Morphological controls in sandy estuaries the inf

Authors: Peter Edward Robins Alan G Davies

Publish Date: 2010/03/16

Volume: 60, Issue: 3, Pages: 503-517

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Abstract

The morphodynamics of shallow vertically wellmixed estuaries characterised by tidal flats and deeper channels have been investigated This paper examines what contributes to flood/ebbdominant sediment transport in localised regions through a 2D model study using the TELEMAC modelling system The Dyfi Estuary in Wales UK has been used as a case study and together with idealised estuary shapes shows that shallow water depths lead to flood dominance in the inner estuary whilst tidal flats and deep channels cause ebb dominance in the outer estuary For medium sands and with an artificially ‘flattened’ bathymetry ie no tidal flats the net sediment transport switches from ebbdominant to flooddominant where the parameter a/h local tidal amplitude ÷ local tidally averaged water depth exceeds 12 Sea level rise will reduce this critical value of a/h and also reduce the ebbdirected sediment transport significantly leading to a flooddominated estuarine system A similar pattern albeit with greater transport was simulated with tidal flats included and also with a reduced grain size This suggests that analogous classifications for flood/ebb asymmetry of the tide in estuaries as a whole may not represent the local sediment transport in sufficient detail Through the Dyfi simulations the above criterion involving a/h is shown to be complicated further by augmented flow past a spit at the estuary mouth which gives rise to a selfmaintaining scour hole Simulations of one year of bed evolution in an idealised flatbottomed estuary including tidal flow past a spit recreate the flood/ebb dominance on either side of the spit and the formation of a scour hole in between The erosion rate at the centre of the hole is reduced as the hole deepens suggesting the establishment of a selfmaintaining equilibrium stateThe authors would like to thank JM Brown at the Proudman Oceanographic Laboratory POL Liverpool for data collection and assistance with TELEMAC Further field data were provided by Aberystwyth University through the Centre for Catchment and Coastal Research CCCR POLCOMS model data were provided by SP Neill at Bangor University The project was partfunded by the Countryside Council for Wales CCW

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