Australia is a land of extremes, we have had 12 years of drought over much of Australia with the hottest decade on record which culminated in the devastating Black Saturday bush fires in summer 2009 when temperatures in Melbourne hit an unprecedented 47degC – my posts regarding this can be seen here.
La Nina has brought a wet, cool, humid 6 months to Victoria and our unseasonally wet summer continues, but this is nothing to what has happened in Queensland over the past few weeks where they have often been receiving Victoria’s annual rainfall in a day and repeated episodes of this. The last major La Nina was in in 1974 which resulted in Australia’s wettest year on record and major flooding as well as cyclone Tracy which devastated Darwin.
Much of Queensland has been covered with flood waters for weeks and all the dams are not surprisingly full with limited capability for further flood mitigation.
Heavy rain and storms continue over Queensland.
Yesterday, the Queensland town of Toowomba was hit by a devastating, surprise “inland tsunami” – an unexpected 8m wall of water rushing through the main streets washing away all before it and resulting in 8 dead and 72 missing as of this morning with that figure expected to “rise dramatically” according to Premier Bligh. This wall of water came from torrential rain (80-200mm in 30 minutes – a ” 1 in 100 year localised event” for that region) in the nearby Lockyer Valley and disappeared almost as fast as it came, to continue on to cause major flooding downstream, washing away houses.
Here is a sample of how fast the creek rose:
To try to understand this type of event, imagine that if you had a 1km wide and long flat paddock with no inflows or outflows other than the direct rain, at 100mm in 30 minutes, the whole paddock will be filled with water 100mm deep (ie. 4 inches). This equates to a volume of 100,000 cubic metres = 100 megalitres.
Now imagine a potential 10m wide creek within a valley which is 1km wide, if there was no run off from the mountain sides, and no inflows or outflows, that creek would become 100mm deep after this amount of rain.
BUT, as a crude calculation, if all the run off from the mountains 0.5km on either side of the creek ran into the creek immediately, then 1 kilometre of catchment draining into 10m wide creek results in a 100x multiplier effect (1000m valley width / 10m creek width) – ie. the creek would rapidly rise to 10m and this wall of water would rush downstream. For every 1km length of valley, 100 megalitres will be added to the flood plains below.
This is why bushwalkers and campers do not camp in dry creek beds in areas at risk of heavy rain in the catchment of the creek bed – which could be a 100km away!
Queensland’s capital city, Brisbane lies on the banks of the Brisbane River which is now at risk of major flooding in the next 36 hours.
Major flood level of the Brisbane River is regarded as being at a level of ~3.6m, with the Wivenhoe Flood Mitigation Dam still managing to delay water flows into the Brisbane River.
At 9am today, the forecast was that the river is expected to peak at 3m tomorrow which will inundate perhaps 500 houses but if levels are more than this perhaps 7000 homes are at risk. Authorities have indicated 30-70 suburbs are at risk.
The Wivenhoe Dam reached full flood mitigation capacity today and further inflows will by necessity be allowed to flood Brisbane.
At 3pm today, this forecast has been revised to give a peak height of the Brisbane river at major flood levels of 4.5m at the high tide tomorrow afternoon, not too far off the 5.45m of the 1974 flood, but levels on Thursday are expected to EXCEED the 1974 flood and “devastating”!
The nearby city of Caboolture on the Sunshine Coast has been advised to evacuate today.
The Wivenhoe Dam was built above Brisbane to prevent a recurrence of the devastating 1974 flood when the Brisbane River hit just over 5m, and it was calculated that it would reduce flood levels in the Brisbane River by ~2m. See history of Brisbane floods here which show the highest flood on record was the Feb 1893 flood when the Brisbane River hit over 8m resulting in boats floating onto their Botanic Gardens.
The Wivenhoe Dam supply capacity is 1.15 million megalitres (“100% capacity)”, and can hold a further 1.45 million megalitres in a flood before it runs risk of failure. On the 6th Jan it was at 100% capacity, and by 9am yesterday it had reached “148%” capacity – filled to half its flood additional capacity in only 4 days!
Remember, assuming 100% run off, as in saturated soils, each 1mm rainfall results in 1 megalitre per square kilometre of catchment.
The dam has a 7000 square kilometer catchment and the recent inflows have equated to water twice the volume of Sydney Harbour EVERY DAY!
This has forced water authorities to release water from the dam which is contributing by necessity to minor flooding of Brisbane River at this stage and moderate flooding later today and tomorrow as levels reach 2-3m.
The ongoing heavy rain and fog is not only hampering rescue efforts by limiting helicopter rescues but it runs the risk of further unprecedented flash flooding throughout Queensland and it was hoped the dam will prevent major flooding of Brisbane itself.
Some areas between Maroochydore and Warwick are expecting further rain today well in excess of 50mm per hour (2 inches per hour)!
Many spent a cold, wet night on their roof tops last night and are still to be rescued this morning.
Ipswich levels will hit 16-18m today (in 1974 they hit 21m) – this has now been revised to be forecast at more than 22m and up to a third of the city is expected to be inundated.
One third of Queensland has been declared a disaster area – that is ~600,000 square kilometres – an area larger than the whole of France.
Best case scenario for many Queenslanders is that they are likely to be isolated in their homes for perhaps a week or more as roads continue to be closed, many others will have severe damage and losses to their homes and businesses from which it will take many months if not years to recover.
Bureau of Meteorology warnings here.
Premier Bligh has announced a revised figure, with up to 40,000 homes now at risk of being affected by the Brisbane floods while 100,000 will lose power for the next few days.
