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., <br /> PUBL/f f�f/O/'rrYS <br /> ]11EMORANUUni <br /> TO: hirk 13rooks� <br /> i <br /> I7tOM: Dan MathiasC_��1� <br /> llATE: August 5, 1992 <br /> SUIiJECT: Bceing North Parking Lot Gxpansion <br /> 1 reviewed Reid Middleton's drainage report for Boeing's North Parking Lot <br /> Bxpansion. This report stales Ihat I'Iukc's pond i3 (which I3oeing now owns) that was <br /> constructed in 1984 would be used for detention for the North Parking Lol Expansion. <br /> The Doeing EIS states that if pond I3 is lo be used lor detention, documcntation must bc <br /> provided to demonstrale Ihat adequate storage volume exists in pond B. This <br /> documentalion wasn't provided, so 1've perl'onned thc analysis using the City's <br /> Stormwater Management MoAel (SWMM). <br /> Yond F3 was originally constructed to servc Flukc phase 1I, 111 and IV expansions. <br /> Only phase II was constructed so, theoretically, some excess srorage is available ro <br /> serve Boeing's North Parking Lot Expansion. <br /> Current City drainage policy requires that the existing peak 2 YR discharge cannol he <br /> exceeded in a 25 YR, 24 HR storm. 1 used SWMM lo modcl thc S I acre area tributary <br /> ro pond I3 to determine thc existing pcak 2 YR rcicasc rate from pond II and thc <br /> required storage volume lo cnsure that the rclease rate doesn't exceed the 2 YR rate in <br /> a 25 YR, 24 HR storm. <br /> Pond B has an existing storage capacity of 2.67 acre-feet, with a design release rale of <br /> 3.8 cfs at a dep�h of ll.6 fect. The existing peak 2 1'R release rate is 1.01 cfs. To <br /> restrict the peak outflow from pond D to I.01 cfs in a 2S YR, 24 HR storm after thc <br /> Norlh Parking Lot Expansion will require modifica�ion of the oullet structure and an <br /> expansion of the �nd ro a slorage capacity of 1,4 acre-fcet. <br /> Ultimately, whcn the entire SI acre ama draining to pond A is devcloped (assuming <br /> 70% imperviousness in Ihc Iributary area), pond li ���ill nced a stora�c capacity of <br /> approximately I 1 acre-fect to mect currenl ('ity drainagc requiremcni�. <br /> ��nr.�s�nr.+i r.� iu�� �i:i <br /> CITY OP f V�HET T � ;i700 C��dar �h��o� • 1�-vrro�I. WA ',�H:'01 • I:`OGi ;SU il:l(lQ • I',ia �20G1 259 AAE32 . . <br />