Subaerial River Bank Processes and Interaction with other Bank Erosion Mechanisms on the River Arrow, Warwickshire, UK

River bank erosion occurs primarily through a combination of three mechanisms: mass failure, fluvial entrainment, and subaerial weakening and weathering. Subaerial processes are often viewed as preparatory processes, weakening the bank face prior to fluvial erosion. Within a river basin downstream process domains occur, with subaerial processes dominating the upper reaches, fluvial erosion the middle, and mass failure the lower reaches of a river. The aim of this paper is to demonstrate that (a) subaerial processes may be underestimated as an erosive agent, and (b) process dominance has a temporal, as well as spatial, aspect.
Bank erosion on the River Arrow, Warwickshire, UK, was monitored for 16 months (December 1996 to March 1998) using erosion pins. Variations in the rate and aerial extent of erosion are considered with reference to meteorological data. Throughout the first 15 months all erosion recorded was subaerial, resulting in up to 181 mm a-1 of bank retreat, compared with 13 to 27 mm a-1 reported by previous researchers. While the role of subaerial processes as preparatory is not contended, it is suggested that such processes can also be erosive. The three bank erosion mechanisms operate at different levels of magnitude and frequency, and the River Arrow data demonstrate this. Thus the concept of process dominance has a temporal, as well as spatial aspect, particularly over the short time-periods often used for studying processes in the field. Perception of the relative efficacy of each erosive mechanism will therefore be influenced by the temporal scale at which the bank is considered. With the advent of global climate change, both these magnitude-frequency characteristics and the consequent interaction of bank erosion mechanisms may alter. It is therefore likely that recognition of this temporal aspect of process dominance will become increasingly important to studies of bank erosion processes.