Evaluation of coastal process impacts arising from nearshore aggregate dredging for beach recharge -Shingles Banks, Christchurch Bay

摘要

Although monitoring is a requirement of all aggregate production licences (within the UK), this generally relates to examination of physical or biological impacts within, or immediately adjacent to, the dredging area. Relatively few licences require detailed monitoring of the coastal zone, as aggregate dredging-areas are usually in relatively deep water (>20m) and some distance from the shoreline. Post-dredging responses of the Shingles Banks dredging area; shoreline; and nearshore bathymetry of Christchurch Bay and Western Isle of Wight, are examined, in relation to a nearshore shallow water aggregate dredging programme for beach recharge of Hurst Spit. Data from an intensive 6-year programme of field monitoring and analysis has been compared with pre-dredging coastal process impact assessments, based largely on numerical modelling. Surveys have been integrated within best practice local and regional schemes of shoreline management and monitoring, to identify patterns of erosion and accretion. Changes have also been monitored at control sites, outside of the potential influence of the dredging area. Conditions in the shelter of the offshore banks have been shown to be less severe than numerical modelling methods have previously suggested. Conversely, the offshore wave climate, to seawards of the dredging area, has been more severe than long-term statistical modelling methods have predicted; this suggests increased storminess over the analysis period. Shoreline responses demonstrate that beaches on the shoreline of the Isle of Wight have not been affected by dredging activity. Beaches have remained stable, despite increased storm activity. Offshore bathymetric surveys have demonstrated considerable movements and continued evolution of a dynamic offshore bank system, in response to wave and current action; this has remained in context relative to historical patterns of movement Some of the risks, uncertainties and conflicts, associated with nearshore aggregate dredging for beach recharge purposes, are highlighted. The benefits of intensive aggregate-licence monitoring requirements, and co-operation between local authorities are demonstrated, providing a robust analytical approach to coastal process impact assessment of nearshore dredging.