U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2011–5047

Prepared in cooperation with Gunnison County

Estimated Probabilities, Volumes, and Inundation Area 

Depths of Potential Postwildfire Debris Flows from 

Carbonate, Slate, Raspberry, and Milton Creeks, 

near Marble, Gunnison County, Colorado
Front cover: Debris-flow deposits on Slate Creek west of Marble, Colorado, in July 2008, just upstream from 

the confluence with the Crystal River (Photograph taken by Mike Rupert, U.S. Geological Survey).

Back cover: View north of the Carbonate Creek watershed and confluence of Carbonate Creek with the 

Crystal River at Marble, Colorado, in July 2008 (Photograph taken by Mike Stevens, U.S. Geological Survey).
Estimated Probabilities, Volumes, and 

Inundation Area Depths of Potential 

Postwildfire Debris Flows from Carbonate, 

Slate, Raspberry, and Milton Creeks, near 

Marble, Gunnison County, Colorado

By Michael R. Stevens, Jennifer L. Flynn, Verlin C. Stephens, and 

Kristine L. Verdin

Prepared in cooperation with Gunnison County

Scientific Investigations Report 2011–5047

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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Suggested citation:

Stevens, M.R., Flynn, J.L., Stephens, V.C., and Verdin, K.L., 2011, Estimated probabilities, volumes, and inundation 

areas depths of potential postwildfire debris flows from Carbonate, Slate, Raspberry, and Milton Creeks, near Marble, 

Gunnison County, Colorado: U.S. Geological Survey Scientific Investigations Report 2011–5047, 30 p.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................3

Study Area..............................................................................................................................................3

Previous Debris-Flow Hazards Investigations.................................................................................3

Debris-Flow Susceptibility Modeling .........................................................................................................4

Debris-Flow Probability........................................................................................................................5

Debris-Flow Volume .............................................................................................................................6

Debris-Flow Susceptibility Model Input Data and Assumptions ..................................................6

Debris-Flow Inundation Modeling ..............................................................................................................6

Hydrograph Estimation.........................................................................................................................7

Inundation Model Implementation ....................................................................................................9

Estimated Probabilities, Volumes, and Inundation Area Depths of Potential 

Postwildfire Debris Flows .............................................................................................................10

Debris-Flow Probabilities ..................................................................................................................10

Debris-Flow Volumes .........................................................................................................................10

Debris-Flow Inundation Area Depths..............................................................................................19

Field Reconnaissance........................................................................................................................19

Limitations and Uncertainties....................................................................................................................23

Summary........................................................................................................................................................27

Acknowledgments.......................................................................................................................................28

References....................................................................................................................................................28

Figures

1. Map showing locations of Marble, Colorado, and Carbonate, Slate, 

Raspberry, and Milton Creek drainage basins.........................................................................2

2. Photograph showing aftermath of Carbonate Creek debris flow in 

Marble, Colorado, 1942 with Gallo Hill in the background.....................................................5

3. Graph showing assumed power functions used to estimate changes in sediment 

concentrations between starting, peak, and ending sediment 

concentrations for the debris-flow inundation simulation ....................................................8

4. Graph showing estimated hydrograph of streamflow (water only) and 

debris-flow sediment concentration with time for the simulation of the 

25-year-recurrence, 1-hour-duration rainfall in the Milton Creek drainage basin............8

5–12. Maps showing:

5. Probability of debris flow estimated for stream channels and flow paths 

in the Carbonate Creek drainage basin in response to the (A) 5-year-recurrence, 

1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall ...............11

6. Probability of debris flow estimated for stream channels and flow paths 

in the Slate Creek drainage basin in response to the (A) 5-year-recurrence, 1-hourduration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall ............................12
iv

7. Probability of debris flow estimated for stream channels and flow paths 

in the Raspberry Creek drainage basin in response to the (A) 5-year-recurrence, 

1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall ...............13

8. Probability of debris flow estimated for stream channels and flow paths 

in the Milton Creek drainage basin in response to the (A) 5-year-recurrence, 

1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall ...............14

9. Volume of debris flow estimated for stream channels and flow paths in 

the Carbonate Creek drainage basin in response to the (A) 5-year-recurrence, 

1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall ...............15

10. Volume of debris flow estimated for stream channels and flow paths in 

the Slate Creek drainage basin in response to the (A) 5-year-recurrence, 

1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall ...............16

11. Volume of debris flow estimated for stream channels and flow paths in 

the Raspberry Creek drainage basin in response to the (A) 5-year-recurrence, 

1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall ...............17

12. Volume of debris flow estimated for stream channels and flow paths in 

the Milton Creek drainage basin in response to the (A) 5-year-recurrence, 

1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall ...............18

13. Contour map showing debris-flow inundation depths estimated for the 

debris-flow volumes generated in reponse to the (A) 5-year-recurrence, 1-hour-duration 

rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall at Carbonate, Slate, 

Raspberry, and Milton Creek drainage basins.......................................................................20

14–18. Photographs showing:

14. Gallo Hill, at the head of Slate Creek in June 2008, a source area for 

prewildfire debris-flow material...............................................................................................21

15. Carbonate Creek (view south, downstream) just upstream from the 

confluence with the Crystal River in June 2008, showing the braided 

stream confined by debris-flow levees on both banks ........................................................21

16. Debris-flow deposits on Slate Creek just east of Marble, Colorado, 

in July 2008...................................................................................................................................22

17. Debris-flow deposits on Slate Creek (showing cobble and boulder-sized 

material supported in a fine grained matrix) west of Marble, Colorado, 

in July 2008, just upstream from the confluence with the Crystal River.

Deposits were incised during recession of the debris flow or by the stream 

after the debris flow...................................................................................................................22

18. Home destroyed by debris flow on Slate Creek, July 2008..................................................23

19. Aerial photograph showing debris-flow source area and an incised 

channel lined by debris-flow levees in the Raspberry Creek drainage 

basin surrounded by forest that could burn in a wildfire.....................................................24

20–23. Photographs showing:

20. Channel of Milton Creek in June 2008 on a debris-flow fan showing 

large amounts of unsorted material available for entrainment 

by debris flows ............................................................................................................................25

21. Milton Creek at the confluence with the Crystal River where debris-flow 

material may potentially block the river and cause additional flooding, 

June 2008 .....................................................................................................................................25

22. Heavy equipment poised to unblock the stream channel near a bridge 

in a subdivision on the Milton Creek debris-flow fan near the confluence 

with the Crystal River, June 2008 .............................................................................................26
v

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2) 0.0002471 acre 

hectare (ha) 2.471 acre

square kilometer (km2) 247.1 acre

square meter (m2) 10.76 square foot (ft2) 

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic meter (m3) 35.31 cubic foot (ft3)

cubic meter (m3) 1.308 cubic yard (yd3) 

cubic meter (m3) 0.0008107 acre-foot (acre-ft) 

Flow rate

cubic meter per second (m3/s) 70.07 acre-foot per day (acre-ft/d) 

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

Pressure

dynes per square centimeter (dynes/cm2) 0.00001450 pounds per square inch (psi)

Velocity

millimeters per hour (mm/hr) 0.03937 inches per hour (in/hr)

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 

(NAD 83).

23. Channel of Raspberry Creek in June 2008 showing debris-flow material 

available for possible entrainment in the future....................................................................26

Table

1. Debris-flow input data and estimated probability and volume at the outlets 

of Carbonate, Slate, Raspberry, and Milton Creeks................................................................4

Abstract

During 2009, the U.S. Geological Survey, in cooperation with Gunnison County, initiated a study to estimate the 

potential for postwildfire debris flows to occur in the drainage 

basins occupied by Carbonate, Slate, Raspberry, and Milton Creeks near Marble, Colorado. Currently (2010), these 

drainage basins are unburned but could be burned by a future 

wildfire. Empirical models derived from statistical evaluation 

of data collected from recently burned basins throughout the 

intermountain western United States were used to estimate the 

probability of postwildfire debris-flow occurrence and debrisflow volumes for drainage basins occupied by Carbonate, 

Slate, Raspberry, and Milton Creeks near Marble. Data for the 

postwildfire debris-flow models included drainage basin area; 

area burned and burn severity; percentage of burned area; soil 

properties; rainfall total and intensity for the 5- and 25-yearrecurrence, 1-hour-duration-rainfall; and topographic and soil 

property characteristics of the drainage basins occupied by 

the four creeks. A quasi-two-dimensional floodplain computer 

model (FLO-2D) was used to estimate the spatial distribution and the maximum instantaneous depth of the postwildfire 

debris-flow material during debris flow on the existing debrisflow fans that issue from the outlets of the four major drainage 

basins. 

The postwildfire debris-flow probabilities at the outlet 

of each drainage basin range from 1 to 19 percent for the 

5-year-recurrence, 1-hour-duration rainfall, and from 3 to 35 

percent for 25-year-recurrence, 1-hour-duration rainfall. The 

largest probabilities for postwildfire debris flow are estimated 

for Raspberry Creek (19 and 35 percent), whereas estimated 

debris-flow probabilities for the three other creeks range from 

1 to 6 percent. The estimated postwildfire debris-flow volumes 

at the outlet of each creek range from 7,500 to 101,000 cubic 

meters for the 5-year-recurrence, 1-hour-duration rainfall, and 

from 9,400 to 126,000 cubic meters for the 25-year-recurrence, 1-hour-duration rainfall. The largest postwildfire debrisflow volumes were estimated for Carbonate Creek and Milton 

Creek drainage basins, for both the 5- and 25-year-recurrence, 

1-hour-duration rainfalls.

Results from FLO-2D modeling of the 5-year and 25-year 

recurrence, 1-hour rainfalls indicate that the debris flows from 

the four drainage basins would reach or nearly reach the Crystal River. The model estimates maximum instantaneous depths 

of debris-flow material during postwildfire debris flows that 

exceeded 5 meters in some areas, but the differences in model 

results between the 5-year and 25-year recurrence, 1-hour 

rainfalls are small. Existing stream channels or topographic 

flow paths likely control the distribution of debris-flow material, and the difference in estimated debris-flow volume (about 

25 percent more volume for the 25-year-recurrence, 1-hourduration rainfall compared to the 5-year-recurrence, 1-hourduration rainfall) does not seem to substantially affect the 

estimated spatial distribution of debris-flow material. 

Historically, the Marble area has experienced periodic 

debris flows in the absence of wildfire. This report estimates 

the probability and volume of debris flow and maximum 

instantaneous inundation area depths after hypothetical wildfire and rainfall. This postwildfire debris-flow report does not 

address the current (2010) prewildfire debris-flow hazards that 

exist near Marble.

Introduction

Few hazards, if any, exceed the potentially devastating 

consequences of debris flow. Debris flows are fast-moving, 

high-density slurries of water, sediment, and vegetative debris 

with enormous destructive power that generally are triggered in response to periods of intense rainfall or, in some 

areas, rapid snowmelt on steep hillsides (Istanbulluoglu and 

others, 2004). The townsite of Marble and the surrounding area, located near the headwaters of the Crystal River 

in western Colorado, has been developed on two coalescing 

debris-flow fans (fig. 1), where damaging floods and debris 

flows have occurred historically (Rold and Wright, 1996). Two 

Estimated Probabilities, Volumes, and Inundation Area 

Depths of Potential Postwildfire Debris Flows from 

Carbonate, Slate, Raspberry, and Milton Creeks, near 

Marble, Gunnison County, Colorado

By Michael R. Stevens, Jennifer L. Flynn, Verlin C. Stephens, and Kristine Verdin
2 Estimated Probabilities, Volumes, and Inundation Area Depths of Postwildfire Debris Flows Near Marble, Colorado

Figure 1. Locations of Marble, Colorado, and Carbonate, Slate, Raspberry, and Milton Creek drainage basins.

Crystal River

Rapid Creek

Milton Creek

Carbonate Creek

Slate Creek

Crystal River

Raspberry Creek

Yule

Creek

107°15' 107°10'

39°06'

39°03'

EXPLANATION

Basin boundary

Forest and shrub

Base from U.S. Geological Survey digital data, 1:24,000

Universal Transverse Mercator Projection, 

Zone 13, North American Datum 1983.

0 0.75 1.5 MILES

0 1 2 KILOMETERS

GUNNISON

COUNTY

DENVER

Marble

Marble
Introduction 3

coalescing debris-flow fans on the opposite side of the Crystal 

River also are being developed by landowners, and show evidence of past debris-flow activity. However, the current (2010) 

debris-flow potential may increase with a wildfire, likely posing greater hazards to residents, communities, infrastructure, 

aquatic habitat, and the water supply. Several years of drought, 

combined with the accelerated spread of the mountain pine 

beetle and fuel accumulations attributed to many years of 

active fire suppression, has made many forests in large areas 

of the Rocky Mountains increasingly susceptible to wildfire 

(Keane and others, 2002). Wildfires can denude hillslopes of 

vegetation and change soil properties that affect watershed 

hydrology and sediment-transport processes. Postwildfire 

rainfalls can cause increased overland runoff that erodes soil, 

rock, and vegetative debris from hillslopes (Cannon and others, 2010). Runoff reaching ephemeral stream channels may 

entrain stored sediment and generate hazardous debris flows 

(Cannon and others, 2010).

Marble is within the wildland/urban interface, where 

homes and businesses are interspersed with forested land at 

risk for wildfire. Consequently, there is a need for a hazard 

assessment based on estimates of the postwildfire probability 

of debris-flow occurrence, the volume of debris-flow material, 

and the potential area inundated by debris flow. During 2009, 

the U.S. Geological Survey (USGS), in cooperation with 

Gunnison County, initiated a study to identify the potential 

for postwildfire debris flows to occur in the Carbonate, Slate, 

Raspberry, and Milton Creek drainage basins near Marble, 

Colorado, if they are burned in a future wildfire (fig. 1). Currently (2010), these drainage basins are unburned but could be 

burned by a future wildfire.

Researchers currently are developing models to assess the 

hazards posed by postwildfire debris flows (Cannon and others, 2010; O’Brien, 2009). By considering the possible effects 

of a hypothetical wildfire, these models can be used to identify 

potential debris-flow hazards to life, property, infrastructure, 

and water resources before wildfires occur. It is important to 

recognize that large and severely burned drainage basins may 

produce large volumes of debris-flow material. Although the 

location, percentage of burned area, severity of wildfire, and 

storm intensity and duration after a wildfire cannot be known 

in advance, hypothetical or designed scenarios, such as those 

used in this report, are useful planning tools for conceptualizing potential postwildfire debris-flow hazards. 

Purpose and Scope

This report provides estimates of probabilities of debrisflow occurrence and the debris-flow volumes that could be 

generated from Carbonate, Slate, Raspberry, and Milton Creek 

drainage basins in response to a wildfire that burns all areas 

vegetated with forest and shrubs at moderate- to high-burn 

severity and two hypothetical postwildfire rainfalls of differing 

intensities. Debris-flow hydrographs defined for the estimated 

volumes are used as input to a model that estimates the areas 

inundated by debris-flow material beyond the outlets of each 

drainage basin. A field reconnaissance of the Marble area to 

inspect debris-flow evidence and debris-flow source areas was 

done. Using the information provided in this report, land and 

water-supply managers can consider where to concentrate 

prewildfire planning. If a wildfire happens in the future, this 

information will help managers identify the drainage basins 

that are most vulnerable to postwildfire debris-flow hazards 

and the areas that could be affected by debris flows.

Study Area

Marble, Colorado is located about 125 miles west of 

Denver, Colorado in Gunnison County, in the Crystal River 

Valley at an altitude of approximately 8,000 ft (fig. 1). Four 

drainage basins occupied by Carbonate, Slate, Raspberry, and 

Milton Creeks, tributaries to the Crystal River, are targeted in 

this study. The drainage basins occupied by the creeks range in 

size from 1.15 square kilometers (km2) (0.444 mi2) to 13.1 km2

(5.06 mi2) (table 1) and generally are forested (ESRI, 2009; 

Gesch and others, 2002; Homer and others, 2007). The upland 

vegetation consists of mixed conifer, aspen, and scrub oak, 

and the higher altitude parts of each drainage basin include 

bare rock and grass or alpine vegetation. Cretaceous sandstones and shales north of the Crystal River are overlain by 

alluvial deposits along the river and by colluvial deposits on 

adjacent valley-side hillslopes (Gaskill and Godwin, 1966; 

Rold and Wright, 1996). Deposits from landslides, debris 

flows, and rockfalls have been mapped on the north side of the 

Crystal River in the areas underlain by sandstone and shale 

(Gaskill and Godwin, 1966; Rogers and Rold, 1972; Rold and 

Wright, 1996). The crystalline, Tertiary-age Raspberry Creek 

Phacolith, which was intruded into the Cretaceous sandstones 

and shales, occurs in the Raspberry and Milton Creek drainage 

basins south of the Crystal River (Gaskill and Godwin, 1966; 

Rold and Wright, 1996). In the tributaries upstream from the 

basin outlets, surficial deposits are thin, except for landslide 

deposits, and generally they were not mapped by Gaskill and 

Godwin (1966). 

Previous Debris-Flow Hazards Investigations

Current and future development is at risk from debris 

flows in the Marble area, even without the occurrence of wildfires (Rold and Wright, 1996). Geomorphic hazard areas have 

been well described and mapped in previous investigations 

of the area (Rogers and Rold, 1972; Rold and Wright, 1996). 

Geology, topography, and erosion processes have produced a 

steady source of soil and rock debris, loading hillslopes and 

stream channels, which are susceptible to mobilization during 

large rainfalls. Episodic debris flows have transported large 

volumes of water, sediment, and debris from hillslopes and 

steep channels onto the debris-flow fans in the Marble area.

Debris-flow fans and debris-flow hazard areas also are 

mapped in detailed descriptions of geologic hazards (Rogers 
4 Estimated Probabilities, Volumes, and Inundation Area Depths of Postwildfire Debris Flows Near Marble, Colorado

and Rold, 1972; Rold and Wright, 1996). Debris-flow fans 

and runout depositional areas are prominent features located 

at the outlets of the four drainage basins detailed in this study. 

Whereas historically active debris flows have been mapped 

as emanating from all four drainage basins, some parts of 

the debris-flow fan deposits indicate little evidence of recent 

activity, especially the central part of the Carbonate Creek 

fan, which includes Marble and much of the surrounding 

developed area. However, the flow paths of debris flows can 

change as stated in a previous investigation (Rold, 1977), “...

over time these channels migrate back and forth across the 

entire fan surface much like a fire hose gone wild.” Devastating debris flows and floods have been documented in historical 

accounts in 1874 (reported by the Hayden Expedition), 1936, 

August 1941 (fig. 2, a photograph taken in 1942), July 1945, 

1958, and 1972 (Rold, 1977; Rold and Wright, 1996). However, no relation between debris flows and wildfire in the area 

has been documented. 

Debris-Flow Susceptibility Modeling

A set of empirical equations (models) developed by Cannon and others (2010) from statistical evaluation of data collected from postwildfire debris-flow sites in the intermountain 

western United States were used to estimate the probability of 

debris-flow occurrence and estimated volumes of debris flows 

for the four drainage basins as a function of drainage-basin 

and soil characteristics, burn severity, and storm rainfall. 

In past studies of wildfire-related debris-flow hazards, 

each drainage basin was identified by a single outlet located 

at the basin mouth. Conditions within the basin area above 

the outlet were used to estimate debris-flow probability and 

volume at the outlet (Cannon and others, 2007, Cannon and 

others, 2010). This study uses the same approach to estimate 

debris-flow probabilities and volumes at the outlets for each 

of the four drainage basins evaluated, but also advances the 

methodology by defining a continuous drainage network 

within each basin having a contributing area greater than 0.01 

square kilometers, and estimating debris-flow probabilities and 

volumes for each 10-meter channel reach along the network. 

Each debris-flow probability and volume is estimated based on 

basin conditions within the area upstream from each channel 

reach. This approach provides a continuum of information 

within the drainage network that can be used to assess the 

potential debris-flow effect to existing infrastructure or yet-tobe built structures.

 In recent studies, the probability and volume equations 

have been applied to basins that might be burned in the future 

by a hypothetical wildfire (Stevens and others, 2008; Elliott 

and others, 2011). In those studies, variables used to estimate 

postwildfire debris-flow probability were derived for single 

drainage basins and a single probability for the basin was 

reported. The probabilities are estimated using input variables averaged for the entire basin area and do not quantify 

the effects of local spatial variability. When conditions in a 

particular drainage basin are fairly homogeneous, the averaged 

model input variables such as slope, burn area, ruggedness, 

and soil properties are considered to be representative of the 

overall characteristics of the entire drainage basin. However, 

if the conditions in the drainage basin are heterogeneous or 

have a heterogeneous distribution, the averaged values of the 

Table 1. Debris-flow input data and estimated probability and volume at the outlets of Carbonate, Slate, Raspberry, and Milton 

Creeks.

[km2, square kilometers; mm, millimeters; mm/hr, millimeters per hour; m3, cubic meters]

Drainage 

basin

Drainage 

basin 

area 

(km2)

 Slopes 

greater 

than 

30 

percent 

Basin 

area with 

slopes 

greater 

than 30 

percent 

(km2)

Basin area 

assumed to 

burn medium 

to high 

severity 

(km2)

Area 

burned 

medium to 

high 

severity 

(percent)

Basin 

ruggedness

Total 

storm 

rainfall 

(mm)

Storm 

rainfall 

intensity 

(mm/hr)

Percent 

clay

Liquid 

limit 

(percent)

Probability of 

debris 

flow 

(percent)

Volume 

of 

debris 

flow 

(m3)

5-year-recurrence interval, 1-hour-duration storm

Carbonate 13.1 73 9.59 5.94 45 0.28 23 23 24 34 3 101,000

Slate 1.15 73 0.84 0.67 58 0.80 23 23 31 40 1 7,500

Raspberry 6.11 92 5.64 4.57 75 0.25 23 23 12 29 19 59,000

Milton 11.9 87 10.31 5.55 47 0.58 23 23 12 29 2 100,000

25-year-recurrence interval, 1-hour-duration storm

Carbonate 13.1 73 9.59 5.94 45 0.28 35 35 24 34 6 126,000

Slate 1.15 73 0.84 0.67 58 0.80 35 35 31 40 3 9,400

Raspberry 6.11 92 5.64 4.57 75 0.25 35 35 12 29 35 74,000

Milton 11.9 87 10.31 5.55 47 0.58 35 35 12 29 5 125,000
Debris-Flow Susceptibility Modeling 5

Figure 2. Aftermath of Carbonate Creek debris flow in Marble, Colorado, 1942, with Gallo Hill in the background 

(Photograph by Muriel Sybil Wolle used with permission from Denver Public Library Western History 

Department, call number X-3707).

input variables may be less representative. Using measures of 

variables that are averaged for an entire basin area may either 

underestimate or overestimate probabilities within subbasins. 

In hazard assessments, underestimating probability may affect 

public safety. 

In this study, the debris-flow probability and volume 

estimates also were determined along the drainage network 

(existing flow paths or channels) using a continuous parameterization technique (Verdin and Greenlee, 2003; Verdin and 

Worstell, 2008). This technique was developed as an alternative to traditional basin characterization approaches, which 

require the selection of outlets and their corresponding basins 

using scientific knowledge or iterative analysis. The continuous parameterization technique also can facilitate the identification of smaller subbasins, with high probabilities for debris 

flow, within a larger basin. The continuous parameterization 

technique, based on a digital elevation model-derived flowdirection matrix, also facilitates faster parameter characterization and adds the ability to characterize basin elements above 

any location, not just at the major drainage-basin outlets, 

which provides added detail.

Using the 1/3-arc-second National Elevation Dataset 

digital elevation model (DEM) (Gesch and others, 2002) 

(10-meter nominal resolution) for the study area and the flow 

structure inherent in the DEM, the independent variables 

driving the debris-flow probability and volume equations were 

evaluated for every grid cell within the extent of the DEM. 

Rainfall total and rainfall intensity, calculated from Miller and 

others (1973), were assumed to be uniform over the entire 

burn area because spatial differences were not sensitive in the 

relatively small areas of the hypothetical wildfire. Values for 

all of the other independent variables (except ruggedness) in 

the predictive equations were obtained using the continuous 

parameterization technique. A separate ArcGIS version 9.3 

program (ESRI, 2009) was used to evaluate ruggedness for 

each grid cell in the study area. Once the surfaces of the independent variables were developed, the debris-flow probability 

and volume equations were solved using map algebra for each 

grid cell. Identification of the probability or volume of a debris 

flow at any location within the drainage basin is possible by 

querying the derived surfaces. For this assessment, raster 

sampling was used to compute the values of debris-flow probability and volume for all possible drainage-basin delineations 

defined by successive pour points (subbasin outlets) along 

existing stream channels for map areas larger than 0.01 km2

(Ruddy and others, 2010).

Debris-Flow Probability

Using a statistical evaluation of data collected from monitoring recently burned basins throughout the western United 

States, Cannon and others (2010) developed the following 

empirical equations (models) to estimate the probability of 

debris flow for a given drainage basin:
6 Estimated Probabilities, Volumes, and Inundation Area Depths of Postwildfire Debris Flows Near Marble, Colorado

 P = e x /(1 + e x), (1)

where P is the probability of debris-flow occurrence in fractional form; and

x = –0.7 + 0.03(%SG30) – 1.6(R) + 0.06(%AB) + 0.07(I) 

 + 0.2(%C) – 0.4(LL),

where %SG30 is the percentage of the drainage basin area 

with slope equal to or greater than 30 percent; R is drainage basin ruggedness, the change in drainage basin elevation 

(meters) divided by the square root of the drainage basin area 

(square meters) (Melton, 1965); %AB is the percentage of 

drainage basin area burned at moderate to high severity; I is 

average storm intensity (in millimeters per hour); %C is clay 

content of the soil (in percent); and LL is the liquid limit of 

the soil (percentage of soil moisture by weight), which is the 

water content at which a soil changes from plastic to liquid 

behavior (Das, 1983).

Debris-Flow Volume

Cannon and others (2010) also developed an empirical 

model that can be used to estimate the volume of debris flow 

that likely would be produced from recently burned drainage 

basins:

Ln V = 7.2 + 0.6(ln SG30) + 0.7(AB)0.5 + 0.2(T)0.5 + 0.3, (2)

where, V is the debris-flow volume, including water, sediment, 

and debris (cubic meters); SG30 is the area of drainage basin 

with slopes equal to or greater than 30 percent (square kilometers); AB is the drainage basin area burned at moderate to 

high severity (square kilometers); T is the total storm rainfall 

(millimeters); and 0.3 is a bias correction factor that changes 

the predicted estimate from a median to a mean value (Helsel 

and Hirsch, 1992). This model has an r2 of 0.83 and a standard error of 0.90. In model validation, the volume equation 

predicted 87 percent of the debris-flow volumes within the 95- 

percent prediction interval (Cannon and others, 2010).

Debris-Flow Susceptibility Model Input Data 

and Assumptions

Susceptibility model input data and estimated debris-flow 

probabilities and volumes at the outlets of Carbonate, Slate, 

Raspberry, and Milton Creeks were obtained from a variety of 

sources. Drainage basins were delineated from 10-m DEMs 

using tools available in ArcGIS version 9.3 (ESRI, 2009). 

Basin outlets were chosen from USGS 1:24,000-scale topographic maps. Drainage basin sizes ranged from 1.15 km2

(0.444 mi2) to 13.1 km2 (5.06 mi2) (table 1), which is within 

the range of drainage basin areas (0.01 to 103 km2) or (0.0039 

to 39.8 mi2) used in the development of the debris-flow models 

(Cannon and others, 2010). 

Areas covered by vegetation were used as a surrogate 

for areas of medium- to high-burn severity. It was assumed 

that all of the forest and shrub cover, which was defined from 

the National Land Cover Database (Homer and others, 2007), 

was burned at moderate- to high-burn severity within each 

of the drainage basins. This assumption provides a consistent 

basis for comparison of debris-flow hazards between drainage basins and provides a worst-case scenario for debris-flow 

prediction. 

High-burn severity is defined by Lindsey (2002) as the 

complete consumption of the forest litter and duff and combustion of all fine fuels in the canopy. A deep-ash layer may be 

present on the forest floor, and the top layer of the mineral soil 

may be changed in color because of significant soil heating 

where large-diameter fuels were consumed. Moderate-burn 

severity is defined as the consumption of forest litter and 

duff in discontinuous patches. Leaves or needles, although 

scorched, may remain on trees. Foliage and twigs on the forest 

floor are consumed, and some heating of the mineral soils may 

occur if the soil organic layer was thin.

The drainage basin areas and percentage of basin areas 

with 30 percent or greater slopes were determined using the 

ArcGIS version 9.3 software (ESRI, 2009) with 10-m DEMs. 

Soil properties data were compiled from the State Soil Geographic (STATSGO) database (U.S. Department of Agriculture, 1991; Schwartz and Alexander, 1995). Where more than 

one value for clay content or liquid limit was included for a 

basin, a spatially weighted value was calculated. Ruggedness 

was computed in ArcGIS from the 10-m DEM.

Rainfall is critical to the generation of postwildfire debris 

flows. Post-fire debris flows studied by Cannon and others 

(2008 and 2010) were generated by rainfalls with recurrences 

ranging from less than 2 years up to 10 years. The FLO-2Dmodel developer recommended that the response to a 25-year 

rainfall also should be considered (J.S. O’Brien, FLO Engineering, Inc., oral commun., 2002). Less frequently occurring 

rainfall, such as the 100-year-recurrence rainfall, might deliver 

so much rainfall that sediment-laden water floods would be 

produced, rather than debris flows. 

In this study, the hydrologic response to a 5-year-recurrence, 1-hour-duration rainfall of 23 mm, and a 25-year-recurrence, 1-hour-duration rainfall of 35 mm (Miller and others, 

1973) was evaluated. In any given year, a 5-year-recurrence, 

1-hour-duration rainfall has a 20 percent chance of occurring, 

and a 25-year-recurrence, 1-hour-duration rainfall has a 4 

percent chance of occurring. 

Debris-Flow Inundation Modeling

To predict where postwildfire debris flows may move on 

the debris-flow fans at the outlets of each of the four drainage 

basins, two models were used. The first model is a watershed model (HEC-HMS) (U.S. Army Corps of Engineers, 
Debris-Flow Inundation Modeling 7

2001), which is used to calibrate a hydrograph for the volume 

of debris-flow material estimated using the Cannon (2010) 

volume equation. The second model is an inundation model 

(FLO-2D) (O’Brien, 2009), which uses the hydrograph from 

the watershed model as input for modeling the downstream 

movement of the debris-flow material for the duration of the 

movement of the material downstream across the debris-flow 

fans. In this study, the duration of debris flow was specified as 

3 hours. 

Hydrograph Estimation 

Modeling the movement of debris flows across the 

debris-flow fans requires estimating the flow rate of debrisflow material over the period of the debris-flow occurrence 

(a hydrograph), which is accomplished by distributing the 

volume of debris-flow material, calculated from the Cannon 

and others (2010) volume equation through time. Rainfall, 

sediment concentration of the debris-flow material, and water 

content of the debris-flow material are estimated for each 

increment in a time-series over the duration of the debris-flow 

occurrence. 

In this report, the total rainfall calculated for the 5- and 

25-year, 1-hour-duration-storms, obtained from the National 

Oceanic and Atmospheric Administration (NOAA) Atlas 2 

methods for Colorado (Miller and others, 1973), was distributed over a 1-hour period using a hyetograph, which usually 

is represented by a table or graph that shows what amount 

or what percentage of rainfall falls during each minute of 

the 1-hour storm. The 1-hour storm hyetograph used in this 

report was developed by the U.S. Army Corps of Engineers 

(USACE) from rainfall data in the Crystal River area (Simons, 

Li & Associates, Inc., 1983). 

After calculating the hyetograph for each storm, the 

HEC-HMS watershed model (U.S. Army Corps of Engineers, 

2001) was constructed using drainage-basin topographic characteristics of basin area, mean basin slope, and longest flow 

path in the watershed. The HEC-HMS model will distribute 

the rainfall over each drainage basin and compute a hydrograph for the runoff produced. HEC-HMS was implemented 

using the Soil Conservation Service (SCS) runoff-curvenumber (RCN) method (U. S. Department of Agriculture, 

1986). The SCS RCN method uses a set of equations that 

predict runoff from a single storm in small watersheds, taking 

into account water retention (surface and infiltration) (Wood 

and Blackburn, 1984; Sen, 2008). The SCS RCN method is 

consistent with empirical data used in its development, but 

has limitations (Grove and others, 1998; Ponce and Hawkins, 

1996). For example, the SCS RCN method is not appropriate 

for modeling runoff with complex factors like evaporation 

(Sen, 2008), conditions when rainfall intensity is less than 

infiltration capacity (Wood and Blackburn, 1984), antecedent moisture conditions (Ponce and Hawkins, 1996), or 

spatial and temporal variations in curve number (Sen, 2008). 

Runoff-curve numbers can be estimated from vegetation, land 

use, and soil categories (Wood and Blackburn, 1984), and 

can be calibrated using runoff data or flood-peak information (Cerrelli, 2005; Livingston and others, 2005; Elliott and 

others, 2005; Springer and Hawkins, 2005). 

To use the HEC-HMS watershed model to assist in the 

estimation of a hydrograph, a volume target was used to 

calibrate the model. Because the material represented by the 

hydrograph will have the water content present during the 

debris flow, assumptions were made to estimate the water 

content. For input to the inundation model, a concentration of 

sediment and the total volume of water and sediment must be 

specified (O’Brien, 2009). 

The sediment concentration of the debris flow is used 

by the FLO-2D model to control rheological properties of the 

flow. These concentrations were used to develop an assumed 

hypothetical sedigraph estimated on the basis of results from 

previous studies (Pierson and Scott, 1985; Pierson and Costa, 

1987; Costa, 1988; O’Brien, 2009). Sediment concentrations 

(Cv) used in debris-flow simulations were varied throughout 

the occurrence, an approach used in the Coal Seam fire and 

Missionary Ridge fire simulations (Elliott and others, 2005): 

“...initial Cv 20 percent, mean Cv approximately 31 percent, 

maximum Cv 48 percent, Cv 43 percent at the time of the 

water hydrograph peak, and Cv 20 percent for the duration 

[remainder] of the event.” The difference in the approach 

used in this study is that sediment concentrations were varied 

during the discharge hydrograph by using a power equation to 

make a smooth transition from 20 percent CV at the beginning 

of flow and 48 percent maximum concentration at 75 percent 

of the time to rise to the runoff peak. Another power equation 

was used to estimate the decrease in sediment concentration 

from 48 percent to 20 percent at the end of the hydrograph. 

Examples of the power equations for the 25-year-recurrence, 

1-hour-duration rainfall for Milton Creek are shown in 

figures 3 and 4.

The volume of debris-flow material estimated from Cannon and others (2010) was assumed to have a moisture content 

of about 20 percent (about midway between the wilting point 

and the field capacity of a loamy soil (International Atomic 

Energy Agency, 2008). This assumption was made based on 

the description of the way volume was estimated in the field 

data for the volume model (Gartner and others, 2008). For the 

debris-flow volume equation data, the debris-flow material 

was assumed to be the missing scoured material from channels 

or the deposited material in a fan or detention basin, which all 

contain some moisture prior to the time of scour, or at the time 

of measurement after the debris flow.

Because the FLO-2D inundation model determines rheology based on water content, additional water is then added 

to the Cannon and others (2010) equation volume estimate 

(which already includes soil moisture) to simulate the total 

debris-flow material water content during the flow of the 

material. To estimate a debris-flow volume that includes both 

sediment and water, the amount of water that would result in 

20-percent moisture content was assumed already to be present and is then subtracted from any water content needed to 
8 Estimated Probabilities, Volumes, and Inundation Area Depths of Postwildfire Debris Flows Near Marble, Colorado

0

10

20

30

40

50

60

0

20

40

60

80

100

120

140

160

0 1 2 3 4 5 6 7

Sediment concentration by volume, in percent

Streamflow, in cubic meters per second

Time from start of rainfall, in hours

Streamflow (water only)

Sediment concentration

EXPLANATION

y = 0.80x

0.77

y = 0.32x–0.63

0

10

20

30

40

50

60

0 0.5 1 1.5 2 2.5

Sediment concentration by volume, in percent

Time from start of rainfall, in hours

Power equation rising-limb 

sediment concentration 

Power equation falling-limb

sediment concentration 

EXPLANATION

Figure 4. Estimated hydrograph of streamflow (water only) and debris-flow 

sediment concentration with time for the simulation of the 25-year-recurrence, 

1-hour-duration rainfall in the Milton Creek drainage basin.

Figure 3. Assumed power functions used to estimate changes in sediment 

concentrations between starting, peak, and ending sediment concentrations for 

the debris-flow inundation simulation (the 25-year-recurrence, 1-hour-duration 

rainfall case for the Milton Creek drainage basin shown here).
Debris-Flow Inundation Modeling 9

bulk up the material to represent the sediment concentration 

during the debris flow. During the debris flow, the material has 

the initial soil moisture content prior to being scoured plus the 

water from rainfall runoff that entrained that material. Thus, 

the debris-flow volume equation is bulked to simulate the 

water content minus the initial moisture content. For example, 

if the concentration of sediment in the debris flow is assumed 

to be 45 percent at an instantaneous time increment during the 

runoff, the flow material contains an assumed 20-percent water 

content (soil moisture) from the equation volume, and another 

25-percent water content to get the sediment concentration 

to the assumed value. The 25-percent proportion of water is 

bulked and added to the equation volume.

The next step is to calibrate the hydrograph to the bulked 

volume estimate. First, a range of assumed Runoff Curve 

Numbers (RCNs) were input to the HEC-HMS model. Then, 

by using an iterative procedure, the HEC-HMS time-series 

distribution (hydrograph) of the volume that was closest to 

the volume predicted by the bulked equation was used in the 

FLO-2D inundation model.

Inundation Model Implementation 

The FLO-2D inundation model used for this study is 

a Federal Emergency Management Agency approved twodimensional computer model capable of simulating channel, 

overbank, and unconfined flooding on an alluvial fan, such as 

the relict debris-flow fans downstream from the basin outlets 

(O’Brien, 2009). To estimate the potential inundation from 

water and debris in response to a hypothetical postwildfire 

storm, the hydrograph produced in HEC-HMS was used as 

input to the FLO-2D model (O’Brien, 2009) to develop estimates of inundation downstream from the drainage basin outlets of Carbonate, Slate, Milton, and Raspberry Creeks (fig. 1). 

Results from FLO-2D modeling provide spatial estimates of 

maximum instantaneous inundation depths that occur during 

the debris flow. 

The FLO-2D model (O’Brien, 2009) is a quasi-twodimensional hydraulic model that simulates unconfined spread 

of material (water, sediment, and debris) on debris-flow 

fans and valley bottoms. The FLO-2D model has previously 

been used to estimate postwildfire inundation in Colorado 

(Elliott and others, 2005). FLO-2D also was used in Yosemite 

National Park to simulate debris flows using field evidence 

to calibrate the model (Bertolo and Wieczorek, 2005). The 

model uses a specified input hydrograph, volumetric sediment 

concentration, existing topography, and roughness estimates 

to route a debris flow from the basin outlet of the originating 

tributary to the depositional area on the debris-flow fan. 

Unconfined flow is allowed in eight directions (quasitwo-dimensional) through each grid cell (the four sides and 

four corners of a square, quasi-two-dimensional). The model 

accepts a user-defined hydrograph and variable time-steps 

are automatically regulated by the model using wave-celerity 

and Froude-number limits. FLO-2D simulates flood-wave 

attenuation and ground-surface detention. Volume-conservation and any unreasonable flow velocities computed by 

the model are monitored by the user as a quality-assurance 

measure (O’Brien, 2009).

For implementation of the FLO-2D model in the Marble 

area drainage basins, 10-meter (100-m2 grid) DEMs were used 

to define the topographic flow surface (debris-flow channel 

and fan deposition zones) onto which the debris flows generated in the upper basin would be discharged. After preliminary 

runs of the model it was discovered that the results were not 

sensitive to differences of scale. The final model was gridded to 20-meters (400-m2

 grid cells) within the preprocessor 

for the FLO-2D model. Other assumptions are needed for the 

model simulation of debris flows. Infiltration and evaporative 

losses were assumed to be zero. The initial Manning’s roughness coefficient was assumed to be 0.07, but within the model 

the roughness increases if the velocity increases such that the 

limit of 0.99 set for the Froude number is exceeded. The limit 

restrained the model run to a subcritical flow regime only, 

which is a reasonable assumption considering the turbulence 

and shallow depth in these channels. Channel and floodplain 

scour and deposition were not simulated in the model.

The hydrograph (previously explained in the “Hydrograph Estimation” section of this report) was assigned to an 

inflow point just upstream from the apex of each alluvial 

fan that corresponds to the outlet of each of the four drainage basins. Putting the debris-flow discharge into the model 

upstream from the fan apex allowed the modeled flow to 

adjust and stabilize after abrupt input at the inflow point before 

discharging onto the fan. 

Yield stress and viscosity in the FLO-2D model are the 

primary controlling factors for rheology of the debris flows, 

along with sediment concentration. Yield stress generally 

describes “the threshold strength...which must be exceeded 

for motion to occur” (Enos, 1977) and generally relates to 

particle friction properties. Viscosity generally describes the 

visco-plastic properties of the sediment-water mixture, which 

relate to pore fluid pressures (small grain-size effect) (Kaitna 

and others, 2007). Viscosity exerts more control on debrisflow runout distances when yield strength is low (Elverhoi and 

others, 2000); yield strength, when relatively high, controls 

runout distance more than viscosity. Flood discharges that 

were bulked for debris-flow sediment concentrations (previously detailed in the “Hydrograph Estimation” section of this 

report) need additional sediment properties to be assumed 

before running the model. The rheological properties of the 

debris-flow material in O’Brien and Julien (1988) were chosen 

from experimental soil data characterizing soils found in the 

Glenwood Springs and Aspen areas of Colorado, located 

22 miles north and 34 miles northeast of Marble. Two soils 

were chosen from O’Brien and Julien (1988) to represent soils 

in the Marble area for the simulation. The “Aspen natural 

soil,” which has a clay percentage equal to 27.5 (O’Brien and 

Julien, 1988), best represents soils eroded from the shales 

of Carbonate and Slate Creeks, which have clay percentages equal to 24 and 31, respectively (STATSGO data: U.S. 
10 Estimated Probabilities, Volumes, and Inundation Area Depths of Postwildfire Debris Flows Near Marble, Colorado

Department of Agriculture, 1991). The “Glenwood sample 4” 

soil (O’Brien and Julien, 1988), which has a clay percentage 

equal to 7.6 percent, is the soil that best represents the clay 

content of material derived from the phacolith that characterizes the geology of the Raspberry Creek and Milton Creek 

drainage basins, which have a clay percentage of 12 percent.

Yield stress of the Aspen natural and Glenwood sample 4 soils 

are 3.83 × 10-2 and 1.72 × 10-3 dynes per square centimeter, 

respectively (with beta coefficients 19.6 and 29.5, respectively) (O’Brien and Julien, 1988). Viscosity of the Aspen 

natural and Glenwood sample 4 soils are 4.95 × 10-3 and 

6.02 × 10-4 dynes per square centimeter, respectively (with 

beta coefficients 27.1 and 33.1, respectively) (O’Brien and 

Julien, 1988).

The FLO-2D model was used to simulate 3 hours of 

debris-flow movement (the length of time for most of the 

material to cross the debris-flow fans in the simulation). An 

inundation area produced from the FLO-2D output showed 

the debris-flow inundation and maximum instantaneous depth 

attained in the flow path on each relict debris-flow fan below 

the outlet in each of the four drainage basins. 

Estimated Probabilities, Volumes, and 

Inundation Area Depths of Potential 

Postwildfire Debris Flows 

The following three sections present the results of the 

study. It is important to recognize that the results are based 

on equations and models applied in hypothetical postwildfire 

and rainfall scenarios, and the estimates need to be considered 

within that uncertainty.

Debris-Flow Probabilities 

Estimated debris-flow probabilities calculated from 

drainage-basin characteristics identified for the entire basin 

area at the outlets of the four drainage basins ranged from 1 

to 19 percent in response to the 5-year-recurrence, 1-hourduration rainfall, and ranged from 3 to 35 percent in response 

to the 25-year-recurrence, 1-hour-duration rainfall (table 1). 

Conditions in Raspberry Creek produced the highest probabilities (19 and 35 percent), whereas probabilities estimated for 

Carbonate, Slate, and Milton Creeks were similar and ranged 

from 1 to 6 percent for both rainfall scenarios. It is not clear 

why the highest probabilities are associated with Raspberry 

Creek, but of the four drainage basins, Raspberry Creek has 

the largest percentage of area burned (table 1). For the 4 drainage basins, the 25-year-recurrence, 1-hour-duration rainfall 

debris-flow probabilities are 2 to 3 times higher than for the 

5-year-recurrence, 1-hour-duration rainfall.

When the debris-flow probability model was implemented continuously along the drainage basin stream networks 

(figs. 5-8) many stream channels within the 4 drainage basins 

show debris-flow probabilities greater than 10 percent (see 

the colored points along the streams) and a few channels in 

the Raspberry Creek drainage basin show debris-flow probabilities exceeding 40 percent (fig. 7), which indicates that 

some stream channels may be more susceptible to postwildfire debris flow than others. As expected, the estimated 

postwildfire debris-flow probabilities are greater in response 

to the 25-year-recurrence, 1-hour-duration rainfall than to 

the 5-year-recurrence, 1-hour-duration rainfall (figs. 5-8). 

In the Carbonate, Slate, and Milton drainage basins (figs. 5, 

6, and 8), only a few locations in each basin indicate probabilities greater than 10 percent in response to either the 5- or 

25-year-recurrence, 1-hour-duration rainfalls. Results for the 

Raspberry Creek drainage basin considering the 25-year-recurrence, 1-hour-duration rainfall, indicate widespread areas with 

postwildfire probabilities of debris-flow occurrence greater 

than 10 percent, and many areas with probabilities greater than 

20 percent (fig. 7). 

Debris-Flow Volumes 

Estimated postwildfire debris-flow volumes are proportional to the drainage area burned. Debris-flow volumes 

estimated at the outlets of the four drainage basins (table 1) 

for the 5- and 25-year-recurrence, 1-hour-duration rainfalls 

were, respectively: Carbonate Creek 101,000 and 126,000 

m3; Slate Creek 7,500 and 9,400 m3; Raspberry Creek 59,000 

and 74,000 m3; and Milton Creek 100,000 and 125,000 m3. 

Carbonate and Milton Creeks had the largest estimated volumes (primarily a function of larger basin areas). These results 

indicate that generally, the 25-year-recurrence, 1-hour-duration 

rainfall increases debris-flow volume at the drainage basin outlet by about 25 percent in comparison to the 5-year-recurrence, 

1-hour-duration rainfall. 

The postwildfire debris-flow volume estimates also were 

made using the same continuous parameterization technique 

that was used to estimate postwildfire debris-flow probability. 

The results for the four drainage basins (figs. 9-12) generally 

indicate that volume of water and sediment increased as contributing drainage area increased. Because all the coefficients 

for the variables are positive, and burned area (potentially the 

strongest variable in the equation) never decreases as contributing drainage area is added in the downstream direction, 

progressively larger debris-flow volumes are estimated. 

It is important to understand that the individual colored 

points shown in the figures are estimated debris-flow volumes 

at that point. An incorrect interpretation would be that successive grid volume estimates are cumulative downstream. 

The volume maps may be useful for interpolating debris-flow 

volumes at a particular point along a stream channel or for 

understanding the amount of debris-flow material that could 

be expected to pass through any location in the subbasins.
107°10'

107°11'

39°06'

39°05'

107°10'

107°09'

107°11'

107°09'

39°06'

39°05'

EXPLANATION

Probability, in percent

0 to 1011 to 2021 to 3031 to 4041 to 5051 to 6061 to 7071 to 80

P

5-Year-Recurrence, 1-Hour-Duration Rainfall 25-Year-Recurrence, 1-Hour-Duration Rainfall

U.S. Geological Survey digital data1/3 arc-second Digital Elevation ModelUniversal Transverse Mercator Projection, Zone 13, North American Datum 1983.

0 0.5 1 MILE0 0.75 1.5 KILOMETERS

0 0.5 1 MILE0 0.75 1.5 KILOMETERS

U.S. Geological Survey digital data1/3 arc-second Digital Elevation ModelUniversal Transverse Mercator Projection, Zone 13, North American Datum 1983.

Probability at outlet 

A B

Figure 5. Probability of debris flow estimated for stream channels and flow paths in the Carbonate Creek drainage basin in response to the recurrence, 1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall. 
107°11’3

39°05’30” 39°05’ 107°11’30” 39°05’30” 39°05’ EXPLANATION Probability, in percent 0 to10 11 to 20 21 to 30 31 to 40 41 to 50 51 to 60 61 to 70 71 to 80 

5-Year-Recurrence, 1-Hour-Duration Rainfall 25-Year-Recurrence, 1-Hour-Duration Rainfall 0 0.25 0.5 MILE 0 0.4 0.8 KILOMET

U.S. Geological Survey digital data 1/3 arc-second Digital Elevation Model Universal Transverse Mercator Projection, Zone 13, North American Datum 1983. U.S. Geological Survey digital data 1/3 arc-second Digital Elevation Model Universal Transverse Mercator Projection, Zone 13, North American Datum 1983. Probability at outlet 

A B 0 0.25 0.5 MILE 0 0.4 0.8 KILOMETER

Figure 6. Probability of debris flow estimated for stream channels and flow paths in the Slate Creek drainage basin in response to the (A) 5-yea1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall.
-107°13’

-107°12’

39°04’

39°03’

39°02’

-107°13’

-107°12’

39°04’

39°03’

39°02’

EXPLANATION

Probability, in percent

0 to 1011 to 2021 to 3031 to 4041 to 5051 to 6061 to 7071 to 80

EXPLANATION

Probability, in percent

0 to 1011 to 2021 to 3031 to 4041 to 5051 to 6061 to 7071 to 80

5-Year-Recurrence, 1-Hour-Duration Rainfall 25-Year-Recurrence, 1-Hour-Duration Rainfall

U.S. Geological Survey digital data1/3 arc-second Digital Elevation ModelUniversal Transverse Mercator Projection, Zone 13, North American Datum 1983.

U.S. Geological Survey digital data1/3 arc-second Digital Elevation ModelUniversal Transverse Mercator Projection, Zone 13, North American Datum 1983.

0 0.5 1 MILE

0 0.75 1.5 KILOMETERS

0 0.5 

0 0.75 

Probability at outlet Probability at outlet

A B

Figure 7. Probability of debris flow estimated for stream channels and flow paths in the Raspberry Creek drainage basin in response to the (A) 5recurrence, 1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall.
Figure 8. Probability of debris flow estimated for stream channels and flow paths in the Milton Creek drainage basin in response to the (A) 5-yerecurrence, 1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall.

107°14’

107°15’

107°15’

39°03’

39°02’

39°04’

107°14’

107°13’

107°15’

107°15’

39°03’

39°02’

39°04’

EXPLANATION

Probability, in percent

0 to 1011 to 2021 to 3031 to 4041 to 5051 to 6061 to 7071 to 80

5-Year-Recurrence, 1-Hour-Duration Rainfall 25-Year-Recurrence, 1-Hour-Duration Rainfall

U.S. Geological Survey digital data1/3 arc-second Digital Elevation ModelUniversal Transverse Mercator Projection, Zone 13, North American Datum 1983.

U.S. Geological Survey digital data1/3 arc-second Digital Elevation ModelUniversal Transverse Mercator Projection, Zone 13, North American Datum 1983.

Probability at outlet

A B

0 0.5 1 MILE

0 0.75 1.5 KILOMETERS

0 0.5 1 MILE

0 0.75 1.5 KILOMETERS
107°10’

107°11’

39°06’

39°05’

107°10’

107°09’

107°11’

39°06’

39°05’

EXPLANATION

Volume, in cubic meters

1 to 20,00020,001 to 40,00040,001 to 60,00060,001 to 80,00080,001 to 100,000100,001 to 120,000120,001 to 140,000

5-Year-Recurrence, 1-Hour-Duration Rainfall

25-Year-Recurrence, 1-Hour Duration Rainfall

0 0.5 1 MILE0 0.75 1.5 KILOMETERS

0 0.5 1 MILE0 0.75 1.5 KILOMETERS

U.S. Geological Survey digital data1/3 arc-second Digital Elevation ModelUniversal Transverse Mercator Projection, Zone 13, North American Datum 1983.

U.S. Geological Survey digital data1/3 arc-second Digital Elevation ModelUniversal Transverse Mercator Projection, Zone 13, North American Datum 1983.

Volume at outlet 

A B

Figure 9. Volume of debris flow estimated for stream channels and flow paths in the Carbonate Creek drainage basin in response to the (A) 5-yerecurrence, 1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall. 
107°11’30” 39°05’30” 39°05’ 107°11’30” 39°05’30” 39°05’ EXPLANATION Volume, in cubic meters 20 to 2,000 2,001 to 4,000 4,001 to 6,000 6,001 to 8,000 8,001 to 10,000 5-Year-Recurrence, 1-Hour-Duration Rainfall 

25-Year-Recurrence, 1-Hour-Duration Rainfall U.S. Geological Survey digital data 1/3 arc-second Digital Elevation Model Universal Transverse Mercator Projection, Zone 13, North American Datum 1983. U.S. Geological Survey digital data 1/3 arc-second Digital Elevation Model Universal Transverse Mercator Projection, Zone 13, North American Datum 1983. 0 0.25 0.5 MILE 0 0.4 0.8 KILOMETER 0 0.25 0.5 MILE 0 0.4 0.8 KILOMETER Volume at outlet 

A B

Figure 10. Volume of debris flow estimated for stream channels and flow paths in the Slate Creek drainage basin in response to the (A) 5-year-r1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall. 
107°13’

107°12’

39°04’

39°03’

39°02’

-107°13’

-107°12’

39°4’

39°03’

39°02’

EXPLANATION

Volume, in cubic meters

183 to 15,00015,001 to 30,00030,001 to 45,00045,001 to 60,00060,001 to 75,000

5-Year-Recurrence, 1-Hour-Duration Rainfall

EXPLANATION

Volume, in cubic meters

231 to 15,00015,001 to 30,00030,001 to 45,00045,001 to 60,00060,001 to 75,000

25-Year-Recurrence, 1-Hour-Duration Rainfall

U.S. Geological Survey digital data1/3 arc-second Digital Elevation ModelUniversal Transverse Mercator Projection, Zone 13, North American Datum 1983.

U.S. Geological Survey digital data1/3 arc-second Digital Elevation ModelUniversal Transverse Mercator Projection, Zone 13, North American Datum 1983.

0 0.5 1 MILE

0 0.75 1.5 KILOMETERS

0 0.5 

0 0.75 

Volume at outlet Volume at outlet

A B

Figure 11. Volume of debris flow estimated for stream channels and flow paths in the Raspberry Creek drainage basin in response to the (A) 5-y1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall.
Figure 12. Volume of debris flow estimated for stream channels and flow paths in the Milton Creek drainage basin in response to the (A) 5-year-1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration rainfall. 

107°14’

107°15’

39°03’

39°02’

39°04

107°14’

107°13’

107°15’

39°03’

39°02’

39°04’

EXPLANATION

Volume, in cubic meters

112 to 25,00025,001 to 50,00050,001 to 75,00075,001 to 100,000100,001 to 125,000

5-Year-Recurrence, 1-Hour-Duration Rainfall

EXPLANATION

Volume, in cubic meters

141 to 25,00025,001 to 50,00050,001 to 75,00075,001 to 100,000100,001 to 125,000

25-Year-Recurrence, 1-Hour-Duration Rainfall

U.S. Geological Survey digital data1/3 arc-second Digital Elevation ModelUniversal Transverse Mercator Projection, Zone 13, North American Datum 1983.

U.S. Geological Survey digital data1/3 arc-second Digital Elevation ModelUniversal Transverse Mercator Projection, Zone 13, North American Datum 1983.

0 0.5 1 MILE

0 0.75 1.5 KILOMETERS

0 0.5 1 MILE

0 0.75 1.5 KILOMETERS

Volume at outlet Volume at outlet

A B
Estimated Probabilities, Volumes, and Inundation Area Depths of Potential Postwildfire Debris Flows 19

Debris-Flow Inundation Area Depths

Results of the FLO-2D inundation modeling of the 5- and 

25-year-recurrence, 1-hour duration rainfalls after a hypothetical wildfire indicate that postwildfire debris flows below the 

drainage-basin outlets of Carbonate, Raspberry, and Milton 

Creeks would flow all the way to the Crystal River. The 

postwildfire debris flow from Slate Creek would nearly reach 

the Crystal River in response to the 5-year recurrence, 1-hour 

duration rainfall, and would reach the river in response to the 

25-year-recurrence, 1-hour duration rainfall. The FLO-2D 

model results indicate that simulated postwildfire debris flows 

of the magnitude predicted in this study utilize pre-existing 

stream channels and flow paths caused by prior floods or 

debris flows. The model results are similar to the hazard areas 

(fig. 13, red outline) delineated by Rogers and Rold (1972) 

and Rold and Wright (1996) (fig. 13, blue outline). The current 

(2010) study provides estimates of debris-flow inundation area 

and depth for a defined rainfall under a postwildfire soil condition, which is not representative of the unburned conditions 

in the drainage basins. The inundation area predicted in the 

current (2010) study is somewhat larger and more elongated 

than the mudflow and hazard zones mapped by Rogers and 

Rold (1972), and the debris-flow areas mapped by Rold and 

Wright (1996). The smaller active debris-flow areas mapped 

by Rold and Wright (1996) may be the result of a debris flow 

from a smaller runoff response than those assumed in this 

report, or from features on the ground that would otherwise 

restrict or alter the debris flow but that may not have been 

captured by the 10-meter DEM topography. Rold and Wright 

(1996) mapped areas in Marble that they considered not in 

“recent or active debris-flow” areas and adjacent areas that 

could be considered for development with “site specific study 

for risk assessment.” There is much uncertainty with respect 

to the postwildfire debris-flow hazard associated with larger 

storm events than were considered in this study, such as the 

100-year-recurrence rainfall, and caution is needed. 

One potential problem with using recent debris-flow 

activity as a hazard indicator is that the debris-flow channels 

may get blocked and large debris flows could greatly exceed 

channel capacity, which could change the course of a debris 

flow or cause a debris flow to reoccupy a previously inactive 

channel or deposition zone. Rogers and Rold (1972) mapped 

and described a diversion point on Slate Creek where the 

channel was altered to its present course, which is away from 

the channel that historically inundated part of the larger relict 

debris-flow fan located just east of Slate Creek (flow path on 

figure 13). Rogers and Rold (1972) states, “According to local 

reports, this channel was artificially plugged and diverted to 

its present course by Marble residents to prevent Slate Creek 

mudflows from entering the town.” This vulnerable diversion 

point could be changed again during a large debris flow and 

may cause inundation on a currently inactive portion of the 

fan. 

Estimated postwildfire debris-flow material exceeded 

depths of 5 meters during the debris flow in some areas 

(fig. 13). These depths are the maximum-attained instantaneous depth during the debris flow, not the remnant depth of 

material when the debris flow stops. Comparing results of the 

5- and 25-year-recurrence interval, 1-hour-duration rainfalls, 

only small differences in the inundation areas and depths of 

debris-flow material were estimated by the FLO-2D model 

(fig. 13). Existing stream channels or topographic flow paths 

seem to control the distribution of debris-flow material, and 

the 25-percent larger debris-flow volume estimated for the 

25-year-recurrence, 1-hour-duration rainfall compared to the 

5-year-recurrence, 1-hour-duration rainfall does not seem 

to substantially affect the areal distribution of debris-flow 

material.

Predicted maximum instantaneous depths of debris-flow 

material along the channel of the Crystal River need to be 

considered coarse estimates because the dilution of the debrisflow sediment concentration from water in the Crystal River is 

not simulated by the FLO-2D modeling, and less concentrated 

sediment mixtures could exhibit different behavior. Also, 

blockages by coarse debris such as trees, houses, vehicles, and 

large boulders might alter the debris flow paths or the channel of the Crystal River; blockages were not simulated in this 

study.

Field Reconnaissance 

USGS identified geomorphic evidence of past debris 

flows on the debris-flow fans and in the stream channels 

downstream from the outlets of each of the four drainage 

basins. The current (2010) unburned source area for debris 

flows in Carbonate and Slate Creeks are the shale and sandstone exposures on Gallo Hill and the relatively barren side 

slopes of incised gullies and channels (fig. 14). A large wildfire 

would expose areas, which are currently (2010) covered by 

woody vegetation in Carbonate and Slate Creeks as well as 

similar areas in Raspberry and Milton Creeks, to the erosive 

effects of rainfall. Carbonate Creek flows through an area of 

historic debris flows that exhibit debris-flow levees (Costa and 

Jarrett, 1981) and a braided stream channel that is indicative of 

an abundant sediment supply (fig. 15). Slate Creek is an active 

debris-flow drainage basin with obvious debris-flow runout 

zones (figs. 16 and 17), and in 2008 a relatively small debris 

flow destroyed a home near Slate Creek (fig. 18). 

The likely source areas for debris flows in Milton and 

Raspberry Creeks are the steep, actively eroding hillslopes 

within the drainage basins (fig. 19). A wildfire on these slopes 

would further expose the area to rainfall and erosion, providing material for debris flows. Raspberry and Milton Creeks 

have terminal relict debris-flow fans and lateral debris-flow 

levees (Costa and Jarrett, 1981) that are well developed but do 

not show much evidence of recent overbank debris flow that 

exceeded the capacity of the existing channel (figs. 20-23). 

However, large debris flows from these drainage basins could 

cause substantial flooding if they were to block the Crystal 

River.
20 Estimated Probabilities, Volumes, and Inundation Area Depths of Postwildfire Debris Flows Near Marble, Colorado

EXPLANATION

Drainage-basin boundary

Areas of active mudflows and hazard zones from Rogers and Rold (1972)

Recent or active debris flow from Rold and Wright (1996)

Flow path before diversion Rogers and Rold (1972)

Maximum debris-flow inundation depth attained 

 during runoff, in meters (this study)

0.1 to 1

1.1 to 2

2.1 to 3

3.1 to 4

4.1 to 5

5.1 to 6

107°13’ 107°12’ 107°11’

39°04’30”

39°04’

107°13’ 107°12’ 107°11’

39°04’30”

39°04’

A

B

Base from U.S. Geological Survey digital data, 1:24,000

Universal Transverse Mercator Projection, 

Zone 13, North American Datum 1983.

0 0.4 0.8 MILE

0 0.7 1.4 KILOMETERS

Milton

Creek

Raspberry

Creek

Slate Creek

Carbonate

Creek

Figure 13. Contour map showing debris-flow inundation depths estimated for the debris-flow volumes generated 

in reponse to the (A) 5-year-recurrence, 1-hour-duration rainfall, and (B) 25-year-recurrence, 1-hour-duration 

rainfall at Carbonate, Slate, Raspberry, and Milton Creek drainage basins.
Estimated Probabilities, Volumes, and Inundation Area Depths of Potential Postwildfire Debris Flows 21

Figure 14. Gallo Hill, at the head of Slate Creek in June 2008, a source area for prewildfire debrisflow material.

Figure 15. Carbonate Creek (view south, downstream) just upstream from the confluence with 

the Crystal River in June 2008, showing the braided stream confined by debris-flow levees on both 

banks (Photograph taken by Mike Stevens, U.S. Geological Survey).
22 Estimated Probabilities, Volumes, and Inundation Area Depths of Postwildfire Debris Flows Near Marble, Colorado

Figure 16. Debris-flow deposits 

on Slate Creek just east of 

Marble, Colorado, in July 2008. 

Blocks of Yule Marble on the left 

(Photograph taken by Mike Rupert, 

U.S. Geological Survey). 

Figure 17. Debris-flow deposits on Slate Creek 

(showing cobble and boulder-sized material 

supported in a fine grained matrix) west of 

Marble, Colorado, in July 2008, just upstream from 

the confluence with the Crystal River. Deposits 

were incised during recession of the debris flow 

or by the stream after the debris flow (Photograph 

taken by Mike Rupert, U.S. Geological Survey).
Limitations and Uncertainties 23

Figure 18. Home destroyed by debris flow on Slate Creek, July 2008 (Photograph taken by 

Mike Rupert, U.S. Geological Survey).

Limitations and Uncertainties

This report provides estimates of potential debris-flow 

probability, volume, and debris-flow inundation downstream 

from areas assumed to be burned by a hypothetical wildfire 

in response to 5- and 25-year-recurrence, 1-hour-duration 

rainfalls. The estimated postwildfire debris-flow hazards likely 

are most significant 1 to 3 years following wildfire (Cannon 

and others, 2007). The 5-year-recurrence, 1-hour-duration 

rainfall is more likely to occur than the 25-year-recurrence, 

1-hour-duration rainfall, and at the time when the burned area 

is most vulnerable to debris flows and floods. Response to a 

25-year-recurrence, 1-hour-duration rainfall illustrates a less 

likely, but potentially more devastating, postwildfire debrisflow scenario. The current (2010) assessment evaluates only 

postwildfire debris flows, but substantial hazards from flash 

floods without debris flow may remain for many years after a 

wildfire. Elliott and others (2005) estimated that burned drainage basins are most vulnerable to increased flood flows during 

the first 4 to 6 years after a wildfire. 

Larger, less frequent rainstorms than those considered 

in this study likely will produce even larger postwildfire 

debris flows. Some areas within the selected basins may have 

higher debris-flow probabilities than indicated in this report, 

and debris flows may not be produced in all basins during 

postwildfire rainfall. However, the estimates and maps in 

this report can be used to prioritize areas within the basins or 

downstream from the basin outlets where emergency flood 

warnings or erosion mitigation may be needed for public 

safety. 

The FLO-2D inundation modeling presented in this 

report is based on the 10-meter DEM, which indicates that 

the debris flows from Raspberry and Milton Creeks might 

converge to form one large debris-flow fan. Field evidence 

observed by Rold and Wright (1996), however, indicates 

that debris flows from Raspberry and Milton Creeks do not 

converge; therefore, the FLO-2D inundation maps presented 

in the current (2010) report might underestimate the easterly 

extent of the debris-flow inundation hazard. 

This assessment is one perspective on debris-flow hazards in the Marble area and needs to be considered with other 

information as an indicator of potential risk. Historically the 

area near Marble has experienced periodic debris flows in the 

absence of wildfire, and this study only estimates the probability and volume of debris flow after wildfire. The postwildfire 

debris-flow model does not address the current (2010) prefire 

debris-flow hazards that exist near Marble, and new scientific 

information may indicate a need to revise current thinking 

about debris-flow hazards near Marble. The assessment is provided on the condition that neither the U.S. Geological Survey 
24 Estimated Probabilities, Volumes, and Inundation Area Depths of Postwildfire Debris Flows Near Marble, Colorado

107°12'10" 107°12' 107°11'50"

39°03'30"

39°03'20"

39°03'10"

39°03'

500 250 0 500 FEET

Base from U.S. Geological Survey digital data, 2009

National Agriculture Imagery Program

Universal Transverse Mercator projection, Zone 13

0 50 100 150 METERS

existing

debrisflow

deposits

and

levees

existing

debrisflow

deposits

and

levees

debris-flow

source area

debris-flow

source area

potential

postfire

debris-flow

source area potential

postfire

debris-flow

channels

Figure 19. Debris-flow source area and an incised channel lined by debris-flow levees in the Raspberry Creek 

drainage basin surrounded by forest that could burn in a wildfire (National Agriculture Imagery Program image, URL: 

http://datagateway.nrcs.usda.gov/). 
Limitations and Uncertainties 25

Figure 20. Channel of Milton Creek in June 2008, on a debris-flow fan showing large 

amounts of unsorted material available for entrainment by debris flows (Photograph taken 

by Mike Stevens, U.S. Geological Survey).

Figure 21. Milton Creek at the confluence with the Crystal River where debris-flow 

material may potentially block the river and cause additional flooding, June 2008 

(Photograph taken by Mike Stevens, U.S. Geological Survey).
26 Estimated Probabilities, Volumes, and Inundation Area Depths of Postwildfire Debris Flows Near Marble, Colorado

Figure 22. Heavy equipment poised to unblock the stream channel near a bridge in a 

subdivision on the Milton Creek debris-flow fan near the confluence with the Crystal 

River, June 2008 (Photograph taken by Mike Stevens, U.S. Geological Survey).

Figure 23. Channel of Raspberry Creek in June 2008 showing debris-flow material 

available for possible entrainment in the future. U.S. Geological Survey topographic maps 

indicate that historically Raspberry and Milton Creeks converged upstream to form a single 

stream that flowed into the Crystal River. In 2010, the streams flow separately into the 

Crystal River (Photograph taken by Mike Stevens, U.S. Geological Survey).
Summary 27

nor the United States Government may be held liable for any 

damages resulting from the authorized or unauthorized use of 

the assessment. 

Summary 

During 2009, the U.S. Geological Survey (USGS), in 

cooperation with Gunnison County, initiated a predictive 

study to estimate the potential for postwildfire debris flows 

to occur in the drainage basins occupied by Carbonate, Slate, 

Raspberry, and Milton Creeks near Marble, Colorado. Currently (2010), these drainage basins are unburned but could 

be burned by a future wildfire. This report provides estimates 

of probabilities of debris-flow occurrence and the debrisflow volumes that could be generated from Carbonate, Slate, 

Raspberry, and Milton Creek drainage basins in response to a 

wildfire that burns all areas vegetated with forest and shrubs at 

moderate- to high-burn severity and two hypothetical postwildfire rainfalls of differing intensities. Debris-flow hydrographs defined for the estimated volumes are used as input to a 

model that estimates the areas inundated by debris-flow material beyond the outlets of each drainage basin. Results from a 

field reconnaissance of the Marble area to inspect debris-flow 

evidence and debris-flow source areas are described. Using 

the information provided in this report, land and water-supply 

managers can consider where to concentrate prewildfire 

planning. If a wildfire happens in the future, this information 

will help managers identify the drainage basins that are most 

vulnerable to postwildfire debris-flow hazards, and the areas 

that could be affected by debris flows.

Empirical models derived from statistical evaluation of 

data collected from postwildfire debris-flow sites throughout 

the intermountain western United States were used to estimate 

postwildfire probability and volumes of debris flows. Input for 

the postwildfire debris-flow models includes drainage basin 

area; area burned and burn severity; percentage of burned 

area; soil properties; rainfall total and intensity for the 5- and 

25-year-recurrence, 1-hour-duration-rainfall; and topographic 

and soil property characteristics of the drainage basins occupied by the 4 creeks.

In this study, the debris-flow probability and volume 

estimates also were determined along the drainage network 

(existing flow paths or channels) using a continuous parameterization technique. This technique was developed as an 

alternative to traditional basin characterization approaches, 

which require the selection of outlets and their corresponding 

basins using scientific knowledge or iterative analysis. The 

continuous parameterization technique also can facilitate the 

identification of smaller basins, with high probabilities for 

debris flow, within a larger basin. The continuous parameterization technique, based on a digital elevation model-derived 

flow-direction matrix, also facilitates faster parameter characterization and adds the capability to characterize basin elements above any location, not just at the major drainage-basin 

outlets, which provides added detail.

Postwildfire rainfalls are a key variable used in the 

models that predict postwildfire debris-flow hazards. In this 

study, the hydrologic response to a 5-year-recurrence, 1-hourduration rainfall of 23 millimeters, and a 25-year-recurrence, 

1-hour-duration rainfall of 35 millimeters was evaluated. A 

5-year-recurrence rainfall has a 20 percent chance of occurring 

in any given year, and a 25-year-recurrence rainfall has a 4 

percent chance of occurring in any given year. 

The estimated postwildfire debris-flow probabilities 

at the outlets of the 4 drainage basins ranged from 1 to 19 

percent in response to the 5-year-recurrence, 1-hour duration 

rainfall, and from 3 to 35 percent in response to the 25-yearrecurrence, 1-hour-duration rainfall. The largest probabilities 

of postwildfire debris flows are estimated for Raspberry Creek 

(19 and 35 percent), whereas estimated probabilities estimated 

for Carbonate, Slate, and Milton Creeks ranged from 1 to 6 

percent. Of the four drainage basins, Raspberry Creek contains the most forest and shrub as a percentage of basin area; 

and therefore, the largest percent of potential burned area. 

The debris-flow probability model indicates that the 25-yearrecurrence, 1-hour-duration rainfall debris-flow probabilities 

are 2 to 3 times higher than those for the 5-year-recurrence, 

1-hour-duration rainfall.

When the postwildfire debris-flow probability model was 

implemented continuously along the drainage basin stream 

networks, many stream channels within the four drainage 

basins show debris-flow probabilities greater than 10 percent, and a few channels in the Raspberry Creek drainage 

basin show debris-flow probabilities greater than 40 percent, 

which indicates that some stream reaches within the basin 

are more susceptible to postwildfire debris flow than others. 

As expected, the estimated postwildfire debris-flow probabilities were greater in response to the 25-year-recurrence, 

1-hour-duration rainfall than to the 5-year-recurrence, 1-hourduration rainfall. In the Carbonate, Slate, and Milton Creek 

drainage basins, only a few locations in each basin indicated 

probabilities greater than 10 percent in response to either the 

5- or 25-year-recurrence, 1-hour-duration rainfall. Results for 

the Raspberry Creek drainage basin, considering the 25-yearrecurrence, 1-hour-duration rainfall, indicate widespread areas 

with probabilities of postwildfire debris-flow greater than 10 

percent, and many areas with probabilities greater than 20 

percent.

 Debris-flow volumes estimated at the outlet of the 4 

drainage basins for the 5- and 25-year-recurrence, 1-hourduration rainfalls are, respectively, Carbonate Creek 101,000 

and 126,000 m3; Slate Creek 7,500 and 9,400 m3; Raspberry 

Creek 59,000 and 74,000 m3; and Milton Creek 100,000 and 

125,000 m3. Carbonate Creek and Milton Creek had the largest estimated volumes. The results indicate that the 25-yearrecurrence, 1-hour-duration rainfall increases postwildfire 

debris-flow volume at the four drainage basin outlets by about 

25 percent compared to the 5-year-recurrence, 1-hour-duration 

rainfall. 

Postwildfire debris-flow volumes also were estimated 

using the continuous parameterization technique. The results 
28 Estimated Probabilities, Volumes, and Inundation Area Depths of Postwildfire Debris Flows Near Marble, Colorado

for the four drainage basins generally indicate that the volume 

of water and sediment increased as contributing drainage area 

increased. Because all the coefficients for the variables in the 

model are positive, and burned area (potentially the strongest 

variable in the equation) never decreases as contributing drainage area is added in the downstream direction, hence progressively larger postwildfire debris-flow volumes are estimated in 

the downstream direction.

Results from the quasi-two-dimensional floodplain 

computer model (FLO-2D) of the 5-year-recurrence, 1-hourduration rainfall indicate that postwildfire debris flows from 

Carbonate, Raspberry, and Milton Creeks would flow to the 

Crystal River, and the debris flow in Slate Creek would nearly 

reach the river. In response to the 25-year-recurrence, 1-hourduration rainfall the postwildfire debris flow from the Carbonate, Raspberry, Slate, and Milton Creek drainage basins were 

estimated to all reach the Crystal River. Comparing results of 

the 5- and 25-year-recurrence interval, 1-hour-duration rainfalls, only small differences in the inundation areas and depths 

of debris-flow material were estimated by the FLO-2D model. 

There is much uncertainty with respect to the postwildfire 

debris-flow hazards associated with larger storm events than 

were considered in this study, such as the 100-year-recurrence 

rainfall, and caution is needed. 

Estimated postwildfire debris-flow materials exceeded 

a depth of 5 meters in some areas. Comparing the FLO-2D 

modeling inundation results of the 5- and 25-year-recurrence, 

1-hour-duration rainfalls, only small differences in the inundation areas and depths of debris-flow material were estimated. 

Existing stream channels or topographic flow paths seem to 

control the distribution of debris-flow material, and the difference in estimated debris-flow volume (about 25 percent more 

volume for the 25-year-recurrence, 1-hour-duration rainfall 

compared to the 5-year-recurrence, 1-hour-duration rainfall) 

does not seem to substantially affect the estimated spatial 

distribution of debris-flow material.

Predicted maximum instantaneous depths of debrisflow material along the channel of the Crystal River need to 

be considered coarse estimates because the dilution of the 

debris-flow material from the river was not simulated by the 

FLO-2D model, and less concentrated mixtures could exhibit 

different behavior. Also, blockages by coarse debris such as 

trees, houses, vehicles, and large boulders might alter the flow 

paths of any of the predicted debris flows or the channel of the 

Crystal River, and these possible affects were not modeled. 

Historically the Marble area has experienced periodic 

debris flows in the absence of wildfire. This report estimates 

the probability and volume of debris flow and maximum 

instantaneous inundation area depths after hypothetical wildfire and rainfall. This postwildfire debris-flow report does not 

address the current (2010) prewildfire debris-flow hazards that 

exist near Marble. The assessment is provided on the condition 

that neither the U.S. Geological Survey nor the United States 

Government may be held liable for any damages resulting 

from the authorized or unauthorized use of the assessment. 

Acknowledgments

The author gratefully acknowledges the useful insights 

provided by Richard Stenson, Environmental Health Official, 

Gunnison County Department of Planning, Building, and 

Environmental Health, and critical reviews by Susan Cannon 

and John Elliott, USGS. 

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Stevens and others—Estimated Probabilities, Volumes, and Inundation Area Depths of Potential Postwildfire Debris Flows, near Marble, Colorado—SIR 2011–5047