Geoscience Reference
In-Depth Information
TABLE 25.5A
Runoff Curve Numbers for Urban Areas
a
Curve Numbers for
Hydrologic Soil Group
Cover Description
Average Percent
Impervious Area
b
Cover Type and Hydrologic Condition
A
B
C
D
Fully developed urban areas (vegetation established)
Good condition (grass cover >75%)
39
61
74
80
Impervious areas
Paved parking lots, roofs, driveways, etc. (excluding right-of-way)
98
98
98
98
Streets and roads
Paved; curbs and storm sewers (excluding right-of-way)
98
98
98
98
Paved; open ditches (including right-of-way)
83
89
92
93
Gravel (including right-of-way)
76
85
89
91
Dirt (including right-of-way)
72
82
87
89
Urban districts
Commercial and business
85
89
92
94
95
Industrial
72
81
88
91
93
Residential districts by average lot size
1/8 acre or less (town houses)
65
77
85
90
92
1/4 acre
38
61
75
83
87
1/3 acre
30
57
72
81
86
1/2 acre
25
54
70
80
85
1 acre
20
51
68
79
84
2 acres
12
46
65
77
82
Developing urban areas
Newly graded areas (pervious areas only, no vegetation)
77
86
91
94
Idle lands (CNs are determined using cover types similar to those in
TR-55
Table 2-2c)
a
Refer to
TR-55
for additional cover types and general assumptions and limitations.
b
Average runoff conditions and
I
a
= 0.2
S
.
Source:
Adapted from
TR-55
Table 2-2a—Runoff Curve Numbers for Urban Areas.
25.6.3.3.3 Runoff Curve Number (RCN) Determination
The soil group classification, cover type, and the hydrologic condition are used to determine the
runoff curve number (RCN). The RCN indicates the runoff potential of an area when the ground is
not frozen. Table 25.5 provides the RCNs for various land use types and soil groups. A more com-
plete table can be found in
TR-55
. Several factors should be considered when choosing an RCN for
a given land use. First, the designer should realize that the curve numbers in Table 25.5 and
TR-55
are for the
average antecedent runoff
or
moisture condition
(ARC). The ARC is the index of runoff
potential before a storm event and can have a major impact on the relationship between rainfall and
runoff for a watershed. Average ARC runoff curve numbers can be converted to dry or wet values;
however, the average antecedent runoff condition is recommended for design purposes. It is impor-
tant to consider the list of assumptions made in developing the runoff curve numbers as provided in
the Table 25.5 and in
TR-55
. Some of these assumptions are outline below.
Note:
The decision to use “wet” or “dry” antecedent runoff conditions should be based on thorough
field work, such as carefully monitored rain gauge data.
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