Flood Volumes

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




Information now available to calculate flood volumes (APRIL 2011)

The Qld Government’s IQQM computer model had completed these calculations Pre devlopment flows Jan 11.jpg (64765 bytes) when considering the 66% required for the Ecology at The Brisbane River mouth. They are done on the basis that no dams or people were present. It is called pre-development flows. This IQQM computer model is written into an Act of Parliament and has the force of Law. That chart is not generally available and is presented here for your inspection. Click to view pre-development inflows

Stream discharge volumes at Savages Crossing. Savages Crossing is below the Wivenhoe Dam. The Wivenhoe Dam was Savages Cross graph DERM.jpg (90362 bytes) not there in 1974 so the unrestricted flow in megalitres is available. The recordings commenced in 1909 and are not available for the 1893 floods. This convenient graph is provided by the Department of Environment and Natural Resources (DERM). The data that we will inspect is monthly.

The Lockyer Creek discharge is included in these figures. On catchment size this Creek can produce 25% of the flow through Savages Crossing. Rainfall figures for 1974, being less than 2011 indicate less than 25%.

Click to view the 1974 stream discharges at Savages Crossing was 4,150,00ML

The IQQM computer model and Stream Discharges are two quite different means of measurement. The IQQM calculations are based on a daily time-step using rainfall and other associated data. The Stream Discharges are the actual water that passes through the Crossing unrestricted except for the Somerset dam.

The Savages Crossing Stream Discharge model for 1974 shows the volume in megalitres to be 4,150,000ML for the 1974 year. The Pre-Development flows of the IQQM computer model shows 4,300,000ML. The Savages Crossing graph starts at October 1973 and ends in September 1974. There is only minor adjustment required to match the the 1974 year. It is examined fully on this page.

This means that the data from both sources can be considered reasonably consistent

Seqwater report on the January 2011 flood event 02/03/2011.  Report on the Operation of the Somerset Dam and the Wivenhoe Dam".

Brisbane City Council "Joint Flood Taskforce Report" (JFTF) of March 2011. The Chair is Professor Colin Apelt who is also a reviewer of the "Operations Manual" used by Seqwater in control of the Dams.

Bureau of Meteorology website has daily rainfall statistics for all rainfall stations in the catchments. They date from the time each station was first opened.

This website concentrates solely on Flood proofing Brisbane with mitigation in Ipswich and Gympie and Drought proofing South East Queensland. It does not seek to review Seqwater's handling of the 2011 flood event. 

However, the pre-development flows calculated by the IQQM computer model and the Stream Discharge flows at Savages Crossing suggests caution before acceptance of the above. The caution relates to the possibility of the floods being higher than represented above.

A brief overview follows. They are examined in detail in "Review of flood volumes" which may be skipped to "How to deal with it".


The volume for 2011 is accurate and accepted except for the fact that it was two floods similar to 1893(2). In both cases a minor flood was followed soon after by a major flood. The minor flood was 560,000ML and the major flood 2,090,000ML.

1974. The volume of the 1974 flood put forward by Seqwater for the 1974 flood is 1,410,000ML. They insist that the 2011 flood of 2,650,000ML was 190 per cent of the 1974 flood. The evidence that we are about to examine suggests a very different figure.

Savages Crossing records volumes of 2,394,206ML for January 1974 and 682,405ML for February 1974. This totals 3,073,611ML.This includes flows from Lockyer Creek estimated on a rainfall and catchment basis at around 500,000ML.

The 1974 flood was late in the month of January and the volume in February, after consideration of rainfall in that month, was almost entirely related to flood runoff. For example, 170,000ML was held in the Somerset flood compartment during the flood.

After allowance of inflow from rainfall in the first 24 days of January, an estimate of the Lockyer Creek flows and a portion of February 2011 there was no less than 2,200,000ML attributable to the 1974 flood.

This is more fully explored in "Review of Flood Volumes"

1893. There were three floods in the month of February. Two were major and one minor between the  two majors. The second flood concatenated (piggy-backed) on the back of the minor flood raising the second major flood to almost the same height. 

This puts the volumes of both major floods at around 5,200,000ML. The IQQM computer model suggests that the 1893 year produced 7,500,000ML. 

The fourth flood for the 1893 year was in June and was minor. There remains a substantial volume of water to be accounted for and could affect the volume presented above as the largest flood.

Further Review of flood volume

The review's purpose is to determine the largest flows from the Wivenhoe/Somerset catchments. It is to confirm or otherwise the volumes laid down by Seqwater which has at its maximum the 1893(1) at 2,744,000ML

The final result of this review at the bottom of this section is the adoption of 3,000,000ML as the volume of the largest flood that we have to deal with.


There were four floods in 1893.

Three occurred in the Month of February and one in June.

Their respective heights above "High Water Springs" were Major 1893 Floods chart.jpg (106195 bytes)flood 1893(1) 7.25 metres, minor flood of 0.75 metres and Major flood 1893(2) of 6.97 metres. Major flood 1893(2) concatenated (piggy-backed) on the minor flood. The fourth flood was a minor flood of 1.5 metres in June 1893. Click to view the February 1893 floods

As I understand it, "High water springs" has no relation to 1893 BOM 4 floods.jpg (90661 bytes) seasons and is when the Sun and moon have their strongest pull on tides

The Brisbane City Council Joint task force place the volume of 1893(1) at 2,744,000ML.

Seqwater also places the volume at 2,744,000ML on page 140 of section 8 of their report. Click to view the Bureau of Meteorology heights gauge when reviewing the 1974 flood


1893 Rainfall stations.jpg (126143 bytes)They make no comment on the volume of flood 1893(2) and the minor flood combined total although the height was close to 1893(1).

The IQQM computer model of pre-development flows has a total of 7,500,000ML for the year.

A reasonable estimate of the volume of 1893(2) would be 2,500,000ML based on the volume of 1893(1). The total of the three floods in February 1893 would have been 5,244,000ML. Click to view rainfall data of February 1893. Below is the 1893 year for Kilcoy Rainfall station.

1893 Kilcoy BOM.jpg (158615 bytes)The variance to be explained by the 1893 June flood and other 1893 rainfall is (7,500,000ML - 5,244,000ML )= 2,256,000ML

The IQQM computer model also tells us that the average pre-development flows at the Brisbane River mouth for the 111 years 1890 to 2000 was 1,641,331ML including all floods. The contribution of the Wivenhoe/Somerset was 58% or 951,971ML

This leaves 1,304,029ML to explain. (2,256,000ML - 951,971ML = 1,304,029ML)

The Kilcoy Bureau recordings for 1893 are attached. The main Stanley River rainfall stations of Mt Mee and Peachester were not in operation in 1893. I have singled out these rainfall stations as the Cromhurst laboratory close to the head water recorded 107 inches for a period of 23 days.

We have seen that the IQQM computer model had a reasonable match with the Savages Crossing stream flows so it is reasonable to assume that it is correct bearing in mind it has the force of Law. The recording of 2,744,000ML appears somewhat below the level reached but not significantly.

For the purpose of determining the flood with the largest volume, I am therefore taking the volume of Flood 1893(1) at 3,000,000ML.


Seqwater has placed this flood at 1,410,000ML. ( Seqwater revised this upwards to 1,724,881ML in the GHD report December 2011)

The Brisbane City Council Joint Flood Task Force (JFTF) also places the flood volume at 1,410,000ML.

It is a reasonable assumption that they are reading from the same information. 

It should be pointed out that in the Seqwater report section 8 1974 Seqwater All Floods.jpg (114993 bytes) page 140, the 1,410,000ML represents flood water volume. It indicates that a further 170,000ML was not released from the Somerset Dam. As we are endeavouring to assess the total inflow, presumably their recorded inflow is 1,410,000ML + 170,000ML = 1,580,000ML.
Click to view Seqwater's assessment of all major flood volumes

While on that schedule, there was no flood in March 1989. It is most likely a misprint and should have been April 1988. It was the combination of the April 1988 and April 1989 events that overfilled the new Wivenhoe Dam. It that condition the Wolfdene Dam, commenced after the Wivenhoe was completed, was cancelled for political reasons at that time.

The Savages Crossing Data has a very different outlook.

The first point that we examine is the graphs of both the pre-development flows and the graph of the flows through Savages Crossing. 

Graph Predevlopment flows.jpg (63446 bytes)

The first point that we examine is the graphs of both the pre-development flows and the graph of the flows through Savages Crossing. 

Graph Predevlopment flows.jpg (63446 bytes)Savages Cross graph DERM.jpg (90362 bytes)Savages Cross 1974 flood calcs.jpg (150401 bytes)





Click to see (1) pre-development flows (2) Savages Crossing flows and (3) Reconciliation of both of those graphs

Supporting information

Savages cross Megalitres 1974.jpg (99553 bytes)Savages cross Megalitres 1974 Page2.jpg (113835 bytes)





The actual data on which Savages Crossing Stream Flows are based.

The Pre-development flows are calculated by the Water Resource (Moreton) Plan 2007 Act of Parliament. It is done on the basis of rainfall and related maters.

Savages Crossing is done of the basis of stream flows at that site. It does include Lockyer Creek. The flow from that tributary in 1974 was less than 2011. The 2011 flows from Lockyer Creek appear to be around 10% of the catchments of the Wivenhoe Somerset

They graphs support each other and therefore should be judged as reasonably consistent.

The calculation page at Savages Crossing reveals the the 1974 flood at around 2,200,000ML.

The 1974 flood is also calculated by comparison with the major flood section of the 2011 flood.

This is best reviewed with the 2011 flood.


Seqwater have confirmed that the 2011 flood volume was 2,650,000ML. The Joint Flood Task Force (JFTF) of the Brisbane City Council confirm this volume.

My calculations, made in the same manner as the the JFTF, came to almost the same conclusion. The exception is that I view the 2011 flood in the same way that the Bureau viewed the minor flood of 1893 preceding the 1893(2) flood. 

In my view, the 2011 flood was similar to the second major flood 2011 Major Minor 2011_0001.jpg (138740 bytes)1893 Floods chart.jpg (106195 bytes)in February 1893 which concatenated (piggy-backed) on the back of a minor flood. 

Click to view Seqwater view of 2011 flood. Yellow additions are mine. The 1893 graph on the right has been compiled by me.

The volume of the minor flood of 2011 was calculated from Seqwater flood event, period 6, information with their inflow volume at 560,000ML. It represents releases of 187,500ML plus held in the dam at 18% being 209,700ML and inflow from rainfall yet to reach the dam of 187,350ML. 

That left a volume of 397,050ML (209,700+187,350) not yet dealt with when overtaken by the major flood.

The volume of the 2011 major flood was 2,090,000ML (2,650,000-560,000). The overall total agrees with Seqwater at 2,650,000ML

This distinction permits us to compare the 1974 flood with the major portion of the 2011 flood. The comparison then becomes 2,090,000ML of 2011 with 1,410,000ML being the stated inflow applied to 1974.

This application reduces both floods to a three day event. The hydrologists engaged by the Insurance Council of Australia seem to recognize this point in their investigations.

For strict comparison, only those rainfall stations that were in operation in 1974 and 2011 were selected. Those few outside this rule and in operation in 2011 were examined for statistical aberration.

The first two aspects to observe are:

2011 Rainfall Minor Major 2011 plus 1974.jpg (131308 bytes) * Was the ground saturated? They were both saturated.
 * How does the low rainfall of the minor flood compare with the high rainfall of the major flood in production of inflow? This is to confirm saturation.

This chart constructed from Seqwater operational periods for 2011 shows that the rainfall into the dams in the minor flood period produced the same relative volume of inflow as the major flood. The percentages are in red. Click to examine detail

This also means that the catchments were saturated from the observable prior rainfall. If there was a loss in both events, then it was consistent. 

1974 daily rain January .jpg (153835 bytes)The rainfall information in the month of January 1974 prior to the flood commencing on the 25th ensured that the catchments were in a similar saturated condition.

Click all three to examine detail.

Turning to the catchment rainfall figures from the various rainfall 2011 Wivenhoe Rain 2011 1974.jpg (129810 bytes)2011 Somerset Rain 2011 1974.jpg (93136 bytes) stations, we see that the total of the minor and major floods are consistent with Seqwater operational periods information.

We now see that the rainfall averages for the 1974 major flood exceed the major flood of 2011 by 15% in the Somerset and 12% in the Wivenhoe catchment. 

With the foregoing in mind it is reasonable to deduce that the inflow in 1974 to the Wivenhoe was 114% of 2,090,000ML being 2,383,000ML.

The variance from the stated flows of Seqwater and JFTF of 1,410,000ML is approximately 1,000,000ML

The IQQM computer model of pre-development flows shows inflows for the 1974 year at 4,300,000ML.

1974 Monthly rain.jpg (115837 bytes)If the calculation of 2,383,000ML is accurate, there remains a volume of inflow of 1,917,000ML to be accounted for in the balance of January 1974 and the remaining 11 months. Click to examine the rainfall for 1974.

This remainder of 1,917,000ML is double the annual flow average of 951,971ML over a period of 111 years. This is in addition to the 1,000,000ML identified above