Abstract
It is observed that the length of barrier islands is inversely related to the tidal range. A physical explanation for this behavior is missing. In Chapter 2 it is studied whether a straight coastline can become unstable for small rhythmic undulations of the position of the coastline. This mimics the
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initial evolution of a straight coast into a coast with barrier islands. The results show that, for typical Dutch shelf conditions, a straight coast is unstable for undulations with a length scale smaller than 8 km. These undulations grow due to a feedback mechanism between the position of the coastline and the shore-parallel tidal currents. When also the influence of sediment transport due to waves only is taken into account, it is found that for increasing magnitude of the shore-parallel tidal currents the length scale of the perturbation which grows fastest, decreases. For increasing influence of the sediment transport due to waves only the length scale of the perturbation which grows fastest is increasing. The results compare favorably well with the observed trend in the length of the islands along the Dutch and German coast. Between two barrier islands a tidal inlet is found. This tidal inlet connects a single backbarrier basin with the coastal sea. In the inlet a deep ebb-dominated channel is located in which the tidal currents during ebb are stronger than during flood. At its seaward end a shallow area is present, the ebb-tidal delta. This delta is flanked by two flood-dominated. A main question is whether the hydrodynamics, sediment transport and the bathymetry in the area of the ebb-tidal delta are such that, averaged over several tidal periods, the bottom patterns are steady (morphodynamic equilibrium). In Chapter 3 it is shown that, by using an idealized model, the symmetric ebb-tidal delta (typically found along the east coast of the USA) is in morphodynamic equilibrium. The modeled bathymetry compare well with that of observed ebb-tidal deltas. Furthermore, the relation between the volume of sand in the ebb-tidal delta and the amount of water that is transported through the tidal inlet during one tidal cycle is recovered with the model. In Chapter 5 the results of Chapter 3 are validated by using a sophisticated numerical model based on the Delft3D software. The results of both models compare well. Furthermore, with the complex numerical model the sensitivity of the results to several processes which were not accounted for in the idealized model, are studied. These include the sensitivity of the results to higher harmonics of the tide, the use of a quadratic bottom shear-stress formulation instead of a linearized one and to wave parameters obtained with a realistic wave model (SWAN). In Chapter 4 the model is applied to the Dutch Wadden Sea. The model results recover the observed asymmetry of the ebb-tidal deltas. The area with ebb-dominated currents is found on the left-hand side of the mid-axis through the inlet (when viewing in seaward direction). Most sediment of the ebb-tidal delta is also located on the left-hand side of the mid-axis
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