The rain comes tumbling down, again

As heavy rain fell on my roof this morning, I pondered another forecast for heavy rain and possible flooding in the local area.

The Bureau of Meteorology issued a severe weather warning that stated:

HEAVY RAINFALL For people in Metropolitan, Illawarra and parts of South Coast, Central Tablelands and Southern Tablelands Forecast Districts. (BOM, 2/7/22)

This brings back memories of early 2022 and the effect of local flooding. There is damage to property and people’s mental health.

People become worried about the unknown. So let’s help clear some of the fog.

What is unique about floods on the Hawkesbury-Nepean River?

 The ‘bathtub effect‘ of the Hawkesbury-Nepean River Valley

The Hawkesbury-Nepean River valley has unique landform features that make flooding in the local area perilous.

The river in flood does not behave like other valleys with wide-open flood plains that allow flood water to spread out and slow down.

The Hawkesbury-Nepean River valley has several pinch points constricting the flow and creating upstream localised flooding. This has been termed the ‘bathtub effect’ by engineering geologist Tom Hubble from the University of Sydney in 2021.

The 2019 H-N Valley Regional Flood Study describes the river valley this way:

 The Hawkesbury-Nepean Valley consists of a sequence of floodplains interspersed with incised meanders in sandstone gorges. (ERM Mitchell McCotter, 1995).[p.6]  [ERM Mitchell McCotter, (1995). Proposed Warragamba Flood Mitigation Dam Environmental Impact Statement, Sydney Water, July 1995.]

The Geography Teachers Association has produced an excellent teaching resource about the river valley, and it states:

The unique geomorphic features of the Hawkesbury-Nepean Valley make it particularly vulnerable to dangerous, fast-rising floods.

The NSW SES says:

The Hawkesbury-Nepean Valley has a long history of dangerous and damaging floods. Since records began in the 1790s, there have been over 130 moderate to major floods in the valley, including 6 major and 21 other serious floods since Warragamba Dam was completed in 1960.

So local people have a right to be worried when the BOM issues flood warnings,

Flood trauma is real.

Floods cause a considerable amount of anxiety in the local area.

The New South Wales Governments website Emotional and trauma support after flood states:

Natural disasters, cleaning up and recovery can take a toll on your mental and physical health. It’s vital people seek support and look after their own and their loved ones’ wellbeing. 

The Black Dog Institute states that after flooding:

We anticipate that Australians living in areas affected by the current New South Wales and Queensland floods are likely to experience psychological distress. While some level of distress is a normal and understandable response to these events, we know from previous disasters that for many this may lead to more chronic mental health problems.

Royal Life Saving Australia says there is grief and trauma after the flooding. It maintains:

Looking after yourself during and following a flood event is an important part of the flood recovery process. If you have lost someone during a recent flooding event, or been rescued, it is especially important to check in with your support network and identify steps to help you get the additional support you may need. Everyone processes grief differently, and there is no one ‘right’ way to grieve, but we all need help in difficult times.

For the nerds

There is a lot of nerdy technical stuff around flooding in the river valley.

Technical details

There is an excellent study called the 2015 Nepean River Flood Study for technically minded people.

The study defines the Upper Nepean as the river upstream of the confluence of the Nepean River with the Warragamba River and is around 1800 square kilometres (p1).

For those who want to read a broader study about flooding across the Hawkesbury-Nepean catchment, I suggest looking at the 2019 Hawkesbury-Nepean Valley Regional Flood Study.

One local sage, community gardener and flood watcher, Steve, commented on Facebook:

There has been some discussion about the possible rain event on its way [1/7/22]. Happy to report that all upstream dams are below capacity as per the Bureau of Meteorology. The [community] gardens are not affected till 8.5 metres, upstream inflows will be monitored and in the event of water reaching 8 metres livestock will be moved to higher ground within the garden where applicable or externally if required under the guidance of any relevant authorities. Note that the lagoon fills slowly from the river via the old creek line. However, if the river reaches 11 metres Macquarie Road floods over. Flooding has typically peaked in Camden 9 hours after Avon Dam Road peaked and 3 hours after Menangle. The last floods #3 peaked @ 20 metres at Avon Dam Road. The previous #2 at close to 17 metres. Note the last flood 12.2 metres in Camden occurred after all dams were also full.

This information comes from the BOM rain and river data site.

Steve was disappointed in his predictions about the size of the weather event affecting the New South Wales East Coast.

The rainfall at Robertson is a good indicator of what might happen in the Upper Nepean River river valley. Up to 9.00am today (3/7/22), Robertson had received 258mm of rainfall; at Menangle Bridge, there had been 185mm of rain. The Upper Nepean River valley is saturated and partly explains the behaviour of the Nepean River at Camden.

Historic river heights at the Cowpasture Bridge, Camden.

The historical records of flood heights at the Cowpasture Bridge provide an interesting comparison of the present flood. The records are contained in the 2016 Camden Local Flood Plan.

Updated 19 May 2023. First posted on 2 July 2022 and called ‘The rain comes tumbling down, again’