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<br />Floodplain Analysis <br />To establish hydrology for the current study, the Natural Resources Conservation Service and <br />Colorado Water Conservation Board developed a set of discharge-frequency estimates using a <br />regional watershed methodology. Seventeen watersheds of similar size and relatively close <br />geographic proximity were studied, and a regression analysis of watershed variables was used to <br />estimate discharge-frequency for Willow Creek. The discharge-frequency estimates were similar <br />to earlier estimates by the Colorado Water Conservation Board, but were significantly smaller <br />than estimates from the Army Corps. The discrepancy has arisen in the confidence placed in <br />historic flood estimates. <br /> <br />The U.S. Army Corps of Engineers' (Army Corps) Hydraulic Engineering Center River Analysis <br />System model (HEC-RAS) was used to model channel hydraulics. HEC-RAS is a one- <br />dimensional, steady flow model that can model mixed flow regimes of sub-critical and super- <br />critical flow as well as bridges, culverts, and weirs. A distance of 3087 feet on Willow Creek <br />from the flume entrance to the confluence, 9629 feet on West Willow Creek, and 3153 on East <br />Willow Creek were modeled in HEC-RAS using a total of 104 cross-sections. <br /> <br />HEC-RAS was used to determine water surface elevations, and these water surface elevations <br />were mapped on the topography to delineate areas that are expected to be inundated by floods. <br />The 100-year floodplain was mapped from the entrance to the flume to above the Commodore <br />Mine in West Willow Creek and above North Creede on East Willow Creek. Floodplains for <br />other flood return intervals were mapped at critical locations. <br /> <br />During large floods in Willow Creek, the flow at many, particularly upstream, locations will be <br />high velocity "supercritical" flow. Supercritical flow is highly erosive, and large floods could <br />potentially erode and damage channel banks, levees, and hydraulic structures, especially if debris <br />blockage occurs. Without any debris blockage or structural failures, 100-yr flood levels can <br />generally be contained by the Willow Creek channel or push up levees. However, flooding and <br />overbank flows will occur at all culverts and pipes in the study reach as well as the weir <br />upstream of the mining museum. Locations of potential flooding are discussed in the following <br />paragraphs. <br /> <br />Overtopping of Weir Above Mining Museum Area <br />The earthen and wood weir upstream of the mining museum area will be overtopped starting at <br />the 25-year flood return interval. Flood flows will enter the depression west of the main channel <br />and flow downstream into the parking area east of the mining museum. Floods larger than the <br />100-year flood will flood the mining museum and fire department tunnels. However, these flows <br />should re-enter Willow Creek and be contained by the levees of the masonry flume. Failure of <br />the deteriorated earthen dam and weir upon overtopping and failure of the earthen push-up levees <br />below the weir is probable and could cause increased flooding at lesser flood return intervals. <br /> <br />Windy Gulch Culvert <br />Windy Gulch forms a sub-watershed within the Willow Creek watershed, and the normally dry <br />channel enters Willow Creek through a culvert just above the levees of the masonry flume. <br />Unfortunately, hydrology of the sub-watershed is somewhat uncertain. However, even with <br />uncertain hydrology, it is clear that the culvert is undersized to pass high flood flows. The <br /> <br />11 <br />