Sand closures

Abstract

Dams are an essential part of the Dutch flood defence works to protect the hinterland from flooding by the sea. Sand closures are the construction of closure dams with sand as the main building material. The rising and falling tide generates a current through the closure gap, which increases in strength as the gap becomes smaller. This current is quite strong and transports a significant amount of sand from the gap to areas where the current is weaker. The sand that is transported away from the closure gap is called the sand loss. The changing tide and constantly moving sand grains result in a dynamic environment. This makes it difficult to estimate the total amount of sand that is necessary to close the dam. It is important to be able to assess the amount of sand loss accurately, because it could make the difference between needing an extra dredging vessel or not. The costs of these dredging vessels are about 1 million euro's a week; therefore, having an unnecessary vessel present due to very conservative sand loss predictions is costly. Laboratory experiments and a numerical model were used to investigate the morphology during bedload transport under a lateral expansion. A streamwise bar formed behind the tip of the lateral expansion for both the laboratory experiments and the numerical simulations. However, it was not possible to replicate the bedforms that occurred during the laboratory experiments with the numerical model. These bedforms are deemed to be important for the morphology during bedload transport. The numerical model was used to expand the research to suspended sediment transport by decreasing the grain size of the sediment. In the obtained morphological patterns, four areas are distinguished where the morphology was found to depend on the flow velocity and the degree of turbulence in the flow. It is expected that a variation of these patterns can be found during a sand closure, although these findings should be validated with a laboratory experiment or in situ measurements. Therefore, it is recommended to set up a laboratory experiment to investigate the morphology under a mixing layer for suspended sediment transport. Based on the results of the laboratory experiments and the numerical model, it is advised to look into the possibility to use the sand deposited as a streamwise bar during a sand closure as an additional borrow pit to aid the closure process, since it would lead to less sailing time for the dredging vessels. For the first stage of a sand closure, it is advised to use a two-dimensional model that incorporates a bedload transport formula to model the sediment transport. It is important to use a parametrization of the bedforms that occur during bedload transport. For the final stage of a sand closure, which is the most critical phase due to high flow velocities, high degree of turbulence and time restriction, it is advised to use a three-dimensional model with a pick-up equation that takes the influence of turbulence on the grains into account, such as the pick-up formula of Okayasu et al.