Australasian Journal of Water Resources ISSN: 1324-1583 (Print) 2204-227X (Online) Journal homepage: http://engineersaustralia.tandfonline.com/ loi/twar20 Large floods in South East Queensland, Australia: Is it valid to assume they occur randomly? Greg M. McMahon & Anthony S. Kiem To cite this article: Greg M. McMahon & Anthony S. Kiem (2018): Large floods in South East Queensland, Australia: Is it valid to assume they occur randomly?, Australasian Journal of Water Resources, DOI: 10.1080/13241583.2018.1446677 To link to this article: https://doi.org/10.1080/13241583.2018.1446677 Published online: 18 Mar 2018. Submit your article to this journal Article views: 610 View Crossmark data Full Terms & Conditions of access and use can be found at http://engineersaustralia.tandfonline.com/action/journalInformation?journalCode=twar20 AUSTRALASIAN JOURNAL OF WATER RESOURCES, 2018 https://doi.org/10.1080/13241583.2018.1446677 Large floods in South East Queensland, Australia: Is it valid to assume they occur randomly? Greg M. McMahona and Anthony S. Kiemb aG.M. McMahon Consultants, Brisbane, Australia; bCentre for Water, Climate and Land (CWCL), Faculty of Science, University of Newcastle, Callaghan, Australia ABSTRACT ARTICLE HISTORY Flood frequency analysis and existing engineering practice assumes that flood events are Received 2 June 2017 randomly distributed. However, as understanding of large-scale ocean-atmospheric processes Accepted 26 February 2018 and associated hydroclimatic variability (and change) increases, the assumption that flood KEYWORDS events are random seems unlikely to be valid. This paper examines historical flood data from Non-stationarity; flood risk; 10 major catchments in South East Queensland (SEQ), Australia with the aim of testing for non- climate variability; climate random patterns in the timing of the largest flood events. The results show that most (~80%) change; flood cycles large floods in the study catchments have occurred within sets of 5-year periods separated by 35 years of lower flood risk. This finding is consistent with previous studies that have reported 20- to 40-year cycles in rainfall, flooding and drought across most of eastern Australia. Based on these insights, an explanation for the nominal 40-year cycle in SEQ flooding is proposed, and implications for describing and managing flood risk are discussed. 1. Introduction (Advisory Body Responsible Authority Review of 1.1. Background Environmental Impact Assessment Study Pine Waters Canal Estate 1983–1984), Brisbane River and its trib- The first half of the 1970s was a time of many severe floods utaries (Approvals City of Brisbane (Flood Mitigation in South East Queensland (SEQ; Figure 1) (Middelmann, Works Approval) Act 1980–1986), and Nerang River Harper, and Lacey 2000; Bureau of Meteorology 2014). (Joint Technical Steering Committee of Nerang River That decade was followed by extensive floodplain studies Flood Study 1984–1987). This experience of analysis and and development projects in the 1980s. A small group of evaluation of the hydrology of these catchments pro- engineers from Queensland Government departments, vided a regional, multi-catchment perspective to the modelling laboratories and principal consultants in hydrology of SEQ (McMahon 2011, 2012). From this Queensland steered and conducted most of these flood experience and perspective, the Roadshow engineers studies and development projects. Meeting each other noticed that three large flood events had occurred about as frequently as they did, at different river, creek and 40 years apart: estuarine communities throughout SEQ, the group came to refer to itself as ‘the Roadshow’. • February 1893, the largest on record for Brisbane Floodplain development approvals required favour- River; able recommendations from regulators in several • February 1931, the largest on record for Pine River government departments which were organised by a and also large for Brisbane River; committee set up by the former Coordinator-General’s • January 1974, the largest on record for Nerang Department. The committee system of coordinating River and also large for Brisbane River. the response by regulators to floodplain development In the 1980s, based on the idea that large floods in SEQ proposals was also used to evaluate modelling con- follow a nominal 40-year cycle with sets of 5-year period ducted by the developers and their consultants. Such of high flood risk separated by 35 years of lower flood studies and reviews included were conducted for catch- risk, some of the Roadshow speculated that: (i) the next ments including the Maroochy River (Maroochy River ‘1974 flood’ could come in 2013, plus or minus a few Flood Study Technical Supervising Committee 1980– years (McMahon 2011, 2012); (ii) there would be a Royal 1985), Mooloolah River (Mooloolah River Hydraulic Commission the next time a ‘1974 flood’ occurred; and Model Technical Committee 1976–1983), Pine River (iii) some Roadshow members would still be in practice CONTACT Anthony S. Kiem [email protected] © 2018 Engineers Australia G. M. MCMAHON AND A. S. KIEM 2 Figure 1. The SEQ study area (approximately indicated by the orange box) and the 10 catchments analysed in this study. and required to stand before such a commission. This of accredited scientific opinion, he was widely rec- Roadshow speculation was realised with large SEQ flood ognised for his successes, especially by farmers and events in January 2011 and January 2013 – with the 2011 other practitioners (http://adb.anu.edu.au/biography/ floods resulting in a Queensland Government Flood jones-inigo-owen-539). Commission of Inquiry (QFCI, www.floodcommission. The principal international hydrology textbook for qld.gov.au/). This suggests that the occurrence of large the 1970s and 1980s, Chow (1964, 8–11), mentions floods in SEQ may not be random – as is assumed in ‘periodicity’ and defines it as a regular or oscillatory existing engineering practice and floodplain develop- form of variations in hydrologic phenomena. Studies ment policies. Research conducted before and after the were quoted where technically trained professionals had 1980s Roadshow speculation reinforces the suggestion detected regular cycles in hydrological data provided to of non-randomness in flood occurrence (refer to Section them (Chow 1964, 9–46). The 11-year sunspot cycle was 1.2 for further details). the only cycle that enjoyed some acceptance regarding an explanation for this periodicity, but Chow (1964, 40–46) 1.2. History stated that quantitative investigations into the relation- ships between sunspot cycles and this hydrological peri- Given scientific understanding and engineering meth- odicity failed to demonstrate any statistical significance. odology and guidelines in the 1980s, thinking a hydro- Whether this means that there is no causal relationship meteorological phenomenon like flooding could be between sunspot cycles and hydroclimatic variability on non-random may have seemed radical at worst or unu- Earth, or that the statistical testing methods used were sual at best. So, was the Roadshow alone in its idea of inadequate, remains an open research question (Franks physically based regularity of large SEQ flooding? 2002; Gray et al. 2010; Kiem 2014). Despite the physical Clement Wragge (1852–1922), commonly regarded mechanisms and causal relationships being a matter of as an eccentric polymath, set up an extensive network controversy (a scientific debate which continues today), of weather stations around Queensland in the 1880s Chow (1964) accepts that the occurrence of hydrologic and 1890s. Using data from these weather stations, and extremes is not random, quoting the work of Hurst, training provided by Wragge, Inigo Jones (1872–1954) Black, and Simaika (1965) on the 1300 year (622–1921 developed long-term weather forecasts based on the 11- A.D.) Nile River flood record. Hurst, Black, and Simaika and 22-year sunspot cycles. Regarding flooding, Jones (1965) noted that both wet and dry years appeared to (1931) followed the Bruckner Cycle for solar activities, occur in groups. which he determined to be 35 years long, but which has In Australia, John Ward and colleagues, from the then since been found to vary from 35 to 45 years (Raspopov Queensland Irrigation and Water Supply Commission, et al. 2000). Although Jones failed to have his methods recognised the statistical difference between large and recognised as soundly based by any substantial body common floods in Queensland and supposed there AUSTRALASIAN JOURNAL OF WATER RESOURCES 3 would be a physical driver (or drivers) for this (Ward and and Verdon 2006; Micevski, Franks, and Kuczera 2006; Harman 1972). John Ward also argued that flood fre- Kiem and Verdon-Kidd 2013; Stevens and Kiem 2014; quency distributions at a location differ because higher Verdon-Kidd and Kiem 2015) and drought risk (Kiem flows originate from a variety of different rainfall mech- and Franks 2004; Verdon-Kidd and Kiem 2009; Kiem anisms – at the time, minimal insight was available into et al. 2016). Klingaman (2012) focused specifically on what those different rainfall mechanisms were. More large-scale climate processes that influence rainfall in recent work by Klingaman (2012) provides comprehen- Queensland and explained that ENSO, IPO and the sive details on rainfall mechanisms for Queensland (see Madden-Julien Oscillation were most important but that Figure 2) and as such (i) provides useful insights into they operate on different temporal scales as illustrated