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<br />Mr. Wayne Bethurum <br />February 3, ] 997 <br />Page 2 <br /> <br />In order to determine the floodp!ain and base flood elevations. the U.S. Army Corps of Engineers <br />HEC-2 computer program was used. Existing mapping. provided by the City of Sterling, was <br />used to generate cross sections along the Pawnee Creek Overflow route from Iris Drive to <br />County Road 29. The mapping provided was at a scale of I" = 400', 'Nith 2 foot contour <br />intervals. This information was supplemented with field survey data, provided by Leibert- <br />McAtee & Associates, depicting a profile of Iris Drive and the highest embanJmlent of the <br />Sterling No. I Ditch, from Iris Drive to Highway 138. The starting water surface at Iris Drive <br />was determined by using an equation for broad--crested weir flow. The water surface elevation <br />determined using this method was 3953.18, which ne-<~Iy matches the 100-year water surface <br />elevation (3953.20) presented in the 1983 study completed by Charles Leaf. To ensure <br />consistent results, 3953.20 was accepted as the 100-year water surface elevation at Iris Drive and <br />was used in our model. The I OO-year discharge used for this analysis was 5,000 cfs. <br /> <br />The water surface profile generate-d using the HEC-2 Model was plotted along with the field data <br />provided by Leibert & McAtee to estimate where overtopping would occur during the 100-year <br />event. A copy of this plot is presented on Sheet 2 of 2 and enclosed with this letter for your use. <br />Based on our evaluation, two locations along the ditch embankment were identified where <br />overtopping would occur. Area one is located just south of Iris Drive. At this location <br />storm water will overtop the ditch embankment arLd will sheet flow to the east along Iris Drive <br />and have depths of less than] foot. The second area where stormwater will overtop the ditch <br />embankment is located just west of the proposed Design Technics Subdivision. To determine the <br />amount of storm water that would overtop the ditch at this 1000ltioIl, rating curves were developed <br />for the section along the ditch and also for HEC-2 cross-section number 40. Based on these <br />rating curves, approximately 600 cfs would overtop the ditch at this location during the 100-year <br />event and sheet flow northeast across the subdivision. Using normal depth, we estimated that <br />this sheet flow would spread to an approximate width of 1,600 feet at a depth of about OJ feet <br />(four inches). The resulting sheet flow area is sho\'m on the map and plotted based on the slope <br />of the existing topography. A reduction of flow for downstream sections were not used, since <br />there was little impact in water surface elevations. <br /> <br />It should be noted that the floodplain show11 on Sheet I of 2 depiclS areas of flooding based on <br />the existing mapping and the assumption that the ditch embankment serves as a levee and does <br />not fail during overtopping. FE\!A has specific criteria regarding levees and how they confine <br />potential flooding. It is possible that FEMA would not consider this ditch to act as a levee and <br />therefore not confine the flows to the east. If specific FEMA map revisions are required, this <br />area will have to be reviewed for conformance by FEMA. <br />