How do you size an area drain?

[raydefan]

I am working on a small site (6000 ft^2) that drains to one area drain. I am relatively new to hydrology, but would like to determine the size of area drain required. I already calculated the Q to be 0.5 cfs. How do I do this?

Thanks,

Jeff

 

[sam74]

try this link to the neenah foundry web site. They have a capacity calculator here and also explain the methodology behind the calculator.

http://www.nfco.com/literature/brochures/gratecapacities.html

 

[sidc]

Jeff-

When you say "area drain" I guess you are talking about a detention / retention basin. The short answer: To determine your requirements, you need to find the difference between pre- and post development flow rates. I recommend using tr-55 or Hydrocad to find the difference. After cranking through the software (inputing curve #'s, slopes, flow characteristics, etc.) the software will give you the volumetric storage needed in acre-feet.

I said the "short answer" because you may have to account for a lot more than meets the eye. 6,000 Sq. ft. is dang near imperceptible as far as being a stormwater influence. Then again, it might be a matter of where you live- our 100 year event is 4.2"

If you are talking about a drain pipe to collect .5 cfs, I would recommend flowmaster. You input cfs, manning #, slope of pipe, diameter, etc. to find unknown variable. We also use flowmaster to determine r.o.w. capacities for 10 & 100 year flood conditions.

Sid.

 

[sam74]

I may have misunderstood. In my locality area drain and catch basin are pretty much synonymous. I had assumed he was looking at the physical dimensions length and width to capture the runoff into a storm system, probably in a sag condition with some sort of maximum depth of water requirement. For instance local requirements here require that catch basins, area drains, and curb inlets on roads be spaced so that at least one half of the road surface would experience no ponding during a 10-yr storm event.

 

[LHA]

I read the post to be looking for what sam74 thought, also, and for the same reason.

If you are only getting .5 cfs to it, and located on a reasonable grade (or in a sag), a standard 24"x48" inlet should do it.

For calculations, FWHA HEC12 is the standard in the US.

 

[cvg]

"area drain" = "grated inlet" in this region. use the weir equation with the length of the weir equal to the perimeter of the inlet. Also apply a safety factor to account for plugging of the grate.