FLOOD00106

• County: Statewide
• Title: Hydrologic Engineering Methods for Water Resources Development Volume 5
• Date: 3/1/1975
• Prepared For: US
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

time distribution within the storm. In general, the greater the degree
of concentration of rainfall in time and space, the greater is the
severity as measured by its flood-producing capability. In the case of
the space distribution, however. there are some areas within a basin
that are more conducive to flood production than others, and it is usual
practice to center the heavier storm rainfall over these areas. Loca-
tions of such storm centers may be different from computation of flood
flows at different points in the basin.
Storm rainfall severity is usually represented by depth-area-dura-
tion relationships such as illustrated in figure 2.01. In the case
of the standard project rainflood, maximum rainfall depths character-
istic of the region should be used for the durations and area sizes
that are critical to the study. Figure 3.01 illustrates an evaluation
of standard project rainstorm magnitude for a 24-hour duration and 200
square mile area in different zonal regions of the United States east
of the 105th meridian. Figure 3.01 indicates that the standard project
rainfall depth has been exceeded by approximately 5 to 16 percent of
the historical events studied. Based on all 400 stonnls, about 10 per-
cent of the events exceeded the SPS 200 square mile 24-hour index amount,
in some cases by as much as 80 percent. This illustrates that the stand-
ard project storm is not of unprecendented magnitude regionally. although
it is definitely of a major category. Amounts of precipitation should
normally be expressed as a ratio to a precipitation potential index
representing potential moisture or topographic effects discussed in
section 2.04 and 2.05.
After depth-area-duration criteria are derived, space and time
patterns for the standard project storm must be developed. These can be
taken from actual storm records, but it is not likely that a single re-
corded storm could produce a space or time pattern that is satisfactoy
for application to a variety of drainage basins. It may be desirable
to devise simplified space and time patterns that are sufficiently
balanced and representative of general storm characteristics. Such
patterns are illustrated in figures 3.02 and 3.03.

3.02