Revisiting Waal River Training by Historical Reconstruction

The Dutch River Waal, a branch of the Rhine, has been trained for centuries to mitigate the effects of ice-jams and improve navigation. The works, started in 1850, involved river straightening and narrowing by a series of transverse groynes. Besides fulfilling their goal, the groynes also created the need to raise flood protection works and caused undesirable channel incision. This study assesses the effectiveness of training the river with a longitudinal wall instead of with groynes. The investigation analyzes the long-term response of the historical river with a two-dimensional depth-averaged (2DH) morphodynamic model. The results show that the wall would create two parallel channels, one becoming deeper and the other one shallower. The former would be as suitable for navigation as an equally-wide channel obtained with groynes. The latter would contribute in conveying water during high flow events and improve the river ecology. Training the river with a wall would also lessen channel incision. The best performance is obtained if the wall is built on the channel centerline, starting just upstream of a point bar top.