Improving Postfire Debris-Flow Hazard Assessments In The Pacific Northwest Through Application Of Debris-Flow Models
As part of the Post-fire Hazards and Impacts to Resources and Ecosystems (PHIRE): Support for Response, Recovery, and Mitigation Project, the PHIRE Debris Flow Hazard team is engaging in several studies to better understand the spatial and temporal drivers of postfire debris flows and improve postfire hazard assessments across northern California and the Pacific Northwest.
Postfire Debris Flows
Wildfires leave steep slopes vulnerable to debris flows, a fast-moving mixture of water, soil, and rock that can cause property damage and loss of life. After a wildfire, emergency managers need rapid answers to the questions: Where in the burn area are debris flows likely? How much rain will it take to trigger a debris flow? And how big will that debris flow be? The USGS is conducting applied research and emergency hazard assessments to answer these questions. The results of this work will be used to develop hazard maps that identify the likelihood and potential volume of debris flows across the burn area and identify rainfall thresholds that must be exceeded to trigger debris flows. These thresholds help inform early warning operations by the National Weather Service.
Improving predictions in northern California and the Pacific Northwest
Northern California and the Pacific Northwest have recently experienced an increase in wildfire activity. In response, the USGS has been conducting debris-flow hazard assessments for wildfires in the region at the request of federal, state, and tribal agencies. Evaluated burn areas included the nearly million-acre 2021 Dixie fire (the largest single wildfire in California state history, Figure 1), the 2020 Labor Day wildfires in Oregon, and many of the 2021 wildfires in Washington. Assessments for new wildfires in the region and throughout the U.S. are conducted each year.
The models used for hazard assessment have been developed and tested in semi-arid parts of the country with a history of postfire debris flows, such as southern California and Colorado. But the models have not been tested extensively in wetter regions like northern California and the Pacific Northwest. Important questions that are being addressed as part of the PHIRE project are: Are the models accurate for northern California and the Pacific Northwest? And are the rainfall triggering conditions for postfire debris flows similar in different climates? The USGS and partners have begun monitoring postfire debris-flow activity in the region to answer these questions. The data are being used to test and subsequently improve the models used for hazard assessment.
Postfire monitoring involves installing rain gauges within the burn area and documenting the flow response following rainstorms. The flow response could be a debris flow, a less hazardous water flow, or no flow at all. The observed flow response in monitored drainage basins is compared to the predicted likelihood of a debris flow for the measured rainfall rates. If the model predicted a high likelihood of debris flow and there was a debris flow, the model got it right. The model is also considered successful if it predicts a low likelihood of debris flow and one did not occur. Systematically evaluating where the model makes correct and incorrect predictions helps identify missing factors that could lead to a better model. The same data also evaluates the accuracy of rainfall thresholds used for early warning.
Recent data collection has created an inventory of over 500 new records of flow activity in northern California and the Pacific Northwest (e.g., Graber, 2023a; Thomas et al., 2023a). The data reveals both similarities and differences in debris-flow processes between the northwest and drier areas like southern California. For example, in the 2021 Dixie fire (northern CA) and 2021 Muckamuck fire (north-central Washington), major debris flows (Figure 2 and Figure 3) were triggered by short bursts of intense rainfall like they are in better-studied semi-arid regions (Kostelnik et al., 2023a; Thomas et al., 2023b). In contrast, debris flows in the 2017 Eagle Creek Fire, Oregon were triggered by prolonged rainfall on nearly saturated soils (Kostelnik et al., 2023b). Those hydrologic triggering conditions are like the triggering conditions for shallow landslides and debris flows in unburned areas (Thomas et al., 2021). Continued observation and testing (e.g., Thomas et al., 2023c) is expected to lead to an improved set of hazard assessment models that better reflect regional variations in susceptibility.
How far? and How Long?
Other frequently asked questions after wildfires are: How far will debris flows travel? What will be impacted? And how long will the burn area remain hazardous? These questions are presently not addressed in current debris-flow hazard assessments, but the USGS is doing research to provide answers soon.
Predicting where debris flows will travel is challenging. Debris flows can have peak flows many times greater than floods. They can also easily plug culverts and bridge underpasses, redirecting dangerous flow into neighborhoods outside the floodplain (Figure 4). The USGS and partners are using recent debris-flow events, such as the 2018 debris flows in Montecito, CA, to test models for predicting debris-flow runout (e.g., Barnhart et al., 2021). The goal is to complement the current set of hazard maps with additional maps of possible debris-flow inundation. The inundation maps could inform emergency response and evacuation plans and identify homes and structures that could be damaged (Barnhart et al., 2024). The USGS is developing this approach with input from emergency managers to ensure the product meets their needs (Barnhart et al., 2023).
How long a burn area will remain susceptible to floods and debris flows largely depends on how long it takes stabilizing vegetation to return. And the rate of vegetation recovery depends on the type of vegetation, local climate, and weather conditions after a wildfire. Some burn areas recover quickly in two years or less. Others, such as those in a drought, may remain locked in a high hazard state for several years after wildfire. New research has shown that when a satellite-measured index of vegetation density reaches two-thirds of its prefire level, debris flows triggered by distributed runoff and erosion are unlikely (Graber et al., 2023). This result indicates that full recovery of vegetation is not necessary to substantially reduce postfire hazards. The vegetation recovery ratio also provides an objective way to determine when expensive warning operations can be ramped down, or when parts of the burn area can safely be reopened for public use.
Funding
Funding for this project is provided by the Robert T. Stafford Disaster Relief and Emergency Assistance Act (42 U.S.C. 5121 et seq.) and supplemental funding acts for Federal disaster relief activities. Through this funding USGS supports recovery efforts in declared natural disaster areas, to aid recovery efforts from widespread wildfires, devastating hurricanes, prolonged volcanic eruptions, and damaging earthquakes. This enables USGS to repair and replace equipment and facilities, collect high-resolution elevation data, and conduct scientific studies and assessments to support recovery and rebuilding decisions.
More Information
Emergency Assessment of postfire debris-flow hazards
Recent Postfire Events
Early Warning For Burn Areas
References
-
Multi-model comparison of computed debris flow runout for the 9 January 2018 Montecito, California post-wildfire event
Hazard assessment for post-wildfire debris flows, which are common in the steep terrain of the western United States, has focused on the susceptibility of upstream basins to generate debris flows. However, reducing public exposure to this hazard also requires an assessment of hazards in downstream areas that might be inundated during debris flow runout. Debris flow runout models are widely availabUser needs assessment for postfire debris-flow inundation hazard products
Debris flows are a type of mass movement that is more likely after wildfires, and while existing hazard assessments evaluate the rainfall intensities that are likely to trigger debris flows, no operational hazard assessment exists for identifying the areas where they will run out after initiation. Fifteen participants who work in a wide range of job functions associated with southern California poEvaluation of debris-flow building damage forecasts
Reliable forecasts of building damage due to debris flows may provide situational awareness and guide land and emergency management decisions. Application of debris-flow runout models to generate such forecasts requires combining hazard intensity predictions with fragility functions that link hazard intensity with building damage. In this study, we evaluated the performance of building damage foreCompilation of runoff-generated debris-flow inventories for 17 fires across Arizona, California, Colorado, New Mexico, and Washington, USA
Summary This data release is an inventory of runoff-generated postfire debris flows compiled from 17 burn areas across five western U.S. states. Debris-flow data from the following fires are included: Arizona: 2017 Pinal and 2019 Woodbury Fires California: 2020 Apple, 2020 Bond, 2015 Butte, 2020 El Dorado, 2014 El Portal, 2018 Ferguson, 2016 Fish (San Gabriel Complex), 2011 Motor, and 2017 ThomasHow long do runoff-generated debris-flow hazards persist after wildfire?
Runoff-generated debris flows are a potentially destructive and deadly response to wildfire until sufficient vegetation and soil-hydraulic recovery have reduced susceptibility to the hazard. Elevated debris-flow susceptibility may persist for several years, but the controls on the timespan of the susceptible period are poorly understood. To evaluate the connection between vegetation recovery and dPostwildfire soil‐hydraulic recovery and the persistence of debris flow hazards
Deadly and destructive debris flows often follow wildfire, but understanding of changes in the hazard potential with time since fire is poor. We develop a simulation‐based framework to quantify changes in the hydrologic triggering conditions for debris flows as postwildfire infiltration properties evolve through time. Our approach produces time‐varying rainfall intensity‐duration thresholds for ruThe rainfall intensity-duration control of debris flows after wildfire
Increased wildfire activity in the western United States has exposed regional gaps in our understanding of postfire debris-flow generation. To address this problem, we characterized flows in an unstudied area to test the rainfall intensity-duration control of the hazard. Our rainfall measurements and field observations from the northern Sierra Nevada (California, USA) show that debris flows resultPostfire hydrologic response along the central California (USA) coast: Insights for the emergency assessment of postfire debris-flow hazards
The steep, tectonically active terrain along the Central California (USA) coast is well known to produce deadly and destructive debris flows. However, the extent to which fire affects debris-flow susceptibility in this region is an open question. We documented the occurrence of postfire debris floods and flows following the landfall of a storm that delivered intense rainfall across multiple burn aColumbia River Gorge Landslides
Extreme rainfall during two atmospheric river events in January 2021 and January 2022 triggered a series of debris flows in the Columbia River Gorge, Oregon. The flows had significant impacts, including multiple highway closures and one fatality. This story map highlights rainfall data and observations of debris flow deposits by the Oregon Department of Geology and Mineral Industries (DOGAMI)...Dixie Fire Post-Fire Debris Flows: A Tale of Two Storms
This new geonarrative (Esri story map) highlights two storm events and their postfire impacts in the Dixie burn area.
As part of the Post-fire Hazards and Impacts to Resources and Ecosystems (PHIRE): Support for Response, Recovery, and Mitigation Project, the PHIRE Debris Flow Hazard team is engaging in several studies to better understand the spatial and temporal drivers of postfire debris flows and improve postfire hazard assessments across northern California and the Pacific Northwest.
Postfire Debris Flows
Wildfires leave steep slopes vulnerable to debris flows, a fast-moving mixture of water, soil, and rock that can cause property damage and loss of life. After a wildfire, emergency managers need rapid answers to the questions: Where in the burn area are debris flows likely? How much rain will it take to trigger a debris flow? And how big will that debris flow be? The USGS is conducting applied research and emergency hazard assessments to answer these questions. The results of this work will be used to develop hazard maps that identify the likelihood and potential volume of debris flows across the burn area and identify rainfall thresholds that must be exceeded to trigger debris flows. These thresholds help inform early warning operations by the National Weather Service.
Improving predictions in northern California and the Pacific Northwest
Northern California and the Pacific Northwest have recently experienced an increase in wildfire activity. In response, the USGS has been conducting debris-flow hazard assessments for wildfires in the region at the request of federal, state, and tribal agencies. Evaluated burn areas included the nearly million-acre 2021 Dixie fire (the largest single wildfire in California state history, Figure 1), the 2020 Labor Day wildfires in Oregon, and many of the 2021 wildfires in Washington. Assessments for new wildfires in the region and throughout the U.S. are conducted each year.
The models used for hazard assessment have been developed and tested in semi-arid parts of the country with a history of postfire debris flows, such as southern California and Colorado. But the models have not been tested extensively in wetter regions like northern California and the Pacific Northwest. Important questions that are being addressed as part of the PHIRE project are: Are the models accurate for northern California and the Pacific Northwest? And are the rainfall triggering conditions for postfire debris flows similar in different climates? The USGS and partners have begun monitoring postfire debris-flow activity in the region to answer these questions. The data are being used to test and subsequently improve the models used for hazard assessment.
Postfire monitoring involves installing rain gauges within the burn area and documenting the flow response following rainstorms. The flow response could be a debris flow, a less hazardous water flow, or no flow at all. The observed flow response in monitored drainage basins is compared to the predicted likelihood of a debris flow for the measured rainfall rates. If the model predicted a high likelihood of debris flow and there was a debris flow, the model got it right. The model is also considered successful if it predicts a low likelihood of debris flow and one did not occur. Systematically evaluating where the model makes correct and incorrect predictions helps identify missing factors that could lead to a better model. The same data also evaluates the accuracy of rainfall thresholds used for early warning.
Recent data collection has created an inventory of over 500 new records of flow activity in northern California and the Pacific Northwest (e.g., Graber, 2023a; Thomas et al., 2023a). The data reveals both similarities and differences in debris-flow processes between the northwest and drier areas like southern California. For example, in the 2021 Dixie fire (northern CA) and 2021 Muckamuck fire (north-central Washington), major debris flows (Figure 2 and Figure 3) were triggered by short bursts of intense rainfall like they are in better-studied semi-arid regions (Kostelnik et al., 2023a; Thomas et al., 2023b). In contrast, debris flows in the 2017 Eagle Creek Fire, Oregon were triggered by prolonged rainfall on nearly saturated soils (Kostelnik et al., 2023b). Those hydrologic triggering conditions are like the triggering conditions for shallow landslides and debris flows in unburned areas (Thomas et al., 2021). Continued observation and testing (e.g., Thomas et al., 2023c) is expected to lead to an improved set of hazard assessment models that better reflect regional variations in susceptibility.
How far? and How Long?
Other frequently asked questions after wildfires are: How far will debris flows travel? What will be impacted? And how long will the burn area remain hazardous? These questions are presently not addressed in current debris-flow hazard assessments, but the USGS is doing research to provide answers soon.
Predicting where debris flows will travel is challenging. Debris flows can have peak flows many times greater than floods. They can also easily plug culverts and bridge underpasses, redirecting dangerous flow into neighborhoods outside the floodplain (Figure 4). The USGS and partners are using recent debris-flow events, such as the 2018 debris flows in Montecito, CA, to test models for predicting debris-flow runout (e.g., Barnhart et al., 2021). The goal is to complement the current set of hazard maps with additional maps of possible debris-flow inundation. The inundation maps could inform emergency response and evacuation plans and identify homes and structures that could be damaged (Barnhart et al., 2024). The USGS is developing this approach with input from emergency managers to ensure the product meets their needs (Barnhart et al., 2023).
How long a burn area will remain susceptible to floods and debris flows largely depends on how long it takes stabilizing vegetation to return. And the rate of vegetation recovery depends on the type of vegetation, local climate, and weather conditions after a wildfire. Some burn areas recover quickly in two years or less. Others, such as those in a drought, may remain locked in a high hazard state for several years after wildfire. New research has shown that when a satellite-measured index of vegetation density reaches two-thirds of its prefire level, debris flows triggered by distributed runoff and erosion are unlikely (Graber et al., 2023). This result indicates that full recovery of vegetation is not necessary to substantially reduce postfire hazards. The vegetation recovery ratio also provides an objective way to determine when expensive warning operations can be ramped down, or when parts of the burn area can safely be reopened for public use.
Funding
Funding for this project is provided by the Robert T. Stafford Disaster Relief and Emergency Assistance Act (42 U.S.C. 5121 et seq.) and supplemental funding acts for Federal disaster relief activities. Through this funding USGS supports recovery efforts in declared natural disaster areas, to aid recovery efforts from widespread wildfires, devastating hurricanes, prolonged volcanic eruptions, and damaging earthquakes. This enables USGS to repair and replace equipment and facilities, collect high-resolution elevation data, and conduct scientific studies and assessments to support recovery and rebuilding decisions.
More Information
Emergency Assessment of postfire debris-flow hazards
Recent Postfire Events
Early Warning For Burn Areas
References
-
Multi-model comparison of computed debris flow runout for the 9 January 2018 Montecito, California post-wildfire event
Hazard assessment for post-wildfire debris flows, which are common in the steep terrain of the western United States, has focused on the susceptibility of upstream basins to generate debris flows. However, reducing public exposure to this hazard also requires an assessment of hazards in downstream areas that might be inundated during debris flow runout. Debris flow runout models are widely availabUser needs assessment for postfire debris-flow inundation hazard products
Debris flows are a type of mass movement that is more likely after wildfires, and while existing hazard assessments evaluate the rainfall intensities that are likely to trigger debris flows, no operational hazard assessment exists for identifying the areas where they will run out after initiation. Fifteen participants who work in a wide range of job functions associated with southern California poEvaluation of debris-flow building damage forecasts
Reliable forecasts of building damage due to debris flows may provide situational awareness and guide land and emergency management decisions. Application of debris-flow runout models to generate such forecasts requires combining hazard intensity predictions with fragility functions that link hazard intensity with building damage. In this study, we evaluated the performance of building damage foreCompilation of runoff-generated debris-flow inventories for 17 fires across Arizona, California, Colorado, New Mexico, and Washington, USA
Summary This data release is an inventory of runoff-generated postfire debris flows compiled from 17 burn areas across five western U.S. states. Debris-flow data from the following fires are included: Arizona: 2017 Pinal and 2019 Woodbury Fires California: 2020 Apple, 2020 Bond, 2015 Butte, 2020 El Dorado, 2014 El Portal, 2018 Ferguson, 2016 Fish (San Gabriel Complex), 2011 Motor, and 2017 ThomasHow long do runoff-generated debris-flow hazards persist after wildfire?
Runoff-generated debris flows are a potentially destructive and deadly response to wildfire until sufficient vegetation and soil-hydraulic recovery have reduced susceptibility to the hazard. Elevated debris-flow susceptibility may persist for several years, but the controls on the timespan of the susceptible period are poorly understood. To evaluate the connection between vegetation recovery and dPostwildfire soil‐hydraulic recovery and the persistence of debris flow hazards
Deadly and destructive debris flows often follow wildfire, but understanding of changes in the hazard potential with time since fire is poor. We develop a simulation‐based framework to quantify changes in the hydrologic triggering conditions for debris flows as postwildfire infiltration properties evolve through time. Our approach produces time‐varying rainfall intensity‐duration thresholds for ruThe rainfall intensity-duration control of debris flows after wildfire
Increased wildfire activity in the western United States has exposed regional gaps in our understanding of postfire debris-flow generation. To address this problem, we characterized flows in an unstudied area to test the rainfall intensity-duration control of the hazard. Our rainfall measurements and field observations from the northern Sierra Nevada (California, USA) show that debris flows resultPostfire hydrologic response along the central California (USA) coast: Insights for the emergency assessment of postfire debris-flow hazards
The steep, tectonically active terrain along the Central California (USA) coast is well known to produce deadly and destructive debris flows. However, the extent to which fire affects debris-flow susceptibility in this region is an open question. We documented the occurrence of postfire debris floods and flows following the landfall of a storm that delivered intense rainfall across multiple burn aColumbia River Gorge Landslides
Extreme rainfall during two atmospheric river events in January 2021 and January 2022 triggered a series of debris flows in the Columbia River Gorge, Oregon. The flows had significant impacts, including multiple highway closures and one fatality. This story map highlights rainfall data and observations of debris flow deposits by the Oregon Department of Geology and Mineral Industries (DOGAMI)...Dixie Fire Post-Fire Debris Flows: A Tale of Two Storms
This new geonarrative (Esri story map) highlights two storm events and their postfire impacts in the Dixie burn area.