Tuesday 30th June 2015
Blog post 4 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 4. The Earth’s landscapes are dynamic. Landscapes and landforms are not static and unchanging, but are dynamic and develop through time.
While there is a tendency to view many Welsh landforms as fixed in form, size and position, in reality all landforms are subject to change over time. Nevertheless, because the rates and timescales of landform change can vary widely, the full extent of this dynamism is not necessarily widely appreciated. Variability in dynamism is well illustrated in Welsh river valleys, many of which alternate between relatively short, steep, fast-flowing reaches in bedrock, and longer, gentler, slower flowing reaches in alluvium (e.g. gravel or sand). In bedrock river reaches, illustrated by these photos from the upper River Severn in Hafren Forest, Powys, changes typically occur so slowly that they are effectively imperceptible during a single human lifetime. During low flows and regular flood events, the shale bedrock remains highly resistant to erosion by the water and any transported sediment particles. Nonetheless, the sharp, angular bedrock faces exposed in the river bed and banks provide clear evidence of local erosion of entire joint-bounded blocks, a process known as hydraulic plucking. Most of this erosion only takes place over short intervals (hours to days) during the rare extreme floods that may occur only once every few decades. Over many thousands or tens of thousands of years, however, the cumulative effect is a lowering of the river bed and upstream retreat of rapids (upper photo) and waterfalls (lower photo). As these rapids and waterfalls retreat upstream, albeit erratically, steep-sided bedrock gorges may be generated downstream, many of which also form spectacular elements of the landscape (Photos: Stephen Tooth).
In many alluvial river reaches, changes tend to occur much more rapidly. The photo from the sinuous upper Severn River near Caersws, Powys, shows that deposition on an inner bend has formed a gravelly point bar, while erosion on the outer bend is maintaining a steep bank. Much of this depositional and erosional work takes place during regular flood events that perhaps occur a few times a year, and cumulatively this leads to lateral migration of the entire bend. Similar erosional and depositional processes characterise numerous other bends in such alluvial reaches (see Google Earth image) and, over time, collectively this results in river migration back and forth across the valley. Although the rare extreme floods may also play a role, they are not an essential part of the dynamics in such reaches, and so significant changes to river size, shape and position can be effected on timescales of years to decades (Photo: Hywel Griffiths. Aerial imagery from Google Earth, 2.5 km across. Blue arrow indicates flow direction (lower left to upper right) and red arrow shows direction of photograph).
Did You Know? The reach of the River Severn at Caersws is one of the most dynamic in Wales. Measured bank erosion rates at this site are typically 35 cm per year, but can be as high as 60 cm per year. Measured bank erosion rates on other Welsh rivers include maximum rates of 31 cm per year on the River Ilston and 0.96 cm per year on the Afon Trannon. These rates are low in comparison to rates on larger rivers; for example, 792 m per year on the Brahmaputra, Bangladesh, and 48.2 m per year on the Mississippi, USA (for these and other rates, see the compilation by Lawler, D. M. 1993. The measurement of river bank erosion and lateral channel change: a review. Earth Surface Processes and Landforms, v.18, pp.777-821). Nonetheless, they can cause significant management problems, including the loss of agricultural land, undermining of bridges and pipelines, threats to roads and railways, and introduction of high sediment loads. In some parts of Wales, sediments may be contaminated as a result of historic metal mining, and the increased loads can raise river bed levels, which may cause flooding problems downstream.