The pipe flowing into the box is an existing 18" rcp, that being said the 18" rcp i have spec'ed for the outflow shouldn't be a problem. I guess I have to keep playing with the grading to get the box to work. Thanks for the tips though!
I'm designing a storm sewer system and one of my structures is very tight on vertical space (dist from an existing 18" RCP invert to the finished grade). I am limited on how much I can raise the finished grade because of the grade of a nearby road. Currently, my structure has 4" of clearance...