Erosion during a sand closure

Erosion during a sand closure: A literature study

Huppes, N.

Labeur, R.J. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2016-10-06

Closure dams are structures that are used to close off basins, usually from the sea. A sand closure is the construction of a closure dam made out of sand. During the last stage of a sand closure, there is a closure gap through which the tide forces a current with flow velocities up to five metres per second. The flow pattern that occurs during a sand closure is three-dimensional. Furthermore, the sediment transport does not reach an equilibrium due to the constantly changing flow conditions and gap geometry. During sand closures, large amounts of erosion occur. Because the main costs of a sand closure lie in the transportation of sand to the building site, it is important to know how much sand is needed for the sand closure and how much sand is loss during the closing process. The quantification of the sand loss still requires attention. The currently used models require calibration factors, which are not known beforehand, to give a reasonable prediction of the sand loss. A model that can be applied to any sand closure and does not require calibration factors is desirable. To achieve this, the different processes that occur during sand closures should be parameterized and need to be quantifiable by a model. In this study the important processes of a sand closure are identified. With the results of this study, the sand loss prediction can be improved by considering the processes that are expected to have the largest contributions to the sand loss. It was found that the amount of turbulence in the flow has a large impact on the erosion. There are two mechanisms that appear to have the largest impact on the erosion during a sand closure. The first mechanism is that during a sand closure, mixing layers originate from the heads of the dams. These have higher turbulence intensities than the main current, which causes strong vertical mixing, so the sediment particles are brought up higher into the water column. It was found that possibly, these higher turbulence intensities also lead to pick-up rates orders of magnitude larger than the ones predicted by the current pick-up equations, although it is not known if extrapolating the formula of the TKE method, designed for river flow, to higher turbulence intensities is justified. Because the magnitude of the turbulence intensities seems to have a large influence on the predicted erosion velocity, it may be important for the total erosion at the closure gap. The second mechanism is that the flow is contracted downstream of the closure gap. This increases the flow velocities of the main current. Unfortunately, it is unknown what the impact of these higher flow velocities is on the turbulence at the origin of the mixing layers. It is desirable to be able to model the important mechanisms that occur during sand closures in a numerical model. The contraction of the flow behind the closure gap is already modelled by most two-dimensional and three-dimensional numerical models. It is advised to look further into the pick-up rates of sediment at higher turbulence intensities. Since there is no satisfactory formulation for the pick-up of sediment that can be implemented into a numerical model directly and the erosion at the origin of mixing layers is found to be most influential, it is recommended to carry out a physical scale model to quantify the pick-up of sediments at the origin of a mixing layer. This will be the subject of a forthcoming MSc thesis.

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(c) 2016 Huppes, N.