Representation of Non-Erodible (Hard) Bottoms in Beach Profile Change Modeling

Abstract

Non-erodible or "hard" bottoms are encountered on beaches along many coasts and are often considered a valuable environmental resource that must be protected. Hard bottoms can consist of natural materials such as limestone, coral, shell, worm rock, sedimentary rock, and clay, as well as anthropogenic materials such as rip rap. A hard bottom may be covered or uncovered by sand at various times during a storm, and it imposes a constraint on the sand transport rate. In this study, the SBEACH numerical model was modified to allow calculation of the response to storm waves and change in water level of a sand beach profile with arbitrary configurations of hard bottom. Predictions of the model were compared with one data set from a large wave tank and with several data sets from mid-scale physical model runs. The modified SBEACH model performed well both qualitatively and quantitatively in reproducing the resultant beach profile change in the presence of hard bottom for both monochromatic and random waves. A "scour attenuation coefficient" was introduced to limit unreasonable scour adjacent to vertical or near-vertical side walls of a hard bottom. To numerically simulate the mid-scale physical model runs, a scaling analysis was performed to determine the appropriate values of empirical coefficients in the numerical model. The dimensionless fall speed parameter emerged as the scaling law governing storm-induced beach profile change. Success in numerically simulating the beach-profile change measured in the mid-scale runs provides indirect evidence of the appropriateness of the governing equations of SBEACH in representing the salient physics of storm-induced beach erosion.