Post-Fire Flooding and Debris Flow

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Fast-moving, highly destructive debris flows triggered by intense rainfall are one of the most dangerous post-fire hazards. The risk of floods and debris flows after fires increases due to vegetation loss and soil exposure. Cases of sudden and deadly debris flow are well documented along the western United States, particularly in Southern California. These flows are a risk to life and property because they can occur with little warning, can exert great impulsive loads on objects in their path, and may strip vegetation, block drainage ways, and damage infrastructure.

Wildfire-related flooding and increased runoff may continue for several years in a burn area, but it is unusual for post-fire debris flows to occur beyond the second rainy season. Some of the largest debris-flow events happen during the first post-fire storm season. It takes much less rainfall to trigger debris flows from burned basins than from unburned areas. In southern California, as little as 7 millimeters (0.3 inches) of rainfall in 30 minutes has triggered debris flows. Any storm that has intensities greater than about 10 millimeters/hour (0.4 inches/hour) poses the risk of producing debris flows.

While multiple factors can affect debris-flow occurrence, post-fire debris flows generally are triggered by one of two processes: surface erosion caused by rainfall runoff, and landsliding caused by rainfall seeping into the ground. Surface-erosion runoff processes are by far the most prevalent contributors to debris flows. This is because fires commonly reduce the rate at which water can seep into the soil. Landsliding processes are much less common causes of fire-related debris flow, but prolonged heavy rains may increase soil moisture even after a wildfire. The wetted soil can then fail, producing infiltration-triggered landslides, which may. These can be shallow or deep-seated landslides (greater than 10-15 feet deep).

In 2014, the Silverado Fire burned approximately 4 km^2 in Orange County, California. After the fire, the USGS installed an automated rain-triggered camera to monitor post-wildfire flooding and debris flow at the outlet of a small 0.6 km^2 basin within the burn area. This video shows the initial surge and peak flow triggered by an intense rainstorm on July 19, 2015. The peak flow occured about 3 minutes after the intial surge.

Hydrologic Monitoring Systems and Wildfire

Monitoring the movement of surface water is essential in burn areas to provide early warning of flash flooding and debris movement. The USGS maintains extensive groundwater and surface-water monitoring systems, collecting data from more than 500 streamgages and 3,000 wells throughout the state.

Many of these gages are Automated Local Evaluation in Real-Time (ALERT) streamgages, meaning they are designed to send warnings when water levels reach a predetermined level or change rapidly. Sudden changes generally create an increased risk of debris flow and flash flooding, posing hazards to downstream communities. Data from ALERT gages are used by the National Oceanic and Atmospheric Administration (NOAA) National Weather Service (NWS), as well as other local, state, and national organizations, to assess the need for emergency preparedness or early warning systems that can help protect life and property.

In 2015, the USGS installed 11 new ALERT streamgages in the Valley and Butte wildfire burn areas in northern California to monitor and assess hazards associated with post-fire conditions. The USGS coordinated with NWS to select the sites, which are designed to give emergency managers data needed to issue flood warnings.

The sites were also selected based on recommendations from the USGS Landslides Hazards group, which considered the probability of debris flow and landslides near the sites. There are currently no plans to install additional gages at other burn sites in California.

Emergency Assessment of Post-Fire Debris-Flow Hazards

Wildfire can significantly alter the hydrologic response of a watershed to the extent that even modest rainstorms can produce dangerous flash floods and debris flows. The USGS conducts post-fire debris-flow hazard assessments for select fires in the western United States. Scientists use geospatial data to understand the size and shape of the watershed, and information on burn severity, soil properties, and rainfall characteristics to estimate the probability and volume of debris flows that may occur in response to various types of storms.

To help emergency managers and others prepare for floods, the USGS delivers a continuous source of streamflow information that provides the scientific basis for decision-making related to protection of life and property from water hazards. Visit our California Flood Science page for additional information.

 

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