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discharge equal to the total stream discharge minus the culvert discharge. Generally, the discharge through these culverts was minor compared to the total stream discharge. 4.3 General Overview of Flood Impacts The 60-inch diameter culvert along Oak Gulch at Motsenbocker Road and the two 72" culverts along Lemon Gulch at Lemon Gulch Road were modeled using HEC-RAS, The 72- inch culvert along stream 200 at Motsenbocker Road is filled with 1.6 feet of sediment on the upstream end and 3,0 feet on the downstream end and was modeled using the HDS-5 nomograph approach, Existing culverts in the study area are generally undersized with respect to future development peak discharges, The aggredation of sediment in the culverts and lack of maintenance add to the flooding potential. A number of shallow "spills" from the main floodplain were estimated to occur, producing areas of shallow flooding hydraulically disconnected from the floodplain. These areas are indicated on floodplain maps, For the purpose of delineating the downstream floodplain, baseline peak discharges were estimated to be undiminished by the spills, There is often little overbank relief along Oak Gulch and tributaries to Lemon Gulch, resulting in shallow flooding outside of the main charmel. Average flood velocities typically range from 3 to 11 feet per second (fps), The higher velocities are typically found in the relatively steep, confined charmels, Lower velocities occur in ponded sections where the small dams are located, The hydraulic analysis included estimation of two floodways, The floodways were based on encroachments resulting in maximum increases in the energy grade line of 0.5 and 1.0 feet The floodway analysis was performed by encroaching upon the flow path using the "blocked obstruction" option in HEC-RAS, Encroachment was limited to the top of bank for stream sections that are incised and well defined, When 100-year flows are able to be conveyed within an incised channel, the floodway is coincident with the floodplain. Likewise, in some cases the 1.0' FW is coincident with the 0.5' FW. The charmels in the study area are erodible and may undergo changes in shape, width, and direction during the flood events, resulting in changes to the floodplain limits. Changes due to development and infrastructure improvements, such as the scheduled bridge and charmel improvements along Lemon Gulch at Crowfoot Valley Road, will also impact the floodplain limits. Changes to the small dam embankments located along many of the streams in the study area will also cause changes in the floodplain and the floodways. Due to these unforeseen changes, flood hazards may exist outside the floodplain limits indicated in this report. 4.2 Results Floodplain and floodway data tables are provided in Appendix 1. The data includes information regarding discharges, water surface elevations, velocities, and widths for floodplains and floodways, In many cases, the floodplain and floodway limits are a result of a pond or roadway embankment, as noted in the tables, Future changes to these embankments will impact the floodplain and floodway limits indicated in this report. Floodplain drawings are provided in Appendix J and consist ofthe following: . Index map (Sheet 1) . Plan view maps (Sheets 2 through 25) indicate limits of the 100-year floodplain . Profile drawings (Sheets 26 through 44) show the lO-year and 100-year water surface profiles . Cross-sections (Sheets 45 through 50) show representative floodplain charmel geometry, discharge, and velocity data 7 Oak Gulch and Lemon Gulch FHAD.DOC