Optimization of a rainwater basin using 3D-CFD simulation

The existing rainwater basin of the city of la Chaux-de-Fonds has a volume of 2000 m³ and an intake- and cleaning capacity of 2 m³/s. During rain events with a short return period, the rain basin cannot collect the inflowing combined wastewater quickly enough and a significant proportion of the runoff flows uncleaned into the receiving water. As a result, visible pollution from coarse and fine substances in the water is regularly observed. Therefore, the intake- and cleaning capacity should be increased up to 8 m³/s to use the existing volume of the rain basin more efficiently. In a first step, the intake structure is being rebuilt and new coarse and fine racks with high cleaning performance are being installed. To achieve the new planned distribution of the inflow, the intake structure was iteratively optimized with a two-phase 3D-CFD simulation (water/air). Another aspect of the optimization was the evenly distribution of the flow through the coarse racks. Finally, the functionality of the entire structure, including coarse- and fine racks, emergency overflow and transfer to the receiving waters was checked for different rain events (Q_max = 40 m³/s).