Tidal Divides

Abstract

In this thesis the position of tidal divides is studied by means of a simplified model and the Dutch Wadden Sea. To study the parameters influencing the location of the tidal divide, two basins are schematized to a rectangular channel with at each side an incoming tidal wave. These two incoming waves can differ in amplitude and have phase difference. The tidal divide is shifted towards the channel end with the largest wave amplitude or the end where the wave enters last. If the phase difference and/or wave amplitude ratio between the two incoming waves is too large, there is no tidal divide at all. It turned out that a distinction between two cases can be made: a linear case with very low wave amplitudes and flow velocities, in which the bottom friction is neglected; and a nonlinear case with higher wave amplitudes and velocities resulting in a dominant role of the bottom friction. The limit of the phase difference for the Wadden Sea means that the barrier islands should have a certain length and/or that the basin should be relatively shallow. In the linear case, the difference in amplitude between the two waves is most important in determining the location of the tidal divide. The contribution of the phase difference is minimal. When bottom friction becomes dominant, the phase difference is governing the position of the tidal divide and the influence of the amplitude ratio is very small. The depth of the channel is an important parameter, because it influences the importance of the bottom friction. In the Wadden Sea, both the phase difference and the difference in tidal range in the inlets is contributing to an eastward location of the tidal divides. In this thesis, a distinction is made between a hydraulic tidal divide and a morphological tidal divide. A hydraulic tidal divide is the line splitting the basins in terms of drainage and can be defined as the position where the standard deviation has a minimum. Engineers are looking for a clear line, because then the hydraulic tidal divide can be used as a basin boundary. In reality there is no line, which makes it difficult and arbitrarily to define a line. This has to be kept in mind when analyzing hydraulic tidal divides. In the Wadden Sea, the morphological tidal divide has the shape of a spine. The centre line of this spine does not necessarily coincide with the highest bed level. The morphological tidal divide is the footprint of the hydraulic tidal divide on the bed and indicates where the hydraulic tidal divide is or has been. The height of the morphological tidal divide can tell something about the equilibrium of the basins. The hydraulic and morphological tidal divide can move due to changing circumstances, like closure of a part of the basin. The hydraulic tidal divide can move instantaneously, but its shift is bounded by the morphological tidal divide. The morphological tidal divide first has to be lowered by increased flow velocities and then the hydraulic tidal divide can move more freely. The morphological tidal divide then heightens again at the new location of the hydraulic tidal divide. This behaviour is illustrated by the tidal divide between Marsdiep and Vlie and shows the importance of the initial bathymetry for the evolution of tidal divides. If the circumstances change more gradually, as happens in the eastern Wadden Sea, the morphological can keep up with the movement of the hydraulic tidal divide more easily.