PREDICTING THE TEMPORAL AND SPATIAL EVOLUTION OF REGULAR AND IRREGULAR RIPPLE GEOMETRY UNDER VARYING HYDRODYNAMIC FORCING

Ripples are undulating features commonly present on the seabed in shallow coastal environments and rivers due to the actions of waves and currents. These features increase the hydraulic roughness of the seabed which leads to an increase in wave energy dissipation and enhanced resuspension of sediments. Recent studies have shown that changes in the ripple’s orientation in relation to the mean current can alter the bed roughness, turbulence and associated sediment transport rates. Wave-generated ripple predictors assume ripples developed at an orientation perpendicular to that of wave propagation and the response of ripple orientation to changes in wave direction has been assumed to be instantaneous, thus ignoring the formation of secondary ripples and delays due to the time required for the bed to respond.

This study presents a 2-D, time dependent ripple model which allows for the prediction of ripple wavelength, height and orientation under varying wave conditions. The model accounts for the bed response time and avoids the assumption that ripples are always in alignment with the wave forcing thus enabling the prediction of multiple ripple trains. The ripple irregularity is further determined through two parameters defined as the normalized wavelength and orientation spectral width. This model’s predictions are compared to three field studies conducted along the coasts of North Carolina, South Carolina and Georgia and show an improved prediction of ripple dimensions during periods of active transition and relict geometry. As the time dependent model uses an equilibrium ripple predictor as the target geometry, a new equilibrium model is derived from ripple measurements published in existing field and laboratory experiments as well as the current field studies.