Floodplain Manager August 2019


The release of the Hawkesbury Nepean Flood Study Report this month marks another milestone in the investigation of flood impacts in what is one of Australia’s highest risk floodplains.  I have been involved in flood investigations in the Hawkesbury Nepean Valley since 1991 and in that time there has not been one event on the river which could be considered to be a flood.  Even the 2012 event in which I was assisting in the NSW SES operations centre petered out into something less than a 20% AEP event.  It is no wonder that successive State and local governments baulk at committing to major flood mitigation investments or landuse planning reforms in the Valley which are going to cost money and/or votes.

Yet a repeat of the 1867 flood would create a flood disaster, the like of which has never been seen in Australia.  There are some houses in the Valley which have floors more than eight metres below that flood level and thousands which would be at least five metres under water.  The 1867 event was probably not much bigger than a 0.5% AEP event and there is evidence that at least one 0.2% AEP occurred some time before the British arrived.  While we report this month that thousands are still waiting to get back into their flood damaged homes six months after the Townsville floods, the wait could be much longer for many more in the Hawkesbury Nepean.

In fact, thousands of houses are likely to be completely destroyed and it is questionable whether they should be rebuilt.  This is what happened in Kempsey in 1949 when a whole street was flattened and never built on again.  We report on the 70th anniversary commemorations of that event.

While a lot of the statistics which are wheeled out about flood impacts relate to the numbers of houses flooded and people displaced, what often gets overlooked are the impacts on infrastructure and the cascading effects that can have.  I touched on that in my June 2019 editorial as it was the issue which started my investigations in the Hawkesbury Nepean and has remained of importance to me.  That is why I read with interest this month some recent international research into this very issue and how to develop a resilience framework for critical infrastructure.

When I first looked at this issue nearly 30 years ago our telecommunications systems were far less sophisticated that they are today and we were less dependent upon them.  However, they were actually much more flood resilient.  For example, the old land lines connected by copper cable would still operate for 24 hours on battery backups when there was a power failure.  With the recent roll out of the NBN and the obsolescence of copper networks our telecommunications systems quickly become inoperable in a power failure.  The way the power grid is built in the Hawkesbury Nepean, a big enough flood could black out tens of thousands of non-flood premises for months.  While I am not suggesting we go back to 30 year old telecommunications systems, we shouldn’t replace them without thinking about how to mitigate the inherent risks which they carry.

These are all salient reminders that as floodplain managers we do our most important and most difficult work when it is not flooding, so that these sort of human made disasters are minimised.

Steven Molino

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