The unique geography in the valley affects the extent and depth of flooding in the region. Most river valleys tend to widen as they approach the sea. The opposite is the case in the Hawkesbury-Nepean Valley, which means floodwaters flow into the valley more quickly than they can flow out, causing them to back up and rise rapidly.
Much like a bathtub with five taps (the major tributaries) turned on, but only one plug hole to let the water out. Watch this video to learn how the ‘bathtub effect’ makes this valley have one of the greatest flood risks in Australia.
The recent March 2021 flood was only the 15th largest recorded flood in the region – though the impacts were felt far and wide. Floodwater severely impacted over 600 homes, over 30 caravan parks, hundreds of farms, and transport routes and essential services such as electricity and water were cut. The estimated damage bill was several hundred million dollars.
However, there have been much larger floods in the valley including the worst flood on record. In 1867, river levels reached 19.1 metres above normal river height at Windsor and the region was decimated. If a similar flood happened now the consequences would be catastrophic – many lives would be at risk and almost 90,000 people would have to evacuate.
Flood are random, naturally occurring events. It’s impossible to predict when the next major flood will happen. History has shown that serious floods can happen many times in a single decade, and not again for many years.