east of the harbour entrance, experienced net erosion and recession. The entrance channel cut into the low clay cliff on the northern side, resulting in 170m of coastline retreat at this point between 1780 and 1840.
Natural change ceased in 1876, when both spits were partially stabilised and the inlet channel much reduced in cross-section and sluiced to effect the final land claim of Pagham Harbour in 1877. Although the resulting insignificant tidal prism and therefore negligible ebb and flood flushing effect helped to create almost 120m of foreshore progradation, a major storm in December 1910 breached the Church Norton spit, creating a 160m wide channel near Church Norton and flooding the newly reclaimed area. Subsequent events have been documented by the sources mentioned above, notably the Environment Agency (1998b and 2012), as follows. Further north-eastwards growth of the Church Norton spit narrowed the entrance and deflected it some 700-800m to the north-east towards Pagham. The deflected entrance began to threaten bungalows built on the Pagham spit in the 1920s, so a new narrow entrance, close to the 1910 breach, was established artificially in 1937 and fixed in position in 1944. The pre-1937 entrance gradually closed becoming marked by a low gravel berm and the isolation of Pagham Lagoon. A further breach over a wide front between the 1937 and pre 1937 inlets occurred in 1955 and by 1958 the entrance was some 700m wide. Rapid drift thereafter led to a further extension of the Church Norton spit across the inlet narrowing it to 250m by 1961. Following continued narrowing a stabilisation of the entrance channel by a training wall along the distal end of the Pagham spit was completed in 1963, and this situation has been maintained subsequently. Starting in the early 1990s there has been punctuated extension as well as substantial (though unsteady) volumetric expansion of the Church Norton spit; between 2003 and 2011 its net volume increase was 170,000m³ (Atkins, 2010; Environment Agency, 2012), with over 900m of north-eastwards growth accompanied by distal recurvature towards the Pagham Estate Beach shoreline. Variable inter-annual rates of accretion are associated with fluctuations of incident wave energy. Although initial spit behaviour partly conformed to the 4 to 5 year migratory cycle proposed by Barcock and Collins (1991), sustained growth continuing to 2014 - which has caused the landwards deflection of the harbour approach channel - indicates that this apparent cycle has at least temporarily stalled.
The high instability of the Pagham inlet results from the relatively small tidal prism of the harbour and the potential for rapid west to east drift along the shoreline (Geodata Institute, 1994). Inlet narrowing and deflection occur when drift exceeds the flushing effects of tidal exchange and the Church Norton spit extends north-eastwards, as has occurred since 2005, the channel has a tendency to fill during the periods of neap tides and then be scoured during succeeding spring tides. Gravel sized clasts in excess of 20mm in diameter cannot, however, be flushed seawards towards the ebb delta (Royal Haskoning, 2009), thus promoting net accretion. Breaching occurs when storm surges overwash the spits and lower them sufficiently to allow tidal exchange that can maintain permanent inlet channels. Land claim, especially in the late nineteenth century, drastically reduced the tidal prism and therefore the size of the tidal delta; this in its turn determined in part the repositioning of the inlet channel (Halcrow, 2002).
East Head Spit
The evolution of this spit can be traced in some detail from the end of the sixteenth century, using old maps, charts, successive Ordnance Survey map editions and (since 1945) aerial photographic cover and other remote sensing imagery. Searle (1975); May (1975); Edwards, (1994); Baily and Nowell (1996); ABP Research and Consultancy (2000) Baily et al., (2002) and Carter (2006) all indicate that East Head has grown, over at least the past 200-300 years, from an embryonic or possibly ancestral gravel spit form following the east-west trend of the immediate updrift shoreline. Growth, however, has been a long-term trend superimposed on short term fluctuations. It is currently more than three times the area (measured from mean low water) that it occupied in circa. 1850 (May, 1975), with a total volume of about 2.2 million m³ of sand and gravel (ABP Research and Consultancy, 2000).
The spit has experienced past events of breaching (for an example revealed by sediment coring refer to Bray and Teasdale, 2007) and also progressive recurvature since about 1880 in several stages of re-orientation, in response to changes in (a) incident wave energy; (b) near and offshore topography and (c) both longshore and nearshore sediment supply. Posford Duvivier (2001) indicate that greatest wave heights are currently associated with winds blowing from the south or south-east over a fetch of some 150km. During this period of growth and establishment, the gravel foundation was overlain by sand, and the current dune field accreted, (refer to Carter (2006) for details of dune morphology and ecology). The events were the product of a change of the local sediment budget, and cannot be wholly ascribed to the impact of updrift protection measures. Changes in planform were particularly marked between 1880 and the early 1950s, but throughout its recent history the spit has been 'fixed' at its proximal point ("The Hinge") whilst rotating clockwise and converting from swash to drift-alignment.
Re-orientation of the spit resulted in exposure of a low sand and gravel intertidal foreshore (the Winner) across central and eastern parts of the widening entrance to Chichester Harbour. It served to dissipate wave energy approaching the spit and its wide intertidal expanse formed a key source for wind entrainment and supply of dune-building sands to the spit. However, since 1923 lowering of the Winner has occurred by up to three metres, allowing increased wave exposure and reducing the intertidal foreshore width in front of East Head. The lowering of the Winner is due mainly to the requirement of the cross sectional area of the Harbour mouth to increase. This increase has been in response to the reduction in littoral drift from the east, which has occurred over at least the 100 years following the widespread provision of defences updrift (ABP Research and Consultancy, 2000).
Since 1945, The Hinge and neck has become progressively narrower, and now constitutes a tenuous, and vulnerable, connection to the updrift coastline. This was made apparent in 1963, when it was breached by storm waves; and again in 1987 when it was overtopped. Some 50m of recession of the seaward face of the neck of the spit occurred between 1946-94 (Burgess, 1994; Associated British Ports, 2000), with subsequent dune cliffing and retreat rates of over 3-5m per year between 1994 and 1998, necessitating the insertion of a concealed rock barrier or sill in 2000. A further 6m of recession occurred between July and October 2000, with an average rate of retreat of 0.57m. each month between March 1999 and early October 2004 (Baily and Bray, 2005) In late October 2004 the Hinge was overwashed during a combined tidal and storm surge, causing it to be narrowed and between November 2004 and January 2006 a fan of fine sand to be deposited across a part of the adjacent upper salt marsh that it normally protects. A breach, however, was avoided due to the presence of the rock sill. Between 1997 and January 2005 the inter-tidal area in front of the Hinge lost 4,050m³ of sand, and the dune cliffs over a frontage of 200m north of the Hinge retreated 6m, October 2004 to January 2005. In May 2005 some 1000 tonnes of sand was excavated from West Wittering beach backshore and dumped below Mean High Water in front of the Hinge. Over the succeeding four years, during which 13,000m³ of sand was transferred from a foreshore borrow area immediately west of the northern spit tip there was net accretion in front of the northern eroded dunes. Further transfers of sand from West Wittering occurred in 2008 and 2009. Between 2006 and 2009 there were alternations of erosion and accretion at the Hinge, with the net effect of loss at the seaward toe of the lower foreshore and gain across the mid and upper beach areas. Overall there was a net loss of sediment at the Hinge of an estimated 30,000m³, April 2005 to December 2010 (Bray, 2010). However, at the “neck” of the spit there was net accretion between 2007 and 2009 as a consequence of the welding onto the mid/upper beach of subdued swash bars. Almost all earlier researchers had ascribed erosion episodes at 'The Hinge' to the very substantial reduction of sediment supply from littoral transport along Bracklesham Bay resulting from the progressive extension of longer, higher and more frequently spaced groynes along the updrift shoreline since the late nineteenth century. A further cause of erosion at and north of the 'Hinge' has been the reduction in height of the adjacent Winner Bank, and the shoreward migration of the intervening nearshore channel which appears to have been continuous since at least the early 1920s (ABP Research and Consultancy, 2000). This has reduced sediment supply and has introduced greater wave energy owing to increased nearshore water depths. It is now apparent that phases of erosional loss at the neck of the spit are a consequence of a reduction or cessation of sediment supply to updrift beaches, and thus the starvation of the littoral drift pathway providing throughput of sand. This in turn relates to episodes when offshore to onshore migrating sand bars originating from East Pole Sands fail to weld to the foreshore of West Wittering and Cakeham Beaches The post-2006 growth of the beach at and north of the spit neck represents a phase of cumulative foreshore bar accretion (welding) and subsequent mid to upper beach elevation updrift, thus suggesting that the morphodynamic behaviour of East Head spit is an expression of cyclic or episodic alternation of excesses and deficits of offshore to onshore sediment supply ultimately linked to the Chichester ebb delta store. Excesses are related either to accentuated deposition on East Pole Sands in response to specific sequences of storms or to non-periodic shifts in the position of the main ebb tidal channel dividing that sandbank from the Wittering and Cakeham foreshores. (Bray, 2010; Fitzgerald, 2012). Northwards longshore drift transfers fresh inputs of sand from migrant welded bars to the Hinge, neck and tip of the spit. HR Wallingford (2000) have modelled the hydraulic conditions that would promote breaching for three specific water level, wave and tidal height combinations. Simulation of wave set-up and water levels revealed potentially high wave energy at The Hinge, with wave-induced northwards net transport along the 'open' shoreline to the north most apparent when spring tides combine with a high oblique angle of wave approach (i.e. from the south-west). Several combinations of conditions were identified that could lead to opening of permanent breaches that had a potential to cause sedimentation effects in the main channel (ABP Research and Consultancy, 2000).
Whereas, overall, between the early 1960s and 2006 the spit neck eroded, the width and volume of the distal part of East Head expanded in stages, with the rapid development of a broad triangular shape between 1911 and 1933. By 2000, East Head, as a whole, was significantly larger than it was in the mid- twentieth century, although the generally bulbous shape has been achieved through major accretion at the head and erosion of the neck. The accretion is the result of northwards longshore drift, subdued bar welding and deliberate management measures (brushwood windbreak fences and vegetation planting) introduced between 1967 and 1980 to stimulate new sand dune growth and stabilise the existing vegetation cover (Searle, 1975; Doark et al. (1990); Baily and Nowell, 1996; ABP Research and Consultancy, 2000; Baily et al., 2002). This trend towards distal enlargement is substantially a function of sand supply by net northwards littoral drift and migratory bar attachment along its beach face, fed by wave and tidal current transported sand at and immediately seawards of the mouth of Chichester Harbour. Some sand also appears to be lost from the extreme northern spit tip and transported either seawards via the Chichester Entrance Channel or north-west towards Pilsey Sands and the Emsworth Channel. These latter pathways are suggested from sediment trend analysis of inter-tidal samples from the western and northern shoreline of East Head (Geosea Consulting, 2000). Following an earlier (1997 to 2004) phase of net erosion, between 2006 and 2009 the sector of the upper beach fronting the distal sector of the spit expanded in width and volume (approximately 20,000m³) consistent with the migration northwards and landwards of the zone of accretion initiated further south. There was also seawards accretion of the dunes, with the creation of new embryo dunes and an increase of their elevation. The behaviour of the foreshore, however, was more variable, recording a loss of nearly 30,000m³ (Bray, 2010; Fitzgerald, 2012). Over this same period there was marginal northwards extension across the sandy gravel foreland at the northern tip of the spit, though 25m of retreat occurred between July 2007 and December 2009 in the area of recurvature to the north-east (Bray, 2010).
Pagham Tidal Delta
A body of sediment has accumulated immediately seaward of the Pagham Harbour entrance forming an ebb tidal delta (Photo 10). Its total volume is estimated to be of the order of 0.5 million m³ (Barcock and Collins, 1991) and results from the complex feedbacks between (a) longshore sediment transport along the spits, and (b) in the nearshore, tidal flushing from Pagham Harbour, with wave refraction set up by the bank itself generating net onshore transport by wave-induced currents (including kelp-rafted shingle). See section 4, EO2 for further details. Halcrow (2002) suggest that the delta has historically fluctuated in size, with reductions closely linked to the main phases of land claim (refer to earlier part of this section). The Owers shoals may be a remnant of a former larger delta complex. The delta acts as a store feeding shingle to Church Norton spit beach, which has migrated across part of it since 2005. Between 2003 and 2011 this transfer is estimated at approximately 65,000m³ (Environment Agency, 2012).
A small flood delta exists just inside the harbour entrance composed of silt, sand and gravel shoreline sediments driven into the harbour during storms in combination with flood tides. It may conceivably become reworked by coastal recession, but otherwise cannot readily contribute sediments back to the open coast and is a small scale sink (Royal Haskoning, 2009). It forms an area of raised and stable topography which has afforded stability to the spit barriers which have migrated landward towards it.
Pagham Harbour: Estuarine Sediments and Sedimentation (EO2)
The total volume of estuarine sediment infill has not been precisely calculated, and coring undertaken to investigate sediment stratigraphy (Hinchcliffe, 1988; Geodata Institute, 1994; Cundy et al., 2002) only penetrated a few metres. The latter two investigations revealed relatively coarse sediments at the mouth, and sands close to the ebb/flood channels, suggesting a small flood delta composed of marine-derived sediment. These progressively fine up-estuary to clays and clayey-silts at the heads of creeks, thus demonstrating the dominance of tidal currents on contemporary mudflat sedimentation (Bray and Cottle, 2003). At an unknown depth, estuarine sediments are replaced by biogenic and minerogenic alluvial/colluvial sediments that accumulated when Pagham Harbour was part of the lower floodplain of the River Lavant (Wallace, 1990). These are known to occupy the buried channel of the proto-Lavant, which extends beneath the offshore tidal delta and thence southwards some 3-500m seawards of the eastern shoreline of the Selsey peninsula (Wallace, 1990).
Geodata Institute (1994) and Cundy et al., (2002) report stratigraphical marker horizons at around 0.5m depths in the northern harbour that are interpreted as the 1876-1909 reclaimed agricultural surface. Subsequent tidal sedimentation has occurred at a rate of between 4.7 - 8.3mm per year, resulting in 41,000m³ of accretion since 1866. This is likely to have been highest during the period of expansion of Spartina anglica-dominated lower saltmarsh, from 1919-1948. Extensive swards of Spartina anglica that colonised from 1919, expanded to cover 130 ha in 1947 and then suffered slow die-back to 102ha in 1971 and 97ha in 1984. Between 1947 and 1965 the extent of saltmarsh decreased by 18% (Cope et al., 2008). Losses were mostly due to recession and fragmentation of the outer marsh margin, especially in the north-eastern area. Unusually for the Solent, the dieback trend appears to have reversed more recently with some renewed expansion of Spartina along the Sidlesham and Norton margins and in the centre of the harbour to cover 107 ha by 2001 (Bray and Cottle, 2003; Baily and Pearson, 2007; Cope et al., 2008). The increase in saltmarsh extent between 1971 and 2001 (latest air photo cover analysed) was estimated at 5.1% by Baily and Pearson (2007) and 6.1% by Cope et al., (2008). Dieback continued at least up to 2001 in other areas and HR Wallingford (1997) noted locally severe losses adjacent to the wall delimiting the northern reaches of the harbour. Mudflats and scattered sand and gravel banks and bars cover 220ha. Their morphology is determined by tidal currents. Posford Duvivier (2001) calculated a maximum significant wave height of 0.6m, occurring at least once a year, adjacent to the wall at Sidlesham, but the contribution of wave abrasion to sediment redistribution in uncertain (see account of upper harbour mudflat accretion below).
As there is negligible freshwater discharge into the harbour, input of terrestrial sediment can be regarded as effectively zero. The sediment budget is therefore determined by the balance between flood tide input and ebb tide output together with any primary production by the flora. Thus, the 0.5m thickness of sediment that has accumulated since 1910 would appear to provide evidence of a net input from marine sources, thereby adding new substrate for the development of mudflats. This may be a primary factor promoting the expansion of the area of saltmarsh in recent decades, taking into account a time lag between consolidation of silt and clay and subsequent colonisation by saltmarsh flora. Clarification of the processes and rates of transport and settlement of fine sediment (principally silt) is given in a series of papers by Burgess, Mitchell and Pope (2002; 2003); Mitchell, Burgess and Pope (2004 a and b; 2006); Mitchell, Tinton and Burgess (2006) and Mitchell, Burgess, Pope and Theodoridou (2008). Moored instrumentation was deployed at several sites in the inner harbour, simultaneously measuring current velocities, near bed suspended sediment concentrations (as turbidity) and salinity. Suspended sediment concentrations increased to an average of 60mg.l-¹ when flood tide velocities were at a maximum. Peak values as high as 220mg per litre occurred during spring tides related to short-lived salinity gradient maxima that inhibited vertical turbulent transport (i.e. promoted stratified flow). Concentrations rapidly decreased at and immediately following slack water due to grain flocculation and settlement. This indicated that more sediment is transported landwards by flood tide currents than is moved seawards on the returning ebb, with recycling at or near tidal limits. Locally generated waves increased turbidity at all stages of the tidal cycle, and at all instrumented locations, suggesting that they are effective in redistributing fine sediment from areas of initial accretion (and perhaps entraining or re-working material from harbour margins). Overall, this research indicates that fresh sediment is accumulating at the heads of the main tidal channels, adding to the margins of mudflats and thus increasing their extent. The main source of sediment is considered to be marine, external to the harbour, with input at its greatest during winter storms. The contribution of freshwater discharge to the fine sediment budget is very small, though it was observed that intermittent pumping created a minor increase in turbidity.
An additional component of this research project was the systematic re-measurement of surface sediment levels recorded by reference pins, at nine sites over two years. Results largely confirmed earlier work (see above), indicating a positive sedimentation rate in the range 4 to 8mm per year. Rates varied between sites, and were highest in the north-west area of the inner harbour, at 50mm per year, reflecting its protection from wave action.
6. Summary of Sediment Pathways
- This coastline is characterised by a dominant west to east directed littoral drift pathway (drift aligned) operating to the east of Selsey Bill and a less well defined east to west pathway operating along the swash aligned shoreline of Bracklesham Bay.
- The drift pathways have been sustained by sediment inputs from the Kirk Arrow spit and the Inner Owers. Rapid coastal retreat in the past has provided important sources of fresh sediment derived from erosion of the sand and gravel sediments of the Selsey peninsula, but these are now almost completely concealed by widespread coastal defences.
- The sediment budget is dominated by storage and transfer of sediments at the shoreline within a system of dynamic beaches, spits and nearshore banks, especially the Chichester and Pagham ebb tidal deltas. The former is an essentially closed sediment circulation system, and both would appear to characterised by quasi-cyclic oscillations of excesses and deficits of storage that are apparent from alternating phases of adjacent foreshore, beach and spit accretion and erosion. Movement of gravels inshore from relic deposits by a kelp rafting mechanism is thought to be an important means by which fresh gravels accumulate as banks (e.g. Kirk Arrow Spit) in water sufficiently shallow for them to be driven ashore by wave action. Following release from storage, most shoreline sediments are transported eastward or westward out of this coastal area by drift and few long-term sinks are evident (except deposition of fine sediments within the harbours). In consequence, the natural budget of shoreline sediments is negative, although this imbalance has been reduced in recent years by the practice of beach recycling and replenishment. It has also been at least temporarily reversed since 2005 at East Head, West Wittering and Cakeham beaches and Church Norton spit. The reasons for this are complex, and are apparently independent of one another east and west of Selsey Bill.
- Intensive management involving the holding of a largely fixed line of coastal defence for the past 100-150 years has inhibited the natural tendency for landward migration of the shoreline. It has greatly reduced the supply of fresh sediments from coastal retreat and extensive groyne fields have intercepted much of the drift of gravels and coarse sand on the upper beaches.
- Beach management operations throughout this shoreline involving gravel recharge, and re-profiling together with control structures now largely determine rates and volumes of sediment transport and beach stability. Given the low-lying and erodible nature of this shoreline, its modest natural sediment supplies and the potential for sea-level rise and climate change impacts there are uncertainties relating to the sustainability of trying to hold the present defence line in the long term. The creation of the Medmerry realignment along the west facing shoreline of the Selsey peninsula is an acknowledgement of this view.
In a fully natural condition this coastline would provide a wide range of mobile and partly mobile shingle habitats together with extensive sheltered estuarine intertidal areas around Pagham, Sidlesham and Medmerry. However long established practices of coastal defence and reclamation together with a historical trend of natural recession, narrowing and steepening of gravel beaches, has had some negative impacts on habitat survival and development. The key contemporary habitats are vegetated shingle (Pagham spits and intermittently along Bracklesham Bay), sand dunes at East Head and intertidal mudflats and saltmarsh in Pagham Harbour and behind East Head spit. Some coastal grazing marshes exist on the low-lying reclaimed land between Pagham Harbour and Medmerry.
Vegetated shingle along Bracklesham Bay and the eastern side of the Selsey peninsular is potentially threatened by squeeze between fixed residential developments to landward and the natural tendency of the beaches to migrate. Re-profiling and recycling of gravel on Church Norton Spit has the potential to disturb existing vegetation communities and prevent communities from re-establishing on the managed shingle. Details of appropriate management and habitat creation techniques for this resource have been set out by Doody and Randall (2003) who include Pagham spits as a case study in Annex 3 of their report. Mapping of the distribution and characteristics of vegetated shingle has been undertaken by the West Sussex Vegetated Shingle Project (2003). The project has sought to increase general awareness of the local resource; it has provided guidance for contractors working on vegetated shingle (relevant to Church Norton spit and Medmerry Beach) with further guidance produced for residents with shingle gardens (relevant to Bracklesham Bay and East Beach Selsey).
In Pagham Harbour, moderate saltmarsh dieback from 130 ha in 1948 to 97 ha in 1984 appears to have reversed recently with some renewed expansion of marsh along the Sidlesham and Church Norton margins to cover a total 107 ha by 2001. However, the Halimione dominated mid-marsh that fronts many embankments is diminishing and Spartina and Salicornia species are encroaching into areas abandoned by Halimione so that the transition between the lower and mid marsh is migrating landward (Bray and Cottle, 2003). Such a process is indicative of coastal squeeze and suggests that the flood defences around the perimeter of Pagham Harbour are affecting habitat quality. It suggests also that opportunities should be sought for habitat creation.
There are several positive opportunities for the managed set-back or re-alignment for parts of this coastline that are assessed in detail in Posford Duvivier (2001) Bray and Cottle (2003) and Cope et al., (2008). In particular, there are opportunities for the expansion of intertidal habitats in areas of land claim, such as the former channel connecting Pagham Harbour to Medmerry and around the perimeter of Pagham Harbour. Alternatively, there is the more radical option of planning for a major inundation so as to reinstate the full extent of the former 17th Century estuary to create an intertidal area some 4-5 times larger than at present (Bray and Cottle, 2003). Indeed, the potential is such that the area could be considered for mitigation projects arising from the need to compensate for losses in adjoining areas such as the harbours of the eastern Solent. The inundation of Pagham Harbour, in 1910, represents an excellent analogue of unmanaged retreat as a means of expanding subtidal, mudflat and saltmarsh environments (French, 1991; Cundy et al., 2002). Immediately following submergence significant sedimentation occurred and lower saltmarsh regenerated rapidly, possibly due to the presence of a seedbank in reclaimed soils. This was accentuated after about 1925 with the arrival and spread of the fertile hybrid cord grass, Spartina anglica. Zostera spp. also became well established over a large area of harbour mudflats (Geodata Institute, 1994). Based on this example the prospects for successful creation of intertidal habitats would appear to be good, although there are a wide range of other critical issues that would need to be addressed (Posford Duvivier, 2001; Cope et al, 2008). Geodata Institute (1994) addressed the issues of ecosystem response to anthropogenically forced changes to vegetation communities in Pagham Harbour. These would apply, with some modifications, to the deliberate re-creation of new, or substitute, habitats.
The sand dune habitat, of East Head, although not entirely natural, exhibits a regionally significant set of ecological gradients and characteristic communities (Doark et al., 1990). The continuation of sand supply to the foreshore is critical to the maintenance of the present scale and variety of dune forms and ecological diversity. A permanent breach at the 'Hinge' would intercept sand and potentially reduce inputs to the established dunes at the head of the spit causing long-term loss of habitat integrity. The present strategy of strengthening this vulnerable area is subject to monitoring and periodic review. This approach may need to be supplemented by detailed ecological modelling of the impacts arising from the breaching scenarios outlined by HR Wallingford (2000). An alternative possibility is to permit retreat of the West Wittering frontage with the aim of improving the longshore sediment supply to East Head. The conservation values of East Head, and the range of alternative options, are examined in further detail in Posford Duvivier (2001) although further feasibility studies would be required to support some of the more radical options. More detailed mapping and frequent monitoring of critical factors, such as soil chemistry, may be needed before a final choice can be made.
7. Opportunities for Calculation and Testing of Littoral Drift Volumes
Data collected by the Defra-funded National Network of Regional Coastal Monitoring Programmes is pivotal for future improvement in estimating beach change. The Programmes consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping. Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.channelcoast.org.
The Southeast Regional Coastal Monitoring Programme commenced in 2002. The Lead Authority is New Forest District Council, with data collection, analysis and reporting led by specialist teams at the Channel Coastal Observatory (CCO), Canterbury City Council and Adur and Worthing Councils. Longer term Coastal Monitoring Programme data, when combined with other data sets, academic research and historical studies may enable sediment budgets, transport rates and directions to be identified and/or validated in the future.
8. Research and Monitoring Requirements
The Southeast Regional Coastal Monitoring Programme commenced in 2002. Analysis of the data between 2006 and 2012/13, combined with other datasets, academic research and historical studies has enabled sediment budgets, transport rates and directions to be identified or verified. However, at certain sites either due to a lack of long-term data, data coverage or sedimentological information (e.g. composition and proportion of beach grade material arising from cliff erosion), quantification of sediment transport rates of gravel and sand has not been possible.
Notwithstanding results from the Southeast Regional Coastal Monitoring Programme, recommendations for future research and monitoring that might be required to inform management include:
- To understand beach profile changes it is important to have knowledge of the beach sedimentology (grain 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.
- The effective application of numerical modelling studies of beach behaviour and sediment transport processes requires the input of high quality nearshore bathymetric survey data to improve tidal modelling and the representation of the nearshore wave climate. This is especially important for those sectors of the near and offshore environments with complex landform and sediment associations, especially around the harbour inlets and ebb tidal deltas. Repeat swath bathymetry surveys would provide temporal and spatial changes of nearshore sediment types and rates of mobility.
- The recent application of several numerical and conceptual models to the quantification of rates and volumes of longshore sediment transport (e.g. HR Wallingford, 1995; Gifford Associated Consultants, 1997; Posford Duvivier, 2001) has resulted in some significant advances of understanding. Nonetheless, empirical data is relatively scarce, and carefully targeted sediment tracing would be of real value in verifying existing theoretical understanding. This approach would need to select (i) critical locations where transport discontinuities exist; and (ii) shoreface zones where historical and recent analyses of beach shape and volume have created uncertainties over the relevance of past to future trends.
