Tuesday, 25 October 2011

Calculating New Zealand IDF data for Autodesk Storm and Sanitary Analysis

The main source of rainfall design for areas in New Zealand is the NIWA Hirds website . However the first thing you will notice on getting data from the Hirds website is that the you do not get the rainfall intensity data for the 5 and 15 minute duration storm and the 1,2,3 and 25 ARI event that SSA asks for.

Figure 1.0 Raw Hirds Data


Figure 2.0 Autodesk Storm and Sanitary Analysis IDF input screen


Thanks to the help of Graeme Mackay at NIWA who explained the equations behind the Hirds data. Detailed on pages 19 and 20 of the Hirds method help file available for download at the bottom of this page.

I have developed the attached spreadsheet to calculate the alternative duration rainfall data.


Here is a short video of the process you need to go though with the spreadsheet and SSA to get up and running
Unable to display content. Adobe Flash is required.

Update thanks to Matt Anderson

SSA allows editing of the Storm Duration and the Return Period. Note the 10 and 20 minute Duration added below.


When you right-click on the durations, you get:


No spreadsheet is needed.


  1. Amazing Justin! This will help a lot of Kiwi Civil3D users to plug away at SSA.

  2. Justin -

    I hope you do realize that the entire duration table is edittable from the Right click menu.


    Matt Anderson, PE

  3. Matt

    I did not realize that I assumed they were uneditable like Hydroflow I couple of years ago. I have added you screen captures to the post



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