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Last modified
1/25/2010 7:15:07 PM
Creation date
10/5/2006 3:46:27 AM
Metadata
Fields
Template:
Floodplain Documents
County
Eagle
Community
Gypsum
Stream Name
Sweetwater Creek
Basin
Colorado Mainstem
Title
Anderson Camps Floodplain Analysis
Date
3/23/1977
Prepared For
Travis Anderson
Prepared By
Gingery Associates, Inc.
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />results indicate a value somewhat below the discharges <br />obtained by Magnitude and Frequency of Floods in <br />Colorado, Part 9. <br /> <br />Due to the critical nature of the flood plain analysis <br />being undertaken, that is that loss of life is a possi- <br />bility if discharges exceed the design value, we have <br />selected the larger value of 2,700 cfs for the 100 year <br />flood discharge and 1,300 cfs as the mean annual flood <br />discharge. <br /> <br />FLOOD PLAIN ANALYSIS, EXISTING CONDITIONS <br /> <br />A formal flood plain hydraulic analysis has been completed <br />on the reach of Sweetwater Creek through the lower Anderson <br />Camp site utilizing field surveyed cross sections and the <br />Corp of Engineers HEC-2 backwater program. The results <br />of this flood plain are illustrated on Sheet 1 of 2, Flood <br />Plain Plan and Sheet 2 of 2, Flood Plain Profile and Cross <br />Sections. A hydraulic channel roughness factor (Mannings <br />"n" value) of 0.066 was adopted for the channel section <br />allowing for the rocks, trees, and brush in the cross sec- <br />tional area. An overbank "n" value of 0.050 was utilized <br />since the majority of the overflow area is through pasture <br />land or parking lot areas. <br /> <br />Observation of the existing channel flow and the history <br />of the stream flow characteristics indicate that flows <br />within the channel often exist at super critical velocities. <br />This appears to be the situation for the mean annual run- <br />off or lesser discharge events. Discharges approaching <br />the 100 year flood rate spread out into the valley cross <br />section as illustrated on the drawings and flow at or near <br />critical velocity.. The 100 year Flood Plain and Profile, <br />existing conditions, shown on the drawings is based on <br />sub-critical flow conditions. The mean annual flood plain <br />and profile are based on super critical flows upstream from <br />the bridge backwater area. <br /> <br />The existing bridge on Sweetwater Creek creates a signifi- <br />cant backwater in the area extending upstream approximately <br />150 feet. Beyond this point, the flood plain is not af- <br />fected by the bridge backwater, but is simply due to in- <br />sufficient channel capacity. As shown on the drawings, <br />the mean annual flood profile is very close to, or above, <br />the existing streambank elevations. Since Sweetwater <br />Creek does not have a history of frequent flooding through <br />the Anderson Camp area, it is likely that the stream chan- <br />nel has aggraded due to debris brought in by the 1976 <br />flood. <br /> <br />-4- <br />
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