Post-Fire Debris Flows

Normally, vegetation absorbs rainfall, reducing runoff. However, large-scale wildfires dramatically alter the terrain and ground conditions, leaving the ground charred, barren, and less able to absorb water. This creates conditions ripe for flash flooding and debris flows, particularly during high intensity and prolonged rainfall events. Higher than usual flows in nearby streams and drainages can result in flooding risks, even for areas not typically prone to flooding.

What is a Debris Flow?

A debris flow is a fast-moving mass of material – a slurry of loose mud, sand, soil, rock, debris and water – that travels down a slope under the influence of gravity. Many factors influence the degree of risk from debris flows, including soil type and condition, slope steepness, and the quality and type of vegetation on area slopes.

graphic depicting debris flow starting at top of a burned slope, its flow path down a stream channel, and spread as it reaches the bottom of the slope/channel — Image source: USGS

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Do Your Part to Prepare Your Property

Following a wildfire, property owners need to be proactive to prepare and protect their properties from heightened flooding and debris flow risks, and to minimize risks to their neighbors and public infrastructure downstream while the burned area recovers.

Mitigating flooding and debris flow risks on and to private roadways and hillsides is the responsibility of the property owner.

Private property owners are responsible for mitigating risks on and to their property, including:

- clearing and maintaining drains and culverts, and
- deploying appropriate protective measures such as wattles, sandbags, or other barriers to minimize and direct runoff responsibly.

Property owners with complex protective needs are encouraged to consult professional firms specializing in erosion and flood control to assist them.

County and City agencies are legally prohibited from using public resources to fortify private properties.

Such actions could be considered a misuse of public funds as well as posing liability concerns.

Mountain Fire Debris Flow Thresholds

It may take several years for vegetation in a burn area to establish water absorption and soil stabilization levels comparable to pre-burn conditions. In the meantime, local public safety partners will monitor the area and evaluate the need for any protective actions prior to and during rain events which could result in detrimental flooding and debris flows.

The following thresholds represent levels of increased risk for potential debris flow events in the area of the Mountain Fire burn scar:

- 0.60″ of rainfall per hour
  - Intensity is sufficient to produce flooding/debris flows.
  - Road access may become restricted due to flooding/debris.
  - Mud and debris may begin to clog drains and culverts.
- >0.80″ of rainfall per hour
  - Sustained intensities are sufficient to result in flooding and debris flows that may be life-threatening.
  - Localized flooding should be anticipated.
  - Roadways will be impacted and possibly be hazardous.
  - Residents should remain alert for rapidly changing conditions and be prepared to leave, or shelter in place if necessary.
- >1.00″ of rainfall per hour carriers the potential to trigger flooding, landslides or debris flows at any location in the County.

Evacuation Warnings and/or Orders may be issued by local authorities when conditions warrant.

Flood Preparedness Resources

- National Weather Service – Oxnard Office
- Ventura County Flood Preparedness Information

Did You Know?

- Debris flows can travel at speeds up to and exceeding 35 mph!
- Following a particularly intense fire, the top 1-3 inches of soil in burned areas can become hydrophobic – essentially repelling water rather than absorbing it.
- Flood risk remains significantly higher until vegetation is restored—which may take up to 5 years after a wildfire.

Town Hall Meeting

In January 2025, Ventura County Sheriff’s Emergency Services hosted a town hall meeting to review the post-fire conditions within the November 2024 Mountain Fire burn scar and discuss applicable preparedness for the winter storm season and emergency response information with local residents. The event featured from public safety leaders, as well as updates from weather and geology experts. To view the recording, click on the video below: