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: Further information

This is the leading paragraph of this section to November 2008.

Pattern of Rainfall

The Mary Valley is the site of the proposed Traveston Dam. Its catchment is adjacent to the Somerset Dam catchment. Its rainfall operates in the same way as the Somerset (picture left). A comparison of the rainfall of the two catchments is attached. In the rare "uncommon events" the Mary Valley has a tendency to be up to 20 percent higher but the remainder are just marginally higher than the Somerset.

The overall variation is 6% or almost the same without "uncommon events" and in the same pattern.

This additional information is added:-

Both the Stanley and Mary Rivers have their source in the same area of Mt Mee. They are the subject of a paper by a CSIRO geologist who notes that it is a "geological structural anomaly" with both Rivers initially flowing to the Coast and then turning inland and eventually flow out to sea both North and South of their respective source. It leaves the coast line with no interruption of Rivers from Brisbane to the Maryborough Region. 

With this common geological occurrence and with exactly the same rainfall pattern (picture above) they lead me to believe that the probable action of the Traveston proposal could be judged by examination of the Stanley and the Somerset dam system. 

To me the primary purpose would be to observe that it could maintain the 70,000ML annual yield without "uncommon events" for a 14 year period with normal summer rainfall only with the exclusion of all "uncommon events". 14 years being the longest period without an uncommon event.

Prior website resumes

Pattern of Rainfall

The Mary Valley is the site of the proposed Traveston Dam. Its catchment is adjacent to the Somerset Dam catchment. Its rainfall operates in the same way as the Somerset. A comparison of the rainfall of the two catchments is attached. In the rare "uncommon events" the Mary Valley has a tendency to be up to 20 percent higher but the remainder are just marginally higher than the Somerset.

River heights

There are no inflow figures as there is no Dam. However the Bureau of Meteorology do have the River heights at Gympie located below the Dam. They effectively replace inflow assessments. In addition their web-site records the major floods and it is also attached. These majors date back to 1893 and are mirrored by the "uncommon events" of the Somerset with the exception of 1968.

You will have noticed that the Bureau labels the "floods" Minor, Moderate and Major. It also gives the River height at Gympie. It is therefore possible to match the rainfall with the River heights.

The pattern of Rainfall in the Mary Valley is therefore proven to be similar to the Somerset. An examination of the Wivenhoe and North Pine elsewhere in this web-site also show a similar pattern. This is understandable as the four catchments are adjacent.

Catchment area

The Borumba Dam operates within the Mary River catchment. It holds 46,000ML currently with plans to increase the height of the Dam wall. It releases 15,000ML into the Mary River annually. Its catchment is 466 Square Kilometres. More on this Dam later. The total area of the Mary Valley covered by the catchment of the proposed Traveston Dam with the exclusion of the Borumba catchment is approximately the same as the Somerset 1503 sq Klms. 

With a degree of tolerance there appears to be the same inflow from both catchments and they follow the same pattern. The exception is the uncommon events. 

Individual rainfall stations

An examination of the Individual rainfall stations used is necessary to eliminate any statistical aberrations. They include the rain gauge at the Borumba Dam for comparison. A glance along the bottom line indicates no significant departure from normal. Notice that the Borumba Dam mirrors the Mary Valley catchment average.

Two other matters of significant interest is that the four Summer Months December to March received 54 percent of the rainfall. They also received 94.9 percent of average summer rains without the 1974 flood aberration. The eight non-summer months received 74.4 percent of average. Except in exceptional circumstances, the non-summer months do not produce inflow. As in the Somerset and Wivenhoe Dams there is no drought in the productive Summer months. The rainfall deficiencies are in the non-productive Summer months.

These figures mirror the Somerset figures.

Long term history of the Mary Valley catchment

SEQWater has pointed out the importance of "uncommon events" in filling large Dams. The Somerset has a backup Dam in the flood mitigation Dam the Wivenhoe. It has a capacity of 1.1 million megalitres. We have seen that in the Somerset/Wivenhoe with 4 uncommon events between 1988 and 1999, the Dams not only filled but took care of a population increase of 700,000 on top of the 1.8 million people present at the start of the Wivenhoe in 1986. We have seen the draw down of these flood waters in the last 6 years and it has been misrepresented as a "drought".

We have also seen that the Wivenhoe is an inefficient Dam in providing water. See "Wivenhoe" button.

Therefore the Proposed Traveston Dam should operate in the same way as the Somerset. The short term history does match and we  can now examine the long term.

You will see that they are visually similar. The uncommon events occur at the same time with only one or two exceptions in 95 years. As we saw before they are up to 20% higher than the Somerset.

The conclusion is that the Mary Valley catchment will mirror the Somerset in its operation on a long term basis. This means that the long periods that separate uncommon events will not be covered by any Dam in the Mary Valley. This is especially so as the Somerset has a very large flood mitigation Dam in the Wivenhoe Dam.