Revision History for U.S. Geological Survey Open-File Report 2010-1244

Probability and Volume of Potential Postwildfire Debris Flows in the 2010 Fourmile Burn Area, Boulder County, Colorado

By Barbara C. Ruddy, Michael R. Stevens, Kristine L. Verdin, and John G. Elliott
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First posted on October 5, 2010

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November 19, 2010 Errata Sheet:

Subsequent to publication of U.S. Geological Survey Open File Report 2010-1244, "Probability and Volume of 
Potential Postwildfire Debris Flows in the 2010 Fourmile Burn Area, Boulder County, Colorado," an error was 
found in the text and on the two plates. In short, the probability and volume analyses were done for a
2-year-recurrence, 1-hour-duration rainfall event, not a 25-year-recurrence, 1-hour-duration rainfall event. 
In the following sentences and headings, the instances in brackets are erroneous and should be replaced by 
the text that immediately follows the brackets.

P. 1 Data for the models include burn severity, rainfall total and intensity for a [25-year] 2-year-recurrence,
1-hour-duration rainstorm, and topographic and soil property characteristics.

P. 1  Several of the selected drainage basins in Fourmile Creek and Gold Run were identified as having 
probabilities of debris-flow occurrence greater than 60 percent and many more had probabilities greater than 45 
percent, in response to the [25-year] 2-year-recurrence, 1-hour-duration rainfall.

P. 2 Such a storm affects the area burned by the Fourmile Canyon fire approximately every [25 years] 2 years 
(a [4 percent] 50 percent chance in any given year) (Miller and others, 1973).

P. 3 In response to the [25-year] 2-year-recurrence, 1-hour-duration rainfall, only 2 tributary basins to Gold Run, 
basins 7 and 167 (plate 1), were identified as having probabilities of debris-flow occurrence greater than 60 percent,
and 7 basins (113, 154, 156, 162, 163, 168, and 170) had probabilities between 46 and 60 percent.

P. 4 This assessment indicates estimates of debris-flow probability and volume for the area burned by the Fourmile 
Fire in response to a [25-year] 2-year-recurrence, 1-hour-duration rainfall (a [4 percent] 50 percent chance in any
given year). Larger, less frequent storms are more likely to produce much larger debris flows. Some areas within 
the selected basins may have higher debris-flow probabilities than those shown on plate 1, and debris flows may 
not be produced from all basins during a [25-year] 2–year rainfall.

Plate 1, the heading (in two places):
Probability of a debris flow (percent) in response to a [25-year] 2-year, 1-hour rainfall of 23 millimeters.

Plate 2, the heading (in two places):
Estimated volume of a debris flow (cubic meters) in response to a [25-year] 2-year, 1-hour rainfall of 23 millimeters.

We apologize for the inconvenience of this errata sheet.
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