AN OBSERVATION OF THE CONFLUENCE OF STATISTICAL ABERRATIONS, DAM RATIOS, TIMING AND UNCOMMON RAINFALL EVENTS THAT COMBINED TO HAVE OUR LEADERS DRAW INCORRECT CONCLUSIONS. THEIR INFLUENCE ON ACTIONS TAKEN.

Author : J. V. Hodgkinson F. C. A. Chartered Accountant : August 2006 to May 2010    

The relationship of the Wivenhoe/Somerset dam system to its rainfall in the catchments has been quite different from the way it has been presented to us since the joint dams’ inception in 1986. The main rainfall supplying our water is the, season ignoring, rain depressions in the form of Cyclones, Monsoons and large scale rain depressions. They generally cover the whole of South East Queensland at the same time. They provide much more water than our December to March “wet season” which is well known to us.

Mr Rob Drury of SEQWater drew our attention to this in his Courier Mail article of the 10th February 2007. Output from the Government IQQM computer model, covering the 111 years 1890 to 2000, has proven Mr Drury correct. In essence 300mm of rain in the catchments in a few days is a flood and, in the right circumstances, can fill the dams from scratch. On the other hand 3 months of 100mm a month is a comparative trickle. These facts can be buried in official overall rainfall statistics the use of which in dam performance can and did throw up statistical aberrations.

Mr Drury called these rainfall events “uncommon events”. Bureau of Meteorology flood records from 1841 reveal that they occur on average every 3.7 years with most below that average. Therefore those above can extend well above that average such as the recent period of 2001 to 2007.

Mr Drury also provided dam level charts which were not published by the Courier Mail. They provided clear evidence of the importance of "uncommon events" and the inability of the "Summer wet season" to provide sufficient water from as far back as 1992. We will examine them.

This is my review based on official statistics and documents. These statistics and documents are presented to you so that you can arrive at your own conclusions.

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SEQWater has provided in recent years the releases from the Wivenhoe/Somerset dams. This graph was in a period where there were no overflowing flood water. The releases appear to cover both our requirements and the River's 66% although it did not become law until March 2007.

Photo : SEQWater releases from the Wivenhoe/Somerset system

QWC information is that 280,000ML has been allocated from the system for our consumption. This calculates out to a little above 23,000ML monthly.

The releases seem to indicate that the River was ignored. 

On the basis of their calculations of 34% for our consumption and 66% for the river and ignoring that the yield is 360,000ML and taking the allocation of 280,000ML, the following should have been the releases:-

280,000ML divided by 34% X 100% = 823,529 annually or 68,627ML monthly. 

That is the way the law is written and it makes no allowances for the sometimes large overflows that occur at flood time. 

Surprisingly, the last major "uncommon event" in May 2009 which had the Premier declaring the "drought" over, had only minor engagement with the catchments. The comparatively small rise of 16% in dam levels did not overflow the dams whereas a full engagement of an event that size would have overflowed the dams for some time. The impact on Moreton Bay, while severe, did not have the contribution of the Brisbane River's major tributaries.

The inclusion of the major floods of 1890, 1893 and 1974 in their calculations based on years 1890 to 2000, have the ability to present a major statistical aberration.

For example, major floods of 1890 (5.3 metres), 1893 (8.5 metres) and 1974 (5.4 metres) are included in this calculation. Floods of that size have the capacity to distort percentage calculations presenting a statistical aberration for annual requirements. It also has the element of a short gap of 3 years and a long gap of 81 years. The current gap is 35 years and continuing. The Brisbane River heights used are Brisbane City heights.

Photo : Combination of Bureau rainfall data and confirming Brisbane City River heights highlighting all major large scale rainfall events.