Only Young Water Professionals Need Attend
G McMahon
Publication Date (Web): 7 May 2018
DOI: https://doi.org/10.21139/wej.2018.024

A ‘roadshow’ of flood engineers in the early 1980s ‘prophesized’ the flooding that occurred in South East Queensland (SEQld) during 2011 to 2015. The State Government engineers met frequently at Committees set up for principal estuarine development proposals at different centres within South East Queensland. The period 2013±2.5 was suggested as the timing of the next '1974 flood'. 

The events of 2011 to 2015 may have given increased credibility to a long-held notion that SEQld may be subject to a 40-year flood cycle. This paper examines the data on flooding within the ten major SEQld catchments, bounded by three mountain ranges and comprising more than 63,000 km2. The purpose was to test the strength of any regularity to flooding in this well populated region

For each catchment, a number of the largest floods were identified, the number depending on the length of flood records and the quality of other flood data. The most downstream source of records with a viable record of flooding, within a catchment, was preferred.
The percentage of ranked floods captured by the Five-Year-Target-Periods [FYTPs] 1890-94;1930-34; 1970-74; 2010-14 was identified.

FYTP captures 100% of those flood events of sufficient size that they are ranked in more than one of the subject catchments. 

In three out of the four FYTPs, two years of the five-year FYTP featured at least one ranked flood.

Of the sixteen events given ranking, seven occurred in January seven in February, and two in March. The six largest of floods came three in January and three in February. This may be a lead to some physical explanation. Additionally, any explanation that emerges as to why the FYTPs are capturing the largest floods could be confirmed by an associated explanation as to why so many major floods occurred during 1942 to 1968 (6 ranked events) or during the eight years 1947 to 1955 (4 events).

The capture, by 12.5% of the record, of 80% of ranked floods, and of 100 % of the largest of floods, merits a rethink on flood frequency analyses. The conduct of a flood frequency analysis just for the twenty years of the combined FYTPs may assist the engineering profession and its clients in appreciating the true flood risk faced in South East Queensland. 

The forecast made by the ‘roadshow’ appears to have been supported by this study. Confirmation of the basis for that forecast may require an explanation for the results obtained, a physical explanation about which the statistics can then inform.

There appears to be an 80% probability that the next very large flood event in South East Queensland will occur during 2050-54, and a 60% probability that two or more significant events may occur in that five-year period. 

The pattern of flooding identified by the study might better be described as ‘irregular’ rather than ‘regular’, but not as ‘random’. 

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