L.B. Brakenhoff1*, M.G. Kleinhans1, B.G. Ruessink1, M. van der Vegt1
Ebb-tidal deltas are subtidal bodies of sand, located seaward of tidal inlets. They are affected by both waves and currents and contain a wide variety of bedforms. Since these bedforms influence bed roughness and sediment transport, it is important to understand their dynamics. This can help in improving predictions of sediment transport and hydraulic resistance used by models like Delft3D. Recently developed predictive formulas for bedform geometry have not yet been tested for the complex hydrodynamic conditions of ebb-tidal deltas. The present study analyses the spatio-temporal behaviour of small-scale bedforms on an ebb-tidal delta and relates these to the hydrodynamic forcing.
In September and October 2017 four frames were installed on the Ameland ebb-tidal delta, which measured amongst others wave heights, current speeds and bedforms. The bedforms were measured hourly on a small spatial scale of 2x2 m with a horizontal resolution of 1 cm by a 3D profiling SONAR. Grain sizes near the frames were determined through box core samples.
Figure 1A shows the bedforms at 6.5 m water depth on the outer shoal for one moment in time. The grain size near this frame was 185.8 μm. The bedforms shown in Figure 1A are highly three-dimensional, indicating the combined influence of both waves and currents. At this moment in time, the wave- and current-related Shields parameters were approximately the same (θw = 0.06 and θc =0.05; red dot in Figure 1B). The associated bedform classification is ‘mixed wave-current ripples’.
Figure 1B also shows predicted bedform types as a function of the wave- and current- related Shields parameters for all other moments during the measurement campaign on the same location. It is visible that most of the time, both waves and currents were important here, although waves are a little more dominant. The associated ripple types are mixed wave-current ripples and hummocks (Kleinhans, 2005). All bedform patterns measured by the four Sonars through time will be classified, in order to find a relation between waves, currents and bedform types.
Figure 1. A: typical example of bedforms as measured on the outer delta shoal. B: Nondimensional wave- (θw) and current- (θc) related Shields parameters throughout the measurement period and predicted bedform types. Red dot indicates the moment visualized on the left. Red lines indicate transition between wave-, wave-current, and current-dominated ripples. Black lines indicate thresholds for ripples vs flat bed and sheetflow. (Lines reproduced after Kleinhans, 2005)
Kleinhans, M.G. 2005. Phase diagrams of bed states in steady, unsteady, oscillatory and mixed flows. EU-Sandpit end-book, Ed. Leo van Rijn, Aqua Publications, The Netherlands, paper Q