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 ID 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.