Southampton Water

1. INTRODUCTION - References Map

Southampton Water and the estuaries of the Test, Itchen and Hamble are drowned remnants of a major tributary of the ancestral "Solent River". This river system became fully established during the Pleistocene when fluctuating sea-level resulted in numerous phases of inundation (marine sedimentation) and emergence (fluvial downcutting). The most recent emergent phase (Devensian glacial stage) is recorded by deposition of a series of gravel terraces during stands in sea level regression (Everard, 1954; Hodson and West 1972; Allen and Gibbard, 1993). During low sea-level, the main channel was excavated to a depth of at least -21m O.D. at Calshot, -14m O.D. at Fawley and -9m O.D. at Southampton Western Docks (West 1980). The river system was inundated during the Holocene transgression and borehole data suggests that the sea had entered Southampton Water prior to deposition of the earliest dated sediments around 6500 to 6800 years B.P. (Hodson and West 1972).

There have been several subsequent minor transgressions, regressions and periods of stationary sea-level (see unit on Quaternary History of the Solent for further details). The estuary has increased in size since the mid-Holocene through overall submergence and erosion of the north-east coast. The past 7,000 years has been characterised by sedimentation of fine material at a rate similar to local relative sea-level rise over the past 5-6000 years of 1.1 to 1.5mm.a^-1, contributing 3-4000 m³ for every 1 mm of sea-level rise (Hodson and West 1972; Long and Tooley, 1995; Long and Scaife, 1996; 2000). Dyer (1980) suggests a spatially variable accretion rate of 2-10 mma^-1 over the past 3-6,000 years. The Pleistocene and Holocene sediment sequence (Everard, 1956; Long and Scaife, 2000) overlies and mostly conceals underlying Eocene bedrock, but the latter are exposed in marginal cliffs on the north-east shore and where the main channel has been incised below -10 m O.D.

The estuary boundaries are Calshot Spit (Photo 1) in the west and the Hamble estuary (Photo 2) and Hook Spit to the east. Southampton Water is therefore a relatively narrow and enclosed meso-tidal estuary, subject to very limited wave action generated in the estuary itself. The west coast is almost totally sheltered from wave erosion in comparison to the opposing shoreline. Thus, tidal currents dominate sediment transport, especially on western shores and in the inner estuary where fine sediments have accreted (Photo 3). The tidal regime is unusually complex controlled by the resonance effect of the eastwards narrowing of the English Channel, but modified by the hydraulic characteristics of the Solent. An initial flood stand lasting approximately two hours is followed by a normal flood, including high water stand of 5 hours; and a shorter ebb period of 5 hours. As a consequence, ebb currents in the mid and lower estuary are significantly more rapid than flood currents, but this asymmetry virtually disappears upstream from the Western Docks. The mean spring tide range is 4.05m and that for neaps is 2.0m. In general terms, Southampton Water may be regarded as a low flux system, whose morphology and sediment budget are no longer in equilibrium with estuary hydrodynamics as a result of marginal land claim and other artificial modifications of the tidal prism (ABP Research and Consultancy Ltd, 1995e). Residence (flushing) time is between 5 to 10 days (according to location), with a partially-mixed water mass stratification (Dyer, 1980; Sharples, 2000).

A wide range of sediments exists in Southampton Water as a result of Holocene regressive and transgressive phases and contemporary fluvial and marine deposition. Over the past 200 years, the system as a whole has been hydrodynamically stable, but there have been substantial changes affecting the vertical and horizontal dimension of the inter-tidal area. Sediment transport and circulation is complex, with poor understanding of interactions with the external Solent system.

2. SEDIMENT INPUTS - F1 References Map

2.1 Marine Sources

F1 Input of Suspended Sediments from the Solent

Entry of coarse sediments into Southampton Water from the Solent is severely limited due to lack of suitable transport mechanisms although suspended sediments can move into the estuary. The asymmetry of the tidal curve results in significantly more rapid, short-lived ebb currents compared to the flood (Webber 1980). Bedload transport of sand and fine gravel is therefore likely to be in a net seaward direction, determined by maximum current velocity. By contrast, net suspended sediment transport is likely to be into the estuary due to the longer duration of the flood current (ABP Research and Consultancy, 2000; Dyer, 1980; Barton, 1979). Limited field evidence of this effect is available from measurements of suspended sediment concentrations, which indicate supply from the Solent. Restricted measurements in the Itchen estuary (Qadri 1984) and Southampton Water (Dyer 1980) suggested that concentrations increase towards potential sources.

Mean suspended sediment concentrations of 2 mg/litre to 50 mg/litre (mean of 35-40mg/litre) are reported by Dyer (1980) and ABP Research and Consultancy, Ltd., (2000c). Concentrations tend to fall in the up-estuary direction, to average values of 5-20mg/litre between Southampton Dockhead and the Container terminal. Seasonal fluctuations are considerable, with winter peaks in excess of 100mg/litre even at the mouth of the River Test. Qadri (1984) reports very low mean concentrations, 0.6 to 2.5mg/litre in the lower Itchen in July. Sampling by Dyer was from the surface and at mid-depth over an 18 month period at three stations, with the highest concentrations recorded at mid-depth at the mouth of Southampton Water. These findings revealed that concentrations within Southampton Water were especially low compared to other estuaries. Gradients in concentrations indicated that the primary source of suspended sediments was the Solent and the English Channel, with especially low summer concentrations being related to reduced wave energy. Humby and Dunn (1975) reported suspended sediment concentrations of 20-200 mg/litre for other Solent estuaries, which were markedly less than concentrations of 300-2000 mg/litre and 2000 mg/litre recorded in the Thames, Mersey and other British estuaries. The unusually low concentrations characteristic of Southampton Water can be attributed its unique tidal regime and small tidal range, but it is probable that other sediment sources, eg. fluvial and sewage, may contribute more significantly elsewhere.

Sediments are removed from suspension in sheltered parts of the estuary, particularly the western side where accretion is facilitated by reduction of flow velocities imposed by mudflats and marshes, swards and very low wave heights and energy. The long still stand at high water assists this process.

Minerological analyses have revealed a clay assemblage characteristic of local Tertiary sediments (Hodson and West, 1972; Algan, et al, 1994). These may have been derived from the Solent but could also have been eroded from either the north-east shoreface of Southampton Water or the catchments of the Itchen, Test and Hamble (Algan, 1993; Algan et al, 1994). Marine input of suspended sediments has not been quantified, but significant net accumulation of fine-grained sediment in the form of mudflats and marshes, has occurred since at least 6,800 BP, according to radiocarbon dating of organic horizons from borehole samples (Hodson and West 1972, Long and Scaife, 1996; 2000).

2.2 Fluvial Input - FL 1, FL2, FL3 References Map

FL1 Test; FL2 Itchen; FL3 Hamble

Three major rivers flow into Southampton Water, the Test, Itchen and Hamble, draining a combined catchment area of a little over 1500 km². Part of the discharge of these rivers is derived from Chalk groundwater which results in fairly stable hydrographs (Webber, 1980). Southern Water Authority and Environment Agency discharge data has been analysed revealing flow of between 3-13m³s^-1 for the Itchen (Wright and Barnard, 1964; Webber, 1980; Qadri, 1984; Rendel Geotechnics and University of Portsmouth, 1996); 6-25m³s^-1 for the Test (Wright and Barnard, 1964; Webber, 1980; Rendel Geotechnics and University of Portsmouth, 1996) and mean flow of 0.6 to 1.3 m³s^-1 for the Hamble (Wright and Barnard, 1964; ABP Research and Consultancy, 2000 c). Both discharge and velocity characteristics are not conducive to bedload transport.

Samples of suspended sediments were collected at four sites on the Itchen estuary between Itchen Bridge (estuary mouth) and Woodmill Bridge (near the tidal limit). Measurements covered spring and neap tides and surface, mid-depth and bottom levels and revealed that suspended sediment concentrations were greatest at the mouth (1.5 - 2.5mg/litre) and declined upstream (0.6mg/litre). It has been suggested that seawater entering Southampton Water on the flood current is much richer in suspended sediments than the fluvial discharge (Qadri, 1984). Rendel Geotechnics and the University of Portsmouth (1996) calculate that the Itchen and Test rivers contribute between 2,400 to 4,300 tonnes a^-1 of suspended load to the upper reaches of Southampton Water. This is less than 5% of potential supply, most of which is diverted into storage in the extensive flood plains of both rivers. The Hamble River is unlikely to supply in excess of 300-350 tonnes a^-1. Webber (1980) calculated that fluvial discharge into Southampton comprised only 1.3% of the neap tidal prism. Information relating to bedload inputs have not been monitored, but supply may be restricted to occasional peak discharges. Figures of estimated potential bedload supply of 3,668 tonnes a^-1 (Itchen) and 11,324 tonnes a^-1 (Test) are an order of magnitude above actual delivery to Southampton Water owing to numerous upstream sources of storage (Rendel Geotechnics and University of Portsmouth, 1996). Bray (2000) proposes a total potential fluvial sediment input of a maximum of 17,000m³a^-1, but this includes bedload materials that would be stored in the lower river channel reaches and upper estuaries without being supplied directly to Southampton Water.

2.3 Coastal Erosion - E1 E2 References Map

Three distinct areas undergo erosion: the shoreline between the Hamble and Itchen rivers; the saltmarsh margins of the south-west shore between Calshot and Eling; and intertidal flats in several areas of the estuary. Potential erosion losses are prevented along some 26 km of estuary-side frontage by defences maintained by a mix of statutory authorities, commercial and private ownerships (Halcrow, 1998).

E1 The North-East Shore

The coast between the Hamble and Itchen rivers comprises low cliffs up to 9m height cut in sandy gravel and medium to coarse gravel sediments of the Pleistocene (Solent River) Plateau and Terrace Gravel sequence overlying mostly arenaceous Eocene (Selsey Sands) bedrock (Photo 4). At Hamble Common, a cliff line is cut into alluvium directly overlying bedrock. Historically, erosion was probably continuous, but over the past 100 years there has been piecemeal construction of defences and significant lengths are now protected to differing standards (Photo 5). At present, unprotected cliffs are actively eroding through basal undercutting and slab failure. Weathering makes a small additional contribution, but there is no evidence of seepage erosion.

Cliff retreat is generally slow and concentrated at Weston Point, and from Netley Castle to Netley Hard (Hydraulics Research, 1987; Posford Duvivier, 1994; 1997; New Forest District Council, 1993). South of Victoria Park, the cliff line is vegetated and partly wooded, thus suppressing potential erosion. Map comparisons over the period 1870-1965 have revealed retreat at a rate of 0.1 to 0.5m.a^-1, (Posford Duvivier, 1994; 1997; 1999) with some coastal advance by land claim at Hamble Point (Hooke and Riley, 1987). Highest recession rates (0.5ma^-1) appear to be at Hamble Common (Photo 6) and between Netley Abbey and the Royal Victoria Country Park. Site observations in 1988 by Southampton City Council revealed a low (2-3m) gravel and clay cliff 250m long, which was at that time retreating rapidly due to basal erosion attributable to sea wall deterioration. Approximately 20-25% of eroded material, released by cliff erosion, constituting approximately 50-100 m³a^-1 is sand and gravel that is retained on local beaches, with the remainder about 400 m³a^-1 presumed to be removed as suspended load (Posford Duvivier, 1997). Although the yield is very low it appears to have been sufficient in the past to supply the bulk of sand and gravel on the adjacent narrow beaches (Hydraulics Research, 1987). Total supply rate from cliff degradation is influenced by intermittent slope destablisation due to the disturbance of vegetation by basal erosion and/or shallow slides. It is also spatially differentiated by protection structures, which vary in their effectiveness. Inter-tidal shoreface abrasion, which mostly contributes a suspended load, is low and is estimated to yield in the range of 4,000 m³a^-1 of mostly fine sediments (Posford Duvivier, 1996, 1999; Halcrow, 1998).

E2 Saltmarsh and Mudflat Erosion

Saltmarsh

Retreat of Spartina marsh and consequent release of fine sediments has been widely reported for the western shore of Southampton Water - see Photo 7 and Photo 8 (Goodman et al, 1959; Hydraulics Research, 1987; ABP Research and Consultancy Ltd, 2000a, 2000c; Halcrow, 1998; Johnson 1996, 2000) and within the Hamble estuary (Goodman et al, 1959; Nature Conservancy Council, 1984c; Hooke and Riley, 1987; Gray, et al, 1993). This erosion commonly takes two forms; (a) frontal retreat resulting in formation of cliffs 0.5m - 1.5m high separating marshes and mudflats; and, (b) internal dissection of the marsh by erosion of channel margins and development of erosion "pans" (Goodman et al, 1959), although this is less developed here than in other parts of the Solent (ABP Research and Consultancy Ltd, 2000c). One cause appears to be "dieback" of Spartina anglica around inter-creek 'pans' which reduces the stability of accumulated saltmarsh sediments, making them susceptible to wave and/or tidal abrasion. A more complete discussion of this phenomenon is given in Section 5.4. Sediments eroded from saltmarshes must be regarded as a largely re-circulating input released from previous storage.

Spartina now appears unable to naturally colonise previously unvegetated mud flats, or to re-occupy "die back" areas. Field trials conducted by Dicks and Levett (1989) only achieved regeneration by transplanting techniques. Sediments released by erosion are therefore unlikely to be re-incorporated into expanding inter-tidal marshes elsewhere, although they could be deposited upon existing marsh surfaces. Evidence of erosion extends back to the 1930s when the 'die-back' phenomenon was initiated (Goodman et al, 1959; CEGB, 1988; Gray and Benham, 1990; Gray et al, 1991). Ordnance Survey map comparisons covering the period 1870-1965 are not an effective means of documenting longer-term saltmarsh erosion because it is uncertain how accurately the marsh margins were mapped at the beginning of this period (Hooke and Riley, 1987). Some of the most reliable map information relates to the Marchwood-Eling segment, where significant marsh growth was recorded for the period 1910-1932 followed by frontal erosion at 1.7 ma^-1 over the period 1932-1965 (Hooke and Riley 1987). A similar rate of erosion was recorded for the edge of the saltmarsh between Ashlett and Calshot over the period 1910-1965, but in other areas map evidence is inconclusive and only indicates increased saltmarsh dissection (Hooke and Riley 1987). Air photo analysis covering the period 1962-1985 revealed saltmarsh erosion at up to 4ma^-1 between Calshot and Fawley (CEGB 1988). This suggests that erosion may have accelerated in recent decades, but analysis only covered a limited spatial area.

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 66ha would remain in 2050 and only 20-30 ha in 2100. On this basis it is clear that fine sediments are likely to continue to be released into the estuary as marsh retreat procededs.

Inter-tidal Mudflat Erosion

Map comparisons have revealed retreat of Low Water Mark at a significantly more rapid rate than High Water Mark, so that the intertidal zone has narrowed and steepened This occurred during the period 1870-1965 and was recorded for all parts of Southampton Water except the reclaimed docks between the Itchen mouth and Redbridge (Hooke and Riley 1987). Although the low gradient of the intertidal zone makes specification of Low Water Mark susceptible to error due to sea level datums being changed over successive map editions, the reality of net erosion was confirmed by the distribution of eroding mudflat margins, evident from field inspection (Hooke and Riley 1987). Intertidal narrowing was quantified by recession rates derived from map comparisons at the following locations: Calshot-Ashlet (1.7ma^-1 - 1870-1965), Fawley Refinery (0.6ma^-1 - 1868-1966); Weston Point (up to 3.2ma^-1 - 1870-1965); and Itchen mouth to Hamble Point (3.2ma^-1 - 1870-1965). Intervening areas showed similar trends but at generally lesser rates (Hooke and Riley 1987, Halcrow, 1998). Comparable rates, are reported by Oranjewoud (1988) and ABP Research and Consultancy (2000c) and are reviewed by Halcrow (1998). Short-term erosion rates as high as 8 m.a^-1 may have prevailed in the 1940s, but the fastest recession rate for recent decades is approximately 4 m.a^-1, between Calshot and Fawley, 1962-1985 (ABP Research and Consultancy, 1995c).

Causes of marsh and mudflat erosion are uncertain but several, probably inter-related, possibilities have been suggested. These include an absolute shortage of sediment (Geodata Unit 1987), on-going relative sea-level rise; land claim, coast defences, channel dredging and the failure of some intertidal marshes and mudflats to store sediment released by co-adjacent erosion. Barton (1979) reported that some deeply dredged navigation channels were as close as 200m to the leading edges of inter-tidal mudflats, e.g. Marchwood-Eling (Photo 8). Here, inter-tidal slopes were frequently much steeper (9-13 degrees) than those more remote from dredged channels and where cut into Pleistocene gravels. This was attributable, at least in part, to failure of oversteepened dredged channel margins. Some localised losses have specific causes, such as oil spillage on Fawley marshes causing marsh plant mortality (Oil Pollution Research Unit, 1994); there has been a positive effort to encourage regeneration at this location, thus compensating for erosion losses. Waves created by shipping movements (48,430 vessel movements in 1998) may make a small input to the erosion of mudflat margins in the mid and upper estuary. ABP Research and Consultancy (2000a) estimate that they contribute about 2% of energy received at Netley Shore, but some 10% at Hythe. Most larger vessel movements are likely at or close to high tide when waves generated are likely to strike saltmarshes rather than be dissipated by the lower foreshore.

Inter-tidal and erosional losses have also taken place in the estuaries of the main tributaries of Southampton Water; however, only the River Hamble has been examined in any detail. Volumetric analysis of bathymetric charts, 1965-1993, (ABP Research and Consultancy, Ltd., 1994a), shows an overall trend for erosional loss downstream from the m²7, railway and Bursledon Bridges, but a slight positive balance upstream. Most of the losses and gains were related to changes in the position and depth of the main channel. However, Cundy and Croudace (1995) reported consistent sub-tidal mudflat erosion adjacent to vertically accreting saltmarshes, but with no correlation between accumulation rates and site elevations. It is uncertain if changes in the sediment budget are primarily the result of channel dredging and marina construction. A net volume loss of subtidal sediment of 156,000m³ is computed based on an average rate of erosional retreat of mean low water mark of 4.7mma^-1.

Changes in the extent of inter-tidal mudflats in the Hamble estuary have not been specifically quantified. Areas of both accretion and erosion have been documented (Gray, et al, 1993; Cundy and Croudace, 1995; Cundy, et al, 1997). Contemporary losses through marsh edge recession, both up and downstream of the three bridge sites, are noted by the Universities of Newcastle and Portsmouth (2000). They are most marked adjacent to inner bends of meanders of the main estuary channel, particularly on the western shore and in the vicinity of marina sites.

Chart comparisons (ABP Research and Consultancy, 2000c) indicate that most sub-tidal areas within Southampton Water have been relatively stable over the past 100-200 years, but some fluctuations that may have occurred during this period cannot easily be detected from this evidence. There is no specific evidence to support the view that erosion of inter-tidal areas has been balanced by gains in the sub-tidal environment. However, most authorities propose that suspended load sediment derived from marshes and mudflat erosion is subject to re-circulation at various timescales, within the estuary. The fate of such material, however, is not understood fully.

Using a largely theoretical approach based on the assumption of a rate of 1mma^-1 vertical erosion, Posford Duvivier (1999) calculate that: (i) approximately 4,500m³a^-1 to 13,500m³a^-1 of fine sediments are removed from the inter and sub-tidal shoreface between Hythe and Fawley; (ii) 900-2600m³a^-1 at Fawley, where there are protection measures and attempts to stimulate Spartina recolonisation; and (iii) 3-9,000m³a^-1 between Fawley and the marshes protected by Calshot Spit. Spatial variations of erosion losses and shoreface width would appear to correlate closely. These losses are in addition to those involving marsh retreat on the mid to upper foreshore

3. LITTORAL TRANSPORT - LT1/2 References Map

LT1, LT2 Hamble and Netley to Weston Point ( see introduction to littoral transport)

Discernible littoral drift is only reported for the beach and foreshore between the mouths of the Hamble and Itchen rivers where narrow upper beaches of gravel and coarse sand are developed (Photo 9 and Photo 10). Limited site observations indicate drift directions from sediment build-up against groynes, outfalls and other foreshore obstructions (New Forest District Council, 1993). Both north-westward and south-eastward drift is indicated, with a possible transient drift divide occurring between Victoria Park and Hamble Common (Hydraulics Research 1987). Webber (1980) considered that bedload and suspended load longshore transport were in opposite directions along this entire frontage, the latter moving northwards. The temporal stability of this dual, divergent transport system has not been assessed and no quantitative details are available. Maximum significant wave heights generated by the longest available fetch of 5140m are between 0.57 and 1.02m, although in reality waves from this narrow window are infrequent and prevailing waves are significantly lower. It is thought that significant drift can only be generated waves combine with peak tidal stream velocities (ABP Research and Consultancy, 2000c; Posford Duvivier, 1999). There is no evidence for longshore transport along the western shore of Southampton Water, where maximum theoretical wave heights are between 0.53 and 0.94m at the Hythe shore, from a maximum south-easterly fetch of 4150m (ABP Research and Consultancy, 2000c). Again, these waves are infrequently generated in reality and the shoreline is shaltered from prevailing southwesterly waves.

4. SEDIMENT OUTPUTS

4.1 Estuarine Transport - EO1 References Map

EO1 Ebb tidal southeastward out of the estuary

Flume studies by Wright and Leonard (1959) simulated a variety of flow conditions in the main channel and determined a series of critical velocities for gravel transport (>13mm size). These were established both across and along a variety of slopes to recreate conditions over dredged channels. The results indicated lower values when currents were normal to slope contours and when slopes were steep; the lowest critical velocity was 0.91ms^-1 on a 32E slope. Although these laboratory studies simplified actual field conditions, the lowest critical velocity was almost twice as great as the maximum bottom velocity measured in the field (0.52ms^-1, Flood 1981). Whilst ebb:flood current asymmetry results in some selective seawards movement of coarser sediment grades, it appears unlikely that any significant gravel output can occur by ebb currents in any part of the main channel even on the less stable sloping margins of dredged channels. However, there is considered to be some potential for output of fine gravel under exceptional conditions. Although the Brambles Bank, in the central Solent could be thought of as the ebb tidal delta of Southampton Water it is unlikely to actually receive much contemporary material flushed out of Southampton Water. Instead, it is either a relic feature, or is has formed from sediment movements within the Solent that are influenced by ebb tidal flows emerging from Southampton Water. The intertidal and subtidal portions of Hamble Spit, approximately 1000m south of Hamble Common could be considered as being derived from the ebb tide delta of the Hamble, composed of coarse silt, sand and fine gravel.

Sediment transport in the main estuary channel is clearly indicated by linear erosion furrows aligned parallel to ebb current flow (See map). These bedforms are developed in predominantly fine sediments, suggesting that only these grades are transported in this area (Dyer 1970, Flood 1981, Ziedler 1990). Furrows reported by Flood (1981) represent the erosion of some 25,000m³ of fine-grained sediment, but over an indeterminate time scale. However, any net output of suspended load clay and silt by tidal currents is at least balanced by input (Section 2.1).

Sediment transport in Southampton Water is thus limited compared to many other English estuaries, due to shelter from wave action, restricted tidal range and a long period of standing water. Tidal current velocities are modest and combined tide-wave interaction is limited by low wave heights imposed by very restricted wave fetch. The net direction of tidal transport differs for suspended sediments compared to bedload sediments, suspended sediment transport being inward and bedload transport outward.

Further evidence of net seaward transport along this pathway has been provided by a series of sandwaves identified by side-scan sonar traverses of the main channel between Fawley and Hythe (Flood, 1981; Alluvial Mining Co. Ltd., 1994). These indicate net seaward transport, with sampling revealing a composition of muddy coarse sands and fine gravels. Current measurements indicate that peak ebb velocities decline upstream to Southampton Docks, so the maximum sediment size transported must also decline in this direction.

In the inner estuary (Dockhead to Redbridge) tidal current asymmetry rapidly reduces, and current velocities are also less than in lower Southampton Water. The Docks are therefore a nodal point for both tidal hydraulics and sediment movement.

4.2 Land Claim - References Map

Significant areas of Southampton Water have been reclaimed beginning in 1836 (Eastern Docks, Southampton) and extending up to the 1980s (Dibden Bay). Land claim at the docks at Southampton is described by Barton (1979) and Long and Scaife (2000), and the western shore at Fawley, Hythe and Marchwood by Coughlan (1979), Hooke and Riley (1987) Hydraulics Research (1987) and ABP Research and Consultancy (1995b; 2000c). The last source gives a comprehensive documentation, with total land claim amounting to over 500 hectares (160 ha, 1927-34; 187 ha, 1935-1978). 125 ha were reclaimed at Fawley, 1928-1963, most of it in 1950-51 to extend the refinery site using dredge spoil from the Calshot approach channel. Around 200ha. were involved in the 1970s to 1980s reclamation of Dibden Bay. Land claim immobilises large volumes of estuary sediment and must therefore be regarded as an output. It also may have other effects, which have been studied using numerical models, calibrated by field measurements, for the evaluation of proposed reclamation schemes (ABP Research and Consultancy Ltd., 2000b; 2000c). Two main effects are recognised: firstly, land claims protruding into the estuary tend to constrict tidal flow and locally increase current velocities and sediment transport potential (Gifford and Partners, 1989). Secondly, reclamation reduces the estuary tidal prism so that current velocities are reduced in the main channels, lowering the potential for sediment input and output, especially bedload output. The current proposal to construct a container terminal at Dibden Bay (ABP Research and Consultancy, Ltd., 2000a; 2000c) involves some land claim of adjacent intertidal marsh and mudflat areas, thus adding to this effect.

It should be noted that the cumulative effects of the historical reclamations have not been evaluated specifically in terms of the intertidal sediments that would have been impounded, or the reductions in tidal prism that have resulted.

4.3 Dredging for Navigation - References Map

Both capital and maintenance dredging of the main channel, dock approaches and berths have been undertaken in Southampton Water for over two centuries. Four main sediment types are removed from the estuary by dredging:

a) Thin veneers of recently deposited clays and silts including re-settled sediments distributed by previous dredging operations, and occasional slumped and degraded sediments derived from oversteepened channel slopes. (ABP Research and Consultancy, Ltd., 2000a).

b) Mid to Late Holocene clays and silts deposited over the past 5000 - 6000 years.

c) Pleistocene gravels deposited as river terraces during the Devensian establishment of the 'Solent River' system.

d) Tertiary (Eocene) sands, silts and clays forming the underlying substrate.

A total of over 11 million m³ of sediment has been removed from Southampton Water during these operations, including 3 million m³ in 1951 and 6.6 million m³ in 1996-7 (Webber 1980). Detailed descriptions are provided by Anon (1951), Anon (1963), Barton (1979), Webber (1980), Long and Scaife (2000), as well as the sources quoted above. Routine maintenance dredging of berths and channels is also conducted and mean estimates of material removed are in the order of 90,000m³a^-1 (Robinson 1963) to 100,000m³a^-1 (Webber 1980). All of these volumes are relatively small compared to dredging undertaken in many other British estuaries, so it may be concluded that natural siltation rates are low, particularly in the main channel. This is in spite of the fact that channel dredging can truncate, and therefore steepen, adjacent sub-tidal profile gradients leading to downslope movements of materials into dredged channels. These modest siltation rates can be attributed to natural sediment scour and transport by the ebb currents (Robinson 1963, Greenwell, 2000), although it may also relate to limited sediment availability as indicated by low suspended sediment concentrations recorded throughout the estuary. Siltation is also relatively slow in sheltered dock berths after dredging (Barton 1979), indicating a low rate of input from suspended sediments, once immediate site disturbance effects have ceased.

At the time of writing, there is a proposal to construct a container terminal at Dibden Bay on land reclaimed between the 1930s and 70s (ABP Research and Consultancy, Ltd., 2000a, b and c). If approved, it will involve the dredging and removal of 13million m³ for the access channel and a further 1.4million m³ as part of the creation of a tidal creek, further south, to accommodate intertidal habitat losses.

A review of the impact of dredging in the Hamble River estuary, 1990-97 (Universities of Newcastle and Portsmouth, 2000) calculated removal of a minimum of 2,900m³ over this period. Most of this was undertaken as maintenance dredging of the various marinas and around jetties, berths and pontoons. A further 2,200m³ appears to have been removed as part of the regular programme of maintenance of the main navigation channel, 1990-2000.

The impact of marina construction, over the period 1975 to the early 1990s, in the lower Hamble estuary has been examined for its effects on tidal flow velocities (ABP Research and Consultancy, 1994b). Overall, increases of velocity are apparent for sites adjacent to the main channel, increasing rates of erosion. However, eddy circulations within marinas have been initiated, resulting in high rates of accretion over co-adjacent areas. This is confirmed by Cundy and Croudace (1995), who report accretion as high as 20cma^-1 immediately following marina construction.

5. SEDIMENT STORES - References Map

5.1 Beaches

Sand and gravel beaches are only present between the Hamble and Itchen rivers mouths because much of the low energy northern and western margins of the estuary are either reclaimed and protected by embankments, or are fronted by saltmarsh and mudflats. The north-eastern coast is most exposed to wave action and is fringed by beaches of varying width and stability, composed of materials from the adjacent eroding cliffs (Hydraulics Research, 1987; Posford Duvivier, 1994)

At Weston Point, a low bank of sand and gravel (reported as unstable) has been artificially replenished (Southampton City Council, 1988). Further east, a stable upper gravel beach backs a small embayment at Weston Park (Photo 10). East of Weston Park, both the foreshore and beach narrow; the latter is intermittently backed by sea walls of various construction types and dates - see Photo 5 (New Forest District Council, 1993; Posford Duvivier, 1994). At Victoria Park, the narrow gravel upper beach has been characterised by falling levels causing undermining and the erosion of sea walls (Photo 9). The foreshore widens towards Hamble Point where land was reclaimed by dumping of rubble in the 1920s and 30s (Hydraulics Research, 1987; New Forest District Council, 1993; Posford Duvivier, 1996).

The lower foreshore is generally composed of silt and clay with overlying patches of gravel, cobbles and occasional boulder-sized clasts. These coarser sediments were probably derived from reworking of Pleistocene terrace gravels (Hodson and West 1972) and became stranded as a 'lag' deposit by coastal recession, rising sea level and selective removal of fine-grained sediment. Their weed covered state suggests that current velocities are insufficient for transport of this coarse sediment.

Information on these beaches is derived from limited site observations and little quantitative data is available, other than map evidence of historical and recent narrowing of the lower intertidal foreshore. Details of short and long-term beach morphodynamics and sedimentology remain uncertain.

5.2 Sand and Gravel Banks

No significant mobile deposits composed of these sediment types have been described for Southampton Water. Gravel and sand are present in some intertidal areas but is immobile and derives almost certainly from reworking of relic Pleistocene terraces. Small concentrations of coarse sand and gravel-sized clasts, with occasional boulders, are present on the eastern foreshore of the lower Test and at Weston Point.

Some shell banks are present near Marchwood Power Station, although it has been stated that shells are not frequent within the estuary (Dyer 1980). Chenier accumulations, consisting of mixed shell, fine gravel and coarse sand, are present at several eroding saltmarsh margins (Photo 7). Their origin is uncertain, but their shelly materials may have been winnowed out of eroding mudflats and saltmarshes prior to sorting and deposition by wave action.

The closest major accumulations of coarse sand and gravel are located out of the estuary at Brambles Bank and Calshot Spit respectively. The latter has been stable in configuration over a long period of time, but there is some debateable historical evidence for periods of both extension and retreat, indeed a breach may have occurred in the early seventeenth century (ABP Research Consultancy, 2000c). These erosive phases could have released some quantities of coarse material into the estuary, but its subsequent pathway is uncertain although some gravel spreads have been located buried by fine sediments immediately behing the spit. At present, there is a substantial store of gravel extending as a lobe shaped inter and sub tidal projection aligned with the main axis of the spit. Evidence suggests that it is currently immobile. Under prevailing hydrodynamic conditions, the input of gravel at the mouth of Southampton Water is thought to be unlikely.

5.3 Mudflats

The majority of intertidal areas are mudflats, up to 1000m in width covering some 1.1x107m² (1100 ha.) and composed of stores of organic-rich silts, silty clays and clays. There is a dense, but apparently stable, network of dissecting creeks. The deposits are substantially of marine origin, moved into Southampton Water by suspended transport, although some organic (peat) and fluviatile sediments have been revealed at depth by boreholes and excavations at the Western Docks, Southampton (Everard, 1956; Hodson and West, 1972; Barton, 1979); Dibden Bay (ABP Research and Consultancy, Ltd., 2000c) and elsewhere. Similar deposits also flank the margins of the main channels and are described in Section 5.5. Overall, mudflats have experienced significant erosion since approximately 1940 as evidenced by retreat of Mean Low Water mark. (Hooke and Riley, 1987). However, the spatial pattern of loss is complex, and in some areas, such as Hythe, net accretion has been recorded in recent decades (ABP Research and Consultancy, 2000c). This, combined with general erosion of upper saltmarsh, has had the effect of flattening of inter-tidal profiles (Gray, et al, 1993; Johnson, 1998). See E2 for further details. It has been suggested (ABP Research and Consultancy (2000a) that mudflats are currently moving towards a more stable morphodynamic condition following the "perturbation" effect of Spartina growth and then decline on adjacent saltmarshes. Much of the fine-grained sediment eroded from marshes and inter-tidal mudflats may be gained by sub-tidal areas (redistribution) or flushed into the Solent and lost to the system.

5.4 Spartina Saltmarsh

Significant areas of Spartina (Cordgrass) marsh are present along the western shore (mostly S. anglica, but notably including small areas of S. martima), but their extent is significantly less than formerly due to land claim and erosion resulting from "die-back" of S. anglica. For Southampton Water as a whole, 449ha of saltmarsh in 1946 had reduced to 191ha in 1996, representing a cumulative loss of 59% (ABP Research and Consultancy, 2000c). Bray and Cottle (2003) quote a decline from 258ha in 1971 to 165 ha in 2001 (excluding Test marshes and the Hamble) based on photogrammetric survey (see E2) and estimate that this decline is likely to continue in the future.

S. anglica is a fertile hybrid of the native S. maritima and the accidentally introduced American Smooth Cordgrass, S. alterniflora. The latter was first observed in the Itchen estuary in 1816, but a sterile hybrid, S. x. townsendii did not appear - at Hythe - until about 1870 (S. x. townsendii can only spread vegetatively). S. anglica colonised large areas between approximately 1890 and 1920 due to its reproductive vigour, spreading quickly into other estuaries of the Solent and beyond. Its decline appears to date from the 1920s, due to a combination of edge erosion and pan "dieback". There are probably a number of causes working together, but anaerobic soil conditions due to waterlogging and genetic change are the more convincing. Decline and loss continues to the present, although S. anglica remains healthy at some locations, eg. between Calshot and Fawley. Native saltmarsh mortality may have been in slow progress at least a century before the appearance of S. anglica, eg. in the lower Test estuary. In this case, some recovery is apparent, with mixed saltmarsh now invading reed swamp.

The colonising swards of cordgrass interrupted water flow and increased sedimentation so that significant quantities of suspended sediments were trapped and stored (Goodman, Braybrooks and Lambert, 1959; Raybould, et al, 2000). Initial reports of degeneration and dieback of S. anglica date from the mid 1920s (CEGB, 1988; Gray, et al, 1990; Raybould, et al, 2000). As root systems of dead plants decomposed, previously bound sediments around pan edges and marsh margins became mobilised and they also became susceptible to lateral erosion by tidal scour and by wave attack. The marsh edge and channel (creek) margins receded, with further release of sediments to lower foreshore and sub-tidal mudflats. Two key processes are thus recognised: (i) frontal erosion of marsh margins and (ii) fregmentation of saltmarsh blocks. Recolonisation of mudflats was negligible and by the mid/late 1950s significant recession and cliffing of the marshes was evident (Goodman, et al, 1959; Gray and Benham, 1990) at rates of up to 2.5m.a^-1. The "dieback" process is also reported from the Hamble River, but was relatively less rapid and extensive on the east bank (Goodman, et al, 1959; Gray, et al, 1993; Johnson, 1998). Rates of erosion of the front edge of the saltmarshes declined in some areas in the 1980s and 1990s, for example to an average of 0.9m.a^-1 in the Hythe-Cadland area (ABP Research and Consultancy, Ltd., 2000c).

Although reasons for loss are uncertain, "dieback" areas have generally been restricted to marshes where the substratum is particularly soft, fine-grained and waterlogged. It has been suggested that sediment input may be insufficient to maintain marginal sedimentation at a rate comparable to relative sea level rise at some sites, thus leading to waterlogging of Spartina roots. (Geodata Unit, 1987; Gray, et al, 1993). A range of other causes has also been examined, (Gray & Benham, 1990; Raybould et. al, 2000; Gray et al, 1991), whose precise applicability to Southampton Water is uncertain. Limited areas have undergone dieback due to contamination by refinery effluents 1km upstream and downstream of the outfall at Fawley. Improvements to effluent quality beginning in 1971 had resulted in some regrowth of degenerate marsh by 1981, although no natural recolonisation of bare mudflats has yet been recorded (Dicks and Levett, 1989; Oil Pollution Research Unit, 1994; May, 2000). This may be due to the historical burden of contaminants trapped in the mudflat sediments, so recovery by Spartina may not be possible until an overlying layer of uncontaminated sediments has been deposited. Vertical accretion of 5-8mm.a^-1 (Hythe) and 2-8mm.a^-1 (Hamble estuary) for recent decades, based on 137Cs, 60Co, 210Pb and other dating methods (Cundy, et al, 1997) indicates that upper saltmarshes may trap some of the fine sediment removed from mudflats and lower elevation marshes subject to longer periods of immersion. These rates are considered to be in response to contemporary sea-level rise (Cundy and Croudace, 1996). Spartina marshes protect sea walls, embankments and low natural cliffs from wave attack, so their recession could ultimately affect the backshore boundary. Depletion of Spartina marshes also releases significant quantities of fine sediment which may be deposited in navigation channels, and is periodically removed from Southampton Water by maintenance dredging. There is likely to be a complex link between saltmarsh and mudflat erosion, with the latter at potentially being partly the result of a sediment deficit created by the dredging of the main estuary channel. Wave erosion is only an especially effective erosional process when high water coincideseasterly and southeasterly winds. Saltmarsh retreat is not universal within Southampton Water, but exceptions are due to special local factors. For example, mean high water at the south west end of Hamble Common has extended seawards some 150m since the early 1930s, following extinction of foreshore saltmarsh between 1910 and 1925. This is apparently due to waste dumping, and has attracted some new colonisation (Halcrow, 1998).

5.5 Channel Sediments

The main channels of Southampton Water have been dredged to their present depths, so sediments of the bed and margins are those of the underlying geological succession with additional cover by a veneer of recently deposited fine sediments (Barton, 1979; ABP Research and Consultancy, Ltd., 1993; 2000b). In upper parts of the estuary where the buried Pleistocene channel lies closer to the surface eg. Western Docks, navigation channels have been cut through its associated gravels -up to 5 m thick- and into the sandy clays of the Bracklesham Formation (Barton, 1979). Between Fawley and Calshot the buried channel is deeper and the navigation channel is generally floored by Pleistocene gravel deposits covered by a relatively thin sequence of Holocene silts and clays (Hodson and West 1972). North of Fawley, the latter are rarely in excess of 5m in thickness (Barton, 1979), but towards Calshot, they increase in thickness to 13m and form the major slopes flanking of the navigation channel (Long and Scaife, 2000). Pleistocene deposits chiefly comprise sub-angular flints and brown sandy clays (Barton 1979) that probably form relatively stable slopes. This is apparent from the evidence of flume experiments in which the majority of gravels could not be moved by simulation of the current velocities measured in the channel (Wright and Leonard 1959). Sidescan sonar and echo-sounding revealed linear erosional furrows (with widths of up to 5m and depths of 1m continuous for nearly 4km (Flood, 1981) from which sediments were sampled by divers. An average size distribution yielded 1% sand, 54% silt and 45% clay. Slightly coarser sediments, including shell layers, were found within the furrows.

A similar range of deposits are present in the Itchen estuary, where silts and clays are the main channel deposits together with some superficial patches of fine gravel, sand and cobbles upstream to Woodbridge (Qadri, 1984). The provenance of these coaser sediments is not known.

Estuarine deposits reach a maximum thickness of 13m at Fawley and decrease upstream to 6m at the Container Docks (Hodson and West, 1972; Barton, 1979). This information is reliable as it was based on numerous boreholes and foundation excavations.

5.6 Siltation Rates

Although the overall sediment transport system in Southampton Water has been relatively poorly studied, research has been conducted into siltation rates due to its impact on navigation access to the Port of Southampton and at Fawley.

This work confirms that net accretion rates in dredged channels in Southampton Water are low because relatively little maintenance dredging is required compared to most UK estuary-head ports (Robinson, 1963; Barton, 1979; CEGB, 1988; ABP Research and Consultancy, Ltd., 1995f and 2000c). Comparison of Southampton Port Authority hydrographic surveys indicated no measurable changes in bed levels over most of Southampton Water between 1970-1985 (CEGB, 1988 and 1990-95 (ABP Research and Consultancy, 1995f). However, sedimentation of 0.3m was measured at a berth at the Container Docks, and similar rates were recorded in the Bury swinging ground (Photo 8) soon after (Barton, 1979) and at Hythe and Ocean Village marinas within 6-9 months of their initial dredging. Some of this may be attributed to resettlement of sediment disturbed by dredging operations. Some sedimentation also possibly occurs by slumping of the banks of navigation channels. In both cases, accretion is attributable to disturbance, thus natural "background" siltation rates are low (Barton, 1979 ABP Research and Consultancy, 2000b; 2000c).

A detailed study of echo-sounding traces recorded over the period 1939-1968 revealed a significant area of sedimentation on the west side of the main channel between Fawley and Hythe (Flood, 1981). The major accumulation area was subject to sedimentation at approximately 0.11m.a^-1 with a maximum of 0.20m.a^-1 in a zone 2-3km long and 100m wide, previously dredged in 1926. A second, delta-shaped accumulation area was identified opposite Eastland Creek and was subject to sedimentation at 0.12m.a^-1 (Flood, 1981). This has been confirmed by more recent surveys (ABP Research and Consultancy, 2000a), albeit at lower conjectured rates. As part of this area of sedimentation is opposite the cooling water outfalls from Fawley, it was suggested that hydrocarbons from the effluent became adsorbed on suspended sediment particles causing local intensification of flocculation and deposition (Knap, 1978). Overall, contemporary siltation is more rapid to the west of the main channel, (eg Marchwood Military Port) whilst that to the east is extremely slow and was measured at 0.02 - 0.04m.a^-1 (Flood, 1981). This may be due to the fact that channel deepening causes artificial steepening of sub-tidal profiles, thus siltation represents an attempt to restore equilibrium.

ABP Research and Consultancy Ltd. (1995e; 2000b) have undertaken numerical modelling studies to evaluate the impact on suspended sediment transport in the upper estuary of a proposal to construct a container terminal at Dibden Bay. Under present circumstances accretion during the period of slack water and erosion on the succeeding ebb phase are approximately balanced, with a tendency towards net erosion on spring tides. Further towards the estuary head, net deposition of suspended load is apparent, estimated at 137 to 161,000 m³a^-1 (ABP Research and Consultancy, 2000c). Maintenance dredging of the upper channel and adjacent to container port berths, 1985-1990, removed 160,000 (uncertainty of 80,000m³a^-1). Net accretion is probably less between Weston and Netley because of greater its exposure to sediment entrainment and removal by wave or combined tidal/wave action.

6. SUMMARY AND SEDIMENT BUDGET - References Map

Within this section key sediment transfer processes are summarised and an initial attempt at preparing a sediment budget for the estuary is presented.

Sediment Inputs

Marine sources are likely to be the most significant, however, analysis of tidal conditions indicates that net input comprises suspended rather than bedload sediments. The quanities of inputs involves have yet to be measured although they have been inferred by sediment budget analysis. Littoral drift is a negligible input because most material transported towards Southampton Water is retained on Calshot and Hook spits or entrained and flushed back into the Solent. Fluvial input is considered small because river discharge is limited, low in suspended sediment concentrations by comparison to tidal exchange and bedload sediments are likely to be stored within floodplains. Erosion of cliffs between the Itchen and Hamble river mouths supply gravel to local beaches, but the quantity is small due to slow retreat and low cliff height. A significant trend for erosion of Spartina marsh margins and narrowing of the intertidal zone has been identified, (ABP Research and Consultancy, Ltd., 2000c; Bray and Cottle, 2003). Whilst this process undoubtedly releases significant quantities of predominantly fine sediments, it does not comprise much input of new sediments but, rather, re-distribution of the existing estuarine sediment store. Although a considerable thickness of sediments is stored within the estuary, only those sediments at the surface and particularly at the estuary margins are susceptible to erosion transport. Sediments immobile under the prevailing hydraulic region can be released into the dynamic estuarine system only by dredging of the navigation channels. Dredged material may re-settle locally or be transported and contribute to siltation further afield.

Sediment Outputs

Natural output is possible by tidal flow, whilst human activities result in output by reclamation (impoundment of mudflat and saltmarsh sediments) and dredging. Ebb current tidal streams are significantly more rapid than corresponding flood currents, so net output occur by bedload transport. This is mostly coarse silt, and sand because tidal current velocities are insufficient to transport coarser sediments. This sediment transport pathway is confirmed by the presence of linear furrows and sandwaves indicating bedload transport out of the estuary although its magnitude is difficult to discern. Land claim and dredging involves direct sediment loss from the estuary and also affects the sediment budget indirectly by interfering with the hydraulic regime of the estuary. Both practices have resulted in localised variation in tidal velocities, but with net reduction of the tidal prism and enlargement of the submerged profile of the channel (Associated British Ports Research and Consultancy, Ltd., 1989, 2000c). These changes suggest possible reduction of output by bedload transport with potential reduction in marine imputs due to the reduced tidal exchange. Historically, sediment inputs have exceeded outputs with consequent sedimentation over the past 6500 years (Hodson and West, 1972). It is possible that this balance has been upset by recent reclamation and dredging so that these practices may be contributory factors in the widely reported erosion of saltmarsh and intertidal areas. Dredged channels reach to within 200m of the edge of the foreshore zone in places. Mudflat erosion may therefore be partly ascribed to sediment "demand" induced by channel deepening over more than 150 years. This, in turn, can be transferred to saltmarshes in an attempt to restore sediment budget equilibrium.

Sediment Stores

Beaches are extremely small and limited to the shore between the Hamble and Itchen rivers. Sedimentological and volumetric information is not available so beach behaviour is uncertain. The main sediments within Southampton Water are estuarine silts and clays, which form the mudflats of the intertidal zones and the flanks of the channels. The main navigation channel has been significantly deepened by dredging and parts are now floored by Pleistocene gravels. These gravels are more resistant to scour and can maintain higher slope angles than the estuarine silts and clays. Siltation following episodes of dredging results in deposition of thin layers of silts and clays over the gravels. Mobile sand and gravel banks have not been recorded in Southampton Water, although shell banks were recorded at Marchwood. It must be concluded that sedimentation over the past 6500 years has mostly comprised silts and clays in Southampton Water. Coarser materials, by contrast, are relatively scarce amongst the surface sediments. Formerly extensive areas of Spartina marsh are now much reduced by land claim and erosion involving estuarine sediment loss and release respectively. Protection afforded by Spartina marshes to defences low-lying land is therefore declining, and is unlikely to improve because natural recolonisation has ceased.

Sediment Transport

Sediment transport is predominantly by tidal currents because wave generation is limited by the narrow and sheltered nature of the estuary. Limited littoral drift is reported on beaches between the rivers Hamble and Itchen, with a drift divergence between Victoria Park and Hamble Common. Bedload and suspended load transport along the axis of the main channel are in opposite directions because of more rapid but shorter duration ebb currents. Suspended sediments such as fine silts and clays undergo net transport up the estuary and into various creeks, channels and saltmarshes, although concentrations within the water column are considerably lower than recorded at many comparable British estuaries. Bedload sediments (mostly coarse silts, sand and fine gravels) are transported down the estuary and result in the formation of bedforms such as gravel sand waves and linear furrows (Dyer, 1970; Flood, 1981). Channels are relatively stable with low natural siltation and stable bedforms (Flood, 1981; Ziedler, 1990; ABP Research and Consultancy, 2000c). Thus, it is generally the view that sediment transport rates are low within the estuary. This conclusion appears accurate for the present, where channels are well aligned to peak tidal flows, but numerical model studies indicate that new channels cut across the natural flow direction could experience siltation at up to 84, 000m³a^-1 (Gifford and Partners, 1989). Sediment transport and subsequent siltation can also increase in the short-term due to disturbance caused by dredging of navigation channels. Widening of channels close to intertidal areas can also lead to drawdown, narrowing of intertidal areas and steepening of foreshore profiles (Barton, 1979). Erosion of fine-grained sediments (mudflats and saltmarshes) may be effective when maximum fetch coincides with the long high water stand. The impact of ship-induced bow waves will also be greater during high water, but has not been quantified fully (ABP Research and Consultancy, Ltd., 2000c). Sedimentation in Southampton Water is generally restricted to the western side of the main channel where rates of 0.12m.a^-1 are characteristic for limited areas (Halcrow, 1998). Overall vertical sedimentation rates over the past 6500 years are estimated at 2mm -10mma^-1 (Dyer, 1980) and 1mm - 2mma^-1 (Hodson and West, 1972). These authors therefore suggest that sedimentation has kept pace with sea-level rise over recent millennia. More rapid 20th Century rates of 4-8mma^-1 have been recorded from the Hythe and Hamble saltmarshes (Cundy and Croudace, 1995; 1996).

Sediment Budget

Despite several areas of uncertainty, together with lack of representative and reliable quantitative data for most elements, ABP Research and Consultancy Ltd (2000c) attempted a provisional 20th century estuary-wide budget. Its elements were critically evaluated by Bray (2000) who has proposed the following based upon the ABP work:

(the cliff inputs are very high compared to estimates of 500 m³a^-1 presented in Section 2.3) (river input refers to potential rather than actual transport)

Some elements are not quantified e.g. sediment stored within the water column, impoundment losses resulting from reclamation. Furthermore, it could be argued that capital dredging alters the sediment storage capacity of the estuary. Note, especially that the marine input is inferred as the amount required for balancing the budget and assumes that the budget actually is balanced. It has yet to be measured or checked independently.

The change in storage elements are summed to give a net value that refers primarily to sediments yielded by saltmarsh and intertidal erosion that would appear not to become re-deposited in the estuary (subtidal gains are small by comparison. It suggests that significant losses have occurred. Posford Duvivier (1999) calculated rather lower erosion of between 8,500 and 25,000m³a^-1 from the shoreface between Calshot and Hythe, but this only covers some 30% of the total estuary shoreline.

Despite the considerable uncertainty, and the wide range of estimated values, it is apparent that the total estuary budget is in deficit given sustained dredging and an apparent failure to retain all of the products of saltmarsh and mudflat erosion. It is partly in this context that the impoundment of intertidal sediments resulting from land claim proposals (e.g. Dibden Bay) must be viewed. Despite these variations, the cumulative effect of land claim within the estuary as a whole has been to reduce the tidal prism. This has had the direct effect of reducing output via bedload transport and potentially reducing input of suspended sediments.

More detailed budget assessments have been prepared for distinct portions of the estuary. For example, the area upstream of the Dockhead is an area of net deposition, with an estimated 40% coming from erosion of inter-tidal areas and 60% from marine sources; most but perhaps not all of this gain is removed via berthside and channel dredging (ABP Research and Consultancy, 2000a) . The mouth of the Itchen appears to have a steady balance between losses and gains, whilst an erosion loss of 780 m³a^-1 is calculated for the lower Hamble estuary (ABP Research and Consultancy Ltd. 2000a).

A clear implication is that marine input is essential to maintain the existing budget although its actual magnitude is not known. Dredging represents a major loss and some of the sediments eroded from saltmarshes and mudflats would also appear to be lost, although it is suggested that further studies and refinements of estimates are required. These initial findings would appear to be contrary to the long established trend for net sedimentation that has prevailed over the past 6000 years.

Examination of the Southampton Water estuary regime reveals that its cross section area at the mouth (and also at intervals further upstream) is larger than the equilibrium value that might be expected for its tidal prism (ABP Research and Consultancy Ltd. 2000c; Bray and Cottle, 2003). The non-equilibrium regime appears to have been inherited from the Test/Itchen valley that was inundated by rising sea-levels some 6,000 years ago. Sediment transport and deposition should normally act to reduce the inlet cross section towards its equilibrium value. However, infilling appears to have operated slowly within most Solent estuaries due to a limited sediment supply and shelter from wave action at their entrances.

7. COASTAL DEFENCE AND HABITAT INTERFACE ISSUES - References Map

Intertidal habitats comprising mudflats (1000 ha) and saltmarsh (250 ha including the Hamble) are the main types present and constitute a major element of the Solent and Southampton Water SPA. The Southampton Water SAC is specifically designated for the distinct upper, middle and lower saltmarshes, each of which has a specific vegetation community. Other notable habitats include coastal grazing marsh (around 120 ha. mostly in the lower Test valley and flanking the Hamble estuary) and reedbeds (30 ha in the Lower Test). The area is also covered by Ramsar designation and numerious specific SSSI designations.

Southampton Water is 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 estuary), 'coastal squeeze' of mudflat saltmarsh and habitat would continue in response to relative sea-level rise. Progressive landward shift of mean low water has been a consistent trend over large areas of Southampton Water since the 1840s and can be anticipated to continue. There is evidence that the rate of recession of the saltmarsh has reduced in recent decades, having been at a maximum in the 1940s and 1950s following the degeneration of Spartina anglica. However, it is unlikely that recession would cease because future increases in sea-level rise would result in intensification of the coastal squeeze process. For this reason, marsh loss is estimated to continue at rates similar to those of the past three decades. Overall, saltmarshes are estimated to diminish to cover just 90-100 ha. by 2050 and 30-40 ha by 2100 with marshes being maintained for longest between Calshot and Hythe Bray and Cottle (2003). An additional significant impact is that breakish and saline habitats are anticipated to migrate further up the lower Test valley.

One pro-active "soft" engineering approach might be mudflat/marsh surface recharge, using appropriate spoil from navigation dredging. This is proposed as part of a creek and inter-tidal habitat creation scheme within the Dibden Bay land claim/dredging scheme currently under discussion (ABP Research and Consultancy, 2000a, b and c). If implemented, it would add 22ha of mudflat, thus compensating for approximately 60 years of projected natural loss in the Hythe-Dibden area at the above quoted rate. There are, however, several unresolved environmental and hydrodynamic problems to address before this can be considered a viable proposition. The removal, or landward realignment, of landward barriers to saltmarsh retrogression is the more attractive option in biophysical terms, but is faced with difficult issues of land ownership. Much of the defended shoreline protects very high value commercial property, services and infrastructure (e.g. Calshot Power Station; Fawley Oil Refinery and petrochemical complex; Marchwood Military port), and is therefore unlikely to be sacrificed.

There are several positive opportunities for the managed set-back or re-alignment for parts of this coastline, that are assessed in detail in Bray and Cottle (2003). In particular, there are opportunities for the expansion of intertidal habitats in areas of land claim, such as Hythe, Bury Marshes and along the margins of the Hamble. These sites are all limited in extent by rising topography behind the shore, but are extremely important for they adjoin some of the most diverse and healthy upper saltmarsh and Cordgrass habitats. In total they amount to 114 ha. Although this would appear to correspond quite well to likely saltmarsh losses it is not directly equivalent for only a portion of the inundated areas would become saltmarsh with the remainder developing as mudflats.

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

The lack of significant wave energy and the poor development of beaches means that shorelines of this frontage are unsuited for definitive studies of drift. There is, however, a need to improve knowledge of drift and beach behaviour along the Netley frontage between the Itchen and Hamble rivers as explained in Section 9.

9. KNOWLEDGE LIMITATIONS AND MONITORING REQUIREMENTS - References Map

The SMP (Halcrow Maritime, 1998) has reviewed much of the available information and mades 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 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 theSouthampton Water. 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. Previous investigations have not regarded sediment distribution and availability as a critical issue, so it is difficult to assess the overall sediment budget. Processes transporting sediment have not been previously perceived as contributing inputs and/or outputs and consequently have received only cursory examination. Comprehensive and quantitative budgetary analysis is not therefore possible at present, and much of the existing knowledge base requires verification by further research to enhance understanding of complex inter-relations between hydrodynamics and sedimentation processes.
2. Outputs and inputs at the estuary mouth are supported by strong direct (eg. tidal current tracking and analysis) or indirect (e.g. bedforms) information, but little integrative quantitative information is available. Details are lacking of both suspended sediment input and bedload output. By far the most important transport mechanism is tidal flow, a process, which can be modelled using appropriate sediment, transport equations and verified by field measurements. A useful first step would be to collect measurements of suspended load concentrations within the water column over a representative range of tidal conditions. This shoulds assist analysis of suspended sediment transport using tidal models and possibly lead to more definitive estimates of the magnitude of marine input.
3. Bedform surveys should be coupled with systematic sediment sampling, so that distribution maps can be compiled. Examination of particle size distribution and sorting patterns can then be used to infer transport directions and the grades of sediment being transported as was undertaken for Chichester Harbour entrance by Geosea Consulting Ltd (1999). Sureys should include Brambles Bank, which is influenced by tidal flows exiting Southampton Water.
4. The major natural process causing concern is narrowing of the intertidal zone and associated saltmarsh erosion. This should be monitored, with saltmarsh perimeters and mudflat levels carefully measured, and volumes of eroded/deposited sediment determined by map comparison of different parts of the estuary. Field monitoring of upper saltmarsh surfaces is needed, to determine the rates of vertical accretion throughout the estuary. This would provide improved estimates of sediment storage that could contribute to sediment budget calculations.
5. Littoral drift has not been quantitatively evaluated, hence the morphosedimentary behaviour of beaches between the Hamble and Itchen rivers is poorly understood. The causes of apparently declining beach levels remain uncertain, yet without this information effective beach management cannot be undertaken. Beach levels can be investigated by beach profiles and short-term drift rates can be determined from volumetric analysis of the data. Short period variability of drift direction can be studied by monitoring the volumes and distribution of sediment building up against obstructions eg. groynes or outfalls. This will allow better resolution of the location of drift divergences.
6. Siltation rates have been measured in the main channels and newly dredged berths immediately following operations, but these are unlikely to be representative of normal conditions in the estuary as a whole because disturbance by dredging causes enhanced localised siltation. Hydrographic chart comparisons for a limited area between Fawley and Hythe, for example, revealed spatially and temporally variable siltation patterns. Analyses of this type could be extended to cover all appropriate parts of the estuary and thus obtain an overall view. Such information would provide a valuable input into the compilation of a sediment budget of the estuary, and perhaps indicate more clearly the fate of sediments released by mudflat and saltmarsh erosion. Constructing a more soundly-based sediment budget for Southampton Water is a major medium to longer term research priority.
7. Studies are needed into the cumulative effects of historical reclamation and dredging upon the estuary. Improved knowledge of past estuary responses to these factors should provide insights into future sensitivity. It is recommended that studies should focus upon reconstructing historical sediment budgets and tidal prism analyses in order to determine the effects of these processes.

10. REFERENCES - Map

ABP RESEARCH AND CONSULTANCY LTD (1993) Southampton Water Siltation and Dredging Study. Report 437. Report to ABP Southampton.

ABP RESEARCH AND CONSULTANCY LTD (1994a) Chart Comparisons of the River Hamble, 1964/6 and 1988/93. Report 457. Report to Hampshire County Council and the Harbour Master, River Hamble.

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