2018-07-24_PERMIT FILE - C2010089 ‎(5)‎

• Permit No: C2010089
• IBM Index Class Name: Permit File
• Doc Date: 7/24/2018
• Doc Name: Pond NHN-002 & Ditch Designs
• Section_Exhibit Name: Appendix 2.05.3‎(4)‎-9
• Media Type: D
• Archive: No

Both the emergency and principal spillway discharge pipes will be buried for a length of over
340 feet north from Pond NHN-002. These pipes will daylight onto the surface, continue east
around the knob before turning north and proceeding downhill to the final discharge. At this final
discharge an erosion control "skirt" will be built. It will consist of a plastic liner on the ground
with 2-12" rock placed on it. The liner will protect the underlying ground from erosion, while the
rocks will dissipate flow from the pipes, which will help protect Meehan Draw from erosion. In
accordance with Rule 4.05.18, the discharge structures for Pond NHN-002 will not be within the
100 foot buffer zone for Meehan Draw. The route of the pipes and discharge point is shown on
Map 2.05.3(4)-9. Portions of the pipe and skirt on the surface will likely require securing to the
ground with stakes or other techniques to be determined in the field. During construction,
temporary sediment control will be in place around all construction areas.
Sediment Pond Capacity Demonstration
Pond NHN-002 should be able to store the 10 -YR, 24 -HR event in addition to sediment storage
below the principal spillway inlet. SEDCAD was used to analyze the hydrology and
sedimentology of the 10 -YR, 24 -HR event and for this scenario, the peak stage (5679.93') is
3.07 feet below the emergency spillway and 1.97 feet below the top of the principal spillway
riser entrance. Also, a minimum 10" pipe diameter is required to convey the 2.30 cfs discharge
from the open weepholes. Therefore, the 18" principal spillway pipe will be more than adequate.
SEDCAD was also used to analyze the 25 -YR, 24 -HR event and for this scenario, the peak stage
(5683.20') is 1.80 feet below the top of structure, providing adequate freeboard. A minimum 21"
pipe diameter is required to convey the combined 12.39 cfs discharge from the principal and
emergency spillways. Since each pipe is 18" in diameter, this should provide adequate capacity.
NHN-002 Ditch Designs
Two collection ditches (NHN-002 North and South) will convey stormwater runoff to Pond
NHN-002. Map 2.05.3(4)-9 shows the ditch routes, design details and criteria and profiles with
anticipated grades. These ditches have been designed to convey the 10 -YR, 24 -HR event flow to
the sediment pond. Only one portion of the NHN-002 South Ditch (7+75 to 11+00) will be steep
enough to require riprap lining. The remaining segment of the NHN-002 South Ditch (0+00 to
7+75) as well as the NHN-002 North Ditch will have an excavated overburden lining. The 10 -
YR, 24 -HR event SEDCAD demonstration shows the ditch designs based on appropriate sub -
watersheds and channel lining. The ditches are designed with a trapezoidal cross section and
have minimum bottom widths and maximum side slopes as indicated in the table below.
Table 2.05.3(4)-9-2 NHN-002 Ditch Design Parameters
Ditch
Station
Cumulative
Slope
Minimum
Left Side
Right Side
Rip Rap
Flow (cfs)
(%)
Bottom
Slope
Slope
D5o (in)
Width (ft)
(HAV)
(H:1V)
NHN-002
0+00 to 7+75
6.64
1-5
6.0
2.0
3.0
N/A
South
7+75 to 11+00
8.14
6-10
12.0
2.0
2.0
1.5
NHN-002
20+80 to 30+30
5.23
1-4
12.0
2.0
2.0
N/A
North
Appendix 2.05.3(4)-9 Page 2 June 2018 (TR -17)