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<br />discharge equal to the total stream discharge minus the culvert discharge. Generally, the <br />discharge through these culverts was minor compared to the total stream discharge. <br /> <br />4.3 General Overview of Flood Impacts <br /> <br />The 60,inch diameter culvert along Oak Gulch at Motsenbocker Road and the two 72" <br />culverts along Lemon Gulch at Lemon Gulch Road were modeled using HEC-RAS. The 72, <br />inch culvert along stream 200 at Motsenbocker Road is filled with 1.6 feet of sediment on the <br />upstream end and 3.0 feet on the downstream end and was modeled using the HDS,5 <br />nomograph approach, <br /> <br />Existing culverts in the study area are generally undersized with respect to future <br />development peak discharges. The aggredation of sediment in the culverts and lack of <br />maintenance add to the flooding potential. <br /> <br />A number of shallow "spills" from the main floodplain were estimated to occur, producing <br />areas of shallow flooding hydraulically disconnected from the floodplain. These areas are <br />indicated on floodplain maps. For the purpose of delineating the downstream floodplain, <br />baseline peak discharges were estimated to be undiminished by the spills. <br /> <br />There is often little overbank relief along Oak Gulch and tributaries to Lemon Gulch, <br />resulting in shallow flooding outside of the main charmeL Average flood velocities typically <br />range from 3 to 11 feet per second (fps), The higher velocities are typically found in the <br />relatively steep, confined charmels. Lower velocities occur in ponded sections where the <br />small dams are located, <br /> <br />The hydraulic analysis included estimation of two floodways. The floodways were based on <br />encroachments resulting in maximum increases in the energy grade line of 0.5 and 1.0 feet <br />The floodway analysis was performed by encroaching upon the flow path using the "blocked <br />obstruction" option in HEC-RAS. Encroachment was limited to the top of bank for stream <br />sections that are incised and well defined. When 100,year flows are able to be conveyed <br />within an incised channel, the floodway is coincident with the floodplain. Likewise, in some <br />cases the 1.0' FW is coincident with the OS FW. <br /> <br />The charmels in the study area are erodible and may undergo changes in shape, width, and <br />direction during the flood events, resulting in changes to the floodplain limits, Changes due <br />to development and infrastructure improvements, such as the scheduled bridge and charmel <br />improvements along Lemon Gulch at Crowfoot Valley Road, will also impact the floodplain <br />limits. Changes to the small dam embankments located along many of the streams in the <br />study area will also cause changes in the floodplain and the floodways. Due to these <br />unforeseen changes, flood hazards may exist outside the floodplain limits indicated in this <br />report. <br /> <br />4.2 Results <br /> <br />Floodplain and floodway data tables are provided in Appendix L The data includes <br />information regarding discharges, water surface elevations, velocities, and widths for <br />floodplains and floodways. In many cases, the floodplain and floodway limits are a result of <br />a pond or roadway embankment, as noted in the tables. Future changes to these <br />embankments will impact the floodplain and floodway limits indicated in this report. <br /> <br />Floodplain drawings are provided in Appendix J and consist ofthe following: <br /> <br />. Index map (Sheet 1) <br />. Plan view maps (Sheets 2 through 25) indicate limits of the 100-year floodplain <br />. Profile drawings (Sheets 26 through 44) show the lO'year and 100,year water surface <br />profiles <br />. Cross,sections (Sheets 45 through 50) show representative floodplain charmel <br />geometry, discharge, and velocity data <br /> <br />7 <br /> <br />Oak Gulch and Lemon Gulch FHAD.DOC <br />