Mechanics of Sediment Transport by the Combination of Waves and Current

Report (1992)

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Copyright: © 1992 Bijker, E.W.
Waves Combination Sediment Transport Longshore Current ICCE 1992
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Published Date

01-10-1992

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Design and Reliability of Coastal Structures, short course during the 23rd ICCE in Venice

Abstract

The motivation to develop a transport formulation for the combination of waves and current has been the necessity to model longshore transport passing tidal in- and outlets. The generally applied C.E.R.C. formula for the longshore transport is not applicable here because that formula relates the longshore transport to the longshore current caused by the breaking waves, without using this current explicitely. For this reason a formulation was looked for, which related the sediment transport to the current taking the wave influence into consideration. It is explicitely assumed that the waves itself do not transport material. As long as the direction of the wave propagation is not close to the current direction this assumption gives no problems. The basic concept is that waves stir up the sediment and the current transports it. Therefore any transport formulation which allows the introduction of this effect adequately, will do. The found formula is a very rough approximation of the reality: 1. It is not correct to average over tau_cw for the computation of the exponential stirring-up term. The averaging should have taken place over the total exponential function. 2. It is assumed that the turbulent diffusion coefficient for the waves is equal to that for the fluid. This is most likely not true. 3. It is not certain whether the increased turbulence near the bed spreads indeed over the total height of the fluid so that for the concentration distribution the increased value of z* should be used. Tests indicate that this is possibly not entirely true. 4. The ripple factor is calculated as the ratio between the shear stress caused by only the grains and the total shear stress. The same value is used in the case for a combination of waves and currents. It should have been better to use for both values those increased by the waves. The influence will be, however, rather limited. 5. The formula is derived for a current as main transporting agency, with waves only as an increasing factor. As soon as the orbital motion at the bed becomes almost equal to the current near the bed the results should be regarded with some suspicion.