Equilibrium coastal profiles: I. Review and synthesis

Applicability of the coastal equilibrium concept depends upon proof of the existence of equilibrium. The present study demonstrates that, on the basis of the sediment continuity equation, three types of equilibrium are possible. Type I equilibrium requires that instantaneous sediment transport rates in both longshore and cross-shore directions are small, representing a final stage of erosion in response to natural processes. Type II equilibrium is reached if there are no variations in the net sediment transport rate in the longshore directions (i.e. zero cross-shore sediment transport). Such a situation occurs if the coastline is straight and there are no alongshore variations in hydrodynamic (i.e. wave and tidal) conditions. Type III equilibrium occurs when there are variations in longshore transport rates but the magnitude of instantaneous transport rate in the longshore direction is small compared with that in the cross-shore directions. In this case, the coastal profile is characterised by parallel advancement or retreat. Disequilibrium occurs if these conditions are not satisfied. Hence, prior to the selection of methods to determine the equilibrium coastal profile and the response time, the type of equilibrium must be identified.