Large-Scale Laboratory Measurements of Longshore Sediment Transport Under Spilling and Plunging Breakers

Total rates and cross-shore distribution of longshore sediment transport under predominantly spilling and plunging breakers were examined in the Large-scale Sediment Transport Facility (LSTF) at the U.S. Army Engineer Research and Development Center. The input waves were long-crested unidirectional irregular waves with broad spectra. Taking advantage of the new state-of-the-art LSTF, a suite of parameters including wave height, longshore current, longshore sediment flux, sediment concentration, and their cross-shore distribution patterns were precisely measured. The main objective of this study was to quantify the influences of different forms of wave breaking on rates and patterns of longshore sediment transport. A significantly greater total rate of longshore sediment transport was measured under the plunging breakers than under the spilling breakers with similar breaker height. The peak longshore transport rate was measured in the swash zone for the spilling breaker case. In the case of plunging breakers, a bi-modal distribution pattern was measured with one peak in the swash zone and one in the vicinity of the breaker line. Similar rates oflongshore transport were measured in the surf-bore dominated mid-surf zone for both cases. The suspended sediment concentration near the breaker line was approximately one order of magnitude greater under the plunging breakers than under the spilling breakers. Except in the inner surf zone, where faster current was measured during the plunging case, a similar cross-shore distribution of longshore current was measured for both plunging and spilling cases. Breaking type has significant influence on the magnitude and pattern of longshore transport. Parameters distinguishing dominant breaker types are important in improving the accuracy of longshore sediment transport predictions