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:

  1. Post-fire debris flows are most often triggered by high-intensity, short-duration bursts of rain.
  2. 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).
  3. 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.