News

A Run on the Banks

08 February 2011

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Those who regularly walk the river banks of the Tyne will know that they are not in good shape. They have had the worst combination of weather events for weakening their battle with the erosive force of flood flows. A memorable impact of the recent wet weather has been the sight of roads awash from surface water flooding, valuable topsoil flowing from arable fields, road drains blocking with 'silt' and individual or groups of homes being flooded, some requiring Fire Service pumps. The harsh winter has not helped - in that the soil structure has been disrupted by freezing temperatures and the weight of a snow pack.

The floods have followed the long freeze-up of December and January. During this time the idyllic views of snow-covered banks, ice floes and the sheer length of the wintry spell all combined to set up some very destabilised river banks and cliffs. Many of us saw ice crystals actually growing from normally wet, sandy river banks. Some saw six-inch columns of ice hurtling into river banks as the January freeze-up ended. Everywhere the action of ice has been to weaken bank materials - the action of frozen, brittle plant roots is not half as binding as that of supple, growing ones.

When the river level rises, it neatly removes what the bigger cliffs have deposited, creating room for more or undermining the base, creating landslides. There is a place on the South Tyne where you hear the banks falling before you see them. On lower banks the rising water soaks the finer bank-top material, making it both heavy and mechanically weak - leading to a collapse during the falling limb of the flood. Floods which cover the bank make their way back into the channel down the bank face, this exposing them to erosion from a new direction: above! These 'return flows' are now better understood as powerful agents of bank erosion.

We too, have had a role - several riverside footpaths understandably received quite a 'hammering' from our footsteps whilst the vistas were so grand during the Christmas break. It is now along the line of our trampling that the bank has failed simply because the frozen ground beneath our feet appeared to bear our weight but was quietly rearranging itself at a granular level: footpaths have cracked as the result of frost.

One upshot of all the new erosion (combined with the 'dirty floods - read more here) is that the streams of the Tyne catchment are now carrying quite a high load of sediments, mainly as sand from bank erosion - but there is also new gravel and some cobbles from the river cliffs. Thus, depositional parts of the river will 'grow' and, if colonised by summer vegetation, may come to be the gain which matches the loss of many metres of bank. Landowners will of course end up being on both sides of this particular 'run on the banks'