AN OBSERVATION OF THE CONFLUENCE OF STATISTICAL ABERRATIONS, DAM RATIOS, TIMING AND UNCOMMON RAINFALL EVENTS THAT COMBINED TO PRESENT OUR DAMS AS VICTIMS OF A SEVERE DROUGHT. THIS IS NOT THE CASE. THEIR INFLUENCE ON CONCLUSIONS REACHED AND ACTIONS TAKEN.

Author : J. V. Hodgkinson F. C. A. Chartered Accountant
August 2006 to April 2008

  UPDATES : November 2008. This website is maintained as it was in April 2008. Currently the Dam levels are around 40% and, with the addition of the Grid, should be maintained around that percentage. The next "uncommon meteorological event" will fill the Dams to overflow. This event will show that that the actions of "uncommon events", being Monsoons, Cyclones or Large Rainfall Depressions that generally cover all of SEQ, is again misunderstood and that they are the major source of our water supply. Our current storage capacity is insufficient to deal with them. The historical fundamental flaw of the cancellation of the Wolfdene Dam (1989) is being repeated in reverse at Traveston Crossing in the Mary Valley. See "Home" page for further updates.

The Wivenhoe Dam is the largest Dam in Queensland. I am indebted to an interested reader for allowing these personal photographs to be published.

Dam under construction Circa 1981.

The relationship of the Wivenhoe Dam to the Somerset Dam.

You will see from the attached contour map that Somerset is on the Stanley river. The Wivenhoe is on the upper reaches of the Brisbane River. The Somerset sits atop of the Wivenhoe and releases water into the Wivenhoe. The reverse does not apply. The Wivenhoe releases to the lower reaches of the Brisbane River.

Relevant statistics are : The Catchment of the Somerset is 27.0 percent of the size of the Wivenhoe. The Dam capacity of the Somerset is 32.5 percent of the Wivenhoe.

The annual rainfall of the Somerset is said by SEQWater to be 1,230 mm and 940 mm in the Wivenhoe. The Somerset therefore receives 30.8 percent more rainfall. These statistics are also at the "Dam Statistics" button. My annual rainfall calculations differ from those of SEQWater but they both agree that the Somerset receives 30 percent more rainfall. This is further evidenced by the charts accompanying this web-site. See " Initial statistics" button.

2004 Summer Rainfall and the efficiency of the catchments.

The 2004 summer rainfall permitted the assessment of the abilities of these two catchments to provide inflow into the Dams. On this infrequent occasion, the rainfall in both Dams were similar in volume and timing of fall.

These two SEQWater charts show the rise in Dam levels for the four months December 2003 to March 2004. Also attached is a summary of every Rainfall Station in the Dams and their collective totals for each month. It shows that the rainfall that occurred was widespread and covered the catchments of both Dams. The exercise shows that the Somerset catchment, although only 27 percent in size of the Wivenhoe, is 300 percent more efficient in producing inflow into the Dams.

The net result is that the same amount of rainfall in both Dams will produce almost the same inflow into the Dams.

Wivenhoe Dam contribution to our water supply.

The first aspect of this assessment is to confirm the advice of SEQWater for events greater than 300mm. The two graphs covering the years 1910 to 2006 are attached. You will see that in the years 1964 to 2006 there was one event. In the years 1910 to 1964 there were no events over 300mm. I dropped the requirement to 250mm to get some measurement. There was, however,  rare non summer months of May 1996, mentioned below, and April 1988 and April 1989 when the 300mm was exceeded by some but not all of rainfall stations in the catchments.

What is quite clear is that the Somerset achieves the requirement of 350mm in tight groups many years apart.

The amount of water supply that the dam provides is discussed later in this section.

The Summer rains in the Wivenhoe for the period 1910 to 2006 support the foregoing.

Analysis of uncommon events in conjunction with the worth of the Wivenhoe Dam to our water supply.

This is a summary of major events that occurred in the first 16 years of the Wivenhoe Dam's life to February 2001. The current period is also shown with any interesting events though minor in comparison.

It shows that in the really big events, the rainfall in the Somerset approximates 50% or more than in the Wivenhoe. It is not for me to calculate the inflow that would occur in these circumstances. However I am of the opinion that this would calculate out well in excess of 100% more inflow.

Again there is a rare exception. The month of May 1996 saw a major rainfall event that produced more rain in the Wivenhoe than in the Somerset. This produced significant inflow into the Wivenhoe of around 32 percent according to the BOM history of floods. I note that the average for the catchment was 285mm with three rainfall stations over the 300mm mark. As there was no releases from the Somerset involved, it can be taken as an accurate assessment of how much rainfall is required to refill the Wivenhoe by its own resources.

An overall view

Viewed together, the long term graphs support the evidence that the Somerset is by far the major supplier of water to our system.  

Estimates of inflow based on normal average rainfall.

 We saw that in the six years 2001 to 2006 it produced an inflow into its Dam of 10 percent in the Summer months of year 2004. February 2003 with an average of 173mm and October 2005 with an average of 176mm would have produced approximately the same as the four months for the 2004 year. October 2005 appears to have produced 5 percent on the SEQWater chart. The 2001 to 2006 Summer period received 99% of the Normal summer rain in the Wivenhoe.

May 1996 is not only rare because it is an uncommon event, it is the reverse of normal uncommon events in that there is significantly more rain in the Wivenhoe. It has been excluded from these calculations. It is a significant aberration and to include it would distort the object of the calculation.

With normal rainfall for the six years producing around 20 percent inflow, it converts to 70 percent inflow of its own resources for the 21 years of its life. That converts to around 110 ML a day towards our water supplies. Major uncommon events are not included.

Flood mitigation and water collection from the Somerset.

Those of us of a reasonable age will, when prompted, recall that the Wivenhoe was a flood mitigation dam. This is not an accurate description. The Dam is both a Storage Dam and a flood mitigation Dam. The capacity of the Dam is 1.12 million ML with the flood holding area of 1.4million ML above normal supply level. It comes into it own in "uncommon events" when it collects major inflows in its own right as well as from the Somerset.

We saw above that the size of the Somerset Dam was only 32 percent of the Wivenhoe. It cannot cope with the much higher rainfall and passes the water to the Wivenhoe. The 1992 and 1999 uncommon events that rivalled the 1974 Flood passed us by with little notice in Brisbane.

These flood waters held by the Wivenhoe Storage are available for our water supplies. They have been drawn upon for the last six years and are below 20 percent of late. With the considerable flood compartments mentioned above, almost all flood water can now be contained and held in reserve in the expanded Borumba Dam of 2,000,000ML.

The Dam also acts as a collector of surplus water from the Somerset from time to time.

Conclusion

The Somerset Dam is by far the dominate partner in the provision of our water supply.

Occurrences in Somerset Dam are the major influence on our water supply. The Somerset figures have been used to un-complicate the viewing of the overall graphical position.