RIVERSIDE COUNTY

ORANGE COUNTY

Santiago

Fire

CALIFORNIA

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§ ̈¦5

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241

241

261

241

133

241

261

El Toro Mcas

Serrano

Place

Lake

Forest

Foothill

Ranch

Trabuco

Canyon

East

Irvine

Portola

Hills

Kathryn

Frances

Modjeska

Silverado

Cowan

Heights

117°45' 117°40' 117°35'

33°45'

33°40'

Pete

rs

Canyon

Wash

Hicks Canyon Wash

Bee Canyon Wash

San Diego Creek

San Diego Creek

Serrano Creek

Oso Creek

F

alls Canyon

Aliso Creek

Santiago

Creek

Santiago

Creek

Santiago Canyon Road

Santiago Creek

Black Star Creek

Silverado Creek

Williams Canyon

Harding Canyon

Modjeska Canyon

Coldwater Canyon

Bedford Wash

Live

O

ak

C

anyon

Dove

Canyon

Tijeras Canyon

Hickey

Canyon

Rose

Canyon

Agua Chinon Wash

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 Santiago Fire in Orange County, 

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 median volume of debris flows that can issue from basin outlets in response to 1.75 

inches (44.45 mm) of rainfall over a 3-hour period. Such a storm has a 10-year return period (Hershfield, 1961). The calculation of predicted debris flow volume is based on a multipleregression statistical model that describes the median volume of material that can be expected from a recently burned basin as a combined 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

In response to the 10-year-recurrence, 3-hour rainstorm, 15 of the 84 basins evaluated in this assessment were identified as having the potential to produce debris flows with median 

volumes less than 1,000 m3, and 59 basins showed the potential to produce debris flows with volumes between 1,001 and 10,000 m3. These are small (0.02 to 4.3 km2) basins that drain 

into Silverado Creek, Harding Canyon, Modjeska Canyon, and Santiago Creek. The remaining 10 basins evaluated showed volume estimates between 10,001 and 100,000 m3. These 

include Williams Canyon, an unnamed canyon that drains into Silverado Creek, the headwaters of Harding Canyon, an unnamed canyon that drains into Modjeska Canyon, and six 

unnamed canyons that drain the southwest side of the burned area into the communities of Foothill Ranch, Frances, and Kathryn.

In addition to buildings, or people within any of the burned basins, the towns of Silverado, Modjeska, Foothill Ranch, Frances, and Kathryn, as well as any of the roads and highways 

that cross the area, can be inundated by debris flows that would pose a significant threat to life and property.

Use and Limitations of the Map

This map shows potential hazards posed by debris flows as estimates of median volumes of material that may issue from the outlets of basins burned by the Santiago 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. This includes Silverado creek and Williams, Harding and Modjeska 

Canyons. 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. 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.

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

Dry wash

Minimum length of main channel that

 can be impacted by debris flows

 generated from delineated basins

Major Roads

Streets

Towns

Volume of debris flow (m3)

(in response to 1.75 inches (44.45 mm) of rainfall in 3 hours)

1–1,000

1,001–10,000

10,001–100,000

>100,000

0 0.25 0.5 1 MILE

0 0.5 0.25 1 KILOMETER

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/2007/1419

EMERGENCY ASSESSMENT OF DEBRIS-FLOW HAZARDS FROM BASINS BURNED BY THE 2007 SANTIAGO FIRE, ORANGE COUNTY, 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

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

OPEN-FILE REPORT 2007–1419

Version 1.0