Groyne field nourishments

Abstract

In the Waal bed degradation occurs, which is mainly induced by a large number of river regulations measures done in the past. Rijkswaterstaat develops possible measures and new techniques to stop riverbed erosion and/or mitigate the negative effects. One of the possible new techniques to reduce riverbed erosion locally is groyne field nourishments. The objective for this research is to deepen insights how groyne field nourishments provide enough sediment to the river such that bed degradation locally does not develop further. The following research question is formulated for this: How can groyne field nourishments act as small feeder nourishment that release sediment slowly to the main channel? To answer the research question a literature study is done and a XBeach model is set-up. The literature study is done to obtain knowledge about the hydrodynamic and morphodynamic processes in a groyne field in case of emerged and submerged groynes. From this literature study it appears that the effect of navigation on the morphology in a groyne field is dominant over the effect of the river discharge when the groynes are emerged: net sediment is transported from the groyne field to the main channel mainly by suction. When the flow conditions increase and the groyne get submerged, the effect of navigation on the morphology in the groyne field decreases rapidly and the net sediment transport is from the main channel to the groyne field induced by the river discharge, mainly the large-scale eddies. Extra literature study is done to possible relevant coastal and river processes to get knowledge of the effects of waves and currents on the behaviour of nourishments in a groyne field since very limited research has been done before on groyne field nourishments. It appears that especially dune erosion and river bank erosion by waves and currents can be used to explain how nourishments erode by the primary and secondary ship wave system. After the literature study a XBeach model is set-up of for a series of emerged groynes on a straight section in the Waal. The model is calibrated and validated with data from Rijkswaterstaat and literature. In this model three possible nourishments with the same volume are applied at different locations in the groyne field: 1) one upstream in the groyne field; 2) One downstream in the groyne field; 3) And one in the middle of the groyne field. From this research it can be concluded that groyne field nourishment at all three locations can work as small feeder nourishment that release sediment slowly to the main channel in the case of emerged groynes for about 200 days per year. The nourishment placed in the centre of the groyne field appears to be the most effective nourishment, since 1) the waves and currents induced by navigation attack the nourishment along all its edges, 2) the centre nourishment causes in almost the whole groyne field an increase of sediment transport to the main channel and 3) the flow pattern in the groyne field is changed favourably for the sediment transport of already suspended sediment to the main channel. This research should be seen as first indication if nourishment could work at all. It is advised to research the material of the nourishment and to simulate the groyne field nourishments for a longer period, such that more flow conditions and more ship passage are included, to get a more complete picture of how groyne field nourishments act as small feeder nourishments. Furthermore a practical test with a centre nourishment is recommended to collected data of the hydro- and morphodynamics in a groyne field with nourishment.