Copyright © MMXVII SCOPAC Sediment Transport Study. All Rights Reserved
Chairperson Councillor Mrs M Penfold MBE, West Dorset District Council.
Technical assistance provided to Councillors by Mr Lyall Cairns (Southern Coastal Group Chair) and Dr Samantha Cope (SCOPAC Research Chair).
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.
This is a coastline of low elevation and relief, with a north-
It is developed in Tertiary (Eocene) rocks and overlying Quaternary fluvial and niveo-
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-
Interpretation of long-
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-
Tidal currents operate parallel or near-
A major new source of coastal data is from the Defra-
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-
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-
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-
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-
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.
The shallow, relatively flat nearshore seabed sediments between Hill Head and Browndown are a complex pattern of gravels, sands and muds, with bar-
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-
Offshore/nearshore barrier and bar topography is both complex and long-
Bedforms are evident further offshore, often located on the flanks of the northwest-
The shallow, relatively flat nearshore seabed is a complex mix of gravels, sands and muds, with a series of apparent swash bar-
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.
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.
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.
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).
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-
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-
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-
Analysis of Coastal Monitoring Programme data supports the spatial variation in erosion rates from the SCOPAC Sediment Transport Study (2004) and indicates 3,000-
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-
Erosion of Saltmarsh and mudflats at the margins of the lower estuary are detailed in section 5.5.
At Meon Shore, replenishment by 3,000m³ of quarry gravel was undertaken in the mid-
Downdrift beaches at Lee-
The partially stabilised form of Hook Spit, which displays a well-
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-
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-
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-
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.
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-
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-
The upper gravel and lower sand-
In 1996, Lee-
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-
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-
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-
The spit at Haslar, encased by urban development since the mid-
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.
Transport along the Portsmouth Harbour tidal channel is south-
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-
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-
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-
Several beaches display consistent patterns of particle size sorting, with coarsest material on the backshore. Mid-
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-
Gosport Borough Council measured beach volumes for an 850m segment of the Lee-
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-
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.
Ordnance Survey map comparisons indicate accretion and northward extension of the spit between 1870 and 1870-
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-
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).
Map comparisons have previously indicated significant erosion of the cliffs, whilst beach volume has showed relatively little net change over the period 1860-
Variable accretion and erosion of the High Water Mark occurred over the period 1860-
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-
Coastal Monitoring Programme data agreed that the overall beach volume fluctuates over time in this area. However, conversely, from 2004 -
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-
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).
Map comparisons reveal a variable pattern of accretion and erosion over the period 1898-
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-
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-
Four marina boatyard sites in the Hamble estuary, occupying approximately 120,000m², have been constructed since the mid-
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-
Refer also to relevant sections of the text on Southampton Water.
Data collected by the Defra-
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.
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: