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FLOOD10389 (2)
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Last modified
11/23/2009 12:47:52 PM
Creation date
7/25/2007 10:12:39 AM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
577
County
Conejos
Community
Manassa
Stream Name
Conejos River
Title
Floodplain Information Report - Manassa Colorado
Date
8/1/2004
Prepared For
Manassa and CWCB
Prepared By
Love and Associates
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />ENGINEERING METHODS <br /> <br />Hvdrolo2ic Analvsis <br /> <br />Basin delineation for Conejos River are shown on Figures 3a and 3b, Catchment Areas <br />and Design Points. The design point used for hydrologic analysis is located downstream <br />of the Town and is shown on the Figure. The drainage area of Conejos River at the <br />design point is about 360 square miles. Flood peaks can be estimated using the USGS <br />Regression equations developed for the Rio Grande Basin (Vaill, 2000). These <br />regression equations have a large standard error (around 50 per cent) and, as a result, the <br />flood peaks should be considered accurate only to an order of magnitude. <br /> <br />Flood peak discharges for the 10-, 50-, 100-, and 500-year events for Conejos River used <br />for the floodplain study are summarized in Flood Peak Discharge Summary table. <br /> <br />Flood Peak Discharge Summary <br /> <br />Location <br /> <br /> <br />Cone 'os River <br />At Manassa <br /> <br />1300 <br /> <br />2100 <br /> <br />2500 <br /> <br />3400 <br /> <br />Hvdraulic Analvsis <br /> <br />Just west of Highway 285 and upstream of Manassa, Conejos River splits into two <br />branches. The name Conejos River identifies the southernmost branch, while the <br />'northernmost channel downstream of the split is called North Branch Conejos River. <br />Please refer to Figure 4. The 100- year flood will be transported in both branches of the <br />River. Since immediately downstream of the split the slope of both parallel channels are <br />similar, about 0.65 per cent, each branch is likely to transport half of the 100-year flood <br />peak, or approximately 1250 cfs. <br /> <br />" <br />!' <br /> <br />A detailed hydraulic analysis of the North Branch Conejos River floodplain was not <br />performed for the study. However, examination of topographic information in the <br />vicinity of the Town provides insight into the characteristics of the floodplain. Just south <br />of the Town, a rise topographically divides the North Branch Conejos River and the <br />Conejos River. Please refer to Figure 4. The topographic map indicates the ground <br />between the toe of the rise and the Town is fairly flat. The minimum distance from the <br />toe of the rise to the Town boundary is approximately 3000 feet as shown on the Figure. <br />The slope of the North Branch Conejos River valley south and adjacent to the Town is <br />approximately 0.44 per cent. Conservatively assuming a flat channel and a roughness <br />coefficient of 0.06, the depth of flow (normal depth) for a discharge of 1250 cfs and a <br />3000 foot wide floodplain is 0.44 feet. Please refer to the calculation in the Appendix. If <br />the full IOO-year discharge of2500 cfs is assumed to enter North Branch Conejos River, <br />the flow depth would be less than 0.7 feet at a 3000 foot width. <br /> <br />5 <br />
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