Since laying out this section I reviewed a
desktop report by the Government appointed
Engineers GHD. In my view, the report supplied contributing evidence in their
section on the Augmentation of the
Wivenhoe Dam. It is contained on page 9 under the tab "Final
Solution & EIS" They reviewed the Department of Natural Resources and Water, DNR
& W, hydrological report of December 2005. This indicated an annualized surplus
of 80,000ML and it largely confirms these calculations.
......................................................................
Different approach
What is markedly different in this approach to
water is the isolation of Uncommon events from normal average Summer rainfall.
It is the timing that has caused incorrect decisions in the past and present and
may yet cause us anguish in the future. They have been isolated in the flow
schedules that appear in this section and rebalanced against expected stated yields by
various Qld Govt Departments.
Most important contribution
The most important contribution to the situation on water 
was
published by the Courier Mail on the 10th of February 2007. The information was
provided by the professionals at SEQWater, the Dam managers. The article
directed us to two matters. The first is that it is the four summer
months December to March that provide the high rainfall necessary to provide
inflow into the Dams. The second is that it takes "uncommon
events" to fill large Dams. We saw from the first matter that the
Dams had been receiving normal Summer rains for the six years to 2006 and that
the reduction in rainfall of 20% existed in the non-productive non-summer months
that have no impact on the Dams.
Same rainfall pattern and timing in the Mary Valley
The course that we are currently on to "drought
proof" and "secure our water supply" ultimately involves the
Traveston Dam near Gympie. It is in the Mary Valley. The Mary Valley has almost
exactly the same rainfall pattern as the Somerset and Wivenhoe Dams. This
means that the timing is the same. Although the Mary Valley has a higher
rainfall, the same result that we are now seeing will occur. The Mary Valley
will have no flood retention until 2035 and even then only partial.
Uncommon events and surplus water
We have seen in this web-site that "uncommon events"
are aptly named as they are uncommon. They appear in tight groups many years
apart. When they occur they can be immense.
If you think about it, the most recent in our memories is the
period of the filling of the Wivenhoe Dam. The Dam commenced from scratch in
1986 to overflow in the years 1999 and 2001. The capacity of that Dam is
1,165,000ML. Add to that 190,000ML being one half of the capacity of the
Somerset. In addition my estimate of overflow from BOM flood information is
around 500,000ML.
The calculation is : if the Wivenhoe dam had been in operation
in 1986 and was at the same level in 2001 as it was in 1986, it devolves
to a total of 1,855,000ML of surplus water would occur. This is over and above the
total requirements of most of the current population on a "business as
usual" basis. Enough to fill the expanded Borumba Dam of 2,000,000ML. The
Borumba Dam was not idle. The Dept of Natural Resources recorded that 908,464ML
was released over the spillway in the same period.
SEQWater reported in the above article that the Dams had four
uncommon events in that 16 year period. That is rare in itself. However you
would have seen that the 1974 period was made up of four years of
high rainfall culminating in the 1974 January flood. It does not require
calculation to see that all Dams including the expanded Borumba would have been
full. Keep in mind that the Wivenhoe and Somerset Dams flood retention has been expanded to
1,950.000ML facilitating pumping requirements.
Previous uncommon events
Previous events in 1955/1956, 1927/1928, 1908 had a
lesser impact. The twelve year period from 1887 to 1898 saw two floods
equivalent to 1974, two other major floods and the well known 1893 flood which
was three floods in the space of a few weeks. While the information originates
from the Bureau of Meteorology, the flood heights attached in the
"Solution/Base" section are at Brisbane City.
Some floods are recorded in the upper reaches of the Brisbane River that do not rate
highly in this recording and vice-versa.
Impact of uncommon events on timing and volume of
surplus water
To observe the interaction of "uncommon events" with
the Dams and population in their current format, I have provided three "cash
flows" type schedules on water inflows and use. Like all "cash
flows", they have been examined in total for sensibility from reliable data.
Their construction base and method has been dealt with
in detail in the "Solution/Base"
button.
All three flows include the full annual
requirement of 150,000ML required of the three stages of the Traveston Dam. In
the third flow it is progressively introduced.
The first deals with the 108 years from 1898 to 2006 on the
basis of the three dams in existence and current population. The Dams would have
been full in 1898. You will see that it is conservative on two counts. The yield
of the uncommon events at 98GL annually is below the stated yield average of
183GL in the first 70 years to 1974 and the yield of 197GL for the period 1974
onwards at 197GL compares with the average yield of 183GL. The evaporation is
taken into account when considering the yield by Qld Govt sources. I have added
an additional evaporation rate for caution.
The second
deals with
the same period with a start of 14% or 500GL to see how it performs over the difficult
70 years period to 1974. Evaporation already taken into account in
the yield is eliminated.
The third deals on the same basis with the current situation
at year 2007 forward where the Dams are almost empty. The Traviston stages are
progressively introduced.
CONCLUSION
There is little doubt that the provision of stable water
supplies from the "Grid" will hold the current position with normal
Summer rain and excluding uncommon events.
It is harnessing these uncommon events that
is the key to securing our water supply well into the future.
These calculations show that the expanded Borumba Dam is a
superior method than the Traveston Crossing Dam when considering the future. I
understand that engineering advice says it is feasible to the 2,000,000ML
capacity and beyond.
It is the move forward that should be ruthlessly examined including
all of this web-site. It is critical for people of the Mary Valley and for
many generations of all our South East Queenslanders.
