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Dempsey Wood Civil Contractors took an innovative approach to surmounting the environmental challenges posed by working in watercourses with the result that the project achieved the highest possible environment ratings from start to finish. BY GAVIN RILEY
Hiring 80 metres for four months from the New Zealand agent, Hydro Response of Rangiora, Dempsey Wood project manager Daniel Taylor says the Geodesign barriers were used in three ways on the Waiuku Stream contract. “Initially, they were used when damming the stream to stop the tidal and stormwater inflows,” he says. “The next purpose was to divert the stream so there was a clear working space on one side of the stream and water diversion down the other side. Then, as we got used to the barriers, we found that if they had a weight of water on them they actually held back the silt water on the side that we were working, which we could then pump out into a silt turkey net – geocloth filled with bark for the water to purify itself as it disperses. “The one thing I liked about the Geodesign barriers was that when high tides or stormwater events ‘topped’ the barriers, the barriers held their place and didn’t move. They allowed the water to go under the plastic if it had to, in order to escape out of the work area after the event had passed. As the works area was stabilized at the end of the working day and prior to stormwater and tidal events we were allowed to let water in.”
“Overall, they worked in perfectly for us and helped us to achieve the highest environment scores from Auckland Regional Council that it’s possible to get. On a scale from one to five (one being the highest), Dempsey Wood scored ‘ones’ right throughout the contract.” Daniel says he’s found the barriers both useful and easy to use. “Once you get the idea of how they work, they’re very simple – very much like building Lego.” Clay Griffin of Hydro Response says the Geodesign barriers have been used in local authority flood response work in New Zealand, and their superiority over sandbags has been demonstrated at weekend flood response courses involving councils, Civil Defence and the Red Cross. “It’s difficult to have the resources with sand-bagging, to get enough sand on site, and it’s also very labour intensive,” Clay says.
Although the barriers used in New Zealand so far, have been mainly in flood response work, Clay says their use in waterway civil works is common in Europe and he sees no reason why that should not happen here.
Contractor Vol.34 No.3 April 2010 |
With damming works comprising a significant component of the Franklin District Council’s $1.4 million Waiuku River Lane Stream upgrade project, Dempsey Wood researched a range of barrier techniques and ended up selecting Geodesign barriers. Simple and robust, Geodesign barriers consist of a galvanised steel frame designed to support aluminium sheeting, waterproof plywood or wooden pallets.
“The dams managed to cope on a low-to-medium rain day but in extreme weather events the water just toppled over them and we stopped work. The next day we were back into it. The water had gone underneath the plastic, the weight had gone back down on them, and the works area was dry again and we could carry on working.
For flood protection, a team of 10 can normally install about 100 metres of barrier an hour in the dry. It takes a little bit longer to get it all set up when doing civil works and sloshing around in mud. Normally, they’re easy to manoeuvre and quick to assemble.”