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Last modified
11/23/2009 12:58:15 PM
Creation date
10/4/2006 10:06:25 PM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
201
County
Boulder
Community
Lyons
Title
Flood Insurance Study - Lyons, Boulder County, Colorado
Date
2/1/1980
Designation Date
8/1/1982
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />Table 2. summary of Discharges <br /> <br />Flooding Sources <br />and Location <br /> <br />Drainage Area <br />'(Square Miles) <br /> <br />Peak Discharges <br />(Cubic Feet per Second) <br />lO-Year 50-Year lOO-Year' 500-Year <br /> <br />, <br /> <br />St~ Vrain Creek <br />At Cross Section F <br /> <br />219 <br /> <br />2,070 <br /> <br />6,000 <br /> <br />10,200 <br /> <br />36,000 <br /> <br />South St. Vrain Creek <br />At Mouth <br /> <br />94 <br /> <br />1,275 <br /> <br />4,150 <br /> <br />6,450 <br /> <br />16,000 <br /> <br />North st. Vrain Creek <br />At Mouth <br /> <br />125 <br /> <br />980 <br /> <br />2,800 <br /> <br />4,900 <br /> <br />20,000 <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of streams in the community <br />were carried out to provide estimates of the elevations of floods of <br />the selected recurrence intervals along each stream studied in <br />detail. <br /> <br />The analyses used field conditions represented by bridge and valley <br />cross sections surveyed in 1971. Water-surface profiles were <br />determined from backwater computations employing the Standard Step <br />Method (Reference 6). Starting water-surface elevations were taken <br />" from the U. S. Army Corps of Engineers report concerning the St. <br />Vrain Creek (References 4 and 5). <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles (Exhibit 1). For streams for which <br />a floodway is domputed (qection 4.2), selected cross section locations <br />are also shown on the Flood Boundary and Floodway Map (Exhibit 2). <br /> <br />" <br /> <br />Flood profiles were drawn showing computed water-surface elevations <br />to an accuracy of 0.5 foot for floods of the selected recurrence <br />intervals (Exhibit 1). <br /> <br />The roughness coefficients (Manning's "n") used in the study were <br />determined by field study and ranged from 0.045 to 0.055 for the <br />main channels and from 0.060 to 0.100 for the overbank. Head losses <br />at bridges were computed using data in Hydraulics of Bridge Waterways <br />published by the U.S. Department of Transportation (Reference 7).' , <br />, <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />DatUlll of 1929 (NGVD). Elevation reference marks used in the study <br />are shown on the maps. <br /> <br />" <br /> <br />" <br /> <br />10 <br />
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