The Forest Service (USFS) Burned Area Emergency Response (BAER)assessment team coordinated early with US Geological Survey (USGS) staff during its evaluation of the Bighorn Fire burned area to strategically assess potential post-fire impacts to the watersheds and predicted debris flow response during damaging storm events. From the USGS website found at https://landslides.usgs.gov/hazards/postfire_debrisflow/.
“Wildfire can significantly alter the hydrologic response of a watershed to the extent that even modest rainstorms can produce dangerous flash floods and debris flows. The USGS conducts post-fire debris-flow hazard assessments for select fires in the Western U.S. We use geospatial data related to basin morphometry, burn severity, soil properties, and rainfall characteristics to estimate the probability and volume of debris flows that may occur in response to a design storm.”
USGS used the BAER team’s runoff potential analysis of the area burned by the Bighorn Fire to estimate post-fire debris-flow hazards, including the likelihood, potential magnitude, and rainfall threshold. The USGS Bighorn Post-Fire Debris Flow Hazard Assessment estimates convey these hazards for the Bighorn burn area as of the morning of June 28, 2020. Results will be updated as the fire expands, new satellite imagery is acquired, and the BAER team updates their runoff potential map.
The interactive map on the USGS website shows the likelihood, potential magnitude, and combined hazard of debris flows that may occur from about a 1/4 inch of rainfall over a 15-minute period. Results from more intense rain events, ranging from about 1/2 inch per hour to 2 inches per hour, are also available as downloadable data from the USGS website. The online interactive map is posted at: https://landslides.usgs.gov/hazards/postfire_debrisflow/detail.php?objectid=281.
USGS Fact Sheet 176-97, entitled “Debris Flow Hazards in the United States” contains information used to interpret the debris flow map and analysis that was incorporated into the BAER assessment team’s anticipated soil erosion and hydrologic response findings. According to the USGS, “Analysis of data collected from studies of debris flows following wildfires can answer many of the questions fundamental to post-fire hazard assessments— what and why, where, when, how big, and how often?” This information is extremely important in assisting the public in increasing their awareness of the areas where there may be a higher increase in flooding, sediment and soil erosion, and a high probability of debris flows –- all of which are potential risks to human life, safety, and property.
SPECIAL NOTE: Everyone near and downstream from the burned areas should remain alert and stay updated on weather conditions that may result in heavy rains over the burn scars. Flash flooding may occur quickly during heavy rain events-be prepared to take action. Current weather and emergency notifications can be found at the National Weather Service website: www.weather.gov/twc/.
Bighorn Post-Fire BAER Assessment information is available at: https://inciweb.nwcg.gov/incident/6796/