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.
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.