Hydraulic Grade Line Analysis Case Study: Flow Conveyance and Volume Storage in Oversized Pipes Under Restricted Outlet Conditions

Claire Wittnebel, AECOM, ON, Canada


The premise of stormwater management design is to provide feasible and cost-effective solutions within the physical constraints imposed by site and external conditions, work area limitations, and competing utility requirements. The presented case study provides insight into the use of PCSWMM to model and evaluate solutions for a particularly problematic location.

As part of a larger transportation improvement project, storm sewers along Highbury Avenue in London, Ontario have been identified for replacement and improvement. The storm sewers at this location drain a low point with no defined overland flow route, and therefore must convey the 100-year event in order to preserve the safety and functionality of the road. The right-of-way does not provide sufficient space to provide surface storage of major event volumes, so robust conveyance is necessary. However, the storm sewers downstream of this location are within an easement through private property, and cannot be upsized at the same time as the roadway sewers. Further downstream, a crossing under a major railway embankment represents an ultimate restriction which is infeasible to upsize or otherwise address. PCSWMM modelling of the area has been utilized to determine the sewer layout within the road which provides the best results under both interim and ultimate conditions, i.e. without and with the upsizing of the downstream sewers within the private property easement.

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