Long-term evolution of sand waves in the Marsdiep inlet. I: High-resolution observations

A seven-year long data set from 1998 to 2005 of sand waves in the Marsdiep tidal inlet in the Netherlands is presented. The sand waves are visible on digital terrain models (DTMs) that were compiled from water depths obtained with a ferry-mounted acoustic Doppler current profiler. This multi-year data set allows to study the spatial and temporal evolution of sand-wave migration, height, length, asymmetry, and orientation. Horizontal migration rates were successfully obtained with a simple cross-correlation technique similar to techniques used in particle tracking. The study area can be divided in roughly two halves. In the southern half of the inlet, the sand waves are of the progressive type with area-mean heights of about 3 m and lengths of about 190 m. In the northern half, the sand waves are asymmetrical-trochoidal with area-mean heights of about 2 m and lengths of about 165 m. Across the inlet, the sand waves migrate in the flood direction with rates between 0 and 90 my(-1). Only the progressive waves in the southern half migrate in directions perpendicular to their crests. A striking observation is the seasonal variability in height and migration of the sand waves in the northern half of the inlet. The sand-wave heights are about 0.5 m higher in fall than in spring and the migration rates are about 30 my(-1) higher in winter than in summer. This paper only presents observations and analysis techniques. In a subsequent paper, the ADCP-current data will be applied to explain the spatial and seasonal variability of the sand waves. (C) 2008 Elsevier Ltd. All rights reserved.