Significance of Bedrock Channel Morphology and Sediment Dynamics in a U.K. Upland River.

Abstract

Bedrock channels in the uplands of the U.K. are widespread but their geomorphology and sedimentology have rarely been studied. The aim of this research is to determine rates and processes of sedimentation in an upland bedrock channel following a large flood event. The main field site is a 300 m bedrock reach of Trout Beck in the Upper Tees drainage basin, Northern England. Sedimentary processes are investigated using morphological evidence (channel cross section surveys, repeat photography and geomorphic mapping) and field tracing of coarse bedload. The significance of the channel processes operating in the study reach are evaluated by mapping the extent of bedrock channel features in the upper Tees catchment. Flume experiments are also undertaken to supplement field data collected in rock step surveys. In the Upper Tees catchment, bedrock channels form 20% of the upland stream network, 10% of channels consist of bare rock with no sedimentation. The stream network was divided into five channel classes from bedrock (1) through to fully alluvial (5). In both Trout Beck and the Upper Tees bedrock channel sections gradual transitions from bedrock to fully alluvial reaches and back again were common. Equally, however very abrupt changes at the end of fully alluvial or bedrock channel section could often be found (e.g. transitions 1-5 and 5-1). Classification of the river channel using this scheme allowed the local importantance of bedrock channel landforms on hydraulic conditions and sedimentation processes to be described. In the study reach both lateral sedimentation and amalgamation of sediment bars were observed as mechanisms of re-sedimentation. The rate and location of sedimentation is controlled by local hydraulics (rock steps), gradient and discharge. The tracer study supports the assumption that sediment movement in river channels is episodic. The rock step survey demonstrated the large variability in the shape and size of these features and their important influence on the hydraulics of the flow.