Downstream change in river bank erosion rates in the Swale-Ouse system, northern England

Abstract

Few studies have considered downstream changes in bank erosion rates and variability along single river systems. This paper reports some preliminary results of an intensive and direct field monitoring exercise of bank erosion rates on 11 sites along 130 km of the 3315 km2 Swale-Ouse river system in northern England over a 14·5 month period. Data were collected at active sites using grid networks of erosion pins read at c. 18–30 day intervals and bank-line resurveys. Erosion rates were relatively high for a river of this scale: spatially averaged bank erosion magnitudes over the 14·5 months varied from 82·7 mm to 440·1 mm, although at one highly mobile reach retreat of 1760 mm was recorded over 4 months. Bank erosion rates tended to peak in mid-basin, possibly because of an optimum combination there of high stream powers and erodible bank materials, as predicted theoretically by Lawler (1992, 1995). The piedmont (upland–lowland transition) zone was especially active. Graphical erosion representations for specific periods, however, showed that bank retreat was often highly localized within individual sites. Strong seasonal variations in erosion rate were also observed with a significant winter (December–March) peak. A novel finding, however, was the apparent downstream increase in the length of the erosion ‘season’, with measurable retreat occurring at the lower sites from September to July. This is interpreted as a reflection of a richer mix of bank erosion processes at the downstream sites, where mass failure, fluid entrainment and weathering processes are all active, with each process group having its own, but overlapping, temporal (seasonal) domain. Copyright © 1999 John Wiley & Sons, Ltd.