Hurst Spit to Calshot Spit (Western Solent Mainland)

1. INTRODUCTION - References Map

The West Solent comprises the tidal channel between Brambles Bank to the east and Hurst Narrows in the west. It has an average width of 4.5km, and is narrowest (1,480m) at its western entrance. It is strongly asymmetrical in cross-section, with a much wider and shallower inter-tidal shoreface (up to 2,000m in width) along the mainland between Hurst Spit and the mouth of the Beaulieu River than on the Isle of Wight side. The main channel is on average 10-15m deep, but reaches 20m in depth at its eastern end and 60m at Hurst Narrows. Included within this unit is the low-lying northern Solent shoreline extending from the fixed, but partial transport boundary of Hurst Spit Photo 1) in the west to Calshot Spit Photo 2) in the east. Both boundaries are by-passed by the transport of fine fine sediment.

1.1 Coastal Evolution

The West Solent represents a submerged and enlarged segment of a previous Pleistocene river system, the Solent River (West 1980; Allen and Gibbard, 1993). This channel, its main tributaries, the Lymington Photo 3) and Beaulieu Rivers Photo 4), and their associated sediments have been strongly influenced by the Holocene sea-level transgression, which has caused submergence, widening, and deposition of sediments by tidal currents and wave action.

In the initial stages of its recent geomorphological development, a Chalk ridge between the Isle of Purbeck (Handfast Point) and the Isle of Wight (The Needles) was breached (possibly initially by fluvial incision) and removed in stages by marine erosion in the late Pleistocene (Wright, 1982; Nicholls, 1987) or early Holocene (Everard, 1954; Velegrakis, et al., 1999; 2000). This had two important effects, which probably developed contemporaneously: (i) rapid erosion of soft Tertiary (Eocene) sands and clays capped by Plateau Gravels, creating Christchurch Bay and (ii) inundation of the channel of the Solent River after the removal of a low elevation isthmus of land connecting the mainland and Isle of Wight shorelines west of the Lymington River (Velegrakis, et al., 1999, 2000; Tyhurst and Hinton, 2001). The general level of the ancestral Solent River channel is approximately -12 to -16m OD in Christchurch Bay and at Yarmouth, Isle of Wight. Nicholls (1987) recognised a former channel, at approximately -14m OD between Hurst Spit and Pennington, based on the recognition of fluvial gravel deposits beneath Pennington as being older than those now exposed at the shoreline. It is hypothesised that the western entrance to the Solent was not created until sea-level transgression reached approximately -12m OD between 8400-6500 years BP (Nicholls and Webber, 1987; Velegrakis et al, 1999). A date of approximately 7,500 BP is now considered probable (Velegrakis et al, 2000). Linkage between the Western Solent and Christchurch Bay transformed the tidal currents in the area from weak to very strong; its channel was subject to tidal scour and rapidly deepened (Webber, 1980). In this way, the original fluvial valley of the lower Solent River was converted into a quasi-estuarine channel.

Rapid cliff recession in Christchurch Bay released large volumes of sediments and it is probable that significant quantities of sand and gravel were swept into the Western Solent prior to the full development of Hurst Spit (Nicholls and Webber, 1987; Dyer, 1980). The floor of the West Solent is underlain by gravel-rich Pleistocene niveo-fluvial terrace deposits associated with earlier incision(s) of the Solent River. Such deposits have been recognised from borehole data at Stone Point (NCC, 1979; Green and Keen, 1987; Allen and Gibbard, 1993; Brown, et al., 1975), Pennington (Allen, et al., 1996; Nicholls, 1987) and Hurst Spit (Nicholls and Clark, 1986; Nicholls, 1987). Investigations suggest that Hurst spit transgressed over low-lying Pleistocene gravel terraces in response to Holocene sea-level rise, but has been relatively stable in both size and position over the past 4000-5000 years since sea-level approached its present position (Nicholls and Webber, 1987). This is also the case for Calshot Spit (Hodson and West, 1972), which overlies saltmarsh deposits beveled by advancing sea-level during the early Holocene, with little evidence of landward migration since approximately 6300 years BP. Further detail on the geomorphological history of the West Solent is provided in the unit on the Quaternary History of the Solent.

1.2 Hydrodynamic Regime

Hurst Spit and the Isle of Wight provide substantial modification to incident waves generated in the English Channel and its Western Approaches. The West Solent wave climate is therefore low energy and at most points fetch limited with significant wave heights between 0.3 and 0.8m (Dyer, 1970b; 1980; Langhorne, Heathershaw and Read, 1982; Ke, 1995; Posford Duvivier, 1994; Halcrow, 1998). Extreme wave heights in excess of 1.6m are however experienced along this shoreline in association with south-east or south-south-east gale force winds operating over the deeper water of the east Solent (Posford Duvivier, 1994; Bradbury, 1995). ERM (1998) indicate that the outer Lymington estuary has a wave climate dominated by this easterly fetch. The shoreline wave climate between Keyhaven and Pitts Deep is becoming increasingly energetic following erosion of protective mudflats and saltmarshes. This process has led to construction of a series of wave screens around tohe Lymington River to compensate for the loss of natural protection (Colenutt, 2002). Large waves from Christchurch Bay can enter at Hurst Narrows, but their energy at the mainland shore is much reduced by diffraction.

Tidal range is small, but water movements are considerable because the range varies markedly over the short distance from Calshot (3.7m mean spring tidal range) to Hurst Point (2.0m mean spring tidal range) with a double high water effect on spring tides (Halcrow, 1998). These differences can set up significant hydraulic gradients that may be further influenced by tidal and surge conditions originating outside the Solent. These tidal factors with the morphological configuration the Solent channels, to influence flow configuration and resistance to flow. Volumetric assessment of tidal flows reveals substantial throughput and a mean east to west residual flow of 1,400 m³s-1, although meteorological influences can reverse this direction (Webber, 1980; Sharples, 2000). Ebb and flood currents are also of differing magnitude and duration with the ebb shorter, and therefore faster, than the flood. These currents frequently flow in distinct pathways and not always in exactly opposite directions (Webber, 1980; Dyer, 1980). Current asymmetry increases progressively eastwards. In contrast to the Eastern Solent and Southampton Water, tidal currents are relatively rapid throughout the length of the Western Solent. Peak surface ebb current velocities of up to 3 ms-1 (1.7 to 2.0ms-1 is more characteristic) have been recorded at Hurst Narrows (Heathershaw and Langhorne, 1988) and up to 1.8 ms-1 at Solent Bank in mid-channel (Hydraulics Research, 1981). These decline to peak ebb velocities of 0.35ms-1, and peak flows of 0.14ms-1 in the inner Lymington estuary (ERM, 1998). The mobility of main channel sediments is therefore much greater and only coarse materials are stable on the bed for any length of time. The West Solent is thus a dynamic environment subject to sediment transport flux dominated by a complex pattern of tidal currents over all but its most easterly sector. Highest energies are achieved when very strong south-easterly winds and waves coincide with peak ebb tidal current velocities.

In contrast to the main channel, the north west Solent shoreline is sheltered by Hurst Spit and the Isle of Wight. The gently shelving offshore and nearshore (shoreface) and inter-tidal gradients generally prevent rapid tidal flow close to the shore. Considerable clay and silt sedimentation has occurred in this low energy environment and extensive saltmarshes have developed (Ke and Collins, 1993). Although accretion has predominated, its spatial and temporal distribution has been variable. The overall sediment budget of the mudflats has been influenced by human activities such as land claim, port development, coast protection and dredging (Ke and Collins, 1993; Bray, et al., 1995).

2. SEDIMENT INPUTS

2.1 Marine Input - F1 F2 F5 F6 F7 References Map

F1 Coarse Sediment Input at Hurst Narrows

Entry of coarse sediments into the West Solent from Christchurch Bay is normally restricted by tidal conditions at Hurst Narrows. Examination of tidal curves for Lymington, Yarmouth (Isle of Wight) and Totland reveal marked asymmetry, because the ebb flow is concentrated into a shorter time period than the flood (Webber 1980). The ebb flow is therefore considerably more rapid than the flood and transport of coarse bedload sediments (sand and gravel) is therefore likely to be in a net seaward direction, determined by peak current velocities.

However, coarse sediments may enter Hurst Narrows during exceptional conditions. A combination of high wave energy and a storm surge from the south-west coincident with peak flood tide velocities be sufficient to transport pulses of coarse sediment into the West Solent against the prevailing net transport direction. Thiswould certainly explain the growth of recurves and the extension of Hurst Point. Due partly to its relative infrequency, this potential input has yet to be studied in a quantitative manner but may derive, in part, from sediments of the shoreface of Hurst Spit and the inshore portions of Shingles Bank. Once within the West Solent, sediment is transported in separate ebb and flood pathways (Dyer, 1980) so these inputs are not immediately moved back offshore by subsequent ebb tidal currents. It has been noted that gravel-sized sediment at the distal point of Hurst Spit can be mobilised by the high flood and ebb tidal stream velocities that operate in the constricted entrance of the Western Solent. This may limit any further lateral growth of the spit. Distal recurvature is the product of waves generated over local fetches, and refracted swell waves. Neither would have sufficient energy to move coarse sediment into the West Solent channel. (Further detail on the morphodynamics of Hurst Spit are provided in the unit on Christchurch Bay).

F2 Suspended Sediment Input at Hurst Narrows

Net suspended sediment transport is likely to be into the West Solent at Hurst Narrows due to the greater duration of the flood current. Thus, it is likely that fine marine sediments and suspended clay sediments derived from cliff erosion become drawn into the West Solent. Remote sensing studies of suspended sediments within Christchurch Bay and the Western Solent support these conclusions (Strisaenthong, 1982; McFarlane 1984).

F5 Suspended Sediment Input to the Lymington and Beaulieu Estuaries

It is probable that some of the suspended sediments circulating within the West Solent are deposited on the north-west shore in sheltered areas such as the lower Beaulieu Photo 5) and Lymington River Photo 3) estuary mouths. In these areas, major intertidal mudflats and saltmarshes have developed although net losses are now occurring owing to both Spartina erosion and "die back" and the migration of the Lymington and Beaulieu River channels (Ke and Collins, 1993; Colenutt 2002). Analysis of fine sediments in the Beaulieu Estuary revealed that the majority were derived from marine rather than fluvial sources (Codd, 1972), via tidal currents (Posford Duvivier, 1994). Erosional scour of the inter-tidal shoreface creates suspended sediment input that may be deposited upon the marsh surface, but is part of the complex sediment budget of the mudflat/saltmarsh system (Ke and Collins, 1993). Quantitative estimations are given by Posford Duvivier (1999), based largely on the application of simple formulae informed by measurements of shoreface width, mean water depth and frontage length (see Section 2.3 for specific figures). There is also a probable, but currently unquantified, input of fine sediment as a consequence of ebb tide scour of dredged channel margins near the entrance to Southampton Water (and possibly upstream of Calshot Spit). The fate and residence time of this material is unknown.

F6 Gravel input to Warren Farm Spit

Studies of the episodic development of Warren Farm Spit, which is composed of coarse gravel, using map comparisons and field survey revealed episodic spit growth and eastward extension between 1898 and 1976 (Human, 1961; Dobson, 1964; Sawyer, 1976; Clark and Gurnell, 1987; Hooke and Riley, 1987; Hydraulics Research, 1987; Williams, 1988; Lobeck, 1995). Occasional storms appear to have been very influential in providing sediment in rapid episodes, thus it has been conjectured that this material has been derived from the sub-tidal bed and channel and driven ashore by the storms. This view would appear to be supported by the intertidal morphology that reveals a variety of swash bars and other gravel features that would be indicative of onshore transport Photo 5). A possible explanation is that the gravel is being driven ashore from a relic portion of the Beaulieu River ebb tidal delta that formed when the lower estuary trended directly southward prior to reclamation of Warren Farm and eastwards deflection of the river mouth.

F7 Gravel input to Calshot Spit

The wide gravel and sand intertidal foreshore in front of Calshot Spit is occupied a variety of swash bars and other gravel features that appear to be indicative of onshore transport that feeds the spit Photo 2). It is uncertain whether this represents fresh material e.g. Pleistocene river terrace gravels eroded from the channel, or whether it could represent a tidal recirculation of existing shoreline sediments.

2.2 Fluvial Input

FL1 Lymington and Beaulieu Rivers

Two main rivers drain into the West Solent, the Lymington and Beaulieu Rivers, for which no long-term measured discharge and sediment input data are available. Rendel Geotechnics and the University of Portsmouth (1996) calculate that both rivers, together with Bartley Water, Dark Water and Avon Water, contribute approximately 785 tonnes a^-1 of suspended load to the West Solent. Sedimentological studies of the Beaulieu river indicate that the upper estuary sediments are sandy muds, much coarser than the fine clays and silts closer to the estuary mouth (Codd, 1972). Both the major and minor rivers, notably the Lymington, possess major impediments to sediment transport, such as dams and weirs. The Lymington causeway was built in 1731, and has removed most potential fluvial sediment input. Codd (1972) concluded that the fine materials of the mudflats that flank the Beaulieu estuary are predominantly derived from the Solent, with only very small quantities of coarser sediments supplied by the river. Subjective and somewhat imaginative assessment of the morphometry of the Beaulieu estuary suggested that it is a tidal palaeomorph as opposed to being a submerged fluvial valley (Geyl 1976a and b). If this interpretation is correct, the meandering channel might have developed in response to tidal currents during a time of fluctuating sea-levels. Although speculative, this hypothesis is partly corroborated by the contemporary sedimentological evidence of Codd (1972) and indicates that marine sediment input is likely to have far exceeded fluvial input throughout its Holocene history.

2.3 Coast Erosion - E1 E2 E3 E4 References Map

E1 Keyhaven to Park Shore (see introduction to coastal erosion)

Qualitative assessments show that this sector is subject to recession of the High Water Mark (Hydraulics Research 1987). Since the majority of this frontage is low lying and protected by seawalls and embankments, potential for sediment supply is limited. Since, Brickearth and gravel-rich Pleistocene terrace deposits underlie much of the frontage, backshore erosion of unprotected areas can potentially supply coarse materials capable of forming stable beach sediments. Between Lymington Spit and Keyhaven, Posford Duvivier (1999) calculate a yield of fine sediment from shoreface erosion of between 16,000 and 27,000m³a^-1 based on an assumption of between 1 and 2mm a^-1 vertical erosion. Hydraulics Research (1991) report that recently settled, unconsolidated mud may be subject to erosion in the centre of mudflats and at creek margins in the Lymington Marshes - see Section 6 for full details of saltmarsh and mudflat erosion. Bow waves and return currents generated by ferries in the Lymington River have the potential to contribute independently to erosion (some 0.3m over 10 years) within the main, dredged channel, at a suggested rate of 0.03m a^-1 Hydraulics Research (1991). A series of nine cross sections measured across the Lymington River between 1993 and 2001 reveal moderate widening and deepening of the subtidal channel and significant lowering by up to 0.5m of the intertidal mudflats (Colenutt, 2002).

E2 Inchmerry House to Lepe and Stone Point (see introduction to coastal erosion)

Active erosion along a 600m long section of coastline between Inchmerry House and Lepe has formed a distinctive series of retreating cliffs up to 6m high cut into Tertiary sands capped by Pleistocene river terrace gravels Photo 6). The eroding cliffs formerly extended further eastward to to Stone Point , but a series of timber and rock revetments in combination with groynes have protected the their toes so that the cliffs are now inactive and vegetated (Hampshore County Council, 2003). Map comparisons covering the period 1868 to 1977 revealed recession of the High Water Mark at a rate of 0.22-29ma^-1 (Hooke and Riley, 1987; Posford Duvivier, 1997; Lock, 1999). Posford Duvivier (1994) propose the higher figure of 0.4ma^-1, sustained since the early 1890s. Posford Duvivier (1994, 1997) note some acceleration of erosion, and falling beach levels, possibly due to the migration of the Beaulieu River channel towards this shoreline (some 30-50m since the early twentieth century). This may be in response to the elongation of Needs Oar Spit. Current erosion yield is calculated at 2,000m³ a^-1, one half of which is coarse (gravel) sediment. Shoreface erosion, at a calculated rate of 0.2mma^-1 over an area some 800m in width, may yield approximately 1800-2500m³a^-1 (Posford Duvivier, 1999).

E3 Stone Point to Bourne Gap (see introduction to coastal erosion)

Marine and sub-aerial erosion along this segment has formed low, discontinuous cliffs at Stone Point and to the east of Stansore Point extending to Bourne Gap. Although they extend to no more than 7m above OD, historical cliff toe recession and sub-aerial mass movement has produced a locally important sediment supply to beaches due to their dominant gravel composition. These cliffs are not inactive and partially vegetated due to the effects of coast protection at Stone Point and the accretion of a wide protective garvel beach along the southern half of Stanswood Bay. Hooke and Riley (1987) calculated that 17m of net accretion of the foreshore between Stone and Stansore Point occurred between 1865 and 1985 thus isolating cliffs from marine erosion. Stratigraphic studies revealed that the cliffs composed of four distinct deposits of mid to late Pleistocene age (West, 1987; Green and Keen, 1987; Allen and Gibbard, 1993). These comprise upper and lower gravel units, each of 2-3m thickness, with an intervening silty horizon. The sequence is overlain by a 1m thickness of Brickearth. Both gravel units are interpreted as terrace deposits of the former River Solent and contain considerable quantities of angular flint clasts. Map comparisons covering the period 1868-1994 reveal cliffline retreat of between 0.20 and 0.27ma^-1 (Halcrow, 1998) although cliff recession would have ceased well before 1994. Cliffs 7m high over a frontage of 1km retreating at this latter rate would have yielded a maximum of 1900 m³a^-1 of sediment. Of this approximately only 50% would be sufficiently coarse to remain on the beach, so maximum supply of less than 1,000 m³a^-1 would have been likely prior to their protection. The shallow shoreface, which narrows to a little over 400m, may yield approximately 800m³a^-1 of suspended sediment as a result of wave and tidal current abrasion (Posford Duvivier, 1999).

E4 Bourne Gap to Hill Head and Calshot Spit (see introduction to coastal erosion)

Low eroding cliffs cut into sand, clay and silt of the Barton Sand (Eocene) and Headon Hill Formation (Oligocene) and Pleistocene gravel are present along this segment (Hinsley, 1990; Posford Duvivier, 1999), with an average height of 6m (maximum 10m). Map comparisons covering the period 1868-1968 indicate overall, long-term retreat (Hooke and Riley, 1987). Coastline recession of 1.5ma^-1 over the period 1967-87 (Hydraulics Research, 1987) and 0.2 to 2.0ma^-1 (Oranjewoud, 1988, 1990; Posford Duvivier, 1999) are reported. Posford Duvivier (1997) give a mean of 0.5m³a^-1 for the period 1898-1976. The late Pleistocene river terrace gravels exposed in the eroding cliffs have a significant potential to supply coarse material to the local beach in Stanswood Bay. Basal erosion and sub-aerial mass movement is active at specific sites, including localised re-entrants due to groundwater seepage. For the cliffline fronting Stanswood Bay, Lock (1999) calculates a potential yield of gravel of 1,420m³a^-1, with over available for beach storage. For ~Stanswood Bay frontage, Posford Duvivier (1997) propose a total yield of 5,000m³a^-1; of which 1,000m³a^-1 is gravel that contributes directly to the local beach.

Map comparisons and air photo analysis have revealed significant foreshore erosion and narrowing of the intertidal zone since the 1950s (Tubbs, 1980; Hydraulics Research, 1987; Hooke and Riley, 1987). This process probably results in significant release of fine sediments; however it is not regarded as sediment input but a redistribution of existing sediment.

Taking the eroding shoreline of the north-west Solent as a whole, Bray et al. (1998) suggest a total cliff sediment yield of some 7,000m³a^-1, but only between 1,000 and 2,000m³a^-1 represents coarse clastic debris that is retained on local beaches. Suspended sediment yield resulting from shoreface erosion is in the range of 2500 to 5,000m³a^-1 (Posford Duvivier, 1999). The fate of this material is unknown, but it is presumed that a large proportion is removed from the West Solent by tidal flows.

2.4 Channel Erosion

The bed of the West Solent is mantled by extensive Pleistocene river terrace deposits of a similar nature to those recognised at Hurst Spit, Pennington and at Stone Point (Nicholls and Clark, 1986; Green and Keen, 1987; Allen and Gibbard, 1993). Significant scour of the West Solent tidal channel has occurred in areas where tidal currents are rapid e.g. Hurst Narrows (Dyer, 1970b;Webber, 1980). This process involves mobilisation of considerable volumes of sand and gravel from these deposits, together with some fines from underlying Eocene bedrock. The coarser sediments have been deposited in a series of banks within the channel of which Solent Bank is the best known. It must be asked whether contemporary erosion of the channel bed continues to supply sediment to the West Solent system. An equilibrium may have developed whereby the bed has become armoured by mobile coarse sediments and further scour is now limited. If this is the case the gravel banks of the Western Solent represent a finite deposit.

Sand and gravel deposits at Solent Bank were dredged between 1950 (Hydraulics Research, 1981) and 1994. Detailed examination of bed levels in the vicinity of Solent Bank involving chart comparisons revealed significant changes, including patches of erosion that lowered the bed by up to 3m, over the period 1963-1973 (Hydraulics Research 1977, 1981). It was uncertain whether this involved bed erosion of insitu Pleistocene gravel deposits or redistribution of existing mobile sediments, although the latter is assumed. I could be that dredging over Solent Bank removed the armoured surface gravel veneer exposing looser or softer materials to scour by tidal currents, so that renewed localised bed erosion became possible. The occurrence of extreme wave and tide conditions may also be important in eroding or re-mobilising the bed materials, but positive evidence is extremely limited. Rapid shoreline gravel accretion was recorded at Warren Farm Spit after a storm in 1952 and an offshore source of supply of this material was postulated (Hydraulics Research, 1987). Such supply may be related to intermittent scour of channel deposits, but this suggestion remains speculative.

Gravel transport within the West Solent Channel is addressed in further detail in Section 4 and dredging of Solent Bank is addressed in Section 5.2.

3. LITTORAL TRANSPORT - LT1 LT2 LT3 References Map

LT1 Drift at Hurst Castle (see introduction to littoral transport)

Detailed studies of the historical pattern of littoral drift on Hurst Spit have indicated a long-term east to west supply from Christchurch Bay and transport to the distal end of the spit (Hurst Point). At Hurst Castle, a divergence of transport was recognised with some material being supplied to the Hurst Narrows tidal channel and the remainder drifting northwards to recurves on the spit at Hurst Point (Nicholls and Webber, 1987). The drift passing around Hurst Castle has led to the growth of Hurst Point (the recurved tip of the spit) to such an extent that it is dredged every five years with the material taken being recycled back on the western side of the spit (Colenutt, 2002).

Net transport in the Hurst Narrows tidal channel is offshore due to the dominant ebb current and this pathway is regarded as a long-term supply to the Shingles Bank. It is widely held that intermittent pulses of sediment can be transported into the western Solent from Hurst Point by wave action during storms (Dyer 1970a, 1971, 1980; Webber 1977; Hydraulics Research, 1987). This view is largely unsupported by direct evidence and no studies have evaluated the periodicity of formative event(s) or how it might relate to accretion of hurst Point. The possible quantitative significance of this input is therefore impossible to evaluate. Detailed sampling of bed sediments has been undertaken from a limited area on the bed of the southern margin of the main channel between Yarmouth and Hampstead Ledge (Langhorne, Heathershaw and Read, 1982). Analysis revealed that roundness of clasts was insufficient for the main supply source to be from relatively well rounded contemporary beach gravels on Hurst Spit. Dyer (1972) stated that Hurst Spit was the main coarse sediment source for the Western Solent, but this view may now be in need of modification. Further details relating to the morphodynamics and sedimentology of Hurst Spit are given in the unit on Christchurch Bay.

LT2 Drift between The Lymington and Beaulieu Rivers (see introduction to littoral transport)

The shoreline between Lymington and Pitts Deep is sheltered from wave action by wide (but eroding) mudflats and saltmarshes so that there is little beach development or effective drift. Exposure is greater along low-lying shores to the east of Pitts Deep where a transgressive upper gravel beach Photo 7) is controlled by a variety of groynes and other coastal defence constructed piecemeal along numerous small private frontages (Venner, 1986a; 1993). A long-term trend for west to east littoral drift is recognised from the historical eastward deflection of the mouth of the Beaulieu River and from the distribution of gravel against groynes and other structures Photo 4 and Photo 5).

Venner (1993) has suggested that the paling groynes established since 1982 along the gravel shoreline of the North Solent National Nature Reserve (Park Shore to Gull Island), are sub-optimal in their efficiency. He suggests that whilst both gravel and coarse sand move predominantly from west to east, there may be some counter drift, close to Mean Low Water, in association with unquantified combinations of steeper than average wave heights and peak flood tide velocities. This observation has been tentatively confirmed by others, elsewhere along the frontage e.g. Posford Duvivier (1994).

Studies of the episodic development of Warren Farm Spit Photo 5), which is composed of coarse gravel, using map comparisons and field survey revealed intermittent growth in an eastward direction between 1898 and 1976 (Human, 1961; Dobson, 1964; Sawyer, 1976; Clark and Gurnell, 1987; Hooke and Riley, 1987; Hydraulics Research, 1987; Williams, 1988; Lobeck, 1995). Occasional storms appear to have been very influential in providing sediment promoting rapid short-term growth, e.g. some 100m of distal extension across former mudflats in 1952/3, (Halcrow, 1998). This pattern of development strongly indicates supply and distribution of gravel by eastward drift. In 1986, a tidal channel (Bulls Gap) separating Needs Oar Point and Warren Farm Spit from Gull Island, first opened in 1727, was closed by building a causeway and through 13,000 tonnes of gravel recharge (Clark and Gurnell, 1987; Venner, 1986b; Lobeck, 1995). After storms in 1987, it is reported that gravel was transported eastward from Warren Farm Spit to supply Gull Island (Williams, 1988), and its eastwards and northwards growth has continued since (Halcrow, 1998). Eastward drift has also been demonstrated by crude tracer studies, using concrete pebbles, undertaken for limited periods on the spit. Although their representativeness was uncertain, these experiments provided direct evidence of eastward drift, with transport rates averaging 0.3m day-1 recorded over a 21-day period (Williams, 1988). Further evidence of net eastward drift is provided by the distribution of sediments within groyne compartments between Thorns Beach and Warren House (Venner, 1986a; 1993).

Thus, a wide variety of evidence measured at timescales ranging from days to hundreds of years supports the conclusion of a long established net eastward drift pathway. Although qualitative evidence is strong, few quantitative details of drift rates and volumes are available. Mudflat growth has occurred in front of Gull Island since 1986, partly as a result of the eastwards extension of this longshore transport pathway (Halcrow, 1998). MLW has moved seawards some 270m between Needs Oar Point and Gull Island, and 600m at the distal tip of Gull Island, since 1975.

LT3 Beaulieu River to Calshot Spit (see introduction to littoral transport)

Little specific information relating to littoral drift between Inchmerry House and Calshot is available, though net beach accretion between Stone and Stansore Points occurred between 1870 and 1994 deduced from map analysis (Hooke and Riley, 1987). Tubbs (1999) has stated that the mouth of the former tidal inlet of Dark Water and the Mopley Stream were closed before 1600, with both valleys rapidly accumulating alluvial sediment. It is uncertain if this is indicative of spit growth fed by longshore drift, as closure might have been effected by barrier migration. Both streams now discharge via seepage through their gravel "dams". Maps produced by Hydraulics Research (1987) indicate a net northeastward drift within Stanswood Bay, but this information is of low reliability because sources of evidence were not indicated. Eastward drift is indicated at Hillhead and along Calshot Spit by the distribution of gravel against groynes and other structures. A small ness-like gravel feature is present on the shore in front of Inchmerry House. Its occurrence is probably indicative of a locally reversed east to west drift operating from Lepe to this point that is fed by gravels released by local cliff erosion Photo 6). In fact, Posford Duvivier (1994) calculate a potential drift rate of 1,900m³a^-1 in the vicinity of Lepe, with net transfer from east to west. Drift reversal can be explained by the local shelter from southwesterly waves produced by the eastward growth of Gull Island and Warren Farm Spit across the mouth of the Beaulieu estuary leaving the shoreline exposed only to waves approaching from the southeast

Historically, rates have been low because gravel was stored on Warren Farm Spit, and could not by-pass the mouth of Beaulieu River. Lobeck (1995) reports a modest increase in drift rates since 1986, suggesting a weakening of the Beaulieu River mouth as a partial drift discontinuity (Bray et al, 1995). Localised and short-term drift reversal may occur when south-easterly waves enter the eastern Solent (Lobeck, 1995). Net accretion at Between Stone Point and Stansore Point is probably the product of a locally complex interaction of refracted waves from different fetch directions. The very small quantity of gravel moving from Stansore Point to Bourne Gap is probably due to storage occurring on the foreshore to the north east of Stansore Point, where the upper gravel beach has accreted to a width of 50m. It has also been suggested that sediments may move offshore at Stansore Point before arriving in Stanswood Bay (Dyer, 1980; Lobeck, 1995). Stansore Point might, therefore, act as a weak sub-cell boundary.

The alignment of Calshot Spit is generally taken to indicate north-eastward drift (Dyer, 1980; Hydraulics Research, 1987; Hinsley, 1990). Long-term spit stability over the past 6300 years (Hodson and West, 1972), including the past 140 years of documentary records (Hooke and Riley, 1987; Oranjewoud, 1990), may indicate constancy of this transport pathway. In an analysis of the morphodynamics of Calshot Spit, Lock (1999) assumed a dominant north-eastwards transport direction, with a potential capacity to move some 5,050m³a^-1 of gravel. Not all of this material would be supplied to the spit, as there is a postulated but unquantified, onshore to offshore feed to Lepe Middle Bank. (Dyer, 1972; 1980). Conversely, there is evidence also of offshore to onshore feed directly to the spit (see F7) The stability of shape and position of Calshot Spit since the construction of the Castle in the 1530s suggests that gains and losses balance in the long term. However, beach steepening (1931-1962) was followed by flattening (1963-1974), but no discernable trend since (Lobeck, 1995).

A modest renourishment of Calshot beach adjacent to the proximal part of the spit was undertaken in 1994/5, but there has been no artificial feed of the updrift inter-groyne compartments created in 1991. Lock (1999) has calculated that the gravel foreshore of the axis of Calshot spit has prograded between 0.09 and 0.15ma^-1 since 1890. Oranjewoud (1990) ascribe the absence of foreshore erosion to the steep slope into the central channel and the width of the gravel-dominated fronting inter-tidal zone. If the erosion yield of the updrift cliffline between Lepe and Hillhead to the littoral drift pathway is in the order of 2,000m³a^-1 (Lock, 1999; Bray, et al., 1998), some 220,000m³ of gravel has been input into Calshot spit since approximately 1870 (date of earliest large scale Ordnance Survey plan). This would indicate a modest net growth of the volume of material stored by Calshot spit over the past 120-130 years, but a more precise figure can only be calculated once losses to the near and offshore environments are quantified (Halcrow, 1998). This information is currently unavailable. The well-defined distal point may be due to erosion by tidal scour, but most studies suggest that Calshot Spit is a quasi-equilibrium form. Recognised accretion phases, e.g. 37m of seaward growth near its proximal point 1867-1910 (Oranjiewoud, 1990) must be balanced by periods of depletion. The latter, however, have not been specifically identified from map evidence. There has been a notable pattern of foreshore accretion along the Calshot Activities Centre frontage since 1971, apparently involving the onshore migration, and accretion, of at least two inter-tidal banks - see Photo 2 (Halcrow, 1998). Accretion is sufficient that material is recycled and used to recharge beaches along the updrift frontage to the west (Colenutt, 2002; Hampshire County Council, 2003).

Lobeck (1995) was able to demonstrate that sets of groynes constructed during the period 1868-1971, between Stansore Point and Calshot, were the direct cause of HWM recession, especially the proximal sector of Calshot Spit between 1910 and 1931. Stability returned between 1935 and 1971 and thereafter, when most groynes had virtually ceased to function due to neglect. Since 1993, backshore "chevron" groynes have been installed, and have helped to maintain stability.

4. TIDAL-CURRENT SEDIMENT TRANSPORT - T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 References Map

1. Broad scale descriptive and analytical survey in the late 1960s by Dr K.R. Dyer Southampton University
2. Dredging of Solent Bank, studied by Hydraulics Research in the late 1970s and early 1980s.
3. Fundamental entrainment processes studied within a range of field and laboratory experiments by the Institute of Oceanographic Sciences in the 1980s.

(a) Sediment Distribution and Mobility by Dr K R Dyer

These surveys revealed that much of the West Solent floor was covered by sedimentary bedforms, mostly sand and gravel waves. Crest orientation was generally at right angles to the trend of the main channel and the larger waves had pronounced asymmetry (Dyer, 1971). This proved a reliable guide to sediment transport direction and in conjunction with sedimentological indicators (e.g. fining in the direction of transport) it was possible to identify net sediment transport pathways. As bedform distribution was generally stable in the short term, it was postulated that sediment transport pathways are also relatively stable. Analysis revealed that sediment moved in different directions on opposite sides of the channel and there was often rapid change in the direction and mode of transport over short distances. Mapping of the transport pathways revealed a general west to east transport in the north part of the channel, with a series of local reversals caused by recirculating current eddies on the opposite side (Dyer, 1971). Measurement of tidal currents revealed small but important differences in both the strength and direction of ebb and flood currents. These often meandered, with small differences between ebb and flood currents resulting in slight net sediment transport. Repeated over many tidal cycles these net movements combine to produce three recirculating eddies of sediment off Egypt Point, Newtown Harbour and Hamstead Ledge, Isle of Wight (Dyer, 1971). Zones of accretion (stores) occur where a recirculating eddy meets an eastward moving transport pathway, and it was suggested that these positions are occupied by Solent Bank and Prince Consort Shoal (Dyer 1971, 1972). Sedimentological analysis indicated that net eastward transport continued to Cowes where diminution of tidal currents causes deposition of coarse sand on Prince Consort Shoal and medium sand on Brambles Bank. It is suggested that these shoals and banks represent sediment sinks for the respective grain sizes.

Analysis of sediment samples revealed a series of grain size zones primarily based upon the relative proportions of the sand and gravel modes (Dyer, 1971). Gravel, with variable proportions of coarse sand and silty clays, was the main bed material. Calculation of peak bed shear stress from tidal current measurements indicated gravel was potentially mobile over most of the West Solent and that sand was unstable on areas of the bed subject to net sediment transport (Dyer, 1970a, 1971, 1972). Further experiments were conducted into the spatial variations of sand distribution, and the ratio of trough to crest shear stress was found to be an important control on sandwave morphometry. Conclusive results could not be achieved due to difficulties of taking measurements over specified parts of bedforms. Limited studies were undertaken by divers to evaluate rates of transport, but data collected could not be converted to general mass transport rates as it was measured over restricted timescales and limited spatial areas (Dyer, 1980).

(b) Detailed Studies of Solent Bank by Hydraulics Research

A second phase of investigation involved further chart comparisons, repeated echo sounding surveys, side-scan sonar survey, current metering and sediment sampling using grab and vibrocore techniques (Hydraulics Research, 1981). Comparison of Admiralty hydrographic charts covering the period 1965-1975 revealed net accretion on the western margin of Solent Bank and erosion to the east. This pattern of change supported evidence of supply by the dominant eastward sediment transport pathway recognised by Dyer (1971). If this interpretation is correct, bed lowering during this period by dredging in the eastern sector of Solent Bank must have intercepted a significant proportion of eastward moving sediment. (See Section 5.2 for details of dredged volumes). Echo sounding surveys repeated on seven occasions between 1979 and 1981 showed a similar pattern of bed lowering but, with lower dredged volumes, replenishment reduced to 45,000 m³a^-1. This supports a mechanism of partial replenishment, primarily by redistribution from neighbouring areas at rates controlled by the size of dredged holes. Alternatively, it can be argued that hydrographic surveys are difficult to repeat precisely, so detailed analysis of volumetric change covering short periods can be inaccurate. It must be concluded that comparison of bathymetric charts and hydrographic surveys can yield valuable information on long-term trends and bed changes, but gives information of a less reliable nature relating to short-term changes. The directions and magnitudes of sediment transport pathways involved in bed changes are extremely difficult to determine without resort to more sophisticated survey and experimental techniques.

Sediment sampling over a range of different tidal states between 1978 and 1981 revealed that surface sediments were mostly mixed sand and gravel, with some patches of sand and silty clays. Vibrocore survey identified an upper mobile sand and gravel layer and a lower, immobile gravel-rich layer composed of Pleistocene fluvial terrace deposits. Side-scan sonar survey showed both deposits to be extensively scoured by dredging, frequently outside licensed areas. Removal of in situ deposits inevitably leads to lowering of the bank and may result in changed hydraulic conditions and alterations to sediment transport pathways. Tidal current measurements at a variety of depths indicated no significant velocity changes over a three-year period, but that direction of flow was critical to net transport (Dyer, 1971). It was concluded that not only are sediment transport pathways poorly understood in the vicinity of Solent Bank, but it remains uncertain whether massive lowering of the bank by dredging could have changed hydraulic conditions and thus altered sediment transport pathways in this part of the Solent. [Dredging ceased in 1994, but there have been no subsequent surveys to reveal if this has had a subsequent impact on hydrodynamic conditions and transport pathways].

(c) Fundamental Gravel Transport Studies by the Institute of Oceanographic Science

Much effort was devoted to acoustic detection of gravel transport. Acoustic energy generated by sediment transport was monitored by hydrophones and calibrated by comparison with observations of sediment transport by underwater TV and simultaneous tidal current measurements. Bursts of acoustic energy were found to be related to large instantaneous shear stresses and rapid transport events. These comparisons demonstrated the viability of this technique, which had the advantage of good temporal resolution of transport events, i.e. instantaneous monitoring (Thorne, Heathershaw and Troiano, 1984). The acoustic detection technique was then employed in a number of fundamental studies into bedload transport in unidirectional flow.

Simultaneous measurement of sediment transport using acoustic detection and current velocities revealed the existence of a "bursting" phenomenon created by currents flowing over gravel beds. Bursts of peak instantaneous shear stress over ten times greater than mean shear stress (measured over 20 minutes) were recorded in conjunction with surges of gravel bedload transport. It was concluded that this was achieved principally by sweep-like motions (dominant horizontal velocity component) in the bottom boundary layer (Heathershaw and Thorne, 1985). Detailed underwater TV recordings were analysed to produce quantitative estimates of sediment transport (Williams, 1990). A mean transport rate of 0.00045 kg m-1s-1 was recorded for a 30-minute period (Thorne, 1986). Such data was of use for calibration of acoustic detection techniques but was of far too limited a spatial and temporal extent for a mass transport calculation for the Western Solent as a whole. The acoustic detection technique was used in conjunction with current measurements to compare the predictive capability of five bedload sediment transport equations. It was found that three of the equations gave results agreeing well with measured transport even though sediment size at the site exceeded the original calibration limits of the formulae (Williams, Thorne and Heathershaw, 1989a). Sediment transport was measured over a series of ebb and flood tides, and rates were calculated over time periods up to 147 minutes of up to 0.00032 kg m-1s-1. Results were again of too limited spatial and temporal representativeness to be applied to the West Solent as a whole but fundamental aspects of bedload transport were more clearly defined. It was concluded that bedload transport is associated with intermittent rushes ("sweeps") of high velocity fluid towards the bed (Williams, Thorne and Heathershaw, 1989b; Williams and Tawn, 1991).

It can be concluded that the observations of gravel waves confirmed earlier work by Dyer (1971) and indicated that the entire size-range of bed sediments were highly mobile. As this information was corroborated by a variety of survey techniques over several years, it must be regarded as highly reliable. Evidence indicated that net bedload transport was eastward and thus supported the major transport pathway recognised by Dyer (1971,1972,1980). Much of the detailed analysis of sediment transport was conducted over very limited timescales and restricted to a single site. The objective firstly was to calibrate the acoustic detection technique and secondly to examine fundamental bed stress:bedload transport relationships. A product of this work was the development of an acoustic measurement technique and calibration of bedload transport equations which could be more widely applied. It is only through the use of such techniques at representative spatial and temporal scales that the complex qualitative sediment transport pathways identified by Dyer (1971) can be verified and quantified. It would appear that net pathways of movement are different on either side of the main channel. Although transport is fundamentally due to tidal currents, both coarse sand and gravel can occassionally be entrained by waves of exceptional height generated by easterly fetch, and moved shorewards. These waves are the product of east or south-easterly winds blowing across the eastern English Channel, and then entering the Eastern Solent.

T1 Eastward Transport from Hurst Narrows (see introduction to tidal current transport)

A general eastward transport of sand and gravel was recognised in the central and northern parts of the main channel (Dyer, 1971, 1972, 1980). This transport pathway extends eastwards from Hurst Spit and terminates at Solent Bank where much material has been removed by aggregate dredging since the late 1950s (Hydraulics Research, 1977, 1981). There is some evidence for channel deepening in the late 1980s (Ke and Collins, 1993). Prior to this, the transport pathway probably extended further east. It has been identified from the asymmetry of bedforms (Dyer, 1971) and its long-term existence was confirmed (Langhorne, Heathershaw and Read, 1982). Direct underwater TV observations of the sea bed indicated a very high degree of sediment mobility along this pathway (Langhorne, Heathershaw and Read, 1982; Williams, 1990). Transport was predominantly by bedload involving material ranging in size from coarse sand to cobbles. Suspended sediment transport was not observed. With the suspension of aggregate dredging in 1994, it is probable that this pathway now extends further eastwards, and may feed transport moving away from Solent Bank.

T2 Eastward Transport from Solent Bank (see introduction to tidal current transport)

Evidence of gravel wave asymmetry indicated continued eastward transport towards Egypt Point (Dyer, 1970a, 1971). Much eastward moving material must have been intercepted at Solent Bank by dredging, but studies by Webber (1977) and Hydraulics Research (1977, 1981) concluded that eastward transport continued beyond this location. Tidal currents become weaker east of Egypt Point and sedimentological evidence indicates that coarse sand is deposited on Prince Consort Shoal and medium sand on Brambles Bank (Dyer 1971, 1980, Hydraulics Research 1977). The fate of eastward moving gravel is less certain; deposition is possible as currents reduce towards Egypt Point, but no gravel banks have been recognised to indicate such a response. Alternatively, gravel may be transported westward back towards Solent Bank in a recirculating eddy (T9) along the southern margin of the main channel (Dyer, 1971, 1980).

T3 Supply to Prince Consort Shoal (see introduction to tidal current transport)

Sand waves in the vicinity of Prince Consort Shoal indicate a node of temporary deposition where an anticlockwise movement of material in recirculation around Egypt Point (T2) meets a clockwise movement set up by a tidal current meander. These pathways were identified by Dyer (1971) and were corroborated by Webber (1977).

T4 Westward Supply to Prince Consort Shoal (see introduction to tidal current transport)

This pathway, only indicated indirectly by bedform asymmetry, reveals sand supply to Prince Consort Shoal from Osborne Bay and the East Solent (Dyer, 1971, 1980).

T5 Westward Transport from Brambles Bank (see introduction to tidal current transport)

An anticlockwise eddy of sediment movement occurs on Bramble Bank, set up by the meandering of tidal currents (Dyer, 1971, 1980). This eddy supplies a westward moving sediment transport pathway which may feed Stanswood Bay or possibly link with the T7 pathway.

T6 Ebb Current Transport from Calshot Spit and Southampton Water (see introduction to tidal current transport)

The dominant ebb tidal current has significant potential to sweep sediment out of Southampton Water (Dyer, 1970a, 1971, 1980; Sharples, 2000). Actual supply from this pathway is difficult to assess because there is relatively little information on coarse bedload sediment transport in Southampton Water. Gravel may be moved from the distal end of Calshot Spit and the adjoining gravel foreshore but this possibility has not been evaluated. The transport pathway probably joins with T5 in the western approaches to Southampton Water, and circulates around Brambles Bank (See units on East and Central Solent and Southampton Water).

T7 Offshore Transport from Stansore Point and Stanswood Bay (see introduction to tidal current transport)

Bedform asymmetry and divers' observations indicate net offshore movement and south-west transport along this pathway (Dyer, 1971, 1980). Sediments may derive from beaches in the vicinity of Stone Point (E3), and may supply Lepe Middle Bank. Sand patches on the seabed between Stansore Point and the proximal part of Calshot Spit, seawards of the foreshore step, are reported by Posford Duvivier (1999). Similar fine sand accumulations occur immediately south of the mouth of Beaulieu River (Codd, 1972) but there is no information on whether they are persistent features.

T8 Onshore to Offshore Transport from Park Shore (see introduction to tidal current transport)

Although indicated by bedform asymmetry (Dyer 1971), this pathway appears to contradict other evidence of rapid accretion of the nearby Warren Farm Spit due to onshore gravel transport (Human, 1961; Hydraulics Research, 1987; Clark and Gurnell, 1987). Transport direction of this pathway may therefore be variable and subject to local and intermittent reversals, possibly in response to incident waves of exceptional height generated by east or south-easterly winds.

T9 Recirculation between Thorness Bay and Solent Bank (see introduction to tidal current transport)

Stationary meanders are located at different positions on the ebb and flood tidal streams, resulting in variations in strength and direction of flow. A major ebb current meander in Thorness Bay causes net westward transport and results in a recirculating eddy of sediment, which transports material to Solent Bank (Dyer, 1971). At this location the recirculation meets the dominant eastward transport pathway (T1) and sediment appears to be deposited. This recirculating pathway was identified from analysis of bedform asymmetry and tidal flow (Dyer, 1971).

T10 Eastward Transport to Newtown Gravel Bank (see introduction to tidal current transport)

A divergence of the dominant eastward transport pathway (T1) is indicated by analysis of bedforms (Dyer, 1971). This pathway appears to supply sediment to Newtown Gravel Bank, located south of Solent Bank. From here sediment may be entrained by the recirculating eddy (T9) and re-supplied to Solent Bank. Chart comparisons covering the period 1963 to 1973 indicated significant accretion of Newtown Gravel Bank (Hydraulics Research, 1977), thus possibly confirming this process.

T11 Westward Recirculation Offshore between Yarmouth and Bouldnor (see introduction to tidal current transport)

Westward transport due to an ebb current meander is indicated by bedform asymmetry. Sediment sampling revealed a westward deflection of a "tongue" of fine material extending offshore from Bouldnor. This may represent sediment supplied by cliff erosion being entrained by the westward flowing recirculating eddy (Dyer, 1971). The pathway meets the dominant eastward flowing transport stream.

T12 Offshore Transport From Lymington Estuary (see introduction to tidal current transport)

Hydraulics Research (1991) undertook a sediment-size analysis in the outer Lymington estuary, revealing gravel and sand in the main channel (in contrast to the silty-clay composition of the banks of the Lymington River). Net seaward transport of fine sediment was inferred and it was demonstrated that shear stresses imparted by waves on recently settled mud was sufficient to re-entrain that material during the low tide period (shallow water depths). Net offshore transport of fines from the outer estuary may also be indicated by some 0.5m increase in depth of the main navigation channel between 1981 and 1991 (ERM, 1998). This is not accounted for by capital dredging, which amounted to only 15,000m³ during this period, or by maintenance dredging off Town Quay. Hydraulics Research (1991) provide the tentative conclusion that silt and mud brought into the estuary as suspended load is flushed seawards by the ebb tidal current, though a proportion of input is trapped in marinas and mooring areas. A series of nine cross sections measured across the Lymington River between 1993 and 2001 revealed continued widening and deepening of the subtidal channel and significant lowering by up to 0.5m of the flanking intertidal mudflats (Colenutt, 2002). Other influences such as increasing wave penetration or perhaps bow-wave or propulsion generated scour by ferries could help to explain this phenomenon.

5. SEDIMENT OUTPUTS - References Map

5.1 Tidal (Estuarine) Output

Tidally-driven sediment outputs are possible at each end of the Western Solent. Studies of tidal currents and bedforms at its eastern limit have revealed that currents diminish towards Egypt Point, gravel ceases to be mobile and sands are deposited from eastward moving pathways onto Brambles Bank and Prince Consort Shoal. These locations are believed to represent sediment sinks (Dyer, 1971, 1980). It is therefore concluded that sediment is not directly transported eastward out of the West Solent. In fact, some sediment may be recirculated back from Brambles Bank by north-westward moving pathways (T5).

EO1 Tidal Output at Hurst Narrows (see introduction to sediment outputs)

Ebb currents are shorter-lived but more rapid at Hurst Narrows (Webber 1980) so potential exists for both suspended and bedload transport westwards out of the Western Solent. It is widely recognised that material moving into Hurst Narrows from Hurst Spit undergoes net offshore transport towards the Shingles Bank in Christchurch Bay (Dyer, 1970b, 1971, 1972, 1980, and Nicholls and Webber, 1987). It can be postulated that a similar fate might apply to sediment reaching the main channel from the opposing, Isle of Wight, shoreline. All previous research suggests a net eastward transport pathway in this area (Dyer, 1980), but this does not fit with the known tidal regime that favours output controlled by dominant ebb tidal currents. The uncertainty relates to the location of the "switchover" from dominant ebb current flushing out of Hurst Narrows to the prevailing eastward transport pathway (T1) that characterises much of the West Solent channel to the east. It is therefore postulated that sediment output driven by tidal current is possible also from the Isle of Wight shore, and may be linked with sediment transported westward by a recirculating eddy (T11). This portion of the pathway remains speculative, and subject to verification.

5.2 Dredging

Dredging has been practised in the Solent for the past century, but it is only since the early 1950s that large-scale aggregate extraction has been undertaken. In the late 1930s dredging within the whole Solent was at a rate of approximately 150,000 m³a^-1 (Shears, 1986). From the 1950s, when dredging effort increased, figures are available relating specifically to Solent Bank. Mean rates of extraction by dredging were 81,000 m³a^-1 for 1950-59, 218,000 m³a^-1 (1960-62), 920,000 m³a^-1 (1963-71) and reducing to 681,000 m³a^-1 for the period 1972-75 (Hydraulics Research, 1977). Thereafter, dredging was further reduced because of uncertainty as to its effects on adjacent sub-cell budgets, finally being suspended in 1994. Dredging records also include extraction at Pot Bank and Prince Consort Shoal but there are no figures specific to the last site. (Pot Bank is located west of the Needles and outside the West Solent).

Steady reduction and final cessation of dredging from Solent Bank is mostly attributable to fears over effects on adjacent shorelines, but it is also possible that the bank was steadily becoming worked out. These concerns resulted in studies of Solent Bank by Webber (1977) and Hydraulics Research (1979, 1981). Chart comparisons covering the period 1847-1973 revealed that the volume of Solent Bank fluctuated periodically, but remained within consistent limits until 1960, when rapid reduction in volume was recorded (Hydraulics Research, 1977). Chart comparisons covering the period 1968-73 indicated lowering of the bank by 3.2m (0.64ma^-1) followed by accretion of net 0.4m between 1974 and 1976 (Webber, 1977). Admiralty hydrographic chart comparisons covering the period 1965-1979 revealed surface lowering at 0.13ma^-1. A series of seven hydrographic surveys over the period 1978-81 revealed lowering at 0.20ma^-1. This suggests diminution of the rate of replenishment, as dredged output was reduced significantly over this period (Hydraulics Research, 1981). The general trend was for significant reduction in the level of the bank following intensification of dredging in the late 1950s, despite some significant natural replenishment. In 1950, Solent Bank was a shoal with a marked plateau within the -13m OD contour. Surveys between 1978 and 1981 showed that only a fragment of this former crest remained, with a mean overall summit/crest depth of -16.5m OD. Side-scan sonar and sediment sampling surveys demonstrated that much of a previous core of Pleistocene terrace deposits had been removed by dredging so the contemporary bank was composed of a relatively thin veneer of mobile sand and gravel overlying in situ Eocene strata (Hydraulics Research, 1981). This suggests that it has been permanently lowered because it is only a temporary resting place for eastward moving and recirculating sediment (Dyer, 1971). (see Section 4b). Replenishment is therefore unlikely to rebuild the bank to previous levels as sand and gravel are highly mobile over the whole bank and losses by throughput must be assumed to be constant.

The transport and sediment budget implications of this well documented volume loss are difficult to determine, but two major possibilities exist: (i) interception of sediment transport pathways and consequent reduction of supply to areas further down the transport pathway, and (ii) alteration of hydraulic conditions over the bank and associated changes to the pattern of sediment transport in its vicinity. These changes are of most immediate concern to the eroding northwest coast of the Isle of Wight. Examination of beach profiles and OS maps indicated that the greatest shoreline changes coinciding with the commencement of intensive dredging were either side of Newtown Harbour entrance. It was concluded that the contribution of Solent Bank dredging to these shoreline changes could not be discounted and it was this consideration that resulted in the withdrawal of dredging licences in 1994. Subsequent bathymetric and sedimentololgical changes have not been monitored.

Dredging for navigation access is undertaken at Lymington River. A study specific to the proposed dredging of Horn Reach (ERM, 1998), which reviewed the local hydrodynamic regime, revealed that the main navigation channel remained constant in width, but deepened by 0.5m, 1981-1992. Much of this erosion was presumed to be natural, though capital dredging removed the modest quantity of 15,000m³ during this period. Net accretion at Horn Reach, close to the causeway, took place between 1992 and 1997. Dredging of the area adjacent to the Town Quay, as well as the marina basins, was not less than about 60,000m³ during the period 1985 to 1995 (ERM, 1998). Colenutt (2001) quotes a mean of 40,000m³a^-1 for the Lymington estuary as a whole attributing siltation to material originating from the eroding mudflats and saltmarshes of the area. Historically, maintenance dredged sediments from the estuary have been disposed of during an ebb tide at a dumping site seaward of Hurst Narrows. It has been suggested that alternative placement sites on mudflats within the estuary could assist mudflat conservation although further appraisals of the feasibility of this option would be required (Colenutt, 2001)

There are no available records of dredging of the Beaulieu and Keyhaven channels.

5.3 Reclamation

Much of the low-lying belt of grazing land between Keyhaven and Lymington Photo 8) was reclaimed in the 18th (used as salterns) and 19th centuries (conversion to grazing meadows) totalling around 260ha. This frontage has been protected by several generations of embankments and seawalls. The corner of land forming some 290 hectares on the western margin of the Beaulieu River mouth around Warren Farm and Needs Ore Point Photo 5) was reclaimed in the 15th Century and is protected by earth embankments. Prior to this latter reclamation, the plan form of the Beaulieu outer estuary would have been similar to that of the Lymington River.

6. SEDIMENT SINKS AND STORES: MUDFLATS AND SALTMARSHES - References Map

A substantial area of inter-tidal mudflat and saltmarsh has accumulated in front of approximately 10km of coastline developed in the rock substrate and overlying Pleistocene sediments. It is most extensive between Keyhaven Photo 1) and the Lymington River Photo 3), but is also present, though less well developed, between the Lymington Ferry terminal and Pitts Deep; and within the Beaulieu River Photo 5), between Lower Exbury and Inchmerry House and extending as narrowing margins up stream towards Beaulieu. Most areas of mudflat and saltmarsh have eroded considerably during the 20th Century. From Pitt's Deep east to the Beaulieu River, saltmarsh is absent, and there is only a relatively very narrow fringe of muddy lower foreshore. Note that considerable areas of saltmarsh and mudflat have been reclaimed (Section 5.3)

Rapid growth in area and elevation occurred between the 1880s and late 1920s or early 1930s. Tubbs (1980, 1999) has recorded in detail the invasion and spread of the hybrid cord grass, Spartina anglica, the prime factor responsible for the trapping and stablisation of a very substantial volume of fine silt and clay. He suggests that the peak of Spartina colonisation was between 1925 and 1929, when there was 2,470ha of monospecific saltmarsh, and a further 430ha of mixed species marsh (a whole Solent estimate?). Colenutt (2001) provides an alternative value of 734 ha. for Keyhaven to Pitts Deep in 1921 based on analysis of hydrographic charts reducing to 297 hectares in 1994 (derived from air photo analysis). The reproductive vigour and rapid vegetative spread of Spartina anglica has been described and analysed by various authorities (e.g. Gray and Benham, 1990; Gray et al, 1991; Raybould et al, 2000). Tubbs (1980) has suggested that the creation of large areas of vacant mudflat may have commenced in the late eighteenth century, as a result of an unexplained loss of species diversity and/or reduction of colonising ability by the native Spartina maritima.

A photogrammetric study of saltmarsh change between 1971 and 2001 revealed continuing major losses in saltmarsh area, primarily as a result of the die-back and loss of Spartina marsh as follows (Bray and Cottle 2003, Baily and Pearson, 2003):

Furthermore, it was estimated that losses were likely to continue in the future so that around 54ha. (total for both areas) would remain in 2050 and only 11 ha in 2100. Colenut (2001) undertook similar predictions for the Lymington to Pitts Deep marshes and drew similar conclusions, except that total marsh loss was estimated by 2090 or sooner. On this basis, it is clear that fine sediments are likely to continue to be released into the estuary as marsh retreat proceeds.

Since the late 1920s, there has been a steady, and cumulatively substantial, decline in the area of Spartina anglica-dominated saltmarsh fronting the shoreline of the north-west Solent Photo 9 and Photo 10). Oranjiewoud (1990) calculate an average retreat of the leading edge of both marshes and mudflats of 4ma^-1, 1867-1968, with acceleration apparent from the early 1950s and Colenut (2002) calculated mean recession of 3ma^-1 for the period 1781-1994. LRDC International Ltd (1993) in a study of the saltmarsh either side of the Lymington River, seawards of the town marina, calculate that there has been close to 300m of retreat of the mudflat edge since approximately 1940 (6ma^-1). However, this research, which was based on detailed analyses of Ordnance Survey maps and aerial photography, indicated a slower recession rate, approximately 1ma^-1, for the upper saltmarsh since 1950. In the Pennington Marshes, to the West, some 640m of onshore movement of MLW has occurred since 1870. Spatial and temporal variation of the rate of loss is thus apparent from these studies; not less than 12% of the original extent of Spartina anglica colonised marsh had disappeared by 1980.

There is convincing evidence that erosion and recession rates have accelerated since the late 1970s. Bradbury (1995), for example, has noted rates as high as 8ma^-1 for two monitored sites at the mouth of the Lymington River, with a mean rate of 3ma^-1 (1992-1994) for this area as a whole. Halcrow (1998) report that MLW has retreated at 7ma^-1 at Pyewell Ploint east of the Lymington River, since 1975, an acceleration by almost 2.6ma^-1 compared to the period 1870 to 1975. However, shoreward movement of the MLW mark of Keyhaven Marshes appears to have been significantly slower, although 300m of recession occurred here in the century between 1867 and 1968. However, accretion has occurred along the margins of, and within, Keyhaven Lake, which necessitates occasional dredging of this minor navigation channel. Recent net accretion along a part of the east bank of the Lymington River has also been recorded (ERM, 1998). Spatial variations in the patterns of past and contemporary erosion and accretion reflect complex interactions between antecedent shoreface width, tidal current velocities and the supply of suspended sediments, as well as the locally very variable patterns of change of saltmarsh and mudflat ecosystem structure. An example of the latter is the loss of Zostera (eelgrass) from areas not taken over by Spartina, as a result of disease, in the 1930s and in the 1990s. In some areas, particularly the mouth of Beaulieu River, the area of Spartina anglica today may be partly controlled by artificial modification of the hydraulic regime, as well in response to deliberate planting (Lobeck, 1995).

The most detailed analysis of the sediment composition and dynamics of the saltmarsh and inter-tidal mudflats of the north-west Solent is by Ke and Collins (1993) and Ke (1995). Both studies noted that inner saltmarshes are elevated above adjacent mudflats, with weakly concave-upward transverse slopes of between 0.2 and 6 degrees. Extensively dissected by creek systems into a virtual pattern of saltmarsh islands, many are abruptly terminated by a bluff or "clifflet" between 0.7 and 1.6m in height fronted by a narrow abrasion platform. Mudflats are characteristically narrower than saltmarshes (the former with a maximum width of approximately 200m), and have higher transverse gradients. Bluff retreat involves the further subdivision of inter-creek blocks, including the temporary creation of small stacks Photo 10). Low relief chenier ridges, composed of a mixture of sand, gravel and shell debris, occur at the edges of several saltmarshes, some at the top of clifflets Photo 3). They are characteristically up to 0.8m in height and 10-20m in width, and are considered to mark a transition from tidally-dominated deposition on mudflats to wave erosion of the saltmarsh edge. Many cheniers rest on erosional surfaces cut into mudflat sediments. They may provide an element of protection (armouring) of the otherwise low erosional resistance of the seaward margins of saltmarsh. Where there is evidence of chernier steepening, an increase in wave erosion (or abrasion) may, perhaps, be inferred (Bradbury, 1995).

The presence of significant quantities of coarse sand, gravel and, particularly, shelly debris indicates wave transport of sediment from nearshore and offshore sources. Much of the sediment composing the saltmarsh is silty clay (0.3 to 0.5 particle diameter), whereas the modal composition of the mudflats is sandy silts and mixed sand, silt and clay. This coarser texture is considered to be due to the mixing of sediment transported by waves and tidal currents from the sub-tidal zone with that removed by landward marsh erosion. At some 2-3m below saltmarsh surfaces, in situ gravel-sized clasts of wood are encountered. These are presumed (Ke and Collins, 1993) to derive from abrasion of offshore (now submerged) peat, probably late Holocene in age. The coarsest silts and sands occur in front of actively retreating clifflets, the result of the selective removal of fines in suspended transport. Sediment texture fines gradually from the main river mouths to inner estuaries, but sand, gravel and shell debris occurs in both the main and many tributary creek channels (Hydraulics Research, 1991). This material is at least partially derived from wave and tidal scour, revealing and releasing underlying substrate sediments.

Ke and Collins (1993) undertook detailed analysis of changes in the position of MHW, and the areal extent of both saltmarshes and mudflats, using successive editions of Ordnance Survey maps back to the early twentieth century. However, they focused on site specific changes since the early 1950s, noting that there has been a landward movement of the 0m isobath of 3ma^-1, 1959-1979, but accelerating to as much as 30ma^-1 since 1980 at a few critical locations. Between 4 and 5ma^-1 of retreat of the outer saltmarsh edge bounding the Lymington River edge occurred between 1959 and 1990; this, however, is only a mean rate, with annual losses of up to 12m having been recorded. Between Oxey and Pennington Lakes, saltmarsh was virtually eliminated between 1900 and approximately 1970, and was replaced by a fringing gravel beach. Lymington Spit was also removed during this period, but rates of edge retreat over Keyhaven Marshes were lower than further east, at approximately 3.5ma^-1. The lowest rates were in the immediate lee of Hurst Spit, at 0.8-1.1ma^-1, 1930-1980. These figures are generally consistent with those calculated by other research work, summarised in preceding paragraphs. However, it was apparent that there had been expansion landwards of the rear boundary of inner saltmarsh in some areas, thus partly compensating for losses at the eroding seaward front. Transverse profiles have also been more or less constant, suggesting that they have accreted vertically whilst retreat has been ongoing. Although more difficult to measure, using mean LWM, Ke and Collins (1993) were able to demonstrate retreat of the seaward margin of sub-tidal mudflats, at an average rate of 4.3ma^-1 (1974-199) for much of the area between Pennington and Pitts Deep. Overall, this evidence of erosion and retreat suggests that the present day inter-tidal morphology is not adjusted to prevailing coastal hydrodynamics.

In contrast to the lateral erosion of both saltmarsh and mudflat, saltmarsh is currently showing net vertical accretion. This is most pronounced in areas where: (a) water depth, at high tide, is less than 0.2m; (b) topographical slope is weak, and (c) there is a continuous vegetation cover. These factors combine to promote the deposition of suspended sediments, which are then more likely to be trapped and retained by vegetation. The success of Spartina anglica in this respect is apparent from 20-50cm thick sequence of finely laminated sediments in the central areas of many marshes where there is no "dieback" around inter-creek pans. Assuming that Spartina anglica has been an effective agent of deposition since approximately 1900, this would give an accretion rate of 2-5mma^-1; actual rates of 3-4cma^-1 are possible in the few small areas where Spartina remains vigorous or has even recently expanded. Ke and Collins (1993) do not provide any field-based measurement as the basis for this inferred rate of accretion, but their conclusion is indirectly confirmed by sample measurements of suspended sediment concentrations. These are in the order of 45-60ppm, with values on spring tides higher than on neaps. They are highest at the edge of saltmarshes, where wave abrasion would be active. This is confirmed by significantly higher suspended sediment concentrations in winter, when waves of over 1.6m in height can penetrate into the western Solent. The greater availability of suspended sediment for potential accretion at and adjacent to marsh edges may be a partial explanation as to why erosional losses here in areas of Spartina "dieback" are no faster than they are in adjacent areas where vegetation cover is unaffected (given similar exposure to tidal current velocities and breaking wave heights).

Ke and Collins (1993) suggest several reasons why there has been substantial, and apparently accelerating, lateral erosion of saltmarsh and mudflats in recent decades. These include: (i) More exposure to winds and waves from the east and north-east; although the potential fetch is limited, waves from these directions can move unobstructed, as demonstrated by the orientation of the widest abrasion platforms; (ii) An increased frequency of storm surge events and significant wave heights since the early 1950s; (iii) Mean sea-level rise; (iv) Progressive northerly shorewards migration of meanders in the main West Solent Channel; and (v) Several independent, but cumulative, human impacts. These include shoreline protection, promoting "coastal squeeze"; ship-generated waves; marina/berth construction; channel dredging; changes in land use management in the New Forest catchments affecting delivery of fluvial sediment input, and the growth and decline of the salt working industry since the early nineteenth century.

Using data on marsh and mudflat recession and landward migration; vertical marsh accretion, and changes in creek morphology, Ke and Collins (1993) have attempted to calculate a budget for fine sediment for Keyhaven-Lymington saltmarshes. Total eroded volume (including losses from dredging) is calculated at 15.4 x 104m³a^-1. Saltmarsh surface accretion, representing approximately 30% of the loss from edge erosion, plus deposition at creek and channel boundaries, amounts to 3.2 x 104m³a^-1. Thus, it is apparent that the overall budget is strongly negative, and represents a net current loss of around 12 x 104m³a^-1. Sediment yield from inter and sub-tidal mudflat erosion averages as 11.6 x 104m³a^-1, of which an estimated 70% is lost entirely as suspended sediment input into the West Solent. The remaining 30% is thought to contribute to channel and marsh accretion.

It is therefore apparent that the sediment store of fine-grained sediment currently "locked up" as saltmarsh and mudflat is being relatively rapidly transferred to suspended load throughput and loss from the Solent system. The implication is that the current hydrodynamic regime strongly favours store depletion, and that mudflats and outer marshes are in disequilibrium with forcing factors. As the causes promoting erosion are unlikely to alter in the foreseeable future, this situation will continue. Indeed, as relative sea-level rise accelerates, it will probably intensify in coming decades.

It is possible to criticise the approach of Ke and Collins (1993), but it provides the best quantitative assessment of the scale of erosional loss of inter-tidal geomorphology and habitat. There is only one other comparable study, undertaken by Hydraulics Research (1991) for a part of the inner Lymington Marshes. Their figure, of close to 500m³ of volume loss for 1981-90 over a much smaller area is somewhat lower than that derived from Ke and Collins (1993) approach, but the latter also established that the landward margins of upper marsh areas are eroding more slowly.

7. SUMMARY - References Map

1. The Western Solent tidal channel is characterised by former marine inundation and erosion of the Solent River valley formed in soft Tertiary and Pleistocene materials. Its shoreline is soft, low-lying and inherently sensitive to inundation and erosion, even when the driving forces are relatively weak.
2. It has experienced mixed behavioural trends since its inundation in the mid to late Holocene. Initially, it was depositional, but this altered swiftly to erosion following the breakthrough of the land/beach connection at Hurst Narrows. This opened up the Western Solent to tidal flows occurring between the East Solent and Christchurch Bay and established its present form of hydraulic regime. Since that time, the channel bed and eastern mainland shoreline and north Isle of Wight shoreline have eroded releasing significant quantities of sediment, however, western parts have accreted and prograded significantly in the lee of Hurst Spit.
3. Suspended sediments enter Hurst Narrows from Christchurch Bay. Eroding cliffs along the northern Isle of Wight shore contribute significant quantities of mainly fine fresh sediments to the Western Solent, but it is uncertain how much crosses the channel to contribute to local saltmarshes as opposed to becoming transported north-eastwards by the residual tidal flow. Several large re-circulating tidal eddies have been identified in the channel that could assist in the wider distribution of fine sediments in suspension.
4. Since the 1930s a portion of the large sediment store of fine-grained sediment currently "locked up" in the Lymington, Keyhaven and Beaulieu saltmarshes and mudflats has been relatively rapidly eroded and transferred to suspended load. Some sediments are re-deposited onto marsh and mudflat surfaces, but the majority become distributed throughout the Solent system, or lost to the English Channel.
5. It is probable that the channel of the Western Solent has yet to achieve an equilibrium adjustment of form to the imposed hydraulic conditions. The variable historic trends recorded along the shorelines including the erosion of its mainland shore mudflats and saltmarshes may therefore be products of this ongoing adjustment.
6. Shoreline drift of sands and gravels is generally from west to east, although discontinuities occur at the Beaulieu River and also at Stansore Point where a weak drift reversal transports sediments westwards along the north east bank of the outer Beaulieu estuary. Drift is not an effective process along the mudflat-dominated Hurst to Pitts Deep frontage except for movement of the chenier deposits. Discrete shoreline units can thus be identified from: (i) Hurst Spit to Pitts Deep; (ii) Pitts Deep to Beaulieu River; (ii) Beaulieu River to Stansore Point and (iii) Stansore Point to Calshot Spit.
7. Between Hurst Spit and Pitts Deep, a low energy fine sediment accretional environment is developed, although mixed accretion/erosion trends have occurred over the past 150 years with rapid erosion in recent decades.
8. Between Pits Deep and the Beaulieu River the low-lying gravel fringed shoreline is transitional between accretion and erosion, but would appear to have altered from accretion dominance towards becoming erosional and transgressive.
9. Moderate erosion, cliff retreat and beach development occur to the east of the Beaulieu River, with a north-eastward trending shore drift pathway delivering small quantities of gravel towards Calshot Spit.
10. The model outlined above is based on maintenance of the stability of Hurst Spit. Major shoreline change and severe erosion of mudflats, saltmarshes and shorelines in its lee would be accelerated further should Hurst Spit suffer breakdown or permanent breaching.

8. COASTAL DEFENCE AND HABITAT ISSUES - References Map

Intertidal habitats comprising mudflats (670 ha.) and saltmarsh (257 ha.) are the main types present and constitute a major element of the Solent and Southampton Water SPA and Solent Maritime cSAC. Other notable habitats include coastal grazing marsh (600ha, mostly at Pennington and around Warren Farm) saline lagoons (21 ha.) and vegetated shingle. The area is also covered by Ramsar designation, several specific SSSI designations and two National Nature Reserves, thus demonstrating the national and international importance of the area as a whole.

The West Solent shores are included within the recent Solent Coastal Habitat Management Plan (CHaMP) produced by Bray and Cottle, (2003). The plan identifies the present distribution and status of coastal habitats and then goes on to predict future habitat changes likely to occur up to 2001, based on an assessment of geomorphological changes. It provides guidance on habitat management and identifies any habitat creation opportunities that could compensate for future losses.

Bray and Cottle (2003) concluded that where Hold the Line is the coastal defence policy (i.e. throughout most of the frontage), 'coastal squeeze' of mudflat and saltmarsh and habitat would continue in response to relative sea-level rise. Almost complete saltmarsh loss and continuing mudflat narrowing are predicted. The dominant issue is therefore identified as the integrity, and long-term survival, of the mudflats and saltmarshes between (i) Keyhaven and Pitts Deep, and (ii) Lower Exbury and Inchmerry House at the mouth of the Beaulieu River (New Forest District Council, 1997). There are, however, numerous other natural and semi-natural habitats associated with the gravel spits and the transitional margins of higher (inner) saltmarsh and reed swamp areas. Some habitats are essentially artificial, such as the saline lagoons within Pennington Marshes; these are the surviving relicts of the formerly extensive salt producing industry.

Several investigators have attempted to measure both local and regional rates of recession of the seaward edge of the mudflats and saltmarshes between Keyhaven and east of the Lymington Rivers (Ke and Collins, 1993; LRDC International, 1993; Tubbs, 1981; Oranjewoud, 1988, 1990; Hydraulics Research, 1991). There is general consensus that current rates are accelerating and the trend towards net loss almost certainly correlates with the commencement of erosion and "dieback" of Spartina anglica. Cause and effect relationships are complex, and involve consideration of changes in wave climate and hydraulic regime, as well as sea-level rise (Ke and Collins, 1993). Uncertainty is compounded by incomplete understanding of the ecological energetics and succession of the area (Gray and Benham, 1990; Gray et al., 1991), as well as the genetics of the Spartina stock (Raybould et al., 2000).

The Keyhaven and Pennington grazing marshes and their adjacent saltmarsh and mudflats to seaward are now effectively decoupled by the presence of seawalls, dykes and bunds (Halcrow, 1998). Despite attempts to preserve ecological diversity during the upgrading of the Pennington sea defences in the early 1990s (Southern Water, 1989; Rainbow, 1990; Martin, 1994), the restoration of natural habitat dynamics will need to involve the partial breaching of these defences and implementation of a strategic option of managed retreat or re-alignment. Without such an approach, complete loss of fronting saltmarshes and severe mudflat erosion would not only result in loss of European designated habitat, but also place the defences themselves at risk due to decreased wave dissipation. Bray and Cottle (2003) identify several potential opportunities for creation of intertidal habitats involving realignment of defences, notably between Keyhaven and Pennington and around Warren Farm. Unfortunately, there is considerable overlap of some of the best sites with existing coastal grazing marshes and saline lagoons that are valuable European designated habitats in themselves.

Renewed efforts to understand the dynamics of marsh and mudflat erosion, involving carefully-integrated process monitoring (Halcrow, 1998; Bradbury, 1995) have been proposed and several ideas have been developed in some detail. Measures such as sediment recharge (including to cheniers and gravel beaches as well as mudflats) have been reviewed (e.g. Colenutt, 1999), but these are likely only to reduce the rate of mudflat loss hather than to halt or reverse it. In conclusion, it will be essential to adopt a combination of approaches to the problem that have minimum impacts and will maintain existing ecological diversity. This may not necessarily involve the maintenance of all existing plant and animal communities, but instead accept a degree of dynamic change. The existing extents of 'Hard' defence solutions are clearly unsustainable and inappropriate over the longer-term (Bradbury, 1995). Such a pro-active approach, if successful, could make a contribution to balancing irretrievable inter-tidal habitat losses elsewhere in the Solent (Johnson, 1996, 2000).

Several investigations have pointed to (though they have not quantitatively demonstrated) the inefficiency of past attempts to retain gravel transported by weak longshore transport currents along the north-west shoreline between Lepe and Calshot (Venner, 1993; Posford Duvivier, 1994; Halcrow, 1998). Recent improvements in groyne design, and increased efficiencies, (New Forest District Council, 1995) might have a beneficial impact on the stability and diversity of plants and invertebrate fauna colonising backshore and crest environments. Similar opportunities for expanding the extent and diversity of shingle habitats exist at Warren Farm spit. General guidance on appropriate management and habitat creation techniques for this resource have been set out by Doody and Randall (2003).

9. OPPORTUNITIES FOR CALCULATION AND TESTING OF LITTORAL DRIFT VOLUMES - References Map

The lack of significant wave energy, modest development of natural linear beaches and prevalence of groynes means that shorelines of this frontage are not well suited for definitive studies of drift. There are, however, opportunities to improve knowledge of drift and beach behaviour. In particular the provision of improved monitoring of beach profiles should allow calculations of changes in beach volumes from which estimates of drift can be made. Locations especially amenable to study include:

1. Park Shore and Warren Farm Spit.
2. Stanswood Bay and Calshot Spit.

10. RESEARCH AND MONITORING REQUIREMENTS - References Map

The SMP (Halcrow, 1998) has reviewed much of the available information and made recommendations for monitoring and research. Some recommendations are in the process of implementation by the Strategic Regional Coastal Monitoring Programme, a consortium of coastal groups working together to improve the breadth, quality and consistency of coastal monitoring in South and South East England (Bradbury, 2001). A Channel Coastal Observatory has been established at the Southampton Oceanography Centre to serve as the regional co-ordination and data management centre. Its website at www.channelcoast.org provides details of project progress (via monthly newsletters), descriptions of the monitoring being undertaken and the arrangements made for archiving and dissemination of data. Monitoring includes wave and tidal recording, provision of quality survey ground control and baseline beach profiles, high resolution aerial photography and production of orthophotos, LIDAR imagery and nearshore hydrographic survey. Not all of these actions are presently planned for this unit. Data is archived within the Halcrow SANDS database system and the aim is to make data freely available via the website.

The recommendations for future research and monitoring here therefore attempt to emphasise issues specific to the reviews undertaken for this Sediment Transport Study and do not attempt to cover the full range of coastal monitoring and further research that might be required to inform management as follows:

1. Surprisingly little analysis has been undertaken of beach profiles in spite of their potential reveal important trends in beach behaviour. Historical beach profiles require thorough appraisal and analysis along this frontage. A potentially invaluable resource is provided by annual Environment Agency ABMS aerial photography since 1979 on this frontage, with subsequent photogrammetric measurement of profiles. There are have been some uncertainties in the past relating to the reliability of parts of the profile data so it has not been used to its fullest extent along this frontage. It is important both to validate the historical dataset and to introduce robust methods for future profile data collection. It is understood that the Environment Agency initiated such work in 2002 and intend to incorporate the new ground control provided by the Strategic Regional Coastal Monitoring Programme into their profile measurement procedures.
2. Once the quality and consistency of the ABMS profile data sets have been assured it will be important to consider how the profiles should best be analysed. It will be important to identify indicators of beach health such as sediment volume, crest or barrier height and position. Different criteria may apply to free-standing barrier beaches or spits, as opposed to beaches retained in front of sea walls or other control structures. Volume is especially important, but can be difficult to monitor reliably using widely spaced profiles on groyned coasts. Furthermore, an error analysis should also be undertaken to identify the minimum volumetric change that can be resolved with the techniques. Past trends in these indicator parameters (decadal, annual and seasonal) need to be established and a system of routine analysis instituted that would provide early warning of "unusual" trends. It may be that local managers could identify critical thresholds, or minimum values of these parameters that could be applied to trigger specific warnings. To effectively interpret the trends recorded, it will also be vitally important to maintain good records of all beach management activities undertaken.
3. To understand beach profile changes it is important to have knowledge of the beach sedimentology (gain size and sorting). Sediment size and sorting can alter significantly along this frontage due to natural processes and beach management. 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. Examination of longshore and onshore-offshore grading of the various sediment parameters can be employed to indicate or confirm directions of transport, sources of sediment and possible residence (storage) timescales.
4. Understanding of inputs of beach forming sediment from coast erosion, would be enhanced by cliff section mapping and sampling of deposits to reveal the detailed thickness, composition and variability of Plateau and Valley Gravels. This material is of particular importance as it is a major local source of beach material.
5. Littoral drift rates and volumes should be calculated using details of cliff, and shoreface erosion inputs and beach volume changes. This information should then be integrated downdrift from the eroding cliff sediment sources to derive drift estimates based upon beach volume change. Such work would only be viable where significant cross-shore exchanges are small, or can be accounted for. Further basic information on littoral transport could be supplied by repeated site observations of sediment distribution in groyne embayments and against other obstacles to drift. Such approaches have proven especially effective in identifying drift reversals and divergences.
6. Wave energy at the eastern end of the Western Solent may have been underestimated as a process contributing to littoral and - possibly - offshore transport. Systematic collection of some field measured data on significant wave heights and periods, would be a valuable input into attempts to validate hindcast local wave climates.
7. Further work is needed to continue the measurement of mudflat and saltmarsh loss, especially to extend studies to the Beaulieu estuary. A detailed monitoring framework has been set out by Bradbury (1995), based partly on the findings of Ke and Collins (1993) and partly on preliminary field investigations. Consideration of options for the commissioning of further research, including a possible demonstration project for realignment of defences was reported by Colenut (2001) and is currently in progress.

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