2013-09-27_REVISION - M1979094HR ‎(3)‎

• Permit No: M1979094HR
• IBM Index Class Name: Revision
• Doc Date: 9/27/2013
• Doc Name: AM01 Drainage Design
• From: Cotter Corp
• To: DRMS
• Type & Sequence: AM1
• Email Name: DMC
• Email Name: TC1
• Media Type: D
• Archive: No

JD -7 Mine – Drainage Design Plan 18
number of 81 corresponding to an herbaceous mix of grass, weeds, and minor brush, in fair condition (Table 9).
For sub - basins containing more than one soil or cover type, weighted average curve numbers were
calculated based on the relative areas of each CN. The spatial distribution of curve numbers across the JD-
7 Mine area for the current condition is shown in Figure 5.
Site curve numbers are listed in Table 9 and the area - weighted average curve numbers for each modeled
sub -basin are provided in Attachment 2.
Table 8. Runoff Curve Numbers for Arid and Semi -Arid Rangelands
(Table 2 -2d of TR -55 [NRCS, 1986])
Cover Description cIl
p
Curve Numbers for
H drolo is Soil Grou
Cover Type
Hydrologic
Condition (2)
A
B
C
D
Herbaceous - mixture of grass, weeds, and low- growing brush,
with brush the minor element
Poor
80
87
93
Fair
71
81
89
Good
62
74
85
Oak - aspen — mountain brush mixture of oak brush, aspen,
mountain mahogany, bitter brush, maple, and other brush
Poor
66
74
79
Fair
48
57
63
Good
30
41
48
Pinyon-juniper—pinyon, juniper, or both; grass understory
Poor
75
85
89
Fair
58
73
80
Good
41
61
71
Sagebrush with grass understory
Poor
67
80
85
Fair
51
63
70
Good
35
47
55
Desert shrub —major plants include saltbush, greasewood,
creosote bush, blackbrush, bursage, palo verde, mesquite, and
cactus
Poor
63
77
85
88
Fair
55
72
81
86
Good
49
68
79
84
Gravel Road
76
85
89
91
Notes:
11) Based on average runoff condition, and Is, = 0.2S.
(2) Poor: <30% ground cover (litter, grass, and brush overstory).
Fair: 30 to 70% ground cover.
Good: > 70% ground cover.
(3) Source for gravel road CN is Table 2 -2a of TR -55 (NRCS 1986)
Table 9. Runoff Curve Numbers for JD -7 Mine
Description CN
Pinyon juniper, grass understory, fair condition, Group C soils 73
Pinyon juniper, grass understory, good condition, Group C soils 61
Gravel road, Group C soils 89
Disturbed ground in mine facilities area 85
Herbaceous -mix of grass, weeds, minor brush, fair condition 81
Sagebrush with grass understory, fair condition, Group C soils 63
Sagebrush with grass understory, poor condition, Group C soils 80
4149A.130926 Whetstone Associates •