SAND TRANSPORT BY WAVES

In coastal engineering practice frequently a distinction is made between two different modes of sand transport: - longshore transport - onshore-offshore transport From a theoretical point of view the longshore transport phenomenon is not as complicated as the onshore-offshore phenomenon. In the longshore mode of transport, the variations in time (wave period scale) are less important since the current velocity component in the longshore direction is nearly constant in time. In the onshoreoffshore direction both the time-variations of (orbital) velocities and sediment concentrations have to be considered in order to be able to compute the resulting net sediment transport. Since our quantitative knowledge of the time variations of the concentration is extremely poor at this moment, realistic calculations of the onshore-offshore transport, based on the actual physics involved, cannot be made. In many practical coastal engineering applications the onshore-offshore transports play an important role, however, and therefore a reliable description is urgently needed. Swart (1974;1976) presented an experimentally based computing method. The Coastal Engineering Group of the Delft University of Technology has been studying the onshoreoffshore transport phenomenon since 1968. Since the beginning of the investigation a rather experimental research method has been used also since the measuring devices lacked to measure real concentrations. The outcome of the present investigation, however, is rather surprising in some aspects. Bijker, Van Hijum and Vellinga (1976) reported on some preliminary results.