Bedform dynamics and net sediment transport paths over a flood-ebb tidal cycle in the Grådyb channel (Denmark), determined by high-resolution multi-beam echosounding

Abstract

High-resolution bathymetric surveys were carried out by means of a high-frequency (455 kHz) multibeam-echosounder (MBES) to determine the total volumetric sediment transport over a tidal period in a tidal channel at the Danish west coast. With the high repetition rate and high accuracy of the MBES, a simple comparison of morphological changes recorded at short time intervals allows an accurate calculation of dune heights and migration rates, the main parameters required for realistic approximations of volume budgets in tidal channels. The net sediment volume of 0.061 m3 per m2 over one tidal period is ebb-orientated, which means an annual net volume export of 44.5 m3 m−2 year−1. The general ebb-oriented export trend recorded for the large, compound dune system is contrasted by a net flood-oriented transport in the ‘active zone’ involving smaller superimposed dunes, which amounts to 0.6 m2 per flow phase on top of the large dunes, and 0.15 m2 per flow phase on the slopes and in the troughs. This gives a mean volumetric sediment transport rate of 0.7697×105 m2 s−1 for the ebb flow, and 1.059×105 m2 s−1 for the flood flow, strongly deviating from earlier calculated transport rates.