Author : J. V. Hodgkinson F. C. A. Chartered
Accountant : Aug 2006 to November 2013
The principal thrust of this
This is my review based on official
statistics and documents. It is done in conjunction with Ron McMah, grazier
of Imbil and Trevor Herse, retired of the Gold Coast
May 2009 : This section has become redundant as I recognized the the Engineers GHD had already determined the 80,000ML\a based on HYNF which means that a great deal more than that is available
Redundant section retained for those that have the time to review my elementary calculations with those of Engineers GHD whom I finally located in their report on the "Augmentation of the Wivenhoe Dam" - by raising the dam wall.
Since laying out this section I reviewed a desktop report by the Government appointed Engineers GHD. In my view, the report supplied contributing evidence in their section on the Augmentation of the Wivenhoe Dam. It is contained on page 9 under the tab "Final Solution & EIS" They reviewed the Department of Natural Resources and Water, DNR & W, hydrological report of December 2005. This indicated an annualized surplus of 80,000ML and it largely confirms these calculations.
What is markedly different in this approach to water is the isolation of Uncommon events from normal average Summer rainfall.
It is the timing that has caused incorrect decisions in the past and present and may yet cause us anguish in the future. They have been isolated in the flow schedules that appear in this section and rebalanced against expected stated yields by various Qld Govt Departments.
Most important contribution
The most important contribution to the situation on water was published by the Courier Mail on the 10th of February 2007. The information was provided by the professionals at SEQWater, the Dam managers. The article directed us to two matters. The first is that it is the four summer months December to March that provide the high rainfall necessary to provide inflow into the Dams. The second is that it takes "uncommon events" to fill large Dams. We saw from the first matter that the Dams had been receiving normal Summer rains for the six years to 2006 and that the reduction in rainfall of 20% existed in the non-productive non-summer months that have no impact on the Dams.
Same rainfall pattern and timing in the Mary Valley
The course that we are currently on to "drought proof" and "secure our water supply" ultimately involves the Traveston Dam near Gympie. It is in the Mary Valley. The Mary Valley has almost exactly the same rainfall pattern as the Somerset and Wivenhoe Dams. This means that the timing is the same. Although the Mary Valley has a higher rainfall, the same result that we are now seeing will occur. The Mary Valley will have no flood retention until 2035 and even then only partial.
Uncommon events and surplus water
We have seen in this web-site that "uncommon events" are aptly named as they are uncommon. They appear in tight groups many years apart. When they occur they can be immense.
If you think about it, the most recent in our memories is the period of the filling of the Wivenhoe Dam. The Dam commenced from scratch in 1986 to overflow in the years 1999 and 2001. The capacity of that Dam is 1,165,000ML. Add to that 190,000ML being one half of the capacity of the Somerset. In addition my estimate of overflow from BOM flood information is around 500,000ML.
The calculation is : if the Wivenhoe dam had been in operation in 1986 and was at the same level in 2001 as it was in 1986, it devolves to a total of 1,855,000ML of surplus water would occur. This is over and above the total requirements of most of the current population on a "business as usual" basis. Enough to fill the expanded Borumba Dam of 2,000,000ML. The Borumba Dam was not idle. The Dept of Natural Resources recorded that 908,464ML was released over the spillway in the same period.
SEQWater reported in the above article that the Dams had four uncommon events in that 16 year period. That is rare in itself. However you would have seen that the 1974 period was made up of four years of high rainfall culminating in the 1974 January flood. It does not require calculation to see that all Dams including the expanded Borumba would have been full. Keep in mind that the Wivenhoe and Somerset Dams flood retention has been expanded to 1,950.000ML facilitating pumping requirements.
Previous uncommon events
Previous events in 1955/1956, 1927/1928, 1908 had a lesser impact. The twelve year period from 1887 to 1898 saw two floods equivalent to 1974, two other major floods and the well known 1893 flood which was three floods in the space of a few weeks. While the information originates from the Bureau of Meteorology, the flood heights attached in the "Solution/Base" section are at Brisbane City. Some floods are recorded in the upper reaches of the Brisbane River that do not rate highly in this recording and vice-versa.
Impact of uncommon events on timing and volume of surplus water
To observe the interaction of "uncommon events" with the Dams and population in their current format, I have provided three "cash flows" type schedules on water inflows and use. Like all "cash flows", they have been examined in total for sensibility from reliable data. Their construction base and method has been dealt with in detail in the "Solution/Base" button.
All three flows include the full annual requirement of 150,000ML required of the three stages of the Traveston Dam. In the third flow it is progressively introduced.
The first deals with the 108 years from 1898 to 2006 on the basis of the three dams in existence and current population. The Dams would have been full in 1898. You will see that it is conservative on two counts. The yield of the uncommon events at 98GL annually is below the stated yield average of 183GL in the first 70 years to 1974 and the yield of 197GL for the period 1974 onwards at 197GL compares with the average yield of 183GL. The evaporation is taken into account when considering the yield by Qld Govt sources. I have added an additional evaporation rate for caution.
the same period with a start of 14% or 500GL to see how it performs over the difficult
70 years period to 1974. Evaporation already taken into account in
the yield is eliminated.
There is little doubt that the provision of stable water supplies from the "Grid" will hold the current position with normal Summer rain and excluding uncommon events.
It is harnessing these uncommon events that is the key to securing our water supply well into the future.
These calculations show that the expanded Borumba Dam is a superior method than the Traveston Crossing Dam when considering the future. I understand that engineering advice says it is feasible to the 2,000,000ML capacity and beyond.
It is the move forward that should be ruthlessly examined including all of this web-site. It is critical for people of the Mary Valley and for many generations of all our South East Queenslanders.