Field Measurement and Modelling of Scour Pit Dynamics in a Sandy Estuary

Field observations and model simulations are presented of the tidal flow over a scour hole at the mouth of the Dyfi Estuary, mid-Wales, U.K. This scour hole is maintained by locally high flow speeds resulting from convergence, and then divergence, of the flow over the hole. Fixed features of the local bathymetry may also have served to stabilise its location (over decades). Model simulations of steady flow over a variety of schematised pits of different elliptical shape on an otherwise flat bed are presented to show that shallow elongated pits can (almost) preserve the upstream flow speed at their centre point, making them potentially selfmaintaining. Deeper pits reduce the flow speed and so will infill. INTRODUCTION A feature of many estuaries and channels is the occurrence of natural pits and scour holes. These fixed features often persist for decades in the erodible seabed sediment. The present paper highlights the dynamics one such scour hole in the mouth of the Dyfi Estuary in the middle of Cardigan Bay, mid-Wales, U.K. Here field observations were made as part of a model validation exercise aimed at demonstrating the ebb-dominant nature of the tidal flow in the Dyfi Estuary. This ebb dominance arises from the extensive areas of drying banks at low tide, together with the discrete channels, that characterise the interior of this predominantly sandy Estuary. The field observations are complemented by modelling work both of the tidal flow in the estuary, here with main emphasis on the area of the scour hole, and also of the flow above an idealized pit that is used to highlight the processes affecting scour-hole dynamics. Coastal Sediments '07 © 2007 ASCE