Here are some of my observations, concerns and conclusions in regard to the extraordinary and tragic flash flooding event that occurred in Toowoomba 10/01/2011.Monitoring in the Toowoomba region is insufficient and certainly not of an acceptable standard. Moreso, it can be argued that lives could well have been saved if a more cohesive monitoring network were in place.
The urban area of Toowoomba encompasses over 120,000 people, with an additional 30,000 living within the council district making it the largest inland city in the country behind only Canberra.
Yet despite these figures Toowoomba has only two official rainfall gauges (Toowoomba Aero #041529 and Toowoomba Alert – Mt Kynoch #540162) which are both situated on the urban fringe and just one flood gauge (Gowrie Creek at Cranley #541093) that is useless for urban flood events as it is located downstream from the city.
The storm itself formed just after 11:00am (AEST) as two cells displaying echoes of moderate intensity merged over Esk to form a single system. It then proceeded to move in a general direction towards the southwest, crossing the ranges near Upper Sandy Creek at 12:30pm and passing over the Toowoomba urban area for approximately an hour between 1:00pm and 2:00pm.
Approximate path of the storm is shown below.
(Yellow lines indicate paths of the cells that merged at Esk to form the system; Pink indicates the edges of moderate to heavy rainfall under radar; Red line indicates approximate path of heaviest falls according to radar)
Transect of the path of heaviest falls from radar imaging, displaying the higher elevation of Toowoomba that intensified rainfall outputs.
The steep incline of the Toowoomba Range fired an intense burst of convection, causing very heavy rainfall on the slopes and urban area. For the 24 hour period to 9am 11/01/2011 Toowoomba Alert and Toowoomba Aero reported 117mm and 123mm falls respectively with 60mm falling in one hour as the storm passed at the latter mentioned station.
However many unofficial totals reported much, much higher figures nearby and as has been reported by the media, the BOM have conceded that this would have been the case: Courier Mail - Jim Davidson quote: WEATHER Bureau chief Jim Davidson described the downpour that caused the fatal Toowoomba and Lockyer Valley floods as a super storm.
Mr Davidson said reports were received of 80mm of rain falling in just over 30 minutes, but he suspected falls of up to 200mm would have occurred in areas where rain gauges were not set up.
As a further piece of evidence to confirm this, NASA have produced an image of the rainfall totals on that day from satellite analysis, showing 200mm+ falls concentrated in Toowoomba area: NASA Satellite SEQld Rainfall
This intense rainfall lead to the infamous “instant inland tsunami” in the Toowoomba business district and a wall of water 8 metres high into the Lockyer Valley, both causing loss of property and life on a disastrous scale.
Let’s take a step further into the event and look into other factors involved in why it occurred the way it did and what measures can be learnt to be put in place to prevent future tragedy.
Firstly the storm never appeared unusually intense upon radar imaging, a misleading situation that caused emergency warnings and procedures to be either delayed or held back entirely and no prior notification of what was coming to those who would be potentially caught up in it. Marburg 128km Radar Archive of Storm Mt Stapylton 128km Radar Archive of Storm
Toowoomba happens to be located on the edge of an area within SE Qld that the BOM acknowledge is lacking in optimal radar coverage as you can see:
Both the Mt Stapylton and Marburg radars failed to detect the exceptional amount of rain falling in the Toowoomba region in regard to its actual intensity, instead only detecting reflectivities of a moderate status. Marburg (in my opinion) drastically underestimates reflectivities and its resolution (being a WSR74 S Band radar) is poor when evaluating intense, localised rainfall events such as this. Mt Stapylton was able to resolve the shape of the storm complex well but failed to pick up the severe intensity of the rainfall. I believe this occurred due to the fact that Mt Stapylton is located 125km away from Toowoomba and at this distance much of the lower level precipitation would be below the scan of the radar and thus undetected, making it appear that the rainfall in the region is lower than what is actually occurring on the ground.
From this alone it could be argued that an additional radar facility is required west of Marburg to provide coverage of Toowoomba and the Darling Downs (Mt Kynoch potential for a site?). I have no doubt that if such a facility was in place, it would have picked up this event earlier and more completely and people would have more informed about the situation and be able to make decisions about any necessary preparations.
The city of Toowoomba itself is located the top of a range at some 700mASL with no major watercourses flowing through it, hardly the expected place for a major flood event to be expected to occur. But factors combined to cause such an event to take place 10/01/2011.
The urban area occupies the edge of the range and the low ridges behind it. Two valleys run north from the southern boundary, each arising from springs either side of Middle Ridge near Spring Street at an altitude of around 680 m. At the confluence of these waterways, East Creek and West Creek, settlers built the first dwellings where water was most accessible and today this area comprises the business district of the city. North of the CBD the watercourse is called Gowrie Creek and is a tributary of the Condamine River.
Additionally the city is located upon red ferrosols derived from mid to late tertiary age basalts which are famously rich in nutrients and near perfect for cultivation purposes. A characteristic of this soil type is that under normal conditions it is well draining, but under intense rainfall it will hold water and cause the water table to rise, resulting in runoff and inundation.ANRA - Ferrosols
So under an event whereby 200mm is falling over a short period, these soil types can be expected to result in significant surface runoff – particularly when over the previous 24 hours the area has been exposed to rainfalls in excess of 100mm in some areas as was the case for Toowoomba region that day.
Furthermore Toowoomba had reported no rainfall on just 16 of the last 55 days prior to the event, and water tables would have already been significantly higher than they would be under normal circumstances.
For those interested here is some further reading: Toowoomba Flood Report - Oct 2009
Considering the CBD is located where the only two watercourses in the city meet, it is surprising and alarming that there are no
flood gauges upstream on East and West Creeks.
In fact, there are NO FLOOD GAUGES IN THE URBAN AREA AT ALL! In the event of an inundation, there is currently no way of knowing unless it is an eyewitness report
. And so it was for the people of Toowoomba – both East and West creeks burst their low drainage banks and sent torrents of water through the city, converging on the CBD where they became the “inland tsunami” and nobody received any prior notice. It is clearly an unacceptable situation to not have flood gauges in multiple locations on East and West Creeks as currently stands. Imagine if say, Kedron Brook had no gauges and a flood was coming down it – that is the situation in Toowoomba. Lives were lost unnecessarily because of this. Flood gauges don’t cost much and are easy to install – the lack of them in this instance is pure negligence.
The only gauge in the city area is as previously mentioned, well downstream at Cranley so its only use is for alerting Oakey of coming events.
Please disregard the peak on the 12th Jan - it is an error in the data. Also note the gauge was likely affected by debris (which would explain why the reading stayed at 0.30m for several hours, 0.70m below the pre-flood height) and the actual peak height experienced at the station could well have been much higher.
Gauges upstream from the CBD area would provide evidence of an event occurring before it impacts so communications and warnings can be sent out to those in its path. However little time is provided, any warning is better than none. It would not have taken long for people to abandon their cars and evacuate to higher ground had they been aware of what was coming – mere minutes could have sufficed.