Schools 2009 SSc1: Site Selection

There is no time frame in regards to the previous development and your site has previously been altered by direct human activity. So I would define it as previously developedPreviously developed sites are those altered by paving, construction, and/or land use that would typically have required regulatory permitting to have been initiated (alterations may exist now or in the past). Previously developed land includes a platted lot on which a building was constructed if the lot is no more than 1 acre; previous development on lots larger than 1 acre is defined as the development footprint and land alterations associated with the footprint. Land that is not previously developed and altered landscapes resulting from current or historical clearing or filling, agricultural or forestry use, or preserved natural area use are considered undeveloped land. The date of previous development permit issuance constitutes the date of previous development, but permit issuance in itself does not constitute previous development.".
The intent of the credit is to avoid inappropriate site development. So as long as you meet the other criteria, I think you are following the intent.
I actually had a project on a urban site, which also had new tree development since the previous buildings were destroyed during the 2. World War. The local regulations required as to protect some trees but other than that the site was consider as previously developed.

USGBC has more specifically defined "previously developed" in its Nov. 2011 addenda. This is very helpful in terms of clarifying that some land alteration and human hands, like agriculture, is NOT considered development. Here is the definition:

Previously developed sites are those altered by paving, construction, and/or land use that would typically have required regulatory permitting to have been initiated (alterations may exist now or in the past). Previously developed land includes a platted lot on which a building was constructed if the lot is no more than 1 acre; previous development on lots larger than 1 acre is defined as the development footprint and land alterations associated with the footprint. Land that is not previously developed and altered landscapes resulting
from current or historical clearing or filling, agricultural or forestry
use, or preserved natural area use are considered undeveloped land. The date of previous development permit issuance constitutes the date of previous development, but permit issuance in itself does not constitute previous development."

The requirement is that you can't develop hardscape on prime farmland. It sounds like you're not going to do that, although it depends on your stormwater design. Given what you've said so far, I'd say you meet the requirement.

This seems like a tough one. Could you check with flood insurance folks to see if they have a sense of things? Though that could lead to a lot of phone calls with no answers. Another thought would be to see if there are areas nearby that have FEMA mapping and compare elevations. But USGBC/GBCI might not accept either of those options.

I would also consider whether it's worth the pursuit of the credit, especially if you don't need the extra point.