Experts: Raising Warragamba Dam wall the best way to flood-proof Western Sydney

A report by NSW’s leading water experts has definitively ruled on whether or not to raise Warragamba Dam wall, saying 10,000 homes could be saved from future flooding. READ to findings

Raising the Warragamba Dam wall is the best way to protect Western Sydney from more devastating floods, according to a landmark report by the state’s top water experts.

An investigation of all feasible options by the Department of Planning and Envir­on­ment has found increasing the height of the wall by ­between 14m and 17m would lower potentially devastating flood levels downstream, give people more certainty about time to evacuate, and cut flood damage to urban and rural properties by more than 70 per cent.

The Hawkesbury-Nepean Valley Flood Management Taskforce report is the strongest indication yet that the NSW government will move to lift the wall.

It also appears to have blown a hole in the NSW ­opposition’s plan to lower the wall, predicting that to do so would trigger an over-reliance on desalination plants to keep Sydney wet and hydrated in times of lower rainfall — and adding an extra $200 per year to the average Sydneysider’s water bill.

The report comes amid fierce debate about the merits of raising the dam wall following four devastating floods in the past two years.

Warragamba Dam spills over following heavy rainfall. Picture: Sam Ruttyn

“In a flood similar to the worst on record in the valley, the number of homes impacted by floodwaters would reduce from around 15,500 to 5000, the number of people requiring evacuation would decrease from around 90,000 to around 14,000, and damages would lessen from $8 billion to $2 billion,” the report on raising the wall, obtained exclusively by The Sunday Telegraph, reads.

“Alternative options to permanently lower the full water supply level have been examined as part of the Greater Sydney Water Strategy. These options would require significant investment to offset the increased risk to Sydney’s water security and drought resilience as a result of the reduced volume of water stored at Warragamba Dam.”

Rescue volunteers patrol around the flooded houses next to the old Windsor Bridge along the overflowing Hawkesbury river in Windsor in March. Picture: AFP

Many Hawkesbury residents have called for water ­levels to be kept much lower than current capacity.

Others have expressed concern that raising the wall would unleash residential development on flood-prone rural properties, while permanently flooding thousands of hectares of pristine bushland.

The DPE report also modelled three scenarios where the 142m wall would be lowered by 5m, 12m and 25m.

Under the 25m scenario — which would offer the same level of flood protection as raising the wall 14m to 17m — an estimated 1400 gigalitres of water storage would be lost. This represents a 67 per cent reduction in storage from Sydney’s main source of drinking water.

“The government’s decision to raise the dam wall is the right thing to do for water sec­urity and the safety of homes across Western Sydney in the event of heavy rainfall,” Water Minister Kevin Anderson said.

“Families don’t have an extra $200 to pay for their water at a time when cost of living pressures are impacting the entire country.”

The Sunday Telegraph last month revealed Labor’s $225 million Western Sydney Floods Resilience Plan would improve public access to Department of Primary Industries water gauges along rivers and creeks to help residents obtain real-time data to monitor rising flood levels.

Mr Minns has also pledged to significantly improve communication infrastructure with self-powered, trailer-mounted, cell towers to be deployed to disaster areas to replace damaged or destroyed mobile phone towers.

Windsor man Travis Frost’s home has flooded five times in the past three years.

He said any change to the dam wall is “going to be too little too late because they’re talking about a September flood”.

“In the short-term we need something done now,” he said.

“They have to do something otherwise the Hawkesbury is in dire straits.”

The NSW government is yet to release its evacuation and flood modelling report for the Hawkesbury and Nepean Valley.

This document will guide necessary flood mitigation works.

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