Representation of non-erodible (hard) bottoms in beach profile change modeling

Non-erodible or "hard" bottoms are encountered on beaches along many coasts and are often considered a valuable environmental resource that must be pr otected. Hard bottoms can consist of natural materials such as limestone, c oral, shell, worm rock, sedimentary rock, and clay, as well as anthropogeni c materials such as rip rap. A hard bottom may be covered or uncovered by s and at various times during a storm, and it imposes a constraint on the san d 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. Pred ictions 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 modifie d SBEACH model performed well both qualitatively and quantitatively in repr oducing the resultant beach profile change in the presence of hard bottom f or both monochromatic and random waves. A "scour attenuation coefficient" w as introduced to limit unreasonable scour adjacent to vertical or near-vert ical side walls of a hard bottom. To numerically simulate the mid-scale phy sical model runs, a scaling analysis was performed to determine the appropr iate values of empirical coefficients in the numerical model. The dimension less fall speed parameter emerged as the scaling law governing storm-induce d beach profile change. Success in numerically simulating the beach-profile change measured in the mid-scale runs provides indirect evidence of the ap propriateness of the governing equations of SBEACH in representing the sali ent physics of storm-induced beach erosion.