2016-10-28_PERMIT FILE - C2010089A ‎(22)‎

• Permit No: C2010089A
• IBM Index Class Name: Permit File
• Doc Date: 10/28/2016
• Doc Name: Pond NHN-002 & NHN-002 DItch Designs
• Section_Exhibit Name: Appendix 2.05.3‎(4)‎-8
• Media Type: D
• Archive: Yes

Sediment Pond Capacity Demonstration
Pond NHN-002 needs to be able to store the 10 -YR, 24 -HR event and pit pumping on top of the
three year sediment volume storage. SEDCAD was used to analyze the hydrology and
sedimentology of the 10 -YR event scenario, and as can be seen in the SEDCAD Model for this
scenario, the top of the water plus sediment storage is 2.98 feet below the emergency spillway,
and 0.98 feet below the top of the principal spillway riser entrance. The stage storage curve on
Map 2.05.3(4)-9 also shows that the design of Pond NHN-002 is more than sufficient for the 10 -
YR event. The SEDCAD Model which shows the 25 -YR, 24 -HR event with the pond design,
shows that the emergency spillway will safely convey the peak stage with over a foot of
freeboard remaining. The entire 25 -YR event discharge will pass out of the pond via the
principal and emergency spillway pipes. These two pipes will each be 18 inches in diameter. As
can be seen in this SEDCAD run, the total pipe diameter needed is 21 inches, therefore a pair of
18 inch diameter pipes will be sufficient.
NHN-002 Ditch Designs
Two ditches will convey stormwater runoff to Pond NHN-002: NHN-002 North and NHN-002
South. Each of these ditches has been designed to convey the 10 -YR, 24 -HR event flow to the
sediment pond. Table 2.05.3(4)-8-1 shows the peak flows for the sub -watersheds that feed each
ditch. Sub -watershed SW2-A feeds NHN-002 South, while SW2C feeds NHN-002 North,
leaving the remainder of SW2 (SW2-B) draining directly into Pond NHN-002. Map 2.05.3(4)-9
shows the anticipated route for each ditch as well as the sub -watersheds. Both ditches are
designed to use a trapezoidal cross section with sideslopes no steeper than 2H:1 V. This design
will facilitate ease of construction given the size and length of the ditches.
Map 2.05.3(4)-9 shows the route of both NHN-002 North and NHN-002 South ditches, profiles
with the anticipated grades of the ditches, and design details and criteria for each. It is anticipated
that only one portion of the NHN-002 South will be steep enough to require riprap lining. The
remaining segments of NHN-002 South and all of NHN-002 North will have excavated
overburden as their lining. The 10 -YR, 24 -HR event SEDCAD Model shows the ditch designs
based on the appropriate sub -watersheds and channel lining.
Table 2.05.3(4)-8-2 NHN-002 Ditch Design Parameters
Ditch
Station
Cumulative
Flow (cfs)
Slope
(%)
Minimum
Bottom
Width (ft)
Side
Slope
(HAV)
Rip
Rap
D50 (in)
NHN-002 South
0+00 to 7+75
12.27
1-5
12.0
2.0
N/A
7+75 to 11+00
14.73
6-10
12.0
2.0
3.0
NHN-002 North
0+00 to 9+00
11.85
1-5
12.0
2.0
N/A
9+00 to 30+30
26.89
1 1-4
12.0
2.0N/A
Appendix 2.05.3(4)-8 Page 3 April 2016 (PR -01)