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About the Study

SCOPAC Committee

Chairperson Councillor Mrs M Penfold MBE, West Dorset District Council.

Vice-Chair Councillor Jackie Branson, Havant Borough Council.

Technical assistance provided to Councillors by Mr Lyall Cairns (Southern Coastal Group Chair) and Dr Samantha Cope (SCOPAC Research Chair).

Introduction & Acknowledgements

Methods

Map Design, Symbols & Reliability

User Guide

Bibliographic Database

The STS 2012 update

The 2012 update of the SCOPAC Sediment Transport Study (STS) was funded by the Environment Agency under FDGiA, grant number LDW 41230, with additional contributions from SCOPAC.  

It is referenced as: New Forest District Council (2017). 2012 Update of Carter, D., Bray, M., & Hooke, J., 2004 SCOPAC Sediment Transport Study, www.scopac.org.uk/sts.

Sediment Transport Study 2012

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River Hamble to Portsmouth Harbour Entrance

LITERATURE REVIEWPHOTOSMAP

1. Introduction

This is a coastline of low elevation and relief, with a north-west to south-east orientation except for the easternmost sector between Gilkicker Point and Fort Blockhouse. It is delimited to the northwest by the Hamble estuary (Photo 1) and in the south east by the entrance to Portsmouth Harbour (Photo 2) forming a distinct sub-cell within the wider Solent context (Bray et al. 2000). The regulated discharge of the River Meon at Hill Head Harbour interrupts the shoreline continuity, but the overall planform is relatively linear with the shallow arcuate form of Stokes Bay, Gosport and a smaller embayment at Hill Head.

It is developed in Tertiary (Eocene) rocks and overlying Quaternary fluvial and niveo-fluvial sediments, which provide sands, silts, clays and gravels of low resistance to wave energy – although this stretch of shoreline experiences more modest erosional energy than areas further east in the eastern Solent. Eocene strata is mainly concealed by drift materials and/or coastal protection structures and outcrop across the foreshore and at the base of the cliffs west of Hill Head (Photo 3). Gravel beaches are continuous along the entire frontage, and accompanied with groynes and promenade protection between Hill Head and western Browndown (Photo 4 and Photo 5). Most of the shoreline to the west of Titchfield Haven is undefended (Photo 6) with sea walls prominent between Gilkicker Point and Portsmouth Harbour entrance (Photo 7) (Oranjewoud, 1992). Natural cliffs, up to 12m in height comprise of Plateau Gravel and Bracklesham Sands, lie between the western end of Lee-on-the-Solent and along to Solent Breezes. In contrast, the former cliff line in the central and eastern parts of Lee-on-the-Solent has been artificially regraded and protected. The seawall east of Gilkicker (Photo 7) is a long-established protection structure that conceals the foundations of the (now built upon) Haslar spit. This spit forms the western entrance of Portsmouth Harbour.

Erosion rates from survey data are commensurate with the findings of seismic studies of the buried channels of the Solent River (Dyer, 1975). These studies indicated that the main channel of the Itchen and Test proto-tributaries ran across what is now the tip of Calshot Spit, which lies on the opposite side of Southampton Water approx. 1.8km west of Solent Breezes. Contemporary erosion rates which average 0.22m per year at Solent Breezes (Hooke and Riley, 1987) indicate that there has been retreat of 1.1km over the past 5000 years, which is slightly less than observed losses. This suggests that erosion may have been more rapid in the past. This scale of retreat would indicate a considerable potential for the supply of sands and gravels from river terrace gravel that mantle the land eroded so that much of the coarse sediments which now comprises Browndown, Spit Sand and Hamilton Bank could have been derived from this source over the past 4-5,000 years.  In support of this hypothesis, Lonsdale (1969) noted the similarity of gravels from the offshore banks to Plateau Gravels (which are exposed in contemporary cliffs). Rates of coastline recession, and thus release of sediment, were probably faster in earlier millennia prior to the development the wide dissipative inter-tidal terrace.

Interpretation of long-term cliff erosion and beach depletion via a geological history is partly interrupted by the development of accretion structures, particularly the cuspate forelands at Gilkicker (Photo 8) and Browndown (Photo 9). The latter have accumulated as a closely-spaced set of both shore-normal and oblique shingle ridges in former shoreline embayments. Both are the product of the final segmentation of long-term onshore barrier and swash bar migration. At Saltern's Park, Hill Head (Photo 5), a smaller inset of the coastline has been cut off, and reclaimed by a small gravel barrier. A similar but smaller feature impounds the mouth of the Brownwich Stream further west, through which it infiltrates. Complex patterns of swash bars and banks, some orthogonal to the coastline, occur in the inter-tidal and nearshore environments between Lee-on-the-Solent and Hill Head (refer to the Quaternary and Holocene evolution of the Solent for further detail).

Wave action along this section of coast is relatively weak, with almost no penetration of residual swell waves (HR Wallingford, 1997). The shoreline between the Hamble and Hill Head is mainly exposed to waves that propagate across the 12km fetch of the western and central Solent, which includes occasional local storm waves which generate significant wave heights in excess of 1.2m (HR Wallingford, 1995; New Forest District Council, 2010).

The Brambles Bank is a large, arrow shaped, 4km in length sandbar which is exposed on low spring tides (CCO, 2013). This extends to within 1.5km of the coastline at Hill Head, and dissipates inshore wave energy. East of Gilkicker Point, the wave climate is influenced by a larger fetch distance than the shoreline to the west. Significant wave height is in the order of 0.6m, but storm waves of up to 1.5m height occur during periods of strong and sustained easterly winds. Waves from this direction can also refract around Gilkicker Point to obliquely strike the shoreline as far to the northwest at Hill Head. Variation in coastline orientation with respect to potential fetch directions and propagation distances are crucial to the wave climate of each specific sector of this shoreline (New Forest District Council, 2010). Wave climates have been constructed by hindcasting and numerical modelling for several points by HR Wallingford (1995) and by Halcrow and Partners (1993) and Halcrow Maritime (2001). Field observations of wave frequencies, wave heights and wind speeds have been collected by the Maritime Coastguard Agency and the former Naval Air Station (HMS Daedalus), both at Lee-on-the-Solent.

Tidal currents operate parallel or near-parallel to the shoreline. Low tidal current velocities (< 1.2m per second) on the flood stream, are the result of dissipation by the wide and shallow shoreface (Price and Townend, 2000). Faster ebb current velocities (≤2.0m per second) have been measured at the mouth of the Hamble estuary and around Gilkicker point (Halcrow, 2002). Sediment transport in the littoral zone is almost entirely due to breaking waves, but the relative contributions of long- and cross-shore movements have not been determined (Wheeler, 1979; Barnett, 1994; HR Wallingford, 1997).

A major new source of coastal data is from the Defra-funded National Network of Regional Coastal Monitoring Programmes. The Southeast Regional Coastal Monitoring Programme commenced in 2002. The Lead Authority is New Forest District Council, with data collection, analysis and reporting led by specialist teams at the Channel Coastal Observatory (CCO), Canterbury City Council and Adur and Worthing Councils. (See the CCO Annual Survey Reports for further details).

The Programmes (nationally) consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping. Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.channelcoast.org. An extensive high resolution, 100% coverage swath bathymetry survey was completed in July 2013. This was commissioned by the Southeast Regional Coastal Monitoring Programme, with the survey coverage extending between Lee-on-the-Solent and Selsey Bill and offshore to abut with the northeast Isle of Wight survey boundary, between Cowes and Bembridge.

The Southeast Regional Coastal Monitoring Programme measures nearshore waves using a network of Datawell Directional Waverider buoys. The nearest directional measurement stations to this cell are at Milford-on-Sea and Hayling Island. The buoy deployed at Milford-on-Sea is in 10mCD water depth. Between 1996 and 2012, the prevailing wave direction was south-southwest, with an average 10% significant wave height exceedance of 1.31m. The buoy deployed at Hayling Island is in 10mCD water depth. Between 2003 and 2012 the prevailing wave direction was south, south, west, with an average 10% significant wave height exceedance of 1.26m (CCO, 2012).

2. Sediment Inputs

The coastal segment between the River Hamble and Portsmouth Harbour Entrance is almost isolated from westward moving littoral drift input by Portsmouth Harbour entrance (Harlow, 1980). The mouth of the River Hamble forms a similar boundary to the northwest. Studies have not ruled out the possibility of littoral drift across the entrance to the Hamble, where tidal currents are likely to predominate over wave action, but drift is weak (Wheeler, 1979; HR Wallingford, 1995). Sediment transport is therefore likely to be along the tidal channel rather than across it. Regular hydrographic surveys close to the Hamble entrance have been undertaken (ABP, 1994b) but these do not provide direct evidence of sediment transport. However, there may be some by-passing of a transient littoral drift barrier at Solent Breezes, as well as the mouth of the Hamble, by suspended sediment (Bray et al., 1995).

Sediment input from fluvial sources is possible from the River Hamble and River Meon, but is minor relative to the coastal inputs. Both rivers derive part of their flow from the Chalk and consequently have fairly stable discharges (Webber, 1980). Supply of suspended sediments is therefore limited to the sub-catchments of both rivers underlain by Eocene sands and clays and drift sediments. Peak discharges may not be sufficient to supply significant quantities of coarse materials by bedload (Rendel Geotechnics and University of Portsmouth, 1996). The Meon flows into Titchfield Haven, a freshwater lagoon and marsh that has formed within the infilled and reclaimed former estuary of the Meon that extended inland to Titchfield up until the mid-seventeenth century (Photo 10). Thus, it is likely that any sediment transported by the river is deposited and stored in this area rather than transferred to the littoral zone. The River Alver discharges via an outfall in Stokes Bay, but provides negligible sediment input; however, the presence of a now relict tidal delta indicates that it was formerly more significant.  

Analysis of Coastal Monitoring Programme 2008 to 2012 lidar, 2003 and 2012 aerial photography and topographic baseline survey data, combined with other datasets, academic research and historical studies has enabled sediment budgets, transport rates and directions to be identified or verified.

2.1 Offshore to Onshore Feed

F1 Salterns Park

The shallow, relatively flat nearshore seabed sediments between Hill Head and Browndown are a complex pattern of gravels, sands and muds, with bar-like features on the lower foreshore between Hill Head and Salterns Park.

The morphology and behaviour of the beach at Salterns Park (Photo 5) suggests that onshore gravel feed may have created a barrier feature at some distance seawards from the cliffline extent (Brumhead, 1963; Korab, 1990). Onshore migration of this feature was measured at a rate of 0.67m per year for the period 1859-1964 (Hooke and Riley, 1987) and 1.9m per year for the period 1954-1962 (Lewis and Duvivier, 1962). Onshore migration, possibly supported by continued sediment supply, must have occurred over the previous 100 to 150 years to create the barrier landform that was described by Brumhead (1963). The source of this sediment is uncertain, but a series of onshore migrating swash bars identified on the lower foreshore indicate potential for onshore feed as seen in the 2003, 2005 and 2008 aerial photography (CCO, 2012).

Offshore/nearshore barrier and bar topography is both complex and long-established, as evidenced by the presence of a Neolithic occupation site at Rainbow Bar (Hack, 1998; 1999).

Bedforms are evident further offshore, often located on the flanks of the northwest-southeast deeper water channel, which runs closest to shore off Browndown and Gilkicker. Analysis of these bedforms suggests eastward movement of sediment around these headlands.

F2 Meon Shore

The shallow, relatively flat nearshore seabed is a complex mix of gravels, sands and muds, with a series of apparent swash bar-like features on the lower foreshore immediately west of Hill Head harbour (Photo 10 and Photo 11). These features could represent residual onshore feed from an abandoned ebb tidal delta on the foreshore that would have been active at a time when Titchfield Haven was an estuary. The extensive inter-tidal mudflats that extend between Hill Head and further north on the eastern shore of Southampton Water are relatively featureless.

2.2 Fluvial Input

FL1 Brownwich Stream

A small, but significant, gravel barrier exists at the mouth of the Brownwich Stream (Korab, 1990). This prevents any direct discharge into the Solent thus minimising any potential sediment input.

FL2 Meon and Hamble Rivers

Rendel Geotechnics and the University of Portsmouth (1996) estimate that the Meon and Hamble would both supply a suspended load of some 2,500 tonnes  per year and a bedload input of somewhat in excess of 700 tonnes  per year. Actual delivery to the coastal zone may be less than 25% of these totals owing to the availability of natural and artificial sediment stores in both river systems. In particular, the Meon flows into Titchfield Haven, a freshwater lagoon and marsh that has formed within the reclaimed former estuary of the Meon (Photo 10), thus, it is likely that any sediment transported by the river is deposited and stored in this area rather than transferred to the littoral zone.

FL3 Alver River

The present day mouth of the Alver to the immediate east of Browndown is associated with a small cuspate tidal delta. As the river drains from a small catchment of negligible relief, sediment input is likely to be small. It is probable that this feature is relict, and was created by the Alver before its mouth was blocked, and its course diverted eastwards via a culvert (due to the growth of the Browndown gravel foreland). A recent tracer study (Eastern Solent Coastal Partnership, 2013) supports littoral drift transport from west to east.

2.3 Coastal Erosion

» E1 · E2 · E3  · E4

Analysis of Coastal Monitoring Programme lidar, 2003 to 2013 aerial photography, and topographic baseline survey data, combined with other datasets, academic research and historical studies has enabled sediment budgets, transport rates and directions to be identified or verified. Low eroding cliffs between Hill Head and the River Hamble are rich in gravel and sand and have been estimated to yield between 5,000 to 8,000m³ per year of materials to local beaches (HR Wallingford, 1997).

E1 Hook Park to Solent Breezes (see introduction to coastal erosion)

Low eroding cliffs of Tertiary sands and silts overlain by Quaternary gravel extend up to 400m west of Solent Breezes. The erosion rate has been calculated between 0.28m and 0.5m per year using comparison of Ordnance Survey maps covering the period 1910-1964 (Hooke and Riley, 1987; Halcrow, 2002). Assuming a mean cliff height of 3m (Wheeler, 1979), coast erosion input was estimated at 112m³ per year of predominantly fine sediments. Posford Duvivier (1997) suggested a higher figure of up to 400m³ per year, based on the estimation of local cliff lithology at 30% gravel and 70% sand-silt. However, analysis of Coastal Monitoring Programme data indicates the finer-grained cliff-derived sediment is not retained on the foreshore and are transported into the Eastern Solent, whilst less than 1,000m³ per year of the coarser fractions is retained on the lower beach. This is a reduction from the 2004 estimated rate of 3,000-10,000m³ per year.

E2 Solent Breezes to Brownwich Stream (see introduction to coastal erosion)

Although the 500m length of the Solent Breezes frontage is currently protected (Photo 12) and probably contributes little sediment, the cliffs immediately to the east, which average 10m high, are actively eroding by both marine and sub-aerial processes (Wheeler, 1979; Hydraulics Research, 1987; Korab, 1990; HR Wallingford, 1995; 1997). The cliffs are composed of sandy clays of the Eocene Bracklesham Formation, capped by a thick unit of Plateau Gravel. A long-term erosion rate of 0.22m per year was calculated by Hooke and Riley (1987) for the period 1910-1964. Measurement of the cliff foot position during beach profiling in 1978 indicated accelerated erosion at 0.57m per year over the period 1964-1978 (Wheeler, 1979). Sampling of cliff materials from three sites yielded a mean composition of 52% gravel (>4mm), 31% sand (>0.125mm) and 17% silt and clay (Wheeler, 1979). Posford Duvivier (1997), however, proposed a lower yield, of between 3,500 and 4,000m³ per year, assuming a cliff composition of 55% gravel and 45% sand and clay. Analysis of Coastal Monitoring Programme data indicates a lower rate of 1-3,000m³ per year of cliff-derived sediment is retained on the lower beach, a reduction from the 2004 estimated rate of 3,000-10,000m³ per year.

E3 Brownwich Farm to Hill Head (see introduction to coastal erosion)

Field observations of the cliffs in this section revealed a mean height of 9m, with western parts eroding strongly (Photo 3) and eastern parts degraded and vegetated, and the lithological composition is similar to the cliffs further west (Korab, 1990). A long-term erosion rate, of between 0.1 to 0.2m per year was calculated by Hooke and Riley (1987) and Bray and Hooke (1997) covering the period 1870-1964. Posford Duvivier (1997; 1999) suggest a rate of between 0.1 to 0.6m per year using the same evidence with the addition of two intermediate map revisions that reveal a tendency for more rapid erosion of western parts.

Analysis of Coastal Monitoring Programme data supports the spatial variation in erosion rates from the SCOPAC Sediment Transport Study (2004) and indicates 3,000-10,000m³ per year of cliff-derived sediment is retained on the lower beach.

At Meon Shore, the cliffs are of lower elevation and almost completely vegetated, being protected by a wide beach and shore platform, which thereby suppresses potential erosion rates.

Further east between Hill Head and Salterns Park a line of degraded and vegetated cliffs is set back from the shingle beach (Brumhead, 1963; Korab, 1990). Although previously subject to marine erosion, the cliffs have been protected by onshore migrating coarse clastic swash bars and cannot have supplied sediment to the beach for over 100 years (Brumhead, 1963). Eastward, at Lee-on-the-Solent, significant cliff erosion occurred until 1959 when the cliffs were regraded and protected by a seawall. Erosion averaged 0.34m per year for the period 1909-1959 (Brumhead, 1963) and 0.28m per year for the period 1860-1964 (Hooke and Riley, 1987). It is not possible to determine precisely the detailed character of previous supply, although Lewis and Duvivier (1954), Everard (1954) and Lonsdale (1969) report that a thickness of up to 5m of Plateau Gravel composed most of the cliff height. All of the coast eastwards to Elmore is protected and therefore yields no sediment from cliff erosion.

E4 Hamble Estuary (see introduction to coastal erosion)

Erosion of Saltmarsh and mudflats at the margins of the lower estuary are detailed in section 5.5.

2.4 Beach Nourishment

At Meon Shore, replenishment by 3,000m³ of quarry gravel was undertaken in the mid-1980s in conjunction with the construction of a new sea wall and groynes. From here eastward to Lee-on-the-Solent, a continuous sea wall and groynes protect the coast, with different parts defended at different times. At Hill Head, groynes were present in the 1950s causing accretion and reduction of eastward drift potential (Lewis and Duvivier, 1954).

N1 Lee-on-the-Solent Replenishment Scheme, 1996 (see introduction to beach nourishment)

Downdrift beaches at Lee-on-the-Solent had been starved since at least the 1920s of sediment supply by defences’ updrift, but were at that time ungroyned and unable to retain sediment; thus beach levels fell and cliff erosion accelerated (Lewis and Duvivier, 1957; Bray, 1993). Construction of a sea wall and groynes halted cliff retreat in the late 1950s, but the latter did not improve beach levels because fresh supply continued to be intercepted by groynes updrift and the cliffs backing the beach no longer contributed sediments. During the early 1970s and 1980s the bedrock beneath the beach was lowered, the seawall was undermined and damaged and the defences also caused terminal scour and a set back of the coastline at their eastern terminal point. Beach replenishment was suggested as the most suitable method by which beach levels could be improved (Lewis and Duvivier, 1957), but this recommendation was not implemented until 1996, when a substantial recharge of 2.3km of the Lee-on-the-Solent frontage was completed to enhance protection of the seawall and promenade (Halcrow and Partners 1996, Bradbury, et al., (2008). This involved the introduction of 300,000m³ of gravel and coarse sand obtained from the dredging of the main navigation channel in Southampton Water, together with profile regrading and the construction of eleven rock groynes (Photo 4). This beach extends across part of the previously exposed foreshore; with wave run up excluded some 30m seawards of the sea wall to provide a high degree of protection (Fowler, 1998; Banyard and Fowler, 2000). Analysis of Coastal Monitoring Programme data and Bradbury, et al., (2008) suggest that this beach has an overall volume that remains healthy with minimal overall volumetric change despite some localised areas of erosion.

3. Littoral Transport

LT1 Hook Spit to Solent Breezes

The partially stabilised form of Hook Spit, which displays a well-developed distal recurve (Photo 1) is clearly the product of wave and tidal current induced north-westwards drift, which extends from an inferred partial, probably transient, littoral transport divergence at Solent Breezes. Evidence of progressive increases in beach levels east and west of this location is provided by Wheeler (1979). HR Wallingford (1995) modelled drift based on a hindcast wave climate covering the period 1971-1991. For Hook Spit this report determined a potential net westwards drift of around 300m³ per year, with mean annual variations from 200m³ per year to the east to 600m³ per year to the west. For Solent Breezes, H R Wallingford (1995) determined a potential net westwards drift of around 500m³ per year, with mean annual variations from 800m³ per year to the east to 1,400m³ per year to the west. Analysis of Coastal Monitoring Programme indicates less than 1,000m³ per year of beach material is predominantly transported westwards between Solent Breezes and Hook Spit, reflecting the very modest input from cliff erosion along this sector. This is a reduction from the 2004 estimated rate of 3,000-10,000m³ per year.

Map analysis reveals that the beach frontage of Hook Local Nature Reserve has accreted a series of gravel ridges since at least 1910 (Wheeler, 1979; Hooke and Riley, 1987; Korab, 1990). Analysis of Coastal Monitoring Programme aerial photography confirms that although Hook Spit is stable in extent and orientation, it has extended slightly into the Hamble River estuary mouth. Between 2003 and 2013 the spit has prograded north north-eastwards approximately 1m per year with successive ‘storm ridges’ stabilising with vegetation establishing. The sinuous shingle bar-like features appear to be long-established features; there is no conclusive evidence of onshore migration or supply of gravel.

LT2 Solent Breezes to Meon Shore (mouth of River Meon)

At Solent Breezes, beach levels are characteristically low (Photo 12), despite the availability of potential input of gravel and sand from actively eroding cliffs. The increase in beach widths to the east of the drift divergence indicates both greater sediment supply from cliff toe and cliff face erosion, and some acceleration of the net eastwards drift rate. Large flint clasts provide a stable surface, often heavily overlain by seaweeds, across much of the low gradient foreshore.

HR Wallingford (1995) modelled drift based on a hindcast wave climate covering the period 1971-1991. At Hill Head, they determined a potential net eastwards drift of around 1,200m³ per year, with mean annual variations from 1,700m³ per year to the east to 500m³ per year to the west. However, analysis of Coastal Monitoring Programme topographic survey data indicates less than 1,000m³ per year of beach material is predominantly transported eastwards between Solent Breezes and Meon Shore.

Historically, gravel accretion has occurred to the west of the culverted sewage/storm water outfall at Bromwich, and some scour and set back of the position of mean high water has occurred to the east (Lewis and Duvivier, 1948, 1954; Wheeler, 1979; Webber, 1979; Brian Colquhoun and Partners, 1992).  

A combined wave and hydrodynamic model study of this shoreline (Price and Townend, 2000) indicated that strong northwest to southeast drift is driven by storm waves generated in the western Solent. This occurred even when maximum flood tide current flow was operating in the opposite direction.  

Net eastwards drift is also evident from the deflection of the mouth of the River Meon in this direction (Photo 10). This process, of marginal lateral spit growth, is also evidenced by historical map analysis (Lewis and Duvivier, 1954; Wheeler, 1979) using records that extend back to the mid-nineteenth century. Archival data from the late seventeenth century indicates the need to frequently remove sediment accumulations at the mouth of the Meon. The original early seventeenth century land claim of the Lower Meon floodplain, was probably facilitated by the presence of a gravel beach extending eastwards from Meon Shore. This may have originated as a barrier spit. Titchfield Haven is a freshwater environment as the river mouth has been dammed since the lower floodplain was drained nearly 300 years ago.

Analysis of Coastal Monitoring Programme indicates less than 1,000m³ per year of beach material is predominantly transported eastwards between Solent Breezes and Hill Head.

LT3 Hill Head to Gilkicker Point

The coastal sector between Hill Head and Browndown is groyned, thereby intercepting transport and preventing potential transport rates from being achieved (Hydraulics Research, 1987; HR Wallingford, 1995; Oranjewoud International BV 1988, 1992a, 1992b). Sediment distribution in groyne compartments indicates a potential for south-eastward drift (Lewis and Duvivier, 1954; Hydraulics Research, 1987; Korab, 1990).

A drift estimate of 3,000m³ per year at Hill Head was made by Halcrow and Partners (1993) as an input for a beach plan shape model study using a hindcast wave climate. The resultant south-eastward net drift predicted at Browndown was 4,000m³ per year implying a tendency for net erosion along the intervening coastline, confirmed by the Regional Coastal Monitoring Programme data collected along this stretch (CCO, 2012). HR Wallingford (1995) modelled drift based on a hindcast wave climate covering the period 1971-1991. At Hill Head, they determined a potential net eastwards drift rate of around 1,200m³ per year, with mean annual variations of 1,700m³ per year to the east to 500m³ per year to the west. At Lee-on-the-Solent, this investigation determined a potential net eastwards drift of around 3,400m³ per year, with mean annual variations of 4,200m³ per year to the east to 900m³ per year to the west. At Hill Head Harbour, depleted beaches immediately to the east (Photo 10) indicate that the Harbour and the River Meon outfall has an intercepting effect on the drift of coarse sediment (Lewis and Duvivier, 1954, 1962; HR Wallingford, 1995; Posford Duvivier, 1997). East towards Seafield, beaches generally show net accretion suggesting supply by eastward drift (Lewis and Duvivier, 1954; Korab, 1990; HR Wallingford, 1995). This could be explained by drift occurring along the lower foreshore so beaches immediately east of the Meon exit are bypassed.

The upper gravel and lower sand-gravel beach at Lee-on-the-Solent has a history of depletion (Bray, 1993), with outgoing eastward drift more rapid than incoming updrift supply from Salterns Park (Lewis and Duvivier, 1957; Hydraulics Research, 1987; Halcrow, 1993). An 800m beach segment at Lee-on-the-Solent was intensively monitored using a series of 38 profiles measured at monthly intervals between June and October 1989 (Gosport Borough Council, 1989). Visual observations and photographs of sediment accumulation in groyne compartments indicated net eastward littoral drift, a trend supported by beach volume calculations between June and July 1989 which showed erosion in the western part of the sector and accretion to the east. Overall, this segment lost 2,600m³ of sediment over the 4 month period, but this was not necessarily all attributable to littoral drift because onshore/offshore sediment exchange was also possible. Other studies documented similar losses suggesting that this process was probably indicative of longer-term trends (Mason, 1993; Bray, 1993; Halcrow, 1993, 1996: Barnett, 1994).  A small, short-term experimental pebble tracing study along a part of this frontage, undertaken in May 2012 by the East Solent Coastal Partnership, indicated predominantly eastwards movement, with by-passing of rock groynes constructed as part of the beach renewal scheme discussed below.   

In 1996, Lee-on-the-Solent beach was substantially renourished (Fowler, 1998; Banyard and Fowler, 2000; Bradbury et al., 2008) with gravel and coarse sand derived from dredging of Southampton Water. Monitoring of subsequent volume changes suggests overall beach stability, but with some localised small losses of volume and continuing net eastward drift across the rock groyne field. Increasing exposure to easterly waves may transport gravel towards western Browndown where coast protection structures terminate, and significant erosion has previously been identified (Hydraulics Research, 1987; Bray, 1993; Halcrow, 1993, 1996). Although similar in lithology to indigenous beach pebbles, the imported flint gravel differs in colour allowing clear visual observation of its transfer downdrift along the Browndown frontage.

The existing literature provides only limited details of littoral drift between Browndown and Gilkicker Point. There are only a few groynes and other control structures along the arcuate planform of Stokes Bay that may indicate drift direction (Gosport Borough Council, 1991). Generalised sediment transport maps by Lonsdale (1969), Dyer (1980), Hydraulics Research (1987) and Bray (1993) indicate net eastward drift. HR Wallingford (1995), using numerical modelling of wave conditions focused on the Alver outfall in Stokes Bay – and determined a potential net eastwards drift of around 3,000m³ per year, with gross annual variations from 3,700m³ per year to the east to 700m³ per year to the west. Open transport conditions occur in Stokes Bay so these drift rates are likely to be achieved, functioning historically to deliver material to the wide accreting gravel beach at Gilkicker Point.

Analysis of Coastal Monitoring Programme data sets indicate 1-3,000m³ per year of beach material is predominantly transported south-eastwards between Hill Head and Gilkicker Point, although the volumes and rates vary along the frontage due to extent and efficiency of groyne structures and prevailing hydrodynamic conditions. Finer-grained fractions are likely to be removed as suspended load and not retained on the foreshore. This is a reduction from the 2004 estimated rate of 3-10,000m³ per year for Lee-on-the-Solent and Browndown.

LT4 Gilkicker Point to Portsmouth Harbour Entrance

The beach between Gilkicker Point and Portsmouth harbour entrance is narrow, falls away steeply over a very short distance, and is subject to periodic drawdown. Hence this area is protected by continuous sea walls and intermittent groynes (Dobbie and Partners, 1987; Harlow, 1980; and Korab, 1990).

High Water Mark has been stabilised, but littoral drift was determined by Harlow (1980) through analysis of the position of Mean Low Water Mark using Ordnance Survey maps covering the period 1863-1972. Although less accurate than techniques utilising the position of mean high water, this approach indicated substantial erosion of the lower beach, indicating net eastward drift of approximately 2,000m³ per year. This is supported by field observations of sediment distribution in groyne compartments (Dobbie and Partners, 1987). The information covered the period 1868-1972 and was thus representative of long-term rates.

Numerical modelling of wave conditions at Haslar (HR Wallingford, 1995) determined a potential net eastwards drift of around 1,600m³ per year, with gross annual variations from 2,200m³ per year to the east to 500m³ per year to the west. This is unlikely to be achieved in reality due to lack of transportable material.

Analysis of Coastal Monitoring Programme survey data from 2003 to 2012 indicates 1000-3,000m³ per year of beach material is predominantly transported north-eastwards between Gilkicker Point and Fort Monkton. This rate is a reduction from the 2004 estimated rate of 3,000-10,000m³ per year. Finer-grained fractions are likely to be removed as suspended load and not retained on the foreshore.

The spit at Haslar, encased by urban development since the mid-eighteenth century, would appear to have a complex origin. Given the limited up-drift feed of sediment, it is patently not a conventional spit (detached beach). The cuspate form of Gilkicker Point suggests barrier emplacement associated with mid to late Holocene sea-level rise, complicated by later spit extension and recurvature at the constricted entrance to Portsmouth Harbour (see section on Quaternary History of the Solent).

4. Sediment Outputs

4.1 Transport in the Offshore Zone

Southampton Water

The 2004 SCOPAC map had an O1 arrow between Calshot Spit and Hook Nature Reserve, representing the analysis of bathymetric survey data which revealed linear furrows adjacent to the banks aligned with tidal current flow in the main channel of Southampton Water, indicating net southward sediment transport (Dyer, 1970; Flood, 1981). Output of sediment at the mouth of the Hamble estuary, where ebb tidal currents reached velocities sufficient to entrain coarse sand made a presumed small separate contribution. Sediments in this area appeared as a complex pattern of gravels, sands and muds, with predominantly finer materials in the eastern part (British Geological Survey, 1989). Thus it was probable that most suspended sediment that is transported adjacent to the Hook to Solent Breezes shore, comprises coarse silt and fine sand. Analysis of tidal streams revealed that suspended sediments are subject to net transport into Southampton Water (Webber, 1980). This arrow contradicted the F1 arrow in the Southampton Water map and there is no further evidence to support it. Therefore the arrow has been removed.

Hamilton Bank and Spit Sands

Transport along the Portsmouth Harbour tidal channel is south-eastward and dominated by ebb current flow from Portsmouth Harbour (Hydraulics Research, 1959; Lonsdale, 1969; Harlow 1980; HR Wallingford, 1995). Offshore transport is apparently westward, but then reverses back towards Gilkicker Point, powered by tidal currents and wave action. Although these banks are believed to be a sediment sink (Harlow, 1980), chart comparisons covering the period 1783 to 1972 reveal some natural erosion since 1893 and significant lowering since 1964 (Fishbourne, 1977). This coincided with dredging of 1,680,000 tonnes over the period 1966-1975. Fishbourne (1977) calculated a theoretical lowering of the sea bed over the dredging licence area based on reported extraction, corroborated by chart measurements over the same period. Comparison of these analyses revealed that actual bed lowering was significantly less than predicted from dredging, which suggested sediment input at 62,000m³ per year. The source of input was probably from other adjoining areas of the seabed. Beaches probably supply relatively little material, because the foreshore between Gilkicker Point and Portsmouth Harbour entrance is depleted and drift rates are low.

The 2004 SCOPAC map had an O2 arrow representing the above. The lack of evidence for the movement described above in the 2004 version of the study coupled with no further evidence to support it now means the arrow has been removed.   

A high resolution, 100% coverage swath bathymetry survey was commissioned by the Southeast Regional Coastal Monitoring Programme. This survey, covered 194km² of seabed between Lee-on-the-Solent and Selsey Bill and offshore to abut with the northeast Isle of Wight survey boundary, between Cowes and Bembridge, was completed in July 2013. The seabed between the Portsmouth harbour channel and the main northwest-southeast East Solent channel is relatively shallow, with sufficient thickness of surficial sediments to mask the underlying bedrock. There is a relative absence of distinct bedforms nearshore, however, bedforms are evident further offshore located on the flanks of the northwest-southeast deeper water channel, which runs closest to shore off Browndown and Gilkicker. Analysis of these bedforms suggests eastward movement of sediment around these headlands.

4.2 Estuarine Sediment Transport

EO1 Portsmouth Habour Entrance Tidal Channel

Sediments transported eastward from Gilkicker Point (LT4) are moved into Portsmouth Harbour entrance tidal channel (Lonsdale, 1969; Harlow, 1980) whereupon the dominant ebb tidal current (Hydraulics Research, 1959; Lonsdale, 1969; Harlow, 1980; HR Wallingford, 1995; 1997) flushes them seaward. The final sinks for these sediments, comprising sand and gravel, appear to be Hamilton Bank and Spit Sands (Lonsdale, 1969; Harlow, 1980; HR Wallingford, 1995). This is supported by a high resolution, 100% coverage swath bathymetry survey was commissioned by the Southeast Regional Coastal Monitoring Programme. This survey, covering 194km², extended between Lee-on-the-Solent and Selsey Bill and offshore to abut with the northeast Isle of Wight survey boundary, between Cowes and Bembridge, was completed in July 2013. The seabed between the Portsmouth harbour channel and the main northwest-southeast East Solent channel is relatively shallow, with sufficient thickness of surficial sediments to mask the underlying bedrock. There is a relative absence of distinct bedforms nearshore, however, bedforms are evident further offshore located on the flanks of the northwest-southeast deeper water channel, which runs closest to shore off Browndown and Gilkicker.

5. Sediment Stores and Sinks

5.1 Beach Sediments and Morphology

The majority of beaches along this coastal segment are composed of a gravel upper berm with a steeply sloping face abruptly terminating on a low gradient, wide, inter-tidal shoreface composed of fine sediments and scattered superficial gravels. Considerable variation exists around this typical beach-type with detailed sedimentology and morphology responding to spatial variations in wave energy, currents, sediment availability and shoreline management.

Several beaches display consistent patterns of particle size sorting, with coarsest material on the backshore. Mid- and high-tide berms are characteristic of the sector between Salterns Park and central Browndown. Beach nourishment, profile regrading and rock groyne construction of the Lee-on-the-Solent frontage in 1996 has substantially altered natural morphodynamics and sedimentology (Fowler, 1998).

5.2 Beach Volumes

The upper beach at Hook Nature Reserve is substantial and includes an accreting series of low gravel ridges that decline eastward to Solent Breezes (Wheeler, 1979; Korab, 1990). Beach volume then increases eastward to Meon Shore (Photo 11). Immediately east of Hill Head harbour (Photo 10 and Photo 13), beach volume is small (Lewis and Duvivier, 1948, 1954), but increases eastward to Salterns Park (Photo 5) where a substantial upper shingle beach has accumulated (Brumhead, 1963; Korab, 1990). By comparison, the beach at Lee-on-the-Solent has been much less substantial (Brumhead, 1963; Hydraulics Research, 1987; Korab, 1990), with a history of steepening and narrowing extending back to the 1870s (Bray, 1993; Halcrow, 1996).

Gosport Borough Council measured beach volumes for an 850m segment of the Lee-on-the-Solent foreshore. A total of 38 profiles were monitored at monthly intervals over a 4 month period in 1989. Beach volume was calculated above Chart Datum (-2.7m OD) and assumed that sediment rested upon a horizontal surface. Total volume was 40,000m³ to 42,000m³ per year (47,000m³ to 49,000m³ per km), with no indication given of the relative proportions of sand and gravel. Beach volumes were also calculated from profiles measured by the National Rivers Authority using aerial photographs covering the period 1984 to 1989 (Gosport Borough Council, 1991). This relatively small, and evidently diminishing, beach volume was the trigger to the renourishment of Lee-on-the-Solent beach in 1996. Total beach volume (above -2mOD) for the 4km segment between western Lee-on-the-Solent and the River Alver outfall in Stokes Bay, Gosport was 841,000m³ to 889,000m³, of which approximately 700,000m³ was stored at Browndown (389,000m³ per km).

The Southeast Regional Coastal Monitoring Programme calculated beach volume above Mean Low Water Springs in 2012. Beach volume was found to vary from around 120m³ per m length of coastline from Solent Breezes to Hill Head, reducing to 107m³ per km from Hill Head to Lee-on-Solent, increasing again to around 240m³ per m along the frontage from Browndown to Gilkicker Point (CCO, 2012).

5.3 Beach Accretion and Erosion

Historic beach changes have been assessed by reference to net advance or retreat of the Mean High and Low Water Marks as indicated on successive Ordnance Survey maps since the mid nineteenth century. Analysis of the Coastal Monitoring Programme survey data from 2003 to 2012 has been used to assess the main areas of accretion and erosion based on the percentage change over that period.

Hook Spit

Ordnance Survey map comparisons indicate accretion and northward extension of the spit between 1870 and 1870-1965 (Hooke and Riley, 1987). Extension into the mouth of the Hamble at a rate of 1.0-1.5m³ per year was reported by Hydraulics Research (1987). Johnson et al. (2007) report that there are anecdotal records of beach overtopping during storm conditions, with some apparent smothering by gravel of salt marsh confined behind the spit. Analysis of Coastal Monitoring Programme survey data from 2003 to 2012 indicates little change to this stretch of coastline over the monitoring period (CCO, 2012).

Hook Nature Reserve

Significant upper beach accretion by successive gravel berms has occurred along this section with a maximum 1.1m³ per year advance of the High Water Mark over the period 1910-1964 (Hooke and Riley, 1987). The intertidal zone narrowed from 500m in 1910 to 300m in 1964 (Hooke and Riley, 1987). A similar pattern of upper beach accretion and lower foreshore narrowing was also revealed from map analyses by Wheeler (1979). Lower foreshore loss did not necessarily indicate major erosion because the offshore gradient was extremely low and small differences in either tidal or lower foreshore levels could have a significant effect on the mapped position of the low water mark. Analysis of Coastal Monitoring Programme survey data from 2003 to 2012 indicates little change over the monitoring period to this stretch of coastline (CCO, 2012).

Solent Breezes

Beach profiles and map comparisons reveal significant erosion and declining beach levels, a feature attributed to coastal protection measures, which at least partly prevented local cliff erosion sediment input and has caused wave reflection at high tide (Wheeler, 1979; Hooke and Riley, 1987). This is supported by the Coastal Monitoring Programme data which shows some indication of erosion in the vicinity of Solent Breezes (CCO, 2012).

Solent Breezes to Brownwich Stream

Map comparisons have previously indicated significant erosion of the cliffs, whilst beach volume has showed relatively little net change over the period 1860-1964 (Lewis and Duvivier, 1954; Hooke and Riley, 1987). The quality of this data is limited by the source documents. Some modest loss of volume between 2003 and 2008 at Solent Breezes is reported (New Forest District Council, 2010).

Brownwich Stream to Meon Shore

Variable accretion and erosion of the High Water Mark occurred over the period 1860-1954 but with no discernible net change in upper beach volume (Lewis and Duvivier, 1954). In contrast, the Low Water Mark receded by 50-100m over the period 1860-1965 (Hooke and Riley, 1987; Bray, 1993) causing narrowing of the intertidal zone between Meon Shore and Hill Head Harbour. Stability of the foreshore was reported by Lewis and Duvivier (1948, 1954) and qualitative observations indicate subsequent net accretion (Korab, 1990). Modest erosion is observed in the Coastal Monitoring Programme data along this stretch of coastline (CCO, 2012).

Hill Head to Salterns Park

The upper beach immediately east of Hill Head harbour was reported as being depleted due to interruption of drift at the harbour entrance (Lewis and Duvivier, 1954, 1962). Further east, groyne compartments in the early 1980s were well filled by gravel, which overtopped some groynes, suggesting accretion since the structures were built (Korab, 1990). Both Brumhead (1963) and Hooke and Riley (1987) described onshore movement of the swash berm at Salterns Park between 1930 and 1960 (Photo 5) but the overall trend of beach volume fluctuation was uncertain. The berm ridge was stabilised by construction of a sea wall and groynes in 1968, with subsequent accretion and filling of groyne compartments (Korab, 1990).

Bray (1993) noted that Low Water Mark recession was a consistent feature of Meon Shore, Hill Head east of the mouth of the Meon (Hill Head Harbour) and Salterns Park, 1870-1964.

Coastal Monitoring Programme data agreed that the overall beach volume fluctuates over time in this area. However, conversely, from 2004 -2012 the trend over this stretch of coastline was one of modest accretion (CCO, 2012).

Lee-on-the-Solent

Beach levels have been generally low and significant sediment loss is indicated since the early twentieth century (Brumhead, 1963; Korab, 1990; Bray, 1993; Halcrow, 1993, 1996). An erosive phase was recognised by Lewis and Duvivier (1954). Recession of the Low Water Mark at up to 2.65m per year and narrowing of the inter-tidal zone were calculated by Hooke and Riley (1987) from map comparisons covering the period 1898-1964. Any sediment loss from the foreshore was critical because the underlying Barton Clay is highly erodible (Lewis and Duvivier, 1957). Between 1984 and 1993, annual beach profiles were measured from aerial photographs. From these, beach volumes were calculated and analysis revealed net accretion of 800m³ per year between 1984 and 1989 (Gosport Borough Council, 1991). This information was contrary to previous investigations and recent site observations, but analysis by Korab (1990) demonstrated that some of the profiles were inaccurately plotted. Comparison of intensive monthly profiles measured in the field from June 1989 to October 1989 revealed initial accretion of 800m³ over the first month followed by loss of 3,400m³ over the following three months (Gosport Borough Council, 1989). Net sediment loss was calculated to be 2,600m³ per year but the study period was too short to be representative of long-term trends. Map evidence for the past 100 years, together with qualitative site investigations for the past 45 years, reliably indicate net sediment loss, a phenomenon supported by beach profiles measured in the field for restricted time periods (Bray, 1993). Taken overall, the inter-tidal profile both shortened and steepened over several decades preceding its transformation as a result of a 300,000m³ of gravel and coarse sand replenishment in 1996.

Analysis of Coastal Monitoring Programme survey data from 2003 to 2012 also indicated some erosion along this stretch of coastline amounting to around 15,000m³ over the 9 year period (CCO, 2012).

Browndown

Map comparisons reveal a variable pattern of accretion and erosion over the period 1898-1965. Accretion at the High Water Mark at the River Alver outfall was 0.50m³ per year over the period 1864-1964 (Hooke and Riley, 1987). Beach profiles measured from annual aerial photographs over the period 1984-1989 reveal relatively stable beach volumes, with net accretion of 1,600m³ per year for the 1.8km segment west of the River Alver (Gosport Borough Council, 1991). Some of this earlier information is not fully reliable due to uncertainties relating to the profiles. Browndown would appear to be a zone of transition between net depletion (west) and net accretion further east towards Stokes Bay. Further slow accretion was recorded during the period 2003 to 2008 (New Forest District Council, 2010) and the period 2008 to 2012 (CCO, 2012).

Stokes Bay

Map comparisons of the western sector revealed fluctuation of the Low Water Mark (with overall recession) and net accretion at the High Water Mark by up to 0.12m per year between 1870 and 1965 (Hooke and Riley, 1987; Bray, 1993). The eastern segment showed High Water Mark accretion of 0.32m³ per year and a stable Low Water Mark (Hooke and Riley, 1987). Overall, this beach, which has few protection structures, maintained an accreting profile and quasi-equilibrium form for several decades up to the late 1990s, but has since experienced a net loss of volume (New Forest District Council, 2010) (Photo 6). Analysis of Coastal Monitoring Programme survey data from 2003 to 2012 indicates minimal change along this stretch of coastline within recent years (CCO, 2012).

Gilkicker Point to Portsmouth Harbour Entrance

Examination of profiles and charts indicates considerable loss of beach materials along the whole frontage over the past 140 years (Dobbie and Partners, 1987). High Water Mark has been stabilised since the eighteenth century by several successive sloping concrete sea walls. Sediment loss therefore resulted in falling beach levels and narrowing of the intertidal zone (Fishbourne, 1977; Harlow, 1980; Dobbie and Partners, 1987; Hooke and Riley, 1987; Halcrow, 1996). These changes resulted in exposure of the vertical sheet-piled toe of the sea wall causing wave reflection, turbulence and scour, with consequent reductions in beach level. It has been reported that beach erosion decreased over the period 1965-1985, but this was probably because very little material remained on the foreshore (Dobbie and Partners, 1987). Significant erosion of the lower foreshore has also been established from comparison of charts for 1783, 1893, 1895, 1934, 1964, 1965, 1972 and 1985 (Fishbourne, 1977; Dobbie and Partners, 1987). Erosion of the lower foreshore was mostly during the period 1885-1965. It can be concluded that aggregate dredging of up to 250,000m³ per year from Spit Sand and Hamilton Bank between 1966 and 1979 significantly contributed to an existing trend for sediment loss. However, declining beach levels were an established trend before dredging began, so this was not necessarily the causal mechanism. Repeated beach topographic surveys between 2003 and 2008 confirm this ongoing trend of volume reduction, except for a small net gain in the immediate vicinity of Gilkicker Point (New Forest District Council, 2010), although surveys since 2008 have demonstrated accretion of 5,600m³ along this stretch of coastline since 2008 (CCO, 2012).

5.4 Reclamation and Dredging

Four marina boatyard sites in the Hamble estuary, occupying approximately 120,000m², have been constructed since the mid-1950s, each of which are sited on former mudflats and saltmarsh (Hamble Estuary Partnership, 2003; Williams, 2006; Cope et al., 2008.) At Mercury Marsh, Hamble Village, 22,600m² was converted to made ground in the 1980s (Bray, 2010). The effect on the estuary tidal prism is unknown. Dredging of the main channel of the estuary and within the several marinas takes place periodically. Refer to the text in Southampton Water for details of estimated quantities removed, most of which is silt and clay sized particles.

5.5 Hamble Estuary: Mudflats and Saltmarsh

Using aerial photography, the Solent Dynamic Coast Project (SDCP) (Cope et al., 2008) calculated that between 1946 and 2000 there was a 41% reduction (0.4% per year) in the area of Spartina saltmarsh (from 61.1ha to 35.7ha). The rate of loss slowed after 1984. Bray (2010), combined map and aerial photographic evidence, to calculate a loss at Hackett’s Marsh of 21% over the period 1946 to 1946. These figures are consistent as that deduced by the SCDP included losses due to reclamation (23% of initial area). Bray (2010) determined a substantial increase of saltmarsh habitat (168%) at Bunny’s Meadow. This could be explained by inundation of previously reclaimed (during the 19th century) land, by breaching of the sea wall in the late 1930s (Hamble Estuary Partnership, 2003). Bray (2010) also calculated an approximate 50% expansion of saltmarsh in the area that is protected from the open coast by Hook Spit although the mechanism for this is not explained. Cope et al. (2008), Bray (2010), Williams (2006) and Baily and Pearson (2007) indicate that the principal cause of the overall loss is erosion by tidal currents and (arguably) wave abrasion of the marsh leading edge and along tidal channels, with a probable contribution from dredging of the sinuous navigation channel and approach channels to marinas and boatyards. The latter directly abut mudflats, some of which are former more elevated saltmarsh. At Mercury Marsh former saltmarsh had converted to Phragmites australis reed swamp by approximately 2000 (Bray, 2010). Whilst the dominant spatial change of saltmarsh and mudflats in the Hamble has been one of recession and fragmentation. Cundy and Croudace (1996) report vertical accretion at 2.8mm per year during recent decades from a monitored mudflat site in the lower estuary. This is probably a response to the release of fine grained sediment into suspension due to edge and creek margin erosion. Further information may be found at http://www.scopac.org.uk/sediment-sinks.html.

Refer also to relevant sections of the text on Southampton Water.

6. Summary and Sediment Pathways

  1. The shoreline between Portsmouth Harbour entrance and the mouth of the River Hamble is bounded at the eastern and western extremities by channels swept by rapid tidal currents, which form effective barriers to lateral sediment transport. It is therefore identified as a discrete littoral drift compartment or cell. Two unequal subsidiary sub-cells are recognised on the basis of a littoral drift divide in the vicinity of Solent Breezes. The larger unit can be differentiated into an erosional shoreline from Solent Breezes to Lee-on-the-Solent and a historically accreting gravel shoreline from Browndown to Gilkicker Point. Overall, the system exhibits classic source-pathway-sink characteristics with low eroding cliffs contributing small quantities of sand and gravel to the shoreline. This sediment is generally moved south-eastwards towards sinks at Browndown and Gilkicker.
  2. Potential sediment input pathways were analysed. Capacity for fluvial input is limited by stable discharge of rivers and small catchments of low erodibility. Onshore feed appears to occur around Hill Head, but is unquantified. Coast erosion is therefore the most obvious sediment input. Low cliffs of Tertiary sands and sandy clays capped by Plateau Gravels eroded at a mean long-term rate of 0.10 to 0.30m per year with localised more recent increase to 0.57m per year. Integration of these erosion rates with cliff height and extent reveals coast erosion input at 5,000m³ per year to 8,000m³ per year between Solent Breezes and Meon Shore. Approximately 80% of this sediment comprises sand and gravel capable of remaining on the foreshore, the remainder is suspended fines liable to offshore loss.
  3. Two distinct and divergent littoral drift pathways are recognised, with the divide located close to Solent Breezes. The westward drift extends to Hook Spit whilst eastward drift transports beach materials along Hill Head, Lee-on-the-Solent, Browndown and Stokes Bay beaches and Gilkicker Point towards the Portsmouth Harbour tidal channel. These pathways have been identified by field observations between 1945 and 2012. Modelling of drift indicates potential eastward drift of around 1,000m³ per year at Hill Head increasing to 3,000m³ per year at Lee-on-the-Solent and in Stokes Bay. Drift at Lee-on-the-Solent was found to also to be moderately sensitive to variations in wave climate direction (HR Wallingford, 1995). Construction of groynes along much of the coast between Hill Head and Browndown has almost certainly reduced the actual drift rate to below the estimated potential. Some recent increases may have occurred due to greatly enhanced sediment availability following large-scale renourishment at Lee-on-the-Solent in 1996. Some of this material is now spilling downdrift to nourish the Browndown frontage.
  4. Early to mid-Holocene invasion of the Meon River valley formed an estuary at least as far inland as Titchfield. Tidal exchange at its inlet would have intercepted drifting sediments and generated an ebb tidal delta. However, following seventeenth century reclamation of the whole of the estuary, this delta became relict and has supplied its sediments to shoaling waves in the form of swash bars that migrate landward over the foreshore. This process continues to the present day and appears focussed on the Hill Head to Salterns Park frontages.
  5. Inter-tidal foreshores between Solent Breezes and Lee-on-the-Solent are likely to continue to narrow, especially in front of defences. Presently active cliffs are likely to erode slightly more rapidly in future due to sea-level rise and climate change and contribute additional sediments that would drift towards the south-east. Groyne fields at Hill Head would probably intercept the majority of these materials leading to some beach accretion. Further downdrift at Lee-on-the-Solent, additional longshore inputs would be negligible, but losses would continue to occur to Browndown so that the newly replenished beach would tend to narrow, albeit at slow rates due to its large volume and controlling rock groynes (Halcrow, 2002; Bradbury, et al., 2008).
  6. The future stability of the low-lying Browndown, Stokes Bay and Gilkicker Point frontages will depend upon the extent to which shore drift from the northwest can be maintained to supply sediments. In the short-term, these areas could benefit from inputs (losses) from the Lee-on-the-Solent replenished beach. In the event of cessation of drift inputs from the northwest, natural re-working of the extensive stored sediments of the Browndown foreland would be likely (Halcrow, 2002).
  7. Analysis of Coastal Monitoring Programme survey data from 2003 to 2012 generally confirmed the conclusions of the 2004 Sediment Transport Study and the new information allowed the drift rates between Solent Breezes and Hill Head and at Stokes Bay to be quantified for the first time, and the drift rate at Hook Spit to be reduced from 1,000-3,000m³ to <1,000m³ (CCO, 2012).

7. Opportunities for Calculation and Testing of Littoral Drift Volumes

Data collected by the Defra-funded National Network of Regional Coastal Monitoring Programmes is pivotal for future improvement in estimating beach change.  The Programmes consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping.  Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.channelcoast.org.

The Southeast Regional Coastal Monitoring Programme commenced in 2002. The Lead Authority is New Forest District Council, with data collection, analysis and reporting led by specialist teams at the Channel Coastal Observatory (CCO), Canterbury City Council and Adur and Worthing Councils. Longer term Coastal Monitoring Programme data, when combined with other data sets, academic research and historical studies may enable sediment budgets, transport rates and directions to be identified and/or validated in the future, although the lack of significant wave energy and the poor development of beaches means that shorelines in this unit are not suited for definitive studies of drift.

8. Knowledge Limitations and Monitoring Requirements

Notwithstanding results from the Southeast Regional Coastal Monitoring Programme, and the summarised information and assessments from the North Solent SMP2 (New Forest District Council, 2010), recommendations for future research and monitoring that might be required to inform management include:

  1. The effective application of numerical modelling studies of beach behaviour and sediment transport processes requires the input of high quality nearshore bathymetric survey data. This is especially important for those sectors of the near and offshore environments with complex landform and sediment associations, e.g. offshore of Hill Head and off Haslar in the vicinity of Portsmouth Harbour inlet channel and tidal delta.
  2. To understand beach profile changes it is important to have knowledge of the beach sedimentology (grain size and sorting). Sediment size and sorting can alter significantly along this frontage due to beach management, especially the practices of recharge. 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. Such sediment data would also be of great value for future modelling of sediment transport, for uncertainty relating to grain size is often a key constraint in undertaking modelling.
  3. 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.
  4. Understanding of sediment exchanges between the beach and sub-tidal regions is limited at present, and could be improved by repeated swath bathymetry of the nearshore region, particularly after stormy periods.

Index

19. River Hamble to Portsmouth Harbour Entrance

References

Reference Map

Close this panel

​01

Start Point to Berry Head

​02

Berry Head to Hope's Nose (Tor Bay)

​03

Hope's Nose, Torquay to Holcombe

​04

Holcombe to Straight Point (including Exe Estuary)

​05

Straight Point to Otterton Ledge

​06

Otterton Ledge to Beer Head  

​07

Beer Head to Lyme Regis

​08

Lyme Regis to West Bay

​09

West Bay to Portland Bill  

10

Isle of Portland and Weymouth Bay  

11

Redcliff Point to Durlston Head (Purbeck)  

12

Durlston Head to Handfast Point

13

Handfast Point to South Haven Point (Studland Bay)  

14

Poole Harbour

15

Poole Harbour Entrance to Hengistbury Head (Poole Bay)

16

Hengistbury Head to Hurst Spit (Christchurch Bay)

Quaternary History of the Solent

​17

Hurst Spit to Calshot Spit (Western Solent Mainland)  

18

Southampton Water  

19

River Hamble to Portsmouth Harbour Entrance  

20

Portsmouth, Langstone and Chichester Harbours  

21

Portsmouth Harbour Entrance to Chichester Harbour Entrance

22

North West Isle of Wight

23

North East Isle of Wight

24a

South West Isle of Wight  

24b

South East Isle of Wight  

25

East Head to Pagham, West Sussex

26a

Pagham to Littlehampton

26b

Littlehampton to Shoreham-by-Sea  

27a

Shoreham-By-Sea to Newhaven  

27b

Newhaven to Beachy Head  

Introduction & Acknowledgements

Methods

Map Design, Symbols & Reliability

User Guide