Originally Posted by agrostis
I'm not sure what you mean by 'sump area'. Do you mean the area inside the catch basin itself? If that is what you are talking about, it is not important. I think the size of the grate is directly related to how much water you are trying to move, but if it all go's into a 4" pipe, then that really doesn't matter. Keeping the grate (no matter what size it is) free from debris during a rain event is more important and will mean a lot more water get's moved quickly. If you can post picture's of the problem area, maybe i can give you a more definitive answer.
This link might help - http://www.ndspro.com/
Yes the sump area is what nds calls the area that collects sediment. It seems to me that either solution, the pvc glues "t" basins with a 6*6 4inch outlet grate or the spee-d basin with 9 inch grate will work because they will both handle the water...
Here is a sketch of the area. The drop of the grade from fence to building is about 8inchs