Okanagan Basin Floodplain Mapping

In 2020, following atypical freshets that led to flooding in 2017 and 2018, the Okanagan Basin Water Board (OBWB), in collaboration with regional districts, municipalities, and the Okanagan Nation Alliance initiated floodplain mapping updates for the Okanagan River and mainstem lakes. Hydrologic and hydraulic models driven by a 50-climate model ensemble were used to create floodplain maps up until 2100 in the Okanagan Basin. The floodplain mapping project identified several recommended actions including modifications to the lake regulation system to reduce flooding, flood risk assessments, flood mitigation plans, and improving community awareness. To address the latter recommendation, an online interactive floodplain mapping tool and Okanagan Flood Story website was implemented to increase the public’s understanding and preparedness for flood events. Since the launch of the floodplain mapping project and website in 2020, other local governments such as the City of Vernon have completed complementary risk assessments and identified flood mitigation projects. Next steps for the floodplain mapping project include implementing recommended actions and frequently reviewing and updating the flood hazard maps as new science emerges and flood risks change.

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Understanding and Assessing Impacts

In 2017 and 2018, atypical freshets, driven by late season snow accumulation, heavy rain, and rapid snow melt, resulted in widespread flooding across the Okanagan Valley and record high Okanagan Lake levels. These events and the concern that they might become more common with climate change prompted the OBWB, along with regional districts, municipalities, and the Okanagan Nation Alliance to update floodplain mapping for the Okanagan River and mainstem lakes. In 2018 with funding from the National Disaster Mitigation Program, Emergency Management BC, and the OBWB, Northwest Hydraulic Consultants Ltd. (NHC) was contracted to update the floodplain mapping from Penticton to Osoyoos including all the river’s mainstem lakes. The main objective was to update floodplain mapping; however, an additional objective was to improve understanding of flood management options under future climate change scenarios. NHC used ECCC’s CanLEADV1 50-climate ensemble dataset for RCP 8.5 to analyze changes to temperature, freezing periods, precipitation, snowfall/rainfall, and storms. Downscaled ensemble datasets were then used to drive a hydrologic model of the Okanagan Basin from 1950 until 2100 to determine current and future design levels. Design levels are hypothetical floods used for planning and development in the floodplain and reflect society’s tolerance for risk. At this stage, results were analyzed by the NHC and BC Ministry of Forests, Lands, Natural Resource Operations and Rural Development to inform operational modifications to the Okanagan Lake Regulation System (OLRS) to reduce flood risk. The NHC then combined design levels for the lakes with a wave effect model to determine the flood extent for each mainstem lake. They used a river hydraulic model to determine the flood extent of the Okanagan River. Results from the lake and river models were used to create lake and river floodplain maps. Maps included flood inundation levels and extent for mid-century floods, river hazard mapping, and flood construction levels. The final floodplain maps assume that OLRS modifications have been implemented. Based on the NHC report, due to an expected increase in temperature and rainfall, decreased snowfall, and shorter freezing periods, the Okanagan Basin could experience earlier freshets, an increase in peak flows in the Okanagan River, and an increase in peak lake levels between 3-48 cm. Climate change will also make forecasting water flows more difficult.

Identifying Actions

The floodplain mapping project was necessary to understand flood hazards and begin to plan risk reduction strategies in the Okanagan Valley. There were several recommended actions identified from the project report. Firstly, a preeminent action would be to implement the OLRS modifications proposed in the report as the floodplain maps are contingent on these changes. Other actions that could be undertaken by local governments in the Okanagan Valley include completing a flood risk assessment and subsequent risk reduction or mitigation plan. These assessments should examine existing flood mitigation measures and infrastructure, high-risk flood areas, and flood mitigation strategies based on the community’s risk tolerance and needs. Finally, it was recommended to share the flood hazard information with Okanagan communities to increase understanding of the hazard, identify important needs of the community, and build momentum for next steps. Based on a provincial review, while floodplain mapping is the first step for creating community awareness, distribution and education should be the next step and are key components of a comprehensive risk mitigation plan.

Implementation

The Okanagan Flood Story website was built on ArcGIS Hub by NHC and was launched in May 2020, a few months after the floodplain mapping report was released. The Flood Story includes interactive floodplain maps and information on flood history, climate change, responsibility, reducing risk, and how to prepare, respond, and recover to raise community awareness. Since the implementation of the Okanagan Flood Story, the OBWB has kept the website up-to-date.

Outcomes and Monitoring Progress

This project has inspired local governments within the Okanagan Valley such as the City of Vernon to take action against flood risk. To complement the OBWB floodplain mapping project, the City of Vernon also contracted NHC to complete a City-wide flood mapping, risk analysis, and mitigation assessment which was completed in February 2022. The mitigation assessment proposed several projects including emergency response planning, land-use and development bylaws, and infrastructure modifications which have been endorsed by the City of Vernon Council. Similar to the Okanagan Flood Story, a City of Vernon Flood Story was also created to educate Vernon residents on flood risk preparedness.

Next Steps

There are many next steps and recommendations identified including OLRS modifications, flood risk assessments and flood mitigation plans, and increased community awareness of flood hazards in the Okanagan Valley. In the long term, it is suggested that the flood hazard maps, risk assessments, and mitigation strategies are reviewed frequently and updated to reflect any changes to flood risk.

Resources

Link to Full Case Study

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