On-Offshore bed-load sediment transport in the coastal zone

Abstract

Laboratory data and theoretical derivations are presented for cross-shore bed-load transport. The laboratory investigations were carried out in a wave flume with movable bed (sand with D 50=0.022 cm). The tests have shown, inter alia, that bed-load transport in the surf zone is shoreward if single breaking occurs (i.e., on a uniform slope or at a single bar) and is accompanied by offshore transport seawards of the breaking point. Multiple transitions from offshore to onshore transport take place if the wave-breaking process repeats (on beds with more than one bar). The theoretical considerations are based on Bagnold's energetics approach. The sediment transport is assumed to be controlled primarily by asymmetric oscillatory motion of water superimposed on a steady return current, gravity, and the critical conditions of incipient sediment movement. The parameters intervening in the theoretical formulae have been identified. Good agreement between theory and experiments has been reached, in particular for short waves and beds with no more than one underwater bar. The formulae derived have been found to be acceptable for Ursell numbers S ⩽ 60.