Tide-driven variations of sediment suspension over large bedforms in a tidal inlet channel

In the presence of bedforms, erosion and vertical mixing of suspended sediment is governed by the bedform-induced turbulent flow field. Subject to strong velocity gradients and regular reversion of the flow, analysis of suspended-load patterns in tidal inlets requires high-resolution in-situ observations of bedform morphology, turbulent flow field and suspended sediment variability on short time scales such as a tidal cycle. Aim of this study is to quantify the relative contribution of suspended load to the sedimentary budget of a bedform field in the high-energy environment of a tidal channel. Serving as a major transport pathway to and from the inlet, suspension processes within the channel are of vital importance for the overall inlet morphology. In the Danish Wadden Sea, ship-based in-situ measurements of combined acoustic Doppler current profiling (1200 and 300 kHz ADCP) and multibeam echosounding (MBES) were carried out along a longitudinal bedform transect and stationary above the bedform stoss-side and crest. Simultaneously, a multi-sensor probe equipped with laser in-situ scattering transmissometry (LISST) and conductivity, temperature, depth sensor (CTD) was lowered into the water column and coupled online with the ADCPs in the ViSea Plume Detection Toolbox (©Aquavison). Combining the acoustic and optical backscatter signals of these instruments enabled real-time detection of suspended sediment structures. Water samples were taken from within the structures by means of a water pump located on the multi-sensor probe. Surveyed ebb-directed bedforms were on average 6.5 m high and 200 m long (lee-side angles of 12-18 °) and located in approximately 15 m water depth. The tidal range comprised 1.8 m with asymmetric currents (max. ebb flow: 1.5 m/s and max. flood flow: 1.1 m/s) following a plateau-shaped signature of strong velocity gradients during acceleration and deceleration. The topographic forcing of the flow was consistently evident: flow acceleration occurred along the stoss-side and flow deceleration in the dune lee. High fluctuations of suspended sediment concentration (from 0.04 to 0.12 g/l) and diameter (median from 40 to 90 μm) characterize the water column during accelerating flow conditions. Maximum SSC values were observed in the dune lee and linked to distinct turbulent flow patterns; decelerated upward-directed flow structures immediately coupled with decelerated downward flow are believed to originate through the flow deceleration/expansion behind the bedform crest and erode sediment from the dune troughs. Within these structures a general increase of sediment concentration and grain size was visible throughout the acceleration period. Under maximum flow conditions fluctuations in concentration and grain size of sediments diminished, the homogenous mixture apparently being decoupled from the existence of macro-turbulent structures, which prevailed during the entire tidal cycle. Deceleration of the flow leaded to settling of the suspended sediment and accretion in the bedform field.