Australian Rainfall and Runoff (ARR) is a national guideline used to estimate flood risk – critical for ensuring we have working drainage infrastructure that doesn’t result in flooding of roads and houses. It includes a set of ‘design rainfalls’ – not data sets that are necessarily fabulous but are in fact: probabilistic estimates of the likelihood of a specific rainfall depth being recorded at a particular location within a defined duration. Design rainfalls are therefore not real (or observed) rainfall events; they are values that are probabilistic in nature.
ARR is used by engineers (and hydrology nerds) around Australia to provide a consistent approach to collecting, analysing and managing flood information.
However, the last updated was done in 1987 when calculations were still mostly done by hand.
Supported by the Commonwealth of Australia through Geoscience Australia, the first major update to the guidelines in nearly 30 years was published in 2016 to consider the significant technological advances in rainfall and runoff assessment since the previous edition. ARR 2016 is also based on Australian data, when previously it was based on USA data.
A two-day workshop was held in Perth this month to introduce all these ARR updates and the latest applications and attended by resident Urbaqua Engineer, Afshin Nikrouh. As expected, the workshop attracted all the major water industry players (hydrologists, civil and environmental engineers, scientists, policy makers from local and state governments, authorities, land developers and consultancies).
The first day of the workshop included an introduction and overview of ARR, concepts and methodology, changes since 1987 and implications of the changes.
New IFDs and New Approaches in Runoff Routing
Using local data, new design rainfall intensity-frequency-duration (IFD) systems are now provided by the great collector of climate data, our beloved Bureau of Meteorology (BoM), to better represent local conditions. ARR 2016 introduces what is known as the Ensemble and Monte Carlo approaches to better capture flood variability and move away from the simpler burst approaches. ARR 2016 also recognises the importance of climate change in flood estimation and has explicit advice on the incorporation of future climate on flooding.
Data Hub; an Interactive “FREE” Web Page for Input Data
Most of the inputs for flood estimation are spatially based. As part of the ARR updates a free tool known as Data Hub was developed that allows for easy access to the design inputs needed in Australia: http://data.arr-software.org/.
RFFE instead of the traditional Rational Method
ARR 2016 has made a major advance with the inclusion of a comprehensive Australia wide Regional Flood Frequency Estimation (RFFE) procedure. The ARR research group worked with state water authorities across to Australia to collect and review stream flow data to ensure its accuracy and completeness. Based on this analysis, a procedure to estimate design flood quantiles is available for any location in Australia which can be implemented with the RFFE software on the ARR website: http://rffe.arr-software.org/.
Application of ARR 2016 and new approaches
The second day of the workshop focused on design applications of ARR using the new design inputs and looking at comparisons of simple, Ensemble and Monte Carlo simulations (simple, Ensemble and Monte Carlo simulations are all rainfall-based procedures to estimate design floods). The Ensemble method considers variability in temporal patterns while the Monte Carlo is a more complex method which allows a better understanding of variability of model processes. Selection of an appropriate method depends on the availability of data and the design context, often more than one method can be used to increase confidence in results.
Design inputs for catchments can now be produced through the ARR Data Hub online service. The design inputs available at Data Hub include IFDs, losses (suitable for rural areas), Areal Reduction Factors, pre-burst rainfall, temporal and spatial Patterns, baseflows, and climate change conditions.
In summary, ARR 1987 was once a very valuable document that provided quality advice for flood estimation for many years. However, the new version of ARR provides significantly greater reliability and more accurate flood estimation. In other words:
……..ARR1987 is dead! Long Live ARR2016!