Restrictor release rate by Manning or pressure flow

[Anaximandar]

Hello, I'm a Structural Engineer, but I have to do a storm water detention calculation for one project and I'm not sure how to calculate the actual release rate at the manhole prior to connection to the main sewer line. I'm using a 4" DIP pipe restrictor. I considered Manning's modified formula Q=A*1.486/n*R^(2/3)*S^(1/2) and also flow resulting from gravity pressure Q=Co*Dr^2/4*(pi)*(2*g*H)^(1/2) Can anybody tell me how to determine how much water will restrictor release. I'm also using about 6" surface storage in the form of a volume of cone frustrum and I assume that at the design storm (100-years 24-hour) underground storage (oversized pipes) as well as surface storage will be full. This should give me minimum required underground storage volume.
Your input is very much appreciated. Thank you.

 

[WILDWINGS27]

I WOULD NOT USE MANNINGS TO DO THIS. INSTEAD USE
Q=CA((2GH)^.5).
Q=FLOWRATE
C=DISCHARGE COEFICIENT OF RESTRICTOR (0.82 FOR THICK WALL, SQUARE EDGED ENTRY, 0.61 FOR A PLATE, ETC.)
A=AREA OF RESTRICTOR
H=HEAD ON RESTRICTOR, FROM HIGH WATER TO CENTER OF RESTRICTOR
G=GRAVITY, 32.2FT/SEC^2

THIS WILL GIVE YOU AN ACURATE MEASURE OF YOUR RELEASE RATE

 

[LHA]

For a user-friendly - and free - method, go here and download:

http://www.hydrocad.net/sampler.htm

Create a Pond, enter pipe and A/G storage volume
Define Outlet as a culvert.
Route a Device to that 4" orifice.
Create a Subcatchment, with CN and Tc
Enter a rainfall depth OR
If you already know design storm hydrograph, enter manually as a Link.
Route Subcatchment or Link to Pond.
Double click on Pond.
All will calculate, with dynamic outflow as outlet switches between orifice, culvert controls.
You can readily edit while reading the resulting changes, if you want.
You can export table to Excel, if you want.
You can export graph as an image, if you want.

Engineering is the practice of the art of science - Steve

 

[Anaximandar]

Thank you for your time, I also think the pressure formula is the one to go with. I'll check that program, too, thanks Steve.