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HydroCAD Stormwater Modeling - Since 1986

IDF Data

IDF Rainfall Data

Click for complete self-training materialsWhen 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.

With HydroCAD-10, you can also download local Precipitation Frequency Data for direct use with HydroCAD.  Details here.

Note: Changes to published rainfall data do not require changes to the HydroCAD software.  New rainfall data can be accommodated by creating an updated IDF data file as described below.

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 1: Starting with HydroCAD-10, you can download local precipitation frequency data from NOAA, NRCC, or other rainfall sources and use it directly in HydroCAD.  Details here.

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

IDF Curve ReportIf you need to create an IDF file by hand, you'll find complete details by selecting Help > IDF Info.  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.

New Jersey DEP: Discrete values. Includes new "water quality" event and Frequency Factors.

Austin, Texas: Example of coefficient-based IDF curves

Arlington, Virginia: Uses new "B-D-E" coefficients based on NOAA Atlas 14.

Saint John's Airport, Newfoundland:  Uses only two coefficients.

Florida DOT (from 2004 Hydrology Handbook):  Zone 1  2  3  4  5  6  7  8  9  10  11

Connecticut DOT (from Oct.2000 Drainage Manual):  Table B-1  Table B-2
(Both files include frequency factors for 25, 50, and 100-year events)

Moncton, New Brunswick, Canada

Toronto, CanadaFrom Toronto Wet Weather Management Guidelines

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