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: 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.