If the design objective is to meet and provide peak flow control for storm events, it is necessary to plan beyond the 1:100-year peak flow, and instead plan for the new norm of a 1:1000-year flood event. Planning for the appropriate peak flow is crucial to building climate resilience and meeting the demand over the full lifecycle of the infrastructure. If an inadequate peak flow formula is used it could result in significant additional costs to the City if it has to repair or tear up failing infrastructure to rebuild and increase capacity before it has reached its end-life. “Even a 1000-year return period has a 5% risk of being equalled or exceeded in a 50-year period.”
It is also important to emphasize the importance of planning for a warming climate by building resilience into whichever alternative/s are chosen. In this vein, planning should be based on at least a 1:200-year storm event, and preferably a 1:1000-year flood event for true resilience to climate change.