Stormwater Calculations - Mathematics and Statistics for the Environment - page 766

TABLE 25.1
Variations of Duration and Intensity for a Given Volume

Duration (hr) Intensity (in./hr) Volume (in.)

0.5 3.0 1.5

1.0 1.5 1.5

1.5 1.0 1.5

6.0 0.25 1.5

Source: Virginia Stormwater Management Handbook , Virginia Department
of Conservation and Recreation, Division of Soil and Water
Conservation, Richmond, 1999.

TABLE 25.2
Variations of Volume, Duration, and Return Frequency
for a Given Intensity

Duration (hr) Volume (in.) Intensity (in./hr) Frequency (yr)

1.0 1.5 1.5 2

2.0 3.0 1.5 10

3.0 4.5 1.5 100

Source: Virginia Stormwater Management Handbook , Virginia
Department of Conservation and Recreation, Division of Soil and
Water Conservation, Richmond, 1999.

is different, as shown, for example, in Table 25.2. It, therefore, becomes critical for any regulatory
criteria to specify the volume (or intensity) and the duration for a specified frequency design storm.
Although specifying one combination of volume and duration may limit the analysis, with regard to
what is considered to be the critical variable for any given watershed (erosion, flooding, water, qual-
ity, etc.), it does establish a baseline from which to work. (This analysis supports the SCS 24-hour
design storm because an entire range of storm intensities is incorporated into the rainfall distribu-
tion.) Localities may choose to establish criteria based on specific watershed and receiving channel
conditions, which will dictate the appropriate design storm.

25.3.1.1 Frequency
The frequency of a specified design storm can be expressed either in terms of exceedence probabil-
ity or return period. Exceedence probability is the probability that an event having a specified vol-
ume and duration will be exceeded in one time period, which is most often assumed to be one year.
Return period is the average length of time between events having the same volume and duration.
If a storm of a specified duration and volume has a 1% chance of occurring in any given year, then
it has an exceedance probability of 0.1 and a return period of 100 years. The return period concept
is often misunderstand in that it implies that a 100-year flood will occur only once in a 100-year
period. This will not always hold true because storm events cannot be predicted deterministically.
Because storm events are random, the exceedence probability indicates that there is a finite prob-
ability (.01 for this example) that the 100-year storm may occur in any given year or consecutive
years, regardless of the historic occurrence of that storm event.