Contemporary and historic river channel change at Swinhope bum, Weardale: a study of river response to flood events in an upland, gravel-bed stream

Abstract

The response of upland gravel-bed streams to floods has long been associated with high levels of erosion and deposition, often resulting in major changes in channel form. This largely stems from observations of upland channels immediately following large floods. The aim of this thesis is to identify the importance of flood events in contemporary and historic river channel change in an upland gravel-bed stream. Swinhope Bum, in upper Weardale, Northem Pennines is used as a basis for this study. An assessment of channel planform change over the historical period was made using historical maps and air photographs. The study reach has retained a stable meandering pattern over a period of 180 years, with a temporary but dramatic change to a straight, low sinuosity, partly braided channel identified in the 1844 Tithe Map. The probable cause is an increase in coarse sediment supply generated by floods in the 1820's and upstream mining activities. The passage of a major flood through Swinhope Bum in February, 1997 produced very little channel change with erosion being the same order of magnitude as deposition, indicating that the study reach is stable even during overbank flows. A sediment tracing experiment designed to assess the importance of sediment exchanges between the bed and lateral inputs from eroding banks and bluffs demonstrates the importance of within channel movements and the minor local influence of lateral sediment sources. This study shows that contemporary channel response to flood events is through vertical rather than lateral adjustment in channel form which is substantiated by channel planform stability over the historical period. Long-term channel stability is attributed to the presence of a local base-level imposed by the Greenly Hills moraine, which has resulted in a low channel gradient which inhibits coarse bedload transport and frequent, major channel change.