Rainfall and Landslides in Northern California Active
A summary of recent and past landslides and debris flows caused by rainfall in northern California, and links to current shallow landslide monitoring.
California is well-known to be susceptible to landslides. These range from small, but potentially fatal, shallow landslides that may mobilize into rapidly moving debris flows, to larger, deep-seated landslides that are capable of moving entire houses downslope. Coastal cliff collapses and cliff erosion also present landslide hazards along the coast of northern California, and debris flows from burned areas are a particular concern following wildfires in the region. Landslides in the state generally occur due to precipitation, and to a lesser extent, earthquakes. Historically, large winter storms have caused the most damage, and in the highly developed San Francisco Bay area in northern California these impacts have sometimes been quite severe, causing both fatalities and significant property damage (for example the January 1982 winter storm - see Landslides, Floods, and Marine Effects of the Storm of January 3-5, 1982, in the San Francisco Bay Region, California - USGS Professional Paper 1434). More recently, storms in January and February of 2017 caused widespread landsliding throughout the East Bay region of the San Francisco Bay area and elsewhere. In Marin County, north of San Francisco, a debris flow destroyed several homes following a particularly intense storm on February 14, 2019 (see Data from the Sausalito, California debris flow). Elsewhere, in the Sierra Nevada foothills of Tuolumne County, a small but intense storm on March 22, 2018 caused hundreds of landslides that led to severe erosion and sedimentation of the Tuolumne River and a downstream storage reservoir (Don Pedro Reservoir).
The USGS conducts active research on identifying the triggering mechanisms and hazards associated with landsliding. In northern California, research efforts have predominantly focused on the San Francisco Bay area, and a number of products are available that showcase the landslide effects of previous large winter storms on the region, as well as current efforts to identify the soil moisture conditions that make the region susceptible to shallow landslides and debris flows at particular times of the year. The USGS also works on rock fall hazards in northern California with efforts primarily focused on assisting the National Park Service in Yosemite National Park.
Shallow Landslides and Debris Flows
Shallow landslides are generally those less than 3-5 m (10-15 ft) in depth. When shallow landslides are sufficiently wet, they may move rapidly and can be highly mobile over long distances – these are termed debris flows. Whereas shallow landslides can cause property and infrastructure damage, and clog streams and drainages, debris flows are more likely to cause injuries and fatalities through their ability to move quickly and with large amounts of debris (soil, boulders, trees, etc.). Shallow landslides and debris flows are most often generated by intense rainfall, with rainfall rates measured in tens of millimeters per hour (few to several inches per hour).
Shallow landslides can occur at any point during a winter season in the San Francisco Bay area, but most often occur once the ground is nearly saturated – this typically occurs after the first few winter storms come in the November and December timeframe. These conditions, termed the “antecedent moisture conditions” have been correlated with seasonal rainfall and maps are available based on previous storms (see Landslides, Floods, and Marine Effects of the Storm of January 3-5, 1982, in the San Francisco Bay Region, California - USGS Professional Paper 1434 Plate 1).
Generally, researchers have found that at least 25 cm (10 in) of rainfall is needed to nearly saturate the ground on San Francisco Bay area hillslopes. Because antecedent conditions do not account for drying and evaporation between storms, USGS researchers have been developing methods to monitor the soil moisture of landslide prone hillslopes directly. This research project aims to quantify the soil moisture conditions responsible for shallow landslides and consequent debris flows.
It is often hard to identify exactly where shallow landslides and debris flows will occur. Instead, researchers use models to develop debris flow susceptibility maps that are, in turn, based on measurements of soil type and depth, and topographic slope and shape. Debris flow source area maps based on measured topographic parameters are available for the entire San Francisco Bay area (see San Francisco Bay Region Landslide Folio Part E – Map of debris-flow source areas in the San Francisco Bay Region, California - USGS Open File Report 97-745E) – these indicate areas with hazard potential for debris flows should seasonal cumulative rainfall and storm rainfall intensity thresholds be exceeded. Storm rainfall thresholds have also been calculated for the entire San Francisco Bay area (see San Francisco Bay Region Landslide Folio Part F – Preliminary maps showing rainfall thresholds for debris-flow activity, San Francisco Bay Region, California - USGS Open File Report 97-745F).
Soil Moisture Monitoring for Shallow Landslide Initiation in the San Francisco Bay Area
The USGS conducts active monitoring of the soil moisture in several locations throughout the San Francisco Bay area. These data provide an indication of the “soil wetness” (that is, the antecedent soil moisiture conditions) that may lead to widespread shallow landsliding and potentially debris flows if a storm of sufficient intensity impacts the region. If the soil moisture is low, even high rainfall intensity may not lead to landslides. On the other hand, high soil moisture and high rainfall intensity are more likely to cause widespread landsliding with consequences such as road closures, property damage, and potential fatalities. Residents concerned with the potential for landslide hazards where they live can find helpful information here.
- Current soil moisture conditions for the East Bay region
- Current soil moisture conditions for Marin County region
- Current soil moisture conditions for the San Francisco region
- Current soil moisture conditions for the San Mateo County region
Note: Soil moisture monitoring sites provide highly site-specific information and do not necessarily exemplify the regional conditions.
Deep-seated Landslides
Deep-seated landslides are generally those greater than 3-5 m (10-15 ft) in depth. These landslides are often generated by prolonged above-average rainfall which can occur during El Niño years, although even “normal” precipitation years in northern California can lead to landslide initiation. Typically, deep-seated landslides occur towards the end of the winter season (March, April, May) due to the time it takes for seasonal rainfall to reach the bottom “slip surface” of the landslide. However, heavy rain earlier in the season can also have this effect. Often, deep-seated landslides lay dormant for lengthy periods of time.
Many deep-seated landslides have been mapped in the San Francisco Bay area (see San Francisco Bay Region Landslide Folio Part C – Summary distribution of slides and earth flows in the San Francisco Bay Region, California - USGS Open File Report 97-745C), and often a qualified geotechnical engineer or engineering geologist may be able to determine if particular properties are on a landslide or susceptible to landslide movement. In 1997, the USGS developed a simple way to determine the locations of landslides that have occurred in the past throughout the San Francisco Bay Area by referencing published topographic quadrangle maps (see San Francisco Bay Region Landslide Folio Part D – Index to detailed maps of landslides in the San Francisco Bay Region, California - USGS Open File Report 97-745D). The California Geological Survey currently tracks the location of deep-seated landslides using an inventory.
Generally, deep-seated landslides do not cause injuries or fatalities; rather, they move slowly and can severely distort and damage buildings and infrastructure such as roads and pipelines. Sometimes, smaller shallow landslides can initiate from the bottom (or toe) of deep-seated landslides. In these cases, the hazard of the shallow landslides turning into debris flows should be assessed.
Recently Burned Areas
Steep, recently burned areas are especially susceptible to debris flows. Even modest rain storms during normal, non-El Niño years can trigger post-wildfire debris flows. In many respects, the hazards associated with debris flows from burned areas are similar to those from shallow landslides, although the precipitation thresholds for initiation are often lower. In addition, there is typically no antecedent soil moisture condition required for initiation. The USGS has conducted hazard assessments for post-wildfire debris flows for recent fires in northern California, as well as numerous fires across the Western U.S. including southern California.
- 2019 Kincade Fire, Sonoma County
- 2019 Briceburg Fire, Mariposa County
- 2018 Camp Fire, Butte County
- 2018 Mendocino Complex – River Fire, Lake and Mendocino Counties
- 2018 Mendocino Complex – Ranch Fire, Lake and Mendocino Counties
Coastal Cliff Erosion
Many areas of coastal California are subject to cliff erosion and coastal landslides. Hazards from these types of landslides can occur both at the bottom of cliffs (from burial) and at the tops of cliffs (from falling over). During the winter season in California, beaches typically erode thereby allowing waves to reach further inland and to inundate the bottoms of coastal cliffs. Wave energy is also typically higher during the winter, and particularly during El Niño events, thereby exacerbating the potential for coastal erosion. Coastal cliff failures may also occur simply as a result of heightened precipitation – wave action makes cliffs inherently unstable, and rainfall may be the ultimate trigger for failure, even during times with little to no wave action.
During and just after storms, existing coastal landslides may become reactivated and seemingly stable coastal cliffs may erode and fail rapidly. Background rates of coastal cliff erosion are variable along the California coast (see National Assessment of Shoreline Change Part 4: Historical Coastal Cliff Retreat along the California Coast - USGS Open File Report 2007-1133) and tied to the rock or soil strength of the cliffs among other factors, but these measurements of historic coastal cliff retreat provide indications of places most susceptible to coastal landslides.
Data and Mapping of Effects of Previous Landslide-Triggering Storms in the San Francisco Bay Area
Data supporting USGS efforts to address landslide hazards in the San Francisco Bay area and elsewhere in northern California can be found here and by following the links below.
- Effects of the 1968-1969 El Niño (Misc Field Studies Map 327)
- Effects of the 1972-1973 El Niño (Misc Field Studies Map 679)
- Effects of the January 3-5, 1982 Storm (Prof Paper 1434)
- Preparation for the 1997-1998 El Niño (Open-File Report 97-745)
- Effects of the 1997-1998 El Niño (Misc Field Studies Map 2325-A-J)
- Effects of the February 2-3, 1998 Storm (Misc Field Studies Map 2384)
- Effects of the 1997-1998 El Niño in the East Bay (Scientific Investigations Map 2859)
- Effects of the January-February 2019 Storms in Marin County (Data Series 1112)
(last updated June 16, 2020)
Below are publications associated with this project.
Terrestrial lidar data of the February 14, 2019 Sausalito Boulevard Landslide, Sausalito, California
Variability in soil-water retention properties and implications for physics-based simulation of landslide early warning criteria
Monitoring subsurface hydrologic response for precipitation-induced shallow landsliding in the San Francisco Bay area, California, USA
Spatially explicit shallow landslide susceptibility mapping over large areas
Overview of the ARkStorm scenario
The U.S. Geological Survey, Multi Hazards Demonstration Project (MHDP) uses hazards science to improve resiliency of communities to natural disasters including earthquakes, tsunamis, wildfires, landslides, floods and coastal erosion. The project engages emergency planners, businesses, universities, government agencies, and others in preparing for major natural disasters. The project also helps to
National assessment of shoreline change, part 4: Historical coastal cliff retreat along the California coast
Preliminary soil-slip susceptibility maps, southwestern California
Landslides, Floods, and Marine Effects of the Storm of January 3-5, 1982, in the San Francisco Bay Region, California
- Overview
A summary of recent and past landslides and debris flows caused by rainfall in northern California, and links to current shallow landslide monitoring.
California is well-known to be susceptible to landslides. These range from small, but potentially fatal, shallow landslides that may mobilize into rapidly moving debris flows, to larger, deep-seated landslides that are capable of moving entire houses downslope. Coastal cliff collapses and cliff erosion also present landslide hazards along the coast of northern California, and debris flows from burned areas are a particular concern following wildfires in the region. Landslides in the state generally occur due to precipitation, and to a lesser extent, earthquakes. Historically, large winter storms have caused the most damage, and in the highly developed San Francisco Bay area in northern California these impacts have sometimes been quite severe, causing both fatalities and significant property damage (for example the January 1982 winter storm - see Landslides, Floods, and Marine Effects of the Storm of January 3-5, 1982, in the San Francisco Bay Region, California - USGS Professional Paper 1434). More recently, storms in January and February of 2017 caused widespread landsliding throughout the East Bay region of the San Francisco Bay area and elsewhere. In Marin County, north of San Francisco, a debris flow destroyed several homes following a particularly intense storm on February 14, 2019 (see Data from the Sausalito, California debris flow). Elsewhere, in the Sierra Nevada foothills of Tuolumne County, a small but intense storm on March 22, 2018 caused hundreds of landslides that led to severe erosion and sedimentation of the Tuolumne River and a downstream storage reservoir (Don Pedro Reservoir).
The USGS conducts active research on identifying the triggering mechanisms and hazards associated with landsliding. In northern California, research efforts have predominantly focused on the San Francisco Bay area, and a number of products are available that showcase the landslide effects of previous large winter storms on the region, as well as current efforts to identify the soil moisture conditions that make the region susceptible to shallow landslides and debris flows at particular times of the year. The USGS also works on rock fall hazards in northern California with efforts primarily focused on assisting the National Park Service in Yosemite National Park.
Shallow Landslides and Debris Flows
Shallow landslides are generally those less than 3-5 m (10-15 ft) in depth. When shallow landslides are sufficiently wet, they may move rapidly and can be highly mobile over long distances – these are termed debris flows. Whereas shallow landslides can cause property and infrastructure damage, and clog streams and drainages, debris flows are more likely to cause injuries and fatalities through their ability to move quickly and with large amounts of debris (soil, boulders, trees, etc.). Shallow landslides and debris flows are most often generated by intense rainfall, with rainfall rates measured in tens of millimeters per hour (few to several inches per hour).
Shallow landslides can occur at any point during a winter season in the San Francisco Bay area, but most often occur once the ground is nearly saturated – this typically occurs after the first few winter storms come in the November and December timeframe. These conditions, termed the “antecedent moisture conditions” have been correlated with seasonal rainfall and maps are available based on previous storms (see Landslides, Floods, and Marine Effects of the Storm of January 3-5, 1982, in the San Francisco Bay Region, California - USGS Professional Paper 1434 Plate 1).
Generally, researchers have found that at least 25 cm (10 in) of rainfall is needed to nearly saturate the ground on San Francisco Bay area hillslopes. Because antecedent conditions do not account for drying and evaporation between storms, USGS researchers have been developing methods to monitor the soil moisture of landslide prone hillslopes directly. This research project aims to quantify the soil moisture conditions responsible for shallow landslides and consequent debris flows.
It is often hard to identify exactly where shallow landslides and debris flows will occur. Instead, researchers use models to develop debris flow susceptibility maps that are, in turn, based on measurements of soil type and depth, and topographic slope and shape. Debris flow source area maps based on measured topographic parameters are available for the entire San Francisco Bay area (see San Francisco Bay Region Landslide Folio Part E – Map of debris-flow source areas in the San Francisco Bay Region, California - USGS Open File Report 97-745E) – these indicate areas with hazard potential for debris flows should seasonal cumulative rainfall and storm rainfall intensity thresholds be exceeded. Storm rainfall thresholds have also been calculated for the entire San Francisco Bay area (see San Francisco Bay Region Landslide Folio Part F – Preliminary maps showing rainfall thresholds for debris-flow activity, San Francisco Bay Region, California - USGS Open File Report 97-745F).
Soil Moisture Monitoring for Shallow Landslide Initiation in the San Francisco Bay Area
The USGS conducts active monitoring of the soil moisture in several locations throughout the San Francisco Bay area. These data provide an indication of the “soil wetness” (that is, the antecedent soil moisiture conditions) that may lead to widespread shallow landsliding and potentially debris flows if a storm of sufficient intensity impacts the region. If the soil moisture is low, even high rainfall intensity may not lead to landslides. On the other hand, high soil moisture and high rainfall intensity are more likely to cause widespread landsliding with consequences such as road closures, property damage, and potential fatalities. Residents concerned with the potential for landslide hazards where they live can find helpful information here.
- Current soil moisture conditions for the East Bay region
- Current soil moisture conditions for Marin County region
- Current soil moisture conditions for the San Francisco region
- Current soil moisture conditions for the San Mateo County region
Note: Soil moisture monitoring sites provide highly site-specific information and do not necessarily exemplify the regional conditions.
Deep-seated Landslides
Deep-seated landslides are generally those greater than 3-5 m (10-15 ft) in depth. These landslides are often generated by prolonged above-average rainfall which can occur during El Niño years, although even “normal” precipitation years in northern California can lead to landslide initiation. Typically, deep-seated landslides occur towards the end of the winter season (March, April, May) due to the time it takes for seasonal rainfall to reach the bottom “slip surface” of the landslide. However, heavy rain earlier in the season can also have this effect. Often, deep-seated landslides lay dormant for lengthy periods of time.
Many deep-seated landslides have been mapped in the San Francisco Bay area (see San Francisco Bay Region Landslide Folio Part C – Summary distribution of slides and earth flows in the San Francisco Bay Region, California - USGS Open File Report 97-745C), and often a qualified geotechnical engineer or engineering geologist may be able to determine if particular properties are on a landslide or susceptible to landslide movement. In 1997, the USGS developed a simple way to determine the locations of landslides that have occurred in the past throughout the San Francisco Bay Area by referencing published topographic quadrangle maps (see San Francisco Bay Region Landslide Folio Part D – Index to detailed maps of landslides in the San Francisco Bay Region, California - USGS Open File Report 97-745D). The California Geological Survey currently tracks the location of deep-seated landslides using an inventory.
Generally, deep-seated landslides do not cause injuries or fatalities; rather, they move slowly and can severely distort and damage buildings and infrastructure such as roads and pipelines. Sometimes, smaller shallow landslides can initiate from the bottom (or toe) of deep-seated landslides. In these cases, the hazard of the shallow landslides turning into debris flows should be assessed.
Recently Burned Areas
Steep, recently burned areas are especially susceptible to debris flows. Even modest rain storms during normal, non-El Niño years can trigger post-wildfire debris flows. In many respects, the hazards associated with debris flows from burned areas are similar to those from shallow landslides, although the precipitation thresholds for initiation are often lower. In addition, there is typically no antecedent soil moisture condition required for initiation. The USGS has conducted hazard assessments for post-wildfire debris flows for recent fires in northern California, as well as numerous fires across the Western U.S. including southern California.
- 2019 Kincade Fire, Sonoma County
- 2019 Briceburg Fire, Mariposa County
- 2018 Camp Fire, Butte County
- 2018 Mendocino Complex – River Fire, Lake and Mendocino Counties
- 2018 Mendocino Complex – Ranch Fire, Lake and Mendocino Counties
Coastal Cliff Erosion
Many areas of coastal California are subject to cliff erosion and coastal landslides. Hazards from these types of landslides can occur both at the bottom of cliffs (from burial) and at the tops of cliffs (from falling over). During the winter season in California, beaches typically erode thereby allowing waves to reach further inland and to inundate the bottoms of coastal cliffs. Wave energy is also typically higher during the winter, and particularly during El Niño events, thereby exacerbating the potential for coastal erosion. Coastal cliff failures may also occur simply as a result of heightened precipitation – wave action makes cliffs inherently unstable, and rainfall may be the ultimate trigger for failure, even during times with little to no wave action.
During and just after storms, existing coastal landslides may become reactivated and seemingly stable coastal cliffs may erode and fail rapidly. Background rates of coastal cliff erosion are variable along the California coast (see National Assessment of Shoreline Change Part 4: Historical Coastal Cliff Retreat along the California Coast - USGS Open File Report 2007-1133) and tied to the rock or soil strength of the cliffs among other factors, but these measurements of historic coastal cliff retreat provide indications of places most susceptible to coastal landslides.
Data and Mapping of Effects of Previous Landslide-Triggering Storms in the San Francisco Bay Area
Data supporting USGS efforts to address landslide hazards in the San Francisco Bay area and elsewhere in northern California can be found here and by following the links below.
- Effects of the 1968-1969 El Niño (Misc Field Studies Map 327)
- Effects of the 1972-1973 El Niño (Misc Field Studies Map 679)
- Effects of the January 3-5, 1982 Storm (Prof Paper 1434)
- Preparation for the 1997-1998 El Niño (Open-File Report 97-745)
- Effects of the 1997-1998 El Niño (Misc Field Studies Map 2325-A-J)
- Effects of the February 2-3, 1998 Storm (Misc Field Studies Map 2384)
- Effects of the 1997-1998 El Niño in the East Bay (Scientific Investigations Map 2859)
- Effects of the January-February 2019 Storms in Marin County (Data Series 1112)
(last updated June 16, 2020)
- Publications
Below are publications associated with this project.
Terrestrial lidar data of the February 14, 2019 Sausalito Boulevard Landslide, Sausalito, California
On February 14, 2019, just before 2:56 am local time (Pacific Standard Time), a landslide initiated from the hillslopes above the Hurricane Gulch section of the City of Sausalito, Marin County, California. The landslide, specifically classified as a debris flow, overran a road (Sausalito Boulevard) immediately below the landslide source area and impacted a residential structure that subsequently tAuthorsBrian D. Collins, Skye C. CorbettVariability in soil-water retention properties and implications for physics-based simulation of landslide early warning criteria
Rainfall-induced shallow landsliding is a persistent hazard to human life and property. Despite the observed connection between infiltration through the unsaturated zone and shallow landslide initiation, there is considerable uncertainty in how estimates of unsaturated soil-water retention properties affect slope stability assessment. This source of uncertainty is critical to evaluating the utilitAuthorsMatthew A. Thomas, Benjamin B. Mirus, Brian D. Collins, Ning Lu, Jonathan W. GodtMonitoring subsurface hydrologic response for precipitation-induced shallow landsliding in the San Francisco Bay area, California, USA
Intense winter storms in the San Francisco Bay area (SFBA) of California, USA often trigger shallow landslides. Some of these landslides mobilize into potentially hazardous debris flows. A growing body of research indicates that rainfall intensity-duration thresholds are insufficient for accurate prediction of landslide occurrence. In response, we have begun long-term monitoring of the hydrologicAuthorsBrian D. Collins, Jonathan D. Stock, Lisa C. Weber, K. Whitman, N. KnepprathSpatially explicit shallow landslide susceptibility mapping over large areas
Recent advances in downscaling climate model precipitation predictions now yield spatially explicit patterns of rainfall that could be used to estimate shallow landslide susceptibility over large areas. In California, the United States Geological Survey is exploring community emergency response to the possible effects of a very large simulated storm event and to do so it has generated downscaled pAuthorsDino Bellugi, William E. Dietrich, Jonathan D. Stock, Jim McKean, Brian Kazian, Paul HargroveByEnergy and Minerals Mission Area, Natural Hazards Mission Area, Energy Resources Program, Groundwater and Streamflow Information Program, Landslide Hazards Program, Mineral Resources Program, National Laboratories Program, Science and Decisions Center, Geologic Hazards Science Center, Geology, Minerals, Energy, and Geophysics Science CenterOverview of the ARkStorm scenario
The U.S. Geological Survey, Multi Hazards Demonstration Project (MHDP) uses hazards science to improve resiliency of communities to natural disasters including earthquakes, tsunamis, wildfires, landslides, floods and coastal erosion. The project engages emergency planners, businesses, universities, government agencies, and others in preparing for major natural disasters. The project also helps to
AuthorsKeith Porter, Anne Wein, Charles N. Alpers, Allan Baez, Patrick L. Barnard, James Carter, Alessandra Corsi, James Costner, Dale Cox, Tapash Das, Mike Dettinger, James Done, Charles Eadie, Marcia Eymann, Justin Ferris, Prasad Gunturi, Mimi Hughes, Robert Jarrett, Laurie Johnson, Hanh Dam Le-Griffin, David Mitchell, Suzette Morman, Paul Neiman, Anna Olsen, Suzanne Perry, Geoffrey Plumlee, Martin Ralph, David Reynolds, Adam Rose, Kathleen Schaefer, Julie Serakos, William Siembieda, Jonathan D. Stock, David Strong, Ian Sue Wing, Alex Tang, Pete Thomas, Ken Topping, Chris Wills, Lucile JonesByNatural Hazards Mission Area, Coastal and Marine Hazards and Resources Program, Earthquake Hazards Program, Science Application for Risk Reduction, Geologic Hazards Science Center, Pacific Coastal and Marine Science Center, Big Sur Landslides, Reducing Risk, San Francisco Bay and Sacramento-San Joaquin Delta EstuaryNational assessment of shoreline change, part 4: Historical coastal cliff retreat along the California coast
Coastal cliff retreat, the landward migration of the cliff face, is a chronic problem along many rocky coastlines in the United States. As coastal populations continue to grow and community infrastructures are threatened by erosion, there is increased demand for accurate information regarding trends and rates of coastal cliff retreat. There is also a need for a comprehensive analysis of cliff retrAuthorsCheryl J. Hapke, David ReidPreliminary soil-slip susceptibility maps, southwestern California
This group of maps shows relative susceptibility of hill slopes to the initiation sites of rainfall-triggered soil slip-debris flows in southwestern California. As such, the maps offer a partial answer to one part of the three parts necessary to predict the soil-slip/debris-flow process. A complete prediction of the process would include assessments of “where”, “when”, and “how big”. These maps emAuthorsDouglas M. Morton, Rachel M. Alvarez, Russell H. Campbell, Kelly R. Bovard, D. T. Brown, K. M. Corriea, J. N. LesserLandslides, Floods, and Marine Effects of the Storm of January 3-5, 1982, in the San Francisco Bay Region, California
A catastrophic rainstorm in central California on January 3-5,1982, dropped as much as half the mean annual precipitation within a period of about 32 hours, triggering landslides and floods throughout 10 counties in the vicinity of the San Francisco Bay. More than 18,000 of the slides induced by the storm transformed into debris flows that swept down hillslopes or drainages with little warning. DeAuthorsGerald F. Wieczorek