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.
Uncommon events
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:
STORM inflow for the record
 |
(1) Storm 30/08/11: Start 86.7 % : Finish
87.0 % (02/09/11): Rise 0.3 %. |
| (2) Storm 08/10/11:
Start 85.9 % : Finish 86.1 % (10/10/11): Rise 0.2 % |
| (3) Storm 13/10/11: Start 86.0
% : Finish 86.1 % (14/10/11): Rise 0.1 % |
| (4) Storm 15/10/11
Morning : Start 86.1 % (15/10/11) |
| (5) Storm 15/10/11 Afternoon. :
Finish 86.2% (17/10/11) : Rise storms 4 & 5 = 0.1%. |
| (6) Storm 30/10/11. Start 85.8
% : Finish 85.7% (01/11/11) Fall 0.1%
|
| (7) No storms in November |
| (8) Storm 01/12/11. Start 81.0%
: Finish 80.7% (05/12/11) Fall 0.3%
|
| (9) Storm 10/12/11. Start
81.3% |
| (10) Storm 11/12/11 Finish
81.5% Joint Rise 0.2%
Dam levels not shown over weekend. Rain estimate on 09/12/11 0.5%
deducted. Total rise of rain and two storms Friday to Monday 0.7%. |
| (11) Storm 27/12/11. Start
81.2% : Finish 82.3% Rise 1.1%
|
| (12) Storm 07/01/12. Start
81.5% : Finish 81.4% Fall 0.1%
|
| (13) Storm 08/02 12 Start ):
81.4% |
| (14) Storm 12/02/12 Start ) |
| (15) Storm 13/02/12 Start )
Finish 83.1% Rise for 3 storms 1.7%
|
| Total inflow
for storms 1 to 15
inclusive = 3.2% |
|
May 2011 : Start 85.8 % : End
87.7 % : Increase 1.9 % |
|
June 2011 : Start 87.7 % : End
87.7 % : Increase 0.0 % |
|
July 2011 : Start 87.7 % : End
86.8 % : Decrease 0.9% |
|
August 2011 : Start 86.8 % : End
86.9 % : Increase 0.1% |
|
September 2011 : Start 86.9%
: End 86.2% : Decrease 0.7%
|
|
October 2011 : Start 86.2% :
End 85.9 % : Decrease 0.3%
|
|
November 2011 : Start 85.9% : To
85.2% 14/11/11 before dam releases. Decrease 0.7%.
Level after releases 24/11/11 81.7% To 81.0% 30/11/11. Decrease 0.7%
Full decrease for November ex planned release 1.4%
|
|
December 2011. Start 81.5% : End
82.1% : Increase 0.6% includes cyclone Fina (off shore) Increase 0.6%
|
|
January 2012. Start 82.1% : End
85.5% : Increase 3.4%.
There were releases from Wivenhoe . The volume is unknown but an estimate is
from 5% to 10% reflecting the two events that did not come West enough. They
had the ability to fill Wivenhoe from scratch to overflow.
|
|
February 2012. Start
85.5% : End 88.7%.
Total increase May 11 - February 2012 = 85.8%
to 88.7% = 2.9% Ex planned release November 2011, and three in
February 2012 with approximate total of 212,000ML or 14.1%.
|
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)
