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Book II, Section 1, Vol. 1, of ARR (2001) describes the procedures for determining IFD design rainfall information. Two equivalent procedures are described, one algebraic (in Part A of Clause 1.3), the other graphical (in Part B). In both cases, the procedure has up to eight steps, depending on the requirements. The procedures can be programmed using standard spreadsheet software or commonly used computer languages. There are a number of these programs used in Australian practice, all of which require the nine basic parameters obtained from Volume 2 of Australian Rainfall and Runoff. Steps 1 to 4 from ARR lead to the determination of design rainfall intensities for the basic durations of 6 minutes, and 1, 12, and 72 hours and for the basic ARI of 2 and 50 years.
Steps 5 to 7 extend these intensity values and plots to standard durations:
5, 6, 10, 20, 30 minutes, 1, 2, 3, 6, 12, 24, 48 and 72 hours
and standard ARI's:
1, 2, 5, 10, 20, 50 and 100 years.
Step 8 fits a sixth degree polynomial to the intensity-frequency duration values.
Maps 1 to 6 in Vol. 2 give intensities for ARI = 2 and 50 years and duration equals 1, 12, and 72 hours.
The values given were derived by fitting log-normal distributions to annual series for the various durations, then adjusting (theoretically) the values for ARI < 10 years to the corresponding partial series values. These were based on the use of the Langbein Equations discussed in Bridge Waterways Chapter 2.
In most locations, the skewness of the log transformed data was > 0 so Log Pearson III (LPIII) distributions fit the data better. To enable the log-normal (approximate) values to be converted to LPIII (more accurate) values, "regional" skew coefficients are given on Maps 7a to 7d in ARR Vol. 2. These were obtained by spatial and temporal smoothing of station skew coefficients for the various rainfall durations.
A frequency curve for a duration of 6 minutes can be obtained from empirical equations relating 6 minute intensities to one hour intensities for the basic ARIs of 2 and 50 years. These equations A(3.1) and A(3.2) in ARR include geographically varying empirical coefficients, given in Maps 8 and 9 of Vol. 2.
In summary, nine parameters need to be read off maps contained in Volume 2 of ARR for application of a generalised procedure for determining design rainfall intensity-frequency -duration (IFD) data relevant to a particular location. These nine parameters are:
2yr ARI 1hr Duration rainfall
2yr ARI 12 hr Duration rainfall
2yr ARI 72hr Duration rainfall
50yr ARI 1hr Duration rainfall
50yr ARI 12hr Duration rainfall
50yr ARI 72hr Duration rainfall
Coefficient of skewness
F2 for determining 6 minute rainfall of 2yr ARI
F50 for determining 6 minute rainfall of 50yr ARI
The rainfall intensity information required for the estimation of peak flow for small to medium size catchments by using methods such as Rational Method which are largely based on flood frequency analysis of the same set of flow data. The Intensity Frequency Duration (IFD) tables for Perth, Geraldton, Albany, and Port Hedland can be accessed by using the following links.
For larger areas and long storm durations, the user is referred to the Bureau of Meteorology for more specialist input from their hydrometeorologists.
References Australian Rainfall And Runoff; A Guide to Flood Estimation, Volume 1, 2001. Australian Rainfall And Runoff; A Guide to Flood Estimation, Volume 2, 1987.