Detailed CSO modeling and evaluation of green infrastructure impacts on global planning

The City of Montreal is currently developing a global stormwater management (SWM) plan aiming to mitigate combined sewer overflows (CSOs) and minimize local flooding. Reducing CSOs is a particularly important issue considering that Montreal is surrounded by the St-Lawrence River and other watercourses and that upcoming projects of shore valorization and urban beaches are planned.

The SWM plan is based on detailed modeling of the sewer networks using PCSWMM, with calibration and validation ensuring that the results to develop adequate solutions. One specific element is that the model must be able to reproduce small events that occur frequently as well as larger events that are less frequent.

Lasalle|NHC has developed for a large area a modeling approach which resulted in an accurate reproduction of measured dry and wet weather flows and observed CSOs. This approach uses multiple GIS layers in order to detail the model so that elements with a significant difference in their hydrological responses may be treated individually. Streets, roofs, commercial areas, residential lots and industrial lots were all modeled separately. The resulting level of discretization thereafter produced good calibration and validation results. Many steps of the process are automated, using a combination of a complementary GIS software and PCSWMM’s built-in tools.

Once calibrated, the model was used to conduct an evaluation of the network’s capacity as well as to identify sections of the network which were the main cause for surface overflows because of their limited capacity. A simple scoring method using PCSWMM’s attributes (and additional attributes) was used to identify these sections. The project also required the evaluation of the impact of green infrastructure (GI) on the reduction of CSO and network resilience. A specialised tool was developed to automatically map and implement GI based on multiple design restrictions and factors.

This presentation will discuss the approach and the results that were achieved during the calibration and validation process, the methodology used for network analysis and the specialised GI mapping tool.