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 December 2009

The Underlying cause of our depleted dams had the appearance of "drought". However most people would not class 80 percent of rainfall compared with the Bureau of Meteorology long term average 1961 to 1990 as a "drought". This is the case in the catchments. Even less people would class it as a "drought" when closer examination of the rainfall shows that the inflow producing four Summer months had been quite normal with 99.7% in the Wivenhoe and 91.3% in the Somerset catchments. The 20% deficiency was in the low inflow producing eight non-summer months.

This incorrect conclusion of "drought" has clouded the real underlying cause of our depleted dams. It is the random nature of large scale rain depressions and they are our main water supply. They occur on average every 4 years and have the capacity to fill our dams in a few days. The last gap was six years 2001 to 2007 and caused a major and expensive panic. Further evidence is available in the tab DEPLETED DAMS or Click here

Dealing with those large scale events that extend beyond the 4 year average requires a very different approach than dealing with a "drought". The current water resource strategy has ignored the short history of the Wivenhoe dam.

May 2009: The Environmental Impact Study has created problems. Public advice is that it may be up to several years before the project is suitable for processing by the Federal Government.

This section is best read in conjunction with the "Mary V Rain pattern"

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The Traveston Dam proposal

Various reports indicate that the proposed Dam will operate in 3 stages.

Stage 1. Prudent yield of 70,000ML annually converting to 191ML per day. Planned capacity of 181,000ML. Cost estimated $ 1.7 billion with an additional infrastructure of $900 million. Completion date 2011.

Stage 2. Raising of the Borumba Dam. Additional yield of 40,000ML converting to 109ML per day when operated in conjunction with stage 1. No cost supplied. Completion date 2025.

Stage 3. Raising of Stage 1 with an additional 40,000ML converting to 109ML per day. Planned capacity of 660,000ML. Construction to follow Borumba Dam. May not be completed until 2042.

Total annual yield of 3 stages 150,000ML converting to 410ML per day.

Catchment area

I have had difficulty in obtaining an accurate definition of the proposed catchment area. There has been recent publications that suggest it is 2000 sq Klms including the Borumba Dam catchment.

The Bureau of Meteorology contributes that the catchment of the entire Mary Valley is 7,000 sq klms.

Deputy Premier Ms A. Bligh is reported in the Gympie times on the 4th April 2007 as stating that the catchment is about one third of the Wivenhoe/Somerset catchment. The Wivenhoe is defined by SEQWater as 5,554 sq klms and the Somerset as 1,503 sq klms making a total of 7,057 sq klms. That calculates to 2,352 sq Klms. 

One other contributor to the Senate enquiry put the catchment as 2,110 sq klms.

Both of these assessments include the Borumba Dam with a catchment of 466 sq klms.

Comparison of catchment areas

We will see that catchment areas are being used for comparison by our leaders. It is important to keep in mind the place that the Wivenhoe Dam holds. Its catchment is three times as large as the Somerset but only one third as efficient. This was measured with significant inflows of the summer months of year 2004 with the rare occasion of similar rainfall in quantity and timing. In lesser rainfall months there is insufficient rain to make the widespread creeks flow.

Therefore the Wivenhoe catchment can be taken as equal to the Somerset catchment. You have also seen in the "Wivenhoe" button that the Somerset receives 30 per cent more rainfall than the Wivenhoe.

The Wivenhoe Dam catchment is very much a minor player in the provision of inflow into the dams even in "uncommon events".

This point is not understood as revealed by documents before the Senate enquiry comparing the Mary Valley rainfall with the Wivenhoe rainfall. They include the Qld Government. We have recently seen this common error resurrected in the EIS report.

Comparison of inflows into the ''Dams"

The statement by Ms Bligh in the 4th April article that the Traveston receives more than 40 percent more rain does not specify this year or any period and is therefore not verifiable. We have seen in the "Mary Valley" button that the Traveston catchment does receive more rainfall than the Somerset but is marginal in the years that do not include "uncommon events" and overall is somewhat around 10 percent. Uncommon events, of course go over the top of stage 1 and are not held.

The Mary Valley catchment Summer rainfall chart is again produced to examine the rainfall for the last 46 years. The Summer Rains have been demonstrated consistently throughout this web-site to be the main provider of inflow into the Dams. Where there is a variation, it is noted as in this chart.

This chart has been rearranged into rainfall order to assess what happens against the benchmark "calendar" year of 2006 with a mark of 387mm for the Summer months.

The save the Mary Valley Committee

This committee is organised and my observations are that they are quite adequately dealing with assessment of the consultants. They have accurate data and sophisticated computer models that contradict the official conclusions. 

The direction of this web-site

This proposed Dam is part of the Emergency legislation and as it involves the future, it attracted my interest.

Because of similar rainfall patterns it could not be expected to provide for long stretches between uncommon events. It will eventually fail as the Wivenhoe/Somerset system has failed. The Grid system will hold the situation for the time being until an uncommon event but with water restrictions in place.

The only possible long term solution is the Borumba Dam expanded to 2 million ML and the surplus water in the Wivenhoe/Somerset system that now goes over the spillway transferred to it. You will eventually see in my submission on the EIS alternative methods proposal that I have obtained Engineering assistance and it is feasible.