Lower Wivenhoe

Author : J. V. Hodgkinson F. C. A. Chartered Accountant : Aug 2006 to November 2013    

The principal thrust of this website is
FLOOD PROOFING BRISBANE from damaging floods to the point of extinction. MITIGATING flooding in Ipswich and Gympie. Putting REAL MEANING into "Drought proofing SEQ" and ensuring our water supplies by natural means well into the future

This is my review based on official statistics and documents. It is done in conjunction with Ron McMah, grazier of Imbil and Trevor Herse, retired of the Gold Coast




2. Will we lower the dams or will we not lower the dams? A topical question 14/11/2011

( Dams were lowered by 5% to 75% 21/11/11)

This may give you an insight into this question and the permanent answer.

The warnings from the Bureau of Meteorology that the State Government and LNP appear to be heeding is that rainfall above average of 75% is likely to occur in the three months November to January. They also include cyclone activity above average. To this end the releases from the Wivenhoe are tidying up the level from 80.4% now (14/11/11) down to 75%. Somerset is kept at 100%.

Rainfall 75% above average – November to January

What we do know from the official IQQM computer model is that of the pre-development flows (no dams, no people) annual average over 111 year period 1890 to 2000 is 1,641,331ML at the Brisbane River mouth. This average includes the floods of 1890, 1893(a), 1893(b) and 1974

It also tells us that the Wivenhoe/Somerset contribution is 56.5% of that average or 927,363ML for a year.

Our most recent experience was that the dams were full in March 2010 well before the La-Nina event took hold. In October 2010 a fill of 339,873ML was released.

Dealing with the November to January period to match the Bureau, a fill of 324,982ML occurred in December and was released. Seqwater account of January 2011 covered three inflows that totalled 2,650,000ML. So the over fill for the November to January period totalled 2,974,982ML.

This overfill of 2,974,982ML is 321 % of the annual average of 927,363ML This total is well above the Bureau “Chance of exceeding” of 75%.

Concentrated rain from low pressure systems

These averages ignore the fact that our dams are filled by concentrated rain from “low pressure systems”.

Storm activity: We have seen the result of 6 storms below with a rise of just 0.6% in our dams. So their flood implications are localized and over quickly. They also provide little water for our use.

Normal summer rain: There has been no rise in dam levels in the last 6 Wivenhoe dam levels red line.jpg (112485 bytes) months. It is the experience of the Wivenhoe Dam since it began operations in 1986 that the maximum rise in dam levels from summer rainfall was 15.6% in the December 2003 to March 2004 right in the middle of the so-called “drought”. This aspect is highlighted in this Wivenhoe Dam level graph. They have been inadequate for our needs since 1992.

Low pressure system activity: These events come every 3.7 years on average with most below that average which means that those above can be lengthy. (See graph) We have seen since 2001 their departure for 7 years. Since then we have seen 4 events filling the Dams to March 2010. Therefore exceedence of 75% above average, as outlined by the Bureau, can only cause us concern if all that rain fell within a short space of time in the form of a “low pressure system”.

Cyclone activity: All of the major floods in the Brisbane River were associated with cyclones with the only exception year 2011 when three large “low pressure systems” combined and concatenated. An event that is unmatched in recorded history.

Therefore the most important aspect of the Bureau warnings is the cyclone activity. With cyclone activity we do get some warning but not much. In 2009, cyclone “Hamish” was a category 5 cyclone that was tracked for days near to the coast but fortunately backtracked on itself and did not make landfall. Our most recent cyclone “Yasi” was tracked for days across the Pacific.

Will we lower the dams or will we not lower the dams?

The answer to this vexing question lies in this web-site. Expansion of the holding capacity via the expanded Borumba Dam to double the size of the Wivenhoe and its integration by two-way pipeline.