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. . . Mr. Wayne Bethurum February 3, 1997 Page 2 In order to determine the floodplain and base flood elevations, the U.S. Army Corps of Engineers HEC-2 computer program was used, Existing mapping, provided by the City of Sterling, was used to generate cross sections along the Pawnee Creek Overflow route from Iris Drive to County Road 29, The mapping provided was at a scale of I" = 400', with 2 foot contour intervals, This information was supplemented with field survey data, provided by Leibert- McAtee & Associates, depicting a profile of Iris Drive and the highest embankment of the Sterling No. I Ditch, from Iris Drive to Highway 138, The starting water surface at Iris Drive was determined by using an equation for broad-crested weir flow. The water surface eleyation determined using this method was 3953.18, which nearly matches the 100-year water surface elevation (3953.20) presented in the 1983 study completed by Charles Leaf, To ensure consistent results, 3953,20 was accepted as the 100-year water surface elevation at Iris Drive and was used in our model. The I OO-year discharge used for this analysis was 5,000 cfs. The water surface profile generated using the HEC-2 Model was plotted along with the field data provided by Leibert & McAtee to estimate where overtopping would occur during the lOO-year event. A copy of this plot is presented on Sheet 2 of 2 and enclosed with this letter for your use. Based on our evaluation, two locations along the ditch embankment were identified where overtopping would occur. Area one is located just south of Iris Drive. At this location stormwater will overtop the ditch embankment and will sheet flow to the east along Iris Drive and have depths of less than I foot. The second area where stormwater will overtop the ditch embankment is located just west of the proposed Design Technics Subdivision, To determine the amount of storm water that would overtop the ditch at this location, rating curves were developed for the section along the ditch and also for HEC-2 cross-section number 40, Based on these rating curves, approximately 600 cfs would overtop the ditch at this location during the 100-year event and sheet flow northeast across the subdivision, Using normal depth, we estimated that this sheet flow would spread to an approximate width of 1,600 feet at a depth of about OJ feet (four inches), The resulting sheet flow area is shown on the map and plotted based on the slope of the existing topography, A reduction of flow for downstream sections were not used, since there was little impact in water surface elevations, It should be noted that the floodplain shown on Sheet I of 2 depicts areas of flooding based on the existing mapping and the assumption that the ditch embankment serves as a levee and does not fail during overtopping. FEMA has specific criteria regarding levees and how they confine potential flooding. It is possible that FEMA would not consider this ditch to act as a levee and therefore not confine the flows to the east. If specific FEMA map revisions are required, this area will have to be reviewed for conformance by FEMA.