Synthetic Rainfall Generation
HydroCAD-10 provides the ability to generate a
synthetic mass curve from any IDF file, including PFD files
from NOAA or other similar sources. This provides the ability to use local
rainfall data instead of a regional rainfall distribution, such as the Type II
and Type III storms.
Background
Since the inception of the SCS/NRCS runoff procedure, most studies have been
performed using a predefined synthetic rainfall distribution, such as the Type
I, IA, II, or III. Although these distributions have been used to
successfully model watersheds throughout most of the US, there is growing
interest in using local precipitation frequency data for these studies.
With HydroCAD-10, you can generate a synthetic
rainfall distribution (like the SCS storms) based on local PFD data from NOAA,
NRCC, or other sources. This allows the watershed model to more accurately
reflect local rainfall conditions, especially for locations that are near the
boundaries between the traditional distributions.
The ability to use local PFD also facilitates the use of the SCS/NRCS method
for (international) locations where predefined synthetic rainfall distributions
are not readily available.
HydroCAD-10 also allows PFD downloads to be used directly as a
Rational method IDF file, eliminating the need for
manual file creation.
Important Note
Before using these
procedures, please consult your local stormwater regulations to determine the
exact rainfall data that should be used for your project. Despite the
availability of site-specific rainfall data, most stormwater
regulations continue to employ one of the standard rainfall
distributions provided with HydroCAD. In these situations,
you can still utilize the new rainfall data to manually define the total 24-hour
rainfall depth for each event without using a new mass curve.
Generating a Synthetic Rainfall
Note: Before you proceed with synthetic mass curve generation, please review
the full range of rainfall setup options to be sure
this is the most appropriate solution for your specific requirements. You
will also need to obtain a local IDF or PFD file for the
desired location.
Once you have the local IDF or PFD file you can create a
synthetic rainfall distribution. There are several ways to access the conversion screen in HydroCAD:
From the
View|IDF screen, click More IDF Data and select the
Convert tab -or-
From the
Rainfall tab of the Settings|Calculation
screen, click More Storms and select the
Convert tab.
On the
Convert tab:
1) Select the desired IDF file
2) Select and desired return period(s)
3) Verify the Options (no changes are required in
most situations)
4) Click Create Mass Curves to generate a mass
curve for each selected return period
Depending on how you launched the conversion process, HydroCAD will select
the new file automatically, and will offer to automatically define the rainfall
events based on the new file.
Technical Notes
Synthetic storms typically have a duration of 24-hours. Since a 24-hour storm
contains intensity information for all storms of 24 hours or less, shorter storm
durations are not typically required, even for the smallest watersheds. Longer
durations are required only for very large watersheds, where the response time
exceeds 24 hours.
The peak intensity of a synthetic storm usually occurs at the mid-point of
the storm. So a 24-hour storm will usually peak at 12 hours. Other peak times
may be used for special requirements.
A synthetic storm is generated by placing the rainfall intensity for the
shortest duration event (e.g. 5 minute) at the center of the storm. Successively
longer durations (and lower intensities) are placed on alternating sides of the
peak, until a complete curve is developed. The result is sometimes referred to
as a "nested storm", since the depths for all possible durations are nested
inside each other.
To prevent irregularities in the final intensity curve, a smoothing algorithm
is normally applied to the IDF data. One procedure is to use the 5-minute,
1-hour, and 24-hour values from the IDF curve and use a log-log interpolation to
determine the intensity for all intermediate durations.
Generation Options
HydroCAD provides the following options for the creation of a synthetic mass
curve. Note that most applications do not require any changes to the
default settings.
Storm Duration: Specifies the total rainfall duration. Defaults to 24
hours.
Peak Time: Specifies when the peak intensity will occur. Defaults to
one-half of the storm duration, so the peak occurs at mid-storm.
Time Increment: Specifies the time spacing between the calculated mass
curve points. Defaults to 1/10 hour. Shorter increments usually have no benefit,
since the original IDF file rarely contains intensity information for shorter
durations, and longer increments will reduce the peak intensity of the resulting
mass curve.
Smoothing: Controls the process for extracting intensity values from
the IDF data.
S0-None: Uses the original IDF data without adjustment. This preserves the
original data, but may produce anomalies in the resulting mass curve depending
on the accuracy of the original IDF data.
S1-3-point: Applies a 3-point smoothing algorithm in order to compensate
for irregularities that are often present in the IDF curve. This procedure
preserves the 5-minute, 1-hour, and 24-hour intensities, but uses a log-log
interpolation for all intermediate intensities. If the Storm Duration is
different than 24-hours, the intensity for the specified duration is preserved
in addition to the three standard durations.
Smoothing is enabled by default, unless the selected IDF file has already
been smoothed, which disables further smoothing. The smoothing setting (S0 or
S1) is included in the file name for later reference. The letter "P" is added if
the IDF data has already been smoothed. This allows the identification of
double-smoothing (S1P), which may produce undesirable results and is not
recommended.
Save Result To: Converted rainfalls are normally stored in the same
folder as the current HydroCAD project. You also have the option to save the
results in the master HydroCAD rainfall folder.
Save Events To: When converting multiple events, all the resulting
mass curves are normally stored in a single file for ease of use. You
also have the option to save each mass curve in a separate file for
compatibility with versions prior to HydroCAD 9.2
For further information on read about Precipitation
Frequency Data.
