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.
..................................................................................
This section calculates the base for examination of the feasibility
of movement of surplus water between the Somerset/Wivenhoe system to and from
the Borumba Dam expanded to 2 million ML. It then proceeds to show the method of
construction of the " flow schedules". Flow schedules show the volume of water in the
Dams at the start and end of the year together with the inflow and use for the
year.
It is extensive. However there was no alternative to
methodically dealing with each item of construction of the flow schedules.
The calculations are included in the "Calculations"
button which follows this section.
Thirteen items that contribute to to these
flow schedules are listed after "the method of calculation"
THE METHOD OF CALCULATION
Annual calculation
( A ) Water volume in Dams at start of year + inflows - use =
water volume in the Dams at end of year. Carry forward end of year
to start of next year etc.
Inflow
Borumba
( B ) Yield - from Borumba Dam.
Wivenhoe/Somerset
( C ) Yield - Inflow from Average rainfall.
( D ) Yield - Inflow from uncommon events.
( E ) Yield - Less evaporation and seepage.
New water not Rainfall
( F ) Inflow or water redirected from Grid system.
Use
( G ) Requirements of the population.
( H ) The full requirements of the Traveston Dam proposal in all three
stages.
( I ) Evaporation above the yield for conservative purposes.
Comments and verification methods
(A) These flow schedules are prepared on the basis that
all three Dams were in operation for the period under review and the population
was the same as it is in year 2007.
First flow schedule
Second flow schedule
The second schedule is prepared on the basis that the Dams were 14 percent full
in 1898. A little less than the current levels.
Third schedule
The third schedule tracks the estimated flows from the year 2007 to 2045 being
the completion of the third stage of the proposed Traveston Dam.
Inflow
( B ) Yield - from Borumba Dam.
Item 12 is used in this calculation. It records releases from the Dam
from 1962 to 2002 when the Dam was taken over by Sunwater. The years prior are
estimates based on these actual records.
( C ) Yield - Inflow from Average rainfall.
In item 10 we see that the last six years 2001 to 2006 has been quite
normal productive Summer rain when compared to the previous 42 years.
In item 11 we see that this normal rain without uncommon events supplies
480ML per day which converts to 175GL annually. I have used 500ML per day or
183GL annually.
In item 7 we see that the Summer rain in both Dams has been consistent
for the whole of the Century while ignoring the uncommon events.
Throughout these schedules under the heading of average
rainfall excluding uncommon events, the 500ML per day or 183GL per year is used.
( D ) Yield - Inflow from uncommon events.
Here we have a number of aids to assist us in determining the
timing and extent of uncommon events.
Item 1 calculates the surplus water from uncommon
events 1988 to 1999 and 2001. SEQWater web site and Bureau of Meteorology information on flood releases.
Item 3 Is a self evident truth of the volume of surplus
water available in the 1972 to 1974 period.
Item 5 are the Bureau of Meteorology flood heights at
Brisbane City which gives us a synopsis of the last 160 years.
Item 6 are the Bureau of Meteorology single month
rainfall records for most of the Twentieth Century.
Item 7 is an overall view of the Bureau of Meteorology summer rainfall from 1911 to 2006 for both Wivenhoe and Somerset
Dams.
Item 8 Single month records are matched with uncommon
events drawn from these records
Item 9 Bureau of Meteorology flood information relating
to the Wivenhoe and Somerset catchments. It dates from 1887 to 2001, its last
update.
Yield balanced against Average Rainfall and
Uncommon events
Item 4 must now be considered after the determination
of (C), the yield of average rainfall ex uncommon events and (D), the yield from
uncommon events. This should match the yield for the total system which, of
course, allows for evaporation.
The yield expected from the Wivenhoe/Somerset system is
376GL annually. See SEQRWSS report in item 4.The yields from average
rainfall and uncommon events should now be compared against this report. You
will see that the yield from uncommon events for the first 70 years falls well
short of its allocation of the overall yield. It was a difficult period and in
the region that I expected. It moved up slightly over the expectation from 1970
onwards.
Evaporation
Evaporation is taken into account when determining the yield.
The yield expressed by SEQRWSS is after evaporation.
( F ) Inflow or water redirected from Grid system.
The contribution to new water under the emergency legislation
calculates to 500ML per day with the addition of minor actions but with the
exclusion of the proposed Traveston Dam. This converts to 183GL annually, about
the same as average rainfall with the exclusion of uncommon events.
Average rainfall plus the Grid system should hold the current
situation until an uncommon event.
Use
( G ) Requirements of the population.
Item 11 reminds us of the calculation in the
"Requirements" button. It calculates to 1003ML per day for both
business and household use for those drawing water from the Wivenhoe/Somerset
system. This calculates to 366GL which in turn almost agrees with the sum of
average rainfall and the Grid system without uncommon events.
( H ) The full requirements of the Traveston Dam proposal in all three
stages.
The expanded Borumba Dam proposal is linked with harnessing
surplus water from uncommon events. For it to be superior to the Traveston Dam
proposal, it must provide for the output from that proposal and build reserves
into the future. All three stages of the Traveston proposal are included. In the
schedule going forward from year 2007, it is phased in at the rate of that
proposal.
( I ) Evaporation above the yield for conservative purposes.
Yield is determined after evaporation and this item has no
place in the schedules. However it has been included where specified for
conservative purposes.
Click to refer to "The
Solution" for the final calculations
Items that contribute to the calculations
are
now listed :-
(1 ) 1988 to 2001 surplus water
The Wivenhoe Dam filled from scratch to overflow in 12 years
1988 to 1999 and 2001. The capacity of that Dam is 1,165,000ML. In addition you
will read that significant water was released in 1999 and moderate in 2001. If
the Dam had been in operation in 1988 and remained at the same level that it
started with, the total available water surplus to our population needs would be
1,700,000ML. Sufficient to fill the expanded Borumba Dam.
Reason for inclusion : To show
that there has been considerable surplus water in our recent history.
( 2 ) Flood storage capacity.
The flood storage capacity was expanded in the last few years
to 1,450,000ML for the Wivenhoe and 524,000ML for the Somerset. Sufficient to
fill a complete expanded Borumba Dam should the occasion present itself.
Reason for inclusion : The
ability to hold water until pumped to Borumba Dam.
( 3 ) 1972 to 1974
There is little need to dwell on this period. With the storage
capacity and the early years of pumping to the Borumba dam, all Dams would have
been full.
Reason for inclusion : To show
that there has been considerable surplus water in our recent history.
( 4 ) Yield of the Somerset/Wivenhoe System
SEQWater
provided a yield graph in 2002
which is attached. Of late they have not reaffirmed this yield. The web-site
records it as "under review".
The Gold Coast City Council has provided a definition of
"yield" for a Dam. "The safe yield of a Dam is the daily or
annual amount of water that can be drawn from it on a long term basis without
running it dry, irrespective of the weather conditions". The calculation
includes evaporation losses before the yield is struck.
Reason for inclusion : The flows
of water, which are the final calculation of this section, must balance back to
the stated yield. If the inflow calculated is lower than the Government stated
yield, then the inflow will be calculated on a conservative basis and vice
versa.
The
SEQRWSS stage 2 interim report is attached. It advises that the yield for the
Wivenhoe/Somerset System is 374,000ML or 374 Gigalitres. The water allocation
from the system stood at 286GL.
Reason for inclusion : Official
confirmation of the yield used in the calculations.
( 5 ) Flood heights at Brisbane City. Bureau of Meteorology
information
These
gauge heights are a reasonable synopsis of what happened in the last 160
years. It does not cover all the events in the catchments and vice versa.
What it clearly shows is that the events of the last 60 years
of the Nineteenth Century are very different to the first 70 years of the
Twentieth Century. There is a resumption of sorts since 1970.
Reason for inclusion : Any
calculations for either the Mary Valley Traveston Dam proposal or the expanded
Borumba Dam proposal, must stand up to the first 70 years of the Twentieth
Century which includes only three groups of relatively minor uncommon events. It
also contributes in determining the timing and extent of the uncommon events.
( 6 ) Rainfall over 250mm in a single month
Rainfall in this category is useful in determining the inflow
from uncommon events. The graphs for the whole three Dams are attached. They
cover the years 1913 to 2006.
Reason for inclusion :
Contributors to the timing and extent of uncommon events.
( 7 ) An overall view of summer rains for the years 1911 to
2006 for Somerset and Wivenhoe Dams.
Another view of the century that we are about to traverse.
Reason for inclusion :
Contributors to the timing and extent of uncommon events.
( 8 ) Matching of single month rainfall with uncommon
events
Under this heading, the single months are matched with
uncommon events to aid in assessing the percentage of inflow.
Reason for inclusion :
Contributor to the timing and extent of uncommon events.
( 9 ) Flood information prepared by the Bureau of Meteorology.
This information is contained in the same section of the
Bureau's web-site that contained the flood heights above. Extracts that are
relevant to the catchments have been set out. The web address is on the bottom
of the flood heights section above.
Reason for inclusion :
Contributor to the timing and extent of uncommon events.
( 10 ) Average Summer rainfall as demonstrated by the years
2001 to 2006
This aspect has been viewed in "Rainfall 2001-06"
button. The schedules are shown for completeness. Notice the comparison to
average in the bottom right hand corner.
Reason for inclusion :
Determination that the summer Rainfall has been normal and average for the six
years to 2006. The deficiency has occurred in the non summer 8 months that do
not normally produce inflow. This then matches the "Requirements of the
current population" below.
( 11 ) Requirements of the current population
This is fully dealt with in the "requirements"
button. The relevant aspects in calculating the flow of water are :-
These two aspects calculate our ordinary requirements plus the
business requirements associated with the population. It also shows the
shortfall of average Summer Rainfall, without uncommon events, against this
requirements. The final requirements are verified via multiple measures.
Reason for inclusion : It is the
basis of water withdrawn.
( 12 ) Yield from Borumba Dam
This is fully dealt with in the "Borumba Dam"
button. The 
Department
of Natural Resources releases are attached for ease of consultation. Estimates
aligned with this yield are made for the period prior to the Dam commencement in
1962.
( 13 ) The "Grid" system
With no uncommon events, the importance of the Grid
system 
comes
into play. Of the 1003ML per day required, average Summer rains provide 480ML.
The Grid system will make up the difference when all is in place with the
Traveston Proposal excluded.
Reason for inclusion : It places
the system of inflows and withdrawals in balance. With no drought in the Summer
months and no uncommon events, the Dams should stay stationery.
Click to refer to "The
Solution" for the final calculations
