Determination of overflow risk for stormwater systems

Stormwater systems 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 low duration or high return periods. A two-module (hydrological and statistical) model is developed in this paper to test the hydraulic performance for stormwater systems by determining their overflow risk.

The hydrological/hydraulic module selects the critical rainfalls series from a long recorded rainfall time series as well as the runoff maximal discharge, the conduits maximal velocities and the flooding maximal volume. The statistical module performs a frequencies analysis series to determine the drainage system flooding risk to the exceedance probability of the design rainfall.

The application of the model to a study case of the Nuns’ Island in Montreal has showed that the exceedance probability of the design rainfall is not negligible. The studied minor drainage system presents a real flooding risk.