Saturday, April 4, 2009

A natural defence against flash floods in Yorkshire

Yorkshire Post (UK): In 1935, Britain was still in the financial crisis of the "hungry thirties" but managed to enjoy a hot summer. Few were concerned about a huge fire which ravaged vast areas of heather cloaking the highest parts of the North York Moors. But more than 70 years later that same moorland is finally being repaired. It is said that the worst economic stormclouds since the 1930s are now gathering, but the reason for the moorland work is the accumulation of real clouds, ones that produce monsoon-like torrential rainfall.

These heavy rains used to occur once every 40 or 50 years but because of global warming they are predicted to be perhaps a one-in-ten year event. And they have usually happened during summer months. If there is more vegetation and peat on the moors, however, the rain is likely to be soaked up, as if by a huge sponge. This water can be held back for sometimes days or weeks, and released slowly into rivers.

But because fire erodes moorland right down to the bare mineral soil and rocks, the heavy rainfalls now being experienced immediately wash down from the moors to create flood surges which were described by one eye-witness a couple of summers ago as being "like a tidal wave of water coming downstream". That's one of the basic principles behind the work now going on at Cockayne Head about eight miles north of Helmsley and – at 1,500 feet – one of the highest points in the North York Moors National Park.

…Jeff Pacey, the Agency's Ouse Catchment Manager, believes a crucial factor in flooding after intense rainfall is speed…"It's the sheer speed at which the water is running off those upland areas, getting into the river and coming downstream. If we can hold up that water on the moorland for even just five or six hours it enables the water that's fallen lower down the catchment to run out of the system."

…With financial aid from Natural England's Higher Level Stewardship scheme, local contractor Ian Fletcher is minimising the effects of those old moorland fires by harvesting the heather – which will regrow – and spreading this so-called "brash" on the bare soil. It is then pegged down beneath coir matting, made from coconut husks, and a mixture of grass and heather seed is planted and fertilised to kick-start the moor's regeneration.

Another major element of the work at Cockayne Head and Bransdale Moor is the blocking of drainage channels – so-called "grips" – which were dug many decades ago to drain the moorland for sheep grazing and grouse shooting. These grips exacerbate the water run-off from the moors, so they are being blocked up with huge bales of heather to allow the moorland to naturally revert to its age-old function of water sponge….

North York Moors national park near Chopgate, shot by Bernard LeprĂȘtre, Wikimedia Commons, under the Creative Commons Attribution ShareAlike 2.5 License.

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