WIND-WAVES AND MOVEABLE-BED BOTTOM FRICTION

Effects of moveable-bed bottom friction for wave observations and wave modeling are investigated using a state-of-the-art bottom friction mo del. This model combines the hydrodynamic friction model of Madsen et al. with a moveable-bed roughness model based on Grant and Madsen. Ana lyzing the present model for idealized swell cases, it is shown that s well might result in wave-generated sand ripples. The large change of roughness corresponding to initial ripple formation results in a prefe rred wave height for swell, related to bathymetric scales as generally occur in shelf seas away from the coast. The corresponding wave-gener ated bottom roughness is not defined by the local wave conditions, but is related to the overall energy balance of the wave field. Sediment data thus is imperative for the interpretation of observed decay rates and friction factors for swell. For idealized depth-limited wind seas , near-bottom wave motion is expected to generate partially washed-out ripples and moderate sheet-flow roughness. A comparison with previous models explains the apparent success of several models that do not ex plicitly account for moveable-bed effects. Such models, however, are n ot expected to reproduce the above preferred wave-height concept.