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

Few studies have considered downstream changes in bank erosion rates and va riability along single river systems. This paper reports some preliminary r esults of an intensive and direct field monitoring exercise of bank erosion rates on 11 sites along 130 km of the 3315 km(2) Swale-Ouse river system i n 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 an d bank-line resurveys. Erosion rates were relatively high for a river of th is scale: spatially averaged bank erosion magnitudes over the 14.5 months v aried 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 i n mid-basin, possibly because of an optimum combination there of high strea m 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, sh owed that bank retreat was often highly localized within individual sites. Strong seasonal variations in erosion rate were also observed with a signif icant winter (December-March) peak. A novel finding, however, was the appar ent downstream increase in the length of the erosion 'season', with measura ble retreat occurring at the lower sites from September to July. This is in terpreted as a reflection of a richer mix of bank erosion processes at the downstream sites, where mass failure, fluid entrainment and weathering proc esses are all active, with each process group having its own, but overlappi ng, temporal (seasonal) domain. Copyright (C) 1999 John Wiley & Sons, Ltd.