The span is 12'.
The reason for this is that I am very suspicious of the supplier. The design calls for 2-16"wide x60long" grated areas (1 in each lane) to drain directly through the top of the culvert to the waterway below. The supplier showed me a sample grate that was only 3/4" thick and...
The 50 year drought period, I believe, is figured as a ratio from the drop in the water table. If the water table drops 2 feet, the pond level should drop approximately 6 inches. I am not sure if .25 is the correct ratio, but I believe it is close.
There is a lot of good slot and trench drain out there. Granted I work for a company that makes them (Zurn), but they can do a lot in terms of draining a situation such as yours. They come with a built in slope so the surface can remain flat. They can be designed to take any loading, ADA...
Every one of the 90 degree bends is killing your efficiency. If they can be replaced with 22.5 or even 45 degree bends, you will see an immediate rise in the volume that the pump can deliver.
I am trying to design a box culvert cover to be traffic rated and heelproof. For the heelproof, I have found that a gap of 1/4" or less will meet the local standards. The "traffic rated" part has me concerned. I have looked at many manufacturers of grating and all will supply me a grate that...