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A question still exists of what flood level should be used to design structures. Analysis of areas just upstream of the masonry flume will be oriented towards ensuring that floodwaters will enter the flume. Designing upstream structures to pass capacities beyond what the flume can actually convey through downtown Creede is not valuable. Therefore, the maximum capacity of the flume may also form a logical analysis point. The Army Corps originally designed the flume with a maximum capacity of 1800 cfs, and this flow level was also considered in hydraulic analyses near the flume entrance. The slight uncertainty in area hydrology may warrant consideration of this flood flow level for safety of designed structures. Windy Gulch forms a sub-watershed within the Willow Creek watershed. Figure 2.2.1 shows the sub- watershed. The creek bed enters Willow Creek through a culvert just upstream of the Army Corps flume entrance approach levees. The culvert may be a concern for flooding and should, therefore, be considered in the current study The hydrologic information available for Windy Gulch is uncertain. One approach to estimate flood flows would be to use a simple watershed area ratio using hydrology determined for the smallest sub- watershed with hydrology (West Willow Creek at Nelson Creek). The watershed area of Windy Gulch is about 1181 acres with a maximum elevation of 11480 feet. The sub-watershed of West Willow Creek has an area of 7872 acres with a maximum elevation of 13291. Table 2.2.4 includes flood flow estimates using this method and estimates using a different method. Figure 2.2.1. Windy Gulch sub-watershed Precipitation amounts may often be lower in Windy Gulch than in the upper portion of West Willow Creek as elevations are lower. However, flood frequencies in the much larger watersheds of East, West, and Upper Willow Creek are governed primarily by snow melt events. Large floods in the much smaller Windy Gulch watershed would probably be caused by a large thunderstorm event that could be able to cover nearly the entire watershed. Therefore, flood estimates using an area ratio method may be quite low. Table 2.2.4. Estimated flood levels for Windy Gulch Flood Return Interval 100 r Flood Flow - watershed area ratio method(cfs) 57 Flood Flow - rainfall/runoff method cfs 239 179 139 57 14 Recently, a study for the Homestake Mining Company's Bulldog Mine Reclamation Project by Water Management Consultants used an SCS curve number and rainfall/runoff model approach to estimate flood levels in the Windy Gulch watershed. Peak outflows at the watershed outlet at the culvert are also included in Table 2.2.4 (adapted from Homestake Mining Company 2002): 2-11