Rainfall Duration
 Background
Most rainfall distributions are developed for a specific rainfall duration.
Due to the historic practice of reading rain gauges on a daily basis, most
rainfall distributions use a 24-hour duration. Although shorter durations are
sometimes used for special purposes (such as water quality events),
most studies are performed with a 24-hour nested synthetic storm.
Because these storms contain intensity data for all events up to 24-hours,
there is generally no need to use a shorter storm duration for smaller
watersheds.
While most of the rainfall distributions used with HydroCAD are 24-hour
duration, HydroCAD supports the use of any storm duration, from short
water-quality events up to multi-day historic events. HydroCAD also
supports dimensionless storms, such as the Huff distributions, which are
scaled to a specific duration as required for each project.
Setting the Rainfall Duration
For most rainfall distributions the duration is set automatically and
should not be changed. As soon as you select a rainfall distribution
(Storm Type on the rainfall screen at right) the Storm Duration is
automatically set to the predefined default value for that storm.
For dimensionless rainfalls (such as the Huff distributions) the Duration
Mode is automatically set to "Scale", and the appropriate Storm Duration must
must be entered by hand.
Changing the Rainfall Duration
For applications that require a different rainfall distribution, you can
specify a new duration by setting the "Duration Mode" to "Trim" or "Scale".
 Trim can be used to reduce
the rainfall duration by "zooming in" to the desired duration and trimming
away the portion of the storm that lies outside this time period. The
resulting storm is centered within the original time span.For example, if
you have a 24-hour storm with the peak intensity at 12 hours, and you trim
to 12 hours you will get a 12-hour storm with the peak at 6 hours. The
portions of the original mass curve before 6 hours and after 18 hours are
"trimmed" and not used, while the 6-18 hour portion is retained and becomes
the new 12-hour distribution, as shown in the illustration at the right.
Most nested synthetic rainfall distributions are candidates for trimming, in that the
peak rainfall intensity is preserved in the resulting reduced-duration event. |
 Scale can be used to increase or decrease the rainfall duration by
rescaling the entire mass curve. This procedure is most commonly used
with the dimensionless Huff distributions, or with the "Constant Intensity"
storm that is pre-installed with HydroCAD.Important: Storms with a
preset rainfall duration generally should not be scaled to a different
duration, since the scaling operation will alter the peak intensity in
direct proportion to the change in duration!
For example, if you have a 24-hour storm with the peak intensity at 12
hours, and you scale to 12 hours you will get a 12-hour storm with the peak
at 6 hours. The entire mass curve is retained, but the 24 hour time
scale is compressed to 12 hours, increasing the peak intensity two-fold! |
History
The "Trim" option was introduced in the HydroCAD 10.1
update. This makes it possible
to trim storms to a shorter duration, eliminating the need for multiple versions
of a given distribution, such as the Type II 6, 12, and 24 hour storms that have
been preinstalled with HydroCAD. But the primary benefit is the
ability to generate reduced-duration versions of any rainfall
distribution, without having to maintain a separate rainfall definition for each
duration.
Since previous versions of HydroCAD did not have the "Trim" capability,
changing the rainfall duration would always result in the storm being rescaled.
These projects will continue to produce the exact same results when
opened in the new version, which will automatically use the "Scale" option.
However, a new Hint 35 will be generated whenever a fixed duration storm is
rescaled, indicating that you should review the rainfall settings and consider
using the new "Trim" option instead of rescaling.
Project files using the "Scale" setting are backwards compatible with earlier
versions. Project files using the "Trim" option are not backwards
compatible, since this capability is not present in earlier versions.
References
The "Trim" procedure is further described in Example 4-5 of the
National Engineering Handbook, Part 630, Hydrology, Chapter 4, revised March
2017.
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