Erosion Prediction of Soft Cliff by Oblique Breaking Waves

Abstract:

Soft cliff (bluff) normally contains both cohesive and cohesionless sediments. The sediment mixture may have been consolidated in the geologic past. The consolidated sediment mixture may also exist below a thin layer of cohesionless sediment (e.g., sand) on the beach in front of soft cliff. The consolidated sediment mixture is eroded slowly and might be treated as a fixed structure for a short duration in comparison to the erosion process of dune that generally comprised of cohesionless sediment. Erosion of soft cliff is difficult to predict reliably without a predictive model that quantifies the consolidated sediment resistance against the hydrodynamic forces. The cross-shore numerical model CSHORE (Kobayashi 2016) calibrated many cases of dune erosion is extended to investigate the recession of soft cliffs using available wave basin data by Damgaard and Dong (2004). The measured cliff recession rates under oblique breaking waves for cliffs built of wet sand and a sand/clay mixture (90.8% sand by volume) can be reproduced by the numerical model CSHORE which is modified to account for sand loss associated with the alongshore gradient of longshore sand transport. The computed cliff recession rates depend on the incident wave angle and the alongshore gradient. An approximate method based on an equivalent alongshore distance is introduced to predict sand loss of a single cross-shore line. The effect of sediment cohesion on cliff erosion is examined using CSHORE that extended to cohesive sediment containing sand. For cohesive sediment with weak resistance against wave action, the cliff recession rate is limited by the rate of sand removal by longshore and cross-shore sand transport. The recession rate decreases when the resistance of the cliff material exceeds a critical value.