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West Willow Creek Bridge Hydraulic analyses of the reinforced concrete bridge over Willow Creek indicated that the bridge should be able to pass the 100-year flood. Figure 2.2.20 shows flood elevations on the downstream side of the bridge. The bridge opening does not significantly constrict the channel. Significant bank erosion was noted on the east side of Willow Creek just downstream from the bridge. A photo of this erosion is provided in Figure 2.2.21. Finer material was used to build up the roadway area and it is not able to resist the high velocity flow below the bridge. 8968 Le gend WS 100 yr . WS 50 yr , WS 25 yr WS 10 yr . WS 5 yr WS 2 yr E WS 1 .25 yr . Grou nd 8966 8964 8962 8960 8958 8956 8954 o 20 40 60 80 100 120 140 Station (ft) Figure 2.2.20. HEC-RAS analysis of bridge Figure 2.2.21. Erosion downstream of bridge West Willow Creek Culvert Just upstream of the bridge below the Commodore Mine, the Bachelor Loop road crosses a 7- foot diameter steel "boiler tank" culvert for West Willow Creek (Figure 2.2.22). Hydraulic analyses show that the culvert will cause overtopping of the roadway parking area during the 100-year flood. Figure 2.2.23 shows flood water elevations looking in the downstream direction. Figure 2.2.24 shows the expected path of the flooding. The overbank flooding may cause localized damage to the road, but should reenter the channel downstream of the culvert. A large amount of timber debris is present just upstream of the culvert. Therefore, flood flows lower than the 100-year level will probably cause plugging of the culvert and damage to the roadway. Figure 2.2.22. West Willow culvert I . . , "j' ....... p - t ~ I ~... i.~ 2-30 Figure 2.2.23. Flood elevations at culvert 9010 9005 Le gend WS 100 yr . WS 50 yr 9000 , WS 25 yr WS 10 yr 8995 . WS 5 yr WS 2 yr 8990 E WS 1 .25 yr . Grou nd 8985 o 20 40 60 80 100 120 Station (ft)