Thursday 23rd July 2015
Blog post 5 of 10 about the geomorphology of Wales. Click on images to view larger versions in separate windows. Parallel blog in Welsh at http://hywelgriffiths.blogspot.co.uk/
Reason 5. Landscape dynamics are often complex. In addition to changing tectonic, geological, climatic or ecological conditions, internal readjustments can also drive landform and landscape development.
Many Welsh landforms and wider landscapes have developed, and continue to develop, in response to external factors, including changing tectonic, climatic, sea level or ecological conditions (see Reasons 1-4). Nonetheless, some landforms and landscapes can develop as a result of internal readjustments that occur independent of changes to external factors. The alternating stability and instability of debris on hillslopes provides a good example, as shown in a photograph looking north along the upper Twymyn valley, Powys. Here, weathering of rocky outcrop (mainly shale) on the upper part of the western valley slope has generated abundant coarse, angular debris, and this has been transported downslope under the action of gravity and running water to accumulate on the mid slope. The debris has gradually built up over time, forming a ‘scree slope’ that tends to reach a stable inclination (‘the angle of repose’) that is governed by the size and angularity of the debris (typically 30-45°). Continued accumulation of debris, however, oversteepens the slope, raising the inclination above the angle of repose and making the scree unstable. Small-scale mass movements (e.g. shallow debris slides) therefore occur periodically, leading to further downslope transport of debris and reducing the inclination back to or below the angle of repose. These changes can occur more-or-less independently of changes to external conditions, although the mass movements on the oversteepened, unstable slopes may be triggered by extreme rainfall events. On the lower slope, the debris is more stable owing to a slightly lower inclination and the binding action of grass, shrub and tree roots.
Along some Welsh alluvial river channels, meander bend cutoffs can result from internal readjustments that can also occur independent of changes to external factors. Even under conditions of approximately steady flow and sediment transport, ongoing bank erosion (see Reason 4) can result in adjacent parts of a meander bend eventually meeting. When this happens, the channel straightens, and subsequent flow and sediment transport effectively bypasses and abandons the former bend. This process is known as a ‘neck cutoff’. Abandoned bends may remain full of water for many years and are termed ‘oxbow lakes’, but gradual infilling with fine sediment and organic material means that the lakes eventually disappear. The photograph (taken in April 2008) shows an example from the Afon Carno, a tributary of the upper River Severn near Caersws, Powys. Ongoing bank erosion had left only a narrow (1-2 m wide) strip of floodplain between two adjacent parts of a meander bend (red arrows indicate flow direction), so a neck cutoff was imminent. In such instances, the final cutoff event may occur during a large or extreme flood, but high flows are not essential, as the process is simply the inevitable end result of ongoing bank erosion. On the Afon Carno, this cutoff has now occurred.
Did You Know? Neck cutoff is just one of the many processes by which meandering rivers abandon former bends. Chute cutoff is one alternative process, and occurs when overbank floodwaters take a more direct path across the floodplain between adjacent parts of a meander bend. These floodwaters carve a straighter channel into the floodplain, and over time this channel deepens to take more and more of the flow and sediment transport, eventually also leading to bypass and abandonment of the former bend. While bends abandoned by neck cutoff tend to be tightly curved and form nearly complete circles in plan view, chute cutoffs tend to result in abandonment of more open, less circular bends. More than 30 years ago, an extensive survey of 964 km of major river valleys in Wales and the Borderlands identified 145 cutoffs. 16% involved simple neck cutoffs, 55% involved simple chute cutoffs, while the remaining cutoffs were the result of more complex processes involving ‘multiloop’ (13%) and ‘mobile bar’ forms (11%), or were due to artificial channel straightening or realignment (5%). For individual rivers, cutoff rates were found to be highly variable but the dataset as whole indicated that one cutoff occurred every 5 years in the period 1880-1900 and nearly every other year in the period 1950-1970 (Source: Lewis, G.W. and Lewin, J., 1983. Alluvial cutoffs in Wales and the borderlands. Special Publication of the International Association of Sedimentologists, v.6, pp. 145-154). Given recent concerns over alterations to flow regimes in Welsh rivers resulting from climate change and human impacts, it would be an interesting exercise to update this study and compare cutoff types and rates from the 1970s to the present.