Composite river bank erosion in mid-Wales, UK

Some of the most active river reaches in the UK can be found in the piedmont zone which surrounds the Plynlimon massif in mid-Wales, where rivers such as the Severn, Wye and Rheidol emerge from steep headwater areas onto lower gradient wide valley floors (Figure 1). River banks in this region often have a composite structure in which coarse-grained material (typically gravel and cobbles with interstitial sand) from relic channel bars is overlain by a layer of silt/clay that has been deposited by overbank flows and on top of emergent bars (Thorne and Lewin, 1979). The boundary between the upper and lower sections of composite river banks is usually well defined (Figure 2), and the position of the interface relative to the present river channel reflects the degree to which the river is incised into its floodplain, the local height of relic channel bars, and the depth of scour close to the bank (Thorne and Lewin, 1979).

A lack of cohesion between sediment particles in the lower section of composite river banks makes them highly susceptible to erosion. Basal scour can oversteepen the lower bank section and lead to slip failures, while subaerial and subaqueous erosion can also reduce the packing density of particles and destroy any imbrication (Thorne and Tovey, 1981). In contrast, material in the upper section of composite river banks is typically far more resistant to erosion due to the existence of electromechanical cohesive forces between silt-clay-sized sediment particles, reinforcement by the roots and rhizomes of bank top vegetation, and infrequent inundation as a result of its elevated position (Thorne and Tovey, 1981). This disparity in erodibility is reflected in retreat rate data from study sites on the River Severn. Mean erosion rates exceeding 350 mm yr^-1, and locally as high as 600 mm yr^-1, have been recorded in lower bank sections, compared to an average of just 28 mm yr^-1 in sampled upper bank sections (Thorne and Lewin, 1979). Over time, the cohesive upper section of the bank is undermined, producing a cantilever (Figure 3) that ultimately suffers mechanical failure when a critical state is reached (Thorne and Tovey, 1981).

When cantilevers fail, sizeable blocks of cohesive material are transferred to the lower bank region (Figure 4). While some of these blocks are immediately entrained by the flow, or break up on impact, others will remain there due to their own weight and any cohesion between their undersides and the bank face below. Subsequent weathering and erosion can increase the stability of these blocks by increasing cohesion between the bank and their bases, and by reducing drag through rounding and streamlining (Thorne and Tovey, 1981). Vegetation densities can also increase due to enhanced water availability and this can help to further secure blocks in place. Stable failed blocks often display a root-reinforced toe, buried several centimetres below the river bed, which appears to build up over multiple seasons (Micheli and Kirchner, 2002). Vegetation has also been found to lessen the effectiveness of fluid erosion by between one and two orders of magnitude (Lawler et al., 1997). It reduces shear stresses by reducing near-boundary velocities and by damping turbulence (Lawler et al., 1997).

Failed blocks of cohesive material can act as a form of natural bank toe protection by consuming and diverting flow energy that may otherwise be used to scour the lower bank region (Wood et al., 2001), creating a semi-stable state where further bank retreat is reliant on the removal of basal accumulations by high flow events (Thorne and Tovey, 1981). Recent data indicate that blocks are often ultimately removed via a similar mechanism to the one which supplies them to the lower bank region. A study by Henshaw et al. (in prep.) on the River Wye found that, at high discharges, flow divergence around failed blocks (Figure 5) creates regions of high lateral shear stress, leading to scour of non-cohesive material in their immediate vicinity and their eventual undermining. The extent of scour around failed blocks is directly related to the flow structures created in their proximity, meaning that the position of the block in the lower bank region and the magnitude and duration of high flow events are likely to be important controls of composite bank stability and rates of retreat.

• Lawler, D.M., Thorne, C.R. and Hooke, J.M. (1997) Bank erosion and instability. In Thorne, C.R., Hey, R.D. and Newson, M.D. (eds.) Applied Fluvial Geomorphology for River Engineering and Management, Chichester: John Wiley & Sons Ltd, 137-172.
• Micheli, E.R. and Kirchner, J.W. (2002) Effects of wet meadow riparian vegetation on streambank erosion: measurements of vegetated bank strength and consequences for failure mechanics. Earth Surface Processes and Landforms, 27, 687-697.
• Thorne, C.R. and Lewin, J. (1979) Bank processes, bed material movement and planform development in a meandering river. In Rhodes, D.D. and Williams, G.P. (eds.) Adjustments of the Fluvial System, Debuque, Iowa: Kendall/Hunt, 117-137.
• Thorne, C.R. and Tovey, N.K. (1981) Stability of composite river banks. Earth Surface Processes and Landforms, 18, 835-843.
• Wood, A.L., Simon, A., Downs, P.W. and Thorne, C.R. (2001) Bank-toe processes in incised channels: the role of apparent cohesion in the entrainment of failed bank material. Hydrological Processes, 15, 39-61.