First United-CS 990403Calculations for Drainage Plan:
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GIVEN: Approx. 5.0 acres are being developed (some development currently exists). Tl~area
of concern covers about 3.0 acres. Thc pre-development area has a current flow of 11.70 cfs into
thc alley. The proposed design would put 14.39 cfs into the alicy.
*Calculations d~termined using Hydro-S5 Int~ns#les
DETERMINE: Required detention area to ke~p the flow into ti~ alley at ll.70cf,.
SOLUTION: Using tho Modified Rational Mcthod .Ik'temion l~in Dezign Example in thc City
of Dallas Drainage Design Manual.
The area of concern is divided into 2 sections.
Present conditions Q = CIA
sec I c = 0.35
Itoo = 7.6 in/hr Iloo = 7.6 in/hr
A = 1.22 acres A = 1.71 acrca
Qloo = 3.25 cfs Q~oo = 8.45 cfa
Total Qloo = 11.70 cfa
Fut~ conditions Q = CIA
Sec I C = - 0.50 Sec2 C = 0.75
Tc = 15 min. Tc = 15 min.
Iloo = 7.6 in/hr I~oo == 7.6 in/hr
A = 1.22 acres A = 1.71 act,
Q~oo = 4.64 cfs Q~oo = 9.75 cf,
Total Q~oo = 14.39 cfs
Check various duration storms
15 min. I = 7.6 Q~ = 0.5 x 7.6 x 1.22 = '4.64 cf, '
Qa = 0.75 x 7.6 x 1.71 = 9.75 cfs
QT = 14.39 cfa
20 min. I = 7.0 Q~ = 0.5 x 7.0 x 1.22 = 4.27 cf~
Qa = 0.75 x 7.0 x 1.71 = 8.98 cfa
QT = 13.25 cfs
30 min. I = 5.8 Q~ = 0.5 x 5.8 x 1.22 = 3.54cfs
Qa = 0.75 x 5.8 x 1.71 = 7.44 cfa
QT = 10.98 cfs
40 min. I = 5.0 Qz = 0.5 x $.0 x 1.22 = 3.05 cfs
Q2 = 0.75 x 5.0 x 1.71 = 6.41cfs
QT -- 9.46 cfs
50 min. I -- 4.4 Q~ -- 0.5 x 4.4 x 1.22 -- 2.68 cfs
Q2 -- 0.75 x 4.4 x 1.71 -- 5.64 cfs
Q~r -- 8.32 cfs
60 min. I = 4.0 Q1 = 0.5 x 4.0 x 1.22 -- 2.44cfs
Q2 -- 0.75 x 4.0 x 1.71 = 5.13 cfs
QT -- 7.57 cfs
70 min. I -- 3.7 Q~ -- 0.5 x 3.7 x 1.22 -- 2.26cfs
Q2 = 0.75 x 3.7 x 1.71 = 4.75 els
Q~r = 7.01 cfs
80 min. I -- 3.4 Q~ -- 0.5 x 3.4 x 1.22 -- 2.07cfs
Q~ = 0.75 x 3.4 x 1.71 = 4.36 cfs
QT = 6.43 cfs
Maximum Storage Volume is determined by deducting the volume of runoff released during the
time of inflow from the total inflow for each duration.
Inflow -- Storm duration X respective peak discharge X 60 sec/min
Outflow -- Half of the respective inflow duration X control release discharge X 60 sec/min
15 min. Storm Inflow 15 x 14.39 x 60 sec/min = 12,951 cf
Outflow 0.5 x 30 x 11.70 x 60 sec/min = 10~530 cf
Storage = 2,421 cf
20 min. Storm Inflow 20 x 13.25 x 60 sec/min = 15,900 cf
Outflow 0.5 x 35 x 11.70 x 60 sec/min = 12,285 cf
Storage = 3,615 cf
30 min. Storm Inflow 30 x 10.98 x 60 sec/mn = 19,764 cf
Outflow 0.5 x 45 x 11.70 x 60 sec/min = 15,795 cf
Storage = 3,969 cf
40 min. Storm Inflow 40 x 9.46 x 60 sec/min = 22,704 cf
Outflow 0.5 x 55 x 11.70 x 60 sec/min = 19,305 cf
Storage = 3,399 cf
Maximum volume required is 3,969 cf at the 30 min. Storm duration.