Under a New South Wales government proposal Sydney’s main water supply storage, Warragamba Dam is to be raised by 14 metres, ostensibly so that downstream flooding can be mitigated by temporarily holding large volumes of incoming flood waters.

As is the way with dam proposals, it has attracted a great deal of controversy.

The proposed raised dam is located within the world heritage-listed Blue Mountains national park, would cost as much as the Sydney football stadium rebuild and would periodically inundate Aboriginal cultural heritage and environmentally sensitive ecosystems.

It is backed by the Berejiklian government in the name of reducing the risk of calamitous flooding in the western Sydney regions of Penrith and Richmond-Windsor.

Despite an election over the weekend, voters lacked the critical information needed for any meaningful evaluation of the political parties’ positions on the proposal.

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Developers are a beneficiary of the policy with land that was previously flood zoned potentially becoming safe to build properties on.

Will the Coalition government seek developer contributions for dam-raising costs and to compensate for damage to environmental assets and cultural heritage? And while raising the dam is designated for flood mitigation only, will it become an enlarged supply storage by stealth, thus encouraging further development and population growth?

In the November 2018 parliamentary debate on enabling legislation, Greens MLCs Justin Field and Cate Faehrmann, as well as NSW Labor deputy leader Penny Sharpe, were united in their scepticism of the government’s stated motivation and their horror at the harm to environmental and cultural heritage that would ensue from inundation during large floods. However, they could not dismiss the possibility that the environmental impact statement (EIS), due in mid-2019, may ultimately support the dam raising.

The dam debate has suffered from a dearth of sound technical information on both downstream flooding risks and upstream inundation damage.

The government fleshed out its proposal with the mid-February 2019 release of the Taskforce Options Assessment Report. The report was unheralded, with no media release, despite stating that it “makes available considerably more detail”, “is expected to be of interest to valley communities and stakeholders” and will inform the EIS process. Unfortunately, the lack of publicity has meant there’s been very little written about it.

The 279-page report outlines the risks for a range of operational configurations for both the current and the proposed raised dam. These include downstream flooding, potential property damage, time required for evacuation and the number of people involved, and potential lives that could be lost, and for upstream of the dam, the duration and extent of inundation.

The report concluded that raising the dam by 14 metres would sufficiently reduce the likelihood of catastrophic damage from the most severe floods, those expected to occur from perhaps once in a lifetime to once in a millennium.

Questionably, the report dismisses the Bureau of Meteorology’s capacity to issue reliable flood forecasts, citing a June 2016 flood forecast which didn’t eventuate to the degree expected. However, the bureau’s forecasting capacity continues to improve through scientific advancement and increasing computing power. The supercomputers coming into the bureau’s operations this year are 50 times faster than those used in 2016.

Dismissing the forecasting, based on one “failed” forecast in 2016, does not seem credible.

Given reliable flood forecasts, rather than keeping a large volume of the dam permanently empty for temporarily storing floodwater, as proposed, space could be created by releasing water when a flood is forecast but before it arrives. These releases could be restricted to the volume that will very likely arrive, reducing any risk to Sydney’s water supply.

Pre-flood releases were modelled for the existing dam but apparently not for the raised dam. Instead of raising the dam by 14 metres, raising it by around nine metres and using pre-flood releases could achieve the same level of flood mitigation downstream. As a corollary, with pre-flood releases a dam raised by 14 metres could store water five metres higher than current full supply (almost a year’s worth) and would also have the same level of flood mitigation.

Another option would be to lower the full supply level by five metres (this was modelled by the taskforce and showed no great risk to Sydney’s water supply) and raise the dam by only five metres. Combined with pre-flood releases, this could also deliver sufficient flood mitigation but with less damage to the area upstream by reducing the extent and duration of inundation. Engineering constraints related to dam stability and release capacity may mean some extra costs would be incurred.

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Whatever is done, raising the dam, creating space for floods with the present dam, building a network of evacuation roads, raising bridges and so on, will have a significant monetary cost. It will also enable further development, for instance, the report mentions the projected need to evacuate an extra 68,000 people in 2041 should there be a twice-in-a-millennium level flood and no mitigation.

So would the government be willing to make developers contribute to the considerable costs of flood mitigation? Special infrastructure contributions are already paid by developers for infrastructure that supports growth such as schools, regional roads, health facilities and some public transport. The government could add a component to cover the cost of flood mitigation.

It may already be too late to inform the electorate about the taskforce report. However, it’s not too late to ensure that any shortcomings in the existing report are not repeated in the EIS.