Tide-driven dune migration and sediment transport on an intertidal shoal in a shallow estuary in Devon, UK

摘要

The migration of subaqueous dunes was investigated over a 3-month period on an intertidal shoal near the mouth of the shallow mixed wave/tide dominated Avon Estuary on the Channel coast of southwest Britain. The dunes, characterised by heights and lengths of 0.05-0.2 m and 5-10 m, respectively, migrated 10-20 m over the 3-month period, representing an average volumetric sediment transport rate of 0.5 m~3 per unit meter width. Compared to equilibrium models of dune dimensions based on the water depth, the dune height was predicted well, but the dune length was under-predicted. Sediment transport on the intertidal shoal was controlled by a distinct tidal asymmetry, with maximum flow velocities during flood (0.4-0.5 m s~(-1)) significantly stronger than during ebb (0.2-0.3 m s~~(-1)). Predicted bedload transport based on theory underestimates the actual transport and this is attributed to the modest flow velocities encountered and, perhaps, the importance of suspended load transport. However, when empirically calibrated and related to tide range, the transport equation adequately accounts for observed rates. Using nearby tidal observations, the predicted annual volumetric sediment transport rate is 2.6 m~3 per unit meter width. This estimate compares favourably with the annual volumetric sediment transport based on dune measurements (2.0 m~3). Using the notion that the transport rate decreases on the intertidal shoal in the up-estuary direction to zero at its up-estuary end, a vertical accretion of 0.5 cm yr(-1) was predicted. The main implication of our study is that accurate and well-designed process measurements of dune dynamics and tidal currents can provide useful information on the longer term evolution of intertidal shoals in sandy, shallow (flood-dominant) estuaries.