Exploring the Validity of Design Storms as Tools to Size and Design Stormwater Infrastructure for Urban Sewersheds

Four conceptual stormwater systems have been designed using City of Toronto and City of Pickering standards. The four systems have been inputted into EPA SWMM 5.1 and simulated against IDF curve-based design storms, as well as Toronto’s historic rainfall data ranging from 2005 to 2015. From analyzing the simulation outputs, it is noted that many of the design assumptions lead to systems performing below expectations when tested directly against the design storm. In some cases, peak flow rates, flow velocities and flow areasall exceed the expected values from the initial design. There are several factors that may contribute to this. The first notable factor would be an incorrectly assumed time of concentration, leading to a design storm that is not representative of an actual “worst case scenario” storm. The second notable factor is the inclusion of all pipes in a dynamic network, as opposed to the simplistic Manning’s approach taken by the design. These two factors are explored and their potential impacts are discussed in relation to real world situations where storms are far less likely to mimic the chosen design storm.