8. Physical Processes - Southampton VTS

Environmental Statement for Port of Southampton:Main Channel Widening (Marchwood) Works

Volume 1: Main Report

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8. Physical Processes

Executive Summary: Chapter 8. Physical Processes A package of technical evaluations, based on standard practice, has been used to determine the magnitude and extent of physical changes that are likely to result from the proposed dredging works. The assessment is based upon conservative assumptions to offer a realistic worst-case scenario. 1) Dispersion of Sediment during Dredging During dredging, heightened suspended sediment concentrations and a slight increase in sedimentation will occur due to local mechanical disturbance of sediments. The magnitude of the change, with respect to background conditions, is considered to be negligible to small. It is considered that these changes will not affect the physical functioning of the estuary. The impact on the estuary is therefore considered to be insignificant. Impacts on other receptors are considered elsewhere. 2) Bed Accumulation and Dispersion of Sediment during Disposal Modelling studies show that the deposit of the capital dredge sediments from the proposed dredging will be widely dispersed from the Nab Tower Deposit Ground, with all effects of the disposal returning to background conditions within one or two weeks after cessation of the disposal operations. The magnitude of the change is small, particularly given ongoing maintenance and capital dredge deposits at the site. Overall, the impact on physical functioning of the estuary is therefore considered to be insignificant. Impacts on other receptors are considered elsewhere. 3) Changes to Hydrodynamics Attributable to Dredging Only small changes in tidal range are anticipated, which are confined to the widening area. Importantly, the assessment of water levels shows there is no predicted change along the lower edges of the designated Dibden Bay and Bury Marsh (areas of nature conservation). The overall effects are considered negligible compared to the normal cyclic variation of the tide and disturbance caused by episodic events such as storm waves and surges. The impact on the hydrodynamic regime of the estuary will therefore be insignificant. Impacts on other receptors are considered elsewhere. 4) Changes to Sediment Regime Attributable to Dredging The proposed dredging works are predicted to result in a slight redistribution of sedimentation, with a slight increase in the dredged area, the Upper Swinging Ground and Berth 201/202, and the Marchwood Basin, with very small reductions in the outer dredged area of Marchwood Military Port and the Marchwood Channel. No detectable changes are predicted down-estuary of Dock Head. Additional deposition attributable to the proposed dredge in the areas currently requiring maintenance dredging is considered to be negligible. It is considered that the very small changes in sedimentation attributable to the proposed dredge will not affect the physical functioning of the estuary. The impact is therefore considered to be insignificant. Impacts on other receptors are considered elsewhere.



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5) Changes to Maintenance Dredging Commitment The overall maintenance dredging requirement for Southampton Water is not likely to increase. A greater dredging volume is likely to be required from the Western Docks area, with a reduced volume further up estuary (around the container terminal approaches). The change to the maintenance dredging commitment is considered to be insignificant. Conclusion The physical changes that are predicted to occur during capital and maintenance dredging operations associated with the proposed works are shown to be negligible and near impossible to measure directly in the field. The impact to the physical functioning of the estuary and coastal waters in the vicinity of the disposal site is therefore considered to be insignificant.

Introduction 8.1 Standard modelling approaches were applied to:

(1) Represent the baseline condition; (2) Provide the means to quantify the extent, magnitude and direction of any change

brought about by the channel widening works; (3) Determine the physical effects of the dredging to complete the channel widening

works; and (4) Inform the interpretation of any impact of these changes.

8.2 This modelling approach has been developed in line with current best practice recommended in

‘The Estuary Guide’ (www.estuary-guide.net). This body of work is recognised 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). The models are evidence-based, having been underpinned by the latest-available datasets. Further details of these methods are provided in Appendix J.

Baseline Information 8.3 The purpose of the baseline review is to describe the present day morphology (form) and

understand the processes operating in the physical environment which may be affected by the proposed dredging works. In considering the baseline, the physical features are defined by a number of morphological features across the study area. The function is partly described by the physical processes, which drive the movement of water and influence the transport of sediments, which in turn interact with the morphological elements. The natural variability of these processes is reviewed in relation to changes that are likely to occur irrespective of any future development.

http://www.estuary-guide.net/



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Overview of the Study Area 8.4 The proposed widening will involve dredging the edge of the existing navigation channel in

Southampton Water opposite the Western Docks over a length of approximately 900 m. The proposed development area is entirely sub-tidal with a maximum depth of removal of approximately 11 m. The channel base is proposed to be widened by around 30 m at the existing maintained dredge level of 12.6 m below Chart Datum (CD). Figure 3.1 illustrates the area of the widening and Figure 1.2 identifies the local features of relevance.

8.5 Available borehole and vibrocore data show that the range of material types that will be

required to be dredged include (in the order surface to dredging base):

Soft silty mud (recent alluvial deposits); Silt and clay; Gravel; and Varying strength firm to very stiff grey/brown sandy/silty clay.

8.6 Siltation in the region mainly comprises fine marine sediments that are transported into the

estuary during flood tides and subsequently deposited for a period of approximately 3 hours from the initial high slack water. Peak tidal flow speeds of around 1 m/s occur during the ebb tide.

8.7 Suspended sediment loads typically reduce through Southampton Water with measured

maximum concentrations at Calshot of around 80mg/l reducing to typical concentrations of around 10mg/l in the upper reaches of the estuary.

8.8 The disposal area at the Nab Tower Deposit Ground is located in a locally deep area 12 km off

the Isle of Wight coast, in a south-easterly direction from Whitecliff Bay. The area of the site is around 11.3 km2 and water depths are generally greater than 30 m below CD, with maximum depths of 43 m. The area is exposed to strong tidal flows of around 1 m/s which sweep across a seabed composed of sandy gravels in an east-north-east direction on the flood tide and a west-south-west direction on the ebb. Stronger flows occur during the ebb tide, with flows exceeding 1 m/s. With the exception of storm conditions waves are thought unlikely to exert significant influence over sediment mobilisation and transport due to the deep water and to natural sediment consolidation over time which acts to raise the incipient entrainment threshold of the sediments.

8.9 Tides are the primary influence on water movements in the study area, and control both bed

shear stress responsible for re-suspension of bed sediments and the subsequent advection of sediments in suspension.

Past Development and Reclamation 8.10 The present day morphology of Southampton Water has evolved during the 10,000 years since

the Pleistocene glaciation when sea level rose by approximately 120 m and drowned the former Solent River basin. A more comprehensive explanation of the geological history of this area is provided by Hodson and West (1972).



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8.11 Southampton Water is a relatively narrow, spit-enclosed meso-tidal estuary, subject to very limited wave action and draining a catchment of around 1,630 km2 (ABPmer, 2007), and defined as a coastal plain estuary according to the JNCC classification.

8.12 Land reclamation in Southampton Water has occurred since 1836 (Eastern Docks,

Southampton) and extending up to the 1990s (Berth 207). In general reclamation of land has tended 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 in 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 8ha 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 80ha 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 for the reclamation of Dibden Bay 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

8.13 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 capital dredging are summarised in Table 8.2. 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 undertaken.

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.



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Date Development

1922-27 Continuous operations to widen and deepen stretches of the channel between Fawley and the Docks and from Calshot out towards Bramble 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, 2007)

Southampton Water Bed Sediment Distribution 8.14 The bed of Southampton Water consists of geological strata that have been recently exposed

by dredging in the main navigation channel. The sediment varies from sandy clay near the Upper Swinging Ground to sands and gravels at Fawley. 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.

Contemporary Processes 8.15 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 reflects the dominance of the tidal processes, and to a lesser extent the variable locally wind-generated wave climate. Port development has also modified the natural function of the estuary.

Waves and Water Levels

8.16 Owing to the dominant wave approach directions and to the short local fetch, the proposed

dredging area is very sheltered from waves. 8.17 The tidal characteristics at the proposed Marchwood widening typify those found at other

locations in Southampton Water. The tide is asymmetric with a flood phase of around 9 hours to the second HW lasting longer than the 3.5 hour ebb phase. On spring tides an important feature of the tide develops during the flood phase, around two hours after low water (LW),



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known as the ‘Young Flood Stand’. This phenomenon is evident as a pause in the rising tide and creates a further slack water period.

8.18 Water levels are recorded routinely at three locations in Southampton Water, namely at:

Calshot at the entrance to Southampton Water; Berth 37 (Dock Head), the Standard Port; and Berth 206 (Redbridge).

The readings demonstrate that there is general consistency in the phase and amplitude of the tide, with only a moderate increase in range moving upstream.

8.19 At Berth 206 (Redbridge), the closest site to Marchwood, the general tidal regime is as follows (relative to CD):

Mean HW Spring = 4.4m Mean HW Neap = 3.6m Mean Tide Level = 2.5m Mean LW Neap = 1.7m Mean LW Spring = 0.4m

8.20 The estuary thus has a mean spring tidal range of 4 m and a mean neap tidal range of 1.9 m.

During surge conditions the tidal range normally reaches 5.1 m CD. However, the majority of tides fall in the range 2 m to 4 m (i.e. mesotidal).

8.21 Two water level records occurring on the 26 December 1999 and 10 March 2008, during storm

surges with amplitudes of 1m are particularly notable. Both events caused the peak water level to reach approximately 5.6 m CD (at Dock Head)

8.22 Another important contribution to extreme water levels and tidal range in the estuary is

attributable to the lunar nodal cycle, which operates over a period of 18.61 years. Long-term data from Portsmouth (ABPmer, 2007) indicate that this effect causes variation in tidal range of the order of 0.25 m. The trough of this cycle last occurred in about 2006/2007 and, therefore, tidal range, and possible extreme water levels, can be expected to increase over the next eight years and reduce again thereafter.

8.23 Historic water level records at Portsmouth suggest a mean sea level rise of 1.7 mm/year in the

region (ABP Research, 2001). Climate change predictions, published by UKCP09, suggest that over the next 100 years or so the rate of sea level rise is likely to be greater at around 3.6 mm/year but within a range of uncertainty bounded between 1 mm/year to 8.1 mm/year.

Tidal Flows

8.24 The direction of tidal flows within Southampton Water is influenced primarily by the main

navigation channel. Winds, surges and freshwater inflows can each have local and secondary influences on flows.



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8.25 The unique characteristics of the tidal curve results in locally-complex flows within Southampton Water which give rise to sediment transport mechanisms which promote the movement of fine sediments onto intertidal areas.

8.26 The asymmetry of the tide, with a longer duration on the flood phase than ebb, creates stronger

flows on the ebb which reach 0.5 m/s to 0.75 m/s in the Marchwood channel widening area.

Freshwater Inputs 8.27 Freshwater inputs into the estuary from the Rivers Test, Itchen and Hamble contribute to the

flow regime and result in a down-estuary salinity gradient. 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. 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 23.7 m3/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.

8.28 Although the estuary is well-mixed, freshwater inputs from the main tributaries influence salinity

throughout Southampton Water. Concentrations around the northern extent of the proposed dredge area are typically in the range 29 ppt to 31 ppt.

Local Sediment Regime

8.29 The primary input of suspended sediments to Southampton Water is from marine sources in

the Solent. River sediment loads are secondary and fluctuate with discharge events to create local variations at the head of the tributaries.

8.30 Low velocity flows combined with prolonged slack water periods provide the potential for local

siltation. Although low suspended loads mean that the present levels of siltation remain low, they are sufficient to require annual maintenance dredging. In addition mobilisation and redistribution of accreted sediments can occur due to the local flows attributable to vessel movements.

Present Maintenance Dredging

8.31 Although maintenance dredging of the navigation channel in Southampton Water is undertaken

biannually during the spring and autumn, additional dredging does occur if the need arises. Not all areas of the navigation channel in any case require dredging during every campaign.

8.32 Maintenance dredging is undertaken in two stages. 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 Tower 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.



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Future Morphology 8.33 The principal factor influencing 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 are important but are likely to remain secondary. Even less certain are possible the changes in background suspended sediment concentration in the Solent. These are likely to be affected by increases or decreases in wave action and changes in the release of sediment into coastal waters by coastal erosion processes.

8.34 Further conceptualisation of how different parts of the study area and wider estuary may

change over time are summarised in ABPmer (2007). Impact Assessment 8.35 This impact assessment describes changes to the baseline morphology and physical

processes that are likely to occur as a result of implementing the proposed works. Changes to the baseline morphology and physical processes may be short-term and limited to the period of dredging or may occur over longer periods after the dredge. The impact assessment presented here considers the relevant impact time-scales and aims to address specific stakeholder concerns, as identified during project scoping and consultation.

8.36 Information to enable this impact assessment has been gained from:

The Southampton Approach Channel Dredge (SACD) Study (for the much larger proposed works to the estuary) (ABPmer, 2008a);

The Environment Statement (ES) for dredging of nearby Berths 201 and 202 (ABPmer, 2008b) ; and

The hydrodynamic modelling reproduced in Appendix C for the Marchwood and Middle Swinging Ground Widening (ABPmer, 2012). Here the model used is an up-date of the model reported in ABPmer (2008a).

8.37 The new modelling study which has been undertaken to quantify the suspended sediment

concentrations and sedimentation resulting from the proposed dredge activity at Marchwood uses the models developed for the SACD study (ABPmer, 2008a). Although no changes are made to the model itself, the bathymetry had been up-dated and the source terms describing the potential release of material as a direct result of the dredger operations are defined according to the most likely foreseeable dredge scenario. The disposal of dredged material at Nab Tower Deposit Ground is not re-modelled. Instead, the results of the modelling undertaken for the SACD are used directly and are appropriately scaled. This scaling is possible for the disposal activities since the dredged materials taken from the proposed dredge area (gravels, alluvium and stiff clays), and the frequency of visits to the disposal site by the dredger and barges both remain the same as in the SACD. The only anticipated difference is the size of the dredger and the volume of dredged material it can carry.

8.38 Modelling results identified above have been used to assist with quantifying the potential

impacts arising from the proposed dredge, and include details related to:



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Hydrodynamics: tidal water levels and flow speeds (over two consecutive spring tides); and

Suspended sediment dynamics: concentrations (over a spring-neap cycle). 8.39 This assessment uses this information and focuses on two pivotal modelling results required for

a comprehensive assessment of physical impacts of the proposed dredging operations. These are:

Sediment plume dispersion modelling from the dredging operations; An assessment of net sediment accretion arising from the dredging operations.

8.40 Specifically, model outputs have been employed in three ways to assess the dredging scenario

described below:

To quantify the spatial differences in suspended sediment concentrations in the plume associated the proposed dredging works. This is achieved through the use of maps in Figures 8.1 to 8.3 which show difference plots (i.e. widening scenario minus baseline) at selected times in the tidal cycle including: the beginning of the dredge; four days from the start of the dredge; and at 3 days after suspension of all dredging activities. The maps are presented at approximately the times of peak ebb and flood tidal flows, and at High Water (HW, -3 hours) and Low Water (LW, + 5 hours), where the times refer to tidal levels at Dock Head.

To quantify the temporal differences in suspended sediment concentrations during the proposed dredging operations. This information is presented as time-series plots of suspended sediment concentration (absolute values) extracted from selected locations (Figures 8.4).

To quantify the spatial distribution of cumulative sediment accretion associated with all dredging activities. These results are presented in Figure 8.5.

Dredge Scenarios 8.41 The following section describes the dredge scenarios implemented in the modelling. The focus

of the modelling work has been on the backhoe dredging scenario which, due to the potential sediment release and intensity of this activity, will have the most significant impacts on the physical environment. The overspill from TSHD dredging has also been accounted for in the modelling of cumulative sediment accretion arising from the proposed dredging activities.

8.42 There are three materials likely to be dredged from the proposed dredge area, each of which

has been modelled using the most realistic anticipated scenario. These sediments are largely the same as those which were considered in the SACD study, namely gravel, stiff clay and alluvium. The gravel and alluvium are likely to be removed by TSHD. This assumption is made on the basis of the low mechanical strength of that material. The stiff clay is likely to be removed by backhoe.

8.43 For the gravels, the volume of fine material contained within the gravel layers is less than 6% of

the dredged gravel volume. The primary mechanism for sediment release from the dredging of gravel is the overspill of fine material interspersed amongst the gravel. The amount of material that overspills into the surface water is assumed to be 12kg for every m³ of material lifted from



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the bed. This release rate of material disturbed during the dredging activities is estimated using the S factor methods of Kirby and Land (1991).

8.44 TSHD removal of alluvium will not involve deliberate overspill and once the hopper is full the

suction pipe is lifted from the sea bed. The primary mechanism by which alluvium is released during removal from the sea bed is the small incidental, but predictable, disturbance by the suction head as it is drawn along the sea bed. The amount of material released at the sea bed is assumed to be 3kg for every m³ of material lifted from the bed. This release rate of material disturbed during the dredging activities is estimated using the S factor methods of Kirby and Land (1991).

8.45 The backhoe removal of stiff clays will be continuous with the exception of the time required to

switch a full attendant barge with an empty barge. The time to switch over and fill a barge is approximately 2.5 hours (modelled at 35 minutes and 88 minutes, respectively). Multiple attendant barges determine that there is no waiting time while a barge returns from the disposal ground. The mechanism by which sediment is released during backhoe operations centres around the ‘washing’ of material from the sides of the backhoe bucket, during the cutting action at the bed, the lift to the surface, the run-off during slewing to and from the attendant barge and finally the lowering through the water column on the return to the sea bed for the next bucket cycle. The majority of the bucket content is anticipated to remain as a solid mass. The small proportion of material released is that which becomes fluidised at the margins of the clay inside the bucket. The model assumes that the amount of material that is released is 12kg for every m³ of material lifted from the bed. This release rate of material disturbed during the dredging activities is estimated using the S factor methods of Kirby and Land (1991). In practice this material is released throughout the water column. To simplify the modelling source term, the material is released at exactly mid-depth.

8.46 The dredge scenarios are implemented so that by the time the total volume of sediment has

been released into the model, all locations at the proposed dredge site have been ‘visited’ by the dredger. This ensures that complete coverage of the, albeit small, dredge site is achieved in the modelling.

8.47 The following assessment presents the results from the backhoe dredge scenario, which is the

scenario where the most sediment is released into the water column over the shortest period of time. The sediment volume anticipated to be released by the backhoe is considered to be the largest and thus is a good representation of the worst-case scenario. The results for net sediment accretion also incorporate the results of the TSHD removal of alluvium scenario.

Key Impact Pathways 8.48 The potential impacts on physical processes in the study area as a result of the proposed

works arise primarily from the direct morphological change to the channel and to the disturbance of sediment during the approximately 13-week period of dredging and disposal.

8.49 The key impact pathways during the construction and operational phases of the proposal are

now addressed in the following sections:

Dispersion of Sediment during Dredging; Bed Accumulation and Dispersion of Sediment during Disposal;



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Changes to Hydrodynamics; Changes to Sediment Regime; and Changes to Maintenance Dredging Commitment.

Dispersion of Sediment during Dredging 8.50 The sediment released in the model is transported, as a plume, by the tide. The concentration

of the plume is estimated by the model and is subject to change through time. When the transported sediment experiences conditions suitable for deposition (e.g. during a period of slack water or transport into an area of very low tidal current speed) the model estimates the thickness of sediment deposited on the bed. Subsequent changes in the local tidal current speeds may then re-suspend the recently deposited sediment and transport it to another location. Over time the sediments will either deposit permanently in areas where hydrodynamic conditions remain below the threshold for re-suspension or they may leave Southampton Water.

8.51 It is not anticipated that this project will be undertaken simultaneously with any other

reasonably-anticipated dredging activity in Southampton Water. Therefore, no in-combination dredging scenarios are considered here.

Suspended Sediment Concentrations

8.52 The modelling results presented as spatial maps in Figures 8.1 to 8.3 show that during the

proposed dredging works, the depth-averaged SSC during backhoe operations are not expected to exceed 10 mg/l above background in most of the sub-tidal areas of the estuary, with peak increases of the order of 15 mg/l expected around the middle area of the channel running alongside the Western Docks (Figure 8.2).

8.53 At the start of the dredge (Figure 8.1), SSCs of the order of 10 mg/l above background

concentration are predicted in the areas immediately surrounding, but not directly at, the location of the dredge activity7. Beyond the immediate vicinity of the dredge, SSCs reduce to around 4 mg/l to 6 mg/l above the background concentration. These local concentrations are generally maintained for the duration of the dredging activity. At +4.5 hours from the beginning of dredging, the ebb tide results in the downstream extension of the relatively narrow dredge plume over a distance of approximately 1 km. During the subsequent flood tidal flows, the plume is displaced to the head of the estuary and is characterised by confined peak SSC values of around 10mg/l. SSCs in this region then declines slightly at HW slack (+11 hours). Subsequent re-suspension of deposited sediments during the following ebb tide results in a plume with maximum concentrations of around 6 mg/l. This plume extends over a greater distance than that developed during the first ebb tide at +4.5 hours and is subsequently swept back up the estuary before maximum concentrations again reduce slightly to around 5 mg/l at the next HW slack at +22.5 hours.

8.54 The extent of the plume and its associated SSCs after a period of 4 days from the beginning of

dredging are shown in (Figure 8.2). At the end of the ebb tide (+6.5 hours) Figure 8.2 shows that the sediment plume is predicted to extend to the area of Fawley and the entrance to the River Hamble. Peak SSCs do not exceed 4 mg/l to 5 mg/l above the background

7 It is noted that within approximately 50m of the dredge, SSCs of up to 50mg/l may be possible.



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concentrations. Peak SSCs in the upper reaches of the estuary remain close to those predicted shortly after the beginning of the dredge (Figure 8.1). Overall Figure 8.2 illustrates that the increase in the spatial extent of the plume is consistent with the gradual introduction of more sediment during the backhoe dredging activities.

8.55 The extent of the plume and its associated SSCs approximately 4 days from the cessation of

dredging activity is shown in Figure 8.3. SSCs of the order of 2 mg/l to 4 mg/l above background result from re-suspension of the sediments and are now typical of the well-dispersed plume throughout Southampton Water during peak ebb (+14.5 hours) and flood (+21 hours) tidal flows. Infrequent and transient concentrations of 6 mg/l to 8 mg/l above background occur locally. These are typical in the shallow margins of Marchwood and at other locations throughout Southampton Water. Although these transient peaks in SSC re-occur at the same locations during subsequent spring tides, their magnitude declines further through time as material is either deposited permanently, or is lost from the estuary and into the Solent where, beyond Calshot, SSCs are not anticipated to be above 4 mg/l to 5 mg/l at any time.

8.56 Time-series of SSC above background are shown in Figure 8.4 for the following locations: Bury

Swinging Ground; Bury Reach; Marchwood; Husbands (Marchwood); Dock Head; between Hythe and Fawley; Fawley Terminal; Hamble Entrance and Calshot. It should be noted that high concentration peaks at Marchwood are artefacts of the model and are unrealistic. These time-series show the characteristic modulation of SSC with tidal currents as the sediments are deposited and subsequently re-entrained on the following ebb or flood tide. Peak SSCs are less than 10 mg/l at all locations. Towards the end of the time-series Figure 8.4 shows an increase in SSC. This reflects an increase in the re-suspension of bed sediments in response to increased tidal flow speeds. Owing to permanent accretion of the dredged sediments at locations in the model domain, and to losses of sediment to the Solent, the magnitude of SSC peaks at this time are lower than those predicted to occur during the dredging operations.

8.57 When interpreting these results, it should be noted that the predicted SSC values in areas of

shallow water on the intertidal may be misleading as even a small amount of material held in a small volume of water may infer an unrealistic high concentration. It is considered therefore, that modelled peak concentrations over the intertidal areas are not wholly representative.

Net Sediment Accretion

8.58 The results presented here are for the plume dispersal simulation presented above and

encompass hydrodynamic effects of a 15-day spring/neap tidal cycle and a further 6 days with no dredging activity.

8.59 Sediments released by dredging have the potential to be permanently deposited where the

ambient flow speeds are below the local entrainment threshold. Modelling results in Figure 8.5 show predicted net accretion over the present 18 day model simulation. It is important to note that the predicted net accretion shown in Figure 8.5 comprises sediment released by backhoe dredging and overspill from TSHD operations and thus represent the worse case scenario with respect to sediment accretion.

8.60 Minimal deposition is predicted to occur at Bury, Upper and Middle Swinging Grounds,

Marchwood and Cracknore intertidal areas and Marchwood Military Port, existing dredged areas such as the entrance to Ocean Dock and berths 30 to 36, Mayflower Park, Hythe Pier



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and the Hythe-Dibden-Fawley Marshes, and further down-estuary at Calshot Marshes, BP Terminal, Hamble Beach and Hamble approaches. At these locations a total accretion depth of around 6 mm is typical with some localised local areas receiving up to 30 mm.

8.61 It is noted that the predicted areas of greatest accretion are in areas of the maintained dredge

channel. It is quite possible that the material deposited in the Upper and Middle swinging grounds, and the area in between, could be re-eroded and redistributed by propeller wash.

8.62 Other locations may also experience deposition of the released sediments but modelling results

indicate that this is transitory, with the bulk of deposition occurring during slack water being re-suspended and transported when the tidal currents increase on the following tide. The model indicated that the thickness of these transitory deposits is likely to be less than 1 mm. The period during which temporally deposited material stays on the bed is predicted to be no more than 2 hours. However, as noted above, the model makes a number of assumptions about the properties of the deposited sediment and the actual behaviour of these materials may be different from that in the model. In particular cohesive forces may be higher than those simulated in the model, and thus re-suspension may result in lower SSCs than those predicted by the model.

8.63 At most locations the accreted sediments may only be present for a few hours depending on

the location and the tidal range. In reality there is some degree of uncertainty in the predictions of accretion by the model attributable to the temporally- and spatially-varying physical properties and behaviours of the sediments with respect to cohesion, settling velocity, entrainment threshold and the degree of bed consolidation. Although the present model cannot reproduce all the complex physical and chemical behaviour of dredge-related sediments, the present parameterisation is considered to represent well a worse case scenario.

8.64 Whether increased sedimentation actually occurs in areas predicted by the model will depend

on numerous factors, including the behaviour of waves, surges and vessel movements, sufficient sediment supply, and the relative strength of flood and ebb flows to cause re-suspension and erosion occurring at the time. It is quite possible that the actual change in sedimentation as a result of the dredge may not cause a noticeable change in overall sedimentation volumes but more likely a redistribution of material.

Time-Series Analysis

8.65 At nearly all locations both accretion and erosion occur at different states of the tide and during

different tidal ranges thus adding complexity to the interpretation of sediment dynamics. An increased potential for sedimentation is indicated over all sub-tidal and most shallow intertidal areas. However, this may be reduced considerably in some areas due to the effects of natural and anthropogenic disturbance noted above.

Assessment

8.66 The predicted sedimentation as a result of the dredge operation is considered to be small in

magnitude compared to natural variability and existing maintenance dredging. The consequence of the change in sedimentation on various receptors is assessed elsewhere (Chapters 10–13).



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Dispersion of Sediment during Disposal 8.67 Modelling results from the SACD study (ABPmer, 2008a) are used here to assess the effects of

disposal for the appropriate material types to be dredged and dredge methodologies. The scaling assumptions made to enable the use of the existing model results are detailed below.

8.68 It is estimated that around 450,000 m3 of in situ capital material will need to be disposed of

from the proposed works at the existing deposit ground at the Nab Tower Disposal Ground, unless a beneficial use is identified for the material.

8.69 Volumes dredged from the proposed works may be dredged by TSHD and/or backhoe. Whilst

a range of material (sand, gravel and fines) is encountered, the largest sediment plumes during disposal are generated by the fine alluvial material, which, although accounting for approximately one third of the total proposed dredge material, can be assumed to be available for re-suspension and dispersion immediately after disposal.

8.70 The use of the backhoe dredger is likely to produce consolidated lumps of sediment. At the

existing deposit ground, much of this will fall to the seabed and, initially, remain as lumps. Those remaining lumps on the seabed could potentially form projections of the order of 2m high. These will be subjected to erosion over time with the sediments released being dispersed into the water column and subsequently transported in suspension or as bedload. Although the rate of sediment dispersal will be slower than that assumed in the modelling study it is likely to last for a longer period defined by the nature of the erosion processes operating, and the mechanical resistance of the material which at present remains unknown.

8.71 The SACD disposal plume modelling is based on a volume of 262,000 m3 of material being

disposed over a two-week spring-neap cycle. This represents 58% of the proposed dredge volume for Marchwood. The SACD modelling of TSHD deposits was based on a hopper capacity of 10,000 m3, whereas the current proposed works are more likely to use a smaller dredger, due to the smaller scale of the works, with a hopper capacity of 2,000–4,000 m³. Through scaling arguments it is reasonable to assume that the suspended sediment concentrations will be 40% of those predicted by the SACD dredge disposal modelling. The modelling assumes that the disposals occur over a 5 minute period and the total number of disposals over a spring/neap cycle is distributed uniformly over the area of the disposal ground. Each model run started and finished at peak spring tide ranges, and for a further 7 days to determine continuing change in SSC or deposition/erosion following cessation of the disposal activities. However, the proposed works are expected to take 13 weeks (6–7 spring-neap cycles), and thus, in reality, this allows sediments to disperse and dilute further than predicted by the model simulation. For the purpose of dispersion modelling a worst case has been assumed whereby all the disposed material is considered to be in its individual particulate form for instantaneous dispersal. This is a conservative approach to the modelling of sediment dispersal and thus the modelling represents a worst-case scenario for the dispersion of the sediment during disposal.

8.72 The modelling indicated that a proportion of the material would not immediately disperse and

would accumulate at the disposal ground. Depending on the material type, and rate of disposal, this could vary from a few hours to a few weeks. In a worst-case scenario initial mounds of gravel up to 4 m high could be deposited beneath a 10,000 m3 capacity dredger before eventually spreading over a wider area. However, the model predicts that sands, silts and clays



R/4073/3 8.15 R.1989

in particulate form would disperse within a spring/neap cycle once disposal has finished. Maximum accumulations over the disposal ground of these materials are not expected to be more than 25mm (assuming that each deposit is taken to a different sector (gridding) of the deposit ground to minimise ‘mounds’ on the sea bed).

Suspended Sediment Concentrations

8.73 Modelling disposal of the dredged sediments from the SACD indicated most of the spoil would

be removed and widely dispersed from the Nab Tower Disposal Ground within about one spring/neap cycle following the cessation of disposal.

8.74 Increased SSC will be greatest at the immediate site of the disposal and will reduce rapidly as

the sediment remaining in the water column is dispersed, predominantly along the axis of the flood and ebb flows.

8.75 Through downscaling of a single disposal from 10,000m³ every 6 hours to 2,000-4,000m³ every

6 hours, the SACD results indicate a high initial SSC of the order of 4,000 mg/l at the point of disposal (ABPmer, 2008a). This is predicted to decay rapidly at the point of disposal to below 400 mg/l within 20-30 minutes following each disposal. The maximum width of the plume characterised by concentrations in excess of 400 mg/l was predicted to be of the order of 250m.

8.76 Sediment dispersal takes place on flood and ebb tides, although sedimentation occurs around

HW and LW before the disposed sediment is again re-suspended. Material moves to and fro on a NE to SW axis, migrating with time towards the Isle of Wight whilst the SSC continues to decay.

8.77 The scales disposal of the dredged alluvial material results in SSC in excess of 200 mg/l

occurring at isolated locations during a spring/neap cycle in the vicinity of the Nab Tower Disposal Ground. Elevated SSC concentrations extend up to 12 km in a south-west direction, with a peak SSC of 80 mg/l approximately 2 km south-west of the disposal ground. Seven days after the start of disposal operations, tidal range reduces from springs to neaps, and SSC may exceed 200 mg/l in some coastal areas. At this time, changes in bed thickness up to 10mm extend 10 km to the north-east and 20 km to the south-west of the disposal ground and the main sedimentation and SSC plume area move inshore towards Sandown Bay.

8.78 The modelling suggests that 12 days after the last disposal (21 days from the start of the

modelling simulation), a plume with scaled SSC in excess of 80 mg/l is still present across the south-east coast of the Isle of Wight. The plume is smallest at LW, with scaled a SSC of 40–80 mg/l occurring offshore in Bracklesham Bay. During the peak flood tide, the south-east Isle of Wight plumes expand. These comprise a large plume adjacent to the coast, and another further south. A plume with scaled SSC of 40–80 mg/l also occurs adjacent to Selsey Bill, although this diminishes on the falling tide. Although the plume concentration will slowly decline over time, its persistence cannot be predicted with certainty.

8.79 The plume migrates towards the eastern shores of the Isle of Wight, where the effects of the

disposal are evident for around 4 days following the initial disposals. SSC values are higher on spring tides, with peaks of around 60 mg/l and tidal averages of around 8 mg/l. The maximum



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accumulation thicknesses will be around 0.5mm and will only be on the bed for short periods around HW and LW.

8.80 Since gravel and sand will cause smaller sediment plumes and lower SSC levels, and comprise

a larger proportion of the overall dredged material, the SSC impacts relate predominantly to the disposal of alluvium which represents a worst-case scenario.

Accretion/Erosion

8.81 After a single day of disposal operations, the sediments are expected to spread approximately

8 km north-east and 10 km south-west, with the bulk of the material about 2 km north-east of the deposit ground, where accumulated depths at the bed are up to 10 mm. Small amounts of sediment may return to Southampton Water with a lag of about 17 days. Maximum transient accumulations over 1km from the disposal site will be between 1–10mm.

Assessment

8.82 In the worst-case scenario the maximum extent of the disposal plume extends 40 km in a west-

south-west direction, and 20km in a north-east direction (limit of plume, with SSC over 4 mg/l above background), with the main plume (SSC maxima at any one time of 100–400 mg/l) extending 20km south-west and 5km north-east. It should be noted that these maxima are short-term, transient concentrations and the average SSC concentrations will be substantially lower.

8.83 The magnitude of change in SSC resulting from the proposed works at the deposit ground over

background levels is small, compared to existing maintenance and capital dredge deposits of similar sediments at the site and in relation to annual maintenance deposits at the site from all the estuaries and harbours in the Solent. Larger deposits have not raised concern in the past. The sensitivity and importance of the site are also considered to be low. The consequence of the change in suspended sediment concentration on various receptors is assessed elsewhere (Chapters 10–13).

Changes to Hydrodynamics 8.84 This section draws on the modelling results presented in Appendix C (ABPmer, 2012) and the

reader is directed to this report for further information. As the changes to hydrodynamic conditions as a result of the proposed works are all judged to be very small the intention here is simply to state the magnitude of changes attributable to the proposed dredge.

Water Levels

8.85 Within the widening area the largest changes in tidal levels are small and are confined to the

area dredged. The maximum changes in water levels in this area are +0.033m to –0.011m at any state of tide, with increased HW levels of +0.001 to –0.005m and decreased LW levels in the range –0.004 to –0.010m depending on location within deepened/widened area.

8.86 The water level difference results show there is no change predicted along the lower edges of

the designated Dibden Bay and Bury Marsh. The modelling indicates that the proposed widening will have no significant effect on other intertidal areas within the estuary.



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8.87 In front of Marchwood there is a slight increase in local LW levels in the Marchwood Basin (up

to 0.002m). Although HW levels are marginally increased in this area, no change is predicted over the remaining area.

8.88 Any changes in water level are attributable to the greatly increased water depth, circa 11m in

some locations, which significantly reduces the friction effects of the bed. As a result, the tide is able to flow more freely and its momentum at HW and LW is not dissipated so quickly allowing what is effectively an ‘overshoot’ to occur thus causing a small phase change.

Tidal Flows

8.89 A reduction in flood tide flow speed attributable to the proposed works is predicted within the

main channel, operational berths and either end of the newly dredged area. 8.90 Slight increases in flow speed occur at the centre of the newly-dredged area which spread over

the shallower sub-tidal area in front of Marchwood for periods of the flood tide. Most changes, whether positive or negative, are small, and for the most part are well below 0.05m/s.

8.91 Flow speeds at the southern end of the area of the widening area are largely unaffected. The

greatest change is between the two HWs, where flow speeds are lowered by around 0.05 to 0.1m/s on a spring tide. This effect is related to the increased depth and reduced bed friction.

8.92 Over the HW period, the dredge footprint attracts more of the flow towards it, causing locally-

increased flows up to a maximum of 0.1m/s through the Marchwood Channel and Basin. On the flood, the maximum flow speeds are reduced by around 0.2m/s (up to about 25%) around the outer edge of the existing Marchwood Yacht Club moorings.

Time-Series Analysis

8.93 Time-series analysis comparisons at Bury Reach and opposite Ocean Dock indicate that

changes to water levels due to the channel widening dredging are negligible in relation to background tidal variations.

8.94 Changes in tidal flow speed attributable to the widening are around ±0.03m/s at Bury Reach

and opposite Ocean Dock. Although, in general, flow speed differences between the baseline condition and post-dredge increase with tidal range and decrease with distance from the works, the changes are no more than a few percent and are probably not measurable.

Assessment

8.95 The modelling reported in Appendix C (ABPmer, 2012) indicates that changes to the tidal range

are confined locally to the proposed dredge area and are negligible compared to the normal cyclic variation of the tide, and disturbance caused by episodic events such as storm waves and surges. The impact of the predicted water level changes on the hydrodynamic regime is, therefore, considered to be insignificant.

8.96 Changes to tidal flow speed following the proposed dredging works are likely to be confined to

an area between Bury Reach (off Berth 203) and Ocean Dock (Eastern Docks, Dock Head).



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Changes to the characteristics of the tide and associated flow regime are not expected. The predicted flow speed changes of up to ±0.03m/s will not have a measurable effect on the hydrodynamic working of the estuary and are considered to be insignificant.

Changes to Sediment Regime 8.97 This section draws on the modelling results presented in Appendix C (ABPmer, 2012) and the

reader is directed to this report for further information. The modelling results for the widening (Appendix C), suggest that the widening is unlikely to have any significant effects on SSC values at any state of tide down-estuary from Dock Head.

8.98 Background suspended sediment concentrations in Southampton Water are controlled by the

estuary-wide processes, and therefore they remain largely unchanged throughout the area of the widening, being reduced by about 0.5mg/l at the time of peak spring tides and no change during neaps. In the Marchwood Channel and Basin, this is equivalent to a change of approximately 2–3%, depending upon the state of the tide.

Accretion/Erosion

8.99 Whether increased sedimentation actually occurs in areas predicted by the model will depend

on numerous factors, including the behaviour of waves, surges and vessel movements, sufficient sediment supply, and the relative strength of flood and ebb flows to cause re-suspension and erosion occurring at the time. The actual change in sedimentation following the dredge is unlikely to create a noticeable change in overall sedimentation volumes but more likely a redistribution of material.

8.100 Reductions in sedimentation of 4% and 9% are predicted in the outer dredged area of

Marchwood Military Port and the Marchwood Channel, respectively. This is accompanied by an increase of 0.004m/yr (3.5%) in the Marchwood Basin. At the upper and central areas of the widening, the modelling indicates potential for 0.0022m increase in sedimentation over a spring-neap cycle (under 0.06m per year).

8.101 The general reduction in flow speeds leads to a general but also very small net increase in

sedimentation in the main channel and Western Dock Berths. The slight increase in sedimentation expected at the Western Docks Berths 107 over the duration of a spring-neap cycle is (+0.0008m) and 109 (+0.0012m).

8.102 Within the Upper Swinging Ground and Berth 201/202, particularly the northern end,

sedimentation will be increased by just over 0.01m/year (circa 10%).

Assessment 8.103 The predicted changes to the sedimentation patterns are considered to be small in magnitude

for the estuary as a whole and probably undetectable from natural background SSC variations.

8.104 For sedimentation patterns within the estuary overall, the effect of the proposed dredging works

will be insignificant.



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Changes to Maintenance Dredging Commitment 8.105 After the proposed dredging works have been completed, the quantity of maintenance dredging

required is not likely to increase. Rather, a slight redistribution in maintenance dredging is predicted. Sedimentation increases slightly at Western Docks, with a corresponding small reduction further up-estuary (see discussion in section ‘Changes to Sediment Regime’ above). Other disturbance factors not accounted for in the model (e.g. waves and vessel disturbance), however, may affect the actual distribution of sedimentation.

8.106 The overall maintenance dredging requirement for Southampton Water is not likely to increase.

The changes predicted are not likely to be perceptible when the natural variability of the system is taken into account

Assessment

8.107 Following the dredge, the exposure to a change in maintenance dredging will be low and

confined to the vicinity of the widening, with a slight redistribution in dredging rather than an overall increase. The change to the maintenance dredging commitment is therefore, considered to be insignificant.

Conclusions 8.108 The potential changes to physical processes have been assessed in accordance with best

practice. The predicted hydrodynamic and sediment transport changes resulting from the widening of the channel are negligible in magnitude, with maximum changes predicted to occur where the direct physical changes are proposed. The changes predicted will be virtually impossible to measure directly in the field, because they will be indistinguishable from natural variation and will be below the working accuracy of standard recording instrument. During the approximately thirteen week 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 very short-term, and variable in magnitude and location. Overall, the impact to the physical functioning of the estuary will be insignificant.

8.109 The magnitude of change in SSCs resulting from the proposed works at the Nab Tower Deposit

Ground over background levels is predicted to be small, compared to existing disposals of similar sediments from a range of estuarine and harbours in the Solent region.

References ABPmer, 2001. Review of a Further Channel Deepening Scenario. ABP Research & Consultancy Ltd. Research Report No. R.857. ABPmer, 2007. A Conceptual Model of Southampton Water. ABP Marine Environmental Research Ltd. Prepared for the Estuary Guide as a case study supporting document by I. Townend http://www.estuary-guide.net/pdfs/southampton_water_case_study.pdf ABPmer, 2008a. ABP Southampton Approach Channel Dredge Environmental Statement. ABP Marine Environmental Research Ltd, Report No. R.1464.

http://www.estuary-guide.net/pdfs/southampton_water_case_study.pdf



R/4073/3 8.20 R.1989

ABPmer, 2008b. Environmental Statement for Port of Southampton: Berth 201/202 Works. ABP Marine Environmental Research Ltd. Report No. R.1494. ABPmer, 2012. Marchwood and Middle Swinging Ground Widening Modelling. ABP Marine Environmental Research Ltd. Report R.1953. Defra 2006. Flood and Coastal Defence Appraisal Guidance. FCDPAG3 Economic Appraisal. Supplementary Note to Operating Authorities – Climate Change Impacts. October 2006. Gifford and partners, 1989. Effects of proposed ensign marina on Southampton water, report to ABPmer. Hodson, F., and West, I.M., 1972. Holocene deposits of Fawley, Hampshire and the development of Southampton Water, Proc Geol Assoc, 83(4):421-442. Kirby, R; Land, J.M. 1991. Impact of Dredging – A comparison of natural and man-made disturbances to cohesive sedimentary regimes. CEDA-PIANC conference: Accessible Harbour, Paper B3, Amsterdam. Lowe, J.A.; Howard, T.; Pardaens, A.; Tinker, J.; Holt, J.; Wakelin, S.; Glenn, M.; Leake, J.; Wolf, J.; Horsburgh, K.; et al. UK Climate Projections Science Report: Marine and Coastal Projections; Met Office Hadley Centre: Exeter, UK, 2009; p. 99. Velegrakis, A and Collins, M. 2000. Expert analysis mode of an impacted estuary: Southampton Water. Paper 16. 103-155. In modelling estuary morphology and processes. Emphasys consortium. Estuaries research programme, phase 1. Final report. December 2000. Webber, N.B., 1980. Hydrology and water circulation in the Solent, In: The Solent Estuarine System, An Assessment of Present Knowledge. The Natural Environment Research Council, Publications Series C No. 22:25-36.

Concentration

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Figure 8.1

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Documents

8. Physical Processes - Southampton VTS