Morphodynamic responds of groyne fields to the lowering of crest level of the groynes in the Waal River, The Netherlands

The lowering of the crest of the groynes in the River Waal is one of the measures being applied in the Netherlands to reduce the water levels at high water conditions. A total of 750 groynes will be lowered lengthwise by 1 to 2 meters. An expected side-effect is sedimentation in the main channel, due to redistribution of the discharge in the cross-section. Also, the flow through the groyne fields will increase, so erosion in the groyne field is to be expected. In order to get insight in the morphological developments after lowering the groynes, a pilot project has been conducted. It consisted of the lowering of the 70 groynes from April 2009 till September 2009. A monitoring program was implemented, including multi beam measurements of the groyne fields and the main channel. The measured groyne fields included river sections with and without lowered groynes. The measurements were carried out during a high water event, with a return period of once per year, in March 2010. Measurements of the bed levels for this high water event, were conducted at three moments: before the high water event, during the peak and after. The patterns of erosion and sedimentation in the groyne fields were analyzed and the effect of the lowering of the groynes on the morphology of the groyne field was quantified. This paper addresses the morphological developments of the groyne fields and near the head of the groynes