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HydroCAD Stormwater Modeling - Since 1986

Rainfall Generation

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.


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.

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