B Rosier

Semi-empirical model for channel bed evolution due to lateral discharge withdrawal

Side weirs and overtopable levees are widely used to increase flood routing along a channel or river. The lateral loss of water reduces the sediment transport capacity leading to the formation of a local sediment deposit close to the overflow. Since the extent of the morphological bed changes is not known a priori, the design discharge is increased by this flow–sediment transport interaction in an uncontrolled way. Based on a systematic flume study, a semi-empirical model to predict the three-dimensional bed evolution of the aggraded channel reach in the vicinity of the overflow is developed. The shape of the deposit is modelled by an adapted Maxwell-type distribution function. The main input parameters of the model are expressed in terms of dimensionless parameters accounting for main channel and side overflow geometry as well as flow and sediment transport characteristics. The application of the empirical model in numerical flow calculations predicts 90% of the measured overflow.

Influence of Lateral Water Withdrawal on Bed Form Geometry in a Channel

In flood protection engineering, side weirs or overflow dams are used to divert water in a controlled way into flood plains as soon as the discharge capacity of the main-channel is reached. Because of the lateral loss of water, the sediment transport capacity is reduced, resulting in local sediment deposition near the side overflow. Moreover, bed form characteristics such as length, height, steepness, and stoss and lee slope angle are affected by the lateral water withdrawal. Both phenomena are responsible for an increased side overflow intensity compared with plane bed conditions. The results from a systematic flume study show that the shape of observed bed forms is highly threedimensional and that three distinct regions along the channel axis can be identified. The first one extends from the channel entrance to the upstream weir corner, the second one comprises the reach of the weir, and the third one represents the reach downstream of the weir. The description of bed form shape by approaches from literature shows reasonable agreement with measured bed form geometry

One-Dimensional Numerical Modelling Of Mobile Bed Evolution In A Flume With A Side Weir

Abstract Side weirs are widely used to control flow processes along a channel or river. The lateral loss of water reduces the sediment transport capacity in the main-channel leading to local sediment deposition near the overflow. The design discharge to be diverted is increased in an uncontrolled way by this flow-sediment interaction. The flow-sediment interaction is investigated numerically and compared with results from a flume study. The numerical tool is based on the continuity and momentum equations and performs the coupled hydrodynamic simulation of 1D flow behaviour and bed-load transport. The friction head loss is computed according to the Manning formula. The outflow is determined by applying the general equation of weirs. With an appropriate formula for bed-load transport capacity and an adequate roughness function to account for form roughness, the geometry of the sedimentary deposit, as well as the side overflow discharge, is reproduced with reasonable accuracy.