You don't build dams to protect people in the rare instances of flooding. Dry rivers & tributaries during the (most of the time) drier periods will be the result. Man trying to control nature comes back to bite!
I totally agree, Wysiwyg.
There are numerous consequences for the local ecosystems when a large dam is built. The most obvious example of which we already suffer supply deficiencies is Fish. Many fish and marine life need to navigate from the upper reaches to the sea in their breeding and life cycle. Also, it's often not enough to just release a steady trickle downstream for the enviornment.
The weather works in cycles for a reason and many humans fail to grasp that and adjust their circumstances accordingly, especially in the government planning agencies, such as allowing too much urban development in known flood planes without the equal amount of risk management protocols, such as emergency evacuation systems. The SES is just so unresoursed.
Too often the local damage is amplified by development, such as sealing large areas with bitumen and concrete and not allowing sufficient additional runoff resources for water that would ordinarily soak into the ground.
Then we have meddled with the watercourses usually restricting their capacity with concrete channels and pipes.
But probably the biggest problem is over clearing vegetation from flood plains. This allows the water flow to acelerate in the upper reaches of river systems and arrive at densly populated towns and cities down stream on the coast a bit quicker... then slow down and back up as it strikes development in the flood plain, such as bridges, reclaimed swamp land used for warfs and industrial facilities and general urban and business buildings on the flood plain.
I doubt the Traverston dam would totally mitigate Gympie and Maryborough on the Mary River from flood damage any more than the Wivenhoe dam will totally mitigate Brisbane form flood damage. Wivenhoe has been frantically releasing water for some weeks to try to avoid overflowing the spillway which could happen very soon with another decent rain event. Then the 'Flood Mitigation' effect will be worthless and the damage potentially worse if town planning lowered the 'flood' level for development purposes.
The proposed Traverston Dam had a planned capacity of 153GL, it would have been full prior to this event as almost all of the dams in the
SE QLD catchment are already full. The Wivenhoe dams stated capacity is about 1000GL and it is currently at 2000GL, which is probably close to the limits of it's flood mitigation capacity. So if the Traverston Dam had been built it would have been like it wasn't there yesterday.
FYI: the flow rates of rivers is commonly expressed in multiples of Sydney Harbour. 1 Sydney Harbour = ~500GL. The Fitzroy flow rate when Rocky was flooding was around 1.5 Sydney Harbours per day = 750GL/day. That would have fillled an empty Traverston Dam in about 5 hours and a bone dry Wivenhoe Dam to it's current level in about 2.6 days.
PS:Thanks derty, I see you have done the technical stuff derty for my arguement.
That easy to remember ratio and conjunction with local flow rates is worth repeating to permanently imprent in our psysic for consideration of our next elected officials and their development and planning policies.