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FLOOD09559
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Last modified
1/26/2010 10:09:41 AM
Creation date
10/5/2006 4:27:49 AM
Metadata
Fields
Template:
Floodplain Documents
County
Larimer
Community
Fort Collins
Basin
South Platte
Title
Flood Insurance Study - Town of Evans
Date
3/18/1996
Prepared For
Fort Collins
Prepared By
FEMA
Floodplain - Doc Type
Historic FEMA Regulatory Floodplain Information
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<br />In 1983, the NRCS published a floodplain management study for Boxelder Creek <br />(Reference 27). The NRCS study indicated that the main channel, about 1.5 miles north <br />of CR-50, has a flow capacity of 150 cfs. The portion of all flood discharges upstream <br />from this point that exceed 150 cfs overtops the right bank and flows into the overflow <br />channel. <br /> <br />During the process of examining the study reach for Boxelder Creek, the USBR <br />determined that there are several locations where discharges will leave the overflow <br />channel, enter the 1-25 median. flow to the Cooper Slough basin (Cooper Slough) to the <br />west, or re-enter the main channel upstream from SH-14. All of these flow situations are <br />caused by: (1) culverts in the northbound lanes of 1-25 north of CR-50 that intercept <br />discharge from the overflow channel; (2) high-road and railroad embankments <br />(perpendicular to the overflow channel) that cause ponding and flow over low portions <br />of 1-25 or through underpasses; (3) discharge intercepted by the canal from the 1-25 <br />median or overflow channel, which either stays in the canal or overtops the canal banks <br />to flow into Cooper Slough or the overflow channel south of the canal; and (4) water- <br />surface elevations in the overflow channel that are higher than road-surface elevations <br />(1-25), which allow a flow-over-the-road situation to exist. The topography of the <br />overflow channel slopes to the west and south. This sloping makes discharge flow in <br />those directions, 1-25 is, in effect, a dike, Once the general flow patterns for Boxelder <br />Creek were determined, the next step was to quantify it at each splitting or combining <br />point. This was accomplished by making use of culvert- and road-rating curves that were <br />developed by the study contractor. <br /> <br />Peak discharge-drainage area relationships for the Cache La Poudre River, Cooper <br />Slough, Spring Creek, Dry Creek, and the Boxelder Creek Overflow Channel are shown <br />in Table L <br /> <br />3,2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of flooding from the sources studied were carried <br />out to provide estimates of the elevations of floods of the selected recurrence intervals, <br /> <br />For the Cache La Poudre River upstream of Horsetooth Road, water-surface elevations <br />were computed using the HEC-2 hydraulic model (Reference 28), All bridges were <br />considered to be unobstructed and head losses due to bridges were computed using the <br />Special Bridge Routine in HEC-2. <br /> <br />The original HEC-2 hydraulic models used in the USACE's 1989 Special Flood Hazard <br />Report were modified from models created by Simons, Li & Associates, Inc, (Reference <br />29). Channel cross sections for these models were field surveyed in 1987 by Simons, Li <br />& Associates, Inc. Significant changes were made from the models used in the 1989 <br />study. <br /> <br />The Cache La Poudre River channel in the study area was restationed from that used in <br />the 1989 study. The floodflow patterns are complicated in this area because some of the <br />flows separate from the main flow path during certain flood events. Within the study <br />reach, there are both divided-flow and split-flow conditions. <br /> <br />11 <br />
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