Interaction of side weir overflow with bed-load transport and bed morphology in a channel

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. Since the interaction of side overflow with bed-load and bed morphology in a channel has not been studied so far, systematic tests have been performed. Three test series and one reference experiment without side weir have been carried out in a 20.00 m long, 1.50 m wide and 1.20 m high rectangular flume. The first test series consisted of a 3.00 m long side weir, the second one had a 6.00 m long weir and the third series was characterised by two weirs of 2.50 m length each. The approach discharge varied between 0.098 m3/s ≤ Q1 ≤ 0.222 m3/s. The overall flow regime has been subcritical. The average initial bottom slope was 0.21 %. The mobile bed was characterized by a median particle size of d50 = 0.72 mm. During the experiments the water surface, the 2D-velocity field, the side overflow discharge and sediment supply were measured. The final bed morphology has been recorded by means of digital photogrammetry. Based on the systematic experimental flume study a one- and a two-dimensional empirical model for the prediction of the mobile bed evolution near the side weir have been developed. The models allow a simple and straightforward estimation of the interaction of a side overflow with bed-load transport and bed morphology in engineering practice. The 1D-model represents the overall mobile bed evolution in the weir reach. The model takes into account a deposit being uniform over the channel width. The height of the deposit only varies in longitudinal direction. In addition to the 1D-approach the 2D-model incorporates the variation of the deposit over the channel width. For the parameterisation of the two models a Maxwell-type distribution function is applied. Input parameters for both models such as the location and height of the maximum bed elevation and a shape factor are expressed in terms of non-dimensional geometric channel and side weir variables as well as hydraulic parameters and bed load transport relations. For the 2D-approach an additional relationship considering the spanwise variation is developed. To implement the models in numerical flow simulations expressions for the location of the empirical deposit relative