Landslides & Debris Flow

Wildfire can significantly alter the hydrologic response of a watershed, to the extent that even modest rainstorms can produce dangerous flashfloods and debris flows. After a fire, slopes are stripped of vegetation and the root systems within the soil, which allows sediment to move downhill. Channels can quickly become laden with sediment, and culverts can plug, increasing flood risks. 

Some of the largest debris flow events happen during the first post-fire rainy season. It takes much less rainfall to trigger debris flow within a burned hillside than it would prior to the fire. Any storm that has intensities greater than about 10 millimeters per hour (0.4 inches/hour) poses the risk of producing debris flows. 

Debris flows can take homes off of their foundations and carry things like vegetation, trees, large boulders, and vehicles.3

Additionally, the ashy slopes left denuded by wildfires in California are especially susceptible to “mudslides” during and immediately after major rainstorms. Those who live downslope of a wildfire area should be aware of this potential for slope failure that is present until new vegetation rebinds the soil.  

The USGS conducts post-fire debris-flow hazard assessments for select fires in the Western U.S. For select fires maps are available that depict the likelihood of debris-flow generation and estimate flow magnitude in locations where debris flows initiate. The models do not predict downstream impacts, potential debris flow runout paths, and the areal extent of debris-flow or flood inundation.  View the USGS hazard assessment here for more information. 

Tips for reducing the hazard posed by landslides & debris flows: 

1. On rainy nights, residents should NOT sleep in lower-floor bedrooms on the sides of houses that face hazardous slopes. Over 100 deaths have occurred in California due to debris flows burying persons who were sleeping in lower-floor bedrooms that were adjacent to hazardous slopes.  
2. Assume that all drainages are capable of carrying debris flows, especially if relatively loose, sandy soils are present in the watershed.  
3. Stay alert to the forecast and the amount of rain falling locally during storms. 
4. Before and during rains, frequent inspection of the slopes (above vulnerable sites) for extension cracks and other symptoms of downslope movement can be a warning of impending failure. 
   1. See the first link below for additional warning signs. 
5. Whenever rainfall has exceeded 3 or 4 inches per day, or ¼ inch per hour, the soil may become over-saturated which can trigger mudflows.  
6. Listen and watch for rushing water, mud, or unusual sounds such as trees cracking or boulders knocking together. A faint rumbling sound that increases in volume is noticeable as the landslide nears.  
7. Stay aware and monitor the land after each rain event until vegetation has been restored. Debris flows and mudflows can occur up to five (5) years after a wildfire.  

Links:

• Landslides & Debris Flow. What to do before, during and after a landslide. 
• Post Wildfire Debris Flow Survival Guide – National Weather Service  PDF 
• Debris Avalanches and Debris Flows in Hillside and Wildfire Areas  PDF 
• Post-Fire Flooding and Debris Flow from USGS.  
• Wildfire and Debris Flow: A Geologic Hazard (ca.gov)
• Quick Video Overview of Debris Flows.

See the Post-Fire Watershed Recovery Frequently Asked Questions for more information.

Click here to return to the Post-Fire Watershed Recovery homepage.