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water surface elevations. In natural streams with erodible beds, channels will often adjust to supercritical flows by developing a series of steps. Water will flow supercritical over the steps, but return to subcritical flow between the steps. Therefore, supercritical flow will probably not persist throughout all the reaches indicated by the model. In this case, water surface elevations may be higher in some areas than indicated by the model. Some hydraulic studies delineate floodplains based on the critical water surface at locations where supercritical flow is indicated. In the HEC-RAS results table, the elevations of the critical water surface, energy gradeline, and left and right overbanks are useful to examine possible problems with supercritical flows. 2.2.3.3 Floodplain Delineation The water surface elevations from HEC-RAS were mapped on the high-resolution topography in GIS to delineate flood inundations. The 100-year floodplain was mapped from the entrance to the flume to above the Commodore Mine in West Willow Creek and above North Creede on East Willow Creek. The 100-year flood inundations are shown in Figures 2.2.3a through 2.2.3d. As previously mentioned, super-critical flows may cause significant erosion and alter the channel geometry and therefore the floodplain inundation. In addition, results are valid only in an unblocked situation. Temporary obstructions due to ice-dams or woody debris could significantly reduce flow capacity and cause additional flooding. Therefore, these results indicate a best-case flow scenario free from debris blockage. Without any debris blockage, 100-yr flood levels can generally be contained by the Willow Creek channel or push up levees. However, flooding and overbank flows will occur at all culverts and pipes in the study reach as well as the weir at cross-section 14. Table 2.2.7 lists overtopping flood flows at modeled structures. The following sections discuss locations of concern in more detail. Table 2.2.7. Overtopping flood flows at modeled structures (cfs) ~ocation ~low level 11800cfs IIOOyr I 50yr I 25yr I lOyr I 5yr I 2yr 11.25yrl Windy Gulch Culvert otal flow 57 49 41 31 24 14 9 overtopping flow 18 12 7 1 Weir at XS 14 otal flow 1800 1073 915 769 586 458 288 185 overtopping flow 521 186 75 2 Culvert at North Creede otal flow 1580 942 807 681 523 412 264 172 overtopping flow 1341 698 570 450 303 200 35 11 Culvert at XS 26.5 otal flow 892 532 457 386 296 232 144 90 overtopping flow 277 16 Commodore Mine otal flow 892 532 457 386 296 232 144 90 ~t entrance to flume overtopping flow 397 197 146 98 2 Pipe through tailings pile overtopping flow 221 154 86 2 Culvert at Amethyst Mine otal flow 641 382 325 273 206 159 96 58 overtopping flow 216 40 11 2-17