Ferry-observed variability of currents and bedforms in the Marsdiep inlet

This thesis presents analyses of unique long-term observations of currents and bedforms obtained with ferry and vessel-mounted acoustic Doppler current profilers (ADCPs) in the Marsdiep tidal inlet, the Netherlands. The general objectives are to determine the influence of tides, density differences, and wind on water transport and currents and explain their relation to sediment transport and bedform migration. In cooperation with the ferry company ‘Texels Eigen Stoomboot Onderneming’ (TESO), the Royal Netherlands Institute for Sea Research (NIOZ) mounted in 1998 an ADCP under the hull of the ferry ‘Schulpengat’, which traverses the inlet up to 32 times per day. The data collected with this ADCP between early 1998 and the end of 2005 are the centrepiece of this thesis. The application of a least squares harmonic analysis in Chapter 2 illustrates that tides explain up to 98% of the variance in the instantaneous water transport and streamwise currents. Moreover, tides also govern the horizontal residual circulation with floodward flow in the southern half of the inlet and ebbward flow in the northern half. It becomes apparent in Chapter 3 that the tides have a smaller influence on the instantaneous transverse, or secondary, currents. In periods with little freshwater discharge from the Lake IJssel sluices, the secondary currents of O(0.01 ms-1) are mainly due to channel curvature. In periods with large freshwater discharge, the secondary currents of O(0.1 ms-1) are largely affected by horizontal transverse and vertical density gradients that are transient within, but recurrent for each tidal cycle. In Chapter 4 a simple analytical model of the connected Marsdiep and Vlie basins is applied to study the subtidal water transport. The model shows that tidal stresses have an important contribution to the time-mean throughflow from the Vlie to the Marsdiep basin, whereas the variability in the subtidal water transport is primarily due to southwesterly and northwesterly winds. Digital terrain models, based on the water depths measured with the ADCP, reveal in Chapters 5 and 6 sand waves with lengths of O(100 m), heights of O(1 m), and floodward migration speeds of O(10 ms-1). In the northern half of the inlet, the height and migration speed feature a seasonal variability of about 0.5 m and 30 my-1, respectively. In the southern half of the inlet, the sand-wave migration agrees with predictions of tidally-driven bedload transport. It is argued that suspended load transport governs the sand-wave migration and seasonal variability in the northern half of the inlet.