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I I I I I I I I I I I I I I I I I I I q - Zone 4 for Tc. of 0.33 = 305 P=6.36 inches R (Figure 3.5) = 2.20 inches Q=qAR = 305 y. 0.67 x 2.20 =450 cfs. Tp = 484/q = 1.59 hours --- ---- Tb = 2.67 Tp = 4.24 hours Volume storm = 4.24/24 x 450/2 x 1,98 = 79 acre feet 100 YEAR STORM: SOIL COVER COMPLEX METHOD DATA: SAME EXCEPT P = 3.00 inches R = 0.32 inches Q = qAR = 305 x 0.67 x 0.32 =65.4 cfs Volume storm = 4.24/24 x 65.4-x:-r.9'8""= 11.4 acre feet. The dam has two spillways, one with a 9.0 foot bottom and with 7 1/2 :1 slopes; the other has a 19.0 foot bottom and has 10:1 slopes. The combined spillways would presently pass the PMP storm, flo'wing at a depth of 1.82 feet, without computing spillway storage. Their capacity was computed useing the broad crested weir formula: Q=3.09wH3/2. Computing spillway storage for the PMP results in a peak discharge at 1.21 feet of depth through the two spillways in 2.5 hours with a peak Q of 198,0 cfs The high water line area is 3'7.26 acres. The regretlsion curve to Q = 0 is a long one, but of little consequence, since both spillways are carved from solid sandstone. It is obvious that, even in the PMP event the present spillways are adequate. The grOSs freeboard is presently 3.4 feet, leaving a net of 1.58 feet freeboard without spillway storage and 2.19 feet with sp~llway storage, both figures exceeding the State Engineer's minimum. (5) not applicable (6) Flood and Erosion Hazard Evaluation. With the reservoir safely storing the peak flow, and reducing the peak Q by 250 cfs at the spillways, the reservoir actually asists in flood Q reduction for the areas below, even during the PMP storm. There is virtually no postlibility of the structure overtopping. The peak Q is of relatively short duration, and the return flow drops into the natural, rock-strewn drainage below the dam. The return floW channel is heavily vegetated and quite rocky, so little or no erosion is anticipated, either in the spillway channels or the waterway below. Flood hazard and erosion are virtually nil for this off channel reservoir. (7) through (12) are not applicable. (13) Both the soil for the embankment and additional rock for 7