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<br />North Creede Culvert <br />A culvert, shown in Figure 2.2.16, conveys East Willow <br />Creek through a portion of the residential area of North <br />Creede. The culvert basically exists to allow access to one <br />log cabin on the south side of East Willow Creek. The <br />cabin is made of chinked log and may be of historical value. <br /> <br /> <br />The hydraulic analysis indicates that the culvert will be <br />overtopped in nearly all flood events. Figure 2.2.17 shows <br />the upstream end of the culvert from HEC-RAS. In the <br />hydraulic analysis, overtopping initiates at even the 1.25- <br />year flood level. Major flooding of structures occurs at the <br />5-year flood level. Flood inundations are delineated in <br />Figure 2.2.19. Three structures will be inundated by flood <br />waters at the 5-year flood level. Personal accounts of <br />flooding indicate that floods typically erode the roadway on <br />top and near the culvert. HEC-RAS considers topography <br />to be fixed and does not consider bank erosion. Therefore, <br />in real flooding events, this road damage may help limit <br />flood damage to the surrounding structures. However, a <br />large flood will probably damage surrounding structures. Figure 2.2.16. North Creede Culvert <br /> <br />The corrugated metal culvert has been deformed to 4.5 feet wide by 6 feet high on the <br />downstream end, but is nearly circular with a 5.5-foot diameter on the upstream end. The <br />flooding at the culvert is caused by the small size, the hydraulic roughness of corrugated metal, <br />the small distance from the culvert top to roadway at the upstream end, and backwater caused by <br />the constricted area just downstream of the culvert. The culvert has a slope and direction <br />transition about 2/3 down its length. Log and timber debris will often get stuck at this point, and <br />the culvert is extremely susceptible to being plugged by debris. A photo of the slope transition <br />and debris that is currently caught at the transition is shown in Figure 2.2.18. <br /> <br />8986 Le gend <br />8984 WS 1 00 Y r <br /> . <br />8982 WS 50 yr <br /> . <br />8980 WS 25 yr <br />8978 WS 10yr <br /> . <br />8976 WS 5 yr <br />8974 WS 2 yr <br /> II <br />8972 WS 1.25 yr <br /> . <br /> Grou nd <br />8970 <br />0 10 20 30 40 50 60 70 <br /> <br /> <br /> <br />Station (ft) <br />Figure 2.2.17 (left). Flood elevations at XS7 <br />Figure 2.2.18 (right). Slope transition and debris in culvert <br /> <br />2-28 <br /> <br />