Impervious Site needs to meet Pervious Pre-Development Conditions

[msf58]

Adding a small 4,000 sf storage warehouse on a ~6 acre developed site. There is a small drainage ditch across the north end of the property which discharges from a 15" CMP into the county ROW. At first, this project was pretty cut and dry. I simply increased the ditch capacity enough to account for the slight increase in runoff. The entire site is currently gravel, which was used in my original pre-development calculations.

However, upon receiving comments back from the county, there has been quite a bit of work done on the site without permit. A little side note, the owner is the contractor in this case. Anyway, the county has no record of 6 acres of gravel on the site, as well as a few of the more recent building additions.

Anyway, to my current problem: I have to match pre-development conditions from the latest site plan the county has on record. This means that I cannot account for the gravel surface currently in place, and have to match the drainage of a pervious site with my almost totally impervious site. I have figured I will need 70,000 cf in additional storage, which I just do not have with the configuration of the site. Any ideas or resources to point me in the right direction? Thanks!