You stated that to be carved out of the basin, the area would need to be raised. You are now changing from discussing elevation, to being an area used for detention of water. 

Other areas of developed land in the area are not raised, and yet are carved out of the basin. This is why they still flood when major flooding events happen, like Harvey, because they are still part of the basin; yet they have been carved out on the political/administrative boundaries that you want to adhere to. Examples include Canyon Gate at Cinco Ranch, Barkers Branch and Barkers Crossing, and pretty much all the housing at Highway 6 and Clay.

So, you propose we don't map whats on the ground, and instead adhere only to administrative/political boundaries.