Predicting the behavior of nearshore feeder berms in the vicinity of Morro Bay, California

Abstract

Being able to predict the final disposition of dredged material mounds is important in the planning of dredging operations. A computerized mathematical model, based on the sediment movement equations of Ackers and White, has been developed by the US Army Corps of Engineers. The model, called Long Term FATE (LTFATE), is a useful tool for making such predictions. The primary objective of this study is to compare the output from LTFATE with the actual movement of dredged material placement mounds placed in the vicinity of Morro Bay, CA in 1990. The secondary objective is to determine a mound shape that would provide the greatest benefit as a feeder berm, causing accretion on the nearby beach. The channel to the harbor at Morro Bay must be dredged on a regular basis, and the dredged material has historically been placed in a high-energy, nearshore area approximately 3 km (1.9 m) from the channel, and 200 m (650 ft) offshore of the MLW line. LTFATE was calibrated by being applied to two geometrically regularly shaped mounds: a cone, 200 m (660 ft) in diameter, and a log-shaped berm, 200 m (660 ft) x 400 m (1310 ft). These mounds were placed on a flat seabed. The model inputs were environmental variables measured during a study at Morro Bay in 1990. The Advanced Ocean Circulation Model ADCIRC was used to generate the wave heights and water levels. The model was then applied to the bathymetry of the mounds placed at Morro Bay, and the output compared to that measured at the site at the end of a five-month study. The mound movement predicted by the model differs from that shown by the final bathymetric study at Morro Bay, both in change of mound height and displacement of mound center of mass. Several reasons for these differences are given in the study. It is also shown that of the three mound configurations, for a given set of parameters, the log-shaped berm has the greatest horizontal displacement, indicating that if placed normal to the prevailing current and the shoreline, it would provide the greatest amount of sediment to accrete an adjacent beach.