Dam bust 'low' but 300 homes face risk

HELEN MURDOCH
Last updated 12:58 28/07/2014

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The proposed 53-metre high Waimea Community Dam is unlikely to fail, but if it did it could potentially flood up to 300 homes, mainly in Brightwater, an assessment says.

A dam failure is rated as a low possibility in the project's assessment of environmental effects (AEE) which is part of the dam's resource consent application out for public consultation.

The Waimea Community Dam Ltd's AEE is legally required to assess the impacts of the dam's construction on the natural environment, biodiversity, residents and roading.

The document says the dam's high potential impact category does not mean the dam has a high risk of failure, and it is designed to the highest level prescribed for dams in New Zealand.

The assessment concludes that the project "has demonstrable positive benefits for the community".

"Consultation undertaken over a 10-year period has indicated that the project has wide support from the community, providing that water can be supplied at an acceptable cost," it says.

Brought to a head by the 2001 drought, when the Appleby River ran dry and the Waimea Plains's water was found to be over-allocated, the proposed dam is touted to not only provide irrigation for a further 1500 hectares but meet urban and industrial water demands for the next 100 years.

The consent only covers the dam's construction and does not address the contentious issues of downstream water takes or use, land values, intensification and in-stream flows - which will be addressed later.

Waimea Community Dam Ltd is the legal entity created under the Waimea Water Augmentation Committee which has applied for the consent and will build and operate the scheme.

If the project goes ahead the entity is likely to become a Tasman District Council controlled organisation/community co-operative, said the report.

Building the dam in the Upper Lee Valley was expected to take 30 months and employ 100 workers during the peak construction period, said the AEE report.

While the build is underway there would be extra 16 truck movements and 20 light vehicle movements daily, on average, on Lee Valley Rd and River Terrace Rd.

The main long-term adverse effect will be the removal of native trees and bush from the dam's footprint. Planned mitigation measures include clearing as little as possible, salvaging logs where possible, replanting and not clearing forest during nesting season.

Aquatic life will be affected and completion of a fish passage is likely to occur after the concreted faced-rock filled dam has been commissioned. It is possible downstream migrating eels will be manually trapped and transferred during peak movement periods.

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The land clearance and construction phases are predicted to have the greatest environmental effect with sediment likely to affect downstream water quality and be most noticeable during low-moderate flows. There is also potential for water quality issues, including elevated pH, related to pouring of concrete.

Some aquatic habitat will be lost as fine sediments settle into the river bed, affecting fish.

While approval to generate electricity at the dam is part of the current application the decision on whether a hydro station will be built has not yet been made and applications to transmit any power from the dam site will be up to future interested parties.

The council has said the construction consent needs to be completed before tenders to build can be called for and thus updated estimates of the dam's cost known.

Public submissions are open until August 15.

- The Nelson Mail

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