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At State Highway 93 (South Broadway) , a split flow analysis was again used to At the lower (downstream) end of Eldorado Springs, numerous dwellings are located evaluate the relative percentages of the 100-year flood passing through the within the 100-year floodplain. Towards the upper end of the study reach, the bridge opening and overtopping the roadway in the west overbank. Approximately floodplain is confined within the existing channel banks. 2100 cfs passes through the bridge whereas 3850 cfs crosses the roadway. The bridge structure itself remains above the 100-year water surface elevation. Floodplain and Floodway Data State Highway 93 to Davidson Ditch Diversion The limits of the 100-year floodplain are depicted on the Flood Hazard Area Delineation maps contained in the report Appendix. Tables 3 and 4 contain Over this 2.3 mile study reach the average 100-year floodplain width varies from tabular data used to establish the floodplain boundaries. These tables list the 300 to 1600 feet. Land use is principally rural-agricultural , however, a group potential flood elevations, floodplain and floodway widths , stream stationing, of residential dwellings is located near the creek channel in the Valle Del Rio and thalweg elevations at each cross-section location used in the hydraulic Subdivision. model . The limits of two 100-year floodways are defined in Table 4. These two floodways are based on encroachment resulting in maximum increases in the energy The overbank areas contain various small gullies and their presence is reflected gradeline of 0.5 foot and 1.0 foot. Boulder County regulates the 1.0-foot rise by the numerous small islands shown on the floodplain maps (Sheets 15 through floodway. In addition, separate floodway data are provided for the Dry Creek 19). The flood profiles for this reach are shown on Sheets 26 through 28. No. 2 Ditch and State Highway 93 split floodplain areas. The data contained in Table 4 provides separate 100-year floodplain and floodway data for these split Bridge crossings consist of a fire access road bridge into the City of Boulder flow reaches. Floodway data presented in Table 4 defines the portion of the Mountain Parks and a culvert crossing to a residence. Diversion structures channel which must remain unobstructed to allow for passage of the 100-year flood include: Dry Creek No. 2 Ditch, South Boulder Ditch, Upper Bear Creek Ditch, and event without significant increases in stage due to potential future development iDavidson Ditch. in areas outside the floodway. The floodway is a high hazard area characterized by relatively deep, fast moving flows. As such, the floodway should be kept free Davidson Ditch Diversion to Eldorado Springs of any development or use which obstruct flows. The uppermost 1.3 mile study reach of South Boulder Creek transitions rapidly Flood Velocities from the broad open valley, characteristic of the high plains, to a narrow, steep-walled mountain canyon at Eldorado Springs. Sheets 17 through 21 show the Mean velocities in the floodway generally range between 3 and 10 feet per second 100-year floodplain limits. Water surface profiles are presented on Sheets 28 (fps) with flow velocities in the channel exceeding 10 fps at certain locations. through 29. Velocities in the overbanks typically range between 0 and 4 fps. This portion of South Boulder Creek is crossed by several small bridges which Velocities in excess of 6 to 8 fps can cause severe erosion of stream banks , and furnish access to residences on the northwest side of the creek. The 100-year are capable of transporting large rocks and debris. Velocities in this range can floodplain is fairly narrow, ranging in width from 300 feet near the Davidson also erode fill around bridge abutments , potentially leading to bridge failure. Ditch diversion to 40 feet at the upstream study limit. Lower velocities of approximately 2 fps or less will deposit debris and silt. 11