Monroe County Drain Commissioner’s Office

Storm Water Detention Methodology 10 Year Event

Commercial Developments of Three (3) Acres or Less

Project Name: ______

Location: ______Township: ______

Date: ______Design Engineer: ______

1. Determination of the maximum allowable discharge:

Existing site tributary area (on-site and off-site) currently draining into the County Drain = ______acres

Qallowable = tributary area (acres) x 0.2 cfs/acre = ______cfs

*Note* Qallowable may be further reduced based upon available Drain capacity per the discretion of the Monroe County Drain Commissioner.

2. Determination of the proposed storm water runoff into the County Drain (detention Pond):

Total proposed site tributary area draining into the County Drain (detention pond) = ______ft2

Pavement: ______ft2 x 0.9 = ______ft2

Building/Roof: ______ft2 x 0.9 = ______ft2

Pond (10-yr elevation): ______ft2 x 0.9 = ______ft2

Gravel/Millings: ______ft2 x 0.9 = ______ft2

Grass/Unimproved: ______ft2 x 0.2 = ______ft2

Total: ______ft2 Sum CA = ______ft2

Cw = _____Sum CA (ft2)______= ______

Total Area (ft2)

CwA (in acres) = __Cw x Total Area (ft2)__ = ______acres

43,560 (ft2 per acre)

3. Determination of the metering line (pipe flow control):

Overflow elevation in the detention pond = ______ft

Case #1: Open Drain (ditch):

Outfall (discharge) pipe crown elevation (top invert) at the outlet to the open Drain or open Drain tailwater if it’s greater = ______ft

Case #2: Closed System (manhole):

Outfall (discharge) pipe crown elevation (top invert) at the manhole or crown elevation of the outletting storm sewer within the manhole if it’s greater = ______ft

Length of the metering line = ______ft

Hydraulic Gradient = S = ___Overflow Elev. – Top Invert Elev.____ = ______

Length of the Metering Line

Conveyance Factor = C.F. = 1.486 x A x R2/3 = ______(see chart below)

n

Pipe Diameter (inches) / Pipe Material & Manning’s Roughness Coefficient ‘n’ / Pipe Material & Manning’s Roughness Coefficient ‘n’ / Pipe Material & Manning’s Roughness Coefficient ‘n’
Smooth Wall PVC-0.010 / Duel Wall PE (N-12)-0.012 / Concrete RCP (CL-IV)-0.013
4 / 2.49 / 2.08 / n/a
6 / 7.29 / 6.08 / n/a
8 / 15.66 / 13.05 / 12.05
10 / 28.44 / 23.70 / 21.88
12 / 46.30 / 38.58 / 35.61
15 / 83.94 / 69.95 / 64.57

4. Determination of actual metered discharge (pipe flow control):

Qdischarge = C.F. x S1/2 = ______cfs

*Note* Qdischarge shall be less than or equal to the Qallowable (see step 1.)

5. Determination of the metering line (orifice control):

Attach orifice calculations.

Provide a detail of the outlet control structure.

6. Storm water quality requirements:

A storm water quality structure (aqua-swirl, storm-ceptor, etc.) can be used. Attach a detail and provide the flow mechanics from the manufacturer.

-or-

Provide for first flush (Vff) runoff by expanding the detention pond and modifying the outlet control structure. First flush runoff shall be computed based upon 0.5 inches of rainfall over the development site drainage area and released over a 24 hour period. Provide all calculations and include a detail of the outlet control structure.

*Note* The first flush volume is in addition to the 10-year storm event volume. Vtotal = V10-yr. + Vff

The following equation can be used to determine the required first flush runoff volume:

Vff = 1815 x A x C

Where: Vff = first flush storage volume (ft3)

A = drainage tributary area (acres)

C = runoff coefficient

5. Determination of the detention volume required:

Where Rainfall Intensity (in/hr) = i10 = ___145___

Tc + 18.5

(1) / (2) / (3) / (4) / (5) / (6) / (7)
Time of Concentration / Rainfall Intensity / CwA / Qinflow (cfs) / Qdischarge (cfs) / Qdet (cfs) / Detention Volume (ft3)
Tc (min) / i10 (in/hr) / (A in acres) / Q = i10 x CwA / Qdischarge / Qin – Qout / (Qin – Qout) x Tc x 60
(from step 2) / (2) x (3) / (from step 4) / (4) – (5) / (6) x (1) x 60
20 / 3.77
30 / 2.99
40 / 2.48
50 / 2.17
60 / 1.85
70 / 1.64
80 / 1.47
90 / 1.34
100 / 1.22
110 / 1.13
120 / 1.05

Design Detention Volume (column 7): V10 = ______ft3

First Flush Runoff Volume (step 6): Vff = ______ft3

Total Detention Volume Required: Vtotal = V10 + Vff = ______ft3

*Note* Vtotal shall be less than Vactual (see step 6.)

6. Determination of detention pond volume available:

Volume of detention pond = Vactual = ______ft3

Attach pond volume calculations (ft3).

*Note* One foot of freeboard shall be required above the total volume required Vtotal.

Design Engineer: ______(printed)

Design Engineer: ______(signature)

State of Michigan Professional Engineer License No: ______

Date of Design: ______

Revised 01/12/17