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<br />-26- <br /> <br />stead of snowmelt flooding. Again, the thunderstorm is unlikely to <br />remain over this basin long because of the proximity to Vail Pass. <br />However, this discharge causes a depth of 3.4 feet which allows two <br />feet of freeboard. <br />The spillway on Black lake No.2 is constructed of concrete and <br /> <br />I <br /> <br /> <br />Black lake No.2 is approximately l~ miles below Vail Pass with <br />a total drainage catchment (including the Black lake catchment) of <br /> <br />1.82squaremi1es. The surface area is 8.6 acres. Because Black lake <br />No.2 has half the surface area and a drainage area three times as <br /> <br />large as the Upper Black Lake, the 500 year projected thunderstorm <br /> <br />produced the most severe discharge of 445 cubic feet per second in- <br /> <br />FIGUREV-l <br /> <br />Upper Black lake Spillway looking East <br /> <br />is a low broadcrested weir which can be seen in Figures V-4 and V-5. <br /> <br />The discharge curve for Black lake No.2 spillway is shown in Figure <br />V.6. A grouted riprap chute conveys water from the spillway to the <br />original channel and also serves to protect the toe of the dam and <br /> <br />deteriorate rapidly due to severe weather and nearly continuous <br />charge without proper maintenance. The spillway will also need <br />periodic maintenance in the future. <br /> <br />dis- I <br /> I <br /> I, <br /> , <br /> " <br /> :1 <br /> " <br /> , <br /> , <br /> , <br /> <br /> <br />'" <br /> <br />-- --" <br /> <br />spillway. This chute presently has a large eroded hole and will <br /> <br />I <br />I <br /> <br />FlGUREV.2 <br /> <br />Upper Bl~ck Lake Spillway Looking Northwest <br />100 Year Water Depth 2.0 feet <br /> <br />I <br />