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Making the switch

Switching the rivers to the new diverted route and freeing up the land is no heavy-handed affair and a precise series of processes must be adhered to.

'We have to do this as painlessly as possible to the ecology of the rivers, ' explains Black and Veatch lead river engineer Dave Palmer.

First off, water will be gently pumped across from several locations to fill the new diversion up to high water level for around a fortnight.

This will allow for clay to swell and the new channels to be monitored. Water levels will then be lowered to match the existing rivers.

Bunds at the top and tail of the gently inclined new channels are then opened. For a period of a couple of minutes, there will be flow in both the existing rivers and their diverted channel, before temporary bunds are built to dam shut the existing routes.

This done, a fish capture programme and silt translocation will kick into action. Plants from the rivers have already been re-planted in the new channels. Although the rivers ran beneath one of the busiest airports in the world through a seeping, highly polluted sewage treatment works, wildlife thrived, with water voles, pike, perch, chub and a healthy variety of creepycrawlies all present. Ensuring this is protected explains the extent of the works being undertaken.

Bird netting will stretch across the new channels to negate any attraction created as flocks of birds can clog up aircraft engines.

Once all wildlife has been rehomed, the existing channels can be pumped dry freeing up the space for the legions of excavators. The Longford river switched to the new channels on 19 February. The Duke of Northumberland's river will switch in April.

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