Effects of silt–clay content on the susceptibility of river banks to subaerial erosion

Abstract

The aim of this paper is to demonstrate how subaerial processes vary with the silt–clay content of river bank soil and to consider this variation in the context of erosion observed in the field. In particular, spatial and temporal variations in erosion, interactions with other bank erosion processes and the implications for bank morphology are explored. A number of soil blocks of known silt–clay content were subjected to a series of freeze–thaw and drying–wetting cycles. Changes in the dimensions of the soil blocks were recorded and the mass of soil ‘eroded’ from each block together with maximum size of eroding aggregates were measured. Limitations to this methodology are acknowledged, including a limited ability to replicate the forces to which a river bank may be subject, restrictions imposed by the available equipment in producing temperature and humidity cycles which reflect those occurring in the field, and issues of scale. The results indicate that river banks with high silt–clay contents are the most susceptible to erosion by subaerial processes. High susceptibility may influence spatial variations in bank erosion processes and rates, including downstream changes in the effectiveness and significance of subaerial erosion. A ‘vertical zoning’ of processes on a river bank is recognised, whereby the upper part of the bank is subject to subaerial erosion and the lower part to fluvial erosion. Given that a high silt–clay content tends to indicate increased susceptibility to subaerial erosion but increased resistance to fluvial erosion, this vertical zoning may have implications for bank morphology.