15. Coastal Defences. Executive Summary: Chapter 15. Coastal Defences

Transcription

1 15. Coastal Defences Executive Summary: Chapter 15. Coastal Defences The potential direct and indirect impacts that will arise from the proposed Berth 201/202 works, either through the reconstruction of the quay wall or changes to the hydrodynamic and sediment regime brought about by the deepened berth pocket, were assessed as: 1) Potential Direct Impact to Coastal Protection Levels The proposed new quay wall will be constructed immediately in front of the existing wall with the top level of the cope at +6.25m Chart Datum (CD) as per the existing level. Flood protection will therefore be maintained throughout the construction period. Following the completion of the Berth 201/202 works, the new quay wall will provide the same level of flood protection as the existing situation. There will be no direct impact on coastal protection levels during and following the construction works. With respect to flood risk assessment, there will be no change to the risk of flooding as a result of the proposed Berth 201/202 redevelopment. 2) Potential Impact Due to Changes in Water Levels The predicted changes in high water (HW) levels are negligible in scale and restricted to the locality of Berth 201/202 in the Test Estuary. In the context of natural variability in the windwave environment, the change is unlikely to be discernable from background conditions. Therefore, the impact of changes in water levels 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 Berth 201/202 works. 3) Potential Impact Due to Changes in the Flow and Sediment Transport Regime The small and localised changes in the flow dynamics will result in negligible changes to existing erosion and sedimentation patterns in the Test Estuary, largely in the area local to Berth 201/202. The predicted level of these changes will not be measurable from natural variation and sedimentation patterns are likely to be redistributed by existing wave disturbance. Therefore, the impact of changes in flow and the sediment transport in this part of the estuary 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 Berth 201/202 works. 4) Potential Impacts Due to Ship Wash Despite the fact that container ships are generally becoming larger and that the deepening of Berth 201/202 will allow deeper draughted vessels to access the berth, the overall impact to ship wash is negligible in the context of background wind-wave energies reaching the shore. Therefore, the impact of ship wash to the integrity of coastal flood protection is considered to be insignificant, with respect to potential bed and bank erosion effects and the risk of overtopping existing coastal defences. 197

2 Conclusion There will be no direct impact to coastal protection levels during construction and operation of Berth 201/202. Following completion of the works, the indirect impacts on flood risk will be insignificant. 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 The Government s national policies on development and flood risk in England are set out in Planning Policy Statement (PPS) 25. This 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 PPS25 and can be considered to be a stand alone Flood Risk Assessment (FRA) The characteristics of the coastal defences and their management types and practices protecting the coastline in the wider study area are presented in Figure 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 (Table 8.10) 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), 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

3 15.5 The following baseline sections review the levels of protection, in the context of existing coastal defence strategies and associated studies, as well as additional information provided on the SCOPAC website, within the general area of direct and indirect impact from the Berth 201/202 works. Baseline Information 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 Length (m) River Test Southam -pton Water West Southam -pton Water East River Itchen River Hamble (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 Total % of Total Concrete wall Shingle ridge Rock armour Steel pipe Steel piling Gabion basket Concrete pile Concrete revetment Rubble revetment Timber pile Stone revetment Sand bags Stone wall Total

4 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. Table 15.2 Summary of natural coastal defence in Southampton Water and its tributaries Parameter Approximate Length of Unprotected Shoreline (m) River Test Southampton Water West Southampton Water East River Itchen River Hamble Total ,180 Total Length of Shoreline (m) 16, ,990 11,460 58,300 % Contribution The following section describes the existing coastal protection and recommended management strategies for the area local to Berth 201/202 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 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 +6.25m Chart Datum (CD). 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 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. Impact Assessment Key Impact Pathways The quay reconstruction associated with the Berth 201/202 works has the potential to affect directly the existing level of protection offered by the quay wall during construction and following 200

5 completion of the works. There is also 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 The key impact pathways with regards to coastal protection are covered in the following sections: Potential Direct Impact to Coastal Protection Levels; 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 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 previous baseline review. Potential Direct Impact to Coastal Protection Levels During Construction During construction of the new quay wall, the existing wall will remain, even after the new one is built. It is considered likely that the cope of the existing wall will be removed prior to completion of the works on the new quay wall. Although the top of both the existing and new walls will be lower than +6.25m CD at this point during construction, the only area at increased risk of flooding will be the works themselves, as they are surrounded by paving at +6.25m CD which will not be affected by the works. Flood protection will, therefore, be maintained throughout the construction period. Therefore, there will be no direct impact on the integrity of coastal protection levels during the construction works. Post Construction Following the completion of the Berth 201/202 works, the top level of the cope of the new quay wall will remain at +6.25m Chart Datum (CD) and will, therefore, provide the same level of flood protection as the existing situation (Chapter 3). There will, therefore, be no direct impact on coastal protection levels during the operational phase of the development. With respect to flood risk assessment, there will be no change to the risk of flooding as a result of the proposed Berth 201/202 redevelopment. 201

6 Potential Impact Due to Changes in Water Levels The levels of calibration achieved by the numerical models that were developed for this assessment have demonstrated a high degree of confidence (Appendix C). The results of the hydrodynamic modelling studies have shown that the proposed berth pocket will cause very small and localised changes to water levels in the Test Estuary (Paras 8.68 to 8.69). The changes in water levels predicted are confined to the vicinity of Berth 201/202, where there will be a maximum reduction in high water (HW) of 2mm on spring tides The existing integrity of the coastal defences in this part of the Test Estuary, namely the quay wall of the container terminal and Western Docks, is not considered to be at risk by these marginal changes in HW levels. If anything, there is a slight reduction in the likelihood of overtopping and load on the structures in the short term. The predicted change in HW levels would be equivalent to offsetting half a year of predicted sea level rise in the area local to Berth 201/202. 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. Overall, the indirect impact of changes in water levels in the locality of Berth 201/202 is considered to be insignificant to coastal flood protection With respect to flood risk assessment, there will be no change to the risk of flooding as a result of the Berth 201/202 works. 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. Potential Impact Due to Changes in the Flow and Sediment Transport Regime The results of the hydrodynamic modelling studies have shown that the proposed berth pocket will cause very small and localised changes to flows in the Test Estuary (Paras 8.70 to 8.76), predominantly over the deepened Berth 201/202 pocket, of up to a maximum of ±0.02m/s. These predicted changes will not have a measurable effect on the hydrodynamic working of the estuary The marginal change in the flow dynamics as a result of the deepened berth pocket will only cause localised changes in existing erosion and sedimentation patterns. In the long-term, the proposed Berth 201/202 works will result in very small and localised changes to potential sedimentation, with the greatest increase of up to ±15mm/year occurring within the deepened pocket. The berth pocket is already maintenance dredged and the effect of this negligible change in sedimentation rates has been reviewed in the context of future maintenance dredging requirements (Paras 8.87 to 8.90) The only predicted effect on sedimentary processes to occur outside of the maintained berth pockets and navigation channel is a very localised and marginal increase in potential sedimentation in the shallow subtidal areas fronting Marchwood Military Port and Cracknore Hard of up to 5mm/year. Although this could have a marginal beneficial effect in terms of affording some additional protection, the changes will be transient throughout tides and likely to be redistributed by existing wave disturbance. The predicted level of these changes would not be measurable from the natural variation and would be close to the accuracy of any recording 202

7 instrument. Therefore, the impact to coastal defence structures and flood protection is considered to be insignificant. Potential Impact Due to Ship Wash The ship wash assessment is presented in Chapter 16. Even though the Berth 201/202 works will allow deeper draughted vessels to access the berth, a negligible increase in average ship wave heights and total incident ship wave energy reaching the Marchwood foreshore is predicted (circa less than 1%). Furthermore, incident wave energy in this part of the estuary opposite Berth 201/202 at Marchwood is almost entirely derived from wind-waves with only a very minor contribution from ship-waves (circa 1%). This is largely due to the very low vessel transit speeds at this location, with ships being predominantly under tug control and preparing to berth. In addition, the change to ship generated waves as a result of the increase in the draught of the largest container vessels that could be accommodated at the Container Terminal following the Berth 201/202 works has been calculated. An increase in container ship size will lead to a slight decrease (circa 0.1%) in the annualised vessel energy in Southampton Water. Historical analyses of wind-wave energies indicates that natural variability in the wind-wave environment exceeds that from any change in vessel generated wave energy brought about by increasing the dimensions of the container ships. Therefore, the overall impact from ship wash is unlikely to be discernable from background wind-wave energies reaching the shore and is considered to be insignificant, with respect to potential bed and bank erosion effects, and the risk of overtopping existing sea defences. Conclusions The potential impacts to coastal defences and flood risk have been assessed in accordance with the Marine Works (EIA) Regulations This chapter demonstrates that the proposed works are consistent with the policies set out in PPS25, and can be considered to be a stand alone Flood Risk Assessment (FRA). There will be no direct effects on the existing level of coastal defence in the study area during or following the proposed Berth 201/202 works. The potential indirect impacts from changes to the hydrodynamic and sediment regime brought about by the deepened berth pocket are considered to be insignificant with respect to coastal defences, and none of the impacts are of a scale that require mitigation. The impacts of ship wash during the operation of the berth is considered insignificant to the integrity of coastal protection. 203

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