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: No change from first update

Associated evidence. The North Pine Dam.

You will see the evidence that the North Pine Dam mirrors the pattern and volume of rainfall of the Borumba Dam catchment. In the first instance we will look at the available information from the professionals at SEQWater.

The comparison of rainfall follows :-

A summary of the comparison of three stations in the Borumba catchment now follows :-

This shows a high degree of consistency of pattern and volume. The Borumba Dam recording station is the same in the Summer months where all of the majors occur.

The SEQWater Dam statistics of the North Pine now follow :-

Of interest is the catchment area of 348 sq klms and a yield of 54,750ML. You will notice that this yield has remained stable whereas the yield from the Wivenhoe and Somerset has been under consideration for some time.

Catchment variation

The calculation is : 54,750ML X 466 Sq Klms of the Borumba catchment / divided by the catchment of the North Pine at 348 Sq Klms.
Equals 73,314ML.

Flood mitigation

The SEQWater information above shows that there is no flood mitigation in the North Pine Dam with all flood water released in a controlled manner. The photo also shows the flood releases of the 1999 event. A glance at the major rainfall figures above shows a similar pattern to all of the Dams and a substantial volume would have flowed over the Dam. We have sufficient information to examine this aspect.

The Borumba Dam is its own flood mitigation if the Dam wall is raised in accordance with Mr R McMah's contribution. He envisages the Borumba capacity to be in the region of 2,000,000ML capacity.

The Courier Mail article of the 17th February 2007 quoted the Dam manager as saying that it would take 600 to 650 mm of intense rain to fill the Dam. Its capacity is 214,960ML ( see Dam statistics)

The attached schedule is a summary of rainfall in that category. Of particular interest is the 1999 rainfall average of 438mm in the North Pine Dam with the photo above clearly showing the flood discharge. The other rainfall events are significantly higher. The higher rainfall can have a much higher impact than a direct correlation.

Catchment efficiency

It is generally recognized that, with its rocky terrain, the Borumba Dam catchment is the most efficient of the Dams. In the area of proof, there is a conflict of information. The Regional Manager at the time advised that the "Peak discharge per day" was 154,300ML. The Department of Natural Resources web-site advises that the total discharge for the month of February 1999 was 167,062ML.

The Qld Government advises that the duration of the flood in the Mary Valley was significantly more than a single day. Examination reveals that the volume of water was most likely to be double the 154,300ML to over 300,000ML.

You will notice that the Dam Manager at the time compared the volume of water in a single day to 1/2 of Sydney Harbour. That converts to 250,000ML a day. However, we should keep in mind that he was making a point.

Using the Dam Manager's advice of the time, the efficiency of the Borumba catchment compared to the rest of the Mary River catchment is above 1.7 times. If the Dept of Natural Resources information is correct, the point is not proven. Refer to my calculations under "Hydrology" button.

Conclusion

With the catchment area of the Borumba being either equal to or more efficient than the North Pine, the Calculation based on the catchment area is stable at 73,314ML. 

The evidence points to a very high volume of inflow not retained by the North Pine because of the lack of flood mitigation or retention availability. This is not so in Mr R. McMah's proposition of the expanded Borumba Dam. This will make a significant addition to the 73,314ML already calculated.