Wind influence on tidal flat sediment dynamics: Field investigations in the Ho Bugt, Danish Wadden Sea

Sediment dynamics on a mixed microtidal flat in a sheltered backbarrier position are shown to vary strongly on short term, tidal, and monthly timescales. Bed level changes were ± 1 mm per tidal period in the absence of winds. Erosion of 10 to 20 mm within a few hours was possible during high combined current and wave shear stress. Mean grain size in the upper 1 cm bed sediment varied from 34 to 155 μm and was consistently coarser than the mean size of trapped suspended matter (7–10 μm). Threshold for resuspension was low at 0.03 N m− 2. No clear correlation was found between resuspension activity and suspended matter concentrations as these concentrations also depend on current induced advection. Due to tidal asymmetry, the net suspended sediment flux was flood directed. On average it was 14 kg m−2 per tidal period in the summer and 3 times higher during November when wave activity generally was stronger. Bed micro topography, was followed on digital photos taken every 1.5 h during the two month study period, showed that (1) ripples started to develop when wave shear stress exceeded 0.15 N m− 2, and (2) their wavelength varied with shear stress.