Determination of overflow risk for stormwater systems

Stormwater systems (SWSs) are designed for “gravity flow” to convey stormwater runoff. The overflow risk appears when the design rainfall intensity is exceeded, i.e. when systems face rainfalls with short duration and/or high return periods. A two-module (hydrological and statistical) method is developed in this chapter to test the hydraulic performance for a SWS by determining its overflow risk. The hydrological/hydraulic module selects the critical events: critical rainfalls series from a long recorded rainfall time series, runoff maximal flows (discharges), maximal velocities in conduits and flooding maximal volumes. Afterwards a series of frequency analyses is done with these critical values in the statistical module to determine the SWS overflow risk linked to the 5-, 10- 20-, 50-, and 100-y critical events. The application of the proposed method to a case study of the Nuns’ Island off the southeast shore of Montreal has shown that the overflow risk of the design storm (rainfall) is not negligible. The studied SWS presents a real flooding risk, depending on the selected time period.