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Matilija dam removal update

The US Bureau of Reclamation (USBR) is currently studying a variety of methods for removing the Matilija dam which was constructed in 1948, near Ventura in southern California, US. The 60m high, 190m long variable radius arch dam, owned by the Ventura County Flood Control District, is one of the largest dams slated for demolition in the US.

As part of the demolition, USBR engineers have to deal with over 5 M m3 of sediment that has collected behind the structure and seriously impaired the usefulness of the dam for flood protection. The concrete in the dam has also been weakened by alkali aggregate reaction.

USBR has been studying the possibility of cutting the concrete in the dam into blocks that can be handled using diamond wire cutting tools. Hydraulic fracturing is also being considered which involves drilling holes into the dam which are pressurised with water — the pressure causes cracks which link the holes, enabling chunks of concrete to be removed.

Alternatives for sediment removal include allowing sediment to wash downstream over a period of 20 years, or to transport it on to the Ventura county beaches via a pipeline. Estimated costs of the sediment removal varies from US$22M by the gradual method, to US$180M through the pipeline.

It is expected that all of the studies will be completed by September this year.