UFC 3-210-10
25 October 2004
10-2.6
Storage Volume Comparison. A comparison of the storage volumes
required for the conventional and LID site designs is given below. The 2-year 24-hour
and the 10-year 24-hour storms are often used as the design storms for channel
protection and adequate conveyance. Although the design objective here is to maintain
the peak runoff rate for the 2-year 24-hour storm, the 10-year 24-hour storm is also
used to further illustrate the differences in peak runoff rate and volume between the two
approaches. In order to determine the storage volume required to maintain the pre-
development peak runoff rate for these design storms, the runoff depths and peak runoff
rates for the existing condition and both proposed conditions were first calculated using
the Graphical Peak Discharge Method from TR-55. Table 10-6 is a summary of those
calculations.
Table 10-6. Summary of Graphical Peak Discharge Results
Peak Discharge (CFS)
Runoff depth (in.)
Condition
CN
Tc
2-year storm
10-year storm
2-year
10-year
(3" depth)
(5" depth)
storm
storm
Existing Condition
63
0.24
2
10
0.4
1.5
Proposed Condition
80
0.22
9
23
1.3
2.9
conventional CN
Proposed Condition
73
0.24
6
17
0.9
2.3
using LID site design
storage volume required to maintain the 2-year 24-hour pre-development peak runoff
rate for both the conventional and LID site designs. As shown in Table 10-6 above, the
target (existing) 2-year 24-hour peak outflow is 2 cfs and the target 10-year 24-hour
peak outflow is 10 cfs. (It is purely coincidental that the values of the return periods
match the values of the discharges.) The results are given in Table 10-7.
Table 10-7. Post-Development Storage Volumes
Conventional site design
LID site design
Design storm
Volume, ac-ft (m3)
Volume, ac-ft (m3)
Depth, inches (mm)*
Depth, inches (mm)*
2-year 24-hour
0.52
(13)
0.28
(347)
0.28
(7)
0.15
(187)
10-year 24-hour
0.85
(22)
0.46
(568)
0.53
(13)
0.29
(354)
* depth of runoff distributed across the 6.5 acre (2.6 ha) area
For the 2-year 24-hour storm, the LID site design results in a 46% reduction in
required storage volume as compared to the conventional site design, and for the 10-
year 24-hour storm, the volume reduction is 38%. The pond shown in Figure 10-7 was
sized using the computed 2-year conventional detention basin storage volume.
Appendix C shows a summary of the computer program results.
43
Ibid.
72
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