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<br />J~hn Andert Parcel Spjjt <br />Roseville, Minnesota <br /> <br />Sheet 3 of ~ <br /> <br />Calculations of runoff r3t~ show Q maximum rat~ 301 lower after <br />deyelopment, because le53 area 1~ drained West and ~oncer.tration <br />ti~e is longer due to the flatter slope for the walk~ut basement. <br />Runoff coefficient used 1s an average weighted by areas drained <br />and uaage. Runcrr qU8Dtity after develo?ment is the same as <br />present usa~e. The quantity per acre 1s higher for the proposed <br />1/2 Acre lot th~n existing One-Acre lot, but this 1s b21anced by <br />D.O~ acres less being drained. Results are given in Table 1 <br />below an~ calculations in Tables 2 and 3. <br /> <br /> Table i RUNOFF COMPARISON <br />E"listing Usage {One- Acre Lot) Pt~posed Development <br />A ...; 0.25 Acres A = 0.21 Acre~ <br />Q = 0.90 c 1".. !!lax. Q = 0.6u ch max. <br />~- <br /> 'OO-yr. 2~ hr rate 100-yr. 2~ hr. rate <br />l :: O.02Q Acre-ft. max. '" 0.02/t Acre-ft. :!lax. <br /> 1QO-y:-. 2~ hr arnt. 10C-yr. 21( IIr 8111t. <br /> <br /> <br />Table 2 RUNOFF RATE CALCUtATYON~ <br /> <br />ExIst US3ge: One-Acre Lot <br />grass & tree~ <br />A = ~.25 Acres to West <br />c = 0.30, sandy soil <br />1 = 11.95 (t : 7.0 ml~. <br />100 rto at 101) <br />Q = 0.25 x 0.30 x ".95 <br />= 0.90 ers ~ax. rate <br />'CO-yr, 24-hr <br /> <br />Proposed Usage: 1/2 Acre Lot <br /> <br />A =: 0.21 Acres to West <br />c = ,). 37 (Avg.) <br />Patio, O.O,~ Ac. flat grade <br />c =: 0.80 <br />Roof, 0.021 Ac. 5te~p gra~e <br />c =: 1.00 <br />La~n. 0.108 Ac. steep grade <br />c=:O .30 <br />La~n, 0.065 Ac. mod. ~rarie <br />C =: 0.20 <br />t = 7.65 <br />~O ft. at 151 =: Q.5 min. <br />.2G ft. at 61 =: n.o min <br />+40 ft. at 2 5J :: -:.0 1!11n <br />':. =: 15.5 min. <br />Q ~ 0.21 x 0.37 x 7.66 <br />~ 0.60 cfs max. rate <br />100-yr, 2~-hr <br /> <br /> <br />