HydroCAD IDF FILE Definition ---------------------------------------------------- Copyright (c) 2010 HydroCAD Software Solutions LLC -------------------- Note: IDF Curves define the Intensity-Duration-Frequency relationship for the Rational Method. You may manually extract this data from the appropriate curve, OR you may define a custom IDF file so that HydroCAD can perform the look-up automatically. To use the manual option you do NOT need to define an IDF file. Just enter the duration and intensity directly on the HydroCAD Rainfall Settings screen. The following instructions apply ONLY if you wish to define a custom IDF file. For additional information, plus other IDF files you can download, please see www.hydrocad.net/idf.htm ******* To create a custom IDF file in HydroCAD 9.2 and later: ************* 1) In many cases you can download a complete NOAA rainfall file for your location. These files can be used directly with HydroCAD 9.2 and later. All you need to do is download the file to your HydroCAD\IDF folder, and be sure the file name ends with a .HCI extension, so that HydroCAD can recognize it as a valid source of IDF data. For complete details see "NoAA Rainfall" in HydroCAD help. ******* To create a custom IDF file in HydroCAD 8.0 and later: ************* 1a) Start HydroCAD and select View|IDF from the main menu. If you have a project open, you may also click the "View IDF" button on the calculation settings screen. 1b) On the IDF report window, select the "AA-Sample County" definition, or any other file that you wish to use as a template. 1c) Click the Edit button to open the selected file in the curve editor. 1d) If you are creating a new file, change the "Name=" line to reflect the new name. (Do not include the file extension.) 1e) Modify the remainder of the file as required to incorporate your actual IDF data. (See details in steps A-I below.) 1f) Click "OK" to save your data under the new "Name=" setting. 1g) You can now select this IDF definition on the Calculation Settings screen of any HydroCAD project. ******* To create a custom IDF file in HydroCAD 7.1 or earlier: ************** 2a) In Windows, navigate to the folder c:\Program Files\HydroCAD\IDF 2b) Open the sample IDF file called "AA-Sample County.hci" 2c) Modify the file to incorporate your actual IDF data, as described in steps A-I below. 2d) Save the file under your custom IDF name using File|Save As. Be sure to include the .HCI file extension. 2e) You can now select this IDF definition on the Calculation Settings screen of any HydroCAD project. ***************************************************************************** The remainder of this document describes the IDF file format in detail, and provides the information you will need to create a custom IDF file. You may also find that the sample file itself provides all the information you need, but additional details are provided here. --------------------------- A) To create your own IDF file, make a copy of the "AA-Sample County.hci" file and modify the contents as required, or edit the sample file and save it under a new name. When naming your file, use something like "TX-Austin County" for easy identification and compatibility with other IDF definitions. When using notepad, be sure to include the .hci extension. B) Each line of an IDF file defines a specific piece of information. For example, the following line defines the name of the IDF file: (this should be the same as the actual file name with no extension) NAME=AA-Sample County C) Each file may contain a comment line, which is commonly used to identify the source of the IDF data, and is displayed when the file is selected in HydroCAD. COMMENT=Sample County Drainage Manual 3/1/03 D) The following line specifies whether depth values will be expressed in INCHES or MM. If this line is omitted, INCHES are assumed. DEPTHUNITS=INCHES E) The following line specifies whether time values will be expressed in HOURS, MINUTES, SECONDS. If this line is omitted, HOURS are assumed. TIMEUNITS=HOURS F) The quotient of the previous two values defines the units used to express intensity vales. The default is INCHES/HOUR. G) The following line specifies the units used to express duration values. This setting allows intensity and duration values to use different units. If this line is ommited, MINUTES are assumed. DURATUNITS=MINUTES H) The following line specifies each of the event durations for which you wish to specify a rainfall intensity. For intermediate durations, HydroCAD will interpolate between the values you provide using a log-log relationship. DURATION=5 10 20 30 1*60 2*60 5*60 10*60 24*60 These values must be in the declared DURATUNITS, and may employ a single math operator as show above. In this example, durations of 5m, 10m, 20m, 30m, 1h, 2h, 5h, 10h, and 24h are declared, with each value separated by at least one space. You may specify any number of duration values, as required to adequately define your IDF curve(s). I) The following line specifies a name for the first curve or event. The name is commonly associated with a given return period, but may also identify an event, such as the "1990-Storm": EVENT=2-Year Although event names can be any length, short names are recommended for maximum readability. You should also avoid using spaces in event names, since this may cause undesired word-wrap in certain reports and column headings. J) The following line defines a frequency coefficient or frequency factor. The line must appear immediately after the corresponding EVENT line (above). If the statement is omitted, a value of 1 is assumed for the current event. Cf=1.25 Note: This feature is available only with HydroCAD 8.5 or later. To activate the frequency factor, select "Automatic Frequency Factor" in the applicable HydroCAD project file. K) The following line defines the actual rainfall intensity corresponding to each DURATION value given above. The number of intensity values must be exactly the same as the number of DURATION values: INTEN=5.1 4.1 3.0 2.3 1.5 .84 .41 .31 .13 At intermediate durations, HydroCAD interpolates using a log-log relationship - the same format in which IDF curves are commonly published. Note that each intensity value must be expressed using the DEPTHUNITS and TIMEUNITS declared above, such as INCHES/HOUR or MM/HOUR. Note: With HydroCAD-9 (or later) simple expressions may be used to represent individual intensity values. This can be useful for converting depth values to intensity. For example, a depth of 2.5 inches over 3 hours could be expressed as 2.5/3 instead of 0.83333 inches-per-hour. L) Additional pairs of EVENT and INTEN lines may be used to define other events. Remember that all INTEN lines must contain the same number of values, corresponding to the single line of DURATION values specified above. In this manner, a singe IDF file may contain any number of curves or events. event=5-Year inten=7.1 5.6 4.2 3.2 1.9 1.1 .57 .33 .18 event=1990-Event inten=12.0 10.0 7.9 6.6 5.7 2.2 1.1 .64 .33 M) If the intensity values are a fixed multiple of the values for a previous event (which is common for IDF curves) subsequent event(s) may be defined using a multiplier: event=100-Year inten=2x This example defines a 100-year event in each intensity value is twice the corresponding value from the first INTEN line in the file. a N) An alternate method of defining the I = ----- Eq.1 intensity is to use the equation: (b+t)^c Where t is the desired duration, and the coefficients a, b, and c are specified by the following line (used in place of the INTEN data): coef1= 72.29 13 0.7475 a O) The following variation of the I = ------- Eq.2 previous equation may also be used: b + t^c With the coefficients a, b, and c specified by the following line: coef2= 10.63 8.1 0.45 2 3 P) Coefficients may also be specified I = a + bx + cx + dx for a polynomial of the form: Eq.3 where: x = ln( t ) With the coefficients a,b,c,d specified by the following line: coef3= 12.0 -1.91 -0.20 0.03 Important ========= When using coefficients of any form, the intensity and duration values must correspond to the units declared above. A DURATION line is still required to specify the range of permissible durations, but only the first and last values are utilized. Attempting to use a duration outside these bounds will cause HydroCAD to use the intensity for the applicable limit.