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.
Side weirs are free overflow regulation and diversion structures commonly encountered in flood protection engineering. The lateral loss of water reduces the bed-load transport capacity in the main channel, leading to local sediment deposition near the side overflow. As a consequence, the design overflow is increased in an uncontrolled way. Since this flow-sediment interaction in such a channel has not been studied so far, systematic flume experiments have been performed. Based on these experiments, a two-dimensional empirical model to describe the longitudinal evolution of the aggraded channel reach near the weir has been developed. In addition, a simple and straightforward approach for direct estimation of the side overflow in presence of bed-load transport has been established. To be generally applicable in engineering practice, all input variables are expressed in terms of dimensionless parameters. Finally, the application of the models is demonstrated in a case study on the Rhone River in Switzerland.Key words: side overflow, sediment transport, bed morphology, deposition, semi-empirical model, laboratory tests. Résumé :Les seuils à déversoir latéral sont des structures de contrôle et de détournement à écoulement libre régulière-ment utilisées en ingénierie de lutte contre les inondations. La perte latérale d'eau permet de réduire la capacité de transport de la charge de fond dans le chenal principal, engendrant ainsi une déposition locale des sédiments près du déversoir latéral. Le débordement servant aux calculs est donc augmenté de manière non contrôlée. Des expériences systématiques de canal sur appuis ont été réalisées puisque cette interaction écoulement-sédiments dans un tel canal n'avait jamais été étudiée. Basé sur ces expériences, un modèle empirique bidimensionnel a été développé afin de décrire l'évolution longitudinale de la portion du canal alluvial à proximité du déversoir. De plus, une approche simple et explicite d'estimation directe de l'écoulement latéral avec transport de la charge de fond a été déterminée. Pour être applicables à l'ingénierie, les variables d'entrée sont exprimées en termes de paramètres sans dimensions. Finalement, l'utilisation des modèles est dé-montrée sur une étude de cas du Rhône en Suisse.Mots-clés : seuil à déversoir latéral, transport de sédiments, morphologie du lit, déposition, modèle semi-empirique, essais en laboratoire.[Traduit par la Rédaction]
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.
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