Bank erosion events and processes in the Upper Severn basin

Abstract

Abstract. This paper examines river bank retreat rates, individual erosion events, and the processes that drive them in the Upper Severn basin, mid-Wales, UK. Traditional erosion pin networks were used to deliver information on patterns of downstream change in erosion rates. In addition, the novel automatic Photo-Electronic Erosion Pin (PEEP) monitoring system was deployed to generate near-continuous data on the temporal distribution of bank erosion and accretion: this allowed focus on the magnitude and timing of individual erosional and depositional events in relation to specific flow episodes. Erosion dynamics data from throughout the Upper Severn basin are combined with detailed information on bank material properties and spatial change in channel hydraulics derived from direct field survey, to assess the relationships between flow properties and bank erosion rates. Results show that bank erosion rates generally increase downstream, but relate more strongly to discharge than to reach-mean shear stress, which peaks near the basin head. Downstream changes in erosion mechanisms and boundary materials, across the upland/lowland transition (especially the degree of development of composite bank material profiles), are especially significant. Examples of sequences of bank erosion events show how the PEEP system can (a) quantify the impact of individual, rather than aggregated, forcing events, (b) reveal the full complexity of bank response to given driving agents, including delayed erosion events, and (c) establish hypotheses of process-control in bank erosion systems. These findings have important implications for the way in which bank erosion problems are researched and managed. The complex responses demonstrated have special significance for the way in which bank processes and channel-margin sediment injections should be handled in river dynamics models.

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

“…This is problematic because the pre-versus post-flow event 'window' is not the same thing as the bank erosion event window, such that it is not usually possible to resolve process thresholds, timing, and rates (Lawler, 2005). To address this limitation, new quasi-continuous bank-erosion sensors based on the use of photoelectronic cells (PEEPs; e.g., Lawler, 1993;Lawler et al, 1997a) and thermal consonance timing (TCT; e.g., Lawler, 2005) have been developed, though they have not yet been widely deployed. While these approaches are promising, the use of sensors can disturb the bank face, while excellent temporal resolution is inevitably obtained at relatively low spatial resolution.…”

Monitoring Riverbank Erosion in Mountain Catchments Using Terrestrial Laser Scanning

“…Riverbank erosion may be caused by different phenomena, occurring with differing magnitudes and frequencies [16,17]. These have been summarized by Lyons et al [18] as: (1) detachment of bank material induced directly by the fluvial activity; (2) mass wasting under the effect of gravity on an undercut or unstable bank; (3) sub-aerial influence as a result of hydrometeorological variations such as freeze-thaw cycles and change in soil moisture; and (4) riverbank destabilization due to groundwater seepage.…”

Reduced fine sediment flux and channel change in response to the managed diversion of an upland river channel

“…Lateral erosion in particular plays an important role in channel migration, meander development and the delivery of fine sediment (< 2 mm) to upland channels (Lawler, 1993;Lawler et al, 1997;Fuller et al, 2003). Documented contributions of bankside sediment sources range from 1.5 % to over 80 % of total fine sediment flux (Bull, 1997;Stott, 1997), with high magnitude episodic events transferring significant volumes of bank-eroded material (e.g.…”