8. Physical Processes - Southampton VTS Oct 2012/SACD... · R/3742/8 49R.2015 8. Physical Processes ... The temporary changes during dredging and disposal will ... Transport mechanism

Environmental Statement for Port of Southampton: Southampton Approach Channel Dredge

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8. Physical Processes

Executive Summary: Chapter 8. Physical Processes The environmental conditions along the approach channel into the Port of Southampton progressively change from open coastal waters into an estuarine environment. Tides are the primary influence on water movements and control the erosion, movement and deposition of sediments. Waves tend to have their main influence at the shoreline and help to develop shoreline features such as the various shingle spits in the Solent. They also increase the energy at the bed enhancing the potential for sediment disturbance; either eroding settled material or preventing sediment from settling, particularly in intertidal areas. The more sheltered area within Southampton Water is conducive to the development of saltmarsh, especially along the western estuary margin, which is in the lee of prevailing winds. These saltmarsh features are mainly fed by fine sediment of marine origin and sediment ‘re-worked’ from within the estuary. In the long-term, the consequence of increased rates of sea level rise will lead to a major loss of saltmarsh coverage due to coastal squeeze. The long-term analysis of the foreshore in Southampton Water suggests that the cause of the erosion in the lower and upper parts of the shoreline profiles is not directly related to the main channel deepening in 1996/97, and is the result of natural and possible other anthropogenic ongoing occurring events. A package of technical evaluations has been used to determine the magnitude and extent of physical changes that are likely to result from the proposed channel deepening. An assessment has been undertaken to understand the physical changes likely to be brought about during the construction and post-construction periods. The assessment is based upon various conservative assumptions to offer a realistic worst-case scenario. 1) Construction Related Impacts The primary issue during the construction period relates to sediment disturbance effects created during the dredging process, and with particular focus on the possible effects of overflow from the TSHD. The nature of the sediment types to be excavated has guided the choice of dredging methods and defined appropriate source terms to model the fate of disturbed sediments. Heightened suspended sediment concentrations will occur locally around each dredger but across the wider area excess concentrations will generally be comparable to background levels. The dredging cycle enables intervening periods of sufficient duration to reduce the excess sediment concentrations and mitigate any risk of cumulative increases. Once the full period of dredging is complete the levels of suspended sediment will return to near background levels within about a two-week spring/neap tidal cycle. The fate of the disturbed sediments is described by the patterns of siltation predicted by the model with berths and intertidal margins providing the major sinks. Maximum accumulations in these sinks are likely to occur during dredging of alluvium material within Southampton Water.



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2) Post-Construction Related Impacts The effects of the completed channel deepening on the hydrodynamic regime are considered to be negligible. The pattern of change is mainly demonstrated in a slight reduction in ebb and flood flows locally within the channel and a slight reduction in the concentration of suspended sediments within Southampton Water, however, any changes remain small and are considered to be well within levels of natural variability. The longer-term trends influencing the future development of Southampton Water remain dominated by sea level rise. The further implications and significance of the construction and post-construction impacts are reviewed in relation to their potential effects on sediment and water quality, nature conservation, fisheries, flood risk and commercial and recreational navigation. Conclusion The physical changes that are predicted to occur following the proposed Southampton Approach Channel Dredge are on the whole negligible and almost impossible to measure directly in the field. The temporary changes during dredging and disposal will be larger in magnitude but highly variable (transient) in time and space. Overall, the impact to the physical functioning of the estuary alone is considered to be insignificant to minor adverse significant.

Glossary of Terms Advection: Transport mechanism of a substance or a conserved property with a

moving fluid.

Amphidrome: A point in the sea where there is zero tidal amplitude due to the cancelling of tidal waves.

Chenier: A large ridge shaped deposit built up of beach material.

Eocene: A geological epoch, which spans the time from the end of the Paleocene epoch to the beginning of the Oligocene epoch.

Holocene: A geological epoch covering the last 11,000 years.

Nodal tidal cycle: A period of time lasting 18.61 years over which the plane at which the moon orbits the earth rotates. This has the effect of periodically changing tidal ranges.

Pleistocene: A geological epoch preceding the Holocene (see above) and stretching between about 1.8 million and 11,000 years before present, the Pleistocene covers the worlds most recent period of glaciations.

Quaternary: A geological time period spanning the last 1.8 million years, the Quaternary comprises the Pleistocene and Holocene epochs.

Transgression: Extension of the sea over the land.



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Baseline Review

8.1 The purpose of the baseline review of physical processes is to identify the form and function of the physical environment, which may be affected by the proposed channel deepening. In considering the baseline, the form is described by the physical layout and morphological features across the study area and the function by the physical processes, which drive the movement of water and influence the transport of sediments, and interact with morphological development. The natural variability of these processes is reviewed and in relation to changes that are likely to occur irrespective of any future development.

8.2 A combination of modelling tools has also been applied to represent the baseline condition, as well as to provide the means to quantify the extent, magnitude and direction of any change brought about by channel deepening and to inform the interpretation of any impact of these changes. This modelling approach has been developed in line with current best practice recommended in the Estuary Guide (www.estuary-guide.net). This was recently extended under a Defra and Environment Agency Flood and Coastal Erosion Risk Management Research and Development Programme (FD2119), bringing together all the existing and new outputs from the Estuaries Research Programme (ERP). Further details related to the application of these methods are provided in Appendix C. The potential impacts of the proposed channel deepening on the baseline condition are discussed from Paras 8.92 to 8.288. Overview of the Study Area

8.3 The proposed channel deepening extends through Southampton Water and the Thorn Channel and with a requirement for small amounts of deepening along East Solent and the Nab Channel. The planned site for disposal of capital spoil is the Nab Deposit Ground. The overall study area is presented in Figure 1.1.

8.4 The environmental conditions along this route change from a relatively sheltered estuarine environment into open coastal waters. Tides are the primary influence on water movements and control the advection of suspended sediments. Waves tend to have their main influence at the shoreline and help to develop features such as various shingle spits evident across the Solent. The more sheltered area within Southampton Water is conducive to the development of saltmarsh, especially along the western estuary margin, which is in the lee of prevailing winds. These saltmarsh features are mainly fed by fine sediment of marine origin.

8.5 The description of the baseline is reviewed in relation to its past, present and future condition. Past Development Geological Evolution

8.6 The present day structure of the Solent and Southampton Water is generally considered to have evolved since the period of the last glaciations (Pleistocene) with rising sea levels drowning the former Solent River basin (Figure 8.1). As water depths increased waves have

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also helped develop the shoreline, but with the Isle of Wight acting to shelter East and West Solent.

8.7 Rising sea levels during the Holocene increased the size of Southampton Water, although its shape was determined by the morphology of the Pleistocene terraces bordering the drowned channel (Hodson and West, 1972). These terraces are important in providing a chronology of marine transgressions and fluvial down-cutting that have occurred throughout the evolution of the estuary. Contemporary Change Reclamation

8.8 Reclamation in Southampton Water has occurred since 1836 (Eastern Docks, Southampton) and extending up to the 1990s (Berth 207). Between 1928 and 1963, 125ha were reclaimed at Fawley, most of it in 1950 to extend the refinery site. Land claims tend to constrict tidal flow and locally increase current velocities and sediment transport potential (Gifford and Partners, 1989). A summary of reclamation history within Southampton Water is provided in Table 8.1. Table 8.1 Reclamation history through Southampton Water

Date Development

1890-1910 Development of Eastern Docks out over the mudflats at the confluence of the Rivers Test and Itchen.

1920s Reclamation of some 8 ha for Marchwood Power Station and the Military Port. 1927-34 Reclamation of the Western Docks between Royal Pier and Millbrook Point.

1920s-60s Dredged Material used to reclaim approximately 80 ha of land around the Exxon Mobil Oil Refinery, the majority of which took place in the early 1960s.

1930s-70 Maintenance dredge spoil used to reclaim Dibden in four stages: Phase 1 – 1930-55; 36ha Phase 2 – 1956-60; 40ha Phase 3 – 1960-62; 36ha Phase 4 – 1962-70; 64ha

1950-51 Fawley Power Station reclamation using material from dredging off Calshot approaches to reclaim some 46ha.

1967-68 Western Dock extension scheme, Phase 1 for Berth 201 reclaimed 6ha. 1970-72 Phase 2 of Western Dock extension scheme created Berths 202-205 by reclaiming 32ha. 1972-75 Phase 3 for Berth 206 involved reclaiming 45ha. 1995-96 Berth 207 construction completed

Dredging

8.9 Both capital and maintenance dredging of the main channel, dock approaches and berths have been undertaken in Southampton Water for over two centuries. Key details relating to past dredging are summarised in Table 8.2. A total of over 11 million m3 was removed from Southampton Water during these operations, including 3 million m3 in 1951 (Webber, 1980) and 6.6 million m3 in 1996/7. Routine maintenance dredging of berths and channels is also regularly undertaken.



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Table 8.2 Historical channel deepening activities

Date Development

1882 Channel dredged to 7.4m below CD in 1889 from Fawley to the docks.

1893 Dredged channel depth increased to 8.6m below CD.

1907 Dredged channel depth increased to 9.3m below CD to accommodate new liners. Length of dredged channel extended.

1922-27 Continuous operations to widen and deepen stretches of the channel between Fawley and the Docks and from Calshot out towards Brambles Bank.

1931 Project to widen channel to 305m and deepen reach below Calshot to 11.1m below CD. From Calshot to the Docks channel was dredged to 10.2m below CD.

1951 The largest single dredging contract at the time was awarded to The Dredging and Construction Company Ltd, to remove 2,900,000m3 of material by: (i) Straightening the western channel; (ii) Restoring a portion of the Calshot channel to 11.1m below CD; (iii) Cutting off a portion of the bend around Calshot Spit; and (iv) Restoring a depth of 10.2m below CD throughout the main channel from Calshot to

the Docks, including the middle and lower swinging grounds, and widening the channel to 610m.

1960s Deepening in the area of the Thorn Channel to 12.6m below CD.

1973-78 Deepening in vicinity of the Container Port.

1996-97 Approach channel between Fawley and the container terminal deepened from 10.2m below CD to 12.6m below CD.

(Source: ABPmer, 2007c)

8.10 Four main sediment types are removed from the estuary by dredging: Thin veneers of recently deposited clays and silts including re-settled sediments

distributed by previous dredging operations; Mid to Late Holocene clays and silts deposited over the past 5000 - 6000 years; Pleistocene gravels deposited as river terraces during the Devensian establishment of

the 'Solent River' system; and Tertiary (Eocene) sands, silts and clays forming the underlying substrate.

8.11 Analysis of foreshore profile data from Weston to Netley suggests that between 1965 and 1996 (prior to the last channel deepening) erosion occurred to varying degrees on the foreshore, intertidal and subtidal areas. Additionally, the most recent data suggest the estuary would appear to be eroding at a similar or slower rate today than in the past.

8.12 Following the 1996/97 channel deepening, a comprehensive monitoring programme was put in place to identify any changes to the intertidal areas of Southampton Water. This involved surveying along a series of transects between 1996 and 2005 on an annual and, at times, quarterly basis using a Kinematic Differential Global Positioning System (DGPS). It should be noted that the results of this monitoring include both the net changes resulting from natural variability and any effects, which have arisen from the last capital dredge.

8.13 During the 10 years of monitoring, foreshore levels have varied by the order of ±0.25m, with no consistent trend monitored, and for the most part this region is considered stable. The largest changes have occurred locally at the base of the shore slope. The eastern shore of



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Southampton Water demonstrates some erosion at both the shoreward and seaward ends of the profiles. In contrast, the western foreshore differs because of the saltmarsh dominance. The western shore has erosion along the outer edges of the saltmarsh in conjunction with chenier retreat. Height and morphology of the saltmarshes have changed very little which is consistent with the previous historic saltmarsh analysis. It is not possible to determine from the monitoring alone whether this rate of change was affected by the channel deepening. The mudflats have, however, generally eroded in the order of 0.15 to 0.25m over the 10-year period.

8.14 These findings suggest that the cause of the erosion in the lower parts of the shoreline profiles is the result of natural and possible other anthropogenic ongoing occurring events and not directly related to the main channel deepening in 1996/97. Furthermore, erosion occurring in the upper part of the profile is more likely to be attributed to wave activity, predominately from weather conditions, than the deepening works. Present Morphology Solent Morphology

8.15 The contemporary morphology of the Solent shoreline is characterised by barrier spits, intertidal flats, saltmarshes, and erosional coastal cliffs, particularly along the coastline of the Isle of Wight and the Solent approaches (Velegrakis, 2000).

8.16 The varied morphology can, to a degree, be explained by the varying wave climate along the shoreline. Within East and West Solent this is essentially low energy and fetch limited, and is responsible for the relative distribution of saltmarshes and mudflats.

8.17 Offshore, the seabed morphology is varied and consists of narrow channels, offshore sand and gravel banks, and generally modest water depths, which rarely exceed 20m. However, there are localised deeps existing in the vicinity of the approaches to the West (e.g. Hurst Narrows) and East Solent.

8.18 The tidal inlets of Hurst Narrows and Southampton Water are bounded by spit features at their entrances (e.g. Hurst and Calshot Spits). These are areas of sediment accumulation and mainly consist of coarse-grained material (shingle). The spits were formed in response to coastal erosion during the early to mid-Holocene sea level rise. Calshot Spit is a recurved barrier spit, which comprises a wide gravel and sand foreshore. The foreshore consists of swash bars indicative of onshore sediment transport. There is some question about the origin of these sediments. This material may be eroded gravel terrace deposits originating from channel erosion, or may be due to recirculation of existing shoreline sediments. The alignment of the spit is indicative of a north-eastward drift (Dyer, 1980). The spit has remained stable over the last 100 years (Halcrow, 1997).

8.19 In the lee of the spits, where the waters are more sheltered and quiescent, intertidal flats and saltmarshes are prevalent (e.g. Lymington Flats). Small ebb tidal deltas are present within Southampton Water at the confluences of the Hamble and the Itchen (Velegrakis and Collins, 2000).



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8.20 Bramble Bank is located at the entrance to Southampton Water and is a feature around which the approach channel turns. It is an extensive, depositional bank, which marks the transition between the coarse sediments in the West Solent and the finer sediments in the East Solent. It also acts as a sediment sink for convergent transport pathways from the East Solent, West Solent, Southampton Water and the Medina. Medium sand is transported in an anticlockwise direction and accumulates on Bramble Bank, which supplies a westward moving sediment transport pathway. Bramble Bank is a relict form inherited from the former Solent River system and is largely immobile under contemporary hydrodynamic conditions. Samples in Bramble Bank demonstrate mixing of minerals originating from river catchments as well as seabed and shoreface erosion. Importantly, the erosion of the Tertiary coastal cliffs behind Bramble Bank suggests that the protective nature of this feature may not be sufficient to prevent coastal erosion (Velegrakis and Collins, 2000). Solent Bed Sediment Distribution

8.21 The sediment characteristics of the Solent, the local river channels and estuaries comprise Pleistocene gravels, which are covered by layers of Holocene sands, and gravels. The offshore sediments rest upon a bedrock erosional surface of complex relief and may be subdivided into buried channel infilling sediments and surficial sediments. The buried channel deposits are predominantly coarse grained.

8.22 The West Solent is predominantly covered by gravel and sandy gravel. This is mixed with varying proportions of sand-sized material. Areas of gravel waves, which are oriented transverse to the dominant tidal flow, cover much of the seabed south of the central channel (Langhorne et al., 1986). Southampton Water Morphology

8.23 Southampton Water is a relatively narrow and spit-enclosed meso-tidal estuary, subject to very limited wave action and draining a catchment of around 1630km2 (ABPmer, 2007c). The JNCC classification of Southampton Water, based on the processes that formed the estuary, is a Coastal Plain estuary. A comprehensive review of Southampton Water is provided in the Estuary Guide website. Southampton Water Bed Sediment Distribution

8.24 The bed of Southampton Water consists of geological strata that have been recently exposed by dredging in the main navigation channel. Here, the sediment varies from sandy clay near the Upper Swinging Ground to sands and gravels at Fawley. At the northern end of the Fawley Terminal the channel consists of southward dipping glacial gravel overlain by Holocene silty clay and above that a shallow band of Solent Marine Shingle. Either side of the navigation channel, clay sediments are found overlying a layer of gravel. Along the western shore, mud deposits are the predominant surface sediment.

8.25 Up-estuary of Fawley, the buried Pleistocene channel lies closer to the surface and the navigation channel cuts through Pleistocene gravel into the laminated silts, clays and the fine to coarse sands of the Bracklesham Beds. The side slopes of the navigation channel and the intertidal areas in the upper part of Southampton Water comprise clays and silts with a maximum thickness of 6m underlain by gravels up to 5m thick.



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8.26 Between Fawley and Calshot, the main channel is floored by Pleistocene gravels and thin Holocene deposits of silts and clays. The gravels comprise sub-angular flint and brown sandy clay forming stable slopes. At Calshot the silts are around 13m thick on the side slopes and intertidal areas. Contemporary Processes

8.27 Present day sedimentary processes vary significantly within the differing estuarine sedimentary environments throughout Southampton Water (Velegrakis and Collins, 2000). Sediment transport processes are dominated by tidal currents, especially on the western coast and in the inner estuary where fine sediments have accreted. The morphology of the estuary boundaries is reflected in the dominance of the tidal processes, and the variable wave climates. The west shore is almost totally sheltered from wave energy in comparison to the opposing east shore, leading to a higher dominance in saltmarsh. The east shore in the outer estuary is bordered by mudflats, which are backed by a series of low cliffs. Waves

8.28 The presence of swell waves in the West Solent and Southampton Water is minimal due to the protection gained from the Isle of Wight, Hurst Spit and the Bramble Bank. The West Solent provides an open fetch for the limited generation of local waves from the predominant south-westerly wind direction. The resultant waves have the greatest impact on the coastline near Lee-on-Solent, an area associated with coastal erosion.

8.29 Wave heights have been monitored over a period of years at Lee-on-the-Solent from which it has been assessed that the maximum significant wave height during the peak autumn and winter months is in the order of 1.2m (Webber, 1980).

8.30 The wave regime within Southampton Water is more sheltered but the relative wave energy dissipation is greater on the Netley shore because it faces the prevailing wind direction, whereas the Hythe to Calshot shore is usually sheltered because it is in the lee of the prevailing winds (ABP Research, 2000a).

8.31 There is significant inter-annual variability in the wind-wave climate. Discrete periods of wind data covering the periods, 1984-91 and 1988-97 demonstrate an increase in wind generated wave energy on the Hythe shore of 17% between the two intervals. This is attributed to the increasing prevalence of easterlies in more recent interval.

8.32 Within Southampton Water, vessel disturbance provides an additional source of wave energy at the shoreline. Water Levels

8.33 The primary influence on water levels across the Solent is due to tides, although strong winds (especially north-easterlies) and surges can have secondary effects leading to occasional increases or decreases on the tidal effect. In the long-term, further and an increased rate of mean sea level is predicted in response to climate change.



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8.34 The Solent’s tidal regime is semi-diurnal and shows a marked increase in range as the tide advances from west to east, with an increasing spring tidal range from 2m at Hurst Point to 3.9m at Portsmouth.

8.35 The local phasing of tidal constituents creates a double high water (HW) across Christchurch Bay, which moderates further into the Solent resulting in an extended HW for Southampton Water lasting for up to 3 hours. Here on spring tides an added feature of the tide develops during the flood phase, and around two hours after low water (LW), known as the “Young Flood Stand”. This phenomenon is evident as a pause in the rising tide. Figure 8.2 illustrates typical tidal curves for Southampton Water for spring and neap tides. The profile of the tide is clearly asymmetric with the flood phase (of around 9 hours to the second HW) lasting longer than the ebb phase (of around 3½ hours).

8.36 Tides are monitored at three locations in Southampton Water and the Test Estuary: Calshot at the entrance to Southampton Water; Berth 37 (Dock Head), the Standard Port; and Berth 206 (Redbridge) at the head of Test Estuary.

8.37 Overall, there is general consistency in the pattern of the tide both in phase and amplitude, with only a moderate increase in range moving upstream.

8.38 At Berth 37 the general tidal regime is as follows relative to CD (Chart Datum): Highest Astronomical Tide = 5.0m Mean HW Spring = 4.5m Mean HW Neap = 3.7m Mean Tide Level = 2.6m Mean LW Neap = 1.8m Mean LW Spring = 0.5m Lowest Astronomical Tide =-0.1m

8.39 This equates to a mean spring tidal range of 4m, a neap tidal range of 1.9m and an extreme range of 5.1m. The majority of tides fall in the range 2 to 4m equivalent to a meso tidal range.

8.40 The maximum recorded HW to date was 5.6m on 26 December 1999 during a surge tide with an amplitude of 1m.

8.41 Another important variation in the tidal range is due to the lunar nodal cycle, which operates over a period of 18.61 years. Long-term data from Portsmouth (ABPmer, 2007c) indicates that this effect alone causes variation in tidal range by of the order of 0.25m. The trough of this cycle last occurred in about 2006/2007 and, therefore, tidal range can be expected to increase over the next circa eight years.

8.42 Analysis of long-term tidal records from Portsmouth demonstrates a present trend in mean sea level rise for the general area of 1.7mm/year (ABP Research, 2001). Increased rates of mean sea level rise are now predicted in response to climate change concerns. Present Defra



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guidance (Defra, 2006) provides revised allowances for South East England of 4mm/yr to 2025, increasing to 8.5mm/yr for the period 2025 to 2055. Flows

8.43 The primary influence on flows through the Solent and within Southampton Water is determined from rates of change in tide levels, within alignment of flows determined by shoreline, channels and banks. Winds, surges and freshwater inflows can have local and secondary influences on flows.

8.44 Given the atypical shape of the tidal curve, the resulting flows within Southampton Water are complex and give rise to important mechanisms, which are conducive to move fine sediments onto intertidal areas. The asymmetry of the tide, with a longer duration on the flood phase than ebb, creates stronger flows on the ebb. This ebb dominance can be illustrated by tidal streams at Calshot where on a spring tide ebb flows reach 1.0m/s whereas on a flood tide flows reach 0.7m/s (ABP Research, 2000a).

8.45 A further unique feature occurs during the flood phase of the tide, which is especially evident on spring tides. The Young Flood Stand creates an initial slack water period, which is then followed, by a further slack water period over the extended HWs. The significance of the tidal hydrodynamics in terms of sediment movement is discussed in Paras 8.78 to 8.80. Freshwater Inputs

8.46 Freshwater inputs to the estuary also contribute to the flow regime and lead to variations in salinity. Primary sources of freshwater are from the Rivers Test, Itchen and Hamble. Rates of freshwater input are highly variable and correlate to the scale of each river catchment and patterns of rainfall, which vary seasonally, with largest flow rates expected during the winter period. Statistically the River Test has the largest average winter (50%ile exceedance December to March) freshwater flows of 16m3/s compared to the Itchen (7m3/s) and Hamble (0.5m3/s) (source: National River Flow Archive). A further smaller tributary that drains into Southampton Water is Tanners Brook, which has an average winter discharge of 0.2m3/s.

8.47 The relative contribution of freshwater input remains small when compared to the volume of water exchanged during each tide (i.e. the tidal prism defined as the difference in water volume between LW and HW). For example, the total river flow rate based on average winter conditions is 23.7m3/s. Over a 9-hour flood phase this provides a volume of less than 0.1 million m3 of freshwater input. The equivalent tidal prism for neap tides is 54 million m3 and 109 million m3 on spring tides.

8.48 Freshwater inputs from the main tributaries influence magnitudes of salinity throughout Southampton Water with concentrations generally ranging from near fully marine conditions at Calshot (measured in the range of 30 to 33.5ppt) to more estuarine levels at Town Quay. The potential remains for localised and short-term stratification may occur in the upper reaches of the Test during periods of high freshwater discharges combined with neap tides, although the estuary is generally considered to be well-mixed overall.



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Present Day Sediment Regime Sediment Sources

8.49 The following sediment sources are discussed: Marine inputs; River load; Intertidal erosion; Subtidal erosion; Cliff erosion; and Saltmarsh erosion Marine Inputs

8.50 The primary input of suspended sediments to Southampton Water is from marine sources with the process of advection-diffusion drawing in the sediments from the Solent. River sediment loads are secondary and fluctuate with discharge events to create local variations at the head of the tributaries.

8.51 Average measured suspended sediment concentrations in Southampton Water vary from around 25 to 40mg/l at the mouth (peak springs about 60mg/l), 10 to 20mg/l at Dock Head and 5 to 10mg/l in the vicinity of the container terminal (ABP Research, 2000b). River Load

8.52 An estimate of the annual sediment inputs from the Rivers Test, Itchen and Hamble equates to 10,000, 6,000 and 1,000m3/yr, respectively (Bray, 2000). This estimate includes some bedload material that could be stored in the lower river channel reaches and also fine sediments on flood plains, both which may not be supplied directly to Southampton Water. Intertidal Erosion

8.53 The rate of intertidal erosion estimated for the sediment budget has been evaluated with the use of modelling and assessment of change from available monitoring data. The increased siltation that occurred during the last capital dredge in 1996/97, as evidenced by an increased in maintenance dredging, updates previous estimates for intertidal erosion. Table 8.3 summarises the present day estimated rate of intertidal erosion. Table 8.3 Estimated intertidal volume changes within Southampton Water, and the

Rivers Test, Itchen and Hamble

Location Rate of Erosion (x103 m3/yr) Southampton Water 53 Test 23 Itchen - Hamble 3



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Subtidal Erosion 8.54 Since 1996, monitoring of the estuary has indicated that there has been a marginal reduction in

the amount of subtidal erosion. The previous capital dredge did not directly increase the tidal prism of the estuary and, therefore, with the deepening the general flow rates decreased. These reduced flows, along with greater depths, have reduced bed shear stresses within the main channel. This linked with the exposure of denser materials at the base of the channel reduced the potential for channel erosion. In the shallow subtidal areas, flow speed reductions will have been proportionally less (compared to the dredged main channel), but the areas remained exposed to erosive forces from waves. The erosive forces in the shallow subtidal areas are not likely to have changed.

8.55 Table 8.4 summarises the present day estimated rate of subtidal erosion. Spatially the pattern seems to be erosion of the lower channel slopes but at a slower horizontal rate than the upper channel. Table 8.4 Estimated subtidal erosional volume changes within Southampton Water

and the rivers Test, Itchen and Hamble

Location Rate of Erosion (x103 m3/yr) Southampton Water 29 Test 0 Itchen 2 Hamble 2

Cliff Erosion

8.56 Cliffs are present along the coast at Netley between the Hamble and Itchen Rivers, these are formed of sandy gravel and medium to coarse gravel sediments and in general do not exceed 8m in height (Halcrow, 1997).

8.57 The erosion rate of Netley Cliff has been estimated at between 0.1 and 0.35m/yr (Halcrow, 1997). Eastleigh Borough Council also carried out cliff top monitoring in 1991 over a period of 8 months, which indicated an erosion rate of 0.5m/yr between Netley Abbey and the Royal Victoria Country Park (Halcrow, 1997). A comparison of historic maps along the entire frontage supports these rates of erosion, showing a retreat of 0.1 to 0.5m/yr between 1870 and 1965 with the highest retreat rate (0.5m/yr) between Netley Abbey and the Royal Victoria Park (Posford Duvivier, 1994, 1996 and 1997).

8.58 It has been suggested that the eroded material from the cliffs consists of approximately 50-100m3/yr of sand and gravel that is retained on the local beaches. A further 400m/yr of finer material is probably removed as suspended load (Posford Duvivier, 1997). Although the amount of sand and gravel supplied from the cliffs is relatively low, it appears to supply an adequate amount of sediment to the adjacent beaches (Hydraulics Research, 1987).

8.59 Historically the cliff was probably eroding at a greater rate along this frontage, but some parts of the cliff have been partially protected through the piecemeal construction of defences, which vary in their quality and effectiveness. The rate of erosion is also influenced by slope destabilisation arising from the disturbance of vegetation which is caused by a combination of



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basal erosion and shallow slides (Bray et al., 2004), weathering also probably makes an additional contribution.

8.60 To estimate the volume of sediments from cliff erosion a length of 7km and an average height of 2m, eroding at a rate of 0.35m/year has been assumed. This gives a sediment input from the eroding cliffs amounting to 4,900m3/yr. Saltmarsh Erosion

8.61 Saltmarsh erosion is primarily related to the retreat of the fronting edge of the feature with quantification primarily derived from aerial imagery based on this lineation.

8.62 When considering saltmarsh losses within the study area, it is useful to consider the issue in the context of saltmarsh losses throughout the wider Solent. Table 8.5 presents the summary of results derived on the basis of aerial photography, excluding losses due to reclamation (Channel Coastal Observatory, CCO, 2008). The results indicate that saltmarsh loss within the study area (Southampton Water, the Hamble and Calshot) has been low in comparison to many areas within the Solent. Table 8.5 Estimated saltmarsh losses throughout the Solent area

Location Data period Average Percentage Saltmarsh Loss Per Year (Excluding Reclamation)

Calshot 1940 – 2001 0.8 Southampton Water 1946 – 2001 0.4 Hamble 1946 – 2000 0.4 Hurst Spit 1971 – 2001 1.2 Keyhaven 1971 – 2001 1.7 Lymington 1946 – 2001 1.1 Pitts and Sowley 1946 – 2001 1.5 Beaulieu 1954 – 2001 1.1 Portsmouth 1946 – 2002 1.1 Langstone 1946 – 2002 1.5 Chichester 1946 – 2002 1.0 Pagham 1947 – 2001 0.2

Southampton Water

8.63 Changes to saltmarsh in Southampton Water have been estimated using aerial photography between 1946 and 1996 at approximately 10-yearly intervals (ABP Research, 2000b). Due to land reclamation in the area, the changes in saltmarsh coverage are confined to the area seaward of the present day defences and walls in Southampton Water. This analysis found that the area of saltmarsh was decreasing (Table 8.6).



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Table 8.6 Change in saltmarsh areas along Southampton Water (1946 to 1996)

Year Fawley and Calshot (ha)

Hythe to Cadland Creek (ha)

Eling and Bury (ha)

Whole Estuary (ha)

1946 304 101 44 449 1954 239 92 41 372 1963 202 84 40 326 1976 134 83 25 242 1986 98 79 20 197 1996 98 75 18 191

8.64 Using a rate of 0.6ha/yr (1986-1996) and a mean saltmarsh depth of 1m, saltmarsh erosion

contributes a contemporary sediment yield estimated at 6,000m3/yr. River Hamble

8.65 Within the River Hamble, the CCO has undertaken analysis of saltmarsh coverage from aerial photographs. The results of this analysis are summarised in Table 8.7 and show that the saltmarsh area within the Hamble has progressively reduced since 1946.

Table 8.7 Change in saltmarsh area within the Hamble (1946 to 2000), (CCO, 2007)

Year Area (ha) 1946 61.0 1971 49.1 1984 38.5 2000 35.7

8.66 Losses between 1984 and 2000 in the River Hamble amount to 0.2ha/yr. Assuming an

average 1m depth this contributes an annual sediment load of 2,000m3/yr. Sediment Sinks

8.67 Sediment sinks are defined as areas where siltation occurs and leads to net deposition.

8.68 Siltation rates in Southampton Water tend to be low and are attributed to limited suspended sediment concentrations, natural sediment scour and transport by ebb currents, but are enhanced in quieter areas outside of the main flows. Redistribution of sediments can occur in relation to vessel movements and dredging activity.

8.69 Highest observed sedimentation is predominately in the container berths, Bury Swinging Ground, around the berths of Dock Head and at Hythe and Ocean Village marinas. Siltation within Southampton Water tends to be more rapid to the west of the main channel, to the east, siltation rates are lower and have been measured at 0.02 to 0.04 m/yr (Flood, 1981).

8.70 The following sediment sinks are discussed: Accretion of saltmarshes; Intertidal accretion; and Dredging.



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Accretion of Saltmarshes 8.71 In order to keep pace with rising sea levels the saltmarsh surface must also be accreting

vertically. DGPS measurements by ABPmer (2007b) between 1996 and 2005 indicate that although the fronting edge of the saltmarshes is eroding, the elevation of the saltmarshes has remained stable with no evidence of vertical erosion. Based on the present day plan coverage and observed rates of sea level rise this equates to a rate of accretion of around 4,000m3/yr. Intertidal Accretion

8.72 Present evidence suggests that intertidal accretion is limited to the River Itchen, at a rate of 2,000m3/yr. Present Maintenance Dredging

8.73 Maintenance dredging of the navigation channel in Southampton Water is undertaken roughly every six months, with a spring and an autumn campaign. In addition, maintenance dredging also occurs at other facilities such as Marchwood Military Port, Fawley Marine Terminal, the BP Jetty, marinas and wharves in the Rivers Hamble and Itchen, the approach to the Hythe Marina and the Intake Channel to Calshot Power Station.

8.74 Maintenance dredging is undertaken as a two-stage process. Regular surveys are undertaken to assess the amount of material above the nominal depth. Material above the nominal depth is subsequently dredged and removed to the Nab Deposit Ground. Bed levelling is then used to smooth out draghead furrows followed by a further survey to assess the interim changes to the seabed.

8.75 Monitoring data has been collected biannually since the previous channel deepening in 1996/7. Dredging volumes are calculated by summing the volumes extracted from each dredging area. Table 8.8 summarises the amount dredged from the study area between 1998 and 2007 from ABP dredging records. Table 8.8 Dredging volumes for the study area between 1998 and 2007

Average Between 1998 and 2007

Bury Swinging Grounds to Dock Head (x103 m3/yr)

Dock Head to Calshot (x103 m3/yr)

Average per year 226 142 Maximum 278 262 Minimum 137 33 Range 141 229 Standard deviation 45 84

8.76 In addition, approximately 59,000m3/yr is dredged from the Exxon Mobil berths, the BP Jetty

and Marchwood Military Port in Southampton Water. Consequently, the net value for Southampton Water is estimated at 201,000m3/yr. Sediment Transfers

8.77 Sediment transport pathways within Southampton Water differ for coarse and fine grained sediments (Figure 8.3).



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8.78 The asymmetric form of the tide provides stronger flows during the ebb phase, which leads to a potential net seaward transport of coarse sediments. This process will lead to natural sorting of sediments along the channel with sands eventually being lost from the estuary and a long-term trend to an overall fining of sediments within the estuary and upriver. Bramble Bank is a potential sink for the exported coarse sediment.

8.79 Fine sediments are carried in suspension with the tide. During the flood phase the fine material brought into the estuary from the Solent has extended periods to settle out before the ebb phase, especially around the HW period. A brief flow reversal also develops during the Young Flood Stand (Figure 8.4) with a pattern of return flows evident across the intertidal area of Hythe Marshes, the shore between the rivers Itchen and Hamble, on the Dibden foreshore and an area of low flow (null zone) between the main channel flow. When the flood tide resumes, the recirculation ceases and flow directions in the intertidal zone returns in an upstream direction.

8.80 Ebb dominance in flow reduces in an upstream direction making areas upstream more susceptible to siltation, as evidenced by the greater dredging requirements upstream of Dock Head. Sediment Budget Summary

8.81 An updated baseline sediment budget has been developed to include present evidence and understanding related to the post-1996/97 channel deepening situation.

8.82 Table 8.9 summarises the component parts of the sediment budget and Figure 8.5 presents these details in schematic form. Table 8.9 Summary of baseline sources and sinks for Southampton Water

(x103 m3/year)

Sources of Sediment Sinks and Removal of Sediment Soton Water 53 Soton Water -

Test 23 Test - Itchen - Itchen 2

Intertidal erosion

Hamble 3

Intertidal siltation

Hamble - Soton Water 29 Soton Water -

Test 0 Test - Itchen 2 Itchen -

Subtidal erosion

Hamble 2

Subtidal siltation

Hamble -

Cliff erosion Soton Water 5 Soton Water 201

Test 10 Test 226 Itchen 6 Itchen 7 River load

Hamble 1

Dredging

Hamble 13 Saltmarsh 8 Saltmarsh 4

Marine import 309 Total 451 Total 451



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Future Change

8.83 The principal factor to influence the future morphological development of the present baseline in the long-term is the predicted increased rates of sea level rise. Further climate change effects such as changes in the intensity of winds and rainfall patterns remain important but are likely to be secondary.

8.84 An assessment of likely change has used a range of methods such as the rollover model (or transgression model). These methods are explained in further detail in Appendix C.

8.85 The rollover model is a conceptualisation of estuary response as a whole, where an estuary will try to move landwards as water level rises. This means that the estuary is trying to get wider and longer, however, this process may be restricted by shoreline developments and underlying geological constraints. In such a constrained system migration still occurs at lower water levels but may be possible at HW, resulting in coastal squeeze. This potentially gives rise to intertidal losses.

8.86 Current Defra guidance (Defra, 2006) has been applied to consider how the estuary might respond to future sea level rise scenarios, and with assessment offered for present day, 20, 50 and 100 years. For the purpose of the exercise no increase to the tidal range has been applied. Table 8.10 summarises the key information. By 2106 the net effect of increased rates of sea level rise is an increase in mean sea level of 1.1m and a predicted loss to the intertidal area of 314ha due to coastal squeeze. Table 8.10 Future mean sea level values based on current Defra guidance and predicted

intertidal losses in Southampton Water

Year 2006 2026 2056 2106 Mean sea level (m CD) 2.60 2.68 2.94 3.61 Predicted intertidal loss (ha) - 45 87 314

8.87 Further conceptualisation of how different parts of the study area may change over time is summarised in Table 8.11. Table 8.11 Summary of how the estuary features are likely to change over different time

scales (ABPmer, 2007c)

Time Scale Feature

Short-Term Change (Tides and Storms)

Decadal Change (10-100 Years)

Solent The tide throughout the system exhibits a marked surface distortion, which increases up-estuary. System orientation will continue to limit fetch lengths and hence waves are limited for the prevailing winds. Sedimentation concentrations are relatively low, although cliffs and intertidal areas are eroding.

The river basin as a whole will respond to the current trend of sea level rise, entailing some form of landward transgression. Beach and cliff erosion around the margins is likely to continue.

Mouth Circulation in the Solent leads to complex interactions at the mouth of Southampton Water.

The spit has a substantial lobe into the Solent, which indicates a sediment



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Time Scale Feature

Short-Term Change (Tides and Storms)

Decadal Change (10-100 Years)

The mouth itself is naturally deep and Calshot Spit limits the width and offers significant protection to Southampton Water and is considered relatively stable.

sink. The spit is unlikely to breach in the foreseeable future.

Southampton Water

Although an ebb dominant system, fine sediments are imported because of the tidal asymmetry and gradient in sediment concentration. The Young Flood Stand produces flow reversals over the intertidal areas, enhancing sediment movement up on to the upper intertidal. Prevailing south-westerlies mean that the area is relatively sheltered with slightly more energy reaching the Netley shore than the Hythe shore. Currently the combined effect is a slight net erosion of shallow subtidal and intertidal areas.

Over the long-term (200 years) the system appears to have been remarkably stable. Erosion on the intertidal has been more marked in recent years and contributes, along with the marine input, to the siltation in the inner subtidal. Saltmarshes in the estuary expanded rapidly in the late 19th Century and have been eroding since the 1940s. The rate of loss appears to be reducing.

Rivers: Test Itchen Hamble

Small sediment volumes and low flows compared to the tidal prism. This may increase during periods of high flow, or the density effects may be causing a greater proportion of the marine supply to be deposited.

Climate changes (particularly rainfall) and developments within the catchment will alter the run-off characteristics. Flow and sediment inputs are, however, likely to remain small. Increased freshwater flow may increase the importance of stratification and density currents.

Impact Assessment

8.88 The impact assessment describes changes to the baseline physical processes that are likely to occur as a result of implementing the proposed works. These changes may be short-term effects that occur during the dredging process or post-construction effects that modify the baseline. The impact assessment aims to address specific stakeholder concerns, as identified during project scoping and subsequent consultation (Appendix A).

8.89 The study methodology has been developed in line with current best practice recommended in the Estuary Guide (http://www.estuary-guide.net/), which has recently been updated under a Defra and Environment Agency Flood and Coastal Erosion Risk Management R&D Programme (FD2119). The suite of models has been applied to consider impacts relating to the proposed scheme at both regional (Solent and English Channel) and local (Southampton Water and component estuaries) scales. The models are evidence based, having been underpinned by the latest available datasets. The assessment is based on the interpretation of results of the numerical models along with a knowledge base gained from new field data e.g. geotechnical, bathymetric and benthic investigations. Further details relating to these investigation methods, configuration, calibration and validation of the numerical models are provided in Appendix C.

8.90 The modelling tools have been applied to support the quantification of the magnitude, extent and duration of potential changes that are likely to result from the proposed channel deepening over the long-term, and during the construction phase in the short-term (i.e. during the dredge

http://www.estuary-guide.net/�



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process). Investigations have also considered the likely response of the physical environment to climate change. The results from these assessments are presented and discussed fully in Appendix C.

8.91 Outputs from the model have been presented as both spatial (plan) plots for key phases of the tide and time-series data at selected locations of interest. The spatial plots represent a fixed moment in time whereas the time-series plots represent at a fixed location but demonstrate changes that may occur over all other states of the tide. The locations at which time-series plots have been extracted were selected to provide the detailed change at locations where the fixed time plan plots of the different parameters indicate maximum change, within areas proposed for dredging, and areas of potential concern identified from project scoping and consultation. In addition, sites were located at the main maintenance dredge areas in order to evaluate any effects that might occur to present maintenance dredge commitments following the channel deepening. The locations of the time-series points are shown in Appendix C, Figure C66 for the local model and Figure C99 for the regional model. Key Impact Pathways

8.92 The potential impact to hydrodynamic conditions and sediment transport in the study area as a result of the proposed channel deepening arise primarily from the direct change associated with the deepening and widening and the disturbance of sediment during dredging and disposal. A change in geometry of the channel within the context of the water body in which it sits leads to direct volumetric changes in the system and potentially causing direct change to the tidal prism. These changes, along with the characteristics of the resulting sea bed, have the potential to modify the way the tide propagates through the area and in relation to changes in water levels, flows, sediment transport (accretion and erosion patterns) and adjustment of the sediment budget. From this understanding, the direct and indirect physical changes to the extent of subtidal and intertidal habitats and morphological evolution can also be assessed. Alongside the assessment of how the estuary may adapt to the proposed channel deepening it is also important to gauge how the system may respond to climate change and sea level rise.

8.93 The key impact pathways are addressed in the following sections: Changes to Water Levels; Changes to Tidal currents; Changes to Sedimentation Patterns from the Channel Deepening; Changes to Tidal Prism, Tidal Propagation, Sediment Balance and Morphology of the

Estuary; Effects of Sea Level Rise; Dispersion of Disturbed Sediment from Dredge Process; Changes to Maintenance Dredging Commitments in the Estuary; Proposed Disposal at the Nab Deposit Ground, Effects on the Bed; and Dispersion of Material from Nab Deposit Ground, Suspended Sediments and Bed

Accumulations.

8.94 The following sections summarise the key outputs that relate to the potential impacts upon the physical environment with an initial comment offered in relation to the scale of any change. The implications and significance of the construction and post-construction impacts are further



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reviewed in relation to their potential effects on water and sediment quality (Chapters 9 and 10), nature conservation (Chapters 11 to 13), commercial fisheries (Chapter 14), coastal defence (Chapter 15) and commercial and recreational navigation (Chapters 16 and 17). Changes to Water Levels Solent High Water

8.95 Within the area of deepening of the Nab Channel little change in water level is evident, particularly at HW. From the Nab to around Ryde Sand HW levels are increased by less than 0.001m (1mm), which then increase through East Solent to the Thorn Channel, to about 0.004mm (4mm). Low Water

8.96 At LW there is consistently small reduction in water level, which starts to slowly increase from about 0.002m (2mm) in magnitude through the Nab Channel, up to about 0.0025m (2.5mm) opposite Ryde Sands and about 0.005m (5mm) as the tide reduces to its minimum range of about 3.25m for the modelled spring tide. The changes that occur at both HW and LW at the Thorn Channel continue at about the same magnitude throughout West Solent.

8.97 This combined HW and LW changes represent a very small (negligible) increase in tidal range of the order of 0.1% in East Solent and about 0.3% at the Thorn Channel.

8.98 When all states of the tide are considered the changes in water level that occur as a result of the approach deepening are generally within ±0.002m (2mm) throughout the Solent, with the exception of the Thorn Channel where marginally larger changes are evident.

8.99 Within the area of deepening of the Nab Channel and the deposit ground little change in water level is evident, particularly at HW. Overall these changes are very small and do not contribute to any change to the phasing and propagation of the tide through the Solent. Tidal Range

8.100 The modelling indicates that the tidal range will be increased by up to about 0.008m (8mm) in the Solent, particularly in the West Solent, and to a lesser extent in the outer section of Southampton Water. These small changes in water levels and this resultant effect on tidal range and, therefore, tidal prism will have an insignificant effect on the hydrodynamic and sediment regime. There will be a small increase in area of intertidal in the form of a thin strip around the LW mark, particularly in the Central and Western Solent (Paras 11.43 to 11.47). Assessment

8.101 The small magnitude of the changes compared to the normal cyclic variation of the tide and disturbance caused by episodic events such as storm waves and surges gives rise to an assessment of negligible exposure from changes to water levels. The impact of water level changes in the Solent on the physical processes is, therefore, insignificant.



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Southampton Water High Water

8.102 From the end of the Thorn Channel the HW levels tend to reduce in a uniform manner with distance into Southampton Water from a reduction of about 1mm at Calshot increasing to about 3mm at the container terminal. Within the Hamble Estuary, in the upper section, between Mercury Yacht Marina and Bursledon Bridge, HW levels are predicted to reduce by up to 10mm, however, this is considered to be a conservative estimate. Low Water

8.103 At LW there is a change from a reduction in level in the Central Solent and outer Southampton Water (average about 0.0025m (2.5mm)) to an increase in level from the approximate location of between the Cadland and Deans Lake navigation buoys at the outer end of the existing maintained channel, just up-estuary of Fawley. Through the area of the widening to Dock Head an increase in LW water level of around 0.002m (2mm) has been predicted. The effect on the LW level then increases in the 2km section up-estuary of Dock Head on both the Test and Itchen estuaries. At Mayflower Park, LW levels are raised by about 0.017m (17 mm), reducing to about 0.01 (10mm) in the container terminal. In the Itchen Estuary, the raised LW levels reach about 0.011m (11mm) at the Itchen Bridge, then reduce further up-estuary. The average reduction in LW level for the estuary as a whole is about 7mm. The detail of these changes along the estuary is shown in Figure C24, Appendix C.

8.104 In the Hamble Estuary the proposed channel deepening will cause reductions in LW levels of up to 5mm (average around 3mm). Tidal Range

8.105 These water level changes, predominantly those at LW, mean the tidal range, and therefore, tidal prism, will be reduced throughout much of Southampton Water, particularly the upper sections and into the component estuaries with the greatest change occurring at LW beyond Dock Head both in the Test and Itchen estuaries. Up-estuary of Cadland, the spring tidal range will be reduced by a maximum of about 20mm in the vicinity of Dock Head, and marginally less through the Test Estuary up to the container terminal. Assessment

8.106 The small magnitude (predominantly millimetric) water level changes that have been predicted by the models for much of the estuary as a result of the proposed channel deepening and widening would be almost impossible to measure directly in the field, mainly because they would not be able to be differentiated from natural variations. The small magnitude of change in water levels relative to the tidal range and longer term cyclic variation in the tides (i.e. the lunar nodal cycle), combined with the high probability that the changes will occur, leads to a low exposure (in EIA terms) to the change.

8.107 The sensitivity of Southampton Water to changes in water levels alone on the physical processes is considered to be high and combined with the low exposure leads to a moderate vulnerability. However, development-induced water level changes in their own right are considered to be negligible. The impact of water level changes in the Southampton Water and its component estuaries on the physical processes is, therefore, considered to be minor/insignificant. The small direct net reduction in intertidal area predicted within



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Southampton Water is assessed in Paras 11.44 to 11.45. The tidal prism implications are considered in Paras 8.158 to 8.162. Changes to Tidal Currents

8.108 Modelling of the change to tidal currents caused by the Southampton Approach Channel Dredge has been undertaken on both the regional and local models. In general, all changes were only detectable within the vicinity of the dredge location with no interaction between effects at the Nab Channel and those around the Central Solent and Southampton Water parts of the development. This is clearly illustrated in Figure C17, Appendix C. The changes to tidal currents have, therefore, been considered separately for the Nab by using the regional model and for Southampton Water and the entrance area by using the local model. The detailed changes from the two methods of data presentation (plan/map plots and time series at specified locations) are set out in Appendix C. Nab Channel

8.109 Figure C17 in Appendix C shows the post-dredge vector map and a comparison before and after the dredge at times of LW-3 (ebb phase) and LW+4.5 hours (flood phase). These times were chosen as they represented times of highly variable flow patterns in the Solent and times of higher flows into and out of Southampton Water. The plot shows the dredge in the Nab Channel has little widespread effect on the flow regime with the majority of change occurring within the area dredged. The changes in close proximity to the Nab Channel are generally less than ±0.025m/s (predominantly reductions), i.e. changes in the order of 5% at the time of the higher flows. The time series results show that the pattern of flow speed and directions is unaffected throughout the tide (Figure C18) with reductions in speed limited to times of peak flows.

8.110 Comparison of spring tide residual flows (the net excursion path of the tide) through the Solent before and after the dredging, along with the predicted differences (Figure C19), indicates that, in general, the pattern of residuals is unaffected by the dredge although very small displacements in the flow are evident particularly in the southern half of the East Solent and throughout the West Solent. In the area of the Nab, where there is negligible residual flow, the dredge has no effects. Little or no change in the residual currents is predicted over the northern section of the Solent and throughout Southampton Water. Southampton Water and Central Solent

8.111 In general, the effect of the Southampton Approach Channel Dredge is a small reduction in

peak flow speeds throughout most of the estuary, and across the full width, on both flood and ebb tides. For the most part, the change is less than 0.04m/s, which is up to about 10% of peak flows. Larger reductions in flows occur in the areas where the channel has been widened and these are flanked by areas of increased flows. This flow gradient results in the movement of water from areas of increased flows towards the widened areas, and is most noticeable in the shallowest water at the top of the side slope of the channel, and toward the edges of the adjacent estuary. Clarify These localised flow speed increases are up to about 0.1m/s in areas where existing peak flows are of the order of 0.8m/s (i.e. a 13% change).



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8.112 Figures C29 and C30 in Appendix C show similar orders of change in the Central Solent, with the maxima occurring within the deepened area and immediately adjacent. Small changes of up to ±0.01m/s extend to the shore of Stanswood Bay and the north shore of the Isle of Wight between Cowes and Gurnard. Nearly all change is confined to a 2.5km section of the Central Solent. Peak Flood Container Terminal to Dock Head

8.113 The most evident changes to the peak flood flow speeds occur across the whole width of the section up-estuary of Marchwood and into the Upper Swinging Ground, with the greatest change occurring where the maximum deepening will take place to widen the entrance to the Swinging Ground and opposite the Mayflower Cruise Terminal (Berths 105/106). In the up-estuary half of the widening section, flow speeds are reduced by over 0.06m/s from existing flows of around 0.4m/s (circa 15%). In the down-estuary part, flow is attracted to the area, raising flow speeds over a deeper depth by up to 0.02m/s. Sideways of these areas the largest change (reductions) in flow speed occur in the main deepened channel, the inner section of the upper swinging ground, and over the shallow foreshore to the west. This tends to indicate flow will be attracted to the widened area from both sides, which continues to increase flows over the outer section of the Upper Swinging Ground. Other changes of note include: In the upper estuary, the reductions in peak flood flows are higher (0.02m/s) at the

edges of the channel, equivalent to about 5% of the channel flows; and In the area of Marchwood moorings flows are reduced up to 0.04m/s on peak flows of

less than 0.4m/s. Dock Head to Hamble Estuary

8.114 Following the dredge, at the end of the widening and the start of the pipeline exclusion zone flow will be drawn to the east side of the channel as it fills the widened area. This will slightly accelerate flow over the existing bed of the exclusion zone before quickly slowing. These changes are in the range of about ± 0.06m/s (circa 10% of the actual flows). The modelling indicates that there will be an increase in flow speeds of 0.04m/s for about 1.5km upstream of the pipeline exclusion zone, equivalent to about 5% of existing channel flows. At the northern end of the widening, the increased flow drawn into the widened channel then moves back to the existing channel increasing flows by up to about 0.02m/s (about 10%) across the entrance to the Itchen Estuary, and in the area fronting Hythe. Within the existing channel, except the west side for 3km up-estuary of the pipeline exclusion area, the deepening will reduce peak flood flow speeds by about 0.02m/s. Either side of the channel flow speeds are reduced, generally by around 0.02m/s, equivalent to around 5% of existing flow speeds. Hamble Estuary to Central Solent

8.115 The differences in flow speeds at peak flood flows in this area as a result of the deepening/widening are relatively small and confined close to the areas dredged. In order to fill this larger cross-section increased discharge will be drawn through the area of the Fawley natural deep, locally raising depth-average flows in the order of 0.03m/s in the east of the channel. This increase is maintained between the BP Jetty and the ExxonMobil Fawley Marine Oil Terminal where there is only a small change to the estuary cross-section. Flow speeds at the western edge of the channel are reduced, particularly in the areas of Hook and Warsash, with a small area of increased flow speed occurring between the two.



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8.116 The greatest change (reduction of up to 0.1m/s circa 15% at peak flows) occurs in the area

proposed for widening for the realignment off Hook. In addition, in this area flow speed changes extend over the complete width of the estuary, reducing speeds by up to 0.04m/s (i.e. of the order of 10%) at the edges of the channel.

8.117 Flow speeds to the west of the channel and across the area of the Fawley Power Station intake Channel are likely to be increased from almost slack water up to 0.04m/s.

8.118 Flow speed changes as a result of the approach channel dredge outside Calshot are small- generally reductions of less than 0.03m/s. To the west of the channel throughout the area of Stanswood Bay, a small reduction up to 0.01m/s can be expected in areas already with slow flows at this state of tide. In the centre of the Thorn Channel where no deepening is required, no change in flood flow conditions will occur. Around the Bramble Turn, the maximum change (reduction) is up to about 0.03m/s around the inside of the bend. Smaller changes extend to the northern shore of the Isle of Wight, and westward into the Solent. In comparison with the flows through the area these small changes are negligible and would have no hydrodynamic or sedimentary effects in the area. Peak Ebb Container Terminal to Dock Head

8.119 The channel widening opposite the berths of the Western Dock affect the local flow directions, particularly on the ebb tide. The fast flows (0.6 – 0.8m/s) moving past the container terminal will be deflected less towards the berths, hence, the flow direction will turn, up to about 20° clockwise from existing conditions, through the outer edge of the turning area. This could slightly enlarge the slack zone in the corner by the King George V Dock. In general, flow speeds in the area are reduced marginally more than on the flood tide and again do not exceed 0.06m/s, and also extend over the shallow subtidal and intertidal in front of Dibden Bay. Localised areas of acceleration occur on the approach and exit of the widened area, which are about double those noted on the flood tide. Dock Head to Hamble Estuary

8.120 Following the deepening, the predominant change is to reduce peak ebb flow speeds throughout the Fawley to Dock Head section of the estuary on average in the order of 0.03m/s (about 3-5% depending on location). At the up-estuary end of the section, just before the widening starts, however, flow speeds increase over the unchanged depths as flow lines are attracted toward the widening, before reducing along the widening itself. Some of the flow is attracted from the adjacent subtidal to the channel immediately at the top of the side slope. The magnitude of the flow speed reduction in the widening area reduces down-estuary. At the end of the widening (start of pipeline exclusion area) flow speeds are reduced by up to 0.1m/s. On the Fawley side some dredging has taken place to ease access to the Number 1 Berth at the ExxonMobil Fawley Marine Oil Terminal. The net effect of these two components of the dredge is to raise flows on the Fawley side of the channel. Further areas of increases in flow of up to 0.03m/s are predicted to occur upstream of the BP Jetty, on the eastern side of the channel. Downstream of the pipeline exclusion zone, flow speeds are again increased by on average 0.02m/s (2% of existing flows) over the width of the estuary for a distance of some 500m.



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8.121 On the ebb tide, both small increases and decreases in flow speeds will occur in parts of the

pipeline exclusion area. This effect is localised, and reductions in flow speed are present either side of the exclusion zone. This might suggest that the flow speed effects are primarily due to redistribution of flow across the section rather than a change to the propagation of the tide.

8.122 In the Hamble, the flow speeds on the ebb tide, similar to the flood, appear unaffected by the dredging. Hamble Estuary to Central Solent

8.123 From the Hamble Estuary to Calshot the effects of the dredge are similar to those that occur on the flood tide. The differences are that with ebb flows, speeds are not increased in the Fawley Power Station intake channel, the area of increased flow speed around Fawley natural deep is slightly larger, and that there is an area of increased flow speed close to the undredged section.

8.124 Overall, in the Central Solent the model results at the time of peak ebb and flood flows (in Southampton Water) show that there is only a very small effect on the flows. Any changes that do occur on the ebb tide will be confined between north to south lines running across the West Solent from Stansore Point to Gurnard Head and in the East Solent from Hillhead to Old Castle Point on the Isle of Wight. On the flood tide, some very minor effects are predicted over the western Solent. The predicted level of these changes (generally less than ±0.01m/s) would not be measurable from the natural variation and would be close to the accuracy of any recording instrument. Time Series Analysis (all states of tide)

8.125 The previous section gives an overview of the effects of the dredge throughout Southampton Water and the Solent based on key phases of the tide, which exhibit maximum flows in and out of the estuary over a spring tide and identify the locations where maximum change caused by the approach dredging is likely to occur. A set of time series has been extracted from the model to determine how representative the spatial analysis is of all tidal ranges and times in the tide.

8.126 The time series plots for these locations within the estuary are shown in Figures C31 to C41 of Appendix C. The following bullet points summarise the main effects specific to these locations. The full detail and interpretation of possible effects these changes are likely to have on the general hydrodynamic and sedimentary regime is presented in Appendix C.

8.127 All three locations, with data extracted over the full range of tides, show the pattern of change to water levels and flows is the same for all tidal ranges; the differences occur in the magnitude of the change. Container Terminal to Dock Head Flow speed changes will be both positive and negative at different states of the tide

throughout the estuary with the greatest magnitude towards the entrance, of the order of 10% at the time of peak ebb flows. At Berth 206 the changes represent less than 1% of ebb flows increasing to around 5% for peak flood flows;



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In the area of the Western Docks, maximum spring tide flow changes (reductions) will be around 0.05m/s (7% of peak speeds);

Within the area of the container terminal, small increases in flood flows occur, but ebb flows are reduced by up to about 0.035m/s;

At Cracknore and towards the entrance to Marchwood Military Port, flow speeds are generally marginally increased, with the exception of the latter part of the rising tide, albeit from very low levels, which only peak on ebb tides at about 0.3m/s;

Between Mayflower Park and Dock Head, flow speeds will be reduced throughout the tide by up to about 0.03m/s (up to 5%).

Dock Head to Hamble Estuary Within the area of the main channel widening between Dock Head and Fawley, flow

speeds are reduced at most states of the tide, with the exception of over the HW stand and very briefly after LW. The most significant changes occur throughout the ebb tide and on the first part of the flood tide before the Young Flood Stand. Maximum flow speed reductions reach about 0.1m/s, just after peak ebb flows (8% reduction). The maximum increase is about 0.02m/s, occurring at times when existing speeds are negligible;

At the entrance to the Itchen Estuary, it is predicted that there will be slight (millimetric) increases in flow speeds. Flow patterns up the estuary will not change;

Flows are, in general, reduced and a small orientation of tidal flows occurs over the lower intertidal Hythe to Fawley foreshore, for most of the tide; and

In the Hamble Estuary, no measurable changes (±0.3cm/s) are predicted to occur to flood and ebb flow speeds as a result of the proposed deepened channel.

Hamble Estuary to Central Solent At the Fawley Natural Deep, the model predicts a marginal net reduction in flow,

averaged over the tide, which reduces further towards the edges of the channel, particularly closer to Calshot, which is likely to be a result of the proposed Hook widening;

Up-estuary of the Hook widening, flows are little affected suggesting that only a small additional amount of water will be attracted into the estuary due to the dredging works;

At Calshot, the largest flow speed changes (reductions) will be by just over 0.1m/s during the spring tide peak ebb flows, with significant increases in flow of up to 0.08m/s during the Young Flood Stand. Overall the magnitude of change is about double on springs compared to neaps.

Assessment

8.128 The modelling results generally indicate that the effect of the Southampton Approach Channel Dredge is to slightly reduce flow speeds at most locations throughout the estuary. In areas where there is no capital dredging, reductions are generally less than 0.04m/s increasing up to about 0.1m/s in the widened areas. Small areas of localised increased flows are predicted at the entrance and exit to the widened areas where a slight change in orientation of the tidal flows will occur. Detailed assessment shows that the magnitude of effect varies at different states of the tide and with tidal range. Magnitudes of change are predicted to be about half on neap tides compared to springs. Overall, the change in flow speed at all states of tide will be less than 10% of existing flows. Outside Calshot, the modelling shows changes to be even



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smaller, therefore, flow induced effects on sedimentation, erosion and general sediment transport patterns will be negligible.

8.129 In their own right, such changes in flows have very little environmental significance but will be an important contributor to the assessment of, for example, sedimentation patterns, dispersal of sediment, water quality and nature conservation issues. These changes to the flow regime are considered in the following sections in relation to other physical process pathways. Changes to Sedimentation Patterns from the Channel Deepening

8.130 Changes to the sediment transport patterns arising from the Southampton Approach Channel Dredge have been investigated for Southampton Water, Central Solent and the area of the Nab Channel. All modelling scenarios were run through a complete spring/neap cycle in order that the full effects of tidal range and water levels are represented. Map plots and time series analysis of the sediment transport modelling, which allow the effects of the deepening on suspended sediment concentrations, accretion and erosion pattern to be assessed against the existing baseline conditions are presented in Figures C42 to C56 of Appendix C. Appendix C also provides a detailed description of the baseline and the changes, which are summarised for the purpose of the EIA in the following sections firstly with respect to suspended sediment concentrations (SSC) and then the erosion and accretion patterns. Explanation of the changes also draws on the results of the hydrodynamic changes presented in Paras 8.108 to 8.129. Suspended Sediment Concentrations

8.131 The Southampton Approach Channel Dredge generally causes a reduction in SSC throughout Southampton Water, with the exception of the immediate entrance, where there is a slight increase following LW. The maximum changes, reductions of up to 2mg/l, against mean average background (or baseline) concentrations of around 30-60mg/l, occur over the HW stand on spring tides, over the shallower subtidal and intertidal areas either side of the channel, along the Hythe and Netley foreshore adjacent to the northern half of the Dock Head to Fawley widening. Up and down-estuary of this section the magnitude of change of SSC reduces with distance being no more than 1mg/l (reduction) at Fawley and the Upper Swinging Ground.

8.132 At the time of peak flood flows the greatest change occurs over the whole estuary width between Dock Head and Fawley with a magnitude of the order of 1mg/l, reducing to no effect just outside Calshot and only a slight change in front of the container terminal. The magnitude of change also decreases with distance up the Test, Itchen and Hamble Estuaries.

8.133 The reduction in SSC is generally lower on the ebb than on the flood, and the greatest change will be just down-estuary of Dock Head at the time of peak flows on the east side of the estuary. The magnitude of effect and variability of this change is greatest for spring tides compared to neaps.

8.134 The time series analysis indicates there will be a mean reduction in SSC (on peak spring tide ranges) of about 0.25mg/l at Calshot, increasing to about 1.6mg/l near Dock Head and then reducing (in magnitude) marginally in the Western Docks.



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8.135 Individual tide analysis generally shows that the SSC reduction will be higher in magnitude over the HW stand compared to at LW.

8.136 Over all tides, these reductions in SSC represent of the order of 1% change at Calshot and increasing to the order of 5% change in the areas beyond Dock Head. Accretion/ Erosion

8.137 Modelling has been used to predict the effect of the approach deepening on the accretion and erosion patterns by calculating the change in bed thickness for each model cell over a complete spring/neap cycle, so that the dynamic effects, over all tidal ranges are taken into account. The data has then been scaled so that it represents an annual accumulation or erosion rate. It is important to note that the models predict potential sedimentation due to the hydrodynamic environment. Whether increased sedimentation actually results in areas where it is predicted will depend on a number of factors, including waves, surges and vessel movements, sufficient sediment supply, and the relative strength of flood and ebb flows to cause erosion. In other words, the actual change in sedimentation following the dredge may not be a significant change in overall sedimentation volumes but more a redistribution of material. Container Terminal to Dock Head

8.138 Up-estuary of Dock Head and in the Itchen Estuary sedimentation rates will be increased within the deepened channel and reduced (circa 5%) in the areas not affected by the dredging works, e.g. Marchwood and Ocean Dock. The only area of predicted increased sedimentation outside the deep channel is over the intertidal area between Cracknore Hard and the Marchwood Yacht Club, where up to about 0.02m/year (+20 – 50% above baseline) potential additional siltation can be expected within 200m of the quay wall, mainly during the flood tide. Consideration of the effects of wave disturbance both naturally and anthropogenically, would most likely reduce the actual rate of settlement.

8.139 The deepening creates a marginal reduction in sedimentation (generally less than 0.005m/year) over the intertidal fronting the saltmarsh at Bury and within the Bury Swinging Ground, whilst no changes are predicted in the container terminal, nor intertidal areas fronting Dibden Bay. A marginal reduction (a few millimetres per year) in the baseline sedimentation rate is also indicated in, and either side of the Hythe Marina entrance channel, as well as locations within the Itchen Estuary, including all areas currently dredged. Negligible ecological effects will, therefore, result from such small changes to sedimentation rates in this area.

8.140 Most of the navigation channel and berths within the Western Docks will be subject to additional accretion, predominantly up-estuary of Berth 102 with a slightly greater thickness around the quay than to the west side. The maximum increase in sedimentation is likely to occur where the greatest depth of dredging has taken place, e.g. the edge of the channel opposite the Mayflower Cruise Terminal where an additional accumulation rate of around 0.04m/year can be expected, which is about double the existing rate. It should, however, be reiterated that vessel movements will redistribute this material to the 'quieter' areas and at some locations the increase in annual depth of accumulation will be greater than indicated by the model due to such re-distribution.



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8.141 An increase in sedimentation potential will also occur in the corner of the Middle Swinging Ground at the intersection of Berth 101 and Mayflower Park from the change in flood tide flows.

8.142 In the Junction Channel, predominantly to the west side, the potential for sedimentation will be very marginally reduced, however, a few millimetres extra per year can be expected slightly further down-estuary at the channel edges, particularly on flood tides, between Town Quay and Dock Head. This redistribution of sediment will require more maintenance dredging in the upper Western Dock area of the main channel, but not in the container terminal. Between Mayflower Park and Dock Head the volume is unlikely to substantially change although it may accumulate slightly further down-estuary than at present. Some of these increases will be offset by the potential for a reduction in the Eastem Docks, entrance to the Itchen Estuary and at Marchwood. Dock Head to Fawley

8.143 Between Dock Head and Fawley the sedimentation potential will be increased over most of the width of the estuary, particularly the widened area at the location of maximum deepening where up to about 0.05 m/year can be expected. This equates to an increase of the order of 50% compared to the baseline case. The northern part of this section of the channel also shows increased sedimentation of the order of 0.02 m/year, i.e. a 20% increase. The changes suggest that maintenance dredging of the channel will be required, particularly down the eastern side, in the widened area.

8.144 To the west of the navigation channel, the annual rate of sedimentation potential in the immediate lee of Hythe Marina is predicted to be reduced by the order of 0.01m/year, representing about a 4% reduction in the existing potential. A slight decrease in sedimentation will also occur along the edge of the saltmarsh, which is likely to arise because of the marginal reduction in tidal range i.e. less sediment can be transported to the area. Potential sedimentation will increase either side of the navigation channel in both the shallow subtidal and intertidal, by about 0.01m/year, an average of about a 25% increase over the area. These areas of increased sedimentation closely correspond to both the areas of reduced flow speeds and reduced SSC. This indicates that the net change in flow pattern allows more settlement of fine sediments from the water column, possibly at a higher level on the mudflat than exists at present, which cannot be re-eroded as a net effect over a spring/neap cycle.

8.145 There will be a marginal reduction in the sedimentation rate of no more than a few millimetres per year in, and either side of, the Hythe Marina entrance channel, as well as all locations within the Itchen Estuary, including all areas currently dredged (maximum reduction from baseline of 5%). Fawley Area

8.146 There will be a marginal reduction in sedimentation potential across the entrance to the Hamble Estuary and over the intertidal of the outer estuary itself, accounting for less than 0.0002m/year (less than 1% of existing rates in these areas). At the channel alignment between Hook to Hamble Spit, which is currently a self-maintaining area with small erosion potential, the modelling predicts an increase in sedimentation potential of up to 0.05m/year, which may require maintenance dredging.



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8.147 Along the ExxonMobil Fawley Marine Oil Terminal increased sedimentation of up to 0.02m/year (10 – 20% for the berth areas) is predicted, marginally less in the berths than at either end. It is highly probable that vessel movements will disturb a proportion of this increase. In the area of the pipeline exclusion zone, however, a slight reduction (millimetres per year) in sedimentation potential is predicted, although this is unlikely to result in practice, as the bed material is coarser than the characteristic size of the material modelled. Fawley to Calshot Turn

8.148 To the north east of the channel, from the Hamble Estuary entrance outwards, there is a marginal potential for a small amount of sedimentation of fine material. However, this is unlikely to result in a net accumulation, as it will be prone to wave activity within this exposed area.

8.149 From Calshot through the Thorn the newly deepened channel is likely to remain self-maintaining, with the possible exception of the outside of the Calshot Turn where some sedimentation could occur on the flood tide. On the intertidal and saltmarsh area between the Fawley reclamation and the lee of Calshot Spit, additional sedimentation of up to 0.05m/year is predicted immediately off Calshot Castle, which extends towards the entrance to the Power Station intake channel.

8.150 Along Stanswood Bay, adjacent to the HW mark, the modelling indicates a small amount of additional sedimentation of up to 0.01m/year. This net accretion is unlikely to remain here due to the exposed nature of the shoreline to wave disturbance. Therefore, it is predicted that there will be no on-going change to the oyster bed sediment regime in this area as a result of the Southampton Approach Channel Dredge. Nab Channel

8.151 There will be slight changes in the overall distribution of accretion and erosion potential at the Nab Channel, with marginally higher potential for accretion of fine sand at the northern end of the channel. Assessment

8.152 In general, the channel deepening has the effect of marginally reducing suspended sediment concentration throughout the estuary at all states of tide. At the same time the sedimentation potential over both the subtidal and intertidal areas of Southampton Water is marginally increased, albeit a very small decrease is predicted in the Hamble and Itchen Estuaries. The maximum increases in predicted sedimentation will occur where the greatest depth of dredging has taken place, i.e. the widening areas. This maximum rate of increased sedimentation is expected to be less than 0.05m/year at any location in the estuary. Within the main navigational areas, this is likely to be redistributed, which could cause a small change to the areas where maintenance dredging might be required.

8.153 There is evidence that the increased sedimentation in the lower estuary is partially as a result is due to the general reduction in ebb flows, but this sediment may not travel further than approximately Fawley. However, the main reason for the increased sedimentation for the estuary as a whole is:



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The marginally greater amount of settlement from the water column, as indicated by the reduced suspended sediment concentrations; and

The reduction of ebb flow speeds, which marginally reduces the erosive potential at the bed, thus, enabling a higher proportion of sediment to remain in the estuary.

8.154 Increased potential for sedimentation is indicated over all shallow subtidal and most intertidal

areas, however, this may not occur in a number of areas due to the effects of natural and anthropogenic disturbance as currently occurs under existing conditions. It should also be noted that at nearly all locations both accretion and erosion occur at different states of the tide and tidal ranges. Any sedimentation is, therefore, transient (intermittent) and this is clearly illustrated by the time series analysis presented in detail in Appendix C.

8.155 The predicted changes to the sedimentation patterns are considered small in magnitude for the estuary as a whole, many of which would not be detectable from background variation. Within the deepened areas, the magnitude of change is considered as medium, but only because the magnitude is negligible in the baseline condition. With respect to sedimentation patterns within the estuary in their own right, the Southampton Approach Channel Dredge will be insignificant for the estuary as a whole and minor/insignificant in localised areas, predominantly adjacent to the areas proposed for widening. The magnitudes and locations of change may, however, have greater significance for other environmental issues. Changes to tidal prism, tidal propagation, sediment balance and morphology of the estuary

8.156 The Southampton Approach Channel Dredge has the potential to change the tidal propagation throughout the estuary, which along with changes to the cross-section geometry and tidal prism will change the estuary hydrodynamics for the longer term. These changes in physical processes will then influence the sediment transport processes resulting to changes in the accretion and erosion patterns. As a consequence, taking account of the availability of sediment, geological, geotechnical and anthropogenic controls, change could be additive to the specific on-going evolution of the estuary. These changes in turn may impact on, for example the future maintenance dredging requirements, ecology and nature conservation interests of the estuary.

8.157 Numerical modelling has been undertaken to determine the short-term change to the hydrodynamics and sediment accretion and erosion patterns, the effects of the deepening being summarised in Paras 8.95 to 8.155 and presented in detail in Appendix C. The model data has also been assessed in terms of the net change in sediment inputs and outputs to different sections of the estuary in volumetric terms to give an indication of the sediment transport processes occurring, how these are modified by the Southampton Approach Channel Dredge and how they influence the sediment budget of the estuary. Further parameters have also been calculated to allow assessment of the long-term change to the estuarine asymmetry and stability of the estuary. This analysis is presented in Appendix C. Tidal Prism

8.158 The Southampton Approach Dredging will remove around 7.76 million m3 of material entirely from the subtidal area upstream of Calshot. Therefore, there will be no direct change to the



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amount of subtidal and intertidal area or the tidal prism of the estuary. The change to the tidal propagation results in an average of about 0.003m lowering of HW levels and increases LW levels (on springs) by about 0.007m, therefore, reducing the spring tide range by about 0.01m. This change will cause a decrease in the mean spring tidal prism of about 245,000m3 (-0.22%). The dredge will increase the permanent estuary volume (at LW) by 7.11 million m3 (+6.5%), whilst at HW (mean springs) the volume reduction will be of the order of 6.89 million m3 (3.1%).

8.159 The dredge carries on through the entrance to the estuary causing an increase in the cross-section area at the entrance by about 3.5%. These effects on the tidal prism and cross-sectional area will reduce the O’Brien ratio (see Appendix C) by about 3.6%. This ratio for the existing estuary tends to indicate the entrance is too large for the tidal prism passing through, therefore, not in the theoretically stable form, however, accretion at the entrance is not apparent.

8.160 It should be noted that between the 1930s and 1970s the tidal prism of the estuary was significantly reduced due to reclamations to form the Western Docks, container terminal, Dibden Bay and Fawley, whilst the capital dredging was again subtidal and did not affect the cross-section of the estuary at Calshot. This suggests that prior to these developments the O’Brien ratio would have been considerably closer to that indicating a stable estuary form. The proposed dredge will, therefore, tend to move the estuary slightly further away from stability and increase the long-term potential for accretion at the mouth.

8.161 Analysis of historical bathymetric records for Southampton Water suggest the effects of perturbations on the natural system may take of the order of 200 years to reaffirm a stable form (ABPmer, 2007c). On this basis the effects of previous developments may still be affecting the evolutionary processes. Any future change would, therefore, be a combination of those developments and the new dredge. It should also be noted that variations would also be occurring due to the cyclic changes as a result of the lunar nodal cycle (18.61 years) as well as those introduced by sea level rise. During the next eight years, the effect of the lunar nodal cycle will increase the tidal prism alone by about 6.125 million m3, i.e. 25 times the effect of the dredge on the magnitude of change of the tidal prism. With respect to the O’Brien ratio this will move the estuary slightly closer to the stable form. This clearly indicates that the effects of the Southampton Approach Channel Dredge are considerably less than the natural variability that exists in the system as a whole.

8.162 On this basis it is considered that although the estuary morphological parameters are likely to move slightly further away from the theoretical stable form, the change to tidal processes coupled with the exposure of different material characteristics at the bed of the estuary will not change the overall relative stability that presently exists. Sediment Transport

8.163 Volumetric analysis of the sediment flux across the entrance section of the estuary at Calshot allows the gross change in sediment transport within the estuary as a result of the approach channel deepening to be assessed on a comparative basis. The analysis (detailed in Appendix C) has been undertaken to determine the net flux across individual cross-sections up the estuary for both pre- and post-dredging scenarios and by considering a complete spring/neap cycle.



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8.164 It should be noted that this analysis only accounts for the effects of tidally induced processes

and cannot take account of additional episodic disturbance factors, such as wave activity, both naturally and anthropogenically produced. Inclusion of such factors is likely to reduce accretion or increase the erosion potential, especially on the eastern side of the estuary with increasing magnitude down-estuary of Dock Head towards the mouth.

8.165 The modelling predicts that the Southampton Approach Channel deepening will cause a net increase (3.7%) in import of sediment across the cross-section at Calshot, which remains in the estuary. Consideration of the flow and sediment modelling results suggests that because the tidal prism is marginally reduced, the depth increases and widening generally reduce flow speeds (particularly the flushing power of the faster ebb flows) and the reduction in suspended sediment concentrations that results is due to an increased proportion of the marine import being deposited within the estuary, combined with a reduction in erosion power, as opposed to an increase in the quantity of sediment imported.

8.166 The greatest percentage change in net sediment retention (+4.6%) occurs up-estuary of Fawley. The most significant changes to the existing sediment regime from tidal processes will occur between Netley and Fawley where the potential for sedimentation derived from tidal flows is increased by 30% above existing conditions. This is the lower half of the area of maximum estuary change due to the widening. Most of this increased accretion potential is likely to accumulate within the area of the widened channel where annual accumulations of up to 0.05m in depth are predicted to accumulate over a year. In reality, however, this actual rate is unlikely to occur, due to vessel-induced redistribution/disturbance allowing more dispersion of the material.

8.167 In the Test Estuary, upstream of Dock Head the net import of sediment will be increased by about 1%, however, net accretion rates between Dock Head and Mayflower Park are little affected. In the section from Mayflower Park through to the container terminal net subtidal and intertidal accretion is predicted to increase by about 4% above existing conditions. This increase in sedimentation is due to the deepening reducing the already relatively low flow speeds in this area. The increased settlement in this area reduces the net import of sediment, into the container terminal and into the Test Estuary (4 – 5%), potentially reducing sedimentation in the container terminal area by about 2.9% per year. Part of this reduction in net import of sediment will be as a direct result of the small reduction in tidal range, which is at its largest in these sections of the estuary. In depth terms, this change represents a reduction in net accumulation potential of less then 5mm per year from tidal sources.

8.168 The capital dredge is predicted to reduce net sediment transport into the Itchen and Hamble Estuaries by 4% and 1.5% respectively. These changes are seen to propagate throughout both estuaries. In depth terms, these changes represent an average reduction in depth of accumulation of less than 1mm and 1.5mm per year in the Hamble and Itchen Estuaries, respectively. Since most accumulations take place within the deepened marina areas, the net effect on the intertidal areas will be smaller i.e. negligible. As a consequence of these reductions in potential sedimentation a marginal reduction in the existing maintenance dredging requirements for the estuaries is likely to result, which will be greatest in those areas nearest to the confluence with Southampton Water.



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8.169 Overall, the volumetric analysis of the modelled sediment transport rates pre and post dredging, indicates a net increase in sedimentation potential within Southampton Water, predominantly in the area of the channel widening between Fawley and Dock Head, particularly the down-estuary reach. Additional accretion is also likely within the Western Docks, but little change between Dock Head and Mayflower Park. These increases in potential accretion from tidal processes alone tend to reduce the net supply of sediment propagating into the Test (container terminal upwards), Itchen and Hamble. These results, however, only represent the changes in the sections of estuary as a whole and give no indication of the relative intertidal/subtidal effects.

8.170 Most of the change over the intertidal areas due to the dredge occurs between Fawley and Mayflower Park. On the west side of the estuary, following the deepening, the erosion potential of the up-estuary section which occurs under existing conditions is marginally increased (0.7%) whilst the accretion potential is reduced by about 4.4% in the lower section. The net effect over the intertidal, however, is still accretional between the reclamations of Fawley and Hythe Marina. On the eastern side, the existing pattern is reversed with small net erosion in the lower half, but accretion up-estuary. This trend is unaffected by the dredge except the erosion potential is marginally increased down-estuary whilst additional accretion potential is predicted towards the Weston Shore. Overall, between Dock Head and Fawley there is a net potential erosion of the intertidal area before the dredge, which may be slightly enhanced following the dredge. At the same time there is an annual 11.5% increase in potential for subtidal accretion in this section.

8.171 Between Dock Head and Mayflower Park, the intertidal fronting Dibden Bay is shown by the modelling to be accreting under existing conditions, which is enhanced by over 50% with the approach deepening. This change represents a potential accretion of the intertidal from about 4 - 6cm per year as a result of the dredge due to tidally induced conditions alone. Monitoring, however, following the last capital dredge in 1996/97 generally indicates that this area has eroded over the period at an average rate of 1 – 2cm/year. This indicates that wave and or vessel motion disturbance effects are of greater significance in determining the sedimentation patterns within the estuary than those generated by tidal effects alone. The modelling does, however, indicate that net erosional effects in this area of the estuary will be reduced following the dredge, but probably not to the extent to create net accretion in the future when all processes are taken into account. Additionally, within the main subtidal channel the potential for tidal induced sedimentation is predicted to reduce. Overall, however, it is likely that the sediment predicted to accumulate on the intertidal is likely to be disturbed, much of which will deposit in the deepened subtidal area. Overall a net increase in maintenance dredging between Dock Head and Mayflower Park is predicted.

8.172 The sub-division of the estuary allows an assessment of the way the sediment tends to move around the estuary transferring from one section to another. In general the net movement of sediment in suspension along the intertidal is down-estuary (on both sides) below Dock Head, which is unaffected by the dredge except for an increase in the volume of sediment being transferred. Above Dock Head in the Test Estuary, sediment is generally transferred upstream over the intertidal.

8.173 Between Netley and Dock Head the patterns of sediment movement indicate a net circulatory transfer of sediment in a clockwise direction on the east side of the estuary and anticlockwise



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to the west. The dredge enhances this pattern of circulation with most notable changes in the order of 20% between Dock Head and Netley. This is the area where the greatest widening and deepening of the estuary will take place.

8.174 The data also shows a general clockwise transfer of sediment on the west side of the estuary along the Dibden Bay foreshore, which moves back towards the subtidal, up-estuary of Marchwood. With the dredging, more material is retained over the intertidal, between Dock Head and Mayflower Park, from both movement up-estuary and across the intertidal/subtidal interface. Within the Western Docks area (above Marchwood) a reduction in loss of material from the intertidal to the subtidal is almost equated by an increased transfer into the area of the Upper Swinging Ground and the subtidal area of the container terminal.

8.175 In summary, the Southampton Approach Dredging does not affect the sediment transfer trends around the estuary, but it does make small changes to the magnitudes of either the predicted tidally induced erosion and accretion of the intertidal in different areas of the estuary. Effectively there is a ‘sediment transport divide' around the location of Dock Head with a down-estuary transfer of sediment over the intertidal downstream and up-estuary above. The analysis indicates there are large volumes of material continually in motion, circulating around the estuary over a spring/neap cycle, which is increased (in volume terms) by the deepening. The net changes, however, are relatively small (equivalent to less than about ±1mm /year) for most intertidal sub-sections, with the exception of the Dibden Foreshore. Here an average increase of about 2cm/year is indicated by the modelling above the existing accretion potential. However, 10 years of monitoring has not indicated accretion in this area, which suggests, wave disturbance, both natural and anthropogenically derived is the more dominant process in this area. This is likely to continue following the dredge, therefore, it is possible this additional accretion volume will be moved up-estuary and into the area of the container terminal, or Upper Swinging Ground, where it will require to be maintenance dredged.

8.176 Overall, the potential for accretion over the intertidal areas is increased by about 8%, the equivalent of less than 2mm/year (average based on 1100kg/m3 bed density) predominantly over the Dibden Foreshore. Tidal Asymmetry

8.177 The asymmetry of the tide and the duration and phase of slack water periods contribute to the net movement of sediments. In Southampton Water the strong ebb flows favour the export of coarse sediments from the estuary whereas the long slack water period favours the net deposition of fines advected into the estuary over the flood tide.

8.178 For this assessment a number of tidal asymmetry parameters have been calculated both for existing water levels and with respect to the effects of sea level rise. These include the Dronkers Asymmetry ratio, the Slack Gradient, Slack Duration and the net length of tidal excursion. This analysis is presented in Appendix C with the results summarised below.

8.179 The tidal curve within Southampton Water is complex, asymmetrical in shape with the ebb phase of shorter duration than the flood. There is also a distinctive ‘stand’ at HW giving rise to a double HW. A period of slack water also occurs mid-flood (referred to as the Young-Flood Stand). These features are more prominent on the spring tides compared to neaps. All of



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these factors have important implications for sediment transport within Southampton Water. These features create ebb dominance in terms of peak velocities, albeit occurring for a shorter period of time.

8.180 The Dronkers Asymmetry Ratio is further parameter commonly used to assess morphological effects on estuaries. This parameter considers area and volumetric data as opposed to flow information. This has been calculated for cross-sections along the estuary both for the existing water levels and with sea level rise 100 years in the future. For this parameter the estuary is defined predominantly as flood dominant (the opposite to what the flow speeds would suggest) except in the section of the estuary between Calshot and the Fawley Power Station. In general, the flood dominance is relatively constant in magnitude until the vicinity of Mayflower Park, where a sudden increase in flood dominance occurs before becoming uniform (although with a slightly increasing trend) but at the higher level for the rest of the estuary. This predominant flood dominance in the Dronkers asymmetry ratio indicates the tidal properties of the estuary are conducive to a net import of fine sediments. With the inclusion of the Southampton Approach Channel Dredge, both alone and in combination with sea level rise, the changes in the asymmetry ratio are negligible. This analysis, therefore, indicates that the approach deepening at the present time or 100 years in the future (with sea level rise) will have no significant effect on the existing evolutionary processes within the estuary, thus generally confirming the conclusion from the O’Brien ratio analysis.

8.181 Calculation of the ratio of the high to LW slack gradient also indicate the estuary is conducive to the import of fine sediment, however, this parameter tends to suggest the flood dominance tends to reduce in an up-estuary direction, particularly in the area of the Western Docks i.e. the opposite of the Dronkers Asymmetry Ratio. With the inclusion of the deepening there is no change in the overall trend and the magnitude and direction of the change is variable along the estuary but generally within the same intersection variability as calculated for the existing condition. With sea level rise there is no significant change in the magnitude or trends indicated by the ratio of high to LW slack gradient. Overall, as with the asymmetry ratio analysis the results do not indicate that there will be a significant change in the existing estuary evolutionary processes in the future, from sea level rise, the dredge or in-combination.

8.182 Another assessment parameter commonly used is the net slack flow duration. In this case it is assumed that most sedimentation occurs during the periods of slack flows and if the duration at HW is greater than at LW there will be a net import of fine sediment. This analysis shows a similar pattern of change along the estuary to that of the Dronkers Asymmetry Ratio, however, is considerably more variable between the entrance to the Itchen Estuary and just upstream of Mayflower Park where the net slack duration method suggests an area of ebb dominance exists. As with the other forms of analysis inclusion of the approach dredging alone and incorporating the effects of sea level rise have no effect on the net slack duration parameter and, therefore, suggests there will be no long-term effect on the existing morphological evolution of the estuary.

8.183 The final asymmetry parameter assessed is the tidal excursion. This is effectively the net area under the curve of peak flow speeds against time and a pre-defined threshold velocity. In this case a threshold value of 0.2m/s was used for analysing the potential movement of the finer sediment. Unlike just looking at the peak flows to determine dominance this parameter takes account of the time at the different flow speeds as well as the direction of flow.



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8.184 The net tidal excursion, similar to the other analysis methods, indicates an overall flood

dominance, indicative of a net import of sediment. The flood dominance reduces in magnitude between the Central Solent and Calshot. Through the area of the ‘Natural Deep’ the excursion length remains relatively stable, but a temporary increase in dominance occurs around the entrance of the Hamble Estuary, returning to the previous level just as the estuary widens (at HW) after the Fawley reclamation. Between here and the confluence with the Itchen Estuary the flood dominance in excursion length increases in an asymptotic manner, before reducing erratically between Dock Head and the southern end of the Western Docks. Thereafter, the excursion analysis reveals no net dominance. This pattern of change, particularly in the upper estuary, is similar to that indicated by the slack gradient analysis.

8.185 Similar to the other parameters the dredge and sea level rise effects on this morphological parameter are very small, however, some trends are apparent, which vary in magnitude of effect at different locations along the estuary. Again, the effects of the dredge are generally of the same order as those introduced by the effects of sea level rise, but in both cases, these effects will only be marginal with respect to the morphological processes that are already at work within the estuary.

8.186 Overall, the analysis of tidal asymmetry shows there is a reasonable level of agreement between the various methods used. In terms of the transport of fine sediments the trend throughout much of the estuary is for flood dominance, this is indicative of a net import of fine sediment. Differences in trend but mostly magnitude of the trend are evident between the methods up-estuary of Dock Head where the section to section variability is also considerably greater. The changes resulting from the proposed channel deepening on all the existing morphological parameters is negligible, indicating there will be no significant change to the morphological evolutionary processes already at work in the estuary. This conclusion is not changed when the effects of sea level rise are also considered. This indicates the estuary will not try to evolve to a substantially different form in the future to that which exists today due to the proposed approach deepening or with sea level rise. Estuary Sediment Budget

8.187 For the current study, a sediment budget for the estuary was updated, taking account of the actual dredging requirements over the last 10 years to provide the current best estimate. The modelling of the sediment transport changes and estimates of the maintenance dredging and its distribution following the Southampton Approach Dredging have been used to give a prediction of a revised sediment budget. The post-construction sediment budget is summarised in Table 8.12 extracted from Appendix C and shown schematically in the same way as the historical and baseline cases in Figure C.121 (Appendix C), with a discussion of the changes made to the budget given below.



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Table 8.12 Summary of sources and sinks for Southampton Water (SW) following the proposed approach deepening (x103 m3/year)

Sources of Sediment Sinks and Removal of Sediment

Soton Water 53 Soton Water - Test 16 Test -

Itchen - Itchen 2 Intertidal erosion

Hamble 3

Intertidal siltation

Hamble - Soton Water 20 Soton Water - Subtidal erosion

Test 0 Test - Itchen 2

Subtidal siltation

Itchen - Hamble 2 Hamble -

Cliff erosion Soton Water 5 Soton Water 228

Test 10 Test 229 Itchen 6 Itchen 7 River load

Hamble 1

Dredging

Hamble 13 Saltmarsh 8 Saltmarsh 4

Marine import 357 Total 483 Total 483

8.188 The modelling results have indicated that following the dredge the annual maintenance dredge commitment for the estuary as a whole is likely to increase by about 30,000m3 per annum of which about 90% will occur within Southampton Water. Down-estuary of Dock Head, however, there will be no significant change in the sedimentation over the intertidal in this area when all processes are taken account. This indicates there will be no change from that previously derived for the sediment budget from this component.

8.189 Overall, the sediment supply to the intertidal in the Test Estuary is predicted to increase by around 7000m3 per annum, which has the potential to reduce the rate of erosion within this area. Within the component estuaries of Southampton Water there will be a marginal reduction, however, in terms of the sediment budget these changes will be insignificant.

8.190 In the subtidal area, modelling tends to indicate a slightly greater potential for accretion (albeit marginal). Flow speeds are generally reduced, particularly between Dock Head and Fawley due to the widening. The tidal range is marginally reduced, which effectively reduces the tidal prism. All these changes indicate that subtidal erosion as shown by the baseline sediment budget will not be enhanced following the dredge and many changes are likely to be due to a reduction in erosion capacity. This effect will be greatest in the widening areas, where the shallow subtidal has been deepened. These are the areas that have been indicated by the previous monitoring programme to be eroding. Since these areas will be removed and flow speeds and bed shear stresses reduced, it is considered that the quantity of shallow subtidal erosion will also reduce. However, the change in tidally induced flows is only one component in determining the erosion potential. In this respect other natural disturbance factors are unlikely to significantly change. The wider and deeper channel, however, does tend to marginally reduce energy over the shallow subtidal areas from vessel related disturbances. This will again reduce the erosion potential primarily due to a reduction in the 'blockage factor’ as vessels pass. Whilst it is not possible to calculate this effect directly, it is estimated that the erosion



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effects could be reduced by as much as 30% for the subtidal area (in sediment budget terms, circa 9,000m3 per annum), down-estuary of Dock Head.

8.191 In considering the budget changes further, the additional intertidal accretion that is indicated by the modelling may not actually occur if sufficient erosion effect from non tidal factors is available to erode more freshly-deposited material, which would accumulate over the consolidated existing foreshore. In this case the material shown by the model to accrete on the intertidal would actually result in subtidal accretion.

8.192 Table 8.12 indicates that to provide a sediment balance for the estuary 357,000m3 of sediment (net) will require to be imported from the marine environment following the completion of the dredge. This compares with the current baseline estimate of 309,000m3, an increase of just under 15%. This, however, does not necessarily indicate that more sediment needs to be in the water column passing through the entrance at Calshot. This would only be the case if all sediment entering were currently settling in the estuary. Since flow speeds are generally reduced the change will be made up of more sediment settling from the water column as indicated by the modelling, where suspended sediment concentrations are marginally reduced (1 – 2mg/l). Also, the erosive potential of the stronger ebb flows is marginally reduced, thus, the potential export of coarser material at the bed will also be marginally reduced.

8.193 The increase in net marine import over the existing required import is assessed as 48,000m3, which represents a 0.3mg/l increase in sediment settling from the water column, based on the average tidal prism of 0.815 x 108m3. Such a small change will not be measurable and there will be no sediment deficiency to accommodate this change.

8.194 Notwithstanding this conclusion, Natural England has requested that ABP undertakes a trial

Sediment Management Scheme (SMS) with the objective of retaining sediment in the Southampton Water estuarine system instead of removing the material for disposal at the Nab Tower Disposal Ground. Analysis of historical and more recent bathymetric change at the edge of the intertidal suggests that most erosional stress (although not continuous) has been in the shallow subtidal and intertidal areas of the western shore between Hythe and Cadland. The SMS will aim to retain a proportion of the predicted increase in maintenance dredging (equating to approximately 3,000 - 4,500m³) in the estuary each year following completion of the SACD capital dredging works. The SMS will be undertaken by means of a controlled overflowing of the dredging vessel in locations designed to maximise the transportation of material to the Hythe to Cadland foreshore. This is also the area which is adjacent to the location where the majority of the increase in maintenance dredging is predicted to occur. Assessment

8.195 The Southampton Approach Channel Dredge has the potential to lead to a small reduction in tidal prism (0.22%) due to the change in tidal propagation marginally reducing the tidal range. This change is of the order of 25 times less than that caused by the lunar nodal cycle variation. At present, the estuary may still be trying to respond to previous developments primarily of the reclamations that have occurred since the circa 1930s and the present entrance cross-section is larger than would be expected for stable form, suggesting that accretion at the entrance should be occurring, however, there is no evidence to suggest that this is the case. The dredging is likely to move the estuary slightly further away from the theoretical stable form,



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however, the change to tidal processes coupled with the exposure of different material characteristics at the bed of the estuary will not change the overall relative stability that presently exists.

8.196 The modelling indicates the deepening will cause a net increase (3.7%) in import of sediment at Calshot, which remains in the estuary. This, however, will be due to an increase in the amount of settlement from the water column combined with a reduction of the scouring effects from ebb flows rather than more sediment entering, particularly as the tidal prism is marginally reduced. This indicates that the existing sediment supply is adequate for the change in processes and, therefore, significant change to the morphology of the estuary is not expected. The majority of this net increase in sedimentation will occur in the area of the channel widening and potentially require dredging. In general, the sediment transfer trends around the estuary are not affected. The dredge increases the potential for accretion over the intertidal areas by about 8%, however, this is unlikely to change the depth of accretion over most of the intertidal by more than 1mm/year.

8.197 With respect to the general estuary sediment budget the dredge indicates that for a balance there requires to be an increase in import of about 48,000m3, which represents a 0.3mg/l increase in sediment settling from the water column over a year.

8.198 Tidal asymmetry analysis shows the estuary is presently flood dominant for the import of fine sediment and ebb for coarse sediment. The dredge at present water levels and in the future makes negligible change to this trend nor the magnitudes. This indicates there will be no significant change to the morphological evolutionary processes already at work in the estuary. This indicates the estuary will not try to evolve to a substantially different form in the future to that which exists today.

8.199 The magnitudes of change in the tidal prism, sediment transport and parameters for long-term evolution are all considered to be small to negligible both under present day water levels an in the future with sea level rise. The sensitivity of most of the parameters assessed to this order of change is considered at the highest “moderate” but mostly “low”, which indicates the vulnerability to be at worst “low”. The overall impact significance is considered to be minor at worst and for the most part insignificant with respect to future change to the estuary physical parameters and morphological evolution. Effects of sea level rise

8.200 The effects of the proposed channel design on the estuary in the context of future sea level rise (SLR) scenarios was assessed using current Defra guidelines (2006). New boundary water levels for present day, 20, 50 and 100-year scenarios were simulated using both the pre- and post-dredge bathymetry. The estuary was divided into sections for analysis of dredge effects on different locations within Southampton Water (upper and lower sections) and the component estuaries (Figure 8.6).

8.201 For the purposes of this analysis, it was assumed that there would be no change in tidal range with SLR and that Southampton Water would not respond morphologically to rising sea levels.



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Predicted Changes to Water Levels with Sea Level Rise

8.202 Effects to water levels as a result of SLR in any year are predicted to be less than 0.015m, with the maximum change being an increase at LW in the present day. The greatest changes are all shown to occur in the Test Estuary upstream of the confluence with the Itchen Estuary. In the early years of sea level rise, the magnitude of effect at HW and LW marginally reduce as the water levels rise to beyond 2056.

8.203 After 100 years (2106), the magnitude of the LW increase continues to reduce but at HW the small reductions change to a small millimetric increase, with a maximum increase of about 0.008m occurring at the upper end of Southampton Water. These changes cause the tidal range to increase again back to a similar level to that which occurs at present, albeit at a higher elevation in the estuary geometry.

8.204 This analysis indicates that the effect of the channel deepening on water levels is two orders of magnitude lower than the effects of SLR, with most changes being millimetric and generally greatest at LW. The change as a result of the deepening will also be an order of magnitude lower than the variations caused by the lunar nodal cycle and variations imposed by wave activity.

8.205 Calculations of the effect of SLR have also been made in the O’Brien ratio. These indicate there will be little change as a result of sea level rise showing there will be little overall impact on the current estuary evolutionary trends. Predicted Changes in Intertidal Habitat with Sea Level Rise

8.206 Changes to the intertidal area have been calculated for each section of the estuary for SLR alone and for SLR with dredge. The results are presented in Figure 8.7.

8.207 The total change in intertidal habitat in the estuary and rivers due to SLR alone (i.e. no dredge) was predicted to be approximately -314ha in 2106 with a 1.1m rise in sea level (Figure 8.7). The loss due to SLR is comparable to that predicted by Bray and Cottle (2003) for the Southampton Water and Solent Coastal Habitat Management Plan (CHaMP). Bray and Cottle (2003) based their predictions on the linear SLR rate of 6.2mm/year, which equated to a 0.62m rise in sea level by 2103. This rise in sea level, therefore, falls somewhere between the 50 and 100-year predictions for this assessment.

8.208 The SLR effects on intertidal losses are shown to be far more significant than the dredge effects (Figure 8.7). In 100 years, the loss of intertidal due to the proposed scheme is expected to be less than 1% of the losses predicted by SLR effects. The total intertidal loss in the estuary due to the capital dredge will very marginally reduce over time. The reason for this is that as the estuary deepens with increasing sea levels, the frictional effects on tidal propagation towards HW are reduced compared to the present day, therefore, reducing the loss of intertidal area. In summary, the relative significance of the dredge will reduce in future when SLR is taken into account, and thus, the predicted areas of intertidal loss presented in the Marine and Coastal Ecology Chapter (Chapter 11) are considered to be conservative estimates.



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Assessment

8.209 The magnitude of change to water levels as a result of the Southampton Approach Channel Dredge will be around two orders of magnitude less than sea level rise, with maximum change less than 15mm at any water level, most of which are increases at LW and decreases at HW. The exception is for the situation 100 years in the future when HW levels at the time will be increased marginally (up to 8mm) in the upper estuary. Sea level rise will occur so the assessment is made on the effect the development will have on this change. In this respect, the water level changes and change to estuary morphological parameters will be small to negligible which leads to a low to negligible exposure. The vulnerability is considered, at worst, low giving an impact assessment of insignificant /minor.

8.210 The effect of SLR alone (i.e. without dredge) on the loss of intertidal area in the estuary will be significant. The losses resulting from the proposed dredge with SLR will contribute less than 1% to those predicted by SLR alone. Furthermore, the loss of intertidal as a result of the dredge will reduce over time with increasing sea levels. The change over time is so small, however, that it is considered to be negligible (around 1ha reduction in loss of intertidal over 100 years). Given the negligible exposure of receptors to this change, the overall impact of SLR effects on intertidal area are considered to be insignificant. Dispersion of disturbed sediment from dredge process (construction phase)

8.211 Dispersion of sediment disturbed from the dredge process has the potential to have a number of environmental effects, which in their own right can have physical effects, e.g. temporary increase in sedimentation in areas outside the dredge and increased suspended sediment concentrations. These physical effects in turn may change conditions of the estuary, which may be significant for ecology, fish and nature conservation interests.

8.212 The Southampton Approach Channel Dredge will require excavation of a large range of sediment types from gravels through to both consolidated and unconsolidated sands, silt and clay, which vary at different locations along the dredge. These materials will require different dredging techniques and will have widely varying production rates and disturbance losses to the local environment. For most of the dredging required above Dock Head, the proposed method of dredging is to use a large backhoe dredger, loading barges for the consolidated material. A trailing suction hopper dredger (TSHD) will remove the alluvial materials, sand and gravel, from the rest of the estuary, the Solent and Nab Channel. The dredger operating with overflow has been simulated to assess the worst-case scenario. The dispersion of this material will also vary from location to location as the hydrodynamic forcing conditions also vary over the extent of the dredging, as do tidal conditions (timing). To assess the effects of these variations a number of specific modelling scenarios have been defined in order to understand the fate of sediment disturbance that will result from the dredge. Appendix C provides details of the derivation of the individual modelling scenarios and gives a detailed discussion of the modelling results. The results presented are for a simulation encompassing the full hydrodynamic effects of a spring/neap tidal cycle whilst dredging is being undertaken and extending into a period with no dredging to understand how quickly effects might decay.

8.213 Spatial plots are presented in Appendix C showing the maximum suspended sediment concentrations (SSC) or bed thickness resulting at any time throughout modelling simulation



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(i.e. the worse case). Time-series plots are also presented to consider the length of time that the maxima occur at specific locations and when it occurs in the tidal phase. The movement and decay/dispersal of the initial dredge plume have also been assessed. The combined results provide the information for an assessment of the potential effects of the actions of the dredging on habitats, fauna and flora, as well as existing users of the estuary. The following sections provide the headline results from the dredge disturbance modelling. Firstly the worst-case scenario resulting from the TSHD dredging of the alluvium from the widening area between Dock Head and Fawley is presented, followed by a discussion of the changes resulting from the dredging of the different material types in the vicinity of Berth 201/2 in the Upper Swinging Ground, the Dock Head to Fawley widening, the Thorn and Nab Channels. TSHD dredging of alluvium - Dock Head to Fawley Suspended Sediment concentrations

8.214 Depth-average SSC will be increased by varying amounts throughout Southampton Water and extending into the Solent during the dredge. In the Solent, the distribution of enhanced SSC is greatest on the eastern side of the entrance to Southampton water, with increases of up to 200mg/l above background levels, mainly in shallower areas, on spring tides, predominantly at the lower water levels. On neap tides, the increase is considerably lower. Maximum increases in SSC levels in the Solent outside the entrance to Southampton water, are generally below 50mg/l, with an average in the region of 30mg/l. This compares with background peak concentrations in the area of around 80mg/l. Within Southampton Water the maximum increase in SSC is an order of magnitude higher particularly between Dock Head and Calshot with maximum concentrations from 200–300mg/l, reducing up-estuary to around 20mg/l within the container terminal. Higher values occur immediately below the dredger and in areas of shallow depth (500–600mg/I). These latter values generally occur at the edges and when the intertidal is in the process of wetting and drying. The peak baseline SSC range from about 80mg/l at the mouth to about 20 mg/l in the area of the container terminal.

8.215 Increased concentrations up to a maximum of about 100mg/l (generally of the order of 50mg/l) are likely to occur within both the lower Itchen and Hamble Estuaries, predominantly at the lower states of spring tides. These changes are in excess of the natural tidal variability within Southampton Water and the component estuaries.

8.216 Detailed analysis of data extracted from the model at different tidal states show that the maxima will only occur for very short periods at specific times. On neap tides, enhanced SSC levels resulting from disturbance of the dredger are considerably lower than the maxima (of the order of 50 mg/l depth-average) between Dock Head and Fawley. On flood flows of lower speeds the initial plume from the dredge remains in close proximity to the source of disturbance and is generally confined to the channel. With peak ebb flows the sediment is spread over a wider area but concentrations are not significantly different and by LW the majority of the sediment is seen to be predominantly within the channel around Fawley but spreading onto the shallow subtidal to the east. The modelling also indicates that the higher ebb flows are not only dispersing the new plume, but also re-eroding material that would have deposited to the bed from earlier TSHD loads, even from up-estuary of the dredge. On spring tides the rate of dispersion of the initial plumes is considerably greater than on neaps (4 to 5 times) with sediment predominantly down-estuary. Up-estuary of Town Quay, in the Test Estuary there is little increase in suspended sediment concentrations except over the HW period of spring tides.



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Re-erosion of settled sediment also occurs during flood, as well as ebb tides. The highest concentrations within the water column (noted above) are generally associated with the highest spring tide ranges during the ebb tide, particularly around LW and the lower half of the tide in general. These concentrations are again 4 to 5 times higher than are likely over the upper half of spring tide water levels. The differential between HW and LW is greater over neap tides. The maxima are predominantly found in the area of Fawley although large increases in concentrations will ‘linger' around the location of overflow of the dredger on the bed around a LW overflow, being re-eroded as flows peak on the early flood tide.

8.217 Suspended sediment distributions observed during the dredge at the beginning of the simulation and after disturbance from the dredge has ceased on similar range tides shows that SSC concentrations do not accumulate to cause an increasing trend following more dredge loads, therefore, concentrations at the beginning and end of the dredging should not vary significantly (for similar range tides). Furthermore, comparison of sediment distributions during the dredging activity with those after dredging has ceased (for similar range tides) indicates that concentrations in the water column will generally decrease quickly, suggesting that elevated SSC will only last for a relatively short period of time following the cessation of dredging. Time series analysis undertaken at all locations shows that concentrations arising from the dredge are both very temporally variable (transient), with peaks often only occurring for a few hours. The following text describes the variability at a number of strategic locations. The full analysis and diagrams is presented in Appendix C.

8.218 In the vicinity of the dredge track peak increases in SSC are predicted to be up to about 600 mg/l on both spring and neap tides whilst dredging was taking place. Such levels were only recorded for short periods of time during the period of dredger overflow. The enhanced SSC directly as a result of the dredger operation tends to decay at an exponential rate, reducing to below 200mg/l (above background) within 40 minutes of the end of the dredge load. Tidal average enhanced SSC along the dredge track was of the order of 100 mg/l on spring tides reducing to below 30 mg/l on neap tides. The pattern of SSC does not vary significantly with time when dredging at different states of tide. Concentrations are shown by the modelling to reduce to near background levels within about a two-week spring/neap tidal cycle following the cessation of the dredging period and there is no trend for increased enhanced levels of SSC with time.

8.219 At Berths 39 and 104 in the Western Docks sediment from the dredge (between Dock Head and Fawley) only reaches the location on spring tides with maximum increased SSC levels of 150-200mg/l and 60-80mg/l around HW at each location respectively, with minimum levels around LW. Spring tide average enhanced values are in the order of 50-60mg/l and 20mg/l respectively with negligible increases over neap tides. Elevated SSC levels of the order of 10mg/l will still occur during the spring tides of the next spring/neap cycle following the cessation of dredging at Berth 104.

8.220 Over the intertidal areas to the east side of the Fawley to Dock Head widening individual locations show peak SSC concentrations will again occur on spring tides, reaching about 500mg/l for short periods (circa 10 minutes) at the time of ebb flows as opposed to around HW within the main channel. The average increase is around 50 mg/l over the HW Stand.



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8.221 On the west side, maximum SSC concentrations are shown to be higher with isolated peaks of more than 900mg/l, but with general levels peaking below 500mg/l. Consideration of the timing of the highest concentrations in the tide reveal that they generally occur when the depths of water over the intertidal are very shallow and about to dry and relate to short-term heightened concentrations prior to settlement.

8.222 At and down-estuary of the oil terminals, towards Calshot, peak SSC reach 200-250mg/l on spring tides and around 150mg/l on neap tides, generally around LW when the water is shallowest. These peaks generally reduce relatively consistently to a minimum around HW and the first part of the ebb tide (less than 50mg/l springs; 20mg/l neaps). Spring tide average enhancement in SSC resulting from the dredging of the alluvium from Dock Head to Fawley is shown to be around 100mg/l in this area. Accretion / Erosion Patterns

8.223 In the detailed description of the modelling of the distribution of sedimentation resulting from the dispersal of dredge disturbed material the depths quoted are representative of the change over a single 15 day spring/neap model simulation. This allows the pattern of dispersal to be described but it does not take account of the full amount of material to be removed by capital dredging. In order to determine the likely requirement for maintenance dredging arising from the capital dredge, the sedimentation has to take account of the dredge volume as well as the distributions indicated by the modelling; this is presented in Para 8.263. The sediment distributions obtained from the modelling scenarios are summarised below.

8.224 The maximum thickness of material that the modelling shows to be present on the bed of the estuary at any time throughout the 15 day spring/neap model simulation resulting from the material disturbed by the dredge and then dispersed by the hydrodynamics is predicted to be 30-50cm. This maximum accumulation occurs along the maintenance-dredged riverside berths within the Itchen Estuary and the entrance to Ocean Dock. For comparison, the model indicates 5-10cm of additional sedimentation in the Marchwood Berths, up to 20cm in the BP Jetty Berth and approximately 5cm in the berths at the ExxonMobil Fawley Marine Oil Terminal. Sedimentation up to about 20cm could also occur in the Fawley Power Station intake channel. Where the dredger is overflowing about 30cm of accretion could be present, however, this will be re-dredged, if not moved by ebb tide erosion. Elsewhere in the channel, accumulations up to about 4cm are shown which would be removed by a final channel cleanup. A similar order of maximum accumulation is shown to occur over the intertidal area particularly between Dock Head and Calshot with slightly less (1-2cm) occurring in the shallow subtidal areas both sides of the channel and extend into both the lower Itchen and Hamble Estuaries. The highest potential for intertidal accretion resulting from the dredge occurs on the middle intertidal either side of the Fawley reclamation with some entering Ashlett Creek. Additional sedimentation potential also exists in the area of Hamble Spit. The modelling indicates that at some point a small accumulation of sediment (at least 1mm) will result throughout the estuary.

8.225 Outside Southampton Water, away from the dredged area of the Calshot Turn, maximum accretion at any time will be negligible and generally less than 2mm, with up to 10mm in the base of the channel.

8.226 The above quantification gives an indication of the conservative worst case and maximum depth of effect only. Time series analysis clearly shows that the flows in the estuary are



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sufficient to cause both accretion and erosion of the dispersed material at different states of the tide with varying magnitudes over different tidal ranges. These sediment accumulations could be re eroded and then re-deposit to different levels on tides of a certain range and/or during flood and ebb flows. The data presented, therefore, in all areas, except those with predominantly slack water conditions, is considered to represent a worst-case scenario, with respect to maximum accumulations of sediment at the bed from the dredging process.

8.227 The detailed description (Appendix C) of the sedimentation patterns at different times throughout the spring/neap cycle shows that the depth of accumulation on the bed is highly transient. Some areas will accrete for a number of days over neap tides, but be completely flushed as the tidal range increases. Other sites vary from flood to ebb on most tides particularly on springs and, therefore, it is difficult to generalise the effects. The amount of accumulation that exists on the bed will, therefore, depend on the state of tide and specific location considered. Accumulation thicknesses of the order presented above for most open areas will only occur at most for a few days over neap tides, with smaller accumulations occurring for a few minutes to a couple of hours on spring tides, predominantly at the time of the HW stand. The way the sediment accretes and erodes is best illustrated by reference to the time series analysis at specific locations.

8.228 Between Dock Head and Fawley within the widening area, during the dredge, accretion will occur on the bed during HW (up to about 15cm). This material will be re- dredged. The model simulation, however, indicates that as the range of tide increases the material would be naturally eroded over a spring tide, predominantly around LW and during the flood. Accumulations of up to 5cm in depth are possible from LW up to the end of the HW Stand. This pattern is typical along the whole length of widening between Dock Head and Fawley and represents the situation most influenced by the actual dredging practice before the effects of the hydrodynamics redistribute the released sediments.

8.229 In the Western Docks (in vicinity of Berth 104) some accretion is likely to occur (5-10mm) on the highest spring tides predominantly during the extended HW, however, this material is re-eroded during the subsequent ebbing of the tide. A similar general pattern occurs further up-estuary but with smaller magnitudes of accretion occurring on less tides for shorter periods of time. The pattern of change is similar opposite Berth 39 except maximum bed accumulation is up to about 12mm. Some accumulation at this location is evident over the Young Flood Stand. This, however, is generally re-eroded by the second phase of the flood tide. That which deposits over the HW stand is eroded on the ebb, and little or no sedimentation occurs at any time on neap tides.

8.230 Within the Hythe Marina Channel, accumulations of up to 6mm could occur during slack water periods but these are likely to be re-eroded on the same tide giving no net accretion within the channel. However, due to the increased SSC all locking operations on spring tides during the period of dredging will result in a larger intake of sediment that has the potential to settle in Hythe Marina.

8.231 On the intertidal, to the east of the Dock Head to Fawley widening erosion of sediment that accumulated during the flood tide and over HW is eroded on the ebb. Over the smallest neap tides net accretion occurs, predominantly at LW, however, as the tidal ranges increase, this material is completely eroded. Some small effects are indicated to continue for the first



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spring/neap cycle following the completion of dredging. To the west side, on neap tides, accretion occurs during the ebb tide and again as the mudflat dries, but on springs the pattern is not consistent. Maximum accumulations are of the order of 15mm, however, they will be transitory and will merge into the background variability within a spring/neap cycle after the end of the alluvium dredging.

8.232 At the oil jetties, accumulations will occur with rates within the berth pockets higher at the BP Jetty than at Fawley. Over neap tides (at the BP Jetty) accumulations up to 18cm could result over a period of a week, predominantly around LW and to a lesser extent at HW, however, assuming no consolidation this material is likely to be removed on both flood and ebb phases of the following spring tides. At the ExxonMobil Fawley Marine Oil Terminal, considerably less accretion occurs over neap tides and maximum changes are only of the order of 2cm. The net accretion in the berths, giving rise to increased maintenance dredging potential, of the order of 5mm (over a spring/neap cycle) over the area of the berths.

8.233 In the area of the Hook to Hamble foreshore, the maximum levels of sedimentation reach about 5mm on springs and 12mm on neap tides after 5 days of accretion. However, on springs there is no net accumulation over a tide and any accumulation occurring on the neaps, is eroded on the following spring tides, particularly once dredging has stopped. In Ashlett Creek, continuous sedimentation will occur up to about 20mm over neap tides and 5mm on springs particularly over periods of slack water. This material is, however, predicted to be re-eroded by the stronger ebb currents, except during a 5-day period during neaps. Within the upper sections of the creek, sedimentation will result which will not be re-eroded on spring tides.

8.234 At the Fawley Power Station, intake accretion occurs predominantly on spring tides with little over neaps, resulting in an accumulation of about 17cm over the period of dredge simulation and continuing for at least the first 7 days following the completion of the dredge. Peak concentrations are around 250mg/l for spring tides and around 100mg/l on neaps, occurring predominantly over the LW period.

8.235 Within the Solent, on some tides 1-2mm will accrete over LW, however, this will be transient and short-lived, being immediately re-eroded resulting in no net change. Gravel Dredging

8.236 For the gravel dredging the simulated dredger track was the same as for the alluvium dredging but with a coarser material being disturbed. Overflow will be essential for the removal of the gravel. The pattern of change of SSC is predicted to be similar to that resulting from dredging the alluvium, except concentrations are higher for the gravel dredge. Increased SSC levels exceeding 50mg/l were modelled adjacent and over the intertidal to the east of the channel, with concentrations peaking to around 300mg/l along the lower intertidal on the western shore either side of the Fawley Reclamation. Due to the coarser nature of the material these peaks are likely to be temporary and restricted to very highest spring tides. At the edges of the estuary, the average increase in SSC is likely to be about one third of the peak concentration on spring tides. These increases represent between 30-133% of the existing spring tide average concentrations and are expected to decrease to background quickly (a few days) once the dredge of this material is complete.



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8.237 The pattern of maximum settlement of sediment is almost identical to that for the alluvium except with a greater depth. The greatest change is that more disturbed sediment will deposit along the length of the channel to be re-dredged. The rates of maximum accumulation on the bed are, however, an order of magnitude lower for the gravel dredging compared with the removal of the alluvium allowing full overflow. In the sedimentation areas where re-erosion of the accumulation is unlikely the complete dredge of the gravel could cause additional sedimentation of the order of 0.025m at Marchwood, 0.135m in the Itchen Berths at Dock Head and up to 0.12m at the BP Jetty. Little or no additional accretion would be expected in the Hamble and Itchen Estuaries, although some accumulation of fine sand could persist on Hamble Spit to a depth of around 0.1m, as a result of this phase of the complete dredge. Backhoe Dredging of Stiff Clay

8.238 The majority of effects will be confined between the middle of the Western Docks to the Calshot Turn area. The maximum enhanced concentrations during the period of backhoe dredging are expected to be around 40-50 mg/l within the subtidal areas over spring tides with an average of around 10mg/l occurring throughout the dredging. Similar to other scenarios, enhanced maximum concentrations occur on the intertidal between Hythe and Calshot, particularly just down-estuary of the Fawley Reclamation, in the area of Ashlett Creek. Concentrations of up to about 500 mg/l could occur for short periods of time (a few minutes) in these areas. The average SSC whilst the intertidal areas are inundated is likely to be of the order of 100-150 mg/l. This level of SSC enhancement can also be expected close to the foreshore between Weston and the Hamble Estuary, particularly around Hamble Spit and the intertidal near to Warsash. Maximum increases in SSC of up to 10mg/l can be expected to reach Port Hamble in the Hamble Estuary. The overall distribution tends to indicate that most of the sediment will be initially transported along the deep channel. This scenario assumes that all disturbed material will have broken down to its particulate state, however, with a backhoe dredger this may not be the case. On this basis the distribution of SSC is considered a worst-case.

8.239 The maximum bed thickness change as a result of this dredging will be similar to that for the gravels and alluvium with a large proportion (20-30mm over a spring/neap cycle) of the settlement occurring within the main channel around the dredger. Most of this will be removed by the stronger ebb spring tide flows or will be re-dredged. Similar depths of accretion are predicted in the Itchen Berths at Dock Head, the BP Jetty, and Admiralty Jetty at Hythe. Once again material is shown to settle in the Power Station intake channel and Hamble Spit. Elsewhere most of the accumulation is of the order of 2-5mm predominantly in the middle intertidal areas at the locations where the maximum concentrations are predicted. Backhoe Dredging in the vicinity of Berth 201/202 in the Upper Swinging Ground

8.240 This dispersion scenario is representative of dredging of the dense/very stiff Greensand materials from the base of the channel (predominantly in the more quiescent areas up-estuary of Dock Head). The modelling predicts that over a spring/neap cycle nearly all subtidal areas of the estuary are expected not to exceed 10mg/l above background, with the exception of the channel opposite the middle of the Western Docks, where peak increases of the order of 30mg/l can be expected. In these areas the background concentrations do not exceed 30mg/l and average considerably less. The detailed analysis of time series from the TSHD dredging of alluvium indicated that maximum values created by dredging could be about 3 times the



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average effect. The average change from the backhoe dredging would, therefore, be in the region of 3–10mg/l in the main channel i.e. representing in the order of 20-60% above background concentrations during the period of the dredge and probably for around 15 days following its completion.

8.241 Higher maximum concentrations are predicted (circa up 150mg/l) on the intertidal and in the creeks of the Bury Marshes. The model also indicates increased concentrations of 30-100mg/l will occur over the intertidal between Cracknore Hard and the Marchwood Yacht Club and all along the Dibden Foreshore. High rates of increased concentration of suspended sediment are also shown to result in the lee of Hythe Marina (up to 200mg/l) and as with all other dredge scenarios over the lower and middle intertidal around the Fawley Reclamation (circa 100mg/l). On the east side the concentrations are generally enhanced less, with the highest adjacent to the foreshore, especially around the 'root' of Hamble Spit. Time series analysis indicates that these maxima only occur for short periods of time at different stages of the tide and predominantly on spring tide ranges.

8.242 Most settlement of the disturbed sediment will be confined to the area up-estuary of Dock Head predominantly within the deepened channel. The most significant accretions (60mm over a spring neap cycle) will be in the slack water (eddy areas) in the Upper Swinging Ground, at the end of Mayflower Park and the furthest up-estuary point of the dredge. This maximum depth is, however, small and will not significantly affect the overall dredge volume. TSHD Dredging from Fawley to the Bramble Turn

8.243 The dredging in this area is required to remove a number of different material types, the largest proportion being marine sands and gravels.

8.244 Within the shallow subtidal areas maximum concentrations reach about 20mg/l above background and 20–30mg/l in the area where dredger overflow occurs. In general the overall increase within the estuary is no more than about 20mg/l. Over the intertidal, particularly between Fawley and Calshot, peak concentrations of 100–200mg/l are predicted in shallow water as material settles out. Within the Solent increases in SSC are relatively consistent at around 15mg/l with marginally higher values in the channel itself associated directly with the dredge. SSC levels will be increased in the area of Stanswood Bay by up to about 10mg/l, but for the most part considerably less, representing a maximum change of about 13% on spring tides. . Maximum increases of around 10mg/l are predicted to occur as far west as Hurst spit, and as far east as the entrance to Portsmouth harbour

8.245 Little deposition of disturbed sediment occurs outside the channel. The maximum increase is about 7mm (over a spring /neap cycle) in the area of the Power Station intake. For the complete dredge in this area a shallowing of the Intake Channel and surrounding mudflat could be in the order of about 0.06m. No change is predicted in bed levels in the other subtidal or intertidal areas. TSHD Dredging from the Nab Channel

8.246 In the Nab Channel sand and gravel will be dredged by a TSHD. The modelling scenario simulated the removal of all the material over a period of about 10 days.



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8.247 The material disturbed can be widely distributed about the Solent mainly to the north east of the

Nab Channel into Bracklesham Bay and around Selsey Bill. Maximum concentrations (up to 150mg/l are predicted to occur around Medmerry Bank for short periods of time, but for the most part less than 50mg/l. Accumulations on the bed will be transient. Small amounts (<5mg/l in channels and <15mg/l in shallow subtidal areas) can be redistributed back to Southampton Water and into the other Solent natural harbours. These concentrations do not cause a build up of sediment anywhere within the Solent, its rivers and natural harbours. The effect is, therefore, negligible and would not be able to be recorded relative the natural variability of the area.

8.248 At the dredge location, away from the immediate source of disturbance maximum SSC does not exceed about 20mg/l above background on spring tides, increasing to 50-75mg/l on neaps, particularly around LW. Disturbed sediments are quickly dispersed causing a relatively fast drop in the initial concentrations both in time and distance from the dredger, with the exception of around LW on neap tides. Elevated SSC levels within the channel will reduce to background levels within 7-10 days following cessation of the dredge. Appendix C also gives detail of the initial plume and how it moves away from the source of the disturbance on the tide. Assessment

8.249 Numerical modelling of the disturbance created by the dredging process has been undertaken at four locations with different hydrodynamic conditions. Scenarios have been run for the range of material types and the different methods of dredging that will be used. Each scenario has been run over a spring/neap cycle allowing the interactive effects of different range tides. Assessment has been made with respect to the maximum predicted concentrations and accumulations of sediment on the bed and how this varies with both tidal range and time in the tide.

8.250 The results show that the worst-case for increases in SSC and bed level accumulation will be when the alluvium from between Dock Head and Fawley is being dredged by a TSHD with overflow. With this scenario, whilst the initial plume is relatively confined to the channel the estuary hydrodynamics over a spring/neap cycle widely distribute the disturbed material over the estuary following periods of accretion and re-erosion. The modelling shows that concentrations and bed thickness change will be highly variable in time and space (transient), often with maxima at any location only lasting for minutes to a few hours on a tide. Accretions occur over HW and LW in different parts of the estuary, but all are generally eroded during the periods of peak ebb and flood flows, particularly on springs, except in ‘quiet‘ areas. At some locations continuous accumulations can occur for a number of days over neap tides but are eroded during the following spring tides. All dredge scenarios tend to deposit some sediment in the areas currently dredged which will require maintenance dredging either during or following the dredge.

8.251 For the alluvial dredging, peak increases in SSC for the main body of water are predicted to be 3 to 4 times the peak baseline concentrations, although these may only last for a few hours, particularly on spring tides. Average values are generally about one third of peak values. In



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shallow water, higher concentrations could occur at the edge of the estuary. For the maximum level of SSC enhancement the magnitude of the change is considered to be medium to large with respect to the background but average levels small to medium. Since the concentrations are highly transient, particularly at peak levels, the probability of occurrence is considered to be medium for the maxima and high with respect to average levels. This leads to an assessment of medium with respect to the exposure to change. In their own right, SSC are not a sensitive feature nor have a great importance, therefore, the impact assessment would be insignificant.

8.252 The magnitude of bed thickness change as a result of the dredge disturbance for the most part is relatively small with the exception of areas which are currently ‘silt traps’, where maintenance dredging is already required. These bed accumulations will occur, however, they may only be continually present for a few hours to a few days depending on location and tidal range. The magnitude of change in sedimentation at the bed during the dredge is considered to be medium to low, but as for SSC this is not particularly sensitive or important in its own right leading to an insignificant impact assessment.

8.253 The medium level of exposure for SSC and medium to low with respect to the levels of bed accumulation, assessed over the estuary, although highly variable in time and space will have important implications in the assessment of other impact pathways, for example maintenance dredging, ecology, fish and nature conservation issues. Nothwithstanding this conclusion, ABP has entered into an agreement with the River Hamble Harbour Authority to monitor sediment levels at specific locations within the Hamble estuary and undertake remedial dredging to remove sediment deposition caused by the SACD project. Changes to maintenance dredging commitment

8.254 Paras 8.78 to 8.76 provides details of the existing average maintenance dredging commitment for the estuary since the last major deepening took place in 1996/7. This will be affected permanently as a result of the dredge but also temporarily during the period of the capital, where additional areas may require maintenance.

8.255 This assessment on the effects of the current proposal on the estuary maintenance dredge commitment has been made using the results from the modelling of various parameters, such as the effect on flow patterns, sedimentation patterns and the volumetric flux analysis. In this assessment the specific modelling assumptions, method and accuracy of calibration have been considered, along with the potential effects of processes not included in the modelling, primarily the natural and anthropogenic disturbance effects (e.g. waves). Comment on the changes to the distribution of sedimentation that could lead to changes in the maintenance dredging commitment is made in Appendix C.

8.256 This overall analysis indicates that the average maintenance dredging commitment following the capital dredge will be increased by about 30,000m3 (8%) of which 90% will be from the new navigation channel down stream of Dock Head. Most of the rest of the increase will be from the area of the berths in the Western Docks. These are all areas within the areas currently dredged by ABP. Post-Dredge



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8.257 Following the dredge, the pattern of sedimentation will remain similar to the existing requirements, however, a small redistribution in the magnitudes will take place. These small changes, however, are considerably less than the annual variability in present maintenance requirements (Table 8.8), therefore, little noticeable change to the maintenance dredge practice will occur.

8.258 Within the Hamble and Itchen Estuaries, maintenance dredging is likely to be marginally decreased. The annual maintenance dredging commitment is estimated to reduce by about 300m3 at Marchwood Military Port and less than 100m3 in the Hythe Marina Entrance Channel.

8.259 At Hythe Pier and the Admiralty Jetty facility, the sedimentation modelling indicates small increases in depth of accumulation, which volumetrically would be the order of 10m3 and circa 500m3 at the two locations respectively. It is considered, however, that with the existing wave disturbance these accumulations may not result in practice.

8.260 Sedimentation rates at the ExxonMobil Fawley Marine Oil Terminal are likely to be marginally enhanced, particularly at the deep front berths. The increased volume is estimated at about 3000m3/year in situ (i.e. 1-2 dredger loads). At the BP Jetty, no change to the existing rates is predicted to occur.

8.261 Between Calshot and Fawley, there are channels leading into Ashlett Creek and the Power Station intake Channel. Following the dredge, the changes to the hydrodynamics and sediment patterns indicate that accretion rates will not be changed from existing conditions in Ashlett Creek, however, a small increase is likely to occur at the outer half of the intake channel. This increase is estimated to be in the order 500m3/year but is likely to be highly variable and may not be detectable from the background variability.

8.262 Within the Solent, continuous maintenance dredging is not expected, but ad hoc removal will be required. Averaged over a number of years the annual rate is estimated at less than 1000m3. During Dredge

8.263 During the capital dredging, sediment will be disturbed and released as a result of the dredging process. Dispersion modelling of this disturbance has been assessed by considering the maximum likely sedimentation that could arise from the capital dredge operation over the estimated 65 week period of the dredge. An approximate area over which this accumulation will take place has also been assessed, which gives rise to the additional maintenance requirement that could result away from the capital dredge area.

8.264 Based on the modelling results, the resulting maximum depth of additional accumulation of sediment that is predicted to occur at berths and marinas around the estuary will range from a few centimetres to tens of centimetres. Over half the additional maintenance dredge requirement will occur in and around the berths at Dock Head, particularly those within the Itchen Estuary. The total additional maintenance dredge required in the areas that are not maintained by ABP is calculated to range between around 105,000m3 and 285,000m3, based on the density assumptions for consolidated and unconsolidated deposition. This would add additional 45-110 dredger loads of material to be removed from over that required for the



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capital dredge, depending on whether the larger dredger carrying out the capital works or the normal maintenance dredge equipment is used to remove the majority of the material.

8.265 This additional (clean up) dredging will increase the overall timescale of the capital dredge unless it is carried out alongside the capital works. It should be noted the model does not take account of the possible erosion and redistribution of material by vessel disturbance effects and occupation of the various berths. Whilst these actual effects cannot be calculated, it is most likely that they will reduce the predicted rates of sedimentation. On this basis, the additional maintenance dredging commitment during the period of the dredge is considered to be a worst-case scenario. Assessment

8.266 During the dredge, disturbance of material will create a maintenance dredging commitment outside the confines of the deepened channel of the same order as the existing maintenance dredging commitment for the estuary. The majority of this material will fall within the existing maintained areas within ABP’s jurisdiction. Additional material will also deposit at the non-ABP facilities around Marchwood, the Hythe Marina Channel, the Hythe Jetty and Admiralty Jetty. Down-estuary additional accumulations of sediment will also occur at the BP and ExxonMobil terminals, with small amounts entering the lower Hamble Marinas, Ashlett Creek and the Fawley Power Station inlet channel.

8.267 The magnitude of this temporary change (over the duration of the capital dredge) is considered to be medium to small depending on the individual location with a medium to high probability of occurrence. This gives rise to a range of exposures spreading from medium to negligible. The majority of the higher exposures are within areas currently maintained by ABP. In the majority of other areas the sensitivity to increased sedimentation is likely to be moderate to high. This results in a vulnerability ranging from high to none for different locations. It is likely that any increase in sedimentation that would need to be removed by dredging will be considered of high importance due to the potential cost of removing it. Therefore, the significance of the impact will be moderate to major for the different maintained locations. Sediment monitoring will be carried out before and after the proposed dredge at locations where significant sedimentation is predicted and concerns have been raised. ABP will take such steps as may be appropriate where it is demonstrated that the dredging works have caused a material increase in sedimentation, above naturally occurring rates of sedimentation, and which as a consequence, has had an adverse impact on marine operations.

8.268 After the capital dredge has been completed, the increase in maintenance dredging for the estuary as a whole is small. All changes are well within the natural variability in the required dredging rates at all facilities over history and over the last 10 years in particular. These changes are so small they are unlikely to be measurable against background. The most significant changes occur in ABP dredged areas and the maintenance dredge commitments are predicted to reduce at a number of non-ABP locations. Following the dredge, the exposure to the change in maintenance dredging will be low. It is, therefore, assessed that the change to maintenance dredging commitment will be minor/insignificant.



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Proposed disposal at the Nab Deposit Ground, effects on the bed

8.269 For the whole capital dredge for the Southampton Approach Channel Dredge, approximately 11.6 million m3 in situ of dredged material will be deposited at the licensed Nab Deposit Ground. This material will be a mixture of fine grained alluvial silts and peat, sand of various size, gravel and fine sand and silts from the consolidated geological strata, a part of which is commonly referred to as Greensand. These materials will be deposited by bottom disposal from barges and a TSHD, with hopper volumes ranging between about 2850 – 10,000m3. The sediment at the point of disposal will vary from a relatively low-density fine particulate form, through non-cohesive sand and gravel to the probability of large lumps of clay, which may be up to the order of 1m in size. Smaller fragments of consolidated Greensand may also result from the backhoe dredging. The rate of disposal will depend on the areas being dredged as well as the method. For the TSHD disposals of the order of 10,000m3 would be deposited at a frequency of 1-2 hours when dredging the Nab Channel but only every circa 6 hours for dredging in Southampton Water. The frequency of deposit from the backhoe dredging will depend on the number of barges available, with the round trip for a barge taking 4-6 hours depending on the location of dredging. Also, for a large part of the dredge the backhoe and TSHD dredging will be in different locations, therefore, it is likely that both operations would occur together, thus increasing the rate of supply to the deposit ground. The overall period of the disposal is at present considered to be around 65 weeks for the complete dredge.

8.270 The total dredge volume deposited will be of the order of 11.6 million m3 in situ, and more in terms of hopper volume due to the breaking-up of sediments during the excavation process. The Nab Deposit Ground is large (12km2), therefore, if all material were retained and spread over the whole area, this deposit would reduce depths by less than 1m in a total depth of over 40m. This uniform distribution, however, will not occur, nor would all the material be retained. The deposit ground is also known to be highly dispersive for much of the sediment to be deposited in its particulate form. The dispersion of the material has been modelled and the results have been reviewed and assessed in the following section. A proportion of the material will not, however, be immediately dispersed and, therefore, will accumulate at the deposit ground. Depending on the material type and the rate of disposal this may vary from a few hours to a few weeks. The dispersion modelling suggests that all the sand, silts and clay that are in particulate form will be dispersed within a spring/neap cycle after the deposits have been finished. Maximum accumulations over the deposit ground of these materials are not expected to be more than the order of 25mm. This evaluation is made assuming that each deposit is taken to a different sector of the deposit ground, to minimise ‘humps’ on the sea bed.

8.271 For the gravel, initial humps are likely to occur under the dredger, which could be circa 3-4m high on the bed. The finer material will then winnow away reducing the height of the humps and spreading over a wider area with time. The deposit of the backhoe dredged material from the barges will also create humps on the sea bed, which are likely to be a maximum of about 3m in height. Any bucket size lumps are likely to ‘roll’ on the bed before coming to rest. Over time these consolidated lumps will break down and eventually disperse from the site over time. Assessment

8.272 Whilst the volume of the material being relocated to the deposit ground is large, the intended spread of the material over the complete area is likely to cause humps on the sea bed up to 3



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to 4m in height at different locations, which will reduce in height over time. Particulate material will be widely dispersed, leaving minimal trace on the sea bed within a few weeks following cessation of the deposits. The deposit site will, therefore, be able to accommodate the amount of material to be deposited without changing the existing physical and hydrodynamic characteristics of the area. The magnitude of the change predicted are considered to be small, therefore, the exposure is low. The sensitivity and importance of the site are also considered to be low, therefore, the impact of the disposal will be minor during the period of disposal, becoming insignificant in a short period of time after completion of the works. Dispersion of material from Nab Deposit Ground, suspended sediments and bed accumulations

8.273 Model disposal scenarios have been defined for various sediment types, volumes and methods of disposal from the four dredge zones used for the dredge disturbance modelling as the input for model runs to assess the dispersion of the different material types from the deposit ground. The derivation of the various scenario parameters is set out in Annex B of Appendix C.

8.274 Each disposal scenario has been modelled over a full spring to neap cycle of disposals at intervals and rates determined by the location of dredging, material type to be deposited and average load of the hopper and barge. At the deposit ground the area has been divided into sectors and each load deposited in a different sector of the deposit ground. This practice ensured that the modelling represented the full extent of dispersion from the deposit ground, without creating the potential for large depth changes at any single location should the material be retained.

8.275 Appendix C presents the model results in detail, showing the dispersion and maximum concentrations over a spring/neap cycle for different component particle sizes from each deposit scenario. The discussion gives detail of the dispersion pattern, the levels of enhanced concentrations, time series at strategic locations and information on the initial plumes predominantly for the resulting worse case, which is likely to arise from the dredging of the sand and gravel from the Nab Channel due to the higher rate of disposal. The following text summarises the main points that emerged from the full set of disposal monitoring.

8.276 The extent of dispersal of sediment from the Nab Deposit Ground covers an approximately rectangular area of around 1500km2 (58.3 by 26.9km), from the east coast of the Isle of Wight eastwards and extending from Bracklesham Bay in a SW direction. In addition sediment is distributed, through the east and west Solent and a small amount moves into Southampton Water and Portsmouth Harbour. The dominant axis of the initial plume away from the disposal area is on a WSW to ENE axis with the majority (ratio 2.5:1) of material initially moving to the WSW, before being dispersed wider over time.

8.277 Detailed modelling of the initial plumes show they will be very linear in form with suspended sediment concentrations of the order of 10,000mg/l extending over 1km from the point of disposal. These SSC, however, decay rapidly at the point of disposal to below 1000mg/l within 20-30 minutes following each disposal. The maximum width of the initial plumes with concentrations in excess of 1000mg/l is predicted to be of the order of 250m.



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8.278 At a distance of 3.5km along the track of the plume peak concentrations of about 1700mg/l occur after about an hour from the time of disposal but reduce to near background levels within 2 hours. Further away peak concentrations are less than 800mg/l, but 'linger’ at 50 – 100mg/l above background for a considerably longer period of time, predominantly due to the fact that the flows slacken around LW. Whilst this modelling gives an idea of the rate of decay it also shows that the actual concentrations at points will vary considerably depending on the location of the disposal within the deposit ground, the volume of material disposed, the time in the tide and the specific range of the tide

8.279 After a single day of disposal operations, the sediment is likely to spread to about 8km NE and 10km SW with the bulk centred about 2km NE of the centre of the deposit ground, with average accumulated depths at the bed of up to 10mm.

8.280 After 7 days of disposal the tidal range in the model simulation changed from spring to neap tides. At this time more material is retained in the deposit ground, increasing the bed thickness to around 115mm with general SSC (away from the immediate point of disposal) of around 1000mg/l. More sediment is likely to cover the bed over an area extending 10km to the NE and 20km to the SW of the deposit ground. The axis of the main sedimentation and SSC plume area also move inshore towards Sandown Bay.

8.281 After the disposal ceases the migration of sediment mass continues both inshore and southwards with peak concentrations falling to below 500mg/l (albeit over a wider area) on springs and then to well below 100mg/l on neaps a week later. The sediment moves via process of continuous accretion, erosion and then transport back and forth on the flood and ebb tides at different rates depending on the tidal range. The modelling of the sand dispersion of the material dredged from the Nab Channel indicates accumulations on the bed will not exceed about 1mm at any time beyond about 15km inshore of the deposit location and enhanced concentrations of 10-50mg/l are likely to result near the shoreline in Sandown Bay for much of the tide. At isolated locations enhanced SSC could be as high as 200mg/l for short periods. Also, within a complete spring/neap cycle most of the deposited (granular) material will be dispersed from the deposit ground.

8.282 This clearly indicates that sand particles between 85-220µm are highly mobile in the area to the east of the Isle of Wight, even on neap tides.

8.283 Modelling of the dispersal of sediments from the other dispersal scenarios indicates that maximum accumulation depths will not be more than 2-3mm at any location at any time outside the deposit site for the full amount of each material to be dredged. This sediment only accumulates on the bed for periods of no more than about an hour around the slack HW and LW flow conditions, with it being in motion throughout the rest of the tide.

8.284 Appendix C shows time series and describes the temporal change in SSC and bed accumulations at selected locations based on the maximum distributions for each parameter. Although not directly modelled, the migratory trend indicated by the various time series locations suggest that the effects of the dredge on SSC would dissipate at the shoreline of the Isle of Wight within one spring/neap cycle (i.e. over 15 days) following the cessation of any disposal activity. A small amount of material is redistributed back through East Solent (lag 12-13 days) and into Southampton Water (lag 17days after the initial deposit). At these locations



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enhanced SSC are generally of the order of 50mg/l, but only persist at the time of peak flood flows for a few minutes. Maxima of 150mg/l could occur for short periods.

8.285 Similar effects can also occur in the entrance to Portsmouth Harbour but with maximum concentrations above background only reaching 30mg/l and then only as short-term pulses.

8.286 A review of all deposit scenarios for the different particle sizes indicate that the deposit of the capital dredge sediments from the Southampton Approach Channel Dredge will only have very temporary effects on the character of the bed occurring within the aggregate extraction areas located to the east of the deposit ground. Settlement will be transient over a tide, with maximum accumulations of sand of less than 20mm, with all effects of the disposal returning to background conditions within one to two weeks of cessation of the disposal operations. Also, the effects from different deposits do not appear to be cumulative, therefore, smothering of benthos or long-term change of character of the sea bed is unlikely to occur. Assessment

8.287 Modelling the disposal of the capital dredged sediments from the Southampton Approach Channel Dredge has shown that the sediment will be widely dispersed from the Nab Deposit Ground with most of the spoil being removed from the area within about one spring neap cycle following the cessation of the dredge. Whilst the initial SSC will be high (circa 10,000mg/l) they will dissipate by an order of magnitude in minutes to hours following the disposal depending on the timing of the deposit in the tide and the particular tidal range. Dispersal of sediment takes place on both flood and ebb tides, although sedimentation occurs around HW and LW before it is re-eroded. This process moves the material to and fro on a NE to SW axis, migrating with time towards the Isle of Wight whilst the SSC continues to decay. Small amounts of sediment may return to Southampton Water with a lag of about 17 days. Maximum temporary accumulations of sediment outside the deposit ground will be less than 20mm.

8.288 At the deposit ground the magnitude of change over the background is large leading to a high level of exposure. The sensitivity of the site to such a perturbation, however, is assessed as low on the basis that the deposit ground as received disposals of similar sediments from both barges and TSHD over a long period from both capital and maintenance dredging, without substantially changing in depth. The annual maintenance deposits at the site are of the order of 10% of the amount to be deposited from the capital dredge, but the rate of supply over short periods will have been of a similar order. Such deposits have not raised concern in the past. In addition, a capital dredge of a greater volume than the current proposal deposited a similar range of materials at the site, albeit predominantly from TSHD’s. Depths at the deposit ground were not compromised, nor were local aggregate areas and no concerns were raised about increases in suspended sediment concentrations during the dredge. This low sensitivity to the dispersion of sediments leads to a moderate vulnerability near the deposit ground but lower further afield. The fact that the site has long been used for disposal, including in the past the deposit of sewage sludge indicates the importance of the site in environmental terms to be low. The overall impact significance is, therefore, considered to be minor/insignificant. Conclusions



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8.289 The potential changes to physical processes have been assessed in accordance with best practice. The predicted hydrodynamic and sediment transport changes resulting from the approach deepening are on the whole small in magnitude and for the most part remain in the sections of the system where direct changes to the channel are proposed. The changes predicted following the dredge would be almost impossible to measure directly in the field, mainly because they would not be able to be differentiated from natural variations and they would be close to (or below) the working accuracy of any standard recording instrument. During the dredging and disposal (construction) phase, the magnitude of change, particularly with respect to increase in suspended sediment concentrations in the water column, will be more notable, albeit short-term, but highly variable (transient) in time and space. Overall, the impact to the physical functioning of the estuary alone will be insignificant to minor adverse significant.

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