Raising the Warragamba Dam wall

Why raise Warragamba Dam for flood mitigation?

Warragamba Dam is on the Warragamba River - one of the major tributaries to the Hawkesbury-Nepean River. Flows from the large Warragamba Catchment are involved in all major regional floods that pose the greatest risk to life and property.

Warragamba is Sydney’s largest water storage dam and provides around 80% of current storage. The dam is not built or operated to manage or mitigate floods.

In developing the Flood Strategy, a range of options was investigated to use or modify Warragamba Dam to provide flood mitigation. It was found that raising the dam wall by around 14 metres is the infrastructure option with the highest benefit – significantly reducing the risk to life downstream, and reducing flood damages by around 75% on average while balancing the impacts of temporary inundation on the upstream environment. It would significantly reduce the risk to life and property, including the worst floods on record, and increase the certainty of time for people to evacuate.

How would a raised Warragamba Dam mitigate floods?

The raised Warragamba Dam wall would mitigate floods by creating ‘airspace’ in a dedicated flood mitigation zone around 14 metres above the current full water supply level. This flood mitigation zone behind the wall would capture and temporarily hold back floodwaters coming from the large Warragamba Catchment. After the flood peak, the floodwaters would be released in a controlled way – reducing flood levels downstream and potentially devastating impacts on downstream communities.

The flood mitigation zone would only be used during floods. The current water supply level would not change.

Can the existing Warragamba Dam be operated to mitigate floods?

Warragamba Dam was not designed or approved to operate as a flood mitigation dam. In developing the Flood Strategy, options were investigated for operating the current Warragamba Dam differently to potentially provide flood mitigation. The options investigated were:

  • pre-releasing water ahead of a predicted flood inflow
  • changing the operation of the gates to temporarily hold back flood water (known as surcharging)
  • permanently lowering the full supply level to provide airspace for flood mitigation.

Pre-releasing water would have limited effectiveness - with minor benefits for small floods, and none for larger floods. Two options to lower the dam’s full storage were examined - five metres, and 12 metres (the maximum possible to the depth of the spillway gates).

The five-metre lowering was found to have limited benefits for the larger floods that pose the most risk to lives and property.

The 12-metre lowering would provide moderate flood mitigation capacity. However, it should be noted that lowering the dam wall by 12 metres would have a significant impact on water security for greater Sydney and on water bills for Sydneysiders – reducing the dam’s storage by around 40% and Greater Sydney’s total water storage by around 30%.

To replace the lost supply, new sources of water would need to be built and the existing desalination plant would need to be operated at its maximum effective capacity.

Would raising Warragamba Dam eliminate flood risk in the Hawkesbury-Nepean Valley?

The dam raising would significantly reduce the flood risk, including the worst floods on record, but not eliminate it completely. It is not feasible to build a dam high enough to capture the most extreme, rare floods possible in the valley. This true for most river valleys.

Flooding is also generated by the other tributaries to the Hawkesbury-Nepean River. However, floodwaters from the large Warragamba Catchment are the major contributor to the most damaging and dangerous floods in the valley. The contributions from the Nepean River and other tributaries downstream of Warragamba Dam contribute to flooding but at much lower levels.

What’s happening with the environmental assessment for the proposal?

WaterNSW, as the owner and operator of Warragamba Dam, is preparing a comprehensive environmental impact statement (EIS) and detailed concept designs for the proposal. The project is considered critical state significant infrastructure under NSW legislation. The NSW Secretary of the Department of Planning and Environment has issued a detailed set of assessment requirements.

Modelling, surveys, technical studies and analysis are under way to inform the EIS, including Aboriginal Cultural Heritage Assessment in consultation with traditional owners, and detailed flora and fauna surveys and assessments. Community and stakeholder consultation is an important part of this process. The EIS will be exhibited for public comment in 2020.

The proposal is also considered a ‘controlled action’ by the Australian Government under the Environment Protection and Biodiversity Conservation Act 1999, and will be assessed in relation to matters of World Heritage, National Heritage and threatened species and communities.

What is the timeline for the proposal?

The EIS for the Warragamba Dam Raising proposal is scheduled to be exhibited in 2020. Subject to environmental and planning approvals, a final business case will be prepared for consideration by the NSW Government in 2021. If approved, it is estimated construction would take approximately four years.

Will the raised dam wall be used to store more water for Sydney?

No. The flood mitigation zone created by the raised dam would only be used to temporarily store floodwaters during floods. The long-term water needs of greater Sydney are met through the mix of water supply and demand measures. This is monitored and regularly reviewed to take account of changes in population growth, water use, climate, technology and other factors. Diversification of supply and demand measures is a key to securing the long-term needs of a growing city.

When full, Warragamba Dam already contributes approximately 80% of Sydney’s stored water supplies. An increased reliance on water supplied from Warragamba Dam was considered and rejected in the most recent water planning review. This was for sound reasons, including:

  • the increased risk to water security of further reliance on a single source of supply – not ‘putting all your eggs in one basket’
  • the potential environmental impacts associated with permanently increasing the level of stored water behind the dam wall.
How would a raised Warragamba Dam affect the Greater Blue Mountains World Heritage Area?

When the exisiting Warragamba Dam is full, areas upstream of the dam can be inundated by several metres for up to three or four days during floods.

Raising Warragamba Dam wall by around 14 metres would temporarily hold back floodwater upstream of the dam from days to around two weeks during large floods.

The extent of temporary flooding would vary according to the size of flood, and the level of the storage. However, preliminary analysis indicates that the extent of Greater Blue Mountains World Heritage Area temporarily inundated upstream could increase by up to a maximum of six hundredths of one percent (0.06%) above the area that would be flooded now with the existing dam wall.

Detailed flood mapping and targeted surveys for all flora and fauna species have been undertaken in accordance with both Commonwealth and State biodiversity assessment requirements. The impacts of the temporary increase in inundation on the upstream environment – and options to manage, mitigate or offset those impacts - will be detailed in the Environmental Impact Statement currently being prepared for public exhibition and comment.

Will Warragamba Dam still be safe if the wall is raised?

Yes. Warragamba Dam is a large concrete gravity dam 142 metres high. Raising a dam in the way proposed for Warragamba Dam has been completed successfully and safely for other large concrete gravity dams internationally.

When dams are modified, the structural integrity and safety of the dam, both during and after the upgrade, is the first and fundamental priority. There are stringent guidelines for building or modifying any large dam. The guidelines have been applied for all previous upgrades at Warragamba Dam and are being rigorously applied to the current process.

WaterNSW, as the dam owner and operator, has let the contract for the detailed design to Stantec/GHD joint venture following a procurement process. The joint venture has extensive relevant experience in major dam projects and is bringing national and international engineering and dam safety expertise to the work. The design project is being overseen by an expert technical panel, and a high-level government steering committee.

Is the Warragamba Dam Raising proposal about increasing development in the floodplain?

There is a very substantial flood risk now facing the current population, homes and businesses in the Hawkesbury-Nepean floodplain. For example, if a flood similar to the valley’s worst since European settlement happened again now, hundreds of lives would be in danger, around 90,000 people would need to be evacuated, and more than 12,000 homes would be impacted.

This existing risk needs to be addressed and raising Warragamba Dam to provide flood mitigation is the most effective infrastructure option to achieve that.

How is the flood strategy responding to land use planning issues in the floodplain?

The Department of Planning, Industry and Environment has released a background paper on ‘Improving flood resilience in the Hawkesbury-Nepean Valley through land use planning’ (October 2019).

The paper explored the new approach to planning in the Hawkesbury-Nepean Valley, as well as outlining the challenges for managing flooding and understanding the flood risk and the land use planning response.

An action under the Flood Strategy is to review the current planning arrangements to account for the high flood risk above the 1 in 100 chance per year flood level in the valley.

This review is underway. It is consistent with best practice flood management and the principles of the Western City District Plan, which includes consideration of the full range of flood risk.

The result will be a new Regional Land Use Planning Framework, that will take account of the cumulative impacts of growth and an Evacuation Roads Masterplan being developed by Transport for NSW.

The development of the Regional Land Use Planning Framework is being drawn from three key elements:

  • flood behaviour
  • the population at risk (using the latest data from the Department of Planning, Industry and Environment and future dwelling forecasts from councils), and
  • an assessment of risk to life – supported by the output of the new Flood Evacuation Model for the floodplain.

Once these three elements are finalised and drawn together, a complete picture of the flood risk to life and property will be known in the Hawkesbury-Nepean Valley and the land use framework will be completed.

Are current planning controls adequate for the Hawkesbury-Nepean floodplain?

The 1 in 100 (or 1%) chance per year flood planning level is suitable for many NSW floodplains, where the difference in flood depth between the 1 in 100 chance per year flood and the largest possible flood (known as the probable maximum flood or PMF) is relatively small, typically less than two metres.

However, the Hawkesbury-Nepean Valley has a unique flood risk. For example, in the Richmond-Windsor region, the largest possible flood is up to nine metres above the 1 in 100 (or 1%) chance per year flood level.

A more risk-based approach is being implemented under the Flood Strategy with the development of the Regional Land Use Planning Framework. This doesn’t mean development will be automatically prohibited in the area of the PMF where development is allowable under current land use zonings and SEPPs, but it does mean that the flood hazard, evacuation capacity and ability to recover from a flood event will be taken into account in land use planning.

What is being done to manage development in the floodplain?

Development in the floodplain needs to be carefully managed, now and into the future.

The NSW Department of Planning, Industry and Environment is leading development of a new Regional Land Use Planning Framework to take account of the cumulative impacts of growth across the floodplain.

More information

For more information view our publications, latest updates or get in touch at floodstrategy@insw.com

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