What is a PMF... really

I’ve been asked a lot recently about what the Probable Maximum Flood (PMF) is, so I thought I’d put this up in as easy to understand way as possible. I’m sure the flood boffins out there will correct me where I’m wrong, but this is intended to be a very high-level, light touch for the non-technocrats. Let’s start with the acronyms.

PMP - The Probable Maximum Precipitation is, theoretically the highest amount of rainfall conceivable at a particular location. Like all rainfall data, it varies spatially (e.g. the PMP in Lismore would be materially different to that in Sydney).

PMF - The Probable Maximum Flood is the largest flood that could conceivably be expected to occur at a particular location, usually estimated from the PMP.

AEP & ARI - In recent times the Annual Exceedance Probability (AEP) has become the standard terminology when referring to return periods. The term Average Recurrence Interval (ARI) has been discouraged as it led to the mistaken belief that a 1 in 100-year ARI storm would “only occur once every 100 years” whereas the 1% AEP storm (the same event) has a 1 in 100, or 1% chance of occurring in any given year.

Hopefully you’re still with me…

The chance of a PMP event occurring is proportional to the size of the catchment. The larger the catchment is, the more frequent the PMP is. Here is the relationship between catchment size and return period from Book 8 of Australian Rainfall & Runoff.

Here, it is important to note that there is some debate over whether or not an AEP can be assigned to a PMP/PMF, AR&R states:

“It should be recognised that there is considerable uncertainty surrounding these recommendations as they are for events beyond the realm of experience and are based on a limited body of information.”

However, applying the “recommended” rates from the above graph to my rough measurements of catchment sizes from aerial contours, you get the following approximate return periods for the PMFs in the Sydney Basin.

Again, this is from an hour’s work on the back of an envelope and should not be used for anything other than this discussion, but you can see from the above that there would be a considerable range in the return period for the various catchments in Sydney.

Now let’s take the Hawkesbury Nepean Catchment as an example, with a PMP return period of around a 1 in 50,000 chance per year (it is the largest catchment and most likely PMP rate from my list). If we look at some historical milestones and see what the statistical likelihood of a PMP occurring over this time period has been, it may help contextualise things.

Hopefully this gives you a high-level (very high-level!!) appreciation of some of the terminology, what it all means and how it might vary based on your circumstances.