Creek enlargement in a low-energy degrading saltmarsh in southern England

摘要

A low-energy saltmarsh in the Beaulieu River Estuary, southern England was investigated. A geomorphological survey revealed that the creek bank geometry was characterized by the presence of cantilevers, a result of the protection provided by roots over the upper part of the bank. The saltmarsh deposits, consisting mainly of clay and organic matter, have a high resistance to flow-induced erosion. Short-term hydrographic deployments revealed that the tidal currents and waves recorded were insufficient to cause erosion of the bank surface. However, pin measurements over a 2-year period revealed that the tidal creeks within the saltmarsh did experience bank erosion, particularly below the cantilevers, in spite of this low-energy setting. Bank face erosion was therefore considered to be related to long-term processes. The bases of the cantilevers were found to be located at a precise level, in relation to the tides. Water level data over a 2-year period were examined against erosion measured below the cantilevers. The results suggest that the erosion on the bank faces, particularly below the cantilevers, is associated with the frequency of water level variations and long-term submergence/emergence cycles, which can decrease the bonds between the particles and enable erosion by low tidal and wind-wave currents within this low-energy environment. This mechanism is probably responsible for tidal creek enlargement via erosion of the bank face and subsequent bank failure, a process which acts over a long temporal scale (1-10 years).