Measurement of Longshore Sediment Transport Rates in the Surf Zone on Galveston Island, Texas

Abstract

Longshore sediment transport in the surf zone on Galveston Island, Texas, was studied to develop a new technique involving optical instruments rapidly calibrated in situ and to compare measured transport rates with those predicted by the well-known Coastal Engineering Research Center (CERC) formula. This method used an instrumented sled equipped with a LISST-100 for particle size distribution determination, four optical backscatter sensors (OBSs) for turbidity measurements, and two velocity sensors for longshore current measurements. The sled was pulled across the surf zone, occupying 10 to 15 stations spaced about 10 m apart, for approximately 3 minutes each.The OBS data were calibrated with the LISST-100 particle size distribution data, thereby overcoming the difficulties associated with the use of these sensors in the presence of a mix of sand and fine particles. Subsequently, these data were fit to a logarithmic profile to determine the average vertical distribution of suspended sand concentration, assuming that the fine particles were vertically well mixed at each station. A logarithmic profile of average longshore current was also computed based on the measured velocity data. The longshore sediment transport rate was calculated as the spatial integral of the product of suspended sand concentration and velocity and related to the wave conditions at the point of breaking. Measured rates ranged from 86,000 to 231,000 m3/y, and transport was found to be greatest in the vicinity of the sand bars. The popular CERC formula gave sediment transport rates significantly greater than the observed values, with the difference between the two on the order of 100%. The average CERC transport coefficient, K1, computed from our measurements was determined to be 0.19 ± 0.12.