HydroCAD® Stormwater Modeling - Since 1986
IDF Rainfall Data
When using the Rational Method, the rainfall duration and intensity must be specified on the Rainfall tab of the Settings|Calculation screen. Although you can look-up the intensity by hand and manually enter the value into HydroCAD, it is usually preferable to have the software do the look-up automatically from an appropriate IDF curve, as illustrated here.
To use this feature, you must define a custom IDF file with the correct format. HydroCAD can use IDF data in several different formats, ranging from a table of discrete IDF values, to equations based on a set of coefficients. Your local stormwater regulations should provide guidance on the appropriate IDF data for your location.
Creating a custom IDF File
Due to the thousands of different IDF curves used in the United States alone, it is not practical to provide local IDF curves with HydroCAD. However, HydroCAD provides several options for using local rainfall data:
Option 2: You can create IDF curves by hand using raw IDF data, which is often included in your state or local stormwater regulations. HydroCAD also supports the use of IDF coefficients, which can produce better results than using tabular IDF data.
Creating IDF files by hand
If you need to create an IDF file by hand, you'll find complete details by selecting IDF Info under Start|Programs|HydroCAD. This document provides instructions on setting up your own file. You can also view the instructions here.
To simplify the process of creating and viewing IDF files, a new IDF curve report was added in HydroCAD-8. To open the report select View|IDF from the HydroCAD main menu.
Defining the Frequency Factor
HydroCAD 8.5 (and later) allows for automatic adjustment of individual C values by the use of a "frequency factor." Some localities prescribe the use of a frequency factor in order to replicate the proportionally higher runoff than tends to occur with larger rainfalls. Although the factor can be set manually on the Settings|Calculation screen, a separate value can be defined for each event by including it in the IDF file, as discussed above.
Selected IDF Files
The following IDF files may be useful when creating your own file definitions. To use one of these files, right-click the desired link and select "Save Target As". Save the file to the folder C:\ProgramFiles\HydroCAD\IDF.
After downloading the file to your IDF folder, you will be able to select the file on the Rainfall tab of the Settings|Calculation screen within any project. To modify one of the files for another location, see the instructions above.
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