Clarifiers help clear Gloucester culverts

Flood risk in Gloucester is being reduced by the Environment Agency, which is using specialist equipment for silt removal in a large culvert. Nathalie Hall of Siltbuster explains

Siltbuster has supplied four Lamella Clarifier HB50 hopper-bottomed units to an Environment Agency project to help clear debris from a large culvert in Gloucester city centre. Work has started to remove silt from a 600m-long culvert where the River Twyver flows underground through parts of the city centre.

Much of the silt in the culvert has accumulated since the 2007 summer floods, when high water levels eroded banks, depositing the resulting silt in the watercourse. Using specialist contractors, the water is being pumped from the underground section of the culvert near Lawrence Way, then diverted through four Siltbuster HB50 units where the water is clarified and returned to the watercourse, which runs under St Oswald's Road to the nearby Persimmons construction works.

OnSite project team leader, Bob Smith, said: "We have a 600m length of culvert to clean and survey that is heavily silted and has long sections between manholes. We have completed several sections and are now cleaning the final section, which is being treated and sent to landfill.

"We are using specialist pumping equipment and four Siltbuster units to remove the silt and gravel to ensure that the culvert will continue to be able to carry the amount of water that needs to flow through it during normal and flood conditions."

Siltbuster's principal environmental scientist, Jonathan Goldsmith, explained the clarifier process: "OnSite are removing contaminates that have ended up in the underground section of the culvert and pumping the polluted water through our equipment for clarification. Our units remove fine particles from water using a hydraulic design that stills the incoming water/solids mix and routes it upwards between a set of inclined plates for separation.

"The particles then settle on to the plates and slide down to the base for collection, while treated water flows to an outlet weir after passing below a scum board to retain any floating material. The clarified water can then be discharged either to sewer or in this case the local water course."

George Tomlin of the Environment Agency concluded: "Keeping culverts clear of blockages is a vital part of our routine maintenance work to keep rivers flowing freely and reduce any future risk of flooding. Rivers deposit silt and gravel naturally and, over time, this builds up on the river bed, reducing the capacity of the channel.

"It is also affected by extreme flood events, as witnessed in 2007, which have pushed large amounts of debris into the culvert. It can often be difficult to get at the silt and debris to remove it, so we need to bring in specialist contractors such as OnSite and Siltbuster to keep it clear."

The culvert cleaning project is part of a series of works designed to improve the standard of flood protection in Gloucester city centre and is being funded by the Environment Agency. The clarifiers were expected to be on site until the end of March 2009.

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