Wivenhoe Dam pictured last Saturday.
Wivenhoe Dam pictured last Saturday. Rob Williams

Government launch dam inquest

A LOOK at January rainfall figures for Wivenhoe Dam goes some way to show why dam operators had such a horrid time deciding on how much water to release before and during the floods.

The State Government has launched a $15 million inquiry which, among other aspects of the flooding, will look at the way Wivenhoe Dam was managed.

South East Queensland Water Grid Management has refused to release any information on inflow volumes at Wivenhoe until after the enquiry.

However, one of several updates released during the peak of the floods mentions that Wivenhoe was receiving more than two Sydney Harbours worth of water per day. One Sydney Harbour is 562,000 megalitres.

This was due to massive rainfall across the Brisbane Valley, which shows up in the Bureau of Meteorology's January figures.

Wivenhoe Dam recorded a phenomenal 340.8mm from January 8 to January 14.

The bureau does not show long-term averages for the area, however the 248.8mm recorded on January 11 was record rainfall to come in one day since records commenced at Wivenhoe in 1986.

During the same week-long period, Somerset Dam was drenched with 310.4mm – nearly three times the 127.4mm average for January.

The biggest day was January 10, when 156mm was recorded.

Showing that the entire Brisbane Valley was soaked, the Esk Post Office weather station recorded 357.4mm over the same week.

Rainfall of 130.8mm and 150.2mm was recorded on January 10 and 11 respectively, showing why the town went under water.

Releases from Wivenhoe Dam between January 8 and January 18 were almost constant, although the volume of water released fluctuated.

On January 8, with Wivenhoe Dam just over 100 per cent but with water coming in from Somerset, water releases were made at a rate of 100,000 megalitres a day.

Those releases were ramped up to 116,000 megalitres per day on January 9, then up to 150,000 megalitres per day on January 10, as the dam reached 140 per cent.

The situation became critical on Tuesday, January 11, when Wivenhoe Dam reached 191 per cent.

Dam managers had to release water from the dam at a rate of up to 645,000 megalitres per day, regardless of the damage being caused downstream.

Of course it was on the following Wednesday and Thursday that the worst of the flooding was experienced in Ipswich and Brisbane.

Releases were cut back to 370,000 megalitres per day on January 12, with Wivenhoe steadily dropping.

By January 18 the dam was back to 107 per cent.

Dam's downpour

January 8: 7mm

January 9: 15.2mm

January 10: 68.4mm

January 11: 248.8mm

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