Creating an integrated dual drainage model in PCSWMM to assess drainage conditions and evaluate alternatives in a 100-hectare urban basin

This paper presents the overall approach, results and recommendations from an Integrated Dual Drainage Study conducted on behalf of the City of Ottawa in 2015. The 2015 Study supports citywide management plans (Comprehensive Asset and Wet Weather Infrastructure), by characterizing the existing drainage conditions and identifying existing levels of service (probability of risk related to surcharging or surface flooding) and constraints and opportunities within the study area. PCSWMM was used to develop and evaluate a pipe-by-pipe model and GIS and spreadsheet analysis were used to prepare input data, perform complex analyses, and render results. Sewer hydraulic grade line and percent pipe full capacity were used to characterize the minor system performance. Major system classification was based on ponding depths/extents; conveyance system – private property crossing points, and level of service within Collector roads. Input rainfall was based on the City’s design storms (various return periods), while impacts of climate change were simulated by increasing storm intensities by 20%. To qualitatively validate the model, simulations of historical storms were run and the results were tested against flooding reports documented by the City. The results indicated the minor and major systems had average levels of service of 1:5 and 1:100 years, respectively. A systematic approach was used to evaluate various alternatives to improve system performance, starting with those most basic and cost-effective and then gradually increasing the cost and complexity until the desired balance was achieved. By restricting flows to the minor system with inlet control devices, average inlet capture rates could be reduced by 40 - 70%, significantly reducing the risk of sewer surcharging conditions during a 1:100 year storm. Reduced inlet capture translated into increases in surface flows and ponding duration, although these were effectively accommodated within the existing capacity of the major system.