Evaluation of the Cerc Formula Using Large-scale Model Data

Longshore transport experiments were conducted in a large-scale physical model to evaluate predictions of the CERC formula with measured longshore sediment transport rates. It was found that the CERC formula produced reasonable estimates if the coefficient K was calibrated and applied to waves with similar breaker type. The calibrated K values are much smaller than values that are commonly used, and there appears to be a strong dependency of transport rate on breaker type. Additional comparisons were made with the formula proposed by Kamphuis (1991). The Kamphuis equation, which includes wave period, a factor that influences breaking, gave good estimates. Examination of the cross-shore distribution of longshore sediment transport indicates that there are three distinct zones of transport: the incipient breaker zone, the inner surf zone, and the swash zone, with each zone contributing a different fraction to the total transport rate. Transport in the incipient breaker zone was influenced by breaker type, transport in the inner surf zone was controlled by depth, and transport in the swash zone showed dependencies on wave height and, in particular, wave period. Swash zone transport was found to have a significant contribution to the total transport rate. INTRODUCTION Total longshore sediment transport rate and its cross-shore distribution in the surf zone are essential to many coastal engineering and science studies. Practical engineering applications such as beach response in the vicinity of coastal structures, beach-fill evolution and re-nourishment requirements, and sedimentation rates in navigation channels all require accurate predictions of longshore sediment transport rates. To maintain navigable waterways along U.S. coasts, the U.S. Army Corps of Engineers regularly applies analytical and numerical models to estimate the total longshore sediment transport rate and its cross-shore distribution in the surf zone. Present predictive tools have been developed based primarily on field studies; however, obtaining high-quality data in the field is difficult (Wang and Kraus, 1999). Arguably the most widely used model for estimating total longshore sediment transport rate is the “CERC” formula (Shore Protection Manual, 1984). The model, based on the assumption that the total longshore sediment transport rate is proportional to longshore energy flux, is given as: