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 
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.
