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 April 2008

  UPDATES : November 2008. This website is maintained as it was in April 2008. Currently the Dam levels are around 40% and, with the addition of the Grid, should be maintained around that percentage. The next "uncommon meteorological event" will fill the Dams to overflow. This event will show that that the actions of "uncommon events", being Monsoons, Cyclones or Large Rainfall Depressions that generally cover all of SEQ, is again misunderstood and that they are the major source of our water supply. Our current storage capacity is insufficient to deal with them. The historical fundamental flaw of the cancellation of the Wolfdene Dam (1989) is being repeated in reverse at Traveston Crossing in the Mary Valley. See "Home" page for further updates.

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.