Monday, February 14, 2011

FLOOD FACTS


Since Brisbane flooded in January 2011, there has been much comment and review on the circumstances of this event in the media – both electronic and print. What one might call ‘the facts’ of the situation remains mixed in a vague scattering of opinion; but there seems to be sufficient information that has been confirmed by repeat reports to allow one to begin a chart of the situation that might loosely be seen as ‘facts’. If there is ever going to be a responsible response to the management of Brisbane’s future, then these facts need to be understood and agreed upon.
One might begin such a list like this:
FACTS
  • There are some hydrologists who have always believed that another 1974 flood was possible for Brisbane – refer Hugh Lunn’s report in The Weekend Australian 12-13 February 2011, INQUIRER, page 4; et al.
  • Hydrologists knew that there had been much higher floods than 1974 and that the history of such events has shown that there is never any reason to believe that these extremes would never be repeated or bettered – see Hugh Lunn report; et al.
  • Wivenhoe Dam has been and still is being promoted as allowing a 2m reduction in 1974 flood levels. No comment is made on higher levels but it seems to be implied that these too can be managed similarly - see seqwater Internet site  http://www.seqwater.com.au/public/catch-store-treat/dams/wivenhoe-dam:
It is anticipated that during a large flood similar in magnitude to that experienced in 1974, by using mitigation facility within Wivenhoe Dam, flood levels will be reduced downstream by an estimated 2 metres.
  • Brisbane Q1:100 design levels were reduced below 1974 levels on the basis of the Wivenhoe Dam. This opened up more low land for development. - see http://resources.news.com.au/files/2011/01/20/1225991/887259-110121-brisbane-flood-study-jun-1999.pdf - This Brisbane River Flood Study was prepared by City Design. Section 6, Flood levels Along The River, notes: At the Port Office gauge the flood level corresponding to the calculated 1 in 100 year design flow of 8,600 cubic metres/second is estimated to be 5.0AHD. The current development design flood level, based on the 1984 study, is 3.8 AHD some 1.2 m lower than the level predicted in this study. From the two flood profiles plotted on Figure 3 it can be seen that the flood levels calculated in this study vary from 1.0 m to almost 3.0 m higher than the current development design flood level in Brisbane.
  • Brisbane has grown substantially since 1974 – various TV reports.
  • Wivenhoe Dam manages only a portion of the Brisbane River catchment with the Bremer River and Laidley Creek catchments lying outside of the dam catchment areas – various TV reports and Hugh Lunn; et al. It is this circumstance, along with others, that is used to support the prediction of the likelihood of yet another 1974.
  • A full dam is equivalent to no dam – Hugh Lunn; et al.

If these ‘facts’ are agreed, then all of these matters need to be reviewed as a set, with implications of one assessed side by side with those of the others.
History, it appears, has shown that the hydrologists are correct – that the Wivenhoe expectations were at best very optimistic, or were based on the dam never filling; or assessed without an analysis of the increased impact of flooding with an increased percentage of the dam filled; and perhaps with all calculations seeming to ignore any parallel impact from catchments outside of the dam. What is what?
QUESTIONS:
Why have the hydrologists’ opinions been put aside and not debated publicly or acted upon? How did this happen? Why?
Similar questions can be asked about the dam itself – was there any study of the likelihood of any increase in risk with increase in fill? – of increase in risk with the parallel increase in risk from other catchments? What work has been done on this?

What to do now?
If another 1974 flood is likely/possible, then what should the flood levels for Brisbane be? Should they be raised? How can this be managed? Can it? What impact? What implications?
Have things now got to a stage of just being too complicated, leaving us to be idiosyncratic Queenslanders – bred tough and able to just keep jumping up for more of the same as we keep doing more of the same?
It is a very difficult circumstance but it must be faced this time so that we might know what to expect in the future.
Being hopeful is just not enough.

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