The River Swale originates as a series of becks 360m above sea level in the Northern Pennines and has one of the steepest gradients, in its upper reaches, of any river in England. On the River Swale there are numerous features characteristic of upland rivers and some examples, eg levees and bank protection works where man has modified the river channel to prevent the natural river processes occurring.

Some Concepts

A river's channel is an efficient conduit for carrying water. The 'discharge' or quantity of water that passes a given point in the channel varies over time. In response most channels are self adjusting, continually modifying their shape to meet these changing conditions. The size of any particular channel cross section reflects the typical stream conditions at that place. However, it may not be large enough to carry exceptionally large flows, which inevitable overtop the river's bank and spreads across the adjoining land which is known as the 'floodplain'.

The flow of water within a channel is not uniform. The speed or 'velocity' of the water decreases towards the bed of the channel and channel walls because of increasing frictional resistance. In sinuous rivers such as the Swale, the zone of maximum velocity swings towards the outside of each bend, becoming closer to the bank as the radius of the curvature of the channel increases. Where river velocity decreases the river does not have enough energy to carry its sediment load and deposition of river gravels occur.

The pattern created naturally by most rivers is a series of bends or 'meanders'. In this way the river minimises resistance to flow and gets rid of its energy from gravity.



Meanders change position regularly, and Reeth meanders are a particularly fine example of the processes at work. Erosion occurs on the outer bend, as the water is travelling fastest at this point and so has energy to erode and undercut the banks, while on the inner bend deposition of gravels occur as the water slows down and no longer has the energy to carry its sediment load. As a result the river channel moves down the valley, subtracting and adding to the land on either side of its river channel.

This process of meanders moving down stream, can be hindered or speeded up by the materials which make up the banks. A harder band of rock or sediment will slow this movement, and softer material will speed it up. Similarly river gravels can create a bar which divides the flow locally, and concentrates the flow elsewhere. As the bar builds up it may become stabilised by vegetation which in itself is more resistant to erosion.

During floods the velocity and amount of water carried by the river increases. The net effect is that it has more energy and can carry a larger load and also cause erosion. Erosion, in this context, is the mechanical loosening, lifting and removal of material by flowing water. Loose material is readily swept away. This loose material, in turn, wears away at the bank sides and river bed.

The Effects

Land adjacent to the main channel and tributaries, which provides essential meadow and pasture land, is lost to the river particularly during flood events. These photos taken at Isles Bridge show how much land can be lost during a single flood event. The effects of erosion are not always easy to predict and can be catastrophic.

As well as causing the loss of land, riverbank erosion causes significant damage to river side paths. During flood events any weakness in the surface is quickly exploited by the water. Such is the scale of damge to individual paths that the resources required for their repair runs into tens of thousands of pounds. It is estimated that 3.3Km of bankside paths have at least one section of damage that requires repair.

Management options

Vegetation, particularly the intricate network of roots, such as grass roots form a tight mesh that holds soil in place, and helps to resist erosion. However, the grazing of grasslands alongside the river leads to short grass and reduced root development [see the importance of trees]. The roots of vegetation are important in binding the sediment of the river bank together, so fewer, and shorter roots means the bank is less stable and therefore less likely to withstand the erosive power of the water.

Research carried out for the Environment Agency in the NW region, has shown that fencing off a bank side from livestock dramatically reduces the incidence of erosion. Tree planting is an optional extra, which will help further to stabilise the bankside.



It is very important that the river processes at the specific location are understood before undertaking any work, particularly in relation to any likely effect downstream. The example of Marble Scar shows what can happen. Here fencing and tree planting were undertaken in addition to hard engineering work. Limestone boulders were placed at the foot of the bank to help provide resistance to erosion but it should be noted that this work was undertaken on the outside of a meander. Unfortunately several floods occurred before the vegetation could become established. The soil and vegetation were washed away, and scouring is occuring around the limestone blocks.

Elsewhere attempts to stabilise the river bank have been more successful. The picture below shows a stretch of river bank where re profiling was carried out some years ago. At Grinton bridge gabion mattresses and willow stakes have been used to stabilise the bank.



Conclusion

The key message is to understand the local river processes at work and only to carry out bank repair work where the bank is not actively eroding. On a dynamic river such as the Swale a low intervention approach is the safest management option, where practicable. Geodata Institute at Southampton University have carried out a 'geomorphology audit' or 'erosion study' of the river Swale and Arkle Beck to help the River Swale Regeneration Project understand these processes.