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FLOOD06904
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Last modified
1/25/2010 7:10:17 PM
Creation date
10/5/2006 2:35:07 AM
Metadata
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Template:
Floodplain Documents
County
Larimer
Basin
South Platte
Title
Hydrology, Geomorphology, and Dam-Break Modeling of the July 15, 1982 Lawn Lake Dam and Cascade Lake Dam Failures, Larimer County
Date
1/1/1986
Prepared For
Larimer County
Prepared By
USGS
Floodplain - Doc Type
Flood Documentation Report
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<br />GEOMORPHIC EFFECTS OF THE FLOOD <br /> <br />35 <br /> <br /> <br />FIGURE 26.-Deeply scoured reach in the Roaring River valley below Lawn Lake dam at river mile 0.28. Ground <br />moraine was scoured 35 feet to bedrock. Man (for scale) in right center of photo. <br /> <br />dipping against the flow direction at an angle of 90 to <br />100. These beds were nearly identical with those de- <br />scribed from flume experiments by Middleton (1965); <br />they indicated upper-flow regime conditions (and per- <br />haps supercritical flow) in the reach immediately below <br />the dam. When the site was visited 1 year later in July <br />1983, the antidune structures were no longer preserv- <br />ed, probably because of erosion in the exposed location. <br /> <br />ROARING RIVER ALLUVIAL FAN <br /> <br />At the mouth of the Roaring River, a large alluvial <br />fan was deposited on the floor of Horseshoe Park at <br />about 0630 MDT on the morning of the flood (fig. 34). <br />Peak flood discharge at that time was estimated to have <br />been 12,000 ft'ts (see "Dam-Break Modeling"). Sedi- <br />ments for the fan were derived from the large lateral <br /> <br />moraine along the Roaring River and Horseshoe Park, <br />just upstream from and including Horseshoe Falls. <br />Boundaries of the alluvial fan were determined from a <br />vertical aerial photograph taken about 4 hours after the <br />flood peak and enlarged to a scale of 1 in.=96.5 ft. The <br />downstream end of the fan was defined as the location <br />where the Fall River assumed a distinct channel. <br />Upstream from this point, the original stream channel <br />was obliterated and buried by sediments. The area of <br />the alluvial fan is 42.3 acres (0.066 mi') (fig. 35). <br />Volume of sediment in the fan was determined by <br />ground measurements of sediment thickness at 14 <br />points on the fan. Thickness was obtained by digging <br />through the sediment to the original ground surface at <br />11 sites. At two additional sites, the original ground <br />surface could not be reached by digging, so the depths <br />were minimal estimates. The final site was the highest <br />and thickest point on the alluvial fan. Thickness was <br /> <br />..... FIGURE 25-A. View upstream at the arrival of the floodwaters at the Big Thompson River at Estes Park gaging station (Site 6) (aerial). <br />Muddy water is arriving under the bridge in the upper left.hand comer. B, View downstream at the Big Thompson River at Estes Park <br />gaging station (Site 6) during the peak flow (aerial), Photograph taken about one-half hour after photograph in figure 2M. Golf green <br />at bottom of figure 2M is in upper left.band corner of photo. Photos courtesy of Zenas Blevins. U.S. Bureau of Reclamation. <br />
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