ALBERT

Description: This study examines tide-dependent variations in the formation and dynamics of suspended sediment patterns coupled to mean flow and turbulence above asymmetric bed forms. In the Danish Knudedyb inlet, very large primary bed forms remain ebb-oriented during a tidal cycle while smaller superimposed bed forms reverse direction with each tidal phase. Hydroacoustic in situ observations reveal pronounced differences in suspended sediment transport patterns between tidal phases caused by the relative orientation of primary bed forms and the mean tidal flow and flow unsteadiness during a single tidal phase. When flow and primary bed form orientation are aligned, water-depth-scale macroturbulence develops in the bed form lee-sides in the presence of flow separation. Macroturbulent flow structures occur at high flow stages and are coupled to increased amounts of sediment in suspension. When flow and bed form orientation are opposed no evidence of flow separation associated with primary bed forms is found. Sediment-laden macroturbulence at high flow velocities is of a smaller scale and attributed to the superimposed secondary bed forms. The flow structures are advected along the primary bed form stoss-side (temporary hydraulic lee-side). The steep primary bed form lee-side (temporary hydraulic stoss-side) however, limits transport capabilities beyond the scale of primary bed forms.

Description: Stationary ship-based data were collected during a period of 19 h, beginning 2014-19-11 16:20:00 CET (UTC +1), in the tidal river part of the Ems estuary, Germany. The ship was moored next to the navigation channel at Jemgum (N53.271114° E07.397424°). Current velocity data was obtained by an ADCP (acoustic Doppler current profiler, RDI 600 kHz, ping rate 0.5 Hz, cell size 0.2 m, 8 pings per ensemble, mode 1), deployed on a floating platform next to the ship. Parts of each current velocity profile were located in a near-bed layer of very high suspended sediment concentration (SSC). These parts were, at times, biased by spikes. Respective parts were marked invalid. The time series of current velocity profiles was first averaged in time, averaging data in corresponding ADCP depth cells, collected during periods of 10 s. The resulting time series of profiles were then smoothed by an 8 min moving average filter. Averages were performed on individual current velocity components (east, north, up) in earth coordinates. Magnitude was subsequently determined from the smoothed components and flagged according to the tidal phase, flood negative. Vertical profiles of salinity and SSC were collected every 30 min by CTD (48M, Sea & Sun) and OBS (optical backscatter sensor, ViSolid 700, WTW, matrix type 2), both deployed on a crane. Optical backscatter was calibrated with respect to SSC by SSC measurements obtained from filtered water samples. Salinity and SSC profiles contain down-cast data only. An average time was assigned to each profile. Current velocity, SSC and salinity data data is referenced vertically to height above the river bed, and interpolated vertically in steps of 0.1 m.