Grain-size control of large compound flow-transverse bedforms in a tidal inlet of the Danish Wadden Sea

Abstract

Flow-transverse bedforms were recorded along a 2-km reach of the northernmost tidal inlet of the Danish Wadden Sea. The bed of the inlet channel is covered with large to very large compound dunes and superimposed medium-sized dunes composed of medium to coarse sands. The mean grain size of the bed material along the centre line of the inlet throat decreases from 0.56 mm in the inner part to 0.3 mm in the outer part. Both dune populations are essentially two-dimensional with straight to slightly curved crest lines. The ebb current is very similar in magnitude along the entire channel axis, whereas the flood current is of similar magnitude in the inner half, but reaches only about half of the maximum ebb current velocity in the outer section. Sediment transport occurs during both the ebb and flood periods, the latter being less vigorous and of shorter duration. As a result, the inlet as a whole is ebb-dominated (dominant ebb current: 1.01 m s −1; dominant flood current: 0.79 m s −1). In agreement with the tidal dynamics, a net ebb-oriented bedform migration rate of 32 m yr −1 was determined from the survey data. The large to very large dunes display distinct flood caps during the flooding tide, whereas the medium-sized dunes reverse their direction during each tide. The downstream dimensional evolution of the large dunes is complex, switching from initial growth to progressive decline, followed by renewed growth in length but progressive decline in height. This latter degradational phase begins in the middle of the survey reach and continues seawards until the large forms disappear completely. At the same time, the outer channel section becomes dominated by medium-sized dunes which grow in size with decreasing grain size. Surprisingly, there is no correlation between dune dimensions and water depth, as would be expected at such shallow depths. The complex evolution of dune dimensions as a function of grain size is discussed in terms of recent tidal bedform stability theory. It is clear, however, that more site-specific observations are required to fully explain the phenomenon.