Myall Lake flood 2021

[no way]

Last week's flooding on mid north coast caused flooding on Myall Lakes above the 1:100 year flood height. 'Fisherman's huts at Tamboy water up to gutter line, pontoon at Legge's pushing cones off tops of piles, houses on eastern side of Nerong harbour with 1m of water in lower floors. The lakes are currently closed due to the continuing high water and pollution and not expected to be open before mid-late April. In the attached pic taken last weekend the tree in the foreground is at the top of the bank which is about 3/4M below the level you see and the normal water level is another 1M plus below that. Is possible to launch boat off Egret St which leads to boat ramp. All boats in Nerong Harbour survived no probs

[Ozzie]

Woh, that’s a ship load of water No Way. It’s interesting that they are now saying the use of the term 1 in 100 year flood, which was standard practice during my tenure in road design probably should be replaced. It is actually a measure of intensity in truth although really a statistical analysis. But, you could technically have a 1 in 100 year flood 3 years in a row.

In the end insurance companies will dictate where we live.

https://www.abc.net.au/news/2021-03-28/ ... /100030144

[Raya]

The 1 in 100 year flood is simply a 1% chance that a body of water will reach a certain level during one year. At the beginning of each year there is 1% chance the river will rise to (say) 10 metres. There will also be a 5% chance that the river will reach 8 metres or a 0.05% that the level will be 12 metres. The estimated 100 year flood level is nominal level initially based on a statistical amount of rainfall over the catchment. The level will change based on actual observations of river level and rainfall.

Ozzie's observation that you can get three hundred year floods in succession is correct and these three floods would change the recurrence interval of the height reached. We have little more than 100 years of record on most of the major river systems in Australia which is far too short an interval for any accurate recurrence calculations.

Most cases of flood damage to houses is caused more by poor planning than by acts of nature. If a river has reached a certain height in the past, it will reach it again and probably surpass it at some time in the future. If you build in a flood plain, you will get flooded. Insurance companies know this.

The 2011 Brisbane flood inundated houses built after the 1974 flood. The flood was three metres lower the 1974 one yet councils had approved new dwellings below the 1974 flood level. Dams can attenuate flood peak heights, they can't stop floods. Blame was slated to the dam operators for the damage while the councils, who approved new housing below flood level, went untouched. I wonder who will get the blame when the next (inevitable) flood occurs.