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H 0 <br />rOH <br />A <br />y � � <br />0 <br />HZ <br />0 rrz i) <br />to H <br />H <br />OqU <br />- g <br />H <br />zH� <br />�HH <br />H <br />c� t7 y <br />�3CO <br />C3 <br />I <br />i <br />I <br />C 3 <br />ru <br />;after the design -,_arm, ground slope and g- iund .cover have <br />been determined L. the site, the allowable �utflaw rate in <br />cfs per acre can be determined by interpolation from tables 2 <br />through 5. If a Lite contains t-do types of ground cover, for <br />example, then a separate interpolation 1s made for each ground <br />cover type and the results added to determine the total <br />allowable outflow rate in cfs per acre. <br />After the allowable outflow rate in oracre <br />the required detention in cubic feet p_ <br />interpolation from table 6 or 7. Again if t <br />types are proposed after development, a separa <br />is made for each ground cover type and the <br />determine the total detention requirement in <br />acre. Thedetention <br />arequirement is then <br />multiplying by <br />e <br />is calculated, <br />Ls determined by <br />wo ground cover <br />=e interpolation <br />result added to <br />cubic feet per <br />determined by <br />For residential subdivisions the actual extent of <br />types will not be )clown during plan review. <br />survey of existing homes in the Everett area, <br />amount of imcer,ricus surface per single family <br />square feet. The applicant should, therefore, <br />by the number cf lots and then add the result <br />ro <br />d 'thin any <br />ground cover <br />Based upon a <br />the average <br />lot is 2900 <br />mult'_ply 2800 <br />o the area of <br />ad constructed <br />impervious surface conta=ne wi <br />for the subdivisiorr to estimate the area of impervious surface <br />that will ultimately be contained within the plat.. <br />Exan-i le : <br />Given: A 3.0 acre parcel located within North Creek <br />Drainage Basin. The slope of overland flow is 3.5%. The <br />existing ground cover consists of 2.0 acres of forested <br />land and 1.0 acre of light grass and sparse trees. The <br />proposed land use is commercial with s% landscaping and <br />95:k impervious ground cover conditions. <br />Solution: From table 1, the design a post development stormr <br />pre -development storm and 25 year <br />From table ` the allowable release o ati nrate <br />for ihe .085 cfs/forested <br />portion of ..he site is by interp <br />ac. The <br />allowable release rate for the grass area is .230 cfs per <br />acre. The total allowable release rate is therefore <br />(2)(.085) + (1)(.23o) or 0.40 cfs. This is .133 cfs Per <br />acre. From table 7, the detention requirement for the <br />landscaping area and impervious area is 885 cf/acre and <br />4009 cf/acre, respectively. The hotal detention <br />requirement is (.95) (3) (4009) -�_ (.05) (3) (8854 -or 11,558 <br />cf. <br />%,r. <br />r F 0 .I bG V. <br />f.'r l <br />