Subaerial river bank erosion processes and their interaction with other bank erosion mechanisms on the River Arrow, Warwickshire, UK

Abstract

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 À1 of bank retreat, compared with 13 to 27 mm a À1 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.

9 Modelling river-bank-erosion processes and mass failure mechanisms: progress towards fully coupled simulations

“…Notable exceptions to this trend include some work that has sought to quantify entrainment thresholds and process rates (e.g., Lawler et al, 1997a;Simon et al, 2000;Dapporto, 2001). The role of weathering as a significant agent of erosion has also started to be recognised (e.g., Lawler, 1993;Prosser et al, 2000;Couper and Maddock, 2001), both in headwater reaches (where weathering may be the dominant mechanism by which sediment is removed from the bank face) and, elsewhere, as a mechanism for enhancing bank erodibility and promoting fluvial erosion. Fig.…”

“…Bearing in mind that these studies have typically sought to isolate the effects of individual process groups, it is noteworthy that they cluster in the mid-to downstream reaches, where process interactions are strongest. Interactions between mass failure and fluvial-erosion processes (as opposed to the role of individual processes acting in isolation) therefore have particular relevance in the context of gravel-bed rivers, as the zone of interaction coincides at least in part with the middle reaches of basins where gravel-beds are typical, and also because the dominance of subaerial processes is generally limited geographically to the headwaters of typical fluvial systems (Couper and Maddock, 2001). This paper therefore seeks to address two objectives.…”

Uncertainty and sensitivity in a bank stability model: implications for estimating phosphorus loading

“…direct entrainment of bank material from flowing water). Other processes such as subaerial erosion (wetting/drying or freeze/thaw cycles) and needle ice formation have been identified but are considered less effective erosive forces and are generally not incorporated into bank erosion models (Couper and Maddock, 2001;Lawler, 1993;Prosser et al, 2000;Thorne, 1982). Although first developed for use with hillslopes, slope-stability relationships exist for both cohesive and non-cohesive soils, resulting in the commonly applied Culmann bank stability relationship for planar failures (Taylor, 1948;Thorne, 1982).…”

“…Although seepage erosion has been directly quantified (Fox et al, 2007b), only early attempts have been made at developing regression models (Fox et al, 2007a(Fox et al, , 2006 and mechanistic models (Chu-Agor et al, 2008aFox and Felice, 2014) to predict erosion rates. Subaerial processes are controlled by freeze/thaw and wetting/drying cycles which loosens exposed soil (Couper and Maddock, 2001;Couper, 2003). Climatological variables, including the number of freeze/thaw cycles and the number of days with frost have been shown to be correlated with subaerial erosion rates (Couper and Maddock, 2001;Pizzuto, 2009), but these variables control soil moisture content, the main driver of subaerial erosion (Thorne, 1982).…”

“…Subaerial processes are controlled by freeze/thaw and wetting/drying cycles which loosens exposed soil (Couper and Maddock, 2001;Couper, 2003). Climatological variables, including the number of freeze/thaw cycles and the number of days with frost have been shown to be correlated with subaerial erosion rates (Couper and Maddock, 2001;Pizzuto, 2009), but these variables control soil moisture content, the main driver of subaerial erosion (Thorne, 1982). Subaerial processes are likely not significant in all cases.…”