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FLOOD11628 (2)
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Last modified
1/26/2010 10:25:17 AM
Creation date
1/5/2009 12:18:10 PM
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Template:
Floodplain Documents
County
El Paso
Community
Manitou Springs, Colorado Springs
Basin
Arkansas
Title
Paleohydrologic Flood Investigations for Streams in Manitou Springs and Colorado Springs
Date
12/3/1987
Floodplain - Doc Type
Floodplain Report/Masterplan
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At the road crosein~s ar the left bank tributary at elev~tions of <br />7, 050 and 7, ~00 feet, very e~tenaive debris-flow deposits occur. The <br />drainage area i~ eetimated to be 2 square miles. There ar~ several <br />100 toat long debris-flow levees With 2 to 3 foot diameter boulders on <br />the top surface. The channel is 50 feet wide and the deposi+~s are 10 <br />or more feet above the streambed. Also, at~several abrup~. channel <br />expansions there are deposits with boulders as large as 10 feet. The <br />boul~era are completely lichen covered and there are many 3 to 4 foot <br />diameter trees growing on the debris-flow deposit surfaces suggesting <br />the deposits are 200 or more years old. The recent channel (~here are <br />n~ indicatic,na of higher water-flowg) is 3 feet wide and 1@f38 than a <br />foot ~eep. The stream banks and channel boulders are c~mpletely <br />coveied with mosa or lichen. Recent peak discharge is e-stimated to be <br />30 cubic feet ~,er secon~. The lack ~f flood evidence indic~tea that <br />flor+s have not exceeded the erosion threshold ~ischarge of 2t~0 to 300 <br />c~ibic feet per second in the laet hundre~ yeare or more. ~ <br />' For about 1/2 mile upstream, to Cryatal Park lake, the hillside <br />i~ extremely steep and probably is the source area for the debris <br />21ow. Several possible source areas for~ the debris f.1ow were <br />considered, but because of the lenc~th of intervening time the location <br />could not be pinpointed without closer inspection. <br />Williama Canvon <br />Williams Canyon drains 2.7 square miles. From the Cave of the <br />Winds road switchback downstream, the canyon has been e~tensively <br />,modified; hence paleoflood studies, particularly to identify large <br />floods, were not done. At an elevation of about 6,840 feet, there is <br />is large flood bar or boulder berm (these result from highly <br />concentrated sediment-laden floods, but not a debris flow) that is <br />about 200 feet long and 20 feet wide (indicative of a larg~ flood). <br />The top of the berm is about 10 feet above the streambed and the width <br />is 70 ieet; the mean flood depth is about 6 feet. The surfa~ce of the <br />berm is covered with boulders 4 to S feet in diameter, many 4 to S <br />inch diameter scrub oak, and several 12 to 15 diameter spruce trees <br />indicatin~ the berm may have be~n deposited about 75 to 100 y~ears ago. <br />These deposits represent the largest flood that has occurred in the <br />channel and suggests that the maximum historic peak discharge is about <br />4,200 cubic feet per second. Given that the flow contained a large <br />amount of sed'i~ment, the wate~r discharge may have only been about a <br />half this discharge. <br />At an elevation of abc,ut 7, 040 feet, there are flood-boulder <br />deposits (boulders up to 3 feet in diameter) in the channel that are <br />about 10 feet above the etreambed (mean depth is about 5 feet? and the <br />channnel width is 25 feet. The decayed organic flood debris, several <br />6 to 10 inch diameter spruce trees suggest that this medium flood <br />occurred about 40 to 50 years ago. The peak diacharge for this flood <br />is about 1,200 cubic feet per second. <br />From this elevation downstream to the switchback in the Cave of <br />the Winds road there is very consistent evidence of recent flows. The <br />channel is about 6 feet wide and the banks are covered with grass and <br />brush. Organic flood debris t.hat is less than about 10 years old on <br />?. <br />
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