De-silting the Helensville raw water dam

How do you de-silt a raw water supply dam while keeping the plant fully operational? The answer was not obvious as the traditional de-silting techniques posed significant risk to the continuity of the drinking water supply. In late January 2010 Downers, in conjunction with Monitoring Technologies Commercial Diving, used divers to utilise a siphon to de-silt 4,500 to 5,000m3 from the primary raw water dam in Helensville. This approach was used not only because it was cost effective but also presented the minimum risk. There were no hydrocarbons on site, little sediment stir, no downtime to operations, and it had the lowest potential environmental impact. The whole process was completely recyclable with the filtered centrate returned to the dam, requiring no special consents and the resulting sludge is likely to be used by the local farmers. This paper outlines the method used and details the risk and environmental impact minimisation deployed by Downers during this sustainable project.