Located at the bottom of an escarpment near downtown Calgary, the community of Sunnyside has suffered a number of floods over the years due to its location in a floodplain below high levels of the Bow River. A berm, constructed with stormwater outfalls running through it, protects the community from flooding. These outfalls have gates which are closed in anticipation of high water levels in the Bow River. However, even with this protection, Sunnyside is vulnerable to four flooding mechanisms:
1. Heavy rainfalls could exceed the capacity of the stormwater system pipes
2. Rainfall could occur when the outfall gates are closed in anticipation of a high water event in the Bow River
3. Groundwater flooding could occur when high water in the Bow River raises the groundwater table
4. Overland flooding could occur when the Bow River overtops the berm
In 2013, Sunnyside was subjected to significant flooding twice over the span of two weeks. The first event was in June when the Bow River overtopped the Sunnyside berms. Two weeks later, a rainfall event occurred while several of the stormwater gates were closed to protect the community from river backup.
Following these events, the City of Calgary retained Associated Engineering to conduct a drainage study of the Sunnyside area. The City wished to analyze the Sunnyside area for flooding due to rainfall runoff and propose improvements to mitigate flood risks.
The City had not previously established a stormwater level of service target for floodplain communities. To assist the City in doing so, Associated Engineering reviewed historical occurrences of concurrent high river level and high rainfall events. Using our analysis, the City selected a dual level of service--one when the outfall gates are open and another when the stormwater outfall gates are closed.
Dual level of service approach reduces risks
Associated Engineering proposed upgrades that provided resilience against multiple flooding mechanisms to reduce the overall flood risk to the community. Using two levels of service and developing multi-purpose solutions were beneficial to the community as the improvements became a higher priority on the City’s investment plan due to their high benefit-to-cost ratio.
Several of the recommended projects are already in the design and construction phases, including: a new stormwater pump station that will protect against stormwater and groundwater flooding, improvements to an existing stormwater pump station to do the same, and a pressurized trunk sewer to convey stormwater from the upper plateau directly to the river, rather than through Sunnyside. This work has been partially supported with funding from the Provincial and Federal governments.
The Sunnyside study received the 2018 Consulting Engineers of Alberta Award of Excellence, in the Studies, Software & Special Services category. Key staff involved in the study included Nadeer Lalji, Andrew Rushworth, Andrew Wiens, Karen Prezelj and Corinne Arkell, as well as many others involved in the design and construction phases.