Index Method

Document No:  67-08-73
Revision:  2A
Date amended:  29-Mar-2019

The information below is intended to reflect the preferred practice of Main Roads Western Australia ("Main Roads"). Main Roads reserves the right to update this information at any time without notice. If you have any questions or comments please contact Eric Cheung by e-mail or on (08) 9323 4402.

To the extent permitted by law, Main Roads, its employees, agents, authors and contributors are not liable for any loss resulting from any action taken or reliance made by you on the information herein displayed.


Revision Register

Ed/Version Number Clause Number Description of Revision Date
1 All Guideline Developed.  19-Jun-2002
2 Header  Contact person changed. 22-Jan-2014
​2AHeader​​Contact person changed.​29-Mar-2019

Table of Content

5. Chapter 5 of 6. INDEX METHOD

5.1 General

In the Gascoyne, Pilbara and Goldfields areas, the techniques mentioned in ARR should be used with extreme caution. Although the Index Flood Method is being used as preferred technique for these regions, the results should be verified by anecdotal evidence particularly in the case of small (less than 1 km2) steep catchments.


5.2 Theoretical Considerations

A regional procedure based on regression analysis of flood frequency curves derived for many locations within Western Australia was developed by the Main Roads for estimation of peak flows up to the 2% Annual Exceedence Probability (AEP) in small to medium rural catchments. The procedure is referred to as the Index Flood Method and is based on multi-variate statistical regression analysis of representative flood peaks to catchment physical and meteorological characteristics. This method is described in detail in Australian Rainfall and Runoff.

In the regional analysis, Western Australia was divided into the four regions of the South West, the Wheatbelt, the North West and the Kimberley. Reliable streamflow data from stream gauging stations within each of these regions were used to derive flood frequency curves. In some regions, the areas are further sub-divided to reflect different soil types and vegetation. For each area, multi-variate regression analysis was carried out to relate the index peak discharge to such catchment characteristics as the catchment area, the mainstream length, stream slope, mean annual rainfall, degree of forest clearing etc. In most locations the 50% AEP or 2 year ARI peak discharge is used as the index flood except for the Wheatbelt and North West regions where the 5 year ARI peak discharge was used. The shape of the flood frequency curves derived for each location were also analysed and normalised to the index flood. The frequency factors so derived express the magnitudes of probabilistic peak discharges for a range of ARI as ratios of the index flood magnitude.

The formulae for the index floods method, corresponding frequency factors and worked examples for the various regions in Western Australia are described in Book IV of ARR Volume 1(p 16-21); 2001.


Australian Rainfall And Runoff; A Guide to Flood Estimation, Volume 1, 2001.
Australian Rainfall And Runoff; A Guide to Flood Estimation, Volume 2, 1987.