The following indicates that the Bremer River and local creeks
that front the Brisbane River were impacted by the Wivenhoe/Somerset catchments.
It relates specifically to the 2011 flood. My Ipswich correspondents assure me that 1974
was quite different.
The impact of the releases from the Wivenhoe Somerset are
clearly seen from these Seqwater produced hydrographs. I have added a red line
at the point where rain ceased.
Photos left to right: Dam releases from the Wivenhoe;
Hydrographs Savages Crossing and Bremer River; Hydrographs Moggill and Brisbane
City; My overlay of Seqwater graph defining the minor and major floods.
First of all the later releases clearly held up the
flood after the peak at all points. This has little relevance to the peak of the
flood but is an indicator of the "barrier" created at the
Bremer River mouth as mentioned in the Ipswich City Council submission.
The second point is that according to the three major
hydrology firms engaged by the Insurance Council of Australia, the major release
from the Wivenhoe created a rise of 7 metres at One Mile Bridge. The graph of
the releases from the Wivenhoe was constructed by me from the Seqwater flood
submission.
One Mile Bridge on the Bremer River is 47 Klm from the Bremer
River mouth.
This, of course, would have a significant bearing on the
flooding in Ipswich from the One Mile Bridge to the Bremer River mouth.
Those Hydrologists ignored altogether the minor flood which
also occurred in the Bremer prior to the major flood. This supports my view
that there were two floods being a minor flood closely followed by a major
flood.
It also contradicts claims at the Flood Inquiry that early
release of the minor flood would not have made any difference.
The combination would have resulted in a minor flood all-round.
Instead, that volume being 400,000ML, based on Seqwater operational submission,
was released at the peak creating creating the scenario outlined by the
Insurance Council Hydrologists.
*******
On flooding in Ipswich the Ipswich City Council had this to
say to the Flood Inquiry
The impact of a Brisbane River flood is twofold.
Firstly, it the direct cause of flooding in the southern areas of the City such
as the suburbs of Redbank and Goodna.
Secondly, a Brisbane River flood can act as a
"barrier" to the Bremer River (and other creeks such as Goodna Creek)
entering the Brisbane River at the junction of those waterways with the Brisbane
River.
As a result water from the Bremer River are then unable to
enter the Brisbane River and are forced back up the the Bremer River.
This exacerbates the flood experience in that river and in
the creeks, such as Bundamba Creek, that feed into the Bremer River near to its
junction with the Brisbane River.
********
While the containment of the Inflow from the Wivenhoe/Somerset
is our goal, the question arises as to just how much water flows to the Brisbane
River from the Bremer River and Lockyer Creek.
The Flood Inquiry has revealed so far that there is no
modelling of the influence of the Wivenhoe/Somerset released water on the flows
of the Bremer River and Lockyer Creek.
Fourteen rainfall stations were reviewed that were in
operation in 1974 and 2011. They covered both catchments.
The combined rainfall for the 1974 and 2011 floods was constant with
only a minor variation.
There was fluctuation in the contributions with the Bremer,
lower in 2011 than 1974 and the Lockyer higher in 2011 and with a larger
catchment.
Impact on flooding in Ipswich
2011 : This has been dealt with above. It seems that it could
have been avoided or largely minimised.
1974 : My correspondents in Ipswich point out that all
historical buildings are all above 1893 flood levels.
There is evidence that the 1974 flood was
higher than 1893.
The David Trumpy Bridge gauge shows 1893 at 24.50 metres and
1974 at 20.70 metres. However that gauge is markedly affected by the Brisbane
River "barrier".
Further examination shows that 1893 is indeed less than 1974
and 2011 at least in some areas. For example, Ipswich in 1893 had rainfall volumes of 236mm for major
1893(1), 104mm for minor flood in between and 355mm for major 1893(2). The 1974
volume was 662mm which far exceeded any flood in 1893.
In the Lockyer Creek, Helidon was the only available Rainfall
station covering 1893, 1974 an 2011. The individual rainfall days were not
available but the monthly totals read 1893 (for 2
floods) 797mm, 1974 448mm and
2011 395mm.
The requirement to mitigate flooding in
Ipswich for a 1974/1893 type flood is still inclusive. My correspondents tell me
that it is not possible. They agree that the creeks that join the Bremer near
the mouth and those that front the Brisbane River will benefit from the
exclusion of the Wivenhoe/Somerset but are doubtful about Ipswich City benefit.
The requirement to eliminate flooding in
Brisbane:
The Bremer River and the Lockyer Creek are the main catchment
areas that would influence flooding in Brisbane.
With the Wivenhoe/Somerset fully contained they are allowed to
run free.
The volumes of flow from the Bremer and Lockyer Creek, on the basis I have
adopted to 1,530,000ML or 131% 
of the Wivenhoe Capacity. The 1974 flood period was a little over three days
requiring 38% of the capacity of the Wivenhoe dam to be dealt with for each of
the three and one half days. The 2011
flood was over a longer period.
If the assessments of 1893 are correct, then the additional
volume should be minimal if any.
****** presently
The impact of the releases from the Wivenhoe Somerset are
clearly seen from these Seqwater produced hydrographs. I have added a red line
at the point where rain ceased.
Photos left to right: Dam releases from the Wivenhoe;
Hydrographs Savages Crossing and Bremer River; Hydrographs Moggill and Brisbane
City; My overlay of Seqwater graph defining the minor and major floods.
Combination of the Bremer/Lockyer/major releases Wivenhoe on the right. Wivenhoe
in red and Lockyer/Bremer in blue. Timing every 6 hours from 5th January 2011
First of all the later releases clearly held up the
flood after the peak at all points. This has little relevance to the peak of the
flood but is an indicator of the " barrier" created at the
Bremer River mouth as mentioned in the Ipswich City Council submission.
The second point is that according to the three major
hydrology firms engaged by the Insurance Council of Australia, the major release
from the Wivenhoe created a rise of 7 metres at One Mile Bridge. The graph of
the releases from the Wivenhoe was constructed by me from the Seqwater flood
submission.
One Mile Bridge on the Bremer River is 47 Klm from the Bremer
River mouth.
This, of course, would have a significant bearing on the
flooding in Ipswich from the One Mile Bridge to the Bremer River mouth.
Those Hydrologists ignored altogether the minor flood which
also occurred in the Bremer prior to the major flood. This supports my
observation that a minor and major flood occurred.
The above is all related to the flood of 2011. My
correspondents from Ipswich assure me that the flood of 1974 had a very
different result.
