The development of a tidal creek system, in a low energy environment, Beaulieu Estuary, southern England

Abstract

The saltmarshes within the Solent area have retreated significantly over the past 120 years. The expansion of tidal creek systems contributes considerably to the net loss of these saltmarshes. The development of a tidal creek system at Exbury Marsh was studied, as being representative of the tidal creeks within the Beaulieu Estuary, with emphasis placed upon the evolution of the crosssectional profiles. The present geomorphology of the studied tidal creek system was investigated, together with the measurements on the bank sediment stability, the parameters of the root systems (Sea Purslane and Sea Rush) and the hydrodynamics. This study aims to understand the influences of various factors, e.g., root systems and bank sediment stability, on the geomorphological evolution of a natural tidal creek system, within a low energy environment. The results reveal that within this low energy saltmarsh, the erosion takes place at a low rate (1-2 cm a-1) on bank faces, greater than the vertical accretion rate of marsh surface (2.5 mm a-1). The geomorphology of the tidal creek system is mainly characterised by the presence of cantilevers. The bases of the cantilevers are found to be located at certain levels, related to submergence/emergence cycles. The cantilever stability analyses suggest that lateral expansion rate of the channel is approx. 2.9 cm a-1, caused by the periodic bank failure. The root systems are central in increasing bank sediment stability, contributing a maximum of 6 Pa to the erosion threshold and a maximum of 7 kPa to the shear strength of the bank sediments. The Sea Purslane root system is more effective than the Sea Rush root system, in retarding the bank erosion. The flow-induced bed shear stress (<0.1 Pa) is lower than the erosion threshold of the bank sediments (>0.5 Pa), under normal weather conditions. The water level frequency analysis suggests that the erosion below the cantilever bases can be achieved by submergence/emergence cycles, aided by low tidal currents, over a long-term period. The evolution of the cross-sectional profiles is controlled mainly by the transformation from mudflat to saltmarsh, the presence of a gravel base, the stabilisation by root systems on bank sediments and the erosion caused by submergence/emergence cycles (together with low tidal currents), over a long-term period. The main contributions of this study to practical application are: (1) root systems (e.g., Sea Purslane) with high resistance to fluid-induced erosion are more efficient than those (e.g., Sea Rush) with high resistance to gravity-induced mass failure, in stabilising tidal creeks. (2) The retreat of the banks, in low energy environments, is associated with long-term processes, e.g., submergence/emergence cycles (together with low tidal currents).

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