Warragamba Dam wall-raising proposal impact statement released

The documents are now able to be viewed at the state government's Planning Portal, encompassing everything from the flood mitigation benefits and biodiversity impacts, to affects to Indigenous sites and construction plans.

The government is proposing to raise the height of the dam wall to mitigate flood risk to residents in the Hawkesbury-Nepean valley in the event of extreme rain inundation.

The executive summary, itself almost 50 pages, notes the dam "historically contributes up to 70 per cent of flows during flooding" in the valley, and raising the wall would reduce that significantly.

Such action would benefit the areas of Wallacia, Penrith, Emu Plains, Richmond, Windsor, South Creek and Eastern Creek, "and to a lesser extent in the Lower Hawkesbury".

However, the adverse effects would be felt upstream in the Wollondilly and the Blue Mountains regions and Indigenous residents, scientists, environmental action groups, local councils, politicians and UNESCO have raised various concerns about the plan.

Raising the height of the wall would cause waters to flow further into the upstream Burragorang Valley area during inundation, which also connects with the Blue Mountains World Heritage Area and largely comprises native bushland.

"The duration of increased inundation would range from hours to around 10 days," the document notes.

"There would be an increased extent and duration of temporary inundation of three threatened ecological communities in the catchment when the flood mitigation zone is operational [and] may affect habitat for threatened flora and fauna species.

"Based on a precautionary approach... up to 76 threatened flora species and up to 16 threatened fauna species could be potentially impacted."

The document states about 300 hectares of the upstream impact area falls within the Greater Blue Mountains World Heritage Area (which is about 0.03 per cent of the total world heritage area).

The government proposes to purchase and manage "additional and appropriate land containing the values of the Greater Blue Mountains World Heritage Area to achieve no net loss" as an offset strategy.

Assessment found a predictive estimation of 174 sites of Aboriginal Cultural Heritage importance (including 43 actually identified sites) within the upstream impact area.

"The archaeological significance of sites identified... are categorised as low, medium or high scientific significance," the document stated.

"However, the sites are all assessed by the [Registered Aboriginal Parties] as high from a cultural significance perspective.

"The cultural values assessment considered 19 dreamtime story sites within the upstream area which were all at least partially impacted by Lake Burragorang."

If the project is to proceed, construction is expected to take five years to complete and would require significant alterations around the Warragamba Dam area to allow access for workers and vehicles.

"Ancillary facilities such as concrete batch plants, materials storage areas, access roads, coffer dams and other temporary features would need to be provided and operated during the construction period," the document reads.

"These would all be located adjacent to Warragamba Dam on land owned by WaterNSW.

"Some out of hours works for key activities may be required.

"During peak construction periods the project would require up to 500 workers and generate daily traffic of about 250 light vehicles and 100 heavy vehicles."

Noise levels during construction would be increased for nearby residents, the study found, and the local road network would likely be congested (though "the local roads and intersections have sufficient capacity").

There would be temporary "minor dust impacts", loss of access to Haviland Park and the Warragamba Dam Visitor Centre, "general community disturbance" and "minor river water quality impacts from sedimentation and erosion during construction works".

The main visual impact from the project would affect the viewing platform and Warragamba Visitors Centre, 18^th Street Lookout and Haviland Park, the document states.

You can view all EIS documents and make submissions at planningportal.nsw.gov. The public has until November 11 to make their submissions.