Grahamstown Dam Risk Assessment Report

Hunter Water began the safety review process in March 2022. The review is a complex, careful process that involves: on site investigations at the dam to gather data; engaging with specialist consultants to undertake a range of modelling and technical assessments; detailed reporting; peer review of the results; and stakeholder consultation.

The safety review is a routine process. It was not undertaken in response to any particular issue at the dam. This is the first time we’ve undertaken a detailed risk assessment of this nature for Grahamstown Dam. The risk assessment is required every 5 years and the safety review is required every 15 years under the Dams Safety Regulation 2019. The risk assessment results are informed by the safety review.

During an earthquake, the sandy shoulders and foundations of the embankments could potentially become unstable through a phenomenon known as ‘liquefaction’.

Liquefaction can occur when wet, sandy sediments temporarily behave like quicksand during the shaking motion of an earthquake. This process could destabilise the sand shoulders of the embankments, impacting the clay core, and lead to the erosion and eventual failure of the embankment.

The highest risk to the dam’s embankments is due to a process called ‘liquefaction’. Liquefaction occurs when wet, loose sandy soil temporarily becomes like quicksand due to the shaking motion of a large earthquake. In the worst-case scenario, this could damage the embankment and allow an uncontrolled release of water to low-lying areas.

Whilst larger earthquakes typically contribute to the risk of liquefication, smaller earthquakes could also contribute damage over time.

Newcastle’s 1989 Magnitude 5.6 (on the Richter scale) earthquake did not trigger liquefaction at the dam. Available evidence indicates that the earthquake’s impact was limited at Grahamstown Dam due to a range of reasons such as the epicentre being far enough away from the dam, or the duration of the earthquake not being long enough to trigger liquefaction (which requires enough cycles to generate excess water pressure in the soil during the event).

There is no way to accurately predict exactly when or where an earthquake will occur. However, based on historical data of past earthquakes and knowledge of geology, scientists can roughly predict how likely it is for an earthquake to occur in a certain region over a timeframe of many decades or thousands of years. These timeframe predictions have been factored into the risk assessment.

The highest risks at Grahamstown Dam are different to those at Chichester Dam. The highest risks at Chichester Dam relate to flooding events that could place considerable stresses on the concrete structure and foundations of the dam, leading to failure.

While the highest risks at Grahamstown Dam are related to earthquake forces that could potentially cause liquefaction of the sandy embankments, leading to failure. These differing risks reflect the differences in each dam’s location, construction, and the ways in which they function.

This risk is not new. This risk has existed since the dam was constructed 60 years ago. However, the use of the latest technology in this recent risk assessment has given us deeper knowledge about how the dam’s embankments perform under all scenarios, particularly earthquakes. This latest information is useful to help us prioritise actions to reduce risks.

Hunter Water has posted letters to residents and owners of properties in certain low-lying areas of Raymond Terrace, Heatherbrae, Medowie and Campvale. We have only written to properties in areas that our modelling indicates could theoretically experience some water inundation in the event of an earthquake-related dam failure.

If you did not receive a letter, your property is likely outside of the potential risk area. The purpose of the letter is to provide residents with the latest risk results and advise on what actions Hunter Water is taking to address those risks.

Hunter Water is contacting all residents/property owners who would potentially be affected an uncontrolled release of water from the Main, Subsidiary or Saddle embankments.

Hunter Water is reducing the water level in Grahamstown Dam by about 10%. This means Grahamstown Dam will be operated to 90% capacity. The interim water level was selected as it will halve the potential consequence of a dam failure while balancing the water needs of the region. The selected water level also considers Hunter Water’s extensive historical data on: climate and rainfall, water storage levels, and community water use. The reduced water level is an interim measure that will be in place until upgrades to the embankments are complete.

There is the possibility that during a large wet weather event the operating level may go above 90% for a short period, but it will be brought back to 90% as soon as possible.

Excess water in Grahamstown Dam will be gradually released in a controlled way through the Irrawang control gates and will return to the Williams River via Irrawang Swamp and a culvert under Newline Road. This water release will not impact people or properties, as the Irrawang control gates have been purposefully designed for the safe, steady release of excess water from the dam.

Our existing Dam Safety Emergency Plan for Grahamstown Dam already guides the actions for emergency services to take if an earthquake were to occur. We are now working with the NSW State Emergency Service (SES) and other emergency response organisations to review this existing plan and incorporate the latest risk information.

Hunter Water and other emergency services will follow the actions set out in the Dam Safety Emergency Plan. If a significant earthquake causes visible damage to the structure of the dam, evacuation of certain areas may be required. Any evacuation orders will be issued by the State Emergency Services (SES) or NSW Police, who will provide direction to the community. Hunter Water is currently reviewing the plan to ensure it considers the latest risk information. We will keep the community updated about any changes to the plan.

Under normal climate conditions, the interim water level in Grahamstown Dam (around 90% capacity) will be adequate to maintain supply for the Lower Hunter region, along with our other water storages such as Chichester Dam, Tomago Sandbeds and Anna Bay Sandbeds. Additionally, by 2028, the Belmont Desalination Plant will be able to significantly contribute to our region’s water supplies. The desalination plant will add up to 30 million litres per day of rainfall-independent drinking water capacity to our system, or about 15 per cent of the region’s average daily needs.

Hunter Water’s total water storages are currently very full. The Belmont Desalination Plant will also provide an important source of water for the region from 2028 onwards.

However, if our region experiences drought while Grahamstown Dam is operating under the reduced, interim water level, Hunter Water will need to apply water restrictions earlier than we would have otherwise.

While Grahamstown Dam is operating under the interim water level, it will be more important than ever to conserve water together by making Smart Water Choices.