| Extent of the 1867 flood in the Hawkesbury (from Hawkesbury Council) |
I hate to say it, but it is inevitable that one day we will have a flood in the Hawkesbury-Nepean as least as serious as the historic one we are currently seeing Queensland, and it won't be until then that we'll get real action on flood mitigation in this area.
Then all the pollies will wail about the loss of property and life, all the residents who've moved into the area and have no idea what it's like to live on a floodplain will ask why they were allowed to build or buy in flood-prone areas, and all the Cassandras will say "we warned you for years, and you ignored us."
The map above shows the extent of the largest historical flood experienced in the Hawkesbury, in 1867. If you live around the Hawkesbury, there are two markers you need to visit. One is a nail in the outside wall of the Macquarie Arms pub at Windsor. A second is in the grounds of Windsor Public School. If you cast your eyes across from either of those points and use your imagination, you may get a sense of the scale of a major flood. The water reached that level in 1867. Now look at that map again. If you live in the Hawkesbury, there's a better than even chance that your home lays in that blue area, since our population is densest around Windsor, South Windsor, Bligh Park, Richmond and so on. And any flood will not have to be of the scale of the 1867 event to be catastrophic. This year marks 50 years since the last big flood in living memory, in 1961. It's a common misconception of statistics to think that a statistically overdue event becomes more likely as time passes, but it underlines that a generation of Hawkesbury and Penrith area residents have little personal experience of a big flood. The terrible things we are seeing on TV today will one day play out in our own back yards. Why should we believe that something that has happened many times before will not happen again?
What will we do? What should be being done now?
5 comments:
It's good to be aware of this. Can they mitigate rapid downpours by using the dam as a buffer, thus lowering the risk?
I believe that Warragamba dam, as built, lessens the maximum probable flood by some degree, and may decrease the frequency of minor flooding. This is because the dam allows a degree of control over the release of water into the downstream river. The problem comes when the dam is full, and 100% of the inflow is obliged to be released straight away. People look at the dam and say "but it's only half full. Surely we can't flood if it's that empty?" What they don't realise is that it would only take a week's rain falling in the right area of its catchment (which extends most of the way towards Goulburn) to get to 100% and then over 100%. Then, a major flood is on.
It's interesting that even after the construction of the Wivenhoe Dam in QLD after the record 1974 floods Brisbane still managed a catatrosphic flood level within 1m of the 1974 floods. This just goes to show that even with the Warragamba Dam in place major flooding of the Hawkesbury is still a very real possibility.
As I am relatively new to the Hawkesbury and after seeing the QLD floods I am particularly interested in researching the impact a major flood would have on the suburb where I am now living. Are there any good websites for researching information like this or is a call to the Hawkesbury City Council the best option?
Apart from the above map, your options are limited.
I was speaking to the chair of the Hawkesbury-Nepean Flodplain Management Committee, Kevin Conolly recently and asked "So, if a local resident wanted to know what elevation their property was at, how would they do it". His reply was "not online. They would have to call to attend the inquiry counter at the Council chambers and ask."
The 1:100 flood building limit is 17.3m, at Windsor. Elsewhere it may vary. Your flood susceptibility will vary on factors like the relative inflow and outflow rates in your suburb, and so the ultimate flood peak will vary from place to place.
Warragamba is not a flood mitigation dam. It has a capacity of 4 sydney harbours. It will only reduce flooding by the amount of storage it has available when it starts raining, which at the moment is 1 sydharb (It is 73% full).
Warragamba's catchment extends from Lithgow to Lake George near Canberra. It could easily fill in 1-2 days. Wivenhoe dam (Brisbane) is a flood mitigation dam, holding 5 Sydharbs, 3 for drinking and 2 for flood mitigation. As you just witnessed this is to reduce frequent little floods. It does little to reduce a major flood event.
The rivers flowing through Rockhampton recently received something like 300+ sydharbs in a month. Talk of flood mitigation in that catchment is just that, talk.
Unfortunately, if the catchment above warragamba received anything like the rainfall recently seen in Qld, or Vic for that matter, the free capacity in warragamba would be of little significance.
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