I apologize for any confusion this post may cause as a result of the questions that I am asking or due to the information that I am presenting, but nonetheless, I appreciate any guidance that can be offered.

Two existing drainage basins combine storm water runoff with the hydrologic properties below:

Basin A - Existing
1. 54 acres
2. 70-minute Tc
3. 87.0 CN value

Basing B - Existing
1. 164 acres
2. 110-minhute TC
3. 92.1 CN value

Combined 100-Year storm water runoff Q - 760 CFS.

A proposed residential development (Basin C) will be located within Basin (B). Storm water runoff from Basin A, Basin B and Basin C will combine at the existing discharge point. The post-development hydrologic properties are below:

Basin A - Proposed
1. 30 acres
2. 37 minutes Tc
3. 91.1 CN value

Basin B - Proposed
1. 156 acres
2. 75 minutes Tc (different path than existing; the existing path is now one of the shorter duration pathways in the basin due to the development)
3. 86.6 CN value

Basin C - Proposed
1. 75 acres
2. 53 minutes Tc
3. 92.3 CN value

Combined 100-year storm water runoff Q (undetained from the development)for Basin A, Basin B and Basin C - 1,225 CFS
Combined storm water runoff Q (100-year)for Basin A and Basin B - 830 CFS

How are scenarios such as the above usually handled? The combined storm water flowrate from Basin A and Basin B for the post-development conditions (830 CFS) is greater than the combined storm water flowrate from Basin A and Basin B for the pre-development conditions (760 CFS), essentially removing any ability for the proposed residential subdivision discharging storm water runoff. Would either the Tc and/or CN value for Basin A be left those for the pre-existing conditions?