2014-04-22_REPORT - M1990057

• Permit No: M1990057
• IBM Index Class Name: REPORT
• Doc Date: 4/22/2014
• Doc Name: Drainage report
• From: Union Milling Company, LLC
• To: DRMS
• Email Name: TC1
• Media Type: D
• Archive: No

Chapter 2 Estimating Runoff Technical Release 56
Urban Hydrology for Small Watersheds
Table 2 -2a Runoff curve numbers for urban areas L
Curve numbers for
Cover description hydrologic soil group
Average percent
Cover type and hydrologic condition impervious area 2/ A
Fully developed urban areas (vegetation established)
Open space (lawns, parks, golf courses, cemeteries, etc.) W.
Poor condition (grass cover < W %) ........... ...............................
Fair condition (grass cover 60% to 75 %) ... ...............................
Good condition (grass cover > 75 %) .......... ...............................
Impervious areas:
Paved parking lots, roofs, driveways, etc.
(excluding right -of -way) .............................. ...............................
Streets and roads:
Paved; curbs and storm sewers (excluding
right -of -way) ................................................. ...............................
Paved; open ditches (including right -of- way) ..........................
Gravel (including right -of -way) .................. ...............................
Dirt (including right -of -way) ....................... ...............................
Western desert urban areas:
Natural desert landscaping (pervious areas only) N .....................
Artificial desert landscaping (impervious weed barrier,
desert shrub with 1- to 24nch sand or gravel mulch
andbasin borders) ....................................... ...............................
Urban districts:
Commercialand business .................................. ...............................
Industrial.............................................................. ...............................
Residential districts by average lot size:
1/8 acre or less (town houses) ........................... ...............................
1/4 acre ................................................................. ...............................
113 acre ................................................................. ...............................
112 acre ................................................................. ...............................
1 acre .................................................................... ...............................
2 acres ................................................................... ...............................
Developing urban areas
Newly graded areas
(pervious areas only, no vegetation) ly
Idle lands (CNs are determined using cover types
similar to those in table 2-20.
B
68 79
49 69
39 61
98
98
83
76
72
63
C D
86 89
79 84
74 80
98 98 98
98 98 98
89 92 93
86 89 91
82 87 89
77 85 88
96
96
96
96
85 89
92
94
95
72 81
88
91
93
66 77
86
90
92
38 61
75
83
87
30 57
72
81
86
25 54
70
80
85
20 51
68
79
84
12 46
65
77
82
77 86 91 94
I Average runoff condition, and L - 0.2S.
8 The average percent impervious area shown was used to develop the composite CN's. Other assumptions are as follows: impervious areas are
directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in
good hydrologic condition. CN's for other combinations of conditions may be computed using figure 23 or 2-4.
8 CN's shown are equivalent to those of pasture. Composite CN's may be computed for other combinations of open space
covertype.
4 Composite CN's for natural desert landscaping should be computed using figures 23 or 2-4 based on the impervious area percentage
(CN = 98) and the pervious area CN. The pervious area CNs are assumed equivalent to desert shrub in poor hydrologic condition.
8 Composite CN's to use for the design of temporary measures during grading and construction should be computed using figure 23 or 2-1
based on the degree of development (impervious area percentage) and the CN's for the newly graded pervious areas.
(210- VI- TR-66, Second Ed., June 1986) 2-5