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.

The Traveston dam is designed to permit the overflow of large scale meteorological events. Refer to Hydrology Mary Valley

Currently the Mary River has ceased flowing above the dam site (CM October 2009). The Mary River mirrors the Stanley River which is our main water supply. It has the backup storage of the Wivenhoe dam and even that dam is proving inadequate to control major events.

It is my view and others that the 85% requirement of all water that flows in the Mary River must reach the River mouth is made up almost entirely of flood water. We now know for certain that flood water is damaging to the eventual point where it enters the sea.

Here again, the use of the "mean annual flow" includes all of this flood water. It is not difficult to envisage a distortion to percentages when most of the water is flood water and especially the very high rainfall events inclusion.

The following give one an appreciation of how our main water supplies fall and why they are our main water supply.

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. 

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.

Photos : Bureau flood heights at Gympie and Maryborough together with major flood heights at various location on the Mary River

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.