Hazard Assessment FAQ - Frequently Asked Questions
The USGS conducts postfire 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 likelihood and volume of debris flows that may occur in response to a design storm.
What do the maps mean?
The maps depict the likelihood and potential volume of debris flows as they exit the mountain front in response to a design storm having a 15-minute peak rainfall intensity of 24 mm/h. The models are designed to assess the potential for debris flow in the locations where debris flows initiate (i.e., where they form and get larger).
Do the maps show where debris flows will travel?
No, the maps do not identify debris-flow runout paths, areas of inundation, or assess potential damage. Modeling debris-flow runout is very challenging, especially in populated areas, and developing predictive runout tools is an active area of USGS research.
It’s been a couple of months since the fire, will the maps be updated?
No, the maps only depict the hazard immediately after the fire. But the danger goes down over time as vegetation regrows and the soil recovers. In general, the debris-flow hazard remains elevated for 2-5 years after a wildfire. Precisely how long the danger lasts depends on many factors, such as how much it rains and how severely the slopes were burned. Quantifying the change in debris-flow hazard as the burned area recovers is an active area of USGS research.
What is the meaning 15-minute peak intensity of 24 mm/h?
A 15-minute rainfall intensity of 24 mm/h is equivalent to the accumulation of 6 mm of rain in 15 minutes. In English units, this is equivalent to approximately 1/4 of an inch of rain in 15 minutes.
Why use a 15-minute peak intensity of 24 mm/h design storm?
This design storm is used for three reasons:
- Postfire debris flows are most often triggered by high-intensity, short-duration bursts of rain.
- A 24 mm/h rain burst is likely to happen in most areas of the western U.S. (i.e. a 1-5 year recurrence interval).
- A 24 mm/h rain burst is known to trigger debris flows at USGS monitoring sites in burn areas.
Are results for other storm scenarios available?
Yes, other design storms are contained in the geospatial data available for download. These storms have peak 15-minute intensities ranging from 12 to 40 mm/h in 4 mm/h increments.
How do I get a post-fire debris flow hazard assessment?
See Assessment Requirements page.
I live near a recently burned area. Am I in danger?
Contact your local authorities to learn about emergency-response and evacuation plans for your area.
Disclaimer - Limitations of Hazard Assessment
The hazard assessments use a design rainstorm with a given peak 15-minute rainfall intensity to predict the probability, volume, and combined relative hazard of debris flows in basins burned by the fire. Differences in model predictions and actual debris-flow occurrence will arise with differences in actual storm duration and intensity. The occurrence of higher rainfall intensities or longer storm durations may increase the probability or volume of potential debris flows.
The models were developed, calibrated, and tested using data from the western United States. The models have not yet been tested in burn areas in the eastern United States, western Oregon, or Washington (west of the Cascade Range). Currently, efforts are being made to validate model predictions in the eastern United States, western Oregon, and Washington.
In addition, this hazard assessment relies upon readily available geospatial data, the accuracy and precision of which may influence the estimated likelihood and magnitude of post-fire debris flows. However, local conditions (such as debris supply) certainly influence both the probability and volume of debris flows. Unfortunately, locally specific data are not presently available at the spatial scale of the post-fire debris-flow hazard assessment. As such, local conditions that are not constrained by the model may serve to dramatically increase or decrease the probability and(or) volume of a debris flow at a basin outlet. The input geospatial data are also subject to error based upon mapping resolution, elevation interpolation techniques, and mapping and(or) classification methods. Finally, this assessment is specific to debris-flow hazards; hazards from flash-flooding are not described in this study and may be significant.
This assessment also characterizes potential debris-flow hazards at a static point in time immediately following wildfire. Studies of post-fire debris flow in the western United States have indicated that debris-flow activity in recently burned areas typically occurs within 2 yr of wildfire. As vegetation cover and soil properties return to pre-fire conditions, the threat of debris-flow activity decreases with time elapsed since wildfire. Conversely, the hazards from flash-flooding may persist for several years after the wildfire.
Finally, this work is preliminary and is subject to revision. It is being provided due to the need for timely "best science" information. The assessment is provided on the condition that neither the U.S. Geological Survey nor the United States Government may be held liable for any damages resulting from the authorized or unauthorized use of the assessment.