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. The measurements show that between the initial situation and the discharge peak, erosion occurred in the groyne field in the first 40 m from the normal line towards the banks for both types of groyne fields. However, the amount of erosion in the groyne fields between lowered groynes, is twice as much as the erosion in the groyne field without lowering. In the period after the peak discharge, in the first 25 m from the normal line towards the banks, sedimentation occurred in the groyne fields without lowering, and erosion in the groyne field with lowered groynes. Between 25 m and 50 m towards the banks, sedimentation occurred in both types of groyne fields, but the sedimentation in the groyne fields between lowered groynes was twice as much as in the groyne field between groynes without lowering. Furthermore, the measurements show that the measured scour-holes near the groyne head, caused by local flow contraction and turbulence, are less deep for the lowered groynes. This is probably due to the