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<br />discharge was about 40 m31s1km2 for the largest known
<br />floods for unburned basins in Colorado (Jarrett, 1990;
<br />Jarrett and Tomlinson, 2000),] The peak discharge
<br />estimate was 450 m3/s (+1-20%) for Buffalo Creek about
<br />0,8 km upstream from its confluence with the North Fork
<br />South Platte River (fig. 1a). The total drainage area at
<br />this site is 133 km2 (Iat. 39"23'27",long. 105"16'15"),
<br />The contributing area at this site for flood runoff
<br />(essentially all from the burned area) was about 19 km2
<br />(unit discharge -24 m31s1km2). Unburned basins with 50
<br />to 75 mm of rainfall had unit discharges less than 0,1
<br />m31s1km2, This is consistent with long-time residents'
<br />observation that no significant flooding had occurred in
<br />the Buffalo Creek area in about 70 years,
<br />The paleohydrologic evidence then were used to help
<br />define the spatial characteristics of the rainstorm and to
<br />draw an isohyetal map (fig. 1a). Rainfall amounts
<br />decreased rapidly outside the bumed area and the
<br />storm footprint within the 25 mm isohyet was about 11 0
<br />km2 (fig. 1a). in conjunction with the Nationai Weather
<br />Service and Colorado Water Conservation Board, study
<br />results were used on Juiy 18, 1996, to help determine
<br />threshold-rainfall amounts that could produce
<br />hazardous flooding.
<br />
<br />5. DISCUSSION
<br />
<br />The South Platte River at South Platte (streamflow-
<br />gaging station 06707500) is located just downstream
<br />from the confluence of the North and South Forks of the
<br />South Platte River (fig. 1 a). The flood of record was 179
<br />m3/s since the gage was installed in 1904. The peak
<br />discharge was about 325 m3/s (+1-25%) on July 12,
<br />1996, was produced by runoff from the total burned area
<br />of about 50 km2, This gage has totai drainage area of
<br />6,680 km2, thus, although less than one percent of the
<br />basin burned, the effects of the fire had a major impact
<br />on flood hydroiogy.
<br />Henz (1998, this proceedings) analyzed Doppler radar
<br />signatures and upper-air observations for the July 12th
<br />storm, but without prior knowledge of bucket data or
<br />these paleohydrologic estimates (fig. 1 a). Henz
<br />estimated maximum rainfall of about 130 mm in about an
<br />hour from the cell located near the head of Spring Creek
<br />with similar core isohyetal patterns, a storm footprint
<br />(for iess than about 50 mm) nearly twice as large, and
<br />oriented slightly different (fig. 1b), Fulton (1999)
<br />evaluated the performance of the Weather Surveillance
<br />Radar-1988 Doppler rainfall estimate for the July 12,
<br />1996 storm. He estimated a maximum of 72 mm of rain
<br />for 2000-21 00 MDT and located about 2 km south of
<br />Buffalo Creek (not shown) and similar size as Henz's,
<br />These comparison suggests that paleohydrologic
<br />techniques provide reasonable estimates of rainfall
<br />amount and spatial coverage, There is some potential
<br />for misinterpretation with the paleohydrologic approach
<br />due to variations in rainfall intensity during a storm and
<br />how they produce variations in the character of
<br />geomorphic evidence and flooding, Paleohydrologic
<br />
<br />estimates were found to be difficult to obtain when the
<br />time between storms is small. This is due in part to time
<br />for ~iIIslope recovery and difficulty discerning HWMs for
<br />different storms when a large flood precedes smaller
<br />floods. Uncertainties in rainfall amounts also can affect
<br />paleohydrologic results. Geomorphic rainfall estimates
<br />had greater uncertainties for rainfall less than about 25-
<br />50 mm in an hour. Combining geomorphic and
<br />hydrologic methods and obtaining field data soon alter a
<br />storm, for various hydroclimatic settings (including
<br />stages of post-fire watershed recovery), and validating
<br />results for numerous storms should help improve the
<br />paleohydrologic estimates, Using all sources of
<br />information (systematic and bucket data,
<br />paleohydrologic, radar, and satellite) should provide the
<br />most reliable estimates of rainfall characteristics.
<br />
<br />6. ACKNOWLEDGEMENTS
<br />
<br />Buffalo Creek residents provided valuable information
<br />about the wildfire, subsequent flooding, flood history of
<br />the area, and bucket-survey data. Larry Tunnell and
<br />Tom Browning helped determine rainfall thresholds.
<br />Ellen Wohl, Jon Nelson, and particularly Ed Tomlinson
<br />provided excellent review comments,
<br />
<br />7, REFERENCES
<br />
<br />Colorado Water Conservation Board, 1997: Emergency
<br />response, flood hazard mitigation, and flood hazard
<br />awareness for residents of Buffalo Creek, Colorado:
<br />Department of Natural Resources, Denver, CO, 18 p.
<br />Fulton, R.A., 1999: Sensitivity of WSR-88D rainfall
<br />estimates to the rain-rate threshold and rain gage
<br />adjustment: a flash flood case study, Wea, Forecasting,
<br />14, 604-624,
<br />Henz, J,F., 1998: The Buffalo Creek flash flood of July
<br />12, 1996, a reconstruction of rainfall and meteorology,
<br />Henz Meteorological Services, Denver, CO, 2B p" 1
<br />appendix, unpublished report.
<br />Jarrett, R,D" 1990: Paleohydrolo9Y used to define the
<br />spatial occurrence of floods, Geomorphology, 3, 181-
<br />195.
<br />Jarrett, RD., 1991: Paleohydrology and its value in
<br />analyzing floods and droughts. U.S. Geol. SUIY. Wat.
<br />Supply Pap. 2375,105-116.
<br />Jarrett, R.D., and Costa, J.E., 1986: Hydrology,
<br />geomorphology, and dam-break modeling of the Lawn
<br />Lake Dam and Cascade Lake Dam failures, Larimer
<br />County, Colorado. U.S. Geol. SUIY. Prof. Pap., 1369,
<br />78 p.
<br />Jarrett, RD" and Tomlinson, E.M., 2000: Regional
<br />Interdisciplinary Paleoflood Approach to Assess
<br />Extreme Flood Potential. Wat. Resour. Res" 36, 2957-
<br />2984.
<br />Miller, J,F., Frederick, R,H, and Tracy, R,J., 1973:
<br />Precipitation frequency atlas of the western United
<br />States, III--Colorado. Silver Springs, MD, NOAA, 67 p.
<br />
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