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#           Post-Fire Flooding and Debris Flow      

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      By California Water Science Center      June 5, 2018    

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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).

                                            - Download Video

View Media Details

### **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.

 

### **Explore the Post-Fire Debris Flow Hazard Assessments >>**

Below are other science projects associated with this project.

link

October 25, 2023

####                 Wildfires in California              

California Water Science Center, Wildland Fire Science

link

October 25, 2023

####   Wildfires in California

Learn More

link

June 5, 2018

####                 Water Quality after a Wildfire              

California Water Science Center, Wildland Fire Science

link

June 5, 2018

####   Water Quality after a Wildfire

Learn More

Below are data or web applications associated with this project.

####                 Post-Fire Debris Flows              

Estimates of the probability and volume of debris flows that may be produced by a storm in a recently burned area, using a model with characteristics related to basin shape, burn severity, soil properties, and rainfall.

Natural Hazards Mission Area, Landslide Hazards Program, Geologic Hazards Science Center

Below are multimedia items associated with this project.

PubTalk 10/2018 — Post-fire debris flow early warning

October 25, 2018

#### PubTalk 10/2018 — Post-fire debris flow early warning

link

Title: Post-Fire Debris-Flow Early Warning: The case for forecast-based warning systems

Natural Hazards Mission Area, Landslide Hazards Program, Communications and Publishing, Public Lecture Series

October 25, 2018

#### PubTalk 10/2018 — Post-fire debris flow early warning

link

#### PubTalk 10/2018 — Post-fire debris flow early warning

link

Title: Post-Fire Debris-Flow Early Warning: The case for forecast-based warning systems

Natural Hazards Mission Area, Landslide Hazards Program, Communications and Publishing, Public Lecture Series

Post-wildfire Flood and Debris Flow: 2014 Silverado Fire

July 19, 2015

#### Post-wildfire Flood and Debris Flow: 2014 Silverado Fire

link

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.

Natural Hazards, Wildfire Hazards, California, Communications and Publishing, Wildland Fire Science

July 19, 2015

#### Post-wildfire Flood and Debris Flow: 2014 Silverado Fire

link

#### Post-wildfire Flood and Debris Flow: 2014 Silverado Fire

link

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.

Natural Hazards, Wildfire Hazards, California, Communications and Publishing, Wildland Fire Science

Dirk Kempthorne Visits Malibu, California 2008

January 22, 2008

#### Dirk Kempthorne Visits Malibu, California 2008

link

**Video Highlights**

* Secretary of the Interior, Dirk Kempthorne, visits the fire ravaged Malibu, California (Jan 14, 2008). Led by USGS scientists, the Secretary was shown how technology and collaborative science are crucial to addressing a number of post-fire threats and concerns.

Communications and Publishing

January 22, 2008

#### Dirk Kempthorne Visits Malibu, California 2008

link

#### Dirk Kempthorne Visits Malibu, California 2008

link

**Video Highlights**

* Secretary of the Interior, Dirk Kempthorne, visits the fire ravaged Malibu, California (Jan 14, 2008). Led by USGS scientists, the Secretary was shown how technology and collaborative science are crucial to addressing a number of post-fire threats and concerns.

Communications and Publishing

USGS and California Wildfires: Post Fire Debris Flow (part 1)

December 30, 2007

#### USGS and California Wildfires: Post Fire Debris Flow (part 1)

link

Jim Bowers, USGS, California Hydrologic Monitoring Program Chief, talks about an historic streamflow site that was buried by debris flows as a result of the 2003 fires.

Communications and Publishing

December 30, 2007

#### USGS and California Wildfires: Post Fire Debris Flow (part 1)

link

#### USGS and California Wildfires: Post Fire Debris Flow (part 1)

link

Jim Bowers, USGS, California Hydrologic Monitoring Program Chief, talks about an historic streamflow site that was buried by debris flows as a result of the 2003 fires.

Communications and Publishing

USGS and California Wildfires: Post Fire Debris Flow (part 2)

December 30, 2007

#### USGS and California Wildfires: Post Fire Debris Flow (part 2)

link

Jim Bowers, USGS, California Hydrologic Monitoring Program Chief talks about the potential for debris flows in central Orange County, Modjeska Canyon, as a result of the 2007 fires.

Communications and Publishing

December 30, 2007

#### USGS and California Wildfires: Post Fire Debris Flow (part 2)

link

#### USGS and California Wildfires: Post Fire Debris Flow (part 2)

link

Jim Bowers, USGS, California Hydrologic Monitoring Program Chief talks about the potential for debris flows in central Orange County, Modjeska Canyon, as a result of the 2007 fires.

Communications and Publishing

Post-fire Debris Flow

January 9, 2004

#### Post-fire Debris Flow

link

Debris Flow in Cable Canyon following the 2003 Old Fire in the San Bernardino Mountains, California.

California, California Water Science Center, Wildland Fire Science

January 9, 2004

#### Post-fire Debris Flow

link

#### Post-fire Debris Flow

link

Debris Flow in Cable Canyon following the 2003 Old Fire in the San Bernardino Mountains, California.

California, California Water Science Center, Wildland Fire Science

Below are publications associated with this project.

August 28, 2018

####                 Fire, flood, and drought: Extreme climate events alter flow paths and stream chemistry              

Climate Research and Development Program, Land Change Science Program, Wildland Fire Science

August 8, 2016

####                 Model simulations of flood and debris flow timing in steep catchments after wildfire              

Natural Hazards Mission Area, Landslide Hazards Program, Geologic Hazards Science Center

January 15, 2014

####                 Emergency assessment of post-fire debris-flow hazards for the 2013 Springs Fire, Ventura County, California              

Natural Hazards Mission Area, Landslide Hazards Program, Geologic Hazards Science Center

October 29, 2013

####                 Emergency assessment of post-fire debris-flow hazards for the 2013 Rim Fire, Stanislaus National Forest and Yosemite National Park, California              

Natural Hazards Mission Area, Landslide Hazards Program, Geologic Hazards Science Center

October 18, 2013

####                 Emergency assessment of post-fire debris-flow hazards for the 2013 Powerhouse fire, southern California              

Natural Hazards Mission Area, Landslide Hazards Program, Geologic Hazards Science Center

October 18, 2013

####                 Emergency assessment of post-fire debris-flow hazards for the 2013 Mountain fire, southern California              

Natural Hazards Mission Area, Landslide Hazards Program, Geologic Hazards Science Center

Below are news stories associated with this project.

USGS Hydrologist Interviewed about Fire Effects on Water Quality, and on Debris Flow

October 2, 2018

####   USGS Hydrologist Interviewed about Fire Effects on Water Quality, and on Debris Flow

As part of an article on the effects of wildfires on the health of infants, Santa Rosa Press Democrat reporter, Guy Kovner, interviewed Scott Wright...

Read Article

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).

                                            - Download Video

View Media Details

### **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.

 

### **Explore the Post-Fire Debris Flow Hazard Assessments >>**

Below are other science projects associated with this project.

link

October 25, 2023

####                 Wildfires in California              

California Water Science Center, Wildland Fire Science

link

October 25, 2023

####   Wildfires in California

Learn More

link

June 5, 2018

####                 Water Quality after a Wildfire              

California Water Science Center, Wildland Fire Science

link

June 5, 2018

####   Water Quality after a Wildfire

Learn More

Below are data or web applications associated with this project.

####                 Post-Fire Debris Flows              

Estimates of the probability and volume of debris flows that may be produced by a storm in a recently burned area, using a model with characteristics related to basin shape, burn severity, soil properties, and rainfall.

Natural Hazards Mission Area, Landslide Hazards Program, Geologic Hazards Science Center

Below are multimedia items associated with this project.

PubTalk 10/2018 — Post-fire debris flow early warning

October 25, 2018

#### PubTalk 10/2018 — Post-fire debris flow early warning

link

Title: Post-Fire Debris-Flow Early Warning: The case for forecast-based warning systems

Natural Hazards Mission Area, Landslide Hazards Program, Communications and Publishing, Public Lecture Series

October 25, 2018

#### PubTalk 10/2018 — Post-fire debris flow early warning

link

#### PubTalk 10/2018 — Post-fire debris flow early warning

link

Title: Post-Fire Debris-Flow Early Warning: The case for forecast-based warning systems

Natural Hazards Mission Area, Landslide Hazards Program, Communications and Publishing, Public Lecture Series

Post-wildfire Flood and Debris Flow: 2014 Silverado Fire

July 19, 2015

#### Post-wildfire Flood and Debris Flow: 2014 Silverado Fire

link

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.

Natural Hazards, Wildfire Hazards, California, Communications and Publishing, Wildland Fire Science

July 19, 2015

#### Post-wildfire Flood and Debris Flow: 2014 Silverado Fire

link

#### Post-wildfire Flood and Debris Flow: 2014 Silverado Fire

link

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.

Natural Hazards, Wildfire Hazards, California, Communications and Publishing, Wildland Fire Science

Dirk Kempthorne Visits Malibu, California 2008

January 22, 2008

#### Dirk Kempthorne Visits Malibu, California 2008

link

**Video Highlights**

* Secretary of the Interior, Dirk Kempthorne, visits the fire ravaged Malibu, California (Jan 14, 2008). Led by USGS scientists, the Secretary was shown how technology and collaborative science are crucial to addressing a number of post-fire threats and concerns.

Communications and Publishing

January 22, 2008

#### Dirk Kempthorne Visits Malibu, California 2008

link

#### Dirk Kempthorne Visits Malibu, California 2008

link

**Video Highlights**

* Secretary of the Interior, Dirk Kempthorne, visits the fire ravaged Malibu, California (Jan 14, 2008). Led by USGS scientists, the Secretary was shown how technology and collaborative science are crucial to addressing a number of post-fire threats and concerns.

Communications and Publishing

USGS and California Wildfires: Post Fire Debris Flow (part 1)

December 30, 2007

#### USGS and California Wildfires: Post Fire Debris Flow (part 1)

link

Jim Bowers, USGS, California Hydrologic Monitoring Program Chief, talks about an historic streamflow site that was buried by debris flows as a result of the 2003 fires.

Communications and Publishing

December 30, 2007

#### USGS and California Wildfires: Post Fire Debris Flow (part 1)

link

#### USGS and California Wildfires: Post Fire Debris Flow (part 1)

link

Jim Bowers, USGS, California Hydrologic Monitoring Program Chief, talks about an historic streamflow site that was buried by debris flows as a result of the 2003 fires.

Communications and Publishing

USGS and California Wildfires: Post Fire Debris Flow (part 2)

December 30, 2007

#### USGS and California Wildfires: Post Fire Debris Flow (part 2)

link

Jim Bowers, USGS, California Hydrologic Monitoring Program Chief talks about the potential for debris flows in central Orange County, Modjeska Canyon, as a result of the 2007 fires.

Communications and Publishing

December 30, 2007

#### USGS and California Wildfires: Post Fire Debris Flow (part 2)

link

#### USGS and California Wildfires: Post Fire Debris Flow (part 2)

link

Jim Bowers, USGS, California Hydrologic Monitoring Program Chief talks about the potential for debris flows in central Orange County, Modjeska Canyon, as a result of the 2007 fires.

Communications and Publishing

Post-fire Debris Flow

January 9, 2004

#### Post-fire Debris Flow

link

Debris Flow in Cable Canyon following the 2003 Old Fire in the San Bernardino Mountains, California.

California, California Water Science Center, Wildland Fire Science

January 9, 2004

#### Post-fire Debris Flow

link

#### Post-fire Debris Flow

link

Debris Flow in Cable Canyon following the 2003 Old Fire in the San Bernardino Mountains, California.

California, California Water Science Center, Wildland Fire Science

Below are publications associated with this project.

August 28, 2018

####                 Fire, flood, and drought: Extreme climate events alter flow paths and stream chemistry              

Climate Research and Development Program, Land Change Science Program, Wildland Fire Science

August 8, 2016

####                 Model simulations of flood and debris flow timing in steep catchments after wildfire              

Natural Hazards Mission Area, Landslide Hazards Program, Geologic Hazards Science Center

January 15, 2014

####                 Emergency assessment of post-fire debris-flow hazards for the 2013 Springs Fire, Ventura County, California              

Natural Hazards Mission Area, Landslide Hazards Program, Geologic Hazards Science Center

October 29, 2013

####                 Emergency assessment of post-fire debris-flow hazards for the 2013 Rim Fire, Stanislaus National Forest and Yosemite National Park, California              

Natural Hazards Mission Area, Landslide Hazards Program, Geologic Hazards Science Center

October 18, 2013

####                 Emergency assessment of post-fire debris-flow hazards for the 2013 Powerhouse fire, southern California              

Natural Hazards Mission Area, Landslide Hazards Program, Geologic Hazards Science Center

October 18, 2013

####                 Emergency assessment of post-fire debris-flow hazards for the 2013 Mountain fire, southern California              

Natural Hazards Mission Area, Landslide Hazards Program, Geologic Hazards Science Center

Below are news stories associated with this project.

USGS Hydrologist Interviewed about Fire Effects on Water Quality, and on Debris Flow

October 2, 2018

####   USGS Hydrologist Interviewed about Fire Effects on Water Quality, and on Debris Flow

As part of an article on the effects of wildfires on the health of infants, Santa Rosa Press Democrat reporter, Guy Kovner, interviewed Scott Wright...

Read Article