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FLOOD07240
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Last modified
1/25/2010 7:11:14 PM
Creation date
10/5/2006 2:49:15 AM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
316
County
San Miguel
Community
Unincorporated San Miguel County
Basin
Gunnison
Title
FIS - San Miguel County, Colorado, and Incorporated Areas: Sawpit and Telluride
Date
9/1/1988
Designation Date
3/1/1991
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />On Cornet Creek, as evidenced in 1914 and 1969, the floodflows <br />carried with them a great deal of debris and mud, and shallow <br />flooding prevailed in Telluride whenever major floods occurred. <br />For this reason, mudflow was considered in the hydraulic analysis. <br />A 20 percent water content in the mudfl.ow was assumed, which in <br />turn yielded a lOa-year mudflow discharge of 1,500 cubic feet per <br />second (ds). Due to inadequate channel capacity, this discharge <br />was routed uniformly across the alluvial fan. Because of highl.y <br />unpredictable flow direction, all possible flood routes were <br />considered. Three cross sections perpendicular to the major axis <br />of the fan were utilized in hydraulic computations for shallow <br />flooding to determine average flood 'depths. Average depths were <br />computed using Sribniy formula (Reference 14), a modified Manning's <br />equation, as follows: <br /> <br /> v = 1.486 H2/3 Sl/2 <br /> ------r- <br /> (0.1948" ) <br />where V = the mean velocity in feet per second <br /> H = the average depth in feet <br /> S = the slope in feet/feet <br /> <br />The average depths and velocities at the three cross sections as <br />shown on the Flood Insurance Rate Map (Exhibit 2) are shown in the <br />following tabulation. <br /> <br />Cross Section <br /> <br />Average <br />Depth <br />(Feet)_ <br /> <br />Mean <br />Velocity <br />(feet per second) <br /> <br />1 <br />2 <br />3 <br /> <br />0.9 <br />2.0 <br />2.7 <br /> <br />2.6 <br />6.8 <br />10.9 <br /> <br />For Cornet Creek, no flood profiles were developed and the 500-year <br />flooding was not studied. The la-year flooding was not studied <br />because the average depth of laO-year flooding was no greater than <br />three feet in this area. <br /> <br />The slope-area method was used for determination of starting water- <br />surface elevations for all other stream studied by detailed <br />methods. Water-surface profiles of selected recurrence intervals <br />were developed using the COE HEC-2 step-backwater computer program <br />(Reference 15). These water-surface profiles were computed to a <br />0.5 foot degree of accuracy. The locations of selected cross <br />sections are shown on both the Flood Insurance Rate Map (Exhibit 2) <br />and the water-surface profiles (Exhibit 1). <br /> <br />Approximate methods were used to study portions of the San Miguel <br />River, a portion of Leopard Creek, South Fork San Miguel River, and <br />Gurley Ditch. Since there is not any previous study in these <br />areas, an area prone to flooding for the recurrence interval of <br /> <br />11 <br />
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