Flood Hazard Mapping for the Similkameen River Prepared for: Dated: March 2021 Ecora File No.: GK-19-548-RDO Flood Mapping for the Similkameen River File No: GK-19-548-RDO | March 2021 | Version 1 Prepared For: Regional District of Okanagan-Similkameen 101 Martin Street Penticton, BC V2A 5J9 Village of Keremeos 702 4th Street, PO Box 160 Keremeos, BC V0X 1N0 Town of Princeton 151 Vermillion Ave. PO Box 670 Princeton, BC V0X 1W0 579 Lawrence Ave, Kelowna, BC V1Y 6L8 | P: 250.469.9757 | F: 250.469.9757 | www.ecora.ca Flood Mapping for the Similkameen River File No: GK-19-548-RDO | March 2021 | Version 1 Executive Summary The Regional District of Okanagan-Similkameen (RDOS), in partnership with the Town of Princeton (ToP) and the Village of Keremeos (VoK), engaged Ecora Engineering & Resource Group Ltd. (Ecora) to undertake a Flood Risk Assessment (FRA), Flood Mapping (FM) and Flood Mitigation Plan (FMP) for the Similkameen River. This report describes the Flood Mapping (Phase 2) portion of the study. Flood hazard maps were developed for three distinct areas within the Similkameen River watershed. Flood mapping areas included part of RDOS Electoral Area B, G, and H, the ToP and the VoK. The areas also included Lower Similkameen Indian Band (LSIB) and Upper Similkameen Indian Band (USIB) reserve lands. The study areas are described as follows. Keremeos Study Area: The Keremeos study area was centred around the Village of Keremeos, BC. The study area extents span 26 km from the Riverside R.V. Park Resort along Crowsnest Highway 3 in RDOS Electoral Area G, through the VoK to approximately 2 km south of Cawston, BC in RDOS Electoral Area B. The study includes portions of several LSIB Indian Reserves including Ashnola 10, Alexis 9, and Narcisse’s Farm 4. The Similkameen River was the subject of the flood mapping in this study area. Princeton Study Area: The Princeton study area was centred around the Town of Princeton, BC at the confluence of the Similkameen and Tulameen Rivers. The study area for both rivers starts approximately 5 km upstream of the confluence in RDOS Electoral Area H, extends through the Town of Princeton, and finishes again in RDOS Electoral Ares H approximately 6 km downstream of the confluence. The study area also includes the USIB Vermillion Forks 1 reserve lands. The Similkameen River and Tulameen River were both the subjects of the flood mapping in this study area. Tulameen Study Area: The Tulameen study area was centred around the communities of Tulameen and Coalmont, BC in RDOS Electoral Area H. The study area starts approximately 3.5 km upstream of Tulameen and extends to approximately 2 km downstream of Coalmont. The study area also includes Otter Creek and a portion of Otter Lake. The Tulameen River, Otter Creek and the south shore of Otter Lake were the subjects of the flood mapping in this study area. A hydrologic assessment of the Similkameen River watershed was completed to establish the design flood flows to be used to create the flood hazard maps. A comprehensive analysis of climate data, snow pillow data and WSC hydrometric data was undertaken. A regional frequency analysis was completed on instantaneous peak and maximum daily streamflows at key WSC streamflow monitoring stations. Through analysis of peak data, it was determined that the largest peak flows typically occur on the Similkameen River during the spring, while the largest peak flows on the Tulameen River tend to occur in the fall/winter. Accordingly, the frequency analysis was completed for the spring and fall/winter for the Similkameen and Tulameen Rivers, respectively. The frequency analysis was used to calculate peak flows for the 20-year, 200-year, 500-year, and 1000-year return period events at key streamflow stations using the HYFRAN v2.2 software. Multiple regional relationships were established to correlate peak flow magnitude with catchment area in order to derive peak flows to be used for model input. In an attempt to refine design flows and estimate the potential impacts of future climate change, a hydrologic model was developed for the Similkameen River watershed using the U.S. Army Corps if Engineers Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS) v4.6.1 modelling software. The model was constructed to represent the hydrologic processes associated with flooding events. Calibration of the model focused on replicating the 2018 streamflow hydrograph during the spring freshet at the Similkameen River near Nighthawk WSC streamflow monitoring station, which represents the hydrologic response of the entire Canadian portion of the watershed. The model results did not show any clear trend in peak streamflow as a result of projected changes in precipitation, temperature, and snowpack. Therefore a 10% increase in all design flood events was applied in Flood Mapping for the Similkameen River File No: GK-19-548-RDO | March 2021 | Version 1 accordance with the Professional Practice Guidelines for Legislated Flood Assessments in a Changing Climate in BC (EGBC, 2018). After establishing the design flows and climate change adjustment factor, three 2-dimensional (2D) unsteady flow models were developed using The U.S. Army Corps of Engineers Hydrologic Engineering Center River Analysis System (HEC-RAS) version 5.0.7. The models were developed for the three study areas described above. Each model was calibrated individually through adjustment of the “Manning’s n” surface roughness parameters assigned to the river channel and floodplain areas. Results were compared to available datasets including designed dike freeboard, high water marks at bridges, Water Survey of Canada (WSC) streamflow monitoring stations, photographs, and anecdotal observations. The models were all calibrated to an acceptable level and then validated against 2018 spring freshet data. Multiple model scenarios were simulated for each of the design events - each with and without a 10% adjustment factor applied to the flow to quantify the potential future impacts of climate change. The scenarios included: 1. Scenario 1: Flow constrained by existing topography (i.e., unaltered DEM terrain). 2. Scenario 2: Flow not constrained by any dikes 3. Scenario 3: Dike breach (Keremeos and Princeton study areas only) Flood depth and inundation maps were produced by simulating the scenarios for the 20-year, 200-year, and 500- year design flood events. Ecora then used the flood mapping results for the 200-year and 500-year design flows to assess the hazard levels in the inundated areas of the floodplain. The hazard calculation was based on the UK Flood Hazard Rating Formula described in the Professional Practice Guidelines for Flood Mapping in B.C. (APEGBC, 2017). Depth and hazard rating maps were presented for Scenario 1, 2, and 3 with Scenario 1 being the primary map set to be used for flood-related decision making. Scenario 2 and 3 maps can also be used for decision making; however, their application should be limited to their specific set of circumstances. The development of the Scenario 2 and 3 maps was important for developing the Regulatory Flood Construction Level and Floodplain maps. The Regulatory Flood Construction Level and Floodplain maps were produced for the 20-year and 200-year events with 0.6 m of freeboard and were based on a composite water surface elevation by combining Scenario 1, 2, and 3. These maps utilized all three scenarios to capture a wide array of flooding possibilities to inform future planning decisions. Ecora notes that the numerical modelling results used to develop the flood maps contain uncertainty. Sources of uncertainty include accuracy of the information used to construct the models, the amount of data available for calibration and validation, and model error. Ecora recommends that municipalities and Regional District should familiarize themselves with the limitations described in this report and listed on the flood map index pages before using the maps. Flood Mapping for the Similkameen River File No: GK-19-548-RDO | March 2021 | Version 1 Limitations of Report This report and its contents are intended for the sole use of the Regional District of Okanagan- Similkameen, the Town of Princeton, the Village of Keremeos, their agents and the applicable regulatory authorities. Ecora Engineering & Resource Group Ltd. (Ecora) does not accept any responsibility for the accuracy of any data, analyses, or recommendations contained or referenced in the report when the report is used or relied upon by any Party other than the Regional District of Okanagan-Similkameen, the Town of Princeton, the Village of Keremeos, their agents, the applicable regulatory authorities or for any project other than that described in this report. Any such unauthorized use of this report is at the sole risk of the user. Where Ecora submits both electronic file and hard copy versions of reports, drawings and other project- related documents, only the signed and/or sealed versions shall be considered final and legally binding. The original signed and/or sealed version archived by Ecora shall be deemed to be the original for the project. Both electronic file and hard copy versions of Ecora’s deliverables shall not, under any circumstances, no matter who owns or uses them, be altered by any party except Ecora. Flood Mapping for the Similkameen River File No: GK-19-548-RDO | March 2021 | Version 1 Table of Contents (Phase 2) 1. Introduction ................................................................................................ 1 1.1 General .............................................................................................................................................1