Stormwater inlet, Precast Pedestal top catch basin, calculating flow capacity

[qrb613]

I'm trying to size a pedestal top catch basin. I have about 40 CFS during the 100 year flood event. The GeagiaDOT storm-water manual doesn't seem to have a design guideline specifically for this situation.

I believe that a weir equation is appropriate for this under certain conditions. I'm not sure which equation to use. I'm looking at the broad crested weir equation now. My manual says that it is appropriate when my upstream head is between 5% and 50% of my weir width. I'm thinking that if the water depth overtops my inlets, this equation may not apply. I think that the weir condition exists when there is stream flow toward the weir inlet but if my top was for example at the bottom of a 3 ft deep "pond" during my 100 year event, the weir condition would not exist.

When dealing with drainage grates, we determine the capacity under both weir and orifice conditions and use the lowest of these two. I think that I may need to consider the orifice equation here as well but i'm not sure.

 

[gbam]

Not sure what a pedestal top CB is? I recommend computing both weir and orifice to determine which produces the greater head water on the inlet. That will tell you which controls.