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V. BLACKLAKEOAMS . The Black Lakes experience high inflow from both thunderstorms and snowmelt. Snowmelt discharges which occur with a diurnal pattern over several days will result in a fairly uniform discharge since the Lakes will effectively buffer the diurnal oscillations. The projected 500 year snowmelt spillway discharges for Black Lake and Black Lake No.2 are ISO and 290 cubic feet, respectively. If the 500 year derived rainstorm is applied to these basins, the spillway discharges are 30 and 445 cubic feet per second. This, however, is an unlikely situation since a thunderstorm ceJl would tend to move eastward rather than remain over the top of the Vail Pass ridge for any length of time. Black Lake lies just below Vail Pass and has only 0.63 square miles of drainage area. This small area, coupled with the I9-acre surface area completely swallows the runoff from a thunderstorm. Thus, the snowmelt discharge which occurs for the better part of a day produces the design discharge. The spillway, as seen in Figures V-I and V-2 is four feet deep and approximately nine feet wide at the bottom with 2:1 side slopes. The spillway is constructed with grouted stones and is currently in reasonably good repair. The spillway depth for the ISO cfs, 500 year projected snowmelt is 3.0 feet, which provides . a one-foot freeboard. A discharge curve for Black Lake can be seen in FigureV_3. Consequently, the Upper Black Lake and spillway are considered adequate for both the 500 year snowmelt and thunderstonm events. -26. Black Lake No. 2 is approximately I~ miles below Vail Pass with a total drainage catchment (including the Black Lake catchment) of I.B2 square miles. The surface area is B.6 acres. Because Black lake No.2 has half the surface area and a drainage area three times as large as the Upper Black Lake, the 500 year projected thunderstorm produced the most severe discharge of 445 cubic feet per second in- stead of snowmelt flooding. Again, the thunderstorm is unlikely to remain over this basin long because of the proximity to Vail Pass. However, this discharge causes a depth of 3.4 feet which allows two feet of freeboard. The spillway on Black lake No.2 is constructed of concrete and is a low broadcrested weir which can be seen in Figures V-4 and V-5. The discharge curve for Black Lake No. 2 spillway is shown in Figure V-6. A grouted riprap chute conveys water from the spillway to the original channel and also serves to protect the toe of the dam and spillway. This chute presently has a large eroded hole and will d~teriorate rapidly du~ to s~vere we<l.ther and "early continuous dis- charge without proper maintenance. The spillway will also need periodic maintenance in the future. ~ . " --, - . , . , ." ..~ -,1 -10