UFC 3-210-10
25 October 2004
of pictures are presented to illustrate how selected LID components would look in a
recently constructed housing development.
10-2.1
Curve Number Calculations For Existing Site Condition. The site being
evaluated has a 2.6 ha (6.5 acre) drainage area. The land is relatively flat and drains to
a small channel with wetlands at the outfall of the drainage area. The slopes are gentle,
averaging 2 percent. The soils are classified as belonging to NRCS Hydrologic Soils
Group (HSG) B. These soils have moderate infiltration rates and moderately fine to
moderately coarse textures. They generally have a moderate rate of water transmission
Approximately 1.5 ha (3.6 acres) near the outfall is classified hydrologically as "Woods
in Fair Condition". The upper portion of the property is classified as "Brush in Poor
Condition". Figure 10-5 is a map of the existing condition.
calculate the composite curve number (CN) for the site. The resulting CN from
Equation 10-3 is 63 for the 2.6 ha (6.5 acres.) Table 10-2 is a summary of those
calculations.
Weighted CN = Sum of Products Drainage Area
Equation 10-3
Table 10-2. Composite Curve Number Calculation for Existing Condition
Hydrologic
CN (Table
Area
Product of CN
Cover Description
Soils Group
2-2 TR-55)
(Acres)
x Area
B
Brush, Poor Condition
67
2.9
194.3
B
Woods, Fair
60
3.6
216.0
Sum of Products
410.3
Drainage Area
6.5
Weighted CN
63
39
NRCS, 1986.
40
Ibid.
67
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