Hawkesbury Floods - Then and Now

My new book ‘Sentenced to Debt: Robert Forrester, First Fleeter’ strongly features the economic impact of the Hawkesbury floods on an early settler in that district.  Page 160 provides some background context:

Two centuries later, it’s difficult to conceive the frequency of the massive flood problems that early settlers like Robert Forrester faced at the Hawkesbury before the Warragamba Dam was constructed. People have become complacent and authorities today recognise the area’s ‘constrained evacuation road network and low levels of community awareness of flood risk’, explaining that: 

Most river valleys tend to widen as they approach the sea. This is not the case in the Hawkesbury-Nepean River. Narrow sandstone gorges between Sackville and Brooklyn create natural choke points. The floodwaters from the five major tributaries back up and rise rapidly, causing deep and widespread flooding across the floodplain. It is much like a bathtub with five taps turned on, but only one plug hole to let the water out. The Insurance Council of Australia considers that the valley has the highest single flood exposure in NSW, if not Australia. (Infrastructure New South Wales website, http://www.infrastructure.nsw.gov.au/media/1525/hnvflooding_factsheet_feb2018.pdf) 

Authors are always encouraged when readers engage intellectually with their work. I’m happy to make public some recent correspondence from Ian Nicholls, who has been a thorough researcher of Hawkesbury history for many years.  Two 'East Coast Low' weather systems have hit the Sydney region within the last two weeks, bringing heavy rain and winds and alerting Ian to the potential flooding consequences for the Hawkesbury:

I’ve had a keen interest in Hawkesbury floods ever since I climbed into a flood boat outside our front door at Freemans Reach one dark night in June 1949. Page 160 reminded me of this experience. 

Freemans Reach, looking upstream, July 2019, photo by Louise Wilson

Complacency is alive and well in the Valley, but it took a jolt in February this year when a 9.2 metre flood arrived, without any water coming from Warragamba.

The Hawkesbury- Nepean River has two distinct catchments. About 20% of the catchment is on the Nepean and 80% on the Warragamba. The Nepean water is fast flowing, about twice as fast as the Warragamba. When there is a large low-pressure system off the coast of NSW the worst scenario is for the low to start at the Hunter and travel down to the south coast. If this happens the Colo and Grose Rivers start to flood just before the water comes down the Nepean from Wollongong and Robertson areas, and it comes fast, within 24 hours.

If the rain pattern extends inland far enough the water feeds into the Warragamba, but it takes longer, 48 plus hours to reach the Valley, assuming Warragamba is full. The only thing holding back the Nepean catchment water are the four dams, Avon [83%], Cataract [72%], Nepean [64%] and the Cordeaux [73%]. Current levels in brackets.

In February this year these four dams were much lower and the Hawkesbury Valley dodged a bullet even though Robertson had 395 mm in one day. As I stated above, the river at Windsor reached 9.2 metres with NO water coming from Warragamba, and the low was off the South Coast. 9.2 metres starts to cover the flood plain, but it could easily have been a 12.0 metre flood if the 4 dams had been full.

The SES has reason to be worried about the Valley, because you could have a 12.0 metre flood without any water from Warragamba and it can happen in 24 hours. If the low-pressure system stays around too long, Warragamba Dam fills to overflowing and adds more misery. The plan to increase the height of Warragamba will not save the Valley from a fast flowing flood from Wollongong/Robertson.

My take on the proposed 14 metre addition to Warragamba is it will hopefully keep the flood height at Windsor below 17.4 metres which is the minimum flood level build height for living area in the Hawkesbury City Council region. Hopefully the extra retention time in the dam will allow the fast flowing Nepean water to get away and not add to it.

My experience in 1961 or 1964 was we had a big flood from the Nepean, Grose etc., then the gates at Warragamba were opened and we had a bigger flood. If there is going to be a big flood, the Nepean will flood first and cut the roads on the flood plain. What happens after that will be caused by Warragamba overflowing, if the rain event in the catchments persists. The new Windsor bridge is open and the north side is now at flood plain level of about 10.5 metres compared to the old bridge level of about 6.0 metres. The North Richmond bridge is to be rebuilt at a higher level as well. So, traffic will be able to cross Windsor bridge in a 10.5 metre flood and access the flood plain on the north side. Wilberforce Road will be cut however, at Buttsworths Creek, at about 8.2 metres. So, the new Windsor bridge buys a few more hours to evacuate the north side.

Warragamba construction was finished in the late 1950s and was designed to supply water to Sydney and has NEVER been used for flood mitigation, i.e., the water level has never been varied to protect downstream areas from flooding.

Because of big floods in the early 1960s, when on one occasion the water continued to rise even though the gates were fully open, several reports were prepared for the Government based upon new thinking about frequency and potential flood heights. Several interesting things came out in these reports. The water capacity of the dam, as built, could only be reduced by 40% by opening the gates. It was calculated that to limit the height of another 1867 flood [19.68 metres] to 16.0 metres at Windsor the dam needed to be at a 10% level before inflow, and there was evidence in the gorges upstream from Penrith that there had been floods approaching 25.0 metres [equivalent at Windsor] before 1788.

Several proposals were made based on the reports. Increase the dam wall height by 15.0 or 23.0 metres. The final decision was to increase by 5.0 metres and build an auxiliary spillway (a rocky wall) containing ‘fuse plugs’. This plan had nothing to do with flood mitigation, it was to stop the water over-topping the dam wall, and it introduced another possible disaster for people downstream. The auxiliary spillway is designed to erode away rapidly [fuse plug] when over-topped, resulting in a wall of water up to 5.0 metres high going into the downstream flow. No flood mitigation here!

The latest proposal, now before Government, is to add another 14.0 metres to the wall height to act as short term retention and not be the new operating height. This is the first proposal to use the dam in flood mitigation. It is controversial because some of the World Heritage Park could be flooded, even if in the short term, until a flood passes. My view is the 14 metres will not save the Valley from a 1867 size flood if the worst case arises, which is, having to release water when the Nepean is still in flood.

P.S. 26 July 2020: I’m sitting here tonight listening to the wind and rain and thinking what it must have been like in 1867. A lot has been written in reports about preparing for the next big flood, but one thing being overlooked is the damage caused by the wind in 1867. There was up to 10 kilometres of open water around Windsor and the wind was gale force from the south-east, causing 1.0-2.0 metre waves to form in the non-flowing backwater, on the south side of Windsor, and roll across and through houses partly submerged in the flood water. Houses which could have survived being flooded were lost, just due to waves, and it could happen again.

Many thanks to Ian Nicholls for providing such a clear explanation of this topic, using his valuable local knowledge and experience.

'Sentenced to Debt' is available for purchase here.