Flood risk assessment of the City of Calgary considering flood control barrier performance

Abstract

The 2013 flood in southern Alberta was one of the worst natural disasters in Canadian history. Heavy rainfall and a melting snowpack in the upstream Rocky Mountains resulted in higher-than-normal runoff in several Alberta watersheds. As the runoff reached downstream to the City of Calgary, the Bow and Elbow rivers overtopped its banks (including at several flood control barriers along the Rivers), causing several deaths, large-scale evacuations and an estimated $6 billion in damages. Responding to this unprecedented flood event, the City of Calgary has implemented a Flood Resilience Plan to help reduce future flood risk. A key component of this plan is to analyze and improve existing flood control barriers (i.e., dike systems) along the rivers to improve performance and reduce the risk of riverine floods in the future. In this research, a probabilistic risk assessment process is developed and applied to assess the overall flood risk on existing dike systems along the Bow River in the City of Calgary. A steady-state hydraulic model of the Bow River is used to determine the water levels along the River for different return periods. The risk of failure of the dike systems (due to slope instability) is quantified using the Morgenstern-Price method and a Monte Carlo scheme that incorporates the uncertainty in the geotechnical properties of each barrier segment. The flood hazard is then quantified for each barrier segment considering each return period, risk of failure of the barrier, or overtopping due to high water levels. Following this, a multi-criteria approach that integrates the flood hazard with flood exposure and vulnerability is implemented to generate the overall flood risk map of the areas protected by the dike systems. The exposure (economic) and vulnerability (socio-demographic) are determined using geospatial data and an analytical hierarchical process. This process propagates the uncertainty from the data to the final probabilistic risk estimates. Results demonstrate the significance of including slope instability calculations when determining overall flood hazard. While dike systems may be designed to protect for a given return period (e.g. 100-year water level), there is a significant risk of failure of the barriers before reaching the designed water level. Additionally, certain protected areas face the compound hazard of overtopping and barrier failure – this hazard is not considered in existing analysis of the flood barriers along the Bow River. The overall risk assessment highlights those areas that have the highest combined risk and indicate the particular segments of the barriers that need design improvements to reach the City of Calgary’s flood resilience goals.