A CONCEPTUAL MODEL OF NON-EQUILIBRIUM BEDFORM EVOLUTION UNDER COMBINED WAVE AND TIDAL CURRENT FORCING

Observations of seafloor morphology under combined wave and current forcing were made at the mouth of the Delaware Bay, in order to develop a conceptual model of bedform evolution in hydrodynamically complex environments. Recorded properties include acoustic backscatter of the bed, directional wave spectra, current velocity through the water column, and near bed boundary layer flow including turbulence. The field area is characterized by significant wave and tidal current energy, which generally alternate in dominance on a semidiurnal time scale. Observed waves and currents both consistently produce bed shear stresses capable of initiating sediment transport and reshaping seafloor morphology. Under typical fair-weather conditions, wave energy is the dominant agent of transport during periods of slack tide, resulting in the formation of wave oriented linear transverse orbital ripples. As tidal current energy increases the ripples persist through the point of maximum velocity, become sinuous, and then rapidly evolve into amorphous hummocks. As current velocity falls, the hummocks persist through the point of minimum velocity until they rapidly evolve into lunate and then finally linear ripples. The morphologic state of the bed generally exhibits a temporal lag between the maximum energy of forcing, and the resultant bed response, during which time new bedforms begin to overprint persistent relict ones. The magnitude of this temporal lag generally scales with the total forcing energy imposed on the bed. For a majority of the time, the high frequency of alternation in the dominant energy source within the system results in an extremely dynamic bed that lacks sufficient time to attain morphologic equilibrium with any given hydrodynamic regime. During rare high-energy conditions, wave transport potential is consistently greater than current transport potential, which results in equilibrium mega-ripples and dunes. After the cessation of high-energy conditions, these features become dormant and persist for multiple tidal cycles, before the bed returns to a typical fair-weather behavior pattern. This conceptual model promises to contribute to an understanding of bedform evolution, which has previously been based of observations of equilibrium conditions under a single dominant source of transport forcing.