Bedload transport in an inlet channel during a tidal cycle

Based on high-resolution swath bathymetry measurements at centimetre-scale precision conducted during a tidal cycle in a dune field in the Grädyb tidal inlet channel in the DanishWadden Sea, a simple tool to calculate bedload transport is presented. Bedload transport was related to simultaneous flow measurements using an acoustic Doppler current profiler. Spatially, bedload transport was higher on the dune crests than in the dune troughs of the large ebb-directed compound dunes, due to higher flow velocities at the crests than in the troughs. Temporally, bedload transport was higher in the troughs of the large ebb-directed compound dunes during the flood tide than during the ebb tide, due to higher near-bed flow velocities in the troughs of the compound dunes during the flood tide as compared to the ebb tide, resulting from flow exposure, i.e. the trough-sheltering effect of the compound dunes being active during the ebb tide, but negligible during the flood tide. Bedload transport was also predicted using five classical and widely used formulae. These predictors were all unable to depict the temporal variation in bedload transport during the tidal cycle. It is suggested that temporal variations in grain-size composition of the mobilised sediment should be taken into account by sediment transport formulae.