Detention Calculator 5dv1e

DETENTION POND DESIGN CALCULATION Percent Imperviousness:

Site Location Cont. Drainage Area (Acres) A

3.00

Proposed Runoff "C" Value Maximum Allowable Outflow (CFS) Storm Recurrence Interval (Yrs)

(L)

B

C

D

E

F

Duration (Minutes)

Duration (Hours)

25-Year Total Rainfall (Inches)

25-Year Rainfall Intensity (Inch/Hr)

Proposed Runoff Flowrate (CFS)

Proposed Runoff Volume (CFT)

5 10 15 20 30 40 50 60 90 120 180 360 720 1080 1440

0.08 0.17 0.25 0.33 0.50 0.67 0.83 1.00 1.50 2.00 3.00 6.00 12.00 18.00 24.00

0.49 0.86 1.10 1.24 1.51 1.65 1.79 1.92 2.15 2.37 2.62 3.07 3.56 3.84 4.09

5.88 5.16 4.40 3.72 3.02 2.48 2.15 1.92 1.43 1.19 0.87 0.51 0.30 0.21 0.17

10.94 9.60 8.18 6.92 5.62 4.60 4.00 3.57 2.67 2.20 1.62 0.95 0.55 0.40 0.32

3,281 5,759 7,366 8,303 10,111 11,048 11,986 12,856 14,396 15,870 17,544 20,557 23,838 25,713 27,387

G

H

I Required Retention Storage (CFT)

J Total Required Storage (CFT)

3,146 5,489 6,961 7,763 9,301 9,968 10,636 11,236 11,966 12,630 12,684 10,837 4,398 -3,447 -11,493

5445 5445 5445 5445 5445 5445 5445 5445 5445 5445 5445 5445 5445 5445 5445

8,591 10,934 12,406 13,208 14,746 15,413 16,081 16,681 17,411 18,075 18,129 16,282 9,843 1,998 -6,048

12,684

Maximum:

18,129

Maximum Required Allowable Detention Outflow Storage (CFS) (CFT)

0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45

60% 0.62 0.45 25

(K) (G)

RETENTION POND DESIGN CALCULATION Retain the 100-Year 24 Hour Design Storm from the Entire Contributing Area (5.5 Inches of Rainfall) 37,135 CFT A) Duration of the storm event in minutes. B) Duration of the storm event in hours. C) Total amount of rainfall during a 25-year recurrence storm event for the given duration in Column A & B (ref.: midwestern climatological center rainfall Atlas-Bulletin 71). D) Average rainfall intensity during the 25-year recurrence storm event. Calculated by dividing Column C by Column B. E) The unrestricted 25-year recurrence discharge flowrate from the proposed site under fully developed conditions. Calculated by multiplying Intensity (D) and Drainage Area (L). F) The unrestricted storm event for the given duration in Column A and B. Calculated by multiplying the Proposed Runoff Flowrate (E) by the Storm Duration (A) and by 60 seconds/minute. G) The maximum allowable discharge from the site is determined by multiplying the drainage area by 0.15 cfs per acre or if the proposed outlet is restrictive by determining the sites share of the existing outlets capacity on a contributing area basis. H) The required retention storage is determined by multiplying the differention flowrate (Inflow (E) - Outflow (G), by the corresponding duration (A) and by 60 seconds/minute. The amount of storage required for various storm durations will vary based on rainfall intensity, the size of the drainage area, and the allowable discharge. The maximum volume of storage for the various storm durations will be the required detention storage volume. This volume of storage will be determined above the required retention volume calculated in Column I. I) The required retention storage is determined by multiplying the drainage area (L) by O.S inches of rain. This volume will need to be retained below the sites outlet. This water will discharge by infiltrating into the ground or by evaporation. J) Total required storage is the sum of Column H and I. K) Proposed percent imperviousness. This assumption will be used to determine the proposed runoff coefficient. Impervious surface will be assumed to have a value of 0.9 and pervious a value of 0.2 L) Contributing Drainage to the proposed detention or retention system

Calculation By: Date:

Spicer Group, Inc. March 2005

Gratiot Method 2005_Guidelines