Stormdrain restriction?

[SMIAH]

The City where I live are requesting 2X storm drain in every low points (with circular 30 inches grate).

They connect both storm drain using 6" PVC pipe but then they connect the 2x 6" PVC pipes to 1 single 8" PVC pipe using a "Y" and a bend then to the main sewer line.

I think (without performing any calculation yet) that the grate capacity might exceed the 8" PVC pipe capacity.

What do you guys think?

 

[gbam]

It depends on the clear open area of the grate. How deep below the grate is the 8" & 2-6" PVC pipes? Remember that the pipe system may be surcharges. What is the allowable pond depth above the grate? The system you describe seems really small also; is it to clean up nuisance flows?

 

[beej67]

Keep in mind when specifying a grate, you usually include a clogging factor.

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

http://www.campbellcivil.com

 

[SMIAH]

It's less about grate capacity than the connection of 2x 6'' pipes to 1x 8'' pipe.

The grate has a net opening of 1.615 sqf. which with 6'' of water above it has a capacity of 5,7 cfs.
If there is 5' of water in the stormdrain, the capacity of a 6'' pipe is ... 2,1 cfs.
So less than the grate capacity.

I think that putting 2 stormdrains in a low point might be a good idea (i.e. clogging, debris, etc.) but not connecting them with 1 single 8'' PVC.

 

[cvg]

flat grates clog easily, usually assume 50% clogged, the 5.7 cfs capacity reduces to 2.8 cfs
6 or 8 inch is way too small, 12 or 15 inch is more common. some agencies allow nothing smaller than 18 inch

 

[SMIAH]

That's what I think too. Even think that putting 2 drain inlets is a little bit useless as the 8'' will control the flow and it won't be able to carry what the grate let through.

A 18 inches seems a bit extreme though as you have to connect this pipe to a main sewer without manhole (?).

 

[cvg]

I agree, 15 inch is good enough in most cases. the agencies that allow minimum size 18 inch laterals also require minimum of 24 inch mainlines. An 18 to 24 connection would require a manhole or a pre-fab tee.