31 SW EVERETT MALL WAY 2018-01-02 MF Import

After the design star=, ground slope and ground •cover have been decersined for the site, the allowable out=low rate in cfs per acre can be dete--zined by interpolation from tables 2 through 5. If a sitz contains two types of ground cover, for example, then a separate inter=olation is made for each ground cover type and the results added to dete_^3ine the total allowable outflow rate in cis per ace. After the allowable outflow rate in cfs/ace is calculated, the recuired detention in cubic feet per acre is deter=ined by inter-oolation from table 6 or 7. Again if two ground cover types are proposed after development, a separate interpolation is made for each ground cover type and the result added to determine the total detention requirement in cubic feet per acre. The detention requirement is then detet3sned by multiplying by the site area in acres. ,v For residential subdivisions the actual extent of ground cover types will not be own during plan review. Based upon a su_^�ey of existing.�homes in the Everett area, the average amount of impervious surface per single Lamily lot is 2800 square feet. The applicant should, therefore, multiply 2800 by the number of lots and then add the result to the area of impervious surfac_, contained within any new road cant=sated for the subdivision, to estimate the area of impervious stLr=aca that will ultimately be contained within the plat, =xamnie Given: A 3.0 acre parcel located within North Creek Drainage Basin. The slope of overland flow is 3.5%. Me existing ground cover consists of 2.0 acres of forested la demand 1.0 acre of light grass and sparse tress. The proposed land use is commercial with 5-1 landscaping and 95t impervious ground*aovE:ecnditions. Solutior_: From table 1, the design story c-_iteria is a 5 year pre -development story and 25 year post development star=. From table 2 the allowable release rate for the forested portion of the site is by inter--olation .osS cfs/ac. The allowable release rate for the grass area is .230 cfs per acre. The total allowable release rate is therefore (2) (.085) + (1) (.230) or 0.40 cfs. This is .133 cfs per 'acre. From table •7, the detention requirement for the landscaping area and impervious area is 885 c_/ace and 4009 cf/acre, respectively. The total detention requirement is (.95) (3) (4009) -� (..QS) (3) (88.5-) .o=' 11,558 cf. .