2015-04-03_PERMIT FILE - C1981041 ‎(3)‎

• Permit No: C1981041
• IBM Index Class Name: Permit File
• Doc Date: 4/3/2015
• Doc Name: A-2 pg CRDA1-1 to RSS-53
• Section_Exhibit Name: Tab 13 Appendix 13-2 Part 2
• Media Type: D
• Archive: No

•
n
LJ
North Decline Final Reclamation
Powderhorn Coal Company
Riprap Size Requirements for North Decline Drainage Channel
SECTION 1
Riprap Size = 12"
Slope = 0.075
Discharge = 470 cfs
Manning's n = 0.0395 x d,,"' = 0.0395 x 116 = 0.0395
At n = 0.0395, V = 10.84 fps (from FLOMASTER program)
Drainage Criteria Manual rock size requirements:
VS'.17 = 5.8
ds' . (S3-1)
where: V = velocity in feet/sec
S = channel slope in feet/foot
Ss = specific gravity of rock
d50 = riprap size
assume: SS = 2.5
d50 = ((10.84 x 0.075017) / ((2.5-1)0.66 x 5.8))2
d5' = 0.85' = 10.2"
therefore, 12" rock is acceptable
SECTION 2
Riprap Size = 6"
Slope = 0.025
Discharge = 470 cfs
Manning's n = 0.0395 x d50116 = 0.0395 x 0.516 = 0.0352
At n = 0.0352, V = 8.26 fps (from FLOMASTER program)
d50 = ( (8.26 X 0.025.17) / ((2.5-1)'-66 x 5.8))'
d50 = 0.34' = 4.1"
therefore, 6" rock is acceptable
NDEC - 13
(New 2-26-92)