Emergency Post Wildfire Geohazard Assessment BGC 2023

BGC Engineering Inc. (BGC) was retained by the Columbia Shuswap Regional District (CSRD) to better understand the preliminary risk, posed by wildfire-related geohazards, to residents that are currently in the area or about to return home following the downgraded wildfire evacuation order. The assessments described in this report are intended to inform ongoing emergency response and support decision-making for preliminary recovery actions within the CSRD, and are not intended to replace detailed post-wildfire risk assessment implemented over a longer time horizon. This report summarizes an emergency post-wildfire assessment of geohazards. Geohazards assessed include debris flow, debris flood, rockfall, and shallow landslide. Debris flow and debris flood are collectively referred to as steep creek hazards. The Post-wildfire Steep Creek Hazard Assessment provides: Post-wildfire Hazard Likelihood ratings for 63 watersheds (Drawing 02) Post-wildfire Geohazard ratings describing the relative risk for 62 identified Potential Hazard Areas (PHAs) (Drawing 03). Fourteen of the PHAs were previously assessed in the Risk Prioritization Study completed for the CSRD (BGC, April 16, 2020) The Post-wildfire Rockfall and Shallow Landslide Assessment (Drawing 04), provides estimates of the Change in Hazard ratings for 20 identified Areas of Interest (AOIs). In the assessments, BGC focused on risk to areas with private property parcels within the CSRD. Assets at risk which are outside of the CSRD, including those belonging to Skwlāx te Secwepemcúl̓ecw and Adams Lake Indian Band, provincial parks, or belonging to the Thompson-Nicola Regional District, or assets which were non-structural or of cultural value were not assessed or focused on in this assessment. Both steep creek hazards and rockfall and shallow landslide hazards were assessed using matrices. For steep creek hazards, Post-wildfire Geohazard Ratings were based on the Post-wildfire Hazard Likelihood of the upstream watershed and the Post-wildfire Impact Likelihood of the alluvial fan. Fifteen PHAs were rated as “Very High” Post-wildfire Geohazard Rating and 23 PHAs rated as “High”. For rockfall and shallow landslide hazard, “Type IV” is assigned to slopes where there is visible evidence that there has been a significant increase in hazard level due to burn conditions given the existing slope geometry or composition, and the asset is at or near the toe of the burned slope with no barriers present. Two AOIs were assessed as having “Type IV” change in hazard level for post-wildfire rockfall and shallow landslide hazards. Download Report