Bedform Morphodynamics Under Combined High-Energy Wave And Tidal Current Forcing

Observations of ripple morphology and evolution under combined high-energy wave and tidal current forcing over a range of conditions, including a tropical storm event, were recorded at the mouth of the Delaware Bay. Measurements of bed geometry were made with a rotating side-scan sonar and measurements of hydrodynamic conditions were made with an upward oriented acoustic Doppler profiler as well as a downward oriented high-resolution pulse coherent acoustic Doppler profiler. Observed flows were characterized by strong semidiurnal fluctuation with peak ebb and flood tidal magnitudes on the order of 1 m/s and peak wave orbital velocity magnitude on the order of 1.5 m/s. The relative orientation between median current and wave direction rotated though 180 degrees during each tidal cycle. Automated analysis of time-lapse bedform imagery yielded probability distributions of ripple wavelength and orientation as well as an estimate of the spatial density of ripple defect features. During periods of prolonged wave or current shear stress dominance, the orientation and wavelength of ripples generally agreed with equilibrium models for bed response to the more energetic forcing mechanism. More commonly, shear stress dominance alternated between waves and currents at a semi-diurnal frequency and lacked sufficient duration to generate equilibrium bedforms. During these periods the bed state lagged hydrodynamic forcing by a period that inversely scaled with total shear stress. Additionally, ripple defect density also fluctuated at a lagged semi-diurnal frequency, decreasing through intervals when the bed state was wave dominated and increasing through periods when it was current dominated. Here, the applicability of existing time-dependent ripple models is evaluated and a conceptual model for bedform evolution under combined wave-current forcing is presented.