Author : J. V. Hodgkinson F. C. A. Chartered
Accountant : Aug 2006 to November 2013
The principal thrust of this
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
Commentary on recent rainfall events or other additions. They give one a fresh understanding of rainfall events that contribute to our water supply. An understanding assists in the construction of flood proofing Brisbane and Ipswich with mitigation in Gympie as well as putting real meaning into drought proofing South East Qld.
1a. On the 25th January we witnessed an "uncommon event" as outlined in 2007 by Mr Drury of Seqwater (see heading above). It involved the "Monsoon", a major supplier of our water. It would have been disappointing to see it not come west enough if our Dams had been low. It had the capacity to fill our dams from scratch.
For example rainfall stations in the Wivenhoe catchment at Blackbutt and Esk had falls of 64mm and 53mm respectively, hardly enough to create flow. In the Somerset catchment, which is closer to the coast where the action was, had recordings at Kilcoy of 57mm and 188mm at Peachester. In previous floods, Peachester recorded 617mm in 2011, 662mm in 1974 and 654mm in 1931.
1b. 18th January 2012. We witnessed a low pressure system quite often called an "East Coast Low", another major contributor to our water supply.
The Low tracked offshore and did not come west enough to provide a fill in the Wivenhoe/Somerset, the dams recording a minor fill of 0.1 per cent with normal minor releases.
1c. 27th February 2012. We witnessed a another low pressure system quite often called an "East Coast Low", another major contributor to our water supply. This is similar to 1b. It created flooding in Gympie however, the Borumba catchment that supplies 35% to 40% of major flood water received only 1/4 of the headwater rain. It provided over 100,000ML inflow to the Wivenhoe/Somerset dams.
1d. 5th March 2012. We witnessed a another low pressure system quite often called an "East Coast Low", another major contributor to our water supply.
The Low tracked offshore and did not come west enough to provide a fill in the Wivenhoe/Somerset, the dams recording a minor fill of around 50,000ML. Gympie was again had minor flooding but again the Borumba Catchment did not come into play.
This summer there has been three minor "uncommon events". They all required some release to keep the dams at around 75%.
It requires a significant increase in inflow and some light to moderate flooding to qualify as an "uncommon events" which provide 90% of our water supply. The average of 3.7 years still stands with one year already past. Most of these events occur below the 3.7 year average which means that those above can be quite lengthy and misunderstood as a "drought".
Storm/"wet season" review (03/09/11)
The Bureau of Meteorology and noted Climatologist Professor Stone have been reported as saying that this "storm season" will have an above average number of storms. These observations have been backed up by a Cabinet briefing ( 12/09/2011) by the Bureau with severe summer storms expected in SEQ. With the final Flood Inquiry report not due until March 2012 it provides an interim to determine the inflow from these storms into the Wivenhoe/Somerset dams.
Storms last for a few hours and are different to "low pressure system" that are not accompanied by lighting. "Low pressure systems" are just that and last from many hours to days with high volume rainfall. The Wivenhoe experience since its opening in 1986 is that they can provide a fill from 20% to over 300% as we have just witnessed.
"Low pressure systems" are our main water supply far exceeding the inflow from summer storms and "wet season" rains. They can appear at any time of the year and are random. It is the control over their inflows that will give us the ability to flood proof Brisbane to the point of extinction, mitigate flooding in Ipswich and Gympie, increase our water supply and put real meaning into drought proofing South East Queensland.
The following percentages are of the joint Wivenhoe/Somerset dams capacities and are taken from Seqwater public records:
DAM levels: for the record (Wivenhoe/Somerset joint levels. Includes storm inflow above)
Assessment of current position
Storms 1 to 12 produced little inflow into the dams and this is the nature of storms.
They present no flood threat from the Wivenhoe/Somerset.
Summer rainfall. On past performance since 1880 the summer months December to March receive 52% of all rainfall. In the life of the Wivenhoe, the largest inflow from normal summer rain for that period was 15.6% for December 2003 to march 2004 in the middle of the "so-called drought". It is clearly visible in this graph. They are inadequate for our needs and this has been observable in this above dam level graph since 1990 when the Wolffdene dam was cancelled. The exclusion of the "low Pressure systems put the dams at a reduction of approximately 5% in our best rainfall period.
hey also present no flood threat from the Wivenhoe/Somerset.
"Low pressure systems" are our main inflow that fill our dams. They regularly overflow our dams with fills between 20 percent and 300 percent that we have recently witnessed. They are random, occurring throughout the year with the majors that produce substantial flooding in January/February. They collectively average every 3.7 years with most below that average which conversely means that those above the 3.7 year average can be up to 11 years with consequences much more severe than a "drought".
Early last year (2011) there were three in January 2011. Preceding was one in December 2010 (26% of Wivenhoe) and another in October 2010 (26% of Wivenhoe). This makes a total of five of varying intensity.
As outlined above there were two systems in January 2012 that missed the catchments and would not have been recorded.
The major flooding "low pressure systems" all occurred on (1) full dams (actual and estimates from BOM rainfall figures) and (2) saturated catchments, a state common to all major floods of 1893(a), 1893(b), 1974 and 2011. All had associated cyclone or monsoon activity
Currently (January 2012)
End of storm/"wet season" review