The Hawkesbury-Nepean Valley is particularly prone to flooding as it is naturally constricted in two places that, in conditions of severe rainfall, results in floodwaters backing up and inundating floodplains in north-west Sydney.
Alternative flood management options have additional benefits for western Sydney, including greater safety for the most flood prone residents, better transport, a more vibrant agricultural sector, a healthier environment and improved water security.
Importantly, half of all floodwaters in the Hawkesbury-Nepean originate from catchment areas that are not upstream of Warragamba Dam [i]. This means that even if a raised Warragamba Dam was to hold back some flood waters, other catchments could still cause significant flooding in the valley. In fact, flood waters from the Grose River alone can cause moderate to major flooding of Richmond in the lower Hawkesbury [ii].
You can read Assoc. Prof Jamie Pittock's full report on the alternatives to raising Warragamba Dam wall here.
[i] Department of Primary Industries (2014), Office of Water. Hawkesbury-Nepean Valley Flood Management Review Stage One. Available Online: https://bit.ly/2JxtchB
[ii] Australian Water and Coastal Studies (AWACS). 1997. Lower Hawkesbury River Flood Study. Prepared for NSW Department of Land and Water Conservation, Sydney.