Evaluation of a depth-averaged numerical model in simulation of side weir overflow variations due to bed morphology changes

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Mohammad Rostami, Hosein Kahani

Friday 3 july 2015

14:45 - 15:00h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 16B. Sediment - Transport

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Side weirs, also known as a lateral weirs, and overflow dams are free overflow regulation and diversion devices commonly encountered in hydraulic engineering. They are set into the side of a channel or river allowing to spill a part of the discharge over their crest when the surface of the flow in the main-channel exceeds a certain level. The lateral loss of water is reducing the sediment transport capacity in the main-channel yielding to aggradation and the formation of a local sediment deposit in the downstream weir alignment. The reduced cross section generates backwater effects and additional contraction and expansion losses. As a consequence, the head over the side weir rises and the side overflow discharge as well. The design discharge to be diverted over the weir is increased by this flow-sediment transport interaction. Based on the systematic experimental flume study a two dimensional sediment transport model, CCHE2D, for the prediction of the mobile bed evolution near the side weir have been used. The Numerical model allows a simple and straightforward estimation of the interaction of a side overflow with bed-load transport and bed morphology in engineering practice. Regarding the impact of the deposit on the intensity of side overflow it has been found out that the spilled discharge might increase by a factor of up to 3 compared to fixed plane bed conditions. In this context about 25 % of the total increase are attributed to effects of form roughness and about 75 % to bed aggradation phenomena. In this regard the height of the deposit represents the most important parameter. Comparison between experimental data and numerical modeling showed to be in a good agreement. The predicted side overflows with CCHE2D model were about 92% of those were obtained in the experimental study.