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15. Coastal Defences

Executive Summary: Chapter 15. Coastal Defences

The proposed channel dredge is consistent with the policies set out in National Planning Policy Framework Planning Policy Statement (PPS) 25, which sets out the Government’s national policies on development and flood risk in England. None of the proposed works are planned to take place upon land and, therefore, this chapter can be considered to be a stand alone Flood Risk Assessment (FRA).

There will be no direct effects on any of the coastal defences in the study area as a result of the proposed works. The potential indirect impacts that will arise from the proposed scheme, either through changes to the hydrodynamic or sediment regime, were assessed as:

1) Potential Impact Due to Changes in Water Levels

The predicted changes in high water (HW) levels across Southampton Water and the Solent are millimetric and negligible in the context of natural variability in the wind-wave environment and would be nearly impossible to measure or observe in the field. Therefore, the impact of changes in water levels across the study area is considered to be insignificant. With respect to flood risk assessment, there will be no change to the risk of flooding as a result of the proposed dredge.

2) Potential Impact Due to Changes in the Flow and Sediment Transport Regime

In Southampton Water, there will be a marginal change in the flow dynamics that will result in minor changes to existing erosion and sedimentation patterns. The predicted level of these changes would not be measurable from natural variation and would be close to the accuracy of any recording instrument. In the Solent, the pattern of flow speed and directions are largely unaffected and the only changes evident occur within the areas proposed to be dredged (i.e. not along the coast). Although the changes to flows and sedimentary processes that are predicted in Southampton Water will potentially improve the situation with respect to coastal flood protection, the overall impact across the study area is considered to be insignificant.

3) Potential Impacts Due to Ship Wash

The total energy reaching the shore and seabed from vessel generated wave activity, water surface drawdown, backflow and vessel propeller wash will be marginally reduced. These changes in ship wash have the potential to marginally reduce intertidal erosion from existing conditions and the risk of overtopping existing sea defences, although this is unlikely to be discernable from background wind-wave energies reaching the shore. Therefore, the overall impact of ship wash is considered to be minor beneficial significant to the integrity of coastal defences, albeit marginal.

Conclusion

There will be no direct impact to coastal protection levels as a result of the proposed Southampton Approach Channel Dredge. Following completion of the capital works, the indirect impacts on flood risk will be insignificant. There will be a marginal reduction in the existing impacts to coastal defences from ship wash, which is considered to be minor beneficial significant.

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Coastal Defence Management

15.1 Operational management of coastal protection structures lies largely with District and Unitary Councils. Under the Water Resources Act 1991, the Environment Agency is responsible for supervision of all matters relating to flood defence and has permissive powers to maintain and improve water levels on main rivers. Natural England has a role in assessing the impact of coastal defence on nature conservation interests and Defra has a supervisory role, which includes providing an overarching policy framework and administering grant aid. Organisations with responsibilities and interests in shoreline protection along the south-central coast of England formed the Standing Conference on Problems Associated with the Coastline (SCOPAC) in 1986, providing a forum for debate on the difficulties relating to coastal defence and protection.

15.2 The Government’s national policies on development and flood risk in England are set out in National Planning Policy Framework (NPPF). The NPPF Planning Policy Statement (PPS) 25. This NPPF PPS aims to ensure that flood risk is taken into account at all stages in the planning process to avoid inappropriate development in areas at risk of flooding and to direct developments away from areas of highest risk. This chapter demonstrates that the proposed development is consistent with the policies set out in NPPF PPS25, and on the basis that none of the development is planned to take place upon land, can be considered to be a stand alone Flood Risk Assessment (FRA). In this way, the risk of flooding arising from the development has been considered and the impacts of climate change according to Defra’s latest guidelines (2006) have been taken into account in the impact assessment.

15.3 The characteristics of the coastal defences and their management types and practices protecting the coastline in the study area are presented in Figure 15.1. New coastal defences and changes to existing coastal defences are designed with future changes to sea level and the potential for overtopping taken into account. The timescale over which designed defences provide protection is typically in the order of 10 to 50 years depending on the coastal area. The saltmarshes and mudflats of the study area act as an additional natural block to wave energy and inhibit erosion of the backing coast and any manmade defences. The presence of these natural features, therefore, enhances the effectiveness of manmade protection works and effectively provides a two-tier coastal defence. The baseline distribution of marine and coastal habitats in the study area has been reviewed in Chapter 11 and the status of the saltmarsh, with respect to erosion and accretion patterns, is reviewed in Chapter 8.

15.4 Coastal defence strategies that cover the coastline of the study area are currently set out in the Western Solent and Southampton Water Shoreline Management Plan (SMP) North Solent Shoreline Management Plan, the East Solent SMP and the Isle of Wight SMP. These management plans are being revised and are expected to be completed by the end of 2009 (see Chapter 5). In addition to these SMPs, there is an Estuary Management Plan (EMP) for the Hamble Estuary. ABPmer have also undertaken an estuary processes study on behalf of Mouchel Parkman for Southampton City Council and backed by Eastleigh and Fareham Borough Councils in support of the coastal defence strategy for the River Itchen, Weston Shore, Netley and Hamble (ABPmer, 2006). The study describes the key issues and problems facing the study area based on current information, supported through the use of the photographs taken during a survey of the study frontage.

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15.5 The following baseline sections review the levels of protection along the key areas of coastline in the context of existing coastal defence strategies and associated studies, as well as additional information provided on the SCOPAC website.

Baseline Information

Southampton Water and Test, Itchen and Hamble Estuaries

15.6 Over 26km of the shoreline in Southampton Water and its component tributaries is protected by a wide range of coastal protection works. Prevailing south-westerlies mean that the estuary is relatively sheltered with slightly more energy reaching the eastern shore than the western shore. Coastal protection works include steel piling and concrete sea walls, stone, rock armour and rubble revetments, shingle ridges, stone walls, gabion baskets, concrete revetments, steel pipes, concrete and timber piles, and sandbanks. Table 15.1 summarises the lengths of coastline in Southampton Water and its component tributaries protected by various engineering structures. Southampton City Council and the Port of Southampton own the majority of these defences. However, defences are also owned by the New Forest District Council, Fareham Borough Council, commercial organisations and private owners.

Table 15.1 Summary of manmade coastal defences in Southampton Water and its tributaries

Southampton Southampton % Length (m) River Test River Itchen River Hamble Total Water West Water East of Total Concrete wall 1640 290 430 1610 1030 5000 19 Shingle ridge 0 890 0 270 0 1160 4.4 Rock armour 2200 0 0 0 0 2200 8.3 Steel pipe 420 0 0 0 0 420 1.16 Steel piling 3510 0 665 2710 1450 8335 31.6 Gabion basket 50 0 160 110 290 610 2.3 Concrete pile 280 0 0 90 0 370 1.4 Concrete revetment 50 0 0 400 90 540 2 Rubble revetment 1150 0 0 120 520 1790 6.8 Timber pile 0 0 75 20 200 295 1.1 Stone revetment 0 0 0 1940 2490 4430 16.8 Sand bags 0 0 250 0 0 250 1 Stone wall 0 0 550 0 420 970 3.7 Total 9300 1180 1880 7270 6490 26370 100 (Source: Western Solent and Southampton Water SMP)

15.7 With respect to natural defences, the majority of the lower intertidal shoreline in Southampton Water and its tributaries comprises mudflat, the western side of the estuary being largely backed by saltmarsh and the eastern side backed by natural low cliffs, such as Netley Cliff. Over 32km of the shoreline is currently not protected by manmade coastal defences (i.e. it is protected by existing natural habitat), which constitutes over 55% of the total length of shoreline. Table 15.2 summarises the lengths and percentage contribution of unprotected coastline in Southampton Water and its tributaries.

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Table 15.2 Summary of natural coastal defence in Southampton Water and its tributaries

Southampton Southampton Parameter River Test River Itchen River Hamble Total Water West Water East Approximate Length of 7190 8190 5110 6720 4970 32,180 Unprotected Shoreline (m) Total Length of Shoreline 16,490 9370 6990 13,990 11,460 58,300 (m) % Contribution 44 87 73 48 43 55

15.8 The following sections describe the existing coastal protection and recommended management strategies for Southampton Water and the tributaries in more detail.

Test Estuary

15.9 There are over 9km of coastal defences managed by various authorities in the section of the Test Estuary that stretches from Hythe Marina to Redbridge. Hythe Marina is protected by 1-2 tonne rock armour revetment with a 3m wide concrete promenade behind. Further north, the 2.2km stretch of embankment defences fronting Dibden Bay are maintained by ABP. The remaining defences along the west shore are mostly under 500m in length in any one section, with the exception of 600m of piling to the south of Redbridge which protects Eling. These defences are managed by the Ministry of Defence, Hampshire County Council, Powergen, New Forest District Council and private owners.

15.10 The defences along the east shore of this area, between Redbridge and Empress Dock, are primarily the quay walls of Southampton Docks, owned by ABP. The crest level of these walls is 3.5m ODN. The majority of the approximately 6km of defences along this stretch of the estuary are composed of steel sheet pile wall with concrete capping across most of its length, and the remaining defences include rubble revetments, concrete walls and stone revetments.

15.11 Defence of this area is largely influenced by the need to protect existing developments. Maintenance, and in some cases upgrading, of existing defences are recommended by the Western Solent and Southampton Water SMP to protect the area from flooding and erosion. Saltmarsh erosion and intertidal narrowing also need addressing in this area, while reclamation activities are not recommended as they can alter the tidal dynamics of the estuary, with potential changes to erosion patterns.

Southampton Water West

15.12 The area of the shore between Calshot Spit and Hythe Marina comprises a number of major national economic assets, including Fawley Power station and ExxonMobil’s Fawley Oil Refinery. The 1.2km of shingle ridge and concrete wall defences in this area of Southampton Water are located along the lee of Calshot Spit, in three individual lengths near Fawley Power Station and along a 1.7km stretch south of Hythe, which is protecting Hythe village. Calshot spit also affords some protection by preserving the low energy environment of the currently eroding saltmarshes.

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15.13 Maintenance and upgrading of existing structures, as well as the construction of new defences where necessary are recommended by the Western Solent and Southampton Water North Solent SMP to prevent flooding and maintain a co-ordinated approach to the management of the whole spit. Continued management of the fronting saltmarshes and mudflats will also ensure the protection of this area. Areas of set-back development, such as those recommended at Hythe, allow for the construction of a retreated defence.

Southampton Water East

15.14 The area between Weston Point and Hamble Common includes the medium density residential areas of Netley and Weston, the latter of which has been recommended for protection by the Western Solent and Southampton Water SMP. The rest of the hinterland is rural with few properties. The 1.9km of defences currently in this area front low eroding cliffs. These defences include over 650m of steel piling, 550m of stone walls, 430m of concrete wall and 250m of sand bags. Cliff retreat is generally slow and concentrated at Weston Point, and from Netley Castle to Netley Hard (Hydraulic Research, 1987; Posford Duvivier 1994; 1997). South of Victoria Country Park, the cliff line is vegetated and partly wooded, thus suppressing potential erosion (ABPmer, 2006). Defences along the Netley cliffs are maintained by numerous organisations, including Southampton City Council, Eastleigh Borough Council, BP Oil UK Ltd. Residents have also erected ad hoc defences along this stretch of coast.

15.15 The SMP recommends that mudflats throughout this area should be maintained, whilst seeking to protect coastal developments. Cliff erosion and tidal currents supply and move sediments, and these processes should be sympathetically managed. The retreat of the low cliffs at Netley poses a significant threat to coastal developments, such as Netley Castle and Royal Victoria Country Park. The Western Solent and Southampton Water SMP suggests some economic evaluation and justification for the maintenance of existing defences and future construction in this area. Protection of the residential area of Weston, for example, is recommended, as is the maintenance of the existing defences fronting the BP-owned Hamble oil terminal.

Itchen Estuary

15.16 The Itchen Estuary is in a sheltered environment where wave action is not considered to be a major factor. The defences here are mainly for protection against overtopping of tidal levels and are all within the Southampton City Council boundary. The hinterland is the city of Southampton, which is a high-density urban area. There are 1.7km of concrete gravity and steel sheet pile walls owned by ABP at the mouth of the river fronting Southampton Docks and Ocean Village.

15.17 Within the Itchen Estuary, cross-channel effects need to be considered, particularly if defences are to be moved, as tidal flows may be affected. The mudflat habitat of the estuary requires sympathetic management and the protection of waterside developments are of primary concern. Consistent with the policy for all other operational facilities of the Port, continued maintenance of the existing defences fronting the Empress Dock is necessary to ensure operations are not interrupted. There is also a need for the continued protection of the entire frontage to the western bank of the Itchen to prevent potential flooding up to 800m inland, as well as the upgrading and maintenance of existing defences on the eastern bank and further upstream.

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Hamble Estuary

15.18 There are almost 6.5km of coastal defences along the shores of the Hamble Estuary, consisting largely of stone revetments (2.5km), steel piling (1.5km) and concrete walls (1km). The main risk for the Hamble Estuary is the potential for variable amounts of flooding. Bunny Meadows on the eastern bank of the estuary has been the cause for growing concern (Hampshire County Council, 2005). Tidal inundation of Bunny Meadows is maintained via a system of culverts through the embankment. The culverts, however, are unable to maintain water levels in the meadows in phase with those in the estuary, and the difference in levels causes stress on the embankment and ultimately erosion, particularly on high spring tides.

15.19 On the west bank of the Hamble Estuary the defences are within the Eastleigh Borough Council jurisdiction, whereas the defences on the east bank are within the responsibility of Fareham Borough Council. They are owned and maintained by a range of other organisations, such as Marina Developments Ltd., as well as privately owned.

15.20 Cross-channel effects of defences in the Hamble Estuary must be considered in relation to tidal flow changes. Intertidal narrowing is a concern and future coastal development should not threaten present sediment deposition rates, particularly in the light of predicted sea level rise. The Western Solent and Southampton Water North Solent SMP considers that continued protection must be afforded to the waterfront developments of Hamble. Retreated defences are recommended in areas where development is set-back, such as Hamble Village. Maintenance and upgrading of existing structures, complemented sometimes by limited new construction is considered necessary in areas of valuable land, such as that fronting the Bursledon and Lower Swanwick developments.

Approaches to Southampton Water

15.21 Eroding cliffs of clay, sand and gravel, which feed Hook Spit to the northwest, front the coastline between Hook and Hill Head. Cliff protection at Solent Breezes currently consists of short lengths of gabion baskets and timber bulkheads. Seawalls, timber groynes and timber and sheet steel piling provide erosion and flood protection at Titchfield Haven. The National Grid has major infrastructure north of Solent Breezes at potential risk from erosion and flooding. Clay cliffs, with pockets of low-lying land, cover the frontage between Hill Head and Lee-on-the-Solent. Littoral drift from the northwest is interrupted by Hill Head Harbour with shingle swept onto nearshore banks and sand transported offshore by tidal currents. There are stretches of seawall and a number of groyne systems, with some areas on the Hill Head frontage having no built protection.

15.22 For this area, the East Solent SMP recommends a mixture of both holding the line in the short- term by maintaining existing defences around Hill Head Harbour and along the road to Meon Shore, a beach recharge programme, and doing nothing along the Brownwich cliffs and along the shingle bank. Further development of property or other assets should be restricted in areas at risk from flooding or erosion. Management operations should minimise impacts on recreational use of the foreshore and damage to intertidal habitats. In the long-term, increasing rates of cliff erosion between Solent Breezes and Meon Shore could result in increased sediment supplies from this source, although material would be intercepted by the groynes at Hill Head.

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North Solent

15.23 There are a range of coastal defence structures protecting the area of the coastline between Browndown and Selsey Bill, including:

. Riprap revetment and groyne system along the northwest end of Browndown Range; . A seawall protecting the coastal road in Solent Bay; . Massive revetment (Haslar Seawall) and the defences from Fort Monkton to Fort Gilkicker protecting MoD property as well as low-lying land; . Seawalls extending from Southsea Castle to the Royal Naval War Memorial; . Masonry walls of historic importance protecting Old Portsmouth; . Gabions and timber groynes protecting the spit facing into Langstone Harbour; . Concrete walls forming a hard defence around Fort Cumberland; . Timber sloping revetment protecting the shingle beach west of the Inn on the Beach at Hayling Island; . A seawall fronted by a recharged beach protecting the main frontage of Hayling Island is protected by seawall fronted by a recharged beach; and . A series of seawalls, groynes and short sections of gabions, as well as beach recharge protecting the rest of the coastline east to Selsey Bill.

15.24 According to the East Solent SMP a mixture of management options are required along this extensive coastline. A combination of doing nothing, holding the line and managed retreat is needed between Browndown and Gilkicker Point, and part of Hayling Island. For the rest of the coastline, a hold the line strategy of maintaining and upgrading existing defences is recommended, as well as implementing a programme of beach recharge in certain areas, such as the shingle beach fronting Fort Gilkicker.

Portsmouth, Langstone and Chichester Harbours

15.25 Defences, in the form of sea walls, earth embankments, dykes and other reinforced barriers occupy over 70% of the harbour shoreline, ranging from sound to very poor in terms of their structural condition and functional efficiency. Much of this frontage is in private ownership. There are many defences throughout the three harbours adjoining low-lying land that are likely to be of marginal future sustainability.

15.26 The East Solent SMP recommends a range of management types for the Solent Harbours. These include non-intervention, maintaining, and/or upgrading coastal protection standards along existing line of defence and possible review of management option when structures become redundant.

Northeast Isle of Wight

15.27 This area covers Old Castle Point to Culver Cliff. It is anticipated that a non-interventionist strategic defence policy will apply to the semi-mature woodland frontage between Old Castle Point and Ryde, and between Priory Bay and Culver Cliff (SCOPAC website). A do nothing strategy is currently adopted offshore of the coastline at Bembridge Ledges.

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15.28 The spits that protect the King’s Quay entrance are vulnerable to overwashing, recession and potentially to breaching as sea level rises. Both spits are vital to the estuary salt marshes as they provide protection from direct wave action from the East Solent. Spit maintenance depends upon continued sand and gravel supply by local cliff erosion.

Western Solent

15.29 This area covers the northern and southern margin of the Western Solent from Hurst Spit to Calshot Spit and the Needles to Old Castle Point respectively. Coastal defence of the northern shore between the spits depends on the maintenance of Hurst Spit, which protects the saltmarsh environment in its lee. The majority of existing defences should be maintained/upgraded. The Solent CHaMP has predicted almost complete saltmarsh loss and continuing mudflat narrowing by the year 2101. The effectiveness of these defences is linked to the existence, and future management, of the areas fronting saltmarshes.

15.30 The existing defences of the southern shore of the Western Solent (northwest coast of the Isle of Wight) are unlikely to be extended to protect presently undefended cliffs. The upgrading of defences at Gurnard, has been undertaken using a detailed environmental assessment of the ecological character of the coastal slope. Although there would be ecological, as well as sediment budget benefits from a relaxation of structural defences at a number of formerly freely eroding cliff sites (e.g. Colwell and Totland Bays), this is probably not an option where significant cliff top residential areas remain.

Impact Assessment

Key Impact Pathways

15.31 No landside works are associated with the proposed scheme and all of the dredging is subtidal, therefore, there are no direct effects on any of the sea defences from the works. There is a potential to impact indirectly coastal protection levels, either through changes to the hydrodynamic environment (water levels and wave climate) or sediment transport regime (erosion/sedimentation patterns). Any increase in water levels across the intertidal, alone and in combination with an increase in wave activity, will increase both the risk of overtopping and the loading on the structures or wave attack at the edge of fronting saltmarsh and mudflat. Any significant increases to tidal currents over shallow subtidal and intertidal areas may result in increased scour and erosion, undermining the integrity of the coastal defences and lowering mudflat or saltmarsh levels, and therefore, affecting the level of protection they provide in dissipating wave energy.

15.32 The key impact pathways with regards to coastal protection are covered in the following sections:

. Potential Impact Due to Changes in Water Levels; . Potential Impact Due to Changes in the Flow and Sediment Transport Regime; and . Potential Impacts Due to Ship Wash.

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15.33 The effects that the proposal have on coastal protection levels, either through changes in water levels, ship waves or through adverse impacts on shoreline erosion/sedimentation patterns have been assessed based on the physical processes assessment and modelling studies (Chapter 8 and Appendix C), and ship wash assessment (Chapter 16), in conjunction with the baseline review above.

Potential Impact Due to Changes in Water Levels

15.34 The deepened and widened channel will marginally alter the tidal propagation in Southampton Water and the Solent, resulting in small and undetectable changes in water levels during high water (HW) periods.

Southampton Water and Test, Itchen and Hamble Estuaries

15.35 Modelling predicts that there will be a slight change in phasing of the tide, occurring mainly around the 3 hours preceding, and the 2 hours following low water. These changes are shown in Figure C.22 of Appendix C. Maximum changes to the level of the tide due to phasing differences will occur in the hour preceding low water (decreases in tidal level coincident between baseline and scheme), and the half-hour following low water (increases in tidal level coincident between baseline and scheme). These changes will be around 5cm in the upper end of the estuary, but will decrease to be near undetectable in the outer estuary.

15.36 The changes in maximum and minimum tidal levels are shown in Figure C.22 of Appendix C. A reduction in the highest HW levels is predicted, ranging from 1mm at Calshot continuing up Southampton Water to around 3mm in the Test Estuary at the container terminal. A negligible reduction in highest HW levels (<1mm) is predicted to occur in the Itchen and Hamble Estuaries. The existing integrity of the coastal defences up-estuary of Calshot is not considered to be at risk by these marginal changes in HW levels. The predicted reductions in HW levels are considered to be marginally beneficial to flood defences in the estuary, as the likelihood of overtopping and load on the structures, as well as potential erosion at the edge of the fronting marsh would be marginally reduced in the short-term. The lowest water levels experienced in the estuary will be lowered by up to 5mm for the majority of the estuary, although in the upper estuary, upstream of Dock Head, the lowest water levels are predicted to be slightly higher by approximately 18mm.

15.37 The magnitude of change in water levels as a result of the complete dredge is, therefore, considered to be negligible and if anything the risk of overtopping and load on the defences would potentially be reduced, albeit marginally. Overall, the impact to coastal flood protection in Southampton Water and its component tributaries is considered to be insignificant.

15.38 The reduction in HW levels would be equivalent to offsetting between ¼ and ¾ of a year of predicted sea level rise. The level of change is so small, however, that it is below the natural variability of the system and would be near impossible to measure or observe in the field. Any changes that are predicted by the modelling to occur under today’s conditions, in the future, would be less noticeable, due to any increases in sea level decreasing the relative magnitude of changes caused by the dredge (Chapter 8). Furthermore, changes in tidal propagation would also be less noticeable during extreme events (e.g. extreme tidal levels).

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15.39 With respect to flood risk assessment, there will be no change to the risk of flooding in Southampton Water and its tributaries as a result of the proposed dredge. The relative impact that any changes in tidal propagation would have on flood risk areas would also be in the order of seconds and millimetres, and therefore would be negligible compared to water level fluctuations caused by meteorological changes. The dredge scheme is predicted to have no impact on fluvial levels.

Solent

15.40 In the Solent, the highest HW levels will be increased marginally in magnitude from a negligible change east of Ryde Sand to around 4mm in the area of the Thorn Channel, continuing through into the West Solent. The marginal increases in water levels are considered negligible in the context of the existing dynamic environment of the more open coast. The area of shoreline most sensitive to a change is in the West Solent, between Hurst and Calshot Spits, as it is mainly protected by natural coastal defences, comprising saltmarsh and mudflat habitat, that are eroding and predicted to be lost in the future due to rising sea levels (Bray and Cottle, 2003). To put into context, the dredging works would add to the effect of predicted sea level rise by the equivalent of 1 year, based on the regional net sea level rise allowances advised by Defra (2006).

15.41 Whilst the front edge of the saltmarsh in the West Solent has been laterally eroding and retreating, reducing the fronting protection with respect to coastal defence, there is evidence to suggest that the vertical height of the remaining saltmarsh has been rising (SCOPAC website). Although the proposed dredge will not alter the patterns of flows in this area and, therefore, the potential for sedimentation, a slight increase in tidal inundation is predicted, which may provide a greater availability of suspended sediment for potential sedimentation. This increased potential for continuing vertical accretion of the marsh, would provide a benefit in the short-term by aiding the protection at the toe of backing defences. As discussed in the following sections, there will be no change to the flow regime or impacts due to ship wash in this area, and so in the long-term, the proposal will have no effect on the existing lateral retreat of the saltmarsh.

15.42 As in the estuary, any changes that are predicted by the modelling to occur under today’s conditions, in the future, would be negligible, due to any increases in sea level decreasing the relative magnitude of changes caused by the dredge. Furthermore, changes in tidal propagation would again be less noticeable during extreme events.

15.43 In summary, the marginal increase in HW levels is considered to be negligible in the context of existing natural variability in the wind-wave environment, although a marginal benefit could result from the potential increase the vertical height of the backing saltmarsh in the West Solent, thus aiding protection at the toe of backing defences in the short-term. Overall, the impact of changes in water levels in the Solent to coastal flood protection is considered to be insignificant. With respect to the flood risk assessment, there will be no change to the risk of flooding in the Solent as a result of the proposed dredge.

Potential Impact Due to Changes in the Flow and Sediment Transport Regime

15.44 The effects of the proposed dredge on the flow regime in Southampton Water and the Solent are, in all cases, very small and predominantly within the deepening and widening areas.

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Southampton Water

15.45 In the approaches to Southampton Water, a small reduction in flow speeds of up to 0.01m/s is expected in areas that already experience slow peak flood and ebb flows, particularly throughout the area of Stanswood Bay. Just upstream of Calshot, the changes in flow speeds will mainly occur within the Hook widening, with subtle changes either side of the channel over the shallow subtidal and intertidal areas. Flows will be marginally reduced by up to 0.04m/s to the east of the widening over the shallow area in front of Hook Shore and marginally increased from almost slack water by up to 0.03m/s to the west of the channel across the area of the Fawley Power Station intake channel. Between Dock Head and the Hamble, peak flood flow speeds will be reduced either side of the channel from around 0.02m/s at the top of the channel side slopes to virtually no change in the shallowest water. Further upstream, between the container terminal and Dock Head, peak flood and ebb flow speeds will be reduced marginally, mainly over the area deepened, with no significant changes in peak flows over the shallow subtidal and intertidal areas.

15.46 A reduction in flow speeds over the shallow subtidal and intertidal areas will increase the potential for sedimentation or reduced erosion, which is mainly predicted to occur between Dock Head and the Hamble. This could have a marginal beneficial effect in terms of affording some additional protection. The marginal increase in flow speeds predicted to the lee of the Calshot Spit, across the area of the Fawley Power Station intake channel, is also likely to lead to an increase in the potential for sedimentation. This is because although the marginal increase in flow speeds from existing almost slack flows would enable more sediment to move into the area, the increase would not be sufficient to maintain the material in suspension.

15.47 The marginal change in the flow dynamics will only cause minor changes in existing erosion and sedimentation patterns. The predicted level of these changes would not be measurable from the natural variation and would be close to the accuracy of any recording instrument. Although the changes to flows and sedimentary processes will potentially improve the situation in Southampton Water from existing conditions, when taking into account the overall scale and magnitude of effect and the sensitivity of coastal defences to change, the impact to coastal flood protection is considered to be insignificant.

Solent

15.48 In the Solent, the pattern of flow speed and directions are largely unaffected and the only changes evident occur within the areas actually dredged, where flow speeds will be reduced by up to around 0.025m/s at the time of peak flows, with little change at the time of minimum flows, which occur around low water (LW). These small changes in the hydrodynamics will only result in subtle changes in the overall distribution of accretion and erosion potential local to the dredge areas in the eastern Solent. Along the coastlines, no change to the flows and sediment transport potential is predicted and, therefore, the impact on coastal defences in the Solent will be insignificant.

Potential Impact Due to Ship Wash

15.49 The proposed deepening and widening of the channel will marginally reduce the total energy reaching the shore and seabed from vessel generated wave activity, water surface drawdown,

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backflow and vessel propeller wash. The ship wash assessment is presented in Chapter 16. These changes in ship wash have the potential to marginally reduce intertidal erosion from existing conditions and the risk of overtopping existing sea defences, although this is unlikely to be discernable from background wind-wave energies reaching the shore. Overall ship wash is, therefore, not predicted to affect the integrity of coastal defences in the study area and the impact is considered to be minor beneficial significant, albeit marginal.

Conclusions

15.50 This chapter demonstrates that the proposed works are consistent with the policies set out in NPPF PPS25, and on the basis that none of the development is planned to take place upon land, can be considered to be a stand alone Flood Risk Assessment (FRA). There will be no direct effects on any of the sea defences in the study area as a result of the proposed works. Following the proposed dredge, the potential impacts from changes to the hydrodynamic and sediment regime brought about by the new channel design are considered to be insignificant with respect to coastal defences, and none of the impacts are of a scale that requires mitigation. There will be a marginal reduction in the existing impacts to coastal defences from ship wash, which is considered to be minor beneficial significant.

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