Spatiotemporal height variations of large-scale bedforms in the Grådyb tidal inlet channel (Denmark): A case study on coastal system impact

In autumn 2004, the dredger Thor R sank in the Grådyb tidal inlet channel (Denmark). After recovery, the dredger had left a large scour hole in the channel, which subsequently acted as sink, being completely refilled in 2008. Since 2002, annual bathymetric surveys were carried out in the outer part of the channel. Based on these surveys, annual height variations of large compound bedforms were determined. In the period 2002-2006, bedform heights were practically stable, just like crest and trough elevations. On the contrary, bedform heights decreased continuously in the period 2006-2008, along with lowering of crests and accretion of troughs. From 2008 to 2009 the trend reversed with an increase in bedform heights, along with accretion of crests and lowering of troughs. We hypothesize that decreasing bedform heights in the period 2006-2008 was due to depletion of coarser bed material fractions in the channel caused by trapping of coarse material, supplied from the Pleistocene substrate in the inner part of the tidal basin, in the scour hole. Such a depletion of coarse material in the outer part of the channel would have resulted in crest fining, reducing the critical suspension velocity of the crest material, potentially leading to lowering of the crests. Potentially, this lead to a decreasing strength of the reverse flows within the recirculation cell in the lee of the bedforms, reducing reverse flow erosivity, and resulting in accretion of the troughs. With the scour hole refilled in 2008, these processes reversed; and the system was reestablishing.