LONGSHORE TRANSPORT EVALUATIONS AT A DETACHED BREAKWATER

Abstract

A field experiment was conducted by the Coastal Engineering Research Center (CERC) to develop correlations between wave characteristics and longshore sediment transport. The waves were measured by two near-bottom mounted pressure transducers and the average longshore sediment transport rates were determined from sequential volumetric surveys behind an offshore breakwater which was regarded as a total trap. The data analyzed herein encompass a period of nine months during which a total accumulation of 675,000 m3 occurred as documented by eight surveys. Spectral analyses of the wave data were conducted and yielded one direction per frequency. The correlations include immersed weight sediment transport rate, I, versus (1) longshore component of wave energy flux at breaking, P&Sf and (2) the onshore flux of the longshore component of wave-induced momentum, S „. The most widely used correlation constant, K, in the relationship I = KPjig is 0,77. The best-fit values found from the data were K = 0.65 and 0.92 for linear and log best-fits, respectively, as based on the p£s values directed toward the trap. The corresponding values of KA (dimensional) relating I and Sxv are 4.98 m/s and 6.37 m/s, respectively. One feature of this type of trap is the potential for overtrapping if the waves are directed nearly normal to shore.