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I I I I I I I I I I I I I I I I I I I At State Highway 93 (South Broadway), a split flow analysis was again used to evaluate the relative percentages of the lOa-year flood passing through the bridge opening and overtopping the roadway in the west overbank. Approximately 2100 cfs passes through the bridge whereas 3850 cfs crosses the roadway. The bridge structure itself remains above the lOa-year water surface elevation. At the lower (downstream) end of Eldorado Springs, numerous dwellings are located within the lOa-year floodplain. TDwards the upper end of the study reach, the floodplain is confined within the existing channel banks. Floodplain and Floodway Data State Highway 93 to Davidson Ditch Diversion The limits of the lOa-year floodplain are depicted on the Flood Hazard Area Delineation maps contained in the report Appendix. Tables 3 and 4 contain tabular data used to establish the floodplain boundaries. These tables list the potential flood elevations, floodplain and floodway widths, stream stationing, and thalweg elevations at each cross-section location used in the hydraulic model. The limits of two lOa-year floodways are defined in Table 4. These two floodways are based on encroachment resulting in maximum increases in the energy grade 1 i ne of 0.5 foot and 1.0 foot. Boul der County regul a tes the 1. 0- foot ri se floodway. In addition, separate floodway data are provided for the Dry Creek No.2 Ditch and State Highway 93 split floodplain areas. The data contained in Table 4 provides separate lOa-year floodplain and floodway data for these split flow reaches. Floodway data presented in Table 4 defines the portion of the channel which must remain unobstructed to allow for passage of the lOa-year flood event without significant increases in stage due to potential future development in areas outside the floodway. The floodway is a high hazard area characterized by relatively deep, fast mDving flows. As such, the floodway should be kept free of any development or use which obstruct flows. Over this 2.3 mile study reach the average lOa-year floodplain width varies from 300 to l600 feet. Land use is principally rural-agricultural, however, a group of residential dwellings is located near the creek channel in the Valle Del Rio Subdivision. The overbank areas contain various small gullies and their presence is reflected by the numerous small islands shown on the floodplain maps (Sheets 15 through 19). The flood profiles for this reach are shown on Sheets 26 through 28. Bridge crossings consist of a fire access road bridge into the City of Boulder Mounta i n Parks and a culvert crossing to a residence. Diversion structures include: Dry Creek No.2 Ditch, South Boulder Ditch, Upper Bear Creek Ditch, and Davidson Ditch. Davidson Ditch Diversion to Eldorado Springs The uppermost 1.3 mile study reach of South Boulder Creek transitions rapidly from the broad open valley, characteristic of the high plains, to a narrow, steep-walled mountain canyon at Eldorado Springs. Sheets 17 through 2l show the lOa-year floodplain limits. Water surface profiles are presented on Sheets 28 through 29. Flood Velocities Mean velocities in the floodway generally range between 3 and 10 feet per second (fps) with flow velocities in the channel exceeding 10 fps at certain locations. Velocities in the overbanks typically range between 0 and 4 fps. This portion of South Boulder Creek is crossed by several small bridges which furnish access tD residences on the northwest side of the creek. The lOa-year floodplain is fairly narrow, ranging in width from 300 feet near the Davidson Ditch diversion to 40 feet at the upstream study limit. Velocities in excess of 6 to 8 fps can cause severe erosion of stream banks, and are capable of transporting large rocks and debris. Velocities in this range can also erDde fill around bridge abutments, potentially leading to bridge failure. Lower velocities Df approximately 2 fps or less will deposit debris and silt. 11