Methods for the Emergency Assessment of 

Debris-Flow Hazards from Basins Burned by the 

Fires of 2007, Southern California 

By Susan H. Cannon, Joseph E. Gartner, and John A. Michael 

 

Open-File Report 2007–1384 

U.S. Department of the Interior 

U.S. Geological Survey 
Frontispiece. Fires burning in southern California at 1:55 p.m. on October 22, 2007. NASA image 

created using data from the University of Wisconsin’s Space Science and Engineering MODIS 

Direct Broadcast facility. 

ii
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2007 

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Suggested citation: 

Cannon, S.H., Gartner, J.E., and Michael, J.A., 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. 

Any use of trade, product, or firm names is for descriptive purposes only and does 

not imply endorsement by the U.S. Government. This report has not been reviewed 

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individual copyright owners to reproduce any copyrighted material contained within 

this report. 

iii
Contents 

 

Abstract ..............................................................................................1 

Introduction............................ ..............................................................1 

Fire-related debris-flow hazards ...................................................................2 

Approach .............................................................................................4 

 Debris-flow volume model................................................................4 

 Hazards mapping ...........................................................................6 

Use and limitations of the maps....................................................................7 

References Cited.....................................................................................8 

Figures 

1. Map showing locations of fires for which hazard maps were generated.....................3 

Tables 

1. Storm rainfall values used in the assessments.................................................6 

2. Relative hazard rank assigned to values of debris-flow volume...........................7 

iv
 

Methods for the Emergency Assessment 

of Debris-flow Hazards from Basins 

Burned by the Fires of 2007, Southern 

California 

By Susan H. Cannon1, Joseph E. Gartner, and John A. Michael 

Abstract 

This report describes the approach used to assess potential debris-flow hazards from 

basins burned by the Buckweed, Santiago, Canyon, Poomacha, Ranch, Harris, Witch, 

Rice, Ammo, Slide, Grass Valley and Cajon Fires of 2007 in southern California. The 

assessments will be presented as a series of maps showing a relative ranking of the 

predicted volume of debris flows that can issue from basin outlets in response to a 3-hour 

duration rainstorm with a 10-year return period. Potential volumes of debris flows are 

calculated using a multiple-regression model that describes debris-flow volume at a basin 

outlet as a function of measures of basin gradient, burn extent, and storm rainfall. This 

assessment provides critical information for issuing basin-specific warnings, locating and 

designing mitigation measures, and planning of evacuation timing and routes. 

Introduction 

The objective of this report is to describe the approach used to perform 

emergency assessments and generate a series of hazard maps that show the potential for 

debris-flow generation from basins burned by the Buckweed, Santiago, Canyon, 

Poomacha, Ranch, Harris, Witch, Rice, Ammo, Slide, Grass Valley, and Cajon Fires of 

2007 in southern California (Figure 1) for a given rainfall event. The assessments identify 

the hazards posed by debris flows as a relative ranking of predicted volumes of material 

that can issue from a basin outlet. Identification of potential debris-flow hazards from 

burned drainage basins is necessary to issue warnings for specific basins, to make 

effective and appropriate mitigation decisions, and to aid decisions about evacuation 

timing and routes. 

1 U.S. Geological Survey, Geologic Hazards Team, Denver Federal Center, MS 966, Denver, CO. 
Fire-Related Debris-Flow Hazards 

Wildfire can have profound effects on a watershed. Consumption of the rainfallintercepting canopy and of the soil-mantling litter and duff, intensive drying of the soil, 

combustion of soil-binding organic matter, and the enhancement or formation of waterrepellent soils can decrease rainfall infiltration into the soil and significantly increase 

overland flow and runoff in channels. Removal of obstructions to flow, such as live and 

downed timber, plant stems, and so forth, by wildfire can enhance the erosive power of 

overland flow, resulting in accelerated stripping of material from hillslopes. Increased 

runoff can also erode significant volumes of material from channels. The net result of 

rainfall on burned basins is often the transport and deposition of large volumes of 

sediment, both within, and down-channel from burned areas (Cannon and others, 2008; 

Cannon 2005a). 

Debris flows are among the most hazardous consequences of rainfall on burned 

hillslopes (Cannon 2005b). Debris flows pose a hazard distinct from other sediment-laden 

flows because of their unique destructive power. They can occur with little warning, exert 

great impulsive loads on objects in their paths; even small debris flows can strip 

vegetation, block drainageways, damage structures, and endanger human life. For 

example, a storm on Christmas Day 2003 triggered debris flows from many of the steep 

basins burned earlier that fall in the San Bernardino Mountains (Brock and others, 2007). 

Sixteen people were killed by this event, and billions of dollars were spent for clean up 

and repairs (Chung and others, 2004). This event clearly illustrates the need for tools to 

rapidly assess debris-flow hazards immediately after wildfires. 

Studies of debris-flow processes throughout the western United States and in 

southern California demonstrated that the great majority of fire-related debris flows 

initiate through a process of progressive bulking of storm runoff with sediment eroded 

from both hillslopes and channels (Cannon, 2000, 2001; Santi and others, 2008). 

Although some infiltration-triggered landsliding can occasionally occur in burned basins, 

and generally in response to prolonged rainfall events, these failures usually contribute a 

small proportion to the total volume of material transported from basins (Scott, 1971; 

Cannon and others, 2001). This finding points to the relative importance of runoffdominated, rather than infiltration-dominated, processes of debris-flow initiation in 

recently burned basins, and indicates that methodologies developed to map landslide 

potential are generally not appropriate for recently burned areas.

 

2
Figure 1. Locations of 2007 fires in Southern California for which debris-flow hazard maps will be generated u

described in this report. 

3
 Approach 

In a study of the erosional response of recently burned basins throughout the 

western United States, including southern California, Cannon (2000, 2001) found that not 

all basins produce debris flows; many burned watersheds respond to even heavy rainfall 

events by sediment-laden flooding, rather than debris flows. However, debris flows 

represent the most destructive end of the potential runoff response spectrum. Because 

debris-flow kinematics are significantly distinct from those of streamflow (Iverson, 

1997), we have taken the approach of developing predictive relations that are specific to 

debris flow. Using data collected from 25 recently burned and debris-flow-producing 

basins throughout southern California, Gartner and others (2008) developed an empirical 

relation that can be used to estimate debris-flow volumes as a function of the area of the 

basin burned, storm rainfall conditions, and basin gradients. In this assessment, we use 

this model to predict the range of potential volumes of material that can be expected at a 

basin mouth in response to a 10-year recurrence, 3-hour duration rainstorm immediately 

following wildfire. 

Debris-Flow Volume Model 

A multiple-regression model developed using data measured from post-wildfire 

debris flows is used to define the range of volumes that can potentially be generated from 

the basins burned by the 2007 fires. The data used in the development of the model 

consists of measurements from 25 recently burned, and debris-flow-producing, basins 

located throughout southern California for which estimates of debris-flow volume had 

been obtained (Gartner and others, 2008). Volumes were determined either through 

detailed surveys of scoured channels or from measurements of material collected in 

debris basins. 

The multiple regression model provides an estimate of the volume of material 

expected at the basin outlet as a function of basin gradient, burned extent, and storm 

rainfall. Because it was not known which measures would best determine debris-flow 

volume, Gartner and others (2008) evaluated a number of different independent variables 

in the regression analyses. Eight measures of basin morphology and drainage network 

were compiled from 30-m or 10-m DEMs, depending on availability. These included: 

• Basin area 

• Average basin gradient 

• Basin relief ratio (the change in elevation between the basin mouth and the 

top of the longest channel extend to the drainage divide, divided by the 

length of the channel (Meyer and Wells, 1997)) 

• Basin ruggedness (maximum change in elevation within a basin divided by 

the square root of the basin area (Melton, 1965)) 

• Basin area with slopes greater than or equal to 30 percent 

• Basin area with slopes greater than or equal to 50 percent 

• Drainage density (the total length of all channels within a basin divided by 

the basin area, (Horton, 1945)) 

• Drainage network bifurcation ratio (average ratio of the number of streams 

of any order to the number of streams of the next highest order (Horton, 

1945)) 

4
Burn severity for each basin was characterized using maps of burn severity 

generated using the Normalized Burn Ratio (NBR), as determined from Landsat 

Thematic Mapper data (Key and Benson, 2005). The maps of burn severity reflect 

changes in biomass from before to immediately after the fire. Gartner and others (2008) 

evaluated the effects of five measures of burn severity, including: 

• Area of the basin area burned at low severity 

• Area of the basin area burned at moderate severity 

• Area of the basin area burned at high severity 

• Area of the basin burned at high and moderate severities 

• Total area of the basin burned at all severities 

Soil properties for each basin were characterized by samples of burned soils 

collected in each basin. The soil properties evaluated included: 

• Sorting of the burned soil grain-size distribution 

• Mean of the soil grain-size distribution 

• Median of the soil grain-size distribution 

• Skewness of the soil grain-size distribution (from Inman, 1952 

The characteristics of storms that affected the monitored basins were determined 

from tipping-bucket rain gages located within 2 km of each basin. For each storm to 

impact a monitored basin, Gartner and others (2008) compiled the: 

• Total storm rainfall 

• Average storm rainfall intensity 

• Peak 10-minute rainfall 

• Peak 15- minute rainfall 

• Peak 30-minute rainfall 

• Peak 60-minute rainfall 

 

Gartner and others (2008) used a series of statistical analyses to determine those 

factors that most strongly affected debris-flow volumes, and to build the most robust 

regression model possible. All possible combinations of independent variables were 

evaluated to determine which combination produced the most effective model. Gartner 

and others (2008) used a combination of statistical measures including the adjusted R2

, 

the residual standard error, and the standard errors of the coefficients to assess the quality 

of each model (Helsel and Hirsch, 2002). 

Gartner and others (2008) found that the volume of debris flows (V, in m3) issuing 

from the outlet of recently-burned basins in southern California can be estimated as a 

function of the basin area with slopes greater than or equal to 30 percent (A30, in km2

), the 

area of the basin burned at high and moderate severities (BMH, in km2

), and the total storm 

rainfall (RS, in mm). These variables form the basis of a multi-variate statistical model of 

the form 

lnV = 6.6 + 0. 6(A30) + 0.6(BHM)

1/2 + 0.3(RS) 1⁄2 (1) 

The adjusted R2

 of this model (0.90) and residual standard error (0.62), coupled 

with additional tests of model quality, indicated that the result is the best possible model, 

5
given the available data. Graphical analyses indicate no apparent correlation between 

average basin gradient and burned area. Model validation using a dataset not used in the 

development of the regression model revealed that measured volumes were within two 

standard errors of the predicted volumes. We used this model in a Geographical 

Information System (GIS) to estimate the median volume of a post-fire debris flow at the 

outlets of individual drainage basins in the 2007 burned areas in response to a single 

storm event. 

Hazards Mapping 

Potential debris-flow volumes were determined for basins burned by the 

Buckweed, Santiago, Canyon, Poomacha, Ranch, Harris, Witch, Rice, Ammo, Slide, 

Grass Valley and Cajon Fires of 2007 (fig. 1). Basins were delineated for the assessment 

by starting with several specific locations identified by the BAER (Burned Area 

Emergency Response) teams and emergency managers as locations of concern in their 

post-fire assessments. Outlets above those specific locations, as well as throughout the 

burned areas, were identified and the basins above those locations delineated. The 

delineated basins range in area between 0.02 and 30 km2

, reflecting the sizes of the basins 

used in the model development. 

For each basin, we compiled values for each of the input variables for the model. 

Measures of basin gradient were obtained from 10-m DEMs. The basin areas burned at 

different severities were characterized from the burn severity mapping, as described 

above. 

The volume of potential debris flows were calculated in response to a 10-year 

recurrence, 3-hour-duration storm for basins within the fires listed in table 1. Storm 

rainfall values used in the models (table 1) were obtained from Hershfield (1963). 

Although there is considerably more variability in storm-rainfall characteristics across 

burned areas than represented by these single values, we used them as representative of 

storm rainfall likely to impact the recently burned areas in the near future. 

Table 1. Storm rainfall values used in assessments (from Hershfield, 1963). 

Fire Name Storm Total for the 10-year 

recurrence, 3-hour duration storm 

Santiago, Ammo, Rice, Harris 1.75 inches (44.45mm) 

Witch, Poomacha, Ranch, 

Buckweed, Canyon 

2.25 inches (57.15 mm) 

Cajon 3.25 inches (82.55 mm) 

Grass Valley, Slide 3.50 inches (88.90 mm) 

To show this information in map form, the calculated values of debris-flow 

volume for each basin will be proportioned into relative rankings (table 2): 

6
Table 2. Relative rankings assigned to predicted values of debris flow volume. 

Calculated 

Volume 

(m3

) 

Volume 

Ranking 

Value 

0–1,000 m3 1 

1,001–10,000 m3 2 

10,001–100,000 m3 3 

>100,000 m3 4 

Use and Limitations of the Maps 

The maps that will be generated using the approach described above will show 

estimates of the relative hazards posed by debris flows that may issue from the outlets of 

basins burned by the Buckweed, Santiago, Canyon, Poomacha, Ranch, Harris, Witch, 

Rice, Ammo, Slide, Grass Valley and Cajon Fires of 2007 in southern California in 

response to a 10-year recurrence, 3-hour duration rainstorm. The maps will identify the 

range of potential debris flow median 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 

those evaluated here are forecast for the area. 

In addition to the potential dangers within these basins, areas downstream from 

the basin outlets are also at risk. In some of these areas, homes were destroyed by the 

fires, and workers and residents may be busy cleaning and rebuilding sites. These people 

may be at high risk for impact by debris flow during rainfall events. In addition, there is a 

great possibility of culverts plugging or being overwhelmed, and of roads washing out. 

Such events can strand motorists 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. 

The potential for debris-flow activity decreases with time because revegetation 

stabilizes hillslopes. A compilation of information on post-fire runoff events reported in 

the literature from throughout the western United States indicates that under normal 

rainfall conditions most debris-flow activity occurs within about 2 years following a fire 

(Gartner and others 2005). If dry conditions prevent sufficient re-growth of vegetation, 

this recovery period will be longer. We thus conservatively expect that the maps 

presented here may be applicable for approximately 3 years after the fires for the storm 

conditions considered here. Furthermore, the assessments presented here are specific to 

post-fire debris flows; significant hazards from flash flooding can remain for many years 

after a fire. 

Lastly, the parameters included in the models are considered to be possible firstorder effects that can be rapidly evaluated immediately after a fire. Other conditions than 

those used in the model may also affect debris-flow production from recently burned 

7
basins in southern California. For example, an abundance of dry-ravel material in a 

specific channel may affect volumes, and the frequently occurring fire–flood sequence 

that characterizes southern California basins may similarly limit material available for 

incorporation into debris flows (Spittler, 1995). However, data necessary to evaluate 

these effects are not currently available. 

References Cited 

Brock, R. J., Cannon, S.H., Gartner, J.E. Santi, P.M., Higgins, J.D., and Bernard, D.R., 

2007, An ordinary storm with an extraordinary response: Mapping the 2003 Old and 

Grand Prix Fire areas in southern California: Geological Society of America Abstracts 

with Programs, v. 39, No. 6, p. 180. 

Cannon, S.H., 2000, Debris-flow response of southern California watersheds recently 

burned by wildfire, in Wieczorek, G.F., and Naeser, N.D, eds., Debris-Flow Hazards 

Mitigation — Mechanics, prediction, and assessment, Proceedings of the Second 

International Conference on Debris-Flow Hazards Mitigation, Taipei, Taiwan, 16–18 

August 2000: A.A. Balkema, Rotterdam, p. 45–52. 

Cannon, S.H., 2001, Debris-flow generation from recently burned watersheds: 

Environmental and Engineering Geoscience, v. 7, p. 321–341. 

Cannon, S.H., 2005a, Southern California—Wildfires and debris flows: U.S. Geological 

Survey Fact Sheet 2005–3106. http//pubs.usgs.gov/fs/2005/3106. 

Cannon, S.H., 2005b, A NOAA-USGS demonstration flash-flood and debris-flow earlywarning system: U.S. Geological Survey Fact Sheet 2005–3104. 

http://pubs.usgs.gov/fs/2005/3104/. 

Cannon, S.H. and Gartner, J.E., 2005, Wildfire-related debris flow from a hazards 

perspective, chapter 15, in Hungr, Oldrich and Jacob, Mathias, eds., Debris-flow hazards 

and related phenomena, Springer-Praxis Books in Geophysical Sciences, p. 321–344. 

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. 

Cannon, S.H., Kirkham, R.M. and Parise, Mario, 2001, Wildfire-related debris-flow 

initiation processes, Storm King Mountain, Colorado: Geomorphology, v. 39, p. 171–

188. 

Chong, J., Renaud, J., Ailsworth, E., 2004, Flash floods wash away lives, dreams: Los 

Angeles Times, Jan 3, 2004, p. B.1. 

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Gartner, J.E., Cannon, S.H., Bigio, E.R., Davis, N.K., Parrett, C., Pierce, K.L., Rupert, 

M.G., Thurston, B.L., Trebish, M.J., Garcia, S.P., and Rea, A.H., 2005, Compilation of 

data relating to the erosive response of 606 recently burned basins in the western U.S.: 

U.S. Geological Survey Open-File Report 2005–1218. 

Gartner, J.E., Cannon, S.H., Santi, P.M., 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. 

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in water resources, Studies in 

Environmental Science 49: Elsevier, Amsterdam, 489 p. 

Hershfield, D.M., 1963, 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. 

Horton, R.E., 1945, Erosional development of streams and their drainage basins — 

Hydrological approach to quantitative geomorphology: Geological Society of America, 

Bulletin No. 56, p. 275–370. 

Inman, D.L., 1952, Measures for describing the size distribution of sediments: Journal of 

Sedimentary Petrology, v. 22, p. 125–145. 

Iverson, R.M., 1997, The physics of debris flow, Reviews in Geophysics, v. 35, p. 245–

296. 

Key, C.H., and Benson, N.C., 2005, Landscape assessment—Remote sensing of severity, 

the Normalized Burn Ratio, in D.C. Lutes and others, eds., FIREMON— Fire Effects 

Monitoring and Inventory System, Ogden Utah, USDA Forest Service Rocky Mountain 

Research Station, General Technical Report, RMS-GTR-164-CD:LA1-LA51. 

Melton, M.A., 1965, The geomorphic and paleoclimatic significance of alluvial deposits 

in southern Arizona: Journal of Geology, v. 73, p. 1–38. 

Meyer, G.A., and Wells, S.G., 1997, Fire-related sedimentation events on alluvial fans, 

Yellowstone National Park, U.S.A.: Journal of Sedimentary Research, v. 67, no. 5, p. 

776–791. 

Santi, P.M., deWolf, V.G., Higgins, J.E., Cannon, S.H., Gartner, J.E., 2008, Sources of 

debris flow material in burned areas: Geomorphology, 

doi:10.1016/j.geomorph.2008.02.022. 

Scott, K.M., 1971, Origin and sedimentology of 1969 debris flows near Glendora, 

California: U.S. Geological Survey Professional Paper 750–C, p. C242–C247. 

9
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Spittler, T.E., 1995, Fire and debris flow potential of winter storms, in Keely, J.E., and 

Scott, T., eds., Brushfires in California wildlands — Ecology and resource management: 

International Association of Wildland Fire, Fairfield, WA.