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FLOOD07246
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Last modified
1/25/2010 7:11:15 PM
Creation date
10/5/2006 2:50:16 AM
Metadata
Fields
Template:
Floodplain Documents
County
Jefferson
Community
Buffalo Creek
Basin
South Platte
Title
Wildfire Flood-Hazard and Watershed-Rehabilitation Monitoring
Date
1/8/1998
Prepared For
CWCB
Prepared By
USGS
Floodplain - Doc Type
Flood Mitigation/Flood Warning/Watershed Restoration
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<br />Maximum water depths as much as 4 m occurred w~hin 30 minutes of the storm's onset in Buffalo Creek <br />(fig. 2), Spring Creek, and the NF and South Platte Rivers. HWMs generally were good-to-excellent <br />comprised of charcoal, leaf, and needle litter, silt, bent grass, and wash lines. The HWMs were used to <br />estimate the water slope and flood depth for each cross section. Peak discharge estimated with the <br />slope-conveyance (SIC) method usually is less accurate than estimates using multiple cross sections. <br />However, estimates provided here reflect an average of several SIC estimates along a reach of channel. <br /> <br /> <br />These estimates probably are more accurate than a single SIC estimate because of a good agreement of <br /> <br /> <br />estimates along a channel. The SIC uncertainty in discharge estimates is caused primarily by n values, <br /> <br /> <br />bulking of flow w~h sediment and debris, and channel changes. Estimates of uncertainties are shown in <br /> <br /> <br />parentheses. The peak discharge estimate was 450 m3ts (+1-20%) for Buffalo Creek near the NF South <br /> <br /> <br />Platte River (fig. 1, s~e 1; fig. 2). This estimate reflects runoff from the bumed area in Buffalo Creek and ~s <br /> <br /> <br />tributaries (notably Sand Draw, Spring Gulch, Shinglemill Creek, and Morrison Creek). Sand Draw, about <br /> <br /> <br />3.6 km2 (fig. 1, s~e 2), had an estimated peak discharge of 200 m3/s (+1-25%). The estimated peak <br /> <br /> <br />discharge was 510 m3/s (+1- 25%) for Spring Creek upstream from ~s confluence w~h the South Platte <br /> <br /> <br />River (fig. 1, s~e 3). Maximum flooding in Spring Creek, in addition to the effects of the fire in other study <br /> <br /> <br />basins, has basin slopes greater than about 30 percent, extensive bedrock exposure, and because the <br /> <br /> <br />storm moved from west to east down the basin. <br /> <br />Flash-flood runoff at the South Platte River streamflow-gaging station (06707500) measures the <br />cumulative runoff from the 1996 burn area (fig. 1, s~e 4), but principally from Buffalo and Spring Creeks. <br />The SIC estimated peak discharge was 325 m3/s (+1- 20 %), reflecting attenuation of flood peaks from <br />Buffalo and Spring Creeks. Unfortunately, the July 12, 1996 peak stage in the South Platte River at <br />South Platte gage stilling well was 1.52 m lower than excellent high-water marks (HWMs) probably due to <br />an insufficient size intake to fill the large-volume, stilling well during the July 12, 1996 flash flood. The <br />peak discharge using the incorrect gage height (estimated during the storm) from the rating curve was <br />only 81 m3ts: ~ is likely the entire flood hydrograph is suspect. Incorrect gage recordings are a serious <br />concern for flash-flood detection and issuing warnings to the public. <br /> <br />9 <br />
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