20200564 Ver 1_JNX PCN - Antidegradation Discussion_20200423

F3c. Per 40 CFR 131.12:
• Existing instream water uses will be maintained and protected.
• Any impacts to water quality are necessary to accommodate important development and
eliminate an existing safety hazard.
As mentioned above, the project is designed to replace a single outfall, as part of a system replacement
necessary to eliminate ponding water. The proposed pipe is larger than the existing pipe in order to
alleviate flooding. No additional runoff will be created by this project.
For the purposes of this project, there are no feasible "non -discharge" alternatives. Without significant
financial burden, there is no re -development alternative which would decrease the rate of discharge.
Detention and peak flow mitigation is not feasible due to the limitations on ponded water at airport
facilities.
The proposed riprap outfall stabilization has been designed per NCDEQ erosion control guidance. See
the nomograph below. The outfall stabilization will provide protection from scour/erosion and provide
some measure of velocity dissipation prior to flow entering the natural receiving channel.

Appendices
m
_N
CO
CL
2
Q
Ir
O
LO
70
10 20 50 100 200 500 1000
Discharge (ft3/sec)
Curves may not be extrapolated.
Figure 8.06a Design of outlet protection protection from a round pipe flowing full, minimum tailwater condition (TW < 0.5 diameter).
Rev. 12/93 8.06.3

0
3Do
I
Outlet W = Do + 0.41-a
pipe 4 i
diameter (Do) � La
_ Tailwa_er , 0.5Do
80
QQ�O 70
60,
20
10
0
120
1
50 100
Discharge (ft3/sec)
Curves may not be extrapolated.
Figure 8.06b Design of outlet protection from a round pipe flowing full, maximum tailwater condition (TW >_ 0.5 diameter).
8.06.4 Rev. 12/93