On the local dynamics of currents in the estuarine Marsdiep basin

Abstract

The estuarine dynamics of the periodically-stratified Marsdiep basin, the westernmost part of the UNESCO world heritage site the Wadden Sea, are investigated through a broad range of observations and numerical model simulations. Spatial surveys and long-term point measurements of velocity, salinity, temperature, density and turbulence production and dissipation are complemented with a 1-D water column model (GOTM) and simulations of the western Dutch Wadden Sea (GETM/GOTM). This thesis aims at providing a better understanding of the factors and mechanisms that determine the vertical structure of velocity in the Marsdiep basin. The motivation for this research originates from the sustainable energy sector. The sustainable energy sector aims at utilizing tidal energy as an alternative for fossil energy and thereby contributing to a broadening of the potential of renewable energy sources. A test site for tidal energy extraction is planned in the Marsdiep basin. For tidal energy extraction to be feasible and profitable, an optimization strategy is required, which is achieved through, amongst others, a profound understanding of the hydrodynamics at the turbine site. The wide range of measurements provides detailed insight into the small-scale variability of currents in the Marsdiep basin. The horizontal and vertical density gradients modify the vertical structure of velocity in a variety of ways, which complicates the estuarine dynamics. The Marsdiep basin is an estuary where a wide variety of estuarine processes occur. Many of these processes are atypical for classical tidal straining estuaries. As a result, the estuarine circulation is characterized by a highly non-steady behavior as a result of the great temporal variability in current speed and in vertical stratification. The tidal current strength, bed friction and density gradients remain the most important underlying factors that drive the hydrodynamics in the periodically-stratified Marsdiep basin, despite the plentiful deviations from standard textbook estuaries.