If you have the room for underground storage in the basin, why not just dig the basin deeper? Seems this would be less costly and easier to maintain. Here in NJ we now have the statewide BMP that we must follow and the pretreatment facilities are really expensive.
Hope this helps.
I am facing the same problem. Only I am designing the repairs for a town. We are going to line the pipes. The best way I have found is to tv inspect the lines to determine the amount of deterioration.
Then have the pipes jet cleaned if possible. If there is too much damage this will just blow...
Here in new jersey we are only allowed to use 150' for maximum sheet flow. Your best bet for clculating tc is to use a dot nomograph. I have never had a drainage analysis come back when I used this figure.