The City of Calgary`s River Morphology Risk Mitigation Program

A 1:75 year return period flood in 2013 caused large scale morphologic changes on the Bow and Elbow Rivers which are wandering gravel bed rivers that flow through The City of Calgary in Canada. After the flood, a review of the rivers morphologic changes occurred to assess potential future risks. The analysis used an enviable data set including orthorectified airphoto’s dating back to 1926, pre and post flood bathymetric digital elevation models, and detailed hydraulic and morphologic models.

Four morphologic risks were assessed including: Valley Wall Failures, Avulsions, Erosion, and Sedimentation. Each of the identified risks was ranked based on their probability and consequence such that a prioritized list of potential interventions to mitigate these risks could be developed. The interventions had to consider numerous complexities outside of the normal cost benefits of civil engineering projects including; morphologic stability, fisheries impacts, navigation, riparian colonization, recreation impacts, and a complex regulatory framework. The City of Calgary’s Triple Bottom Line mandate which requires the consideration of the Economic, Social, and Environmental benefits and drawbacks was than applied to the highest risk projects to confirm their viability. Monetary values were applied to ten economic factors, four environmental factors, and six social factors to determine the overall projects benefit to cost ratio.

The highest benefit to cost projects all revolved intervening on sedimentation locations which formed numerous new gravel bars generally around bridges. The largest risk came not from the gravel bar itself, but from the riparian colonization of the gravel bars by the native balsam poplar trees that create riparian forests and further reduce flood conveyance. A review of past land management practices (gravel mining), the historical hydraulic record, and the impacts of the upstream dams on sediment continuity were all factored into the sediment management program which was developed to address these problem bars while at the same time improving the overall fish habitat and developing a sediment nourishment program. The methodology described in this paper to address the morphologic risks after a large return period flood could be used to guide similar assessments in other jurisdictions.