EMERGENCY ASSESSMENT OF DEBRIS-FLOW HAZARDS FROM BASINS BURNED BY THE 2007 RANCH FIRE, VENTURA AND LOS ANGELES COUNTIES, SOUTHERN CALIFORNIA

By

Susan H. Cannon,1 Joseph E. Gartner, John A. Michael, Mark A. Bauer, Susan C. Stitt, Donna L. Knifong, Bernard J. McNamara, and Yvonne M. Roque

1

U.S. Geological Survey, Box 25046, DFC, MS 966, Denver CO 80225, (303) 273-8604, cannon@usgs.gov

2007

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§¦ ̈5

Naylor

Place

Martins

Del

Valle

Val

Verde

Camulos

Piru

U126

U126

Canton

Canyon

Holser Canyon

Lechler Canyon

Oak

Canyon

Santa Felicia Canyon

Devil Canyon

Sharps Canyon

Turtle Canyon

Reasoner Canyon

Dominguez Canyon

Lime Canyon

Blanchard Canyon

Toms Canyon

34°35'

34°30'

34°25'

118°55' 118°50' 118°45' 118°40'

§ ̈¦5

§ ̈¦5

Sulphur Creek

Spring Canyon Creek

Maple Creek

Elm Creek

Tar Creek

Fourfork Creek

Little Sespe Creek

Hopper Canyon

Sespe Creek

Pole Creek

Piru Creek

Santa

Clara River

Santa Clara River

Castaic

Creek

Real Wash

Tar Creek

Agua Blanca Creek

East Fork Alder Creek

Stone Corral Creek

Piru Creek

Michael Creek

Castaic Creek

Lacosca

Creek

Piru Creek

Ranch

Fire

CALIFORNIA

Lake Piru

Elderberry

Forebay

Castaic

Lake

These maps are not to be used for flood insurance rating purposes under the National Flood Insurance 

Program. For insurance rating purposes, please refer to the currently effective Flood Insurance Rate 

Maps (FIRM) published by the Federal Emergency Management Agency (FEMA). To obtain a copy of the 

FIRM, contact the FEMA Map Service Center at (800) 358-9616, or at http://msc.fema.gov

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial 

standards or with the North American Stratigraphic code.

Any use of trade, firm, or product names is for descriptive purposes only, and does not imply 

endorsement by the U.S. Government.

Permission must be secured from the individual copyright owners to reproduce any copyrighted material 

contained within this report.

For sale by U.S. Geological Survey Information Services

Box 25286, Federal Center, Denver, CO 80225

1-888-ASK-USGS

A PDF of this report is available at: http://pubs.usgs.gov/of/2007/1418

 Introduction

The objective of this report is to present a preliminary emergency assessment of the potential for debris-flow generation from basins burned by the Ranch Fire in Ventura and Los 

Angeles Counties, southern California, in 2007. Debris flows are among the most hazardous geologic phenomena (Turner and Schuster, 1996); debris flows that followed wildfires in 

southern California in 2003 killed 16 people and caused tens of millions of dollars of property damage (NOAA-USGS Debris Flow Task Force, 2005). A short period of even moderate 

rainfall on a burned watershed can lead to debris flows (Cannon and others, 2008). Rainfall that is normally absorbed into hillslope soils can run off almost instantly after vegetation has 

been removed by wildfire. This causes much greater and more rapid runoff than is normal from creeks and drainage areas. Highly erodible soils in a burn scar allow flood waters to 

entrain large amounts of ash, mud, boulders, and unburned vegetation. Within the burned area and downstream, the force of rushing water, soil, and rock can destroy culverts, bridges, 

roadways, and buildings, potentially causing injury or death.

This emergency debris-flow hazard assessment is presented as a relative ranking of the predicted volume of debris flows that can issue from basin outlets in response to 2.25 inches 

(57.15 mm) of rainfall over a 3-hour period. Such a storm has a 10-year return period (Hershfield, 1961). The calculation of debris-flow volume is based on a multiple-regression statistical 

model that describes the median volume of material that can be expected from a recently burned basin as a function of the area burned at high and moderate severity, the basin area with 

slopes greater than or equal to 30 percent, and triggering storm rainfall (Gartner and others, 2008). Cannon and others (2007) describe the methods used to generate the hazard maps. 

Identification of potential debris-flow hazards from burned drainage basins is necessary to issue warnings for specific basins, to make effective mitigation decisions, and to help plan 

evacuation timing and routes. 

Results

Of the 187 basins evaluated in this assessment, five were identified as having the potential to produce debris flows with volumes less than 1,000 m3 in response to the 10-yearrecurrence, 3-hour rainstorm. These are the small (0.04 to 0.07 km2) basins that drain into Lake Piru, an unnamed tributary of Piru Creek east of Turtle Canyon, Agua Blanca Creek, and 

Holser Canyon. One hundred fifty-six of the basins showed the potential to produce debris flows with volumes between 1,001 and 10,000 m3. These are basins with areas between 0.07 

to 3.4 km2 that drain into Pole Creek, Hopper Canyon, Holser Canyon, Piru Creek, Canton Canyon, and Agua Blanca Creek, as well as unnamed canyons north of Val Verde and basins 

that drain into the Santa Clara River Valley between Camulos and Del Vale. Twenty-six of the basins evaluated showed volume estimates between 10,001 and 100,000 m3. From west to 

east, these include the headwaters of Pole Creek; Toms Canyon and three unnamed basins that drain into Hopper Canyon; Blanchard, Lime, Dominguez, Reasoner, Turtle, Sharps, Devil, 

Santa Felcia, Oak, and Lechler Canyons, Michael Creek, and four unnamed basins that drain into Lake Piru and Piru Creek; two unnamed basins that drain into Aqua Blanca Creek; three 

unnamed basins that drain into Canton Canyon; and one unnamed basin that drains into Val Verde, and one that drains into the Santa Clara River Valley. Debris flows issuing from these 

basins can impact highways and roads in the area. In addition, neighborhoods, buildings, and people within, or immediately below, any of the burned basins can be impacted by debris 

flows.

In response to a storm similar to that used in this assessment, Lake Piru can expect significant accumulations of sediment, and associated water quality issues.

Use and Limitations of the Map

This map shows potential hazards posed by debris flows as estimates of the median volumes of material that may issue from the outlets of basins burned by the Ranch Fire of 2007 in 

southern California in response to a 10-year-recurrence, 3-hour-duration rainstorm. The map identifies the range of potential debris-flow volumes that can issue from individual basin 

outlets. This information can be used to issue warning for specific locations, to prioritize mitigation efforts, to aid in the design of mitigation structures, and to guide decisions for evacuation, shelter, and escape routes in the event that storms of similar magnitude to that evaluated here are forecast for the area. 

In addition to the potential dangers within the basins, areas downstream from the basin outlets are also at risk. The danger is particularly high in canyon bottoms (shown as shaded 

areas on the map). In some of these areas homes were destroyed by the fire, and workers and residents may be busy cleaning and rebuilding sites. The potential for debris flows during 

rainfall events places these people at high risk. In addition, if culverts are plugged or overwhelmed by flows, or if roads wash out, motorists may be stranded for long periods of time. In 

some cases, channels cross roads on blind curves where motorists could abruptly encounter debris-flow deposits on the road. 

In addition to the colored drainage basins, small debris flows can be generated from non-colored areas within the burn perimeter. These areas were not included in the analysis 

because they are occupied by either planar hillslopes or basins that are smaller than those used in the model development (Cannon and others, 2007).

We expect that the map presented here may be applicable for approximately three years after the fires for the storm conditions considered. The potential for debris-flow activity 

decreases with time following fire and the concurrent revegetation and stabilization of hillslopes. A compilation of information on post-fire runoff events from throughout the western U.S. 

indicates that under normal rainfall conditions most debris-flow activity occurs within about two years following a fire. If dry conditions slow re-growth of vegetation, this recovery period 

will be longer. Our assessment is specific to post-fire debris flows; significant hazards from flash flooding can remain for many years after a fire.

This assessment is based on the assumption that all basins are equally prone to debris flows (Cannon and others, 2007). Recent work has indicated that, in addition to the volume, the 

probability of debris flow will vary with burn severity, basin gradient, material properties, and storm rainfall. Unfortunately, a determination of debris-flow probability cannot yet be 

incorporated into this hazard assessment.

Further, parts of Hopper Canyon were burned by the 1997 Hopper Fire, the canyons on the west side of Lake Piru were burned by the Piru Fire in 2003, and basins on the north margin 

of the fire were burned in 2006 by the Day Fire. Damaging debris flows and floods were produced from Hopper Canyon and the basins that drain into Piru Creek following the fires 

(Cannon and others, 2008). Although we can say that similar activity is possible from these areas now that they have burned by the Ranch fire, the specific effects of such repeat fires on 

erosion potential, and debris-flow generation, are unknown.

Suggested Actions

People occupying businesses, homes, and recreational facilities downstream of the basins identified as the most hazardous must be informed of the potential dangers from debris flows 

and flooding. Warning must be given even for those basins with engineered mitigation structures at their mouths in the event that the structures are not adequate to contain potential debris 

flows. Site-specific debris-flow hazard assessments ought to be performed upslope from structures and facilities in areas identified as being at risk. 

Because this assessment is specific to post-fire debris flows, further assessment of potential hazards posed by flash floods is needed. Continued operation of the early-warning system 

for both flash floods and debris flows established by NOAA’s National Weather Service and the U.S. Geological Survey (http://www.wrh.noaa.gov/sgx/hydro/debris_flow.php; NOAAUSGS Debris Flow Task Force, 2005) would help local officials make decisions about evacuations and inform the public about potential dangers of debris flows in advance of rainfall 

events. The system consists of an extensive reporting rain-gage and stream-gage network coupled with National Weather Service weather forecasts and radar rainfall measurements. Any 

early-warning system should be coordinated with existing county and flood district facilities. 

An evaluation of the effectiveness of hillslope and channel mitigation approaches focused on the ability of different treatment methods to decrease the potential volume of debris flows 

(deWolfe, 2006; deWolfe and others, 2008). This work found that extensive applications of treatments that promote rainfall infiltration into hillslopes combined with engineered works that 

control incision in low-gradient channel reaches can effectively mitigate debris-flow impacts in basins less than about two km2 in area that are expected to produce debris-flow volumes of 

less than 10,000 m3. Large engineered check dams or collection basins are necessary to effectively mitigate hazards posed by events from basins larger than about two km2 that are 

expected to produce debris-flow volumes greater than about 10,000 m3 ( Hungr and others, 1987; Fiebiger, 1997; Okubo and others, 1997; Heumader, 2000; deWolfe, 2006). 

References Cited

Cannon, S.H., Gartner, J.E., and Michael, J.A., 2007, Methods for the emergency assessment of debris-flow hazards from basins burned by the fires of 2007, southern California: U.S. 

Geological Survey Open-File Report 2007–1384, 10 p. 

Cannon, S.H., Gartner, J.E., Wilson, R.C., and Laber, J.L., 2008, Storm rainfall conditions for floods and debris flows from recently burned areas in southwestern Colorado and southern 

California: Geomorphology, doi:10.1019/j.geomorph.2008.03.019.

deWolfe, V.G., 2006, An evaluation of erosion control methods after wildfire in debris-flow prone areas after wildfire: Golden, Colorado School of Mines, Department of Geology and 

Engineering Geology, M.S. thesis, 186 p.

deWolfe, V.G., Santi, P.M., Ey, John, and Gartner, J.E., 2008, Effective mitigation of debris flows at Lemon dam, La Plata County, Colorado: Geomorphology, 

doi:10.1016/j.geomorph.2008.04.008.

Fiebiger, G., 1997, Structures of debris-flow countermeasures, in Chen, C.I., ed., Debris Flow Hazards Mitigation—Mechanics, Prediction, and Assessment, Proceedings of First International Conference: New York, N.Y., American Society of Civil Engineers, p. 596–606.

Gartner, J.E., Cannon, S.H., Santi, P.M., and deWolf, V.G., 2008, Empirical models to predict debris flow volumes generated from recently burned basins in the western U.S.: Geomorphology, doi:10.1016/j.geomorph.2008.02.033.

Hershfield, D.M., 1961, Rainfall frequency atlas of the United States for durations from 30 minutes to 24 hours and return periods from 1 to 100 years: Technical Paper No. 40, U.S. 

Department of Commerce, Weather Bureau, 61 p.

Heumader, J., 2000, Technical debris flow countermeasures in Austria—A review, in Wieczorek, G.F., and Naeser, N.D., eds., Debris flow Hazards Mitigation—Mechanics, Prediction, 

and Assessment, Proceedings of Second International Conference on Debris Flow Hazards Mitigation: Taipei, Taiwan August 16–18, 2000, A.A. Balkema, Brookfield, Vt., p. 553–564.

Hungr, Oldrich, Morgan, G.C., VanDine, D.F., and Lister, D.R., 1987, Debris flow defenses in British Columbia, in Costa, J.E., and Wieczorek, G.F., eds., Debris Flows/Avalanches— 

Process, Recognition, and Mitigation: Reviews in Engineering Geology vol. VII, The Geological Society of America, Boulder, Colo., p. 201–222.

NOAA-USGS Debris Flow Task Force, 2005, NOAA-USGS debris-flow warning system–Final report: U.S. Geological Survey Circular 1283, 47 p.

Okubo, S., Ikeya, H., Ishikawa, Y., and Yamada, T., 1997, Development of new methods for countermeasures against debris flows, in Armani, A., and Michiue, M., eds., Recent Developments on Debris Flows, Lecture Notes in Earth Sciences, vol., 64, Springer-Verlag, New York, N.Y., p. 166–185.

Turner, A.K., and Schuster, R.L., eds, 1996, Landslides, investigation and mitigation, Transportation Research Board Special Report 247: Washington D.C., National Research Council, 

673 p.

EXPLANATION

Fire perimeter

Intermittent streams

Perennial streams

Minimum length of main channel that

 can be impacted by debris flows

 generated from delineated basins

Highway

Major Roads

Streets

Towns

Volume of debris flow (m3)

(in response to 2.25 inches (57.15 mm) of rainfall in 3 hours)

1–1,000

1,001–10,000

10,001–100,000

>100,000

0 0.5 1 2 MILES

0 1 2 KILOMETERS 0.5

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U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

OPEN-FILE REPORT 2007–1418

Version 1.0