Floods and the Camden ‘bathtub effect’

Flooding of the Nepean River on the Camden floodplain

What is the Camden ‘bathtub effect’?

Not sure – well you are not on your own.

The ‘bathtub effect’ is part of the flooding effect created by the landform that makes up the Hawkesbury-Nepean River system. The river system has a unique floodplain system that creates particular problems for local residents and others along the river.

The Hawkesbury-Nepean River valley has a number of pinch points that constrict the flow and create localised flooding upstream. This has been termed the ‘bath-tub effect’ by engineering geologist Tom Hubble from the University of Sydney in 2021.

The NSW Department of Primary Industry stated in 2014:

The natural characteristics of the Hawkesbury-Nepean Valley make it particularly susceptible to significant flood risk. The combination of the large upstream catchments and narrow downstream sandstone gorges results in floodwaters backing up behind these natural ‘choke points’

The Hawkesbury-Nepean River system has four localised floodplains created by four choke points along the river.  Each of these ‘choke points’ is created by a local gorge along with the river system – Bents Basin Gorge, Nepean Gorge, Castlereagh Gorge and the Sackville Gorge.

Each of the four localised floodplains upstream from the four gorges acts like a ‘bath tub’ in a period of high rainfall, with floodwater flow choked off by the gorges.  The gorge restricts the floodwater flow and the river rises quickly behind the gorge at the end of the local floodplain.

Camden’s ‘bathtub effect’

The 2015 Nepean River Flood Plain Report and the flood maps clearly show how the Bents Basin Gorge acts as a ‘choke point’. The gorge creates a ‘bathtub’ upstream along with the Nepean River floodplain from the entrance of the gorge. The floodplain upstream from the gorge starts around Rossmore, then continues upstream to Cobbitty, to Camden and ends at Menangle.

While the Camden ‘bathtub effect’ is not as dramatic and dangerous as those created in the Penrith-Emu Plains area or the effect of the Sackville Gorge at Windsor and Richmond – it is real.

The 2015 study says (pp1-2) that while floods are ‘rare’  they happen:

 flows escaping from the Nepean River are known to inundate the low lying areas of Camden and certain sections within South Camden and Elderslie. Floodplain areas along many of the tributaries of the river (particularly Narellan Creek and Matahil Creek) are also known to be affected by backwater flooding from the Nepean River during flood events.

Characteristics of local flooding

The 2016 Camden Local Flood Plan says:

Floods are characterized by rapid river rises with flooding commencing as quickly as 6-12 hrs after the commencement of heavy rain if the catchment is already saturated. Under flood conditions, the Nepean River overflows its banks and commences to inundate the low lying floodplain around Camden during floods of 8.5m on the Cowpasture Bridge gauge. (Appendix, pp. A1-A3)

Causes of flooding along the Hawkesbury-Nepean River on the Camden floodplain

The headwaters of the Nepean River floodplain at Camden is the Upper Nepean Catchment. This geographic area drains the Avon, Cataract, Cordeaux and Nepean Rivers, with dams on each waterway.

The catchment of the Nepean River above the Warragamba River junction, below Warragamba Dam, is around 1800km²

The wettest conditions are usually created by low-pressure systems, called east coast lows, that form up off the South Coast of New South Wales. The low-pressure systems moving onshore and the Illawarra Escarpment’s orographic effect can produce heavy rainfall events.

The 2016 Camden Local Flood Plan says:

 Many localities in the catchment have received in excess of 175mm in a 24 hr period. (Appendix, pp. A1-A3)

Largest local floods on the Camden floodplain

The 2016 Camden Local Flood Plan states:

Floods have occurred in all months of the year. The highest recorded flood at Camden occurred in 1873, when a height of 16.5m was recorded on the Camden gauge (approximately a 200yr ARI).  [Cowpasture Bridge, Camden]

Other major floods occurred in 1860 (14.1m), 1867 (14.0m), and 1898 (15.2m). In recent times, major floods have occurred in 1964 (14.1m) and 1978 (13.5m) with moderate to major flooding occurring in 1975 (12.8m) and 1988 (12.8m). (Appendix, pp. A1-A3)

A report of the 1898 flood event at Camden taken from the Camden News 17 February 1898 gives clarity of how quickly the river can rise in the local area:

Near midnight on Saturday rain began to fall, at first with moderation, towards day break gusts of wind sprang up from the South East bringing heavy rain, lowering the crops in its passage, even majestic trees were torn up by their roots and in sheltered paddocks the trees were denuded  of large limbs.

Sunday all day the wind blew with hurricane force; early on Monday morning the storm somewhat abated in its velocity.

Even on Sunday midnight no apprehension of a flood was anticipated by the Camden townspeople the continuous rain and boisterous weather, however made the more Cautious anxious, and one tradesman took the precaution to look after his horses in near paddock when the danger of a flood was manifested to him, the Nepean River had suddenly risen and was flooding the flats.

Camden News 17 February 1898

A report in the Camden News of the 1911 Camden flood event provides further clarity around the behaviour of the river:

The rain of Thursday, it may naturally be expected filled creeks, dams and watercourses to overflowing, but the climax came with a heavy storm between 7 p.m. and 10 p.m., when some four inches [100mm] of rain fell. This brought the local water down from the adjoining hills in torrents, the Main Southern Road and Carrington Road were then covered with some two feet of fast rushing water, and on Druitt Road the local flood was then absolutely impassable..

In the early hours the Nepean River rose rapidly, and before the arrival of the first train the bridge was impassable ; the water continued to rise till about 3.15 in the afternoon, it having then reached it highest point, covering the new embankment between the town and the bridge, running through the Chinese quarters on the one side, and just into the pavilion on the show ground on the other. From near Druitt Road to Beard’s Lane was one long stretch of water….

Camden News, 19 January 1911

Sackville Gorge and the Windsor & Richmond ‘bathtub effect’

In 2012 director of community safety with the State Emergency Service, Steve Opper argues that the Hawkesbury-Nepean Valley has a unique shape that can lead to catastrophic flooding. He describes the effect of the Sackville Gorge on the Hawkesbury-Nepean River:

 “The Hawkesbury-Nepean Valley is throttled down by a narrow gorge down near what’s called Sackville, which is just upstream of Wiseman’s Ferry,” he said.

“The result of that is that the water can flow into the top of the system very, very rapidly, can’t get out, and so you get very dramatic rises in the level of the river.

“So normal river level might be two metres; if you’re at the town of Windsor and in the most extreme thought possible, that could rise up to 26 metres, which is a number that’s quite hard to comprehend.”

John Thomas Smith reported in the Sydney Morning Herald,  2 July 1867 after a flood event that

‘The enormous body of water rushing down with relentless force on its way to the sea could not be easily described, nor its effects conceived. About the neighbourhood of Windsor, now that the waters are fast subsiding, the scene is most dreary, and the destruction caused be -comes every day more apparent. The feeling of bitter anguish expressed not in words but in the blank look of utter despair would move the most hardened.

Conclusion

Flooding is a normal part of the cycle of the Hawkesbury-Nepean River system, as it is for any river basin in Australia.

The particular landform features of the Hawkesbury-Nepean with its four gorges along the river produce four localised floodplains that create a local ‘bathtub effect’ on the local floodplain.

This landform effect of the river gorges creates flooding severity in the local communities.

Updated 4 July 2022. First posted 29 November 2019.