SMALL-SCALE MORPHOLOGY RELATED TO WAVE AND CURRENT PARAMETERS OVER A BARRED BEACH

Abstract

Small-scale morphology (< 5 m horizontal wavelength) was measured by combining CRAB surveys with bed elevations acquired from a 1 MHZ sonic altimeter mounted on the CRAB during the DUCK94 experiment in October 1994. Bedform plan views were recorded simultaneously using a 500 KHz side-scan sonar mounted on the CRAB. Waves and currents were measured at the same time. Three cases are examined in detail: mild waves and weak longshore currents resulting in wave ripples throughout the study area; narrow-band, normally incident waves with a strong rip current resulting in a planar bed except in the throat of the rip where megaripples were measured; and storm waves with strong longshore currents resulting in lunate and straight-crested mega-ripples in the trough of the barred beach. During these strong current days, bed shear stresses calculated from logarithmic velocity profiles are equated to a quadratic bottom shear stress formulation. The associated bed shear stress coefficients vary by more than an order of magnitude across the surf zone (0.0006- 0.012). Bottom roughness was obtained by calculating the wavenumber spectra of the bed. The bed shear stress coefficients are positively correlated with bottom roughness (linear correlation coefficient, 0.68). A higher linear correlation coefficient (0.80) is obtained by subtracting skin friction from the total bed shear stress.