Investigation into Probabilistic Losses for Design Flood Estimation: A Case Study for The Orara River Catchment, Nsw

Abstract

Australian Rainfall & Runoff (Pilgrim, 1987) recommends the design event approach (DEA) as the preferred method for estimating design flood hydrographs, in which a single design event is adopted. More recently, Monte Carlo simulation has been used to allow for the probabilistic nature of input variables in flood modelling. This paper adopts a Monte Carlo framework to evaluate the impact of probabilistic losses on design flood estimates for the Orara River catchment in northeastern NSW. A RORB runoff routing model was used to derive loss values for both the initial loss-continuing loss (IL-CL) and initial loss-proportional loss (IL-PL) models. It has been found that the initial, continuing and proportional losses can be approximated by the Gamma, Weibull and Beta distributions, respectively. When these distributions were compared with non-parametric distributions, differences in the flood estimates were found to be minimal. Another finding was that peak floods estimated using the DEA were more biased for the IL-CL model, than for the IL-PL model. In comparison to the at-site flood frequency curve the IL-CL model produced an overall better fit of the shape of the curve, however, the IL-PL model provided a better fit to the observed flood peaks for mid-range events.

Key Words:

• Monte Carlo simulation
• probability distribution
• initial loss
• continuing loss
• proportional loss
• RORB
• Australian Rainfall & Runoff.

Additional information

Notes on contributors

M Loveridge

Melanie Loveridge is currently in the final year of her PhD research degree at the University of Western Sydney. She holds a Bachelor of Engineering (Civil) from the same institution, where she graduated with first class honours in 2010. Her current research focuses on the analysis of rainfall losses in hydrological models with regards to uncertainties, trends and regionalisation. She is particularly interested in evaluating the impact of these uncertainties on flood risk using a two-dimensional hydraulic model. Melanie has also recently been involved as a graduate engineer in Project 6 (loss models for catchment simulation) of the Australian Rainfall and Runoff revision.

A Rahman

Dr Ataur Rahman is a Senior Lecturer in Water and Environmental Engineering in the University of Western Sydney. He has over 20 years’ experiences in water industries, research and universities in Australia and southeast Asia. He obtained his BScEng degree from Khulna University of Engineering and Technology in Bangladesh, MSc (Hydrology) degree from National University of Ireland Galway, and PhD degree in Hydrology from Monash University, Australia. His research interest includes flood hydrology, urban hydrology and environmental risk assessment. He received The G. N. Alexander Medal from the Institution of Engineers Australia in 2002. He has published over 200 research papers, reports and book chapters in water and environmental engineering field. He is acting as Project 5 Leader (Regional flood methods) in the forthcoming revised version of Australian Rainfall and Runoff (ARR). He has also made notable contributions to ARR Project 1 Development of intensity-frequency-duration information across Australia and ARR Project 6 Loss models for catchment simulation.

P Hill

Peter Hill has 18 years’ experience in flood hydrology. Since joining SKM in 1996 he has worked on a wide range of flood hydrology and dam safety risk analysis projects. He has been involved in hydrologic modelling for more than 100 floodplain management studies and spillway adequacy assessments across Australia. He has recently been involved in a number of studies arising from the 2011 floods in Victoria and Queensland including review of operations of dams for the Victorian Floods Review, review of flood operations procedures for G-MW and assisting Seqwater respond to the Queensland Floods Commission of Inquiry. Peter is currently on the ANCOLD Committee reviewing the Guidelines on Consequence of Dam Failure and is a member of Engineers Australia’s National Committee for Water Engineering. He has authored more than 60 papers and has been awarded 4 prizes for technical presentations.

M Babister

Mark Babister (BE, MEngSci) is the Managing Director of WMAwater. He is a civil engineer with over 25 years’ experience in hydrology and flood estimation. Mark is project manager of the Australian Rainfall and Runoff (ARR) project and was joint editor of the ARR project that prepared the national guidelines on twodimensional hydraulic modelling. Recent projects have included independent expert advice to the Queensland Floods Commission of Inquiry and the Natural Disaster Insurance Review.