Surfzone bedform migration and sediment flux implications to large scale morphologic evolution

Abstract

Field observations of small scale seabed morphology were obtained over 4 weeks at two locations separated 66 m along a cross-shore transect during the 2014 MEGAPEX Experiment conducted as part of the longer term Sand Engine mega-nourishment project along the North Sea Coast of The Netherlands. The seafloor was continuously covered by dynamic bedforms with amplitudes ranging 0.02–0.40 m and wavelengths ranging 0.20–2.5 m. Ripple migration rates were up to 3.6 m/h, dependent on the energy of the waves and currents. Under the assumption of bedload dominant transport, cross-shore and alongshore sediment volume flux by ripples was estimated from observations at the spatially separated imaging locations. The average and maximum ripple sediment volume flux was found to be 0.22 and 1.7 m³/m/day, respectively, with larger fluxes during spring flood tides and storm wave conditions. The daily averaged fluxes were usually oriented about 30° north of shore-normal, moving in the same direction as a nearby transverse sandbar migration direction. Estimated gradients in the sediment flux within the surfzone were computed from bed level change measurements of the inner surfzone including a larger scale transverse sandbar measured from subsequent jetski surveys. We find that the estimated gradients in surfzone sediment flux are conceivably driven by small variations in the sediment flux driven by sand ripple migration, supported by our observations of ripple driven sediment flux at the two ripple imaging stations. A simple conceptual model is presented that shows how small scale bedforms may contribute to the growth and decay of larger scale bathymetric features, such as sandbars. Results suggest that sediment flux by small scale sand ripples and megaripples could significantly contribute to larger scale morphologic development in the surfzone.