Groynes are structures constructed at an angle to the flow in order to deflect the flowing water away from critical zones. They are made of stone, gravel, rock, earth, or piles, beginning at the riverbank with a root and ending at the regulation line with a head. They serve to maintain a desirable channel for the purpose of flood control, improved navigation and erosion control. In the River Rhine, which is considered the backbone of North-western European waterways network, the primary objective of groynes is to provide a fairway of sufficient depth and width. For example the River Waal, the most important branch of the Rhine River in the Netherlands is regulated by around 500 groyne.
Within the framework of the research project "Space for the Rhine Branches" several measures have been devised to achieve a decrease of the water levels at peak discharges, one of those measures, is lowering of the existing groynes. The rationale behind this proposal is that; due to large-scale erosion of the low-water bed through the past decades, the groynes are now higher than necessary for keeping the main channel at depth. Lowering the groynes along certain reaches of the river would result in a reduction of the effective roughness during high water conditions thus, increasing the river's flood conveyance capacity.
If the groynes are lowered, however, the balance of hydrodynamic forces acting on the groyne-fields will change, and there will be a large-scale morphological impact. To identify this impact, a thorough understanding to the effect o f groynes on the morphology of the river is necessary. The sediment exchange between the groyne-fields and the main channel needs to be more comprehensible.
The purpose of this report is to acquire the background knowledge required to study the effect of groynes on a river. The characteristics of the existing groyne-fields along the Waal River are presented. The hydrodynamic and morphological impact of groynes on a river is described. Moreover, because navigation plays an important role in the interaction between the groyne fields and the main channel, the navigation induced water motion and its effect on the flow in groyne-fields is described. Finally, a review of some prediction attempts to the interaction between the groyne-fields and the main channel is presented.