UFC 3-210-10
25 October 2004
hydraulic models, on the other hand, may require the collection of rainfall data in
2-minute intervals over several years to determine the appropriate system design.
4-3.1
LID Precipitation Analysis. An important approach to analyzing the
effectiveness of an LID design is to consider the number of storm events for which the
design will provide enough storage and infiltration capacity to capture all of the
precipitation on-site. This is useful because maintaining the hydrologic integrity or
water balance of a site is better accomplished by managing the frequent smaller events
rather than the occasional large events.
For example, in the Washington, D.C. region there are approximately 80
storm events per year that collectively generate approximately 1000 mm (40 in) of
precipitation. Approximately 75 of these storm events generate 13 mm (0.5 in) or less
of precipitation. Figure 4-3 illustrates this concept.
Figure 4-3. Frequency of Small Storms
4% 2%
Washington, DC -
4%
Reagan National
2001 Daily Rainfall
Frequency (inches)
0.25
0.5
1
>1
90%
Source: NOAA.
This kind of analysis allows the designer to determine the overall storage and
infiltration capacity required to control the desired number of storm events within any
given year or period. The analysis can also be undertaken in terms of the precipitation
depth associated with discrete storm events such as the 1-year 24-hour storm.
4-3.2
Conventional Precipitation Analysis. Conventional practices, as well as
many state and local regulations, often require site engineers to control only specific
events such as the 2-year 24-hour storm events. In the Washington, D.C. area, this
would mean reducing the peak runoff to predevelopment rates for only those events in
which 76 mm (3 in) of rainfall. Events that occur more or less frequently would be less
effectively controlled.
4-4
STORAGE. Precipitation may be temporarily detained within site
depressions or held in the soil. When the capacity of a depression is exceeded, the
water is released as runoff that may be captured further downstream. Water that is not
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