Multiple rainfall events since the end of 2003 have caused residential and commercial backflow and flooding in parts of the Saint-Charles River watershed, more specifically the Lorette River watershed. Tetra Tech CSO was commissioned to produce and update three sectorial drainage master plans, each one representing a separate river: the Lorette and Saint-Charles rivers, the Du Cap Rouge River (Jean-Lesage Airport area), and the Nelson River.
In close collaboration with the City of Québec and the "Centre d'expertisehydrique du Québec" (Quebec’s water expertise center), the stormwater management master plans were create to evaluate existing conditions and capacities required for future development. They were developed through integrated planning and established the capacities of waterways and stormwater networks that could be utilized without risk. The master plan is the management tool that sets the local boundaries of the subcatchment, which permits a global validation of the consequences of local action.
An analysis of the addition of runoffs generated by future urbanization of the area was performed and recommendations were issued. Assessing the level of service for all of the modeled pipes in ultimate development conditions was based on a rainfall event with a 5-year return period and centennial rainfall events. A hydraulic analysis of the Saint-Charles and Lorette rivers played a predominant role as it determined the level of control that needed to be applied in future development areas.
The stormwater watershed covers approximately 6,700 ha. With XPSWMM software, catchments were characterized and pipes and nodes where modeled, while an HEC-RAS model was used for hydraulic analysis of the river. A transfer process was created to integrate inflow hydrographs (generated by XPSWMM) into the HEC-RAS hydraulic river model.
The highlight of this current study was to take into account reactions during rainfall event sewer network due to excess flow in the network and in the river.
Tetra Tech CSO carried out the following activities:
- Inventory of the infrastructure, previous hydraulic and hydrological studies, and interventions;
- Planning a flow monitoring campaign;
- Modeling natural and urban waterways' watersheds;
- Modeling network (XPSWMM) and hydraulic analysis for the river (HEC-RAS);
- Calibration of the hydrological models with observed data from flow meter sites;
- Diagnostic of the stormwater sewer network and waterways;
- Development of a solution;
- Overall plan to ensure optimum development for the area and update for ultimate conditions;
- Analysis of the actual state of the waterways during extreme events (centennial rainfall events);
- Producing work cost estimates and prioritizing the intervention.