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 January 2012    

FLOOD PROOFING BRISBANE from damaging floods to the point of extinction. MITIGATING flooding in Ipswich and Gympie. Putting REAL MEANING into "Drought proofing SEQ"

Preliminary view of our main water supply. To emphasize its importance it appears at the heading in all pages in this web site 

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. 

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.
 
WEB address: wivenhoesomersetrainfall.com
All comments welcome : hodgkin@powerup.com.au

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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.