Implications of bedform dimensions for the prediction of local scour in tidal inlets: a case study from the southern North Sea

The bedforms and the local scour at the base of a cylindrical pile were studied in a tidal inlet in the Wadden Sea, southern North Sea, using high-resolution multibeam bathymetry data from four surveys. The observed changes in scour and bedform dimensions were interpreted in terms of hydraulic forcings varying periodically at different time scales. It appears that bedform orientation reacts to changing flow conditions on a semidiurnal basis, whereas bedform height and steepness reflect the spring-neap cycle as well as seasonal signals. The scour depth carries a strong overprint of the semidiurnal tidal cycle, which is at a maximum during the strongest tidal flow. Subtler variations in scour depth can possibly be attributed to the spring-neap tidal cycle. Based on these data on bedform and scour dimensions, correlation functions were established between scour depth and dune height as well as dune length. In measuring the scour depth under mobile bed conditions, establishing the seabed level based on the trough level of the bedforms nearest to the scour proved useful. These findings suggest that the dimensions of bedforms in dynamic equilibrium with prevailing hydraulic flow conditions can be used to estimate scouring in tidal environments.