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<br />discharge was about 40 rn3/slkm2 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 (+/-20%) for Buffalo Creek aboul
<br />0,8 km upstream from its conftuence with the North Fork
<br />South Platte River (fig, la). The lotal drainage area at
<br />this site is 133 km2 (lal. 39"23'27", long, lOS.16'15'),
<br />The conlnbuting area at this sne for flood runoff
<br />(essentially all from the burned area) was about 19 km2
<br />(unit discharge -24 m3/sJkm2), Unburned basins with 50
<br />to 75 mm of rainfall had unn discharges less than 0.1
<br />m3tsJkm2. This Is consistent with long-time residents'
<br />observation that no significant flooding had occurred in
<br />the Buffalo Creek area in about 70 ye8lS.
<br />The paJeohydrologlc evidence then were used 10 help
<br />define the spatial characteristics of the rainstorm and 10
<br />draw an isohyetal map (fig, la), Rainfall amounts
<br />decreased rapidly outside the burned area and the
<br />storm footprint within the 25 mm isohyet was about 11 0
<br />km2 (fig, la). In conjunction with the Nalional Weather
<br />Service and Colorado Water Conservation Board, study
<br />results were used on July 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 Soulh 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 (+/-25%) on July 12,
<br />1996, was produced by runoff from the total burned area
<br />of about 50 km2. This gage has total drainage area of
<br />6,880 km2, thus, aIlhough less then one percent of the
<br />basin bumed, the effects of the fire had a major impact
<br />on flood hydrology.
<br />Henz (1998, this proceedings) analyzed Doppler radar
<br />signatures and upper-air observations for the July 121h
<br />storm, but without prior knowledge of bucket deta or
<br />these paleohydrologic estimates (fig, la), Henz
<br />estimated maximum rainfall of ab0U1130 mm in about an
<br />hour from the ceO Iocaled near the head of Spring Creak
<br />with similar core isohyetal patterns, a storm footprint
<br />(for less than about 50 mm) nearly IwIce as large, and
<br />orienled slightly different (lig, lb), Fuhon (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 0172 mm of rain
<br />for 2O()().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 10 variations in rainfallln1ensily during a storm and
<br />how they produce variations in the character of
<br />geomorphic evidence and liooding. Paleohydrologic
<br />
<br />J42 AMERICAN METEOROLOGICAL Soclm
<br />
<br />,
<br />
<br />estimates were found to be dillicullto obtain when the
<br />time between storms is small. This is due in part to lime
<br />lor hillslope recovery and difficulty discerning HWMs for
<br />different storms when a large flood precedes smaller
<br />lIoOds. Uncertainties in rainfall amounts, also can aftect
<br />paleohydrologic results, Geomorphic rainfall estimates
<br />had greater uncertainties for rainfall lass than about 25-
<br />50 mm in an hour, Combining geomorphic and
<br />hydrologic methods and obtaining field data soon after a
<br />storm, for various hydroclimatic settings (including
<br />stages of post-lire watershed recovery), and validating
<br />results for numerous stonns should help improve the
<br />paleohydrologic estimates. Using all sources of
<br />information (systematic end bucket data,
<br />paleohydrologic, radar, and sa!elHte) 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, subsequentllooding, flood history of
<br />the area, and bucket-survey data. Larry Tunnell and
<br />Tom Browning helped determine rainfall thresholds,
<br />Ellen Wahl, Jon Nelson, and particularly Ed Tomlinson
<br />provided excellent review comments,
<br />
<br />7. flEFERENCES
<br />
<br />Colorado Water Conservation Board, 1997: Emergency
<br />response, flood hazard mitigation, and flood hazard
<br />awllretless for residents of Buffalo Creek. Colorado:
<br />Deparlment of Natural Resources, Denver, CO, 18 p,
<br />Futton, RA, 1999: Sensillvily of WSR-88D rainfall
<br />estimates to the rain-rate thnt8hold and rain gage
<br />adj~strnent: a flash flood case study, Wea, Forecssting,
<br />14, 604-624,
<br />Henz, J.F" 1998: The Buffalo Creek flash flood of July
<br />12, 1996, a reconstrucllon of rainfall and meteorology,
<br />Henz Meteorological Services, Denver, CO, 28 p., 1
<br />appendix, unpublished report.
<br />Jarrett. R.D" 1990: Paleohydrology used 10 define the
<br />spatial occurrence of floods. Geol7lOlp/JQtogy,3, 181.
<br />195,
<br />Jarrett. R.D., 1991: Paleohydrology and its value in
<br />analyzing floods end droughts. U.S. GsoI, Surv, Wet
<br />Supply Pap. 2375, 105-116.
<br />Jarrett. R.D., end Costa, J.E" 1986: Hydrology,
<br />geomorphology, and dam-breek modeling of the Lawn
<br />LakII Dam and Cascade LakII Dam failures, Larimer
<br />County, Colorado, U,S, GsoI. Surv, Prof. Pap., 1369,
<br />78p.
<br />Jarrett. R,D" and Tomlinson, E,M., 2000: Regional
<br />Interdlsciplinary Paleoflood Approach to Assess
<br />Extreme Aood Potential. Wet Resour. Res., 36, 2957-
<br />2984.
<br />Miller, J,F., Frederick, R.H, and Tracy, R.J., 1973:
<br />Precipitalion frequency atlas of the western United
<br />Slates. III-Colorado. Silver Springs, MD, NOAA, 67 p.
<br />
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