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Sunday 12 December 2010
Thursday 9 December 2010
Willow Spiling To Protect Farmland
Bank erosion of streams pollutes watercourses with fine sediments and phosphates that can disturb the local ecology. It also removes sections of the farm to be deposited downstream, either within the river or on someone else’s land when the river overtops it’s banks. There are numerous methods for controlling this process and so reduce the rate at which soils are lost to rivers. The obvious method is fencing the watercourse to remove stock from the bank side. This allows vegetation to flourish creating a strong root stock that binds soils together. But sometimes just fencing the river bank is inadequate as erosion has taken hold and every flood pulse takes another section of bank and then the watercourse begins to eat into fields.
More drastic hard engineering methods can solve this. Gabion baskets for example create a strong buffer between fields and water. Such methods can be successful but they operate against natural processes and so always look at odds with the landscape. Moreover they too can wash out adding an extra ugliness as they hang into the river spilling their gravel back to the water.
There are intermediate methods that are successful and work with the natural features of a river. Matt Neale, the local ranger for upper Wensleydale, has championed willow spiling to protect banks, and fields, from erosion. This method seems to do the trick on most occasions and adds habitat to the river providing refuge and shade for fish, birds and mammals such as otters. Having visited sites where the Eden Rivers Trust had successfully employed the method Matt identified sections of the Ure that would benefit from this method. The first place he trialed willow spiling was on the river Ure just below Hawes. Here the river was eating into the bank and soil was being lost rapidly from the farm with every high flow event.
The method is simple but effective. Stakes are driven into the river bed close to the bank, spaced at two or three metre intervals. Live willow rods are then woven between the stakes to provide a permeable barrier between the river and the bank. This helps to reduce stream power and thus erosion and it also slows the water enough to allow sediments to drop out behind the willow spiling. The bank then builds up as new sediment is deposited whenever the river overtops the willows. The added benefit with using willow rods is that they take root and flourish in these locations. The roots further bind the soil whilst the new tree growth slows the water down even further and more matter is deposited until the bank becomes completely revegetated. The habitat created by this is valuable and helps to protect farmland creating one of those rare win-win situations.
Since the first trial Matt has carried out the method on several other sections of the river Ure and Duerley Beck. He has refined the process and now builds up coarse gravel behind the spiling to encourage fine sediments to deposit out offering further protection to the willow rods whilst they take root. When possible soil is packed into the coarse sediments providing a substrate for the growth of bank side vegetation, again this further binds the bank providing a more stable environment which offers improved chances that the work will be successful. When it is appropriate the bank is reprofiled to create a less vulnerable slope.
All of the sites that Matt has worked on have been succesful with one exception. This is at a location where a large glacial deposit, possibly a recessional moraine, is eroding badly. The land slip is substantial and the processes causing the slip are not simply undercutting of the bank by the river since the slope has become unstable. These glacial desposits are unconsolidated, porous and permeable meaning that water seeps through them providing a good medium for failure points to emerge. Where the processes causing erosion have been caused by river water the method has been extremely succesful and the habitat created appears natural and undisturbed.
More drastic hard engineering methods can solve this. Gabion baskets for example create a strong buffer between fields and water. Such methods can be successful but they operate against natural processes and so always look at odds with the landscape. Moreover they too can wash out adding an extra ugliness as they hang into the river spilling their gravel back to the water.
There are intermediate methods that are successful and work with the natural features of a river. Matt Neale, the local ranger for upper Wensleydale, has championed willow spiling to protect banks, and fields, from erosion. This method seems to do the trick on most occasions and adds habitat to the river providing refuge and shade for fish, birds and mammals such as otters. Having visited sites where the Eden Rivers Trust had successfully employed the method Matt identified sections of the Ure that would benefit from this method. The first place he trialed willow spiling was on the river Ure just below Hawes. Here the river was eating into the bank and soil was being lost rapidly from the farm with every high flow event.
The method is simple but effective. Stakes are driven into the river bed close to the bank, spaced at two or three metre intervals. Live willow rods are then woven between the stakes to provide a permeable barrier between the river and the bank. This helps to reduce stream power and thus erosion and it also slows the water enough to allow sediments to drop out behind the willow spiling. The bank then builds up as new sediment is deposited whenever the river overtops the willows. The added benefit with using willow rods is that they take root and flourish in these locations. The roots further bind the soil whilst the new tree growth slows the water down even further and more matter is deposited until the bank becomes completely revegetated. The habitat created by this is valuable and helps to protect farmland creating one of those rare win-win situations.
Since the first trial Matt has carried out the method on several other sections of the river Ure and Duerley Beck. He has refined the process and now builds up coarse gravel behind the spiling to encourage fine sediments to deposit out offering further protection to the willow rods whilst they take root. When possible soil is packed into the coarse sediments providing a substrate for the growth of bank side vegetation, again this further binds the bank providing a more stable environment which offers improved chances that the work will be successful. When it is appropriate the bank is reprofiled to create a less vulnerable slope.
All of the sites that Matt has worked on have been succesful with one exception. This is at a location where a large glacial deposit, possibly a recessional moraine, is eroding badly. The land slip is substantial and the processes causing the slip are not simply undercutting of the bank by the river since the slope has become unstable. These glacial desposits are unconsolidated, porous and permeable meaning that water seeps through them providing a good medium for failure points to emerge. Where the processes causing erosion have been caused by river water the method has been extremely succesful and the habitat created appears natural and undisturbed.
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