Grahamstown Dam Risk Assessment Report

August 2024 Earthquake

On Friday 23 August there was a magnitude 5.0 earthquake near Denman, NSW.

As per our dam safety protocols following an earthquake, our teams completed detailed visual inspections of both Grahamstown and Chichester Dams. Upon inspection, there was no visible sign of damage at either site.

The safety of our community is paramount. We monitor the dams daily so they continue to operate safely.

Grahamstown dam consists of a number of structures, including the Grahamstown Spillway, and the embankments (dam walls) – the Main, Saddle, Subsidiary and Irrawang embankments.

A diagram of Grahamstown Dam showing how it consists of a number of structures, including the Grahamstown Spillway, and the embankments (dam walls) – the Main, Saddle, Subsidiary and Irrawang embankments.

Grahamstown Dam is visually inspected every day. We also complete detailed safety, risk, and maintenance reviews every year, every five years and every 15 years.

What the risk assessment and 15-yearly safety review involves

The risk assessment process considers all the possible scenarios that could potentially cause the dam to fail (break) – no matter how rare. It considers the likelihood of each scenario occurring and what the consequence could be if it were to occur and helps dam owners identify potential safety issues so that they can be managed or prevented. Over the past decades, we have made many improvements to the dam as a result of various risk assessments and regular inspections. The process follows the methodology set out in the Dams Safety Act 2015.

Risk assessment results

In late June 2024 we completed our most comprehensive risk assessment. Using the latest technology, we now understand in more detail the risks associated with the major and rare events that may affect the dam. The risk assessment found:

  • The dam continues to operate safely under normal and flood conditions.
  • There is a risk that the dam’s Main Embankment could be damaged by an earthquake. In the worst-case scenario, this could lead to the uncontrolled release of water into lower-lying areas near the Main Embankment, presenting a risk to people and property. The likelihood of this occurring is calculated to be 1-in-3,500, or a 0.03% chance in any single year.
  • The risks are much lower for the Saddle and Subsidiary embankments. The likelihood of the Saddle or Subsidiary embankments failing due to an earthquake is calculated at around 1-in-50,000, or a 0.002% chance in any single year.

This risk isn't new. Earthquake risks have existed since Grahamstown Dam was constructed 60 years ago. The use of the latest technology has helped us to understand these risks in greater detail.

Grahamtown Dam Risk Assessment Fact Sheet

Learn more about our most detailed risk assessment ever completed for the dam.

We are acting quickly to reduce the risk

Although there’s a low likelihood of the dam being damaged by earthquake, we're acting quickly to address the risk.

Reduced water level

In July 2024, we took the interim action of reducing the top water level at Grahamstown Dam to around 90%, since then, a decision has been made to take further steps to increase community safety. The interim top water level in Grahamstown Dam is now being reduced to about 82% capacity, this is so that the risk to the community near the dam is even lower in the event of an earthquake that damages the dam’s embankments.

Emergency plan

We're working with the NSW State Emergency Service (SES) and other emergency response organisations to review our existing Dam Safety Emergency Plan for Grahamstown Dam to incorporate the latest risk information.

Longer term actions

Upgrades to the dam

We’re working with a team of specialist dam engineers to explore options to upgrade the embankments. These upgrades will aim to permanently address earthquake related risks.

Dam upgrades are complex, and will take some time to be scoped, planned, approved and delivered. Indicatively, we’re planning to have upgrades completed within 5 to 10 years. We’ll keep you up to date with our progress on the above actions.

Grahamstown Dam Risk Assessment Summary Report

Grahamstown Dam main embankment

View the Grahamstown Dam Risk Assessment Summary and full Report.

Frequently asked questions

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