Resistance of submerged groynes

Abstract

Groynes are commonly used in the major rivers in the Netherlands. They confine the flow of the river to a main channel but also act as resistance element once submerged and in that way influence the stage discharge relationship of the river. Several formulas exist describing groynes as a drag resistance. Another possible way of determining their resistance is by using a weir formula and fitting the up and downstream water levels to the water level slope of the river. In this thesis a schematized model of the river Waal is set up and progressively expanded. At first seven different drag and weir formulas are compared. There is no consensus for resistance is between them. They are therefor compared to a 2DV computer model of flow over a highly submerged weir in the SWASH software package. Drag resistance, expressed as a function of water depth to groyne height ratio has the same scaling as two weir formulas though they do not match in absolute terms. A 2DH computer model is then used to determine the magnitude of the lateral turbulent momentum exchange between the main channel, groyne fields and flood plain. Finally a 3D computer model is used to determine groyne resistance and the distribution of discharge and momentum around the groyne. Treating groynes as weirs is found to be an acceptable assumption. The schematized model is used to simulate a high discharge of 13.550 m3 s-1. Groynes, when seen as a weir, would be responsible for a 36 cm water level increase. Lateral turbulent momentum exchange increases this by another 34 cm, while using the groyne resistance found in the 3D model added only another 7 cm.