Environmental Statement for Port of Southampton: Main Channel Widening (Marchwood) Works Volume 1: Main Report 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. R/4073/3 8.1 R.1989 Environmental Statement for Port of Southampton: Main Channel Widening (Marchwood) Works Volume 1: Main Report 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. R/4073/3 8.2 R.1989 Environmental Statement for Port of Southampton: Main Channel Widening (Marchwood) Works Volume 1: Main Report 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). R/4073/3 8.3 R.1989 Environmental Statement for Port of Southampton: Main Channel Widening (Marchwood) Works Volume 1: Main Report 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 Development of Eastern Docks out over the mudflats at the confluence of the Rivers Test 1890-1910 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. Dredged Material used to reclaim approximately 80ha of land around the Exxon Mobil Oil 1920s-60s Refinery, the majority of which took place in the early 1960s. Maintenance dredge spoil used to for the reclamation of Dibden Bay in four stages: Phase 1 - 1930-55; 36ha 1930s-70 Phase 2 - 1956-60; 40ha Phase 3 - 1960-62; 36ha Phase 4 - 1962-70; 64ha Fawley Power Station reclamation using material from dredging off Calshot approaches to 1950-51 reclaim some 46ha.