Chapter 2
Figure 2-3
Technical Release 55
Urban Hydrology for Small Watersheds
Estimating Runoff
Composite CN with connected impervious area.
100
Pervious CN =
90
90
80
70
Composite CN
80
60
50
70
40
60
50
40
0
10
20
30
50
40
80
70
60
90
100
Connected impervious area (percent)
Figure 2-4
Composite CN with unconnected impervious areas and total impervious area less than 30%
0.0
N=
40
90
90
80
80
70
60
Composite CN
2–10
1.0
50
60
70
50
40
0
10
20
Total impervious
area (percent)
(210-VI-TR-55, Second Ed., June 1986)
30
(Unconnected impervious)
(Total impervious)
sC
viou
Per
0.5
Estimating Runoff
Chapter 2
Table 2-2a
Technical Release 55
Urban Hydrology for Small Watersheds
Runoff curve numbers for urban areas 1/
------------------------------------------- Cover description ----------------------------------------Average percent
Cover type and hydrologic condition
impervious area 2/
Curve numbers for
-----------hydrologic soil group ------------A
B
C
D
68
49
39
79
69
61
86
79
74
89
84
80
98
98
98
98
98
83
76
72
98
89
85
82
98
92
89
87
98
93
91
89
63
77
85
88
96
96
96
96
85
72
89
81
92
88
94
91
95
93
65
38
30
25
20
12
77
61
57
54
51
46
85
75
72
70
68
65
90
83
81
80
79
77
92
87
86
85
84
82
77
86
91
94
Fully developed urban areas (vegetation established)
Open space (lawns, parks, golf courses, cemeteries, etc.) 3/:
Poor condition (grass cover < 50%) ..........................................
Fair condition (grass cover 50% 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) 4/ .....................
Artificial desert landscaping (impervious weed barrier,
desert shrub with 1- to 2-inch sand or gravel mulch
and basin borders) ......................................................................
Urban districts:
Commercial and business .................................................................
Industrial .............................................................................................
Residential districts by average lot size:
1/8 acre or less (town houses) ..........................................................
1/4 acre ................................................................................................
1/3 acre ................................................................................................
1/2 acre ................................................................................................
1 acre ...................................................................................................
2 acres ..................................................................................................
Developing urban areas
Newly graded areas
(pervious areas only, no vegetation) 5/ ................................................................
Idle lands (CN’s are determined using cover types
similar to those in table 2-2c).
1
2
3
4
5
Average runoff condition, and Ia = 0.2S.
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 2-3 or 2-4.
CN’s shown are equivalent to those of pasture. Composite CN’s may be computed for other combinations of open space
cover type.
Composite CN’s for natural desert landscaping should be computed using figures 2-3 or 2-4 based on the impervious area percentage
(CN = 98) and the pervious area CN. The pervious area CN’s are assumed equivalent to desert shrub in poor hydrologic condition.
Composite CN’s to use for the design of temporary measures during grading and construction should be computed using figure 2-3 or 2-4
based on the degree of development (impervious area percentage) and the CN’s for the newly graded pervious areas.
(210-VI-TR-55, Second Ed., June 1986)
2–5
Chapter 2
Table 2-2b
Technical Release 55
Urban Hydrology for Small Watersheds
Estimating Runoff
Runoff curve numbers for cultivated agricultural lands 1/
------------------------------------------ Cover description --------------------------------------------Hydrologic
Cover type
Treatment 2/
condition 3/
Curve numbers for
------------- hydrologic soil group ---------------A
B
C
D
Fallow
Bare soil
Crop residue cover (CR)
—
Poor
Good
77
76
74
86
85
83
91
90
88
94
93
90
Row crops
Straight row (SR)
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
72
67
71
64
70
65
69
64
66
62
65
61
81
78
80
75
79
75
78
74
74
71
73
70
88
85
87
82
84
82
83
81
80
78
79
77
91
89
90
85
88
86
87
85
82
81
81
80
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
Poor
Good
65
63
64
60
63
61
62
60
61
59
60
58
76
75
75
72
74
73
73
72
72
70
71
69
84
83
83
80
82
81
81
80
79
78
78
77
88
87
86
84
85
84
84
83
82
81
81
80
Poor
Good
Poor
Good
Poor
Good
66
58
64
55
63
51
77
72
75
69
73
67
85
81
83
78
80
76
89
85
85
83
83
80
SR + CR
Contoured (C)
C + CR
Contoured & terraced (C&T)
C&T+ CR
Small grain
SR
SR + CR
C
C + CR
C&T
C&T+ CR
Close-seeded
or broadcast
legumes or
rotation
meadow
1
2
3
SR
C
C&T
Average runoff condition, and Ia=0.2S
Crop residue cover applies only if residue is on at least 5% of the surface throughout the year.
Hydraulic condition is based on combination factors that affect infiltration and runoff, including (a) density and canopy of vegetative areas,
(b) amount of year-round cover, (c) amount of grass or close-seeded legumes, (d) percent of residue cover on the land surface (good ≥ 20%),
and (e) degree of surface roughness.
Poor: Factors impair infiltration and tend to increase runoff.
Good: Factors encourage average and better than average infiltration and tend to decrease runoff.
2–6
(210-VI-TR-55, Second Ed., June 1986)
Estimating Runoff
Chapter 2
Table 2-2c
Technical Release 55
Urban Hydrology for Small Watersheds
Runoff curve numbers for other agricultural lands 1/
Curve numbers for
--------------------------------------- Cover description -------------------------------------Cover type
Pasture, grassland, or range—continuous
forage for grazing. 2/
Meadow—continuous grass, protected from
grazing and generally mowed for hay.
------------ hydrologic soil group ---------------
Hydrologic
condition
A
B
C
D
Poor
Fair
Good
68
49
39
79
69
61
86
79
74
89
84
80
—
30
58
71
78
Brush—brush-weed-grass mixture with brush
the major element. 3/
Poor
Fair
Good
48
35
30 4/
67
56
48
77
70
65
83
77
73
Woods—grass combination (orchard
or tree farm). 5/
Poor
Fair
Good
57
43
32
73
65
58
82
76
72
86
82
79
Woods. 6/
Poor
Fair
Good
45
36
30 4/
66
60
55
77
73
70
83
79
77
59
74
82
86
Farmsteads—buildings, lanes, driveways,
and surrounding lots.
1
2
3
4
5
6
—
Average runoff condition, and Ia = 0.2S.
Poor: <50%) ground cover or heavily grazed with no mulch.
Fair: 50 to 75% ground cover and not heavily grazed.
Good: > 75% ground cover and lightly or only occasionally grazed.
Poor: <50% ground cover.
Fair: 50 to 75% ground cover.
Good: >75% ground cover.
Actual curve number is less than 30; use CN = 30 for runoff computations.
CN’s shown were computed for areas with 50% woods and 50% grass (pasture) cover. Other combinations of conditions may be computed
from the CN’s for woods and pasture.
Poor: Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning.
Fair: Woods are grazed but not burned, and some forest litter covers the soil.
Good: Woods are protected from grazing, and litter and brush adequately cover the soil.
(210-VI-TR-55, Second Ed., June 1986)
2–7
Worksheet 2: Runoff curve number and runoff
Project
By
Date
Location
Checked
Date
Check one:
Present
Developed
1
(cover type, treatment, and hydrologic condition; percent
impervious; unconnected/connected impervious area ratio)
Use only one CN source per line
1
Area
Figure 2-4
CN
Table 2-2
Soil name
and
hydrologic
group
(appendix A)
Cover description
Figure 2-3
1. Runoff curve number
Product
of
CN x area
acres
mi 2
%
Totals
total product = _______________ = ________
CN (weighted) = ____________
total area
;
Use CN
2. Runoff
Storm #1
Frequency ................................................. yr
Rainfall, P (24-hour) ..................................
in
Runoff, Q ..................................................
in
(Use P and CN with table 2-1, figure 2-1, or
equations 2-3 and 2-4)
D–2
(210-VI-TR-55, Second Ed., June 1986)
Storm #2
Storm #3
Worksheet 3: Time of Concentration (Tc) or travel time (Tt)
Project
By
Date
Location
Checked
Date
Check one:
Present
Check one:
Tc
Developed
Tt through subarea
Notes: Space for as many as two segments per flow type can be used for each worksheet.
Include a map, schematic, or description of flow segments.
Sheet flow (Applicable to Tc only)
Segment ID
1. Surface description (table 3-1) ...................................
2. Manning’s roughness coefficient, n (table 3-1) ..........
3. Flow length, L (total L † 300 ft) ................................. ft
4. Two-year 24-hour rainfall, P 2 .................................. in
5. Land slope, s ........................................................ ft/ft
6.
0.8
0.007 (nL)
Tt = _____________
P2 0.5 s0.4
Compute Tt ......... hr
+
=
+
=
Shallow concentrated flow
Segment ID
7. Surface description (paved or unpaved) .....................
8. Flow length, L ...........................................................ft
9. Watercourse slope, s ............................................ ft/ft
10. Average velocity, V (figure 3-1) ............................. ft/s
L
11. Tt = _______
Compute Tt ........... hr
3600 V
Channel flow
Segment ID
12. Cross sectional flow area, a ................................. ft2
13. Wetted perimeter, pw .............................................. ft
a
14. Hydraulic radius, r=
Compute r ......................... ft
pw
15 Channel slope, s ..................................................... ft/ft
16. Manning’s roughness coefficient, n ............................
2/3
1/2
1.49 r
s
17. V = __________
Compute V ................ft/s
n
____
18. Flow length, L .......................................................... ft
L
19. Tt = _________
Compute Tt .............. hr
3600 V
20. Watershed or subarea Tc or Tt (add Tt in steps 6, 11, and 19) ....................................................... Hr
+
(210-VI-TR-55, Second Ed., June 1986)
=
D–3