Long-term evolution of sand waves in the Marsdiep inlet. II: Relation to hydrodynamics

Abstract

A discussion is presented about the mechanisms that govern the spatial and seasonal variability in
sand-wave height and migration speed in the 4 km wide Marsdiep tidal inlet, the Netherlands. Since
1998, current velocities and water depths have been recorded with an ADCP that is mounted under the
ferry ‘Schulpengat’. In this paper, the current measurements were used to explain the sand-wave
observations presented in Buijsman and Ridderinkhof [this issue. Long-term evolution of sand waves in
the Marsdiep inlet. I: high-resolution observations. Continental Shelf Research, doi:10.1016/
j.csr.2007.10.011]. Across nearly the entire inlet, the sand waves migrate in the flood direction. In the
flood-dominated southern part of the inlet, the ‘measured’ (i.e. based on sand-wave shape and
migration speed) and predicted bedload transport agree in direction, magnitude, and trends, whereas in
the ebb-dominated northern part the predicted bedload and suspended load transport is opposite to the
sand-wave migration. In the southern part, 55% of the bedload transport is due to tidal asymmetries and
45% due to residual currents. In addition to the well-known tidal asymmetries, asymmetries that arise
from the interaction of M2 and its overtides with S2 and its compound tides are also important. It is
hypothesised that in the northern part of the inlet the advection of suspended sand and lag effects
govern the sand-wave migration. The relative importance of suspended load transport also explains
why the sand waves have smaller lee-slope angles, are smaller, more rounded, and more
three-dimensional in the northern half of the inlet. The sand waves in this part of the inlet feature
the largest seasonal variability in height and migration speed. This seasonal variability may be
attributed to the tides or a seasonal fluctuation in fall velocity. In both cases sediment transport is
enhanced in winter, increasing sand-wave migration and decreasing sand-wave height. The influence of
storms and estuarine circulation on the sand-wave variability is negligible.

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