Integrated modelling approach of urban wet weather discharges into lakes

With an ever growing population the city of Lausanne has a need to monitor and preserve its main water source, the lake. Permanent discharge from combined sewer over ows (CSO) and the waste water treatment plant have an impact on the Vidy Bay, making it the area the most polluted of the lake (Pote et al., 2008). In this prestudy, we evaluate the impact of the urban wet weather discharge focused on the CSOs, through modelling of sediment transport, as a proxy for pollution (Rossi, 2007b), in commonly observed conditions. The existing hydrodynamic Delft3D Z-model of lake Geneva was adapted to picture sediment transport. After validation of its hydrodynamics on the six rst month of 2014, the resulting deposition/erosion pattern, shear velocity maps and mass balance were analysed. The CSO investigated has two discharge regimes with 90% of discharges under 5 m3/s capable of short range transport, while large discharge for light sediment and strong currents demonstrate long range transport capable of exiting the bay. Hydrodynamics are the main driver of the deposition pattern, leading the sediment plume of the CSO to stretch in multiple direction depending on the wind. Resuspension was identied as a major source of sediment transport events in the bay. These originated from short-, medium- and long-range transport. Over the short period investigated, the region near St-Sulpice was surprisingly identied as a hot spot for suspension in the bay, shaping the modelled deposition pattern after six month in front of the CSO outlet for light sediments. Comparison with in-situ samples from Benejam (2016) show the same zones of in uence for the six months simulated. The proposed and tested modelling approach of this study suggest promising future applications.