Author : J. V. Hodgkinson F. C. A. Chartered Accountant : Aug 2006 to November 2013    

The principal thrust of this website is
FLOOD PROOFING BRISBANE from damaging floods to the point of extinction. MITIGATING flooding in Ipswich and Gympie. Putting REAL MEANING into "Drought proofing SEQ" and ensuring our water supplies by natural means well into the future

This is my review based on official statistics and documents. It is done in conjunction with Ron McMah, grazier of Imbil and Trevor Herse, retired of the Gold Coast



May 2009


Since laying out this section I reviewed a desktop report by GHD P 667.jpg (102189 bytes) 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. 


Section retained for interest ( access to Government calculations have been requested at the invitation of the Minister 01/10/09)

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"


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.


( B ) Yield - from Borumba Dam.
( 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.


( 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.


( 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 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.


( 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

Yields_Annual_SEQWater_web.jpg (114473 bytes)SEQWater provided a yield graph in 2002Dam features from SEQWater Web.jpg (215540 bytes) 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.

Yield Wiv and Som SEQRWSSS Stage 2 interim.jpg (187097 bytes)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

Flood BOM guage heights 1880 to 2006.jpg (164816 bytes)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.
Somerset comp wivenhoe to 1962.jpg (205420 bytes)250 Som Wiv 1963 06.jpg (230506 bytes)250 Som Bor 1913 to 62.jpg (209502 bytes)250 Som Bor 1963 06.jpg (232038 bytes)



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.
Somerset Sum 1911 06.jpg (174272 bytes)Wivenhoe Sum 1911 06.jpg (170052 bytes)


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.
Uncommon 1913 to 62.jpg (191707 bytes)Uncommon 1962 to 06.jpg (193801 bytes)






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.
Flood BOM compare Page 1.jpg (227387 bytes)Flood BOM compare Page 2.jpg (231626 bytes)





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.
Somerset 65 06 Sum and Non.jpg (317322 bytes)Wivenhoe 65 06 Sum Non Sum.jpg (267816 bytes)



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 :-

Requirements.jpg (201573 bytes)Shortfall.jpg (162076 bytes)





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 Bor releases 62 to 2002.jpg (296834 bytes)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 Water Grid Aptil 07.jpg (128872 bytes)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