Rational method setup with 2016 I-F-D data
Although it is not sanctioned by ARR 2019, it is likely that some designers will want to continue working with the rational method, maintaining the methods set out in the 1987 edition of ARR. This can be done in DRAINS with 2016 intensity-frequency-duration data from the Bureau of Meteorology website, which provides 0.5 and 0.2 EY (39% and 18% AEP) rainfall depths and intensities, as well as the usual 50% and 20% AEP values.
The setup process starts with the specification of an ARR 1987 rational method hydrological model in this property sheet, opened from the Project → Hydrological Models option. If you are converting an existing rational method model, this will be specified already.
You can then go to the Project → Rainfall Data option property sheet. For a new model, the following window will appear:
This is used to enter ARR 1987 data. If you are converting an existing rational method model, the boxes will already be filled.
To input the 2016 I-F-D data, click on the Use current BOM IFDs in the Rainfall Data box. If you do this, and click on OK, this window will appear:
You can select the minor and major frequencies as exceedances per year (EY) or annual exceedance probabilities (AEPs). You will need to have gone to the BOM website and obtained I-F-D files as described here. Then click the Import BOM data button to display the window that imports an I-F-D file that you have previously saved.
If you have specified a 0.5 or 0.2 EY minor storm, this message will appear.
If you reply 'Yes' you will be asked to input the second file containing very frequent I-F-D data. Once this is done, the following window will appear, and DRAINS will be able to run rational method calculations, after a drainage system is fully specified. The running process and the results presented are the same as with the ARR 1987 rainfalls.
A curve fit made to the I-F-D data from the BOM is used in rational method calculations.
Related Articles
Rational Method
Developed in the 19th century, the rational method is the most widely-used hydrological procedure for estimation of design flowrates. It employs the equation: Q = C.I.A / F where Q is a peak flowrate in m3/s or L/s, C is a dimensionless runoff ...Extended rational method (ERM)
A hydrological model that produces hydrographs, such as the Horton (ILSAX), RORB, RAFTS and WBNM models, must be applied to analyse the storage routing effects of detention basins. The rational method is not suitable because it only calculates peak ...Synthetic storms for the Extended Rational Method
The extended rational method (ERM) can be applied using the design storm patterns from Australian Rainfall and Runoff 1987 used by the Horton (ILSAX) and storage routing hydrological models or using synthetic patterns derived from the local ...Obtaining 2016 I-F-D Data
The latest rainfall intensities and depths to be used in design and analysis procedures are obtained after completing the previous steps from the ARR Data Hub. If you want to obtain the 2016 IFDs separately to the ARR Data Hub click here. This ...Rainfall data for Rational Method property sheet
This window is used to enter ten values that define the key average recurrence intervals (ARIs) and rainfall intensities required to define the intensity-frequency-duration relationships to be used in rational method calculations. The ARIs entered ...