Relationship Between Longshore and Cross-Shore Transport

Radiation stress principles are shown to control both longshore and cross-shore wave induced stresses which in turn determine the major processes controlling sediment transport. Recent cross-shore theoretical relationships are presented which define equilibrium beach slope as a function of wave height sediment characteristics and beach permeability and it is argued that similar principles will also govern the longshore sediment transport. A series of experiments are described in which the experimental design carefully preserves the same wave attack, but permits beach slope to be varied and longshore sediment transport to be measured. For a range of beach slopes and sediment sizes, the experiments show that total longshore transport increases with beach slope but only slightly with sediment size. The maximum local rate of longshore sediment flow increases both with beach slope and with sediment size. Simple theoretical relationships based on wave power principles are shown to be consistent with these measured sediment transport rates. The cross-shore transport and slope are therefore seen to play a major role in determining the longshore stress levels, the longshore velocity and the resulting intensity of longshore transport.