02-11-1991 Council Packet - Laserfiche WebLink

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i>LfllM/l/inRK ARCHITECTURE I ENGINEERING 11840 V jIIcv View Ra:d. tJen Frame. .MN -^-344 Mail Address Box 12^3 Minneapolis V.N--44''i Telephone 612-b2i?-45CXl. f.AX. 512 •2’i-474 5 November 9, 1990 wrr 'Ji i Mr. Ron Quanb«ck Minnehaha Creak watershed District P.O. Box 387 Wayzata, Minnesota 55391 13 1880 RE:Rick ’s Super Vaiu Navarre, Minr*esota Project No. 1006-13 Dear Ron: The fbilOMing information is submitted as a supplement to the appiication dated November 7, 1990. We have calculated the area draining to the low area lying on both sides of Kelly Avenue and the apparent storage voiume available in It. Our calculations are enclosed. Based on these estimates a 4-hour, 100 year storm will flood to a level Just under 939.0 feet (MSL 1929 USGS adjustment). The proposed retention dike will encroach on the 939.0 storage by about 250 cu. ft. The estimated added retention will be 8,100 cu. ft or a gain of 7,850 cu. ft. The estimated increase In runoff from added roof and hard surface will be from 8.950 sq. ft. converted from a "c" of 0.2 to 0.9, (0.9 - 0.2) X 8,950 t 43,560 x 1.05 x 14,400 = 2,175 cu. ft. This amount of added runoff will easily fit into the 7,850 cu. ft. added retention in this proposal. It is suggested that the outlet of the retention be rather small, so that the water will be stored for a relatively long period time and reduce the effect on the low ground. The a.ioo cu. ft. stored will reduce the flood level by about 0.08 foot. These calculations are based on zero outlet from the low ground. In fact, there is some outflow as svidencad by ths lack of standing water in the area. The rate of tt outflow cannot be easily calculated. Sincerely, Bruce R. Kelley, P.E. /rm Enclosure cc: Lyie Fuller, Mpis. Division Jean Mabueth. Orono John Qerhare'-son, Orono Polly Scott, PLANMARK I00ei3bk/i