Design of Discharge pipe from detention pond 1

[Paul8]

Hi, I am designing a discharge pipe for a detention pond. The pre - construction runoff is 16.5 cfs, so the discharge from the detention pond should not exceed this. The max head is 3.5' in the reservoir. How do I size my pipe so as not to exceed 16.5 cfs. The pipe has 0 slope and is HDPE. I am a little unsure how to determine whether inlet or outlet controlled. Any help is appreciated

 

[cvg]

Use an orifice plate which will operate under inlet control (using the weir equation at low heads and then the orifice equation under higher heads). Mount the plate on the headwall over the end of the pipe. For ease of calculations and fabrication, make the hole rectangular.

 

[Trackfiend]

To answer one of your questions ("How do I size my pipe..."), you can use Manning's equation as shown below:

Q = 1.486/n x A x R^(2/3) x S^(1/2)

where Q = discharge
n = Manning's coefficient
A = cross-sectional area of flow
R = hydraulic radius (divide A by wetted perimeter of pipe)
S = longintudinal slope of pipe (ft/ft)

As far as inlet/outlet control, there are several variables affecting this such as headwater depth, tailwater depth, etc. Will your detention area stay full or do you intend to drain it dry? Does the downstream end of the culvert discharge into a ditch or stream that could overflow and back up into your detention area?

Basically, if water can flow into the upstream end of the culvert faster than it can flow through and out, then the culvert is under Outlet Control. If water can flow through and discharge out of the culvert faster than it can enter the upstream end of the culvert, then the culvert is under Inlet Control.

 

[Paul8]

Thanks Trackfiend. I am strugling with the manning equation because I dont know how full the pipe will be at exit.

I understand the general philosophy on outflow vs inflow but was unsure numerically how to determine.

My pipe discharges in such a way so that there will never be water buildup at the exit.

 

[cvg]

get a free copy of HY8 from the FHWA website and use that to size your culvert. But still recommend an orifice plate may be required to limit the outflow to 16.5 or less.

http://www.fhwa.dot.gov/engineering/hydraulics/software.cfm

 

[beej67]

The orifice plate idea gives you more control over the discharge, and it's what I'd go with. Make the culvert half again as big as the orifice, and size the orifice plate with the orifice equation based on an upstream head of 3.5 feet.

You do realize you'll probably need to route a hydrograph through your pond to determine if its volume is adequate for your design storm, right? Also, don't forget to include some sort of emergency overflow device, for storms that exceed the design storm or if the orifice clogs. If it's a permanent pond, consider an anti-seep collar on the culvert, even if it's dry. If it has a permanent pool, there's water table and wildlife issues. If it's a sediment pond, there's usually state specific rules on how they want it done for sediment capture. If you're doing a hydrology analysis for post development to match predevelopment, don't forget the effects of the watershed that bypasses the pond. Etc.

If you're in over your head, don't be shy about hiring a hydrologist to design your pond for you. A simple hydro analysis with all the necessary bells and whistles is usually only a couple grand, unless you're in a municipality that's overbearingly tedious.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[lidabc]

As cvg said, HY8 is strongly recommended.

If you run the HY8 program for several scenarios, you will be able to understand inlet & outlet control for your project and feel confidence on your design.

I used the program for some projects and included key analysis data in Drainage Reports which were linked to the excel spread sheets for some graphs. It worked pretty well.

 

[TerryScan]

You may also want to check out HydroCad software. (I am a user not affiliated). They have a free "sampler" available with limited nodes and a usage timer:

http://www.hydrocad.net/