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Monday, 17 January 2011

Brisbane (Queensland) flood disaster 2011

Hydrology 101. Study of floods. A layman's take on the Brisbane floods of 2011.

The Catchment at a Glance.
The Brisbane River Catchment area is about 13,500 sq km. This catchment includes the Upper Brisbane, Stanley, Lockyer and Bremer Rivers. It includes 850kms of river and lake banks as well as 50 major creeks.

Over 85% of the catchment has been cleared and developed into various land uses, including rural (grazing, cropping and forested lands) and urban centres (at the lower catchment). This holds the largest population of any catchment in Queensland.

Photo shows an illustration of a typical catchment or drainage basin.
The Brisbane river catchment is equivalent to an area about 135km by 100km or about 70% of the Cordillera, and just slightly smaller than the land area of East Timor. This catchment could cover an area similar to part of the northern Philippines covered in the photo below. (Obviously this area of the Cordillera is NOT just in one, but several catchments).

This is merely illustrative as the Brisbane river catchment is more rough (apparition-shaped) and irregular, than rectangular (below). The path of the inland tsunami went from west to east (Toowoomba to Brisbane) for about 150km.

The critical centres affected by the floods are highlighted further in the photos below.

Before the deluge
According to experts, this summer (wet season down under) marks one of the strongest (if not THE strongest) La Niña events since records began in the late 19th century . The Bureau of Meteorology, as early as October, was warning of substantially increased rain across eastern Australia.
In the previous October and November, the Brisbane river catchments received more than their fair share of spring and summer rains. Brisbane experienced its wettest December in 150 years.
After the last great flood disaster in 1974, Lake Wivenhoe was dammed primarily for flood mitigation purposes and to protect Brisbane from a similar Q100 (one in 100 year event). However the dam was nearly twice its capacity (193%) at the height of the rains, and cannot hold anymore water had the rains kept falling. The other main dam in the catchment, somerset dam was at 160%.

Northern part of Brisbane River catchment

Southern part of Brisbane River catchment

What transpired.
• Three weeks of rains had saturated the city of Toowoomba before it was hit by a line of intense thunderstorms and 36 hours (and more than 160mm) of torrential rains causing sudden or flash flooding on 10 January 2011.

• The massive volume of water generated by the super rainstorm, later described as an 'instant inland tsunami', surged as a devastating wall of water and tore through the city centre of Toowoomba which sits on a plateau in the watershed of the Great Dividing Range, some 700 metres above sea level.

• In the evening, within hours it had claimed many lives as it surged downstream on Lockyer creek rolling past and wiping out chunks of the townships of Murphys Creek, Grantham, Withcott and others in the mostly flat Lockyer valley.

• During the storms, floodwaters from the surrounding hilly catchments on the north and south, including the  major tributaries of the lockyer river (Laidley Creek, Tenthill Creek, Murphys Creek and Ma Ma Creek) added to the ever swelling watery terror.

• This inland tsunami reached heights of 8m and perhaps more than 1000 metres (1km) in width (a serviceman interviewed on ABC tv said between 1-2 kms) as it gathered pace and volume on its way along the lockyer valley towards Ipswich and Brisbane, more than a 100kms and two days away in the east.

• On another tributary, the Bremer River at Ipswich - some 30 kilometres west of Brisbane - reached a height of 19.4 metres on 12 January, inundating the CBD and flooding at least 3,000 houses. One third of the city was flooded and thousands of people were evacuated.

• Already on 11 January 2011 in the afternoon, the Brisbane River broke its banks leading to evacuations in the Brisbane CBD. On 12 January the flood peaked during the afternoon high tide at 4.5m, about 1m less than the previous peak in 1974. Thousands of homes and businesses have already been evacuated.

• The tsunami from Toowoomba hit Brisbane on Thursday. Along the way the raging torrent from lockyer river joined forces with rainwaters from all the sub-catchments. This watery terror, increasing in size all the time, went through and literally obliterated the towns and fields in its way. It merged with the Brisbane river and the swollen waters of the bremer river, just downstream of Wivenhoe dam and again magnified in size as it roared inexorably towards Brisbane and beyond to Moreton bay. On 13 January the floodwaters peaked at 4.46m in Brisbane but still inundated 20,000 houses in 80 suburbs. The rest is history.

The latest figures are 20 dead and 10 missing.And in breaking news, Queensland Premier Anna Bligh has announced a commission of inquiry into the state's devastating floods.The inquiry will be conducted over 12 months with an interin report to be provided in August, in time for the next wet season.
Watch this space.


1. La Niña and low pressure systems in spring, resulted in unusually high levels of precipitation or increased rainfall throughout October to December. These rains have saturated the catchments and the ground can not absorb or hold any more water.

2. The dams built to reduce flood severity (flood mitigation purposes) are nearly overflowing. eg Wivenhoe dam at 193% capacity, Somerset dam 160% capacity.

3. 85% of the Brisbane river catchment has been cleared or developed. Trees and vegetation create space in the soil for rainwater to move in and get absorbed. Development, deforestation or simple land clearing, cause compaction filling up the soil space used by air and water. The result is increased and accelerated runoff and drastic topsoil erosion.

Had it kept raining through to the height of the floods, things would be way way worse. Rains abated around midday on 11 January, and cloudy but mainly fine weather followed from that day.Seventy five percent of Queensland is a disaster zone. (That's more than 4x the size of the Philippines, more than 5x the size of Great Britain or nearly twice the size of the 'big' state of Texas).

Around the state, many other towns (Ingham, Babinda, Gordonvale, Rockhampton, Emerald, Bundaberg, Dalby, Toowoomba, and Ipswich, Condamine, Chinchilla, Theodore, Moura, Rockhampton, Gympie, Goondiwindi) are experiencing their worst floods.

Numerous Roads and highways remain closed and impassable, and the only way to travel between centres is by helicopter. Many regional airports remain closed too. Flood warnings are current for many rivers (Herbert, Fitzroy, Burnett, Condamine/balonne, Mcintyre, Mary).

The great flood of brisbane in 2011 is over. The waters are receding. Let us keep Queensland and Queenslanders in our thoughts as they get back on their feet.
Note too that all around the world, disasters are also happening and continue to happen. Floods are also causing death and destruction in the Philippines, Brazil and Sri Lanka. There are bombings in Pakistan and Afghanistan. Elsewhere in Australia, there are record floods in northern NSW, Tasmania, South Australia and Victoria. All these even as Haiti remembers last year's earthquake. 
Let's put things in perspective.

And if we can live with less of the luxuries, that might be a start. Perhaps mother nature will be less severe next time.

Read Part 2 of this blog here.