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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May 2009: No change

Efficiency of the Borumba Dam catchment. Local residents observations

The local residents have long contended that the topography of the Borumba Dam and its catchment was more efficient than the total Mary Valley catchment. The Borumba Dam and its catchment are in the Mary Valley catchment and the waters of the Yabba Creek, on which it is located, joins the Mary River before the Traveston Dam crossing. We will now examine available evidence.

They are further of the opinion that the rock formations of the Borumba Dam have the ability to provide a Dam with a capacity of up to 2 million megalitres. This is almost twice the size of the Wivenhoe. Reference is Mr. R. McMah's submission to the Senate Enquiry. This is considered in the "Borumba Dam" button along with Mr. McMah's submission.

They also contend that 50mm of rain in the catchment will raise the level of current Dam by 2 meters. The current capacity is 46,000ML. We know from Somerset experience that the minimum is 100mm to create flow. It is much higher in the Wivenhoe.

All of these matters have the capacity to influence the yield of Borumba.

WARNING

These calculations are based on information provided by the Borumba Dam Managers at the time of the 1999 flood. In the area of peak discharges, they differ markedly from the Department of Natural Resources information contained in the "Borumba Dam " button.

Examination of inflow by the Borumba catchment in relation to the total Mary Valley system to the Traveston Crossing.

The Queensland Water Infrastructure Pty Ltd, a division of the Queensland Government, has publicly provided an overview of Gympie Flood Mitigation. It is dated 31st October 2006. It deals with the "Events" of 1999,1992,1989 March,1989 April and 1974.

It provides in cubic meters per second an actual hydrograph of the rainfall and over the hours that it occurred. It was just a short step to convert cubic meters to Megalitres. A sample is attached. All of these events follow later in this section.

A recent update of the proposed Traveston Crossing Dam included the attached information on the Megalitres that the Spillway had to deal with in the various "Events" mentioned above. 

The other information provided enables one to calculate the impact of the Borumba Dam. It is the information provided by an article in the Gympie Times dated 18th February 1999. The " State Water Projects Regional business Manager" has supplied "peak discharge rates per day" for the events of 1999, 1974 and 1989. He also advised that the 1999 event saw one half of Sydney Harbour go over the spillway in a single day. The hydrology shows that 2 days at this rate applied over the 120 hours and it amounts to 500,000 Megalitres.

Calculation

We now have "peak discharge rates per day" for the Borumba Catchment. In addition we have the duration of the floods and can calculate the Megalitres discharged.

The total Megalitres for the entire Mary Valley catchment has been arrived at from two Government sources and is verifiable.

With the known catchment sizes at 2200 sq klms for the entire Mary Valley to the Traveston Crossing and The Borumba catchment of 466 sq klms within that catchment, the volume in megalitres that can be attribute to Borumba can be calculated and compared with what actually occurred.

The calculations follow :-

Conclusion

The local residents are correct. There is a high degree of consistency in the years of 1999, 1989 April and 1974 where the flow across the Borumba spillway is known. The percentage of water through the Traveston Crossing from the Borumba Dam are 1.72, 1.82 and 1.86 respectively higher than the other sources of the Mary Valley catchment. 

The provision of water via Borumba percentage are 34.8, 36.9 and 37.7. As expected, they are consistent.

This information has major implication on the yield calculations of the Borumba Dam and its catchment.

Appendix : other Hydrological graphs mentioned above.

Warning

The 1893 flood height on the Gympie gauge has been ignored most probably for technical reasons. On the Brisbane Gauge it registered much higher than the 1974 flood. We have seen that the catchments of the Wivenhoe/Somerset system rainfall pattern is remarkably similar to the Mary Valley. We see in this Brisbane gauge  that the 1841, 1844 and 1893 are far higher than the 1974 flood shown in this section. The comparison of rainfall in the rainfall stations singled out by SEQWater on the 10th February 2007 is illuminating. Crows Nest Feb 1893 was 769mm and Jan 1974 was 527mm. Kilcoy Feb 1893 was 1,422mm and Jan 1974 was 417mm. Esk Feb 1893 was 1,036mm and Jan 1974 was 633mm.
These original Bureau records can be viewed in the "Uncommon events" button

The rainfall data from the Bureau of Meteorology could not possibly have created the inflow of March 1989. There was a high inflow in December 1988 possibly coupled with the March figures. There may have been overlapping of March and April 1989. I do not have the daily figures and I have therefore ignored March 1989.