Question for GBCMA: I notice that 1:100 flood was being used. I thought the sector had moved to % chance. Is this going to change?
GBCMA response:
The accepted practice is to move away from the concept of using the average recurrence intervals (ARI), e.g., 100-year ARI flood. This is outlined in the Victorian Floodplain Management Strategy (DEWLP, 2016).
The current practice is to move to the annual exceedance probability (AEP) protocol, e.g., the chance (probability) that an event is likely to exceed a threshold or greater. For example, the 1% AEP flood will have the chance of equalling or exceedance the flood magnitude in any given year.
In terms of how to express chance (probability) are akin to “betting odds”. Following research with residence in the Nepean and Hawkesbury Catchments (NSW), the Goulburn Broken CMA has chosen to adopt the 1 in 100 AEP rather than the percentage (1% AEP). Both approaches are in terms of chance.
For large flood events rarer than the 1%, such as a 0.2% AEP becomes more difficult to understand conceptually. Under the adopted approach of the Goulburn Broken CMA this become a 1 in 500 AEP flood event.
The Goulburn Broken Community Flood Intelligence Portal will be updated to reflect the above.
Note: The below two questions were submitted via yoursay@gmwater.com.au prior to the webinar. GMW responded to the questions via email, and also referenced them during GMW’s presentation.
What happens when a large rain event such as the on 2nd December 2017 of around 100mls occurs and the dam is full and you don't have time to make pre-releases?
If you make pre-releases are the pre-releases similar to the ones we have just experienced?
Any releases we make are dependent on inflows to the storage and forecast inflows.
If GMW was subject to Filling Curve Regulations last Year when La Niña was confirmed and retained Water in Lake Eildon, only to quickly release it from 13 October 2022 causing massive River Flooding, how are they releasing Water early, when El Niño and Dry Conditions are almost certain to be confirmed, soon?
GMW develops filling targets for Lake Eildon based on inflows that we expect to receive in 95 years out of 100. We operated to the filling targets last year, and adapted by increasing water releases in late August. La Nina was not confirmed by the Bureau of Meteorology until September 2022, and Lake Eildon did not fill in the previous two La Nina years.
Again this year we have been operating to the filling targets and commenced releasing additional water in April. Releases increased in June to manage rising storage levels, and releases have reduced in the last week as the inflows have receded and the weather forecast favouring dry conditions.