Modeling bottom roughness in the presence of wave-generated ripples

Abstract

Near-bottom wave, current and microtopography measurements have been obtained at the LEO-15 site off the southern coast of New Jersey. The measurements are used in conjunction with a recently calibrated continental shelf bottom boundary layer model to refine estimates of bottom roughness for a rippled bed under combined wave and current flows. In the presence of nearly two-dimensional wave-generated ripples, application of the Grant and Madsen [1979] type bottom boundary layer model produces estimates of the bottom roughness felt by the current that are several times greater than established formulations for rippled beds under waves. Modifying one of the primary length scales in the eddy viscosity profiles returns estimates that are more consistent with the wave roughness results. Application of the roughness model in conjunction with a refined empirical ripple geometry model produces highly accurate estimates of measured shear velocities and apparent roughnesses during a moderate storm. The demonstrated accuracy of this simple combined roughness and ripple geometry formulation reveals its utility when considering applications of coupled bottom boundary layer-shelf circulation models.