Hazard Maps - Frequently Asked Questions

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 variety of rainfall scenarios, also known as design storms. 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).

Learn more about interpreting model results and understanding the debris-flow hazard maps.  (coming soon)

What is a design storm?

A design storm is a specific type of rainfall event used estimate debris-flow hazard.  Design storms are defined by characteristics such as rainfall intensity (millimeters per hour or inches per hour), total rainfall (millimeters or inches), duration, and storm recurrence interval.  

For example, a design storm with a peak 15-minute 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.

Which design storm scenarios are included with assessment results?

The USGS evaluates debris-flow hazard for several design storm scenarios.  These include storms with peak 15-minute rainfall intensities ranging from 16-40 mm/h in 4 mm increments, as well as the intensity corresponding to a 1-year recurrence interval storm based on NOAA Atlas 14 precipitation frequency estimates. 

On the USGS Hazard Assessment Viewer Dashboard, results for the 1-year recurrence interval storm are displayed.  In Oregon and Washington, where NOAA Atlas 14 data is not available, results are show for a storm with a peak 15-minute intensity of 24 mm/h. 

The full set of scenarios -- from 16 to 40 mm/h in 4 mm/h increments  -- is available in the geospatial data available for download. 

Learn more about choosing an appropriate design storm for viewing hazard assessment results.  (coming soon)

Do the maps show where debris flows will travel?

The maps on the Postfire Debris-flow Hazard Assessment Dashboard only display debris-flow likelihood and potential volume.  They do identify debris-flow runout paths, areas of inundation, or assess potential damage. 

The USGS has developed a method (Barnhart et.al, 2026) for mapping debris-flow runout, and runout hazard assessments will be conducted for select fires.  

Contact Jason Kean or Katy Barnhart to inquire about a runout hazard assessment.  

It’s been a couple of months since the fire, will the maps be updated?

Emergency hazard assessment maps show the level of debris-flow hazard immediately after a fire.  Burned areas are generally are most susceptible to debris flows the first year following fire, but the level of hazard changes over time as vegetation regrows and the soil recovers. Recovery varies in space and time.  Some burned areas recover quickly and others experience debris flows for multiple years after fire. A recent USGS study (Graber et.al, 2026) provides a method for updating hazard maps in the years after fire to better reflect how debris-flow hazard change as the landscape recovers.  

Contact Andrew Graber to inquire about recovery and updated hazard assessments.

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.