UFC 3-210-10
25 October 2004
Figure 10-8. Map of Proposed Conditions (LID Design)
Table 10-5. Composite CN Calculation for Proposed Condition Using LID
Hydrologic
CN (Table
Area
Product of CN
Cover Description
Soils Group
2-2 TR-55)
(Acres)
x Area
B
Lawn (good condition)
61
1.8
109.8
B
Woods, Fair
60
2.5
150.0
B
Impervious
98
2.2
215.6
Sum of Products
475.4
Drainage Area
6.5
Composite CN
73
10-2.5
Time Of Concentration For Existing And Proposed Conditions. The time
of concentration (Tc) was calculated for the pre-development and conventional post-
development conditions using the procedures in TR-55. A summary of these
calculations is included in Appendix C. The conventionally developed condition causes
Tc to decrease from 0.24 hours to 0.22 hours, or a 1.2 minute difference. The LID site
design results in a Tc that matches the existing condition; in this case, 0.24 hours.
Additional calculations for flow through the bioretention cells or rougher vegetated areas
were not included in the analysis, but would be expected to further increase post-
development LID Tc.
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