Single Publication

BLACK POST-FIRE

USGS DEBRIS-FLOW HAZARDS ASSESSMENT COMPLETED

BAER SAFETY MESSAGE: Everyone near and downstream from the burned areas should remain alert and stay updated on weather conditions that may result in heavy rains and increased water runoff. Flash flooding may occur quickly during heavy rain events. Be prepared to act. Current weather and emergency notifications can be found at the National Weather Service website:  www.weather.gov/abq/.

The USDA Forest Service (USFS) Burned Area Emergency Response (BAER) assessment team coordinated early with US Geological Survey (USGS) staff during its evaluation of the Black burned area to strategically assess potential post-fire impacts to the watersheds and predicted debris-flow response during damaging storm events.

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 western and southwestern fires. They 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 models estimate the probability and volume of debris flows that may be produced by a storm in a recently burned area, using a model with characteristics related to basin shape, burn severity, soil properties, and rainfall.

USGS utilized the Black Post-Fire BAER team’s soil burn severity (SBS) map to model potential debris flow hazards within the burned area. The data estimates debris-flow likelihood, volume, combined hazard, and rainfall thresholds and are presented in the debris flow map (see below). The combined relative hazard metric is a combination of the likelihood of debris flow (in %) and potential volume of debris flow (in m3 or cubic meters).

One of the maps that USGS developed depicts the likelihood of debris-flow hazard generation and estimates of flow magnitude in locations where debris flows initiate. The models do not predict downstream impacts, potential debris flow runout paths, or the spatial extent of debris-flow or flood inundation. The other two maps show estimated intensity volumes of debris flows, and the estimated probability/likelihood of debris flows.

This information is extremely important to inform our safety awareness of the burned areas, especially where there may be an increased risk of hillslope erosion, flooding sediment delivery to streams, and a higher probability of debris flows – all of which are potential risks to human life, safety, and property.

To access the USGS debris-flow hazards assessment maps for the Black Fire, go to: https://landslides.usgs.gov/hazards/postfire_debrisflow/detail.php?objectid=420.

Note: Click on the map layers interactive icon at the top right side of the map area to toggle on six different assessment map view results.

For information on what to do if you live in a recently burned area where debris flows are possible, and there is a rainstorm - before, during, and after – download the National Weather Service Post Wildfire Flash Flood and Debris Flow Guide (PDF 7.2 MB).

For additional information about the USGS emergency assessment of post-fire debris-flow hazards, go to: Emergency Assessment of Post-Fire Debris-Flow Hazards (usgs.gov).