CEIE 340
Lab 6 Solutions
YHK
Page 1 of 3
Problem 1.
Problem 7.45 on page 419
The weighted curve number for this area is computed by determining the curve number
(CN) for each of the land uses in the area.
The curve numbers for the various land uses and soil
types are found using Table 3.18 on pages 161162.
First equation 3.50 is used to calculate the
CNw for imperviousness.
Equation 7.22 is then used to compute the weighted CN for the entire
area.
Here are the calculations:
Area
(acres)
Area
Fraction
Soil
Group
Imperviousness
(%)
CN
P
CN
w
Fraction x
CN
4.0
0.20
B
24
61
70
14.00
6.0
0.30
C
18
74
78
23.40
7.0
0.35
B
60
61
83
29.05
3.0
0.15
C
75
74
92
13.80
20.0
1.00
Weighted CN for the entire area =
80.25
The weighted CN for the area can then be used to determine the runoff depth using Table
7.13 on page 391.
Here are the results for rainfall depth of 5 inches:
Rainfall
Depth
[in]
CN
Runoff Depth
[in]
5
80
2.89
To compute the runoff depths using the weighted Q method, the runoff depth for each
land use type must be determined.
The weighted average of this runoff depth is then computed.
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 Spring '09
 Choudhury
 Hydrology, Surface runoff, infiltration, Unit peak

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