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Last modified
1/25/2010 6:48:08 PM
Creation date
10/5/2006 1:13:47 AM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
205
County
Larimer
Community
Fort Collins
Basin
South Platte
Title
Master Drainageway Planning - Cooper Slough, Boxelder Creek, Fort Collins, Larimer County, Colorado
Date
8/1/1981
Designation Date
1/1/1983
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />- <br /> <br />I <br /> <br />I <br /> <br />III. HYDROLOGIC AND HYDRAULIC SUMMARY <br /> <br /> <br />Flood Characteristics <br /> <br /> <br />Floods in the study area are usually caused by general rainstorms and <br /> <br />cloudburst storms during the period May through September. Floods resulting <br /> <br /> <br />from prolonged heavy rainfall over the watershed are characterized by high <br /> <br /> <br />peak flow of moderate volume and duration. The cloudburst type is a high- <br /> <br /> <br />intensity, short-duration rainstorm which produces a flood of high peak flow, <br /> <br /> <br />short duration, and small volume of runoff. Peak runoff will usually occur <br /> <br /> <br />within a few hours after a single storm. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />Hydrologic Analysis <br /> <br />A hydrologic analysis was conducted to determine runoff hydrographs from <br /> <br /> <br />one-hour storms of 2-, 10-, 25-, 50-, 100- and 500-year recurrence interval <br /> <br />for Boxelder Creek and Cooper Slough. The "Storm Water Management Model" <br /> <br /> <br />(SWMM) (reference 8) as developed by the Environmental Protection Agency (EPA) <br /> <br />and later modified by the Missouri River Division (MRD) of the U.S. Army Corps <br /> <br /> <br />of Engineers was used to estimate the peak flows and runoff volumes. A storm <br /> <br /> <br />of a given return period was assumed to generate a flood hydro graph and peak <br />. <br /> <br />of the same return period. The parameters used in the analysis were con- <br /> <br />sistent with previous studies in the area, such as the "Fossil Creek <br /> <br /> <br />Hydrology, Larimer County, Colorado" (Fossil Creek) and IIMajor Drainageway <br /> <br /> <br />Planning, Dry Creek, Fort Collins, Larimer County, Colorado" (Dry Creek) stu- <br /> <br /> <br />dies (reference 9 and 10). <br /> <br /> <br />The entire drainage basins for Boxelder Creek and Cooper Slough are shown <br /> <br /> <br />in Figure 1. The shaded area represents nontributary or controlled area. The <br /> <br /> <br />total Boxelder Creek watershed covers 251 square miles of which 60.3 is <br /> <br /> <br />uncontrolled area. The Cooper Slough basin is 10.2 square miles. The <br /> <br />drainage basins were subdivided into sub catchments and channel units. The <br /> <br /> <br />Boxelder drainage was divided into 99 subcatchments and 64 channel units. The <br /> <br /> <br />Cooper Slough basin was divided into 40 subcatchments and 32 channel units. <br /> <br /> <br />Figure 3 shows the subcatchment and channel locations. Subcatchment deli- <br /> <br /> <br />neations were based on the assumption that all irrigation ditches are flowing <br /> <br /> <br />full during floods. Definition of the subcatchments assumed all runoff from <br /> <br /> <br />tributary areas reached the main channel. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />12 <br /> <br />I <br />
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