The measurement of the flow through complex combined sewer overflow structures in the frame of automated monitoring remains difficult. In this paper, a methodology based on the use of computational fluid dynamics (CFD) modelling in order to improve the instrumentation of a downstream-controlled dual-overflow structure is presented. The dual-overflow structure is composed of two combined sewer overflows (CSOs) connected by a rectangular channel and controlled by a downstream gate located at the entry of the Meyzieu waste water treatment plant (close to Lyon, France). The analysis of the CFD results provides: (i) a better understanding of the interaction between the two CSOs – that means the hydraulic operation, the hydrodynamic behaviour, the backflow effect – and (ii) an ability to optimise the location of the water depth sensor. The measured water depth is used to assess the overflow rate by means of a numerical relationship. Uncertainties are also assessed.

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