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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The Traveston dam is designed to permit the overflow of large scale meteorological events. Refer to Hydrology Mary Valley

Currently the Mary River has ceased flowing above the dam site (CM October 2009). The Mary River mirrors the Stanley River which is our main water supply. It has the backup storage of the Wivenhoe dam and even that dam is proving inadequate to control major events.

It is my view and others that the 85% requirement of all water that flows in the Mary River must reach the River mouth is made up almost entirely of flood water. We now know for certain that flood water is damaging to the eventual point where it enters the sea.

Here again, the use of the "mean annual flow" includes all of this flood water. It is not difficult to envisage a distortion to percentages when most of the water is flood water and especially the very high rainfall events inclusion.

The following give one an appreciation of how our main water supplies fall and why they are our main water supply.

Pattern of Rainfall

The Mary Valley is the site of the proposed Traveston Dam. Its catchment is adjacent to the Somerset Dam catchment. Its rainfall operates in the same way as the Somerset. A comparison of the rainfall of the two catchments is attached. 

River heights

There are no inflow figures as there is no Dam. However the Bureau of Meteorology do have the River heights at Gympie located below the Dam. They effectively replace inflow assessments. In addition their web-site records the major floods and it is also attached. These majors date back to 1893 and are mirrored by the "uncommon events" of the Somerset with the exception of 1968.

Photos : Bureau flood heights at Gympie and Maryborough together with major flood heights at various location on the Mary River

You will have noticed that the Bureau labels the "floods" Minor, Moderate and Major. It also gives the River height at Gympie. It is therefore possible to match the rainfall with the River heights.