A.G. Davies, R.G. Wilkinson


This paper describes an experimental study of sand motion on the seabed caused by surface water waves. The observations were made close to a beach, but outside the breaker zone and in a location where steady currents were small. Measurements of water velocity components were made at various heights above both rippled and flat beds, together with measurements of the pressure gradients at the seabed, in order to examine the threshold of motion of the natural coarse sand ( © so ** 1.14mm). This motion was monitored with an underwater television system. The flow conditions were predominantly laminar and no flow separation occurred above the lee slopes of the ripples. Sediment motion was of 'bed load' type and was confined to the crests when the bed was rippled. This motion has been found to be caused by waves having an orbital velocity amplitude (measured in the free stream flow) of about one half of that required to cause motion on a flat bed of the same material. Wherever possible the threshold measurements have been compared with results obtained in the laboratory. The relative importance of forces on the seabed induced by velocity and pressure gradients has been assessed,, It appears that the latter effects are of little or no importance in situations of the kind described, as suggested elsewhere in the literature.


sediment motion; surface waves; wave induced transport

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