2022 – The effect of a changing climate on dam safety in North Queensland – changing design flood hydrology

Ryan Gray – GHD, Cairns, Nick Thomas-Kinsella – GHD, Townsville, Daniel Agius – GHD, Cairns, Nimesh Rajbhandari – GHD, Cairns,

Relatively speaking, few dams in Australia were constructed after 1987. Dam safety assessments, upgrades and, recently, planning have been based on the guidelines at the time. From a flood study perspective, that translates to most of these assessments being based on the prevailing guidelines of the time, from the first edition in 1958 to the 1987 Australian Rainfall and Runoff (ARR87) guidelines. The ARR87 guidelines had a meaningful update in 1999 through Book IV, which introduced amendments to the way the PMF is calculated, changing from a standards-based approach to a risk-based focus, which was further improved with the release of the GTSMR in 2003. These were significant steps, which has led to more widespread risk-based updates recently updated in 2019 (ARR19). This update resulted in updates to design rainfall and modelling techniques that could lead to a change in the design flood estimates used in the design of existing dams. Continuing with this theme of changing design flood estimation, climate change can exacerbate the problem over the coming decades, however, whilst there is some mention of considering climate change impacts in select guidelines, there is limited emphasis in the guidelines requiring projected climate change impacts to be modelled in the design flood hydrology, which may result in a similar situation of potential under-design for a large number of structures in future decades. This may have an unintended impact when undertaken using both standards-based and risk-based approaches to dam safety, as well as cost implications to future dam owners.

Using three case studies, the flood-related dam safety requirements were assessed using three different sets of climate inputs, namely: ARR87, ARR19 and ARR19 + climate change. The outputs mainly focussed on whether there were any clear trends in the results from one set of results to another and how results yielded potential differences in future dam safety requirements.

The aim of the paper is to provoke additional thought and consideration pertaining to risks associated with a changing climate and how these impacts to DFE results could potentially affect long-term dam safety (i.e. spillway capacity and, thus, not satisfying a tolerable risk position in accordance with current standards and
guidelines). Forward actions could include an update to relevant ANCOLD and State Guidelines to consider climate change in current assessments and potential future impacts e.g. in Consequence Category, Risk Assessment and Dam Safety Management Guidelines as a means of providing more informed upgrade requirements (using the standards-based and risk-based approaches).

Finally, there could also be benefits to long-term performance and potential financial benefits through analysis of climate change impacts.