1. Introduction
The North-East Isle of Wight coast forms the southern margin of the East Solent and Spithead. It is widely recognised that this waterway occupies the axial line of an eastward trending Pleistocene drainage system (the Solent River) which was drowned during the Holocene (Flandrian) transgression (Dyer, 1975, 1980; Anon., 1997; Velegrakis, et al., 1999). Inundation was accompanied by erosion of previously extensive Pleistocene fluvial and niveo-fluvial gravel terraces leaving only remnants, such as Sturbridge Shoal. It is likely that the sands and gravels were reworked and contributed significantly to: (i) palaeo-channel infilling (Lonsdale, 1970; Dyer, 1975; Tomalin, 1991; Long and Scaife, 2003); (ii) the contemporary sediment transport pathways of the Solent eventually being delivered to major sinks such as Brambles Bank, Ryde Sand and the Portsmouth Harbour ebb tidal delta (Bray, Carter and Hooke, 1995). The pattern and diversity of offshore, nearshore and intertidal sediments of this coastline reflects in part this inheritance from the Pleistocene and Holocene evolution of the Solent, developed in more detail in the Section covering the Quaternary History of the Solent System.
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.channelcoastal.org.
Two high resolution, 100% coverage swath bathymetry surveys were commissioned by the Southeast Regional Coastal Monitoring Programme. The nearshore zone of the northern coast of the Isle of Wight, extending 1km offshore from the MLW, was completed in April 2011, and an extensive area extending between Lee-on-the-Solent and Selsey Bill and offshore to abut with the northeast Isle of Wight survey boundary, between Cowes and Bembridge, was completed in July 2013.
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).
Tidal currents are less rapid in the East Solent (generally <1ms-¹) compared to the West Solent (>2ms-¹) so that only sediments up to the grade of medium sand are regularly mobile in the moderate water depths (Dyer, 1980; Webber, 1980; Halcrow, 1996; HR Wallingford, 1992, 1993 1995, 1997). Gravels are only mobile within shallow waters especially close to the shoreline and over inshore banks. Net transport into Spithead and the East Solent is indicated by analysis of bedform asymmetry in Hayling Bay and in the vicinity of the Nab Tower (Lonsdale, 1969; Dyer, 1980). Mineralogical analysis of sediments suggests that material may be transported from the south and east Wight coastal zone into the East Solent, whereupon a proportion is deposited offshore the north-east Wight coast, particularly at Ryde Sands (Lonsdale, 1969; Dyer, 1980, Ball, 1985; Algan et al., 1994; Royal Haskoning, 2010). However, experimental and numerical modelling studies undertaken within the South Coast Seabed Mobility Study (HR Wallingford, 1992 and 1993) suggests that there is net transport of sand eastwards on the bed within Spithead and south eastwards in the vicinity of No Man’s Land Fort. Furthermore, eastwards transport is indicated strongly from Sandown Bay with a zone of deposition identified some 5-10km seaward of Culver cliff and the Foreland. These studies are considered to be of medium to high reliability thus implying that any sediment supply to Ryde Sands from the south-east is confined to the nearshore and offshore zone and is largely wave powered.
Irrespective of the net direction of long term transport further seaward, the large quantity of sediments that have accumulated at Ryde Sands exert a significant influence upon transport at the shoreline. The coast east of Nettlestone Point is open to waves generated in Hayling Bay and diffracted swell waves from the English Channel (Posford Duvivier, 1990b). Wave energy is therefore moderate and from a predominantly east or south-east direction. By contrast, Ryde Sands in combination with the presence of the Isle of Wight provides shelter against these waves for the foreshore to the west so that wave energy is significantly lower and locally generated wind waves from west or northwest are more dominant. Analysis of wind speeds and fetch lengths indicates that a significant wave height of 1.2m is rarely exceeded on any part of the shore to the immediate west of Ryde Sands (Hydraulics Research, 1988; HR Wallingford, 1995).
The Coastal Monitoring Programme measures nearshore waves using a network of Datawell Directional Waverider buoys. Between 2003 and 2012, the buoy deployed at Sandown Bay in 11mCD water depth, confirmed the prevailing wave direction is from the South, with an average 10% significant wave height exceedance of 0.98m. In contrast the buoy deployed off Hayling Island in 10mCD water depth, confirmed the prevailing wave direction is South-by-West, and an average 10% significant wave height exceedance of 1.26m, whilst the buoy deployed at Bracklesham Bay in 10mCD water depth, from 2008 to 2012, showed a prevailing wave direction from Southwest–by-South, and an average 10% significant wave height exceedance of 1.47m (CCO, 2012).
Offshore gradients are relatively gentle and the shoreline is not greatly affected by tidal currents except at the estuarine re-entrants of Wootton Creek and Bembridge Harbour. Tidal flow through narrow entrances to these inlets generates rapid currents which interrupt littoral sediment transport causing local circulation effects and associated fluctuations in patterns of sedimentation at, and seaward of, harbour approaches (Posford Duvivier, 1991b; 1994a; 1999; 2000a; Royal Haskoning, 2010). In common with the other tidal inlets of the Solent their hydraulic regimes are ebb-dominant - that is, the tide falls faster than it rises so that ebb currents are shorter in duration but more rapid than their flood tide counterparts. It results in a tendency for net transport of bedload sediments (medium sands and gravels) out of inlets to become stored in ebb tidal deltas and net input of suspended sediments (fine sands, silt and clay) to infill these estuaries. This process has not been derived directly from studies of the Isle of Wight estuaries themselves, but is inferred based on detailed studies of the mainland harbours that have similar tidal regimes (see for example the unit on Chichester Harbour).
Coastal geology comprises a sequence of gently eastwards dipping interbedded Oligocene clays, silts and limestones of the Osborne Beds, Bembridge Limestone and Bembridge Marls units (Halcrow, 1996). Locally, these solid formations are capped by coarse clastic Pleistocene fluvial and marine deposits (White, 1921; Preece, et al. 1990). The clays are soft and readily degraded; the limestones are more durable, but are permeable and act as minor groundwater reservoirs supplying water to the slopes at their coastal outcrop (White, 1921). Much of the coastal slope is therefore potentially unstable, especially during conditions of basal debris removal. Coastal topography is generally low and rises to a maximum of 30m, forming both active cliffs and relic partially stable degraded coastal slopes.
2. Sediment Inputs
Analysis of Coastal Monitoring Programme 2008 to 2012 lidar, 2003 and 2012 aerial photography and topographic baseline survey data, combined with other datasets, academic research and historical studies has enabled sediment budgets, transport rates and directions to be identified or verified.
2.1 Marine Inputs
Seabed sediment sampling studies within the eastern Solent have revealed a predominantly muddy sea bed between Old Castle Point and Ryde (Lonsdale, 1969; Dyer, 1972). On basis of correlations of sedimentological character, it is inferred that much of this mud has been transported into the Eastern Solent by tidal currents from sources in Bracklesham Bay and South and East Wight (Dyer, 1980; Algan, et al., 1994). As evidence is not conclusive, transport pathways cannot be clearly indicated (HR Wallingford, 1993). A substantial proportion may have been derived from local erosion of clay cliffs and possibly the bed within the Solent as a whole (Posford Duvivier, 1999a & 1999b).
Two high resolution, 100% coverage swath bathymetry surveys were commissioned by the Southeast Regional Coastal Monitoring Programme. The nearshore zone of the northern coast of the Isle of Wight, extending 1km offshore from the MLW, was completed in April 2011, and an extensive area extending between Lee-on-the-Solent and Selsey Bill and offshore to abut with the northeast Isle of Wight survey boundary, between Cowes and Bembridge, was completed in July 2013.
The nearshore substrate along the northeast Isle of Wight coastline can be classified as sediment, with small outcrops of rock discernible in the main channel and closer to shore by Ryde Sands and Forelands. Significant sized stores of fine and coarse grained sediments are located along the coastline, with notable accumulations at Ryde Sands and Ryde Middle Bank. These shoals and banks, combined with the abundance of symmetrical bedforms, support an active offshore to onshore transport (F1) between Foreland and Seaview. In addition, there is evidence of onshore supply of material in Priory Bay.
Investigations of bedforms within the Eastern Solent and Spithead indicate predominantly westward transport of sand immediately seaward of the north-east Wight Shoreline (Lonsdale, 1969; Dyer, 1972). This transport pathway probably supplies a proportion of its sediment to Ryde Sands - a store in excess of 300,000m³ - (probably the coarser elements), whilst the remainder may continue further into the East and Central Solent. Mineralogical analysis of sediments at Ryde Sands has revealed a high proportion of limonite, a mineral characteristic of sands derived from the Lower Greensand (Lonsdale, 1969; Dyer, 1972; 1980). Sandown Bay comprises the closest location of such materials and may thus be one of the source areas. The mechanisms of transport and the precise pathways are uncertain although bedload transport in the shallow nearshore waters driven by dominant south and south-east waves can be reasonably postulated on the basis of other established transport routes and hydraulic gradients. Reliability is considered to be medium for whilst the evidence itself is reasonably convincing, it conflicts partially with more recent numerical modelling studies, as explained below. The South Coast Seabed Mobility Study (HR Wallingford, 1992 and 1993) has suggested that there is net transport of sand eastwards on the bed at Spithead and south eastwards in the vicinity of No Man’s Land Fort. Furthermore, eastwards transport is indicated strongly from Sandown Bay towards a zone of deposition identified some 5-10km seaward of Culver cliff and Foreland. These studies are considered to be of medium to high reliability thus implying that any sediment supply to Ryde Sands from the south-east is confined to the nearshore zone and is largely wave powered.
Two high resolution, 100% coverage swath bathymetry surveys were commissioned by the Southeast Regional Coastal Monitoring Programme. The nearshore zone of the northern coast of the Isle of Wight, extending 1km offshore from the MLW, was completed in April 2011, and an extensive area extending between Lee-on-the-Solent and Selsey Bill and offshore to abut with the northeast Isle of Wight survey boundary, between Cowes and Bembridge, was completed in July 2013.
Divergence of the F1 sediment transport pathway is postulated at the Foreland, with a proportion of material being transported northward into Hayling Bay (Dyer, 1972; 1980). Although this possibility is consistent with the mineralogical and sedimentological character of materials on the seabed of Hayling Bay and with the dominant wave direction, the recent available nearshore bathymetry data does not cover this offshore area, and therefore it is not possible to confirm this northward transport).
Asymmetry of the tidal regime within the East Solent results in ebb flow of relatively shorter duration, but greater velocity than corresponding flood flow (Webber, 1980). This phenomenon favours input of suspended sediments into estuaries and has resulted in significant infilling of the Eastern Yar Estuary (and Bembridge Harbour) by fine sediments (Wallace, 1990; Posford Duvivier, 2000a; Royal Haskoning, 2010). Similar tidal conditions operate at Wootton Creek but infilling is only partial, though it is a continuing process.
Entry of coarse bedload sediments (medium sands to gravels) to inlets in the Solent is generally resisted by the ebb dominance of their tidal regimes. At Bembridge, the formerly extensive estuary of the Eastern Yar is now much reduced by successive stages of reclamation dating back to the seventeenth century (Howard, Moore and Dixon, 1988; Cracknell, 2005). The last major phase of reclamation was in 1874, with the construction of the railway embankment in 1879 limiting ingress of tidal waters. This has significantly diminished the tidal prism, reducing the flushing effect of ebb currents in the outer estuary and further seaward, leaving the large ebb tidal delta as a relic feature. Wave action has therefore become relatively more dominant and has transported sands and gravels from the ebb tidal delta towards the shore and into the flood delta of Bembridge Harbour creating a sediment rich environment (Photo 1). Significant growth of Bembridge Point (which accommodates a small but dynamic and possibly expanding dune accumulation) is attributable to this process in conjunction with littoral drift from Foreland.
Accreting sand and gravel banks within the harbour reflects the fact that only fetch-limited locally generated wind waves can operate, and thus provide further evidence of such input (and an ongoing requirement for dredging to maintain navigable channels) (Howard, Moore and Dixon, 1988; Posford Duvivier, 2000a; Royal Haskoning, 2010). Analysis of Coastal Monitoring Programme aerial photography, lidar, and topographic data indicates accretion of sand with low rates 1-3,000m³ per year of onshore transport; a reduction from the 2004 estimated rates of 3-10,000m³ per year. Sediments analysed from a grid of shallow bore sampling sites revealed 50-80% medium sand and 15-20% gravel, the latter being restricted to lenses within the upper 1-1.5m (Posford Duvivier 2000a).
2.2 Fluvial Input
The streams draining catchments on this part of the Island are small and have very limited capacity to deliver sediments to the shore (Rendel Geotechnics, 1996; Royal Haskoning, 2010).
The eastern Yar flows into Bembridge Harbour and sandy sediments have been recognised at its point of entry (Howard, Moore and Dixon, 1988). These may indicate past, if not contemporary, sediment input, although it can be argued that the agitation by river flow simply prevents sedimentation of finer marine sediments introduced by flood tidal currents. Any sediment input is likely to be limited for river discharge is subject to regulation where it enters the harbour (Howard, Dixon and Moore, 1989).
Fluvially transported sediment entering this inlet has been largely but not totally intercepted since circa 1830 by the dam at Wootton Bridge that impounds The Old Mill Pond. The latter continues to accumulate material that would otherwise provide a small input into estuary infilling.
2.3 Coastal Erosion
Most of this coastline is occupied by either active sea cliffs subject to basal marine erosion and mass movement processes or by a steep or moderately steep coastal slope currently removed from the influence of breaking waves. Both are developed in relatively unresistant sandstones, marls and clays which yield readily to both marine and sub-aerial geomorphological processes (White, 1921; Colenutt, 1891; 1893; 1938; Daley and Insole, 1984; Bird, 1997; Royal Haskoning, 2010). Interbedded limestones outcrop at several localities, notably the Bembridge Limestone. This provides somewhat greater resistance and is responsible for the majority of headlands and offshore reef-like platforms. It breaks down into inter-joint blocks and creates a persistent local boulder apron that partly protects the upper foreshore and cliff toe by dissipating some incoming wave energy.
The rock outcrop pattern is determined by geological structure, in particular a series of shallow folds whose axes are roughly parallel with the north coast but are truncated by the approximately north to south alignment of the east coast (White, 1921; Bird, 1997). Stratal dips tend, overall, to be inland, thus contributing to slope stability. Nonetheless, there are several sites of present, or past, slope failure associated with critical pore water pressures in porous or permeable rocks, particularly where they are underlain by rocks which have less capacity for the storage of groundwater. Because the topography of the north-east Isle of Wight is less elevated than in most other areas of the island, coastal cliffs and slopes are modest in height, nowhere exceeding 35m. This factor also helps to suppress the scale and frequency of slope failure and the dynamics of mass movement. The mature woodland cover of much of the north-facing coastal slope also contributes to stability.
Analysis of Coastal Monitoring Programme 2005 and 2012 lidar and 2013 aerial photography, 2005 to 2012 topographic baseline survey data, combined with other datasets, academic research and historical studies has enabled sediment budgets, transport rates and directions to be identified or verified. The overall pattern, and localised rates, of coastal recession were previously calculated from serial analysis of topographic maps from 1863 to 1975, with updating to 2002 from air photographs, where available (Halcrow, 1996; Posford Haskoning, 2004; Royal Haskoning, 2010). These figures are expressed as mean values covering specific historical periods, and conceal fluctuations in time and space; they are selectively quoted in the following summaries for each of the units that currently contribute sediment to littoral transport processes.
Much of the northwest Wight coast is subject to active erosion, but its morphology varies spatially from simple high-angle cliffs, as at Colwell Bay, to compound slopes with multiple scarps and intervening degradation zones, e.g. Headon Hill (Bird, 1997). This is principally related to the mechanisms of mass movement and slope failure. A coastal landslide can be regarded as a transfer of sediment from an area of elevated topography to the foreshore. Slope instability and a semi-continuous sediment cascade is maintained by basal erosion (e.g. Photo 11) which can act in two ways: (i) degraded materials are removed from the base of the slope, which prevents a stable slope angle being achieved; (ii) basal erosion of in-situ strata can undercut the cliff toe so that the slope is steepened to a greater repose angle than would naturally be maintained by the ground-forming materials. From a coastal viewpoint the result is the same, in that sediment is supplied to the littoral zone, and, assuming it is removed thereafter, the coast retreats.
Overall, the longer-term retreat of this cliffed coastline has widened the West Solent estuarine channel and contributed a substantial input of fine sediment to its tributary estuaries. It is probable that much of the finer grained sediment stored in the West Solent itself comes from the same source. The type and rate of coastal slope retreat is controlled by the geology and hydrogeology of outcropping strata, and antecedent topography (height of the coastal slope), thus promoting slope failure through various slide and slip mechanisms (Hutchinson and Bromhead, 2002). All these factors vary spatially, so rates of retreat and volumes and grades of sediment input are also non-constant. Reports of past coastal erosion and landsliding reveal similar rates of activity and landform development to the present day situation (Norman, 1887; White, 1921; Colenutt, 1938; Moorman, 1939; McInnes, 2008). Thus, it is likely that this coast has retreated throughout much of the late-Holocene period following the establishment of interconnection between the West Solent and Christchurch Bay. Evidence of this is provided by recognition of an ancient landslide deposit, extending up to 100m offshore, from a foreshore lobe of boulders off Brickfield Farm (Munt and Burke, 1987).
Spatial variation in sediment yield from eroding cliffs is also, in part, a function of the contrast in hydraulic regime east and west of Fort Albert. To the east, dominant waves are fetch-limited, whilst westwards the more open coast receives attenuated and refracted swell as well as locally propagated waves. H.R. Wallingford (1999) undertook numerical modelling of modified swell waves for Totland Bay, using HINDWAVE applied to synthetic data. For an annual return period, Hs (mean) was computed to be between 0.22 and 1.71m, depending on wave approach. For a 1 in 10 year frequency values are between 0.33 and 2.05m.
The Coastal Monitoring Programme measures nearshore waves using a network of Datawell Directional Waverider buoys. The nearest measurement stations to this cell are at Milford-on-Sea and Hayling Island. The buoy deployed at Milford-on-Sea is in 10mCD water depth. Between 1996 and 2012 the prevailing wave direction was from the southest-by-south, with an average 10% significant wave height exceedance of 1.31m (CCO, 2012).
Much of this coastline is occupied by a steep, but relatively stable (in places graded) wooded coastal slope lacking active cliffing (Roberts and Jewell, 2000; Hutchinson, 1965; Property Services Agency, 1985). Erosion has been most active at Woodside, where a slope failure plane has been intermittently triggered by loss of toe weight following marine erosion since at least the late nineteenth century (Harlow, 1980). Breaches of the now dilapidated defences at certain sites, e.g. Norris Castle, have recently reactivated old mudslides (Roberts and Jewell, 2000). Recession of MHWM averages between 0.15m per year and 0.40m per year (Posford Duvivier, 1994a), with evidence of some recent acceleration (e.g. some 18m of retreat at Woodside Point, 1975-1995). Posford Duvivier (1999a) calculated a total cliff erosion yield of clay, silt and sand of 2,500m³ per year and shoreface erosion of between 2,600 and 7,800m³ per year for this unit; however, analysis of Coastal Monitoring Programme data indicates the fine-grained cliff-derived sediment is not retained on the foreshore. The lack of detailed study information regarding cliff sediment composition and proportions of sediment grain sizes means that it has not been possible to quantify volumes of cliff input yielding shingle or sand grade beach material; a change from the 2004 estimated rates suggesting 3-10,000m³ per year. Where limestones are eroded, they tend to persist as large inter-joint blocks scattered on the foreshore and suffer loss from both solution and abrasion. Accelerating toe erosion together with future sea-level rise and climate change is likely to cause reactivation of landsliding on some of these slopes. It could result in rapid landward extension of the active backscar by up to 100m, together with major increases in sediment delivery to the shore (Posford Haskoning, 2004).
Erosion is active between Fishbourne and Pelhamfield, where cobbles and boulders littering the upper beach represent the evidence of recession, but eastwards the coastal slope has either been incorporated into the built environment or fails to make any marked feature. Small-scale rotational sliding and cliff toppling is currently active at Fishbourne and in front of the Quarr Abbey estate (Photo 2). This has long been a locally active cliff line, as reported by Colenutt (1938) and deduced by archaeological excavation of the adjacent foreshore palaeolandscape (Tomalin, 1991; 1993; Long and Scaife, 2003). Toe erosion of the relic coastal slope and some reactivating slips are apparent eastwards to Binstead behind defences (Photo 4 and Photo 5). Indeed, landslip debris obscuring the cliffline at Ryde, exposed until about 1870, is described by Reid and Strahan (1889). A mean recession rate of 0.05m per year, between 1909-1975, indicated low potential for supply to the shore, although there is evidence of an acceleration, to 0.71m per year, over the period 1975-1995. Posford Duvivier (1999a) proposed a cliff erosion sediment yield of approximately 2,000m³ per year and a shoreface erosion of 9,750m³ per year. However, analysis of Coastal Monitoring Programme data indicates the cliff-derived fine sands, silts and clay sediment is not retained on the foreshore, being removed from the beach as suspended load by waves and currents. The lack of detailed study information regarding cliff sediment composition and proportions of sediment grain sizes means that it has not been possible to quantify volumes of cliff input yielding shingle or sand grade beach material; a change from the 2004 estimated rates suggesting 3-10,000m³ per year.
Coastal slope instability has occurred in the southern part of Seagrove Bay (Hutchinson, 1965; Posford Duvivier, 1998; Royal Haskoning, 2005; Winfield, et al., 2007), where a multiple rotational failure occurred in the mid-twentieth century and more recent reactivation in 2002/3. With the installation of new and improved defences this slope is, at least temporarily, stable. However, softening of the Bembridge Marl [Fishbourne Beds], which accommodates the failure plane, overlying Bembridge Limestone in combination with elevated ground water levels could create further reactivation. Foreshore steepening, progressive beach drawdown and approximately 20m of retreat of MLWS since about 1910, indicated the probability of diminished sediment supply at least partly as a result of defence building. The growth of a nearshore shore-parallel sand bar is likely to be an additional factor in this respect (Sarker, et al., 2007; Winfield, et al., 2007). The progressive deterioration of the seawall between the early 1950s and late 1990s probably induced a small increase in the supply of both limestone clasts and silt/silty sands. In 2002/3 modest beach replenishment was undertaken to obviate a renewed cycle of slope instability due to anticipated deterioration of defence standards. Additional beach recharge, two protective offshore breakwaters and a rock spur to limit losses from littoral drift were suggested for this embayment (Sarker, et al., 2007; Winfield, et al., 2007).
The lack of detailed study information regarding cliff sediment composition and proportions of sediment grain sizes means that it has not been possible to quantify volumes of cliff input yielding shingle or sand grade beach material; a change from the 2004 estimated rates suggesting 3-10,000m³ per year.
The partly unprotected cliffs and shore platform are subject to active erosion, but a significant source of loss of potential feed to the littoral drift pathways may result from on- to offshore transport of fines. Nodes Point is composed of limestone, which breaks down by solution as well as abrasion. An approximate, calculation of coastal recession of 0.3 to 0.5m per year throughout most of the twentieth century would appear to relate to cliff top rather than cliff toe retreat. The presence of privately-constructed defences dating back to the 1930s, although now largely ineffective, have in the past inhibited toe erosion and cliff sediment inputs to the shore. Some sands and limestones would be yielded from renewed cliff toe erosion, although the majority of supply would be clays. Posford Duvivier (1999a) give a figure of between 13,000 and 38,000m³ per year of mostly fine grades for the shoreface erosion of the Pelhamfield to Bembridge frontage as a whole. A supply of coarse materials is available intermittently in the cliffs comprising a thickness of up to 5m of Pleistocene fluviatile gravels at the top of the succession (Samson, 1976). Analysis of Coastal Monitoring Programme data indicates the cliff-derived fine-grained sediment is not retained on the foreshore, being removed from the beach as suspended load by waves and currents. The lack of detailed study information regarding cliff sediment composition and proportions of sediment grain sizes means that it has not been possible to quantify volumes of cliff input yielding shingle or sand grade beach material.
Erosion of the low cliffs of this frontage provides an important source of beach shingle because the cliffs are capped by Pleistocene raised beach and fluvial deposits attaining maximum thickness of 10m and containing rounded flint pebbles (Posford Duvivier, 1990). Sands and clays are supplied from the matrix of the raised beach deposit which overlies the predominantly clay Bembridge Marls. Bembridge Limestone outcrops on the foreshore forming a series of ledges that provide protection to the cliffs against wave attack at low water (Photo 6). Erosion rates are between 0.30m per year and 0.75m per year and vary spatially depending on the presence of coast protection structures and shelter afforded by the presence of a wide, terraced or stepped inter-tidal shore platform (Posford Duvivier, 1983; 1989; 1993b; Halcrow, 1996; Posford Haskoning, 2004; Royal Haskoning, 2010). A retreat rate of 0.3m per year to 0.4m per year is reported for the Warner’s Holidays frontage and 0.5m per year for the coast north of Foreland (Barrett, 1985). The 15m high cliffs between Bembridge Point and Tyne Hall exhibit relatively recent reactivations of a relic wooded cliffline. They evolve by a simple landsliding process in which failures of the backscar result in extension of debris accumulations across the upper beach. The extreme western part of the frontage at Bembridge Point is undergoing net accretion, but erosion at up to 0.15m per year is recorded for the coast eastwards to Tyne Hall (Posford Duvivier, 1983; 1985; 1990). A shoreface sediment yield of 12,500m³ per year for the sector between The Foreland and Ethel Point is proposed (Posford Duvivier, 1999a), but offshore loss of clay and fine sand and cliffs stabilised by coast protection probably significantly reduces the actual input to the beach. It has also been suggested that some 5.0mm per year depth of shoreface erosion is achieved along this coastline, some 2 or 3 times the rate experienced to the north. Analysis of Coastal Monitoring Programme data indicates less than 1,000m³ per year of cliff input yielding shingle or sand grade beach material. This is a reduction from the 2004 estimated rates of more than 20,000m³ per year at Bembridge and 10-20,000m³ per year at Foreland.
The cliffs (approximately 100m high), cut into relatively non-resistant Eocene and Oligocene sands, clays and limestones, are unprotected along most of this frontage. The same varied geological sequences are exposed within Whitecliff Bay as at Alum Bay on the west coast of the Isle of Wight. They are subject to failure creating complex landslide morphologies of scarps and degradation terraces with mudslides developed in the Reading and London Clay (Thames Group) strata in the south of the bay (Photo 7). The small lengths of informal defences in Whitecliff Bay (Posford Duvivier, 1997) are of marginal significance in restraining sediment yield. Analysis of Coastal Monitoring Programme data indicates the cliff-derived fine-grained sediment is not retained on the foreshore, being removed from the beach as suspended load by waves and currents. The lack of detailed study information regarding cliff sediment composition and proportions of sediment grain sizes means that it has not been possible to quantify volumes of cliff input yielding shingle or sand grade beach material. Much of the clay and silt sized sediment mobilised by periodic slope failures and other mass movement processes is probably transferred offshore in suspension. The sand fraction contributes to the wide inter-tidal zone between Culver Cliff and Long Ledge. In the northern part of this unit, a set of curvilinear limestone ledges forms a nearshore-offshore reef, thus inhibiting erosion of the adjacent cliffs. The prominent, oversteepened, chalk cliffs of Culver Down standing at angles in excess of 70 fronted by a boulder-strewn platform form a distinctive, but eroding, southern boundary. This is evidenced by recent falls, bedding plane toppling failures, fresh notching and cave formation on the east facing slope. The northern slope is superficially mantled by colluvial debris subject to mass movement. Historical cliff top recession of some 0.3 to 0.5m per year in Whitecliff Bay (1909-1975) contrasts with a rate of 0.10 to 0.15m per year north of Black Rock.
3. Littoral Transport
Analysis of Coastal Monitoring Programme 2008 to 2012 lidar, 2003 and 2012 aerial photography and topographic baseline survey data, has been combined with other datasets, academic research and historical studies to review and revise sediment budgets, transport rates and directions. Two major drift pathways that converge upon Ryde Sands are identified. Transport is predominantly from west to east at low rates along the Eastern Solent shore powered by waves generated locally in the Solent by prevailing westerly winds. By contrast, the east coast is exposed to wind waves from the south east and diffracted waves from the south and south west that power a more significant net north or north-westward drift. These paths are interrupted to varying extents by several minor headlands and inlets. Some local drift reversals are identified at inlets resulting in zones of divergent transport that are potentially susceptible to beach erosion.
Marked accretion on the eastern side of Cowes breakwater since its construction in 1936/37 indicates a long-term trend for net westward littoral drift from Old Castle Point (Webber, 1981; Posford Duvivier, 1994b; Carter, 1996). Sand and shingle have accumulated on the upper foreshore, with mud on the lower, indicating that all grades of sediment are transported preferentially in the same direction. Analysis of Coastal Monitoring Programme indicates less than 1,000m³ per year of beach material is predominantly transported westwards, although pockets of west to east transport may be a seasonal rather than a long-term trend.
Coastal Monitoring Programme data supports net southeastward littoral drift between Old Castle Point and King’s Quay, although rates of transport along this narrow beach are unquantified, with minimal change in beach levels Twin spits composed of sand and gravel have developed at the entrance to King’s Quay, and have migrated landwards during 2008 and 2012; their orientation is indicative of transport both eastwards and westwards into the entrance (Photo 8). The eastern (westward trending) spit at King’s Quay therefore suggests a very local drift reversal, possibly associated with tidal current and wave interactions at the inlet.
A general net eastward littoral drift is indicated along this unit by: (a) an eastward trending spit that has been pushed into the inlet to form Wootton Hard (the northern spit at the entrance of Wootton Creek); (b) set-back of the east shore compared to the west at Wootton (Photo 9), and (c) widening of the foreshore towards Ryde in the presumed direction of drift (Harlow, 1980; Halcrow, 1996). Comparison with studies of adjacent beaches on the mainland indicate slow drift at less than 1,000m³ per year (Harlow, 1980). Sediment transport is interrupted by Wootton Creek inlet where transport pathways have been identified on the basis of morphological evidence (Harlow, 1980; Robert West and Partners, 1990; Posford Haskoning, 2004).
- An unquantified but small proportion of eastward moving sediment (especially gravels) is diverted into the inlet by littoral drift along the western shore, which supplies the shingle spit of Wootton Hard that has now migrated well into the inlet (Photo 9).
- Coastal Monitoring Programme data supports net eastward transport of the majority of sediment from the western shore and crosses the tidal channel to be driven ashore 400m north of the ferry terminal. At this point transport divides: some of this material is transported south further into the inlet and accumulates against the ferry terminal (Hydraulics Research, 1988; Robert West and Partners, 1990; Bray, 2003). A proportion of this sediment is likely to be deposited within the navigable channel, which is dredged periodically. The majority of material is thought to be transported eastward along the wide foreshore and a series of barrier-like banks on the lower foreshore towards Ryde Sand. A westward trending spit is discernible on the eastern shore of Wootton Creek some 500m up the estuary (Photo 9). It would appear to have migrated to this position having been fed by a local westward-directed drift pathway from the western Quarr frontage. A transient drift divergence boundary may therefore once have operated on the Quarr frontage although recent erosion of this spit (Posford Duvivier, 1994) suggests that the pathway may no longer function as effectively as previously.
Coastal Monitoring Programme data supports net westward drift towards Ryde Sands, attributed to dominant waves from the east and southeast and to diffracted southerly and south-westerly waves from the English Channel, and refracting as depths shallow, as indicated with accretion on the eastern sides of groynes and outfalls at Spring Vale that interrupt the predominantly sandy beaches along this frontage (Photo 10) (Posford Duvivier, 1990b; 2000b; 2000c). Ryde is also exposed to such waves and beach accumulation occurs preferentially on the eastern side of Ryde Marina and other shore structures that intercept transport. This vector is in the opposite direction to that of LT3 so that drift therefore converges at Ryde and the resultant predominantly sandy accumulation, Ryde Sands (Withers, 1979; Dyer, 1980; Gibson and Bone, 1987) represents a sediment sink (Harlow, 1980). Map comparisons undertaken for the design of Ryde Harbour in the early 1990s revealed relatively stable conditions (Gifford and Partners, 1990). Net transport may not be great, as no significant siltation of the dredged channel giving access to Ryde Harbour has been reported. The 700m long pier crossing Ryde Sands to link a ferry terminal to the shore, and the neighbouring Hovercraft operating across the sand flats, are a testament to the scale and extent of the intertidal sediment accumulation at Ryde Sands.
The gravel upper beach of Priory and Seagrove Bays terminate at rock headlands and appear to function as isolated closed–system pocket beaches, although there may be by-passing around Horestone Point of sands and fine-grained fraction forming the extensive shallow barred nearshore and inter-tidal zone. Analysis of Coastal Monitoring Programme data indicates net north-westward longshore drift in Seagrove Bay, and accreting beaches, although the localised weak reversal in the southern part of the bay observed by Winfield, et al. (2007) was not evident. A well-developed nearshore bar in Seagrove Bay is considered to inhibit the supply of sand to the littoral transport pathway, with material preferentially moving offshore (Winfield, et al., 2007). Thus, transport within these areas is probably effective in enabling supply of sands north westward around Nettlestone Point. Both have exhibited progressive drawdown of their beach sediment stores in recent years (Posford Duvivier, 1998; Sarker, et al., 2007). Sediment transport in the nearshore zone, derived from assumed wave climate parameters may be in the order of 9,000m³ per year (Posford Duvivier, 2000a). Interpretation of nearshore bathymetry data provides evidence of onshore supply of material in Priory Bay.
Coastal Monitoring Programme data confirms the continuation of sediment accumulation against the northern sides of groynes on St Helens Duver indicating southward drift (Posford Duvivier, 1988, 1991, and 1996). The south trending alignment of the Duver, a now stabilised sand dune covered spit, suggests that the feature developed during long-term north to south drift. A map dated 1791 showed the Duver spit in more or less its present position and indicates that north to south drift was dominant before this time (Shepard, 1970; Posford Duvivier, 1991, 1996). The present day spit is confined by a sea wall and its stored sediments are no longer available to nourish the foreshore. Beach sediments drift to the southern tip of the spit where they are intercepted by tidal currents within the Bembridge Harbour entrance and flushed offshore by dominant ebb currents (Photo 1). In the past, these sediments were possibly driven back onshore near St Helen’s Church, but dredging of Bembridge Harbour approaches (e.g. 200,000m³ between 1987 and 1989/90) may have interrupted this circulation (Posford Duvivier, 2000a). Beach levels have fallen significantly along the Duver and coast protection measures are aimed at reducing littoral drift so as to minimise further beach losses to the tidal channel. The Duver is particularly vulnerable because a very short drift pathway extending south from a littoral drift divide near Nodes Point supplies its sediments. Fresh sediment sources are therefore limited to local coastal erosion and onshore feed, although further research is required to evaluate the relative importance of each (Posford Duvivier, 2000a; Royal Haskoning, 2010), and quantify rates. Posford Duvivier (2000a) estimated that the total longshore drift flux, passing from Bembridge Point across the harbour approaches towards Priory Bay, may be in the order of 80,000m³ per year. If dredging records and hydrographic charts are acceptable approximations of the local sediment budget, up to 60,000m³ per year may be diverted into bank and channel storage (Posford Duvivier, 2000a; Posford Haskoning, 2004).
Dominant north-westwards littoral drift from Foreland to Bembridge Point is indicated by sediment accumulations on the east side of groynes and outfalls (Barrett, 1985; Posford Duvivier, 1989; 1990a; 1993a; 1993b; Posford Haskoning, 2004). Sediment supplied by local coast erosion is transported along this pathway and deposited at Bembridge Point (Photo 1). Map comparisons covering the period 1862 to 1970 revealed seaward advance here of MHWM by up to 60m, despite periodic shingle removal under licence (Posford Duvivier, 1989; 1990a; 1995b).
Potential longshore drift driven by breaking waves has been calculated at 14,000m³ per year at Bembridge Point and 90,000m³ per year at Colonel’s Hard. Most of this would represent fine sands transported on the shoreface with only a proportion comprising beach drift sensu stricto (Posford Duvivier, 2000a; Posford Haskoning, 2004).
A mean rate of coastal recession of between 0.25 and 0.33m per year for the period 1866-1975 has been suggested (Posford Duvivier, 1983; 1989) for the frontage between Foreland Fields and the lifeboat station. Erosion releases coarse clastic material from the Ipswichian raised beach, thus providing a supply of shingle. Small quantities of shingle from occasional beach replenishments have also supplemented the supply since the late 1970s. A littoral drift rate in excess of 20,000m³ per year may operate over limited periods of time when higher energy waves approaching from the east are operative (Posford Duvivier, 1989). Fine sand and silt is probably transported offshore and bypasses the Foreland. Small quantities of shingle from beach replenishments serve to slightly exaggerate quantitative estimations of drift volumes.
Coastal Monitoring Programme data confirms the northeastward littoral drift between Culver and Foreland, with rates of movement in the order of 1,000m³ per year. The wide sub-tidal extent of the beach suggests cross-shore transport may be active. Cliff recession mostly yields sands and silts, with a potentially significant proportion moved offshore in suspension, although unquantified. The backshore coarse clastic sediment store was severely depleted by removal for aggregate in the early part of the twentieth century (Colenutt and Hooley, 1919), and may not have recovered subsequently, at least in southern and central Whitecliff Bay.
4. Sediment Outputs
4.1 Offshore Transport
Two high resolution, 100% coverage swath bathymetry surveys were commissioned by the Southeast Regional Coastal Monitoring Programme. The nearshore zone of the northern coast of the Isle of Wight, extending 1km offshore from the MLW, was completed in April 2011, and an extensive area extending between Lee-on-the-Solent and Selsey Bill and offshore to abut with the northeast Isle of Wight survey boundary, between Cowes and Bembridge, was completed in July 2013.
The nearshore substrate along the northeast Isle of Wight coastline can be classified as sediment, with small outcrops of rock discernible in the main channel and closer to shore by Ryde Sands and Forelands. Significant sized stores of fine and coarse grained sediments are located along the coastline, with notable accumulations at Ryde Sands and Ryde Middle Bank. Further north and offshore between Ryde and Ryde Middle Bank, the size and abundance of symmetrical bedforms indicates a dynamic sediment transport environment (O2).
Sediment transport within the channels of the Eastern Solent is complex being influenced primarily by the morphology of the main channels, the tidal flow patterns and the availability of seabed sediments (refer to unit on the East and Central Solent for greater detail). Although tidal currents determine the strength and direction of transport, wave action is nevertheless important in mobilising the sea-bed sediments. For this reason it might be expected that bed sediments should be increasingly mobile towards the mainland rather than close to the Isle of Wight shores, which are comparatively more sheltered. The major study of sediment transport in this region is based on a detailed numerical modelling approach that includes the effects of waves and tidal currents and involved application of three alternative sediment transport equations for computations of potential transport rates (HR Wallingford, 1995). It did not however include assessments of the transport occurring around the nearshore and offshore zones of the Isle of Wight, but instead focused on the mainland coast and main Solent channels. The main results were to identify a net long term eastward transport of sand and some gravel south eastwards out of the Eastern Solent towards the Nab Tower. An anticlockwise rotation of sand around the Brambles Bank was also identified. The implications of these findings for the NE Wight were not assessed. As these studies specifically focus on the mainland shores they shall be discussed further in those sections.
Some pathways operating parallel to the shore are identified from earlier studies of bedforms and bed sediments. Although their reliability might be considered uncertain in light of the results of the HR Wallingford (1995) studies, it should be remembered that the latter focused on the mainland and not on the north-east Wight side of the main East Solent channels where there is a comparative information deficit. The following pathways are extracted from the units covering the mainland shores and the East/Central Solent, in each of which there is some reliance on inference rather than direct proof:
Sediment transport in the main channel was determined from analysis of bedforms identified by a series of echo-sounding traverses across the East Solent (Lonsdale, 1969). This information was coupled with details of sediment size, grading and sorting determined by a coordinated programme of sediment sampling involving 110 bottom samples taken using a Shipek grab (Lonsdale, 1969). Sediment grading and to a lesser extent sorting were found to be strongly related to the direction and strength of tidal currents so that sediment transport paths followed the main tidal channels. The main channel in the East Solent is floored by silt and muds with increasing proportion of sand towards the margins where sandwaves were present. Asymmetry of these bedforms indicated east to west sediment transport with divergence of flow around the Ryde Middle Bank, a Tertiary remnant with relatively thick sediment cover (Lonsdale, 1969). Sediments transported were assumed to be primarily muds and silts in the centre of the channel and sands on the margins. This direction partly conflicts with that established by HR Wallingford (1995), although it may suggest that there might be a net transport divergence in the East Solent at Ryde Middle Bank. Sediments east of the bank would tend to move south eastwards and those west of the bank would move north westwards away from this feature. Surveys by Lonsdale (1969) terminated east of Brambles Bank, a major sand accumulation and probable sediment sink for the main channel transport pathway. Studies by Dyer (1980) indicated anticlockwise sediment circulation on this bank, a finding that was confirmed by HR Wallingford’s (1995) numerical modelling studies. Sands in the eastern part of the transport pathway around Sturbridge Shoal were particularly rich in limonite, a mineral that becomes much less concentrated in sediments further west at Ryde Middle Bank (Lonsdale, 1969), but is abundant in Sandown Bay (off the south-east Isle of Wight coast) where similar medium sized, well sorted, sands are derived from erosion of Lower Greensand cliffs and seabed outcrops.
Studies based on bedform and sediment analysis indicated a transport divide offshore from Osborne Bay (Dyer, 1980). The dominant rates of transport and grades of material either described or inferred by Royal Haskoning, (2010) have low quantitative reliability, but suggest a weak transport divergence.
4.2 Estuarine Outputs
Throughout the Eastern Solent the ebb tidal flow is of shorter duration than the corresponding flood flow (Webber, 1980; HR Wallingford, 1995). As a result, ebb currents are of greater velocity than the flood causing net offshore transport of coarse bedload sediments at the mouths of estuaries and inlets.
Examination of the literature has failed to reveal any indication of significant offshore sediment loss either at the foreshore or from inlets. At Wootton Creek, dominant ebb tidal currents approaching 1.0ms-¹ were measured in the main tidal channel and distal ends of spits (0.8ms-¹) adjoining the channel (Hydraulics Research, 1980; 1988). These currents are sufficient to entrain sand and fine gravel, but offshore transport is limited because currents diminish rapidly seaward of the creek entrance (Hydraulics Research, 1980; 1988). Sediments are believed to be deposited within the channel whereupon they are driven onshore by wave action or possibly removed by dredging. Entry of ferries into the creek causes a notable surge effect comprising an elevation and subsequent lowering of water levels by up to 0.30m during low tides (Harlow, 1980; Robert West and Partners, 1990; Bray, 2003). Field investigations revealed that the down slope velocities produced by these surges are sufficient for erosion of both the soft and consolidated muds sampled in the field. Similar tests were conducted to evaluate the erosive power of wind waves and tidal currents but it was concluded that during low tide only ferry surges could generate sufficient shear stress to cause erosion of intertidal mud banks. Lowering of mud banks is well documented within the creek and has apparently occurred since 1938 (Hydraulics Research, 1988). With the introduction of larger ferries in 1982 (Photo 3), cross sections of the channel were measured at frequent intervals and reveal an increase of mud bank erosion, a change directly attributable to ferry surge erosion (Robert West and Partners, 1990). The fate of sediments released by this effect is uncertain because siltation has not increased in the main channel (Hydraulics Research, 1988) and offshore output is unlikely due to the hydraulic regime which favours suspended sediment input (Bray, 2003).
At Bembridge Harbour, offshore surveys involving boreholes and sediment sampling have revealed a substantial accumulation of at least 1.5 million m³ of sand and gravel lying off the entrance (Grontmij, 1972; Posford Duvivier, 2000a). It can be hypothesised that the accumulation is the relic ebb tidal delta comprising material flushed offshore by dominant ebb tidal flow prior to reclamation when the harbour had a larger tidal prism. Since reclamation, this process has ceased and the harbour approach channel has consistently silted up and requires semi-continuous maintenance dredging (Posford Duvivier, 2000a). It can be concluded that ebb currents are now reduced in velocity and transport sediment a shorter distance offshore, where it may be intercepted by dredging or driven back onshore by wave action. Net offshore loss is therefore unlikely (Royal Haskoning, 2010).
There is some evidence of net offshore transport at this location. This is discussed further in the unit covering north-west Isle of Wight.
4.3 Dredging
Dredging comprises the major known sediment output from the north-east Wight coast and is practised for navigational purposes at Wootton Creek and Bembridge Harbour. The approach and entrance channel in Wootton Creek was widened and deepened in 1982 for the introduction of new ferries, and further widening and deepening was undertaken in 1989 and 1993. Maintenance dredging is not required as tidal flow, coupled with regular ferry passage, effectively prevents siltation (Robert West and Partners, 1990). Volumetric information on material removed is not available as a complete and reliable record but dredging may have several effects:
- Widening and deepening of the channel may prevent sediment bypassing the inlet, so the eastern estuary margin and the Quarr Abbey frontage could become depleted (Harlow, 1980). Increasing erosion has been reported in these areas over the past 20 years (Harlow, 1980; Robert West and Partners, 1990).
- Slumping or sliding of sediment into the dredged channel is possible, causing lowering of the intertidal zone. This lowering process was also evident in the 1980s (Robert West and Partners, 1990).
- Removal of sediments by dredging may lead to sediment starvation and reduction of potential for rebuilding mud banks eroded by the ferry surge phenomenon (Robert West and Partners, 1990).
- Dredging results in increased water depths in the channel approaches, which may increase wave penetration and thus transport more sediment further into the inlet (Robert West and Partners, 1990). Increased wave energy at the shoreline could also cause increased erosion (Harlow, 1980) east and west of the inlet.
Dredging at Bembridge has been necessary to maintain a navigable approach channel subsequent to the major reclamation of the harbour completed in 1874. It seems probable that the subsequent change from a straight to a sinuous channel, and the pattern of siltation results from reduction of the effectiveness of the flushing effect of dominant ebb currents due to diminution of the tidal prism (Grontmij, 1972; Posford Duvivier, 2000a; Environment Agency and Isle of Wight Council, 2010; Royal Haskoning, 2010).
Comparison of charts for 1945 and 1967 revealed accretion of up to 1.2m in the approaches to the harbour, 2.5m offshore Priory Bay and up to 2m in Bembridge Harbour (Grontmij, 1972, Alluvial Mining Co. Ltd, 1981). Since 1967, dredging has periodically removed sediment from the harbour approaches, the navigable approach channel at the harbour mouth, a channel within the harbour and a basin at St Helens. Dredged totals are uncertain, but at least 8,000m³ of mud was dredged from the St Helens Basin in the late 1960s (Grontmij, 1972) and 200,000m³ sand and gravel was extracted from the approaches in both 1987 and 1989/90. Between 1990 and 2000 an estimated 130,000m³ of sand and gravel has accumulated in this same area (Posford Duvivier, 2000a). Harbour hydrodynamics are likely to be quite stable due to limited tidal flow and negligible wave fetch. Outside the harbour sediments are more mobile (Grontmij, 1972; Posford Duvivier, 2000a; Posford Haskoning, 2004) with evidence from hydrographic charts of a tendency for the harbour approach channel to rotate in a north-westwards direction, away from the entrance. Drying banks have also shifted progressively north-westwards and westwards since at least the 1890s, whilst the significant amount of accretion on the eastern side of the main approach channel is consistent with by-passing of the littoral drift pathway. Dredging may have had several potential effects:
- Drawdown of sediments from the beach fronting the Duver.
- Interception of sediment which could potentially supply beaches on the Duver.
- Increased water depths could allow larger waves to reach the shore, thereby causing increased nearshore erosion.
- Alterations to bathymetry could cause variations in wave refraction leading to changes of sediment transport.
- Widening of the navigation channel may reduce tidal stream velocities.
These possible effects require further research so as to determine the potential contribution of dredging to shoreline changes, particularly in view of any proposals to realign the main channel and transfer excess sediment to the north-westwards moving littoral drift pathway.
5. Summary of Sediment Pathways and Budget
5.1 A major but as yet unquantified sediment feed to Ryde Sands from the south east, possibly originating from sources in Sandown Bay has been recognised. The exact pathway is uncertain, but it would appear to be an approximately shore-parallel bedload transport process operating in the shallow nearshore waters. Minor marine derived suspended sediment inputs to inlets are also characteristic.
5.2 Coast erosion is the only other significant sediment input as fluvial inputs are extremely small. Compared to the north-west and south Wight coasts, coast erosion is a relatively less active process due to geological and hydraulic factors. It should also be recognised that erosion rates have not been monitored systematically, so the importance of this process as a sediment supply is difficult to quantify precisely. Yield figures remain estimates based on a range of assumptions. With the exception of raised beach deposits at Bembridge, the local geological materials exposed by the cliffs yield mostly fine sediments as they erode and tend to contribute to the suspended sediment load of the Solent rather than to accretion on local beaches. Excluding Ryde Sand, much of the coast has few other sources of supply but local coast erosion is the most likely significant sediment input.
5.3 Two major littoral drift pathways are identified, namely a predominantly eastward drift from Old Castle Point to Ryde and a net north-westward drift from the Foreland to Ryde. These result from variations in wave exposure and shelter from diffracted south-east and south-west waves approaching from the English Channel. At the mouths of each of the main inlets littoral drift is generally in opposite directions converging upon the entrances. Local drift reversals have therefore developed at East Cowes, King’s Quay and the entrance to Bembridge Harbour; a small scale but complex partial sediment bypassing and circulation system operates at the mouth of Wootton Creek.
5.4 Ryde Sands has developed at the convergence of these sediment transport pathways and can be regarded as a sediment sink. Significant convergence of drift also occurs at Bembridge Harbour and a major sediment accumulation close offshore suggests that this may also be a sediment sink. Accumulations of this type should not be regarded as immobile relics for they may display significant short term mobility and may be involved in sediment exchange with nearby beaches.
5.5 Littoral drift divergences are recognised in the vicinity of Old Castle Point, Nodes Point and on the Fishbourne shore of Wootton Creek entrance. These locations are susceptible to sediment starvation and are particularly sensitive to variations in sediment supply.
5.6 Offshore sediment transfers have not been quantified, although it is probable that significant quantities of sediment were flushed offshore and stored within an enlarged ebb tidal delta at Bembridge Harbour prior to reclamation of the Yar estuary. The extent to which this material has been returned to the shore by wave action following reclamation is uncertain.
5.7 Major sediment output is effected by dredging the navigation and approach channels at Wootton Creek and Bembridge Harbour. This practice not only constitutes permanent sediment loss, but may also intercept transport pathways resulting in downdrift erosion. Drawdown of beach materials may also occur into depressions close inshore, and increased depths produced by dredging may increase wave exposure at the shore. Although these factors are relevant to operations at Wootton Creek and Bembridge no conclusive research has been achieved to assess the effects of dredging on sediment circulation and beach erosion and local sediment budgets.
5.8 The contribution of waves and surges produced by ferries to foreshore and mud bank erosion at Wootton Creek has been clearly established by a series of studies incorporating a programme of detailed field measurements. Between 20% and 50% of upper foreshore erosion was attributed to ferry waves and virtually all recent mud bank erosion was considered to be the cause of ferry generated surges.
5.9 There is insufficient quantitative data on sediment inputs, storage and output to calculate formal sediment budgets for either the north-or east-facing sectors of this coastline. Data on sediment yields from cliff erosion is possibly the most definitive element, but even that is based on significant generalisations and assumptions such that its results are insufficiently site-specific. The few available calculations of rates and volumes of littoral drift are largely unchecked and uncalibrated due to lack of beach monitoring data. In general terms, the budget is negative (net loss) on most frontages excepting the inlets, Ryde Sand, Bembridge Point and possibly the Bembridge ebb tidal delta where net accretion may currently be dominant.
6. 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. Although at present a 12 year time series of data has been collected, 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.
The variation over short distances in coastal orientation and wave exposure together with the uncertain role of tidal currents in the transport process makes much of the Solent a difficult coast for the calculation and testing of littoral drift volumes (Bray et al., 2000). This situation is complicated further by the prevalence of headlands and pocket beaches that such beach drift systems are characterised by intermittent or periodic bypassing of headlands, possibly by occurrence of drift in the shallow nearshore waters.
7. 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, and the summarised information collated in the Isle of Wight SMP2 (Haskoning, 2010), recommendations for future research and monitoring that might be required to inform management include:
7.1 Ryde Sands, Bembridge Harbour (and ebb tidal delta) and Wootton Creek are significant sediment sinks in the context of both the local, and Solent-wide, sediment budgets. Whilst some quantitative data on rates of accumulation of sediment in outer Bembridge Harbour is available, there is little reliable information on their dynamics and stability. The provenance of sediments that constitute each of these sinks is uncertain, and it is possible that a proportion of retained sediment is mobilised under extreme hydrodynamic conditions. In that sense Bembridge ebb tidal delta and Wootton Creek, in particular, may function as stores rather than sinks, or traps. Long distance supply pathways originating in Sandown and Hayling Bays remain conjectural.
7.2 The efficacy of tidal currents acting in combination with shoaling or breaking waves in the entrainment, mobilisation and transport of sediment of varying size-ranges is poorly understood, particularly at the entrances of both the larger estuaries and smaller tidal inlets. It is very probable that nearshore tidally-induced transport has been underestimated; enhanced understanding of nearshore morphology may prove critical to resolution of the inferred process of sediment bypassing of both headlands and inlets.
7.3 Littoral drift pathways for several locations have not been experimentally proven, and currently rely on long term observations of sediment accumulations against various artificial and natural barriers. With a few exceptions, there is no reliable and representative data on the rates and volumes of littoral transport. Available figures are based on approximate calculations that use short-term data and assumptions on the width of the active intertidal "envelope"; the continuous availability of transportable sediment and historical trends of coastline change.
7.4 Available data on sediment yield from cliff erosion contains some inconsistencies currently contradictory, and ideally requires more controlled, site-specific analyses and breakdowns of distinct frontages. This deficiency partly reflects the lack of research into, and monitoring of, processes of cliff degradation along most parts of this frontage. The evidence of significant recent acceleration of coastline recession at a number of locations should be a focus of research into its causes and of the fate of sediment thus released. It is especially significant as it may be an early indication of the types of response that could become more widespread with future climate change.
7.5 Beach depletion and drawdown has been reported from several locations, notably those which appear to be semi-confined or otherwise isolated from littoral feed, e.g. Priory, Seagrove and Whitecliff Bays. Research into both local and general causes, carried out in the context of a wider, quantified analysis of sub-cell budgets, would be highly beneficial to informed strategic beach management. The possibility that some elements of contemporary beaches are relict, whilst others reflect the impacts of both formal and informal defences during much of the twentieth century, should be a stimulus to further research.
7.6 Despite several studies in the 1980s and 1990s, there remains considerable scope to evaluate the effects of navigation channel dredging at Wootton Creek, Ryde Harbour and Bembridge Harbour. Analysis of existing bathymetric data obtained from hydrographic charts should be combined with the archive topographic profiles obtained from post-dredging surveys. Whilst a general appreciation of basic patterns and trends of sedimentation is available, little is known of the impacts of dredging on wave refraction, sediment mobilisation in relation to adjacent beaches and nearshore banks, bars and shoals. More sediment analysis using shallow borehole samples would be of considerable value.
