C&G Inlet Design

[zebco]

Hello, I need some help. I'm trying to design the proper number of curb inlets to drain a street. The street is 300' long on a 2.3% longitudinal slope with a 2% cross slope. Standard 30" c&g. The curb inlets have a 4' opening. The north side of the street is approx. 2 acres and drains over the curb and into the street. The design is for the 10 year storm. Using the rational method I get 5.5 cfs at the peak flow. I am coming up with 5 inlets needed. This sounds like a lot for such a short street. Any help out there? Thanks, Herb

 

[cvg]

I wouldnt be surprised if you needed more than 5. Use a longer catch basin. Four feet is too short. Use a local depression and consider adding grates.

 

[zebco]

10-4 cvg, I have thought about a longer inlet and adding a sag curve. This inlet is the Chapel Hill standard that must be used. I guess I could place 2 side by side. I was mainly concerned with my calcs. I guess they may not be too far off.
Thanks

 

[Ryb01]

it may be a good idea also to design the inlets for 50% blockage to account for potential clogging of the inlet.

 

[jgailla]

5.5 cfs seems like a lot of flow for a 10 year storm on a 300 foot long road. I'm in Florida and inlets are generally placed at about 300 feet separation, depending on geometric and topographical considerations.
How wide is the road and right of way? Is there off site flow?
For a 300 foot stretch of 4 lane road plus median, each at 12 feet, I get an area of 18,000 square feet=0.4 acres.
With C=0.98 and i=6 inches, the flow rate is 2.4 cfs.

 

[zebco]

2 acres of offsite flow. I've decided to capture mos of it in yard inlets to reduce the flow on the roadway.
DA=2.0
c=0.4
I=7.22

 

[gbam]

Off roadway inlets are always a good idea. But make sure you clog them by 50% as a safety factor as Ryb01 mentioned. Also, we clog the on-grade curb opening inlets by 20%.

 

[cvg]

clogging factor for offsite drainage inlet sizing is fine and recommended, but I would recommend you design them to be as non-clogging as possible. I have seen grated inlets 100% clogged before with leaves, plastic bags, cardboard etc.

 

[beej67]

The reason it sounds like a lot for a short street is all the non-street surface drainage you're catching with your street.

The smart thing to do here, if you have the ability, is to use a ditch or swale or low point up-gradient of the street and catch all that non-street drainage in a pedistal top inlet or headwall before it ends up in your gutter pan. That will dramatically reduce the number of structures you need in your street.

I would use a pedestal top inlet for something like this. Those still keep neighborhood kids out of the storm drain system but have a very low chance of clogging.

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

http://www.campbellcivil.com

 

[zebco]

Thanks for the comments, two catch basins / yard inlets will handle all the flow with ease.

 

[beej67]

I knew those two years of residential subdivision design would pay off one day.

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

http://www.campbellcivil.com