Sediment Transport Influences on Coarse Bed Wandering Channel Dynamics

Wandering systems are characterized by discrete reaches that feature laterally unstable, anabranched active channels and are characterized by comparatively rapid rates of sediment transfer. They are often viewed as transitional between braided and single-thread sinuous channel states. They are composed of individual anabranches surrounding semi-permanent islands with locally high rates of bed sediment redistribution creating a dynamic morphology. Sensitivity to the flood regime has been reported as high with increased dynamism seen as a response to an increased frequency of geomorphologically effective floods.

This paper reports on the influence of legacy sediment deposit type on wandering character reviewing examples from Northern England and Southern Scotland and presenting data from the Wooler Water in Northumberland. The location of wandering zones is strongly correlated with deposits of mixed fluvio-glacial sediments containing very coarse lag deposits. Lateral erosion of these sediment is possible during floods due to the relatively low material shear strength with individual flood event related erosion equivalent to several channel widths recorded historically, however, flood energy appears insufficient to transport coarse clasts released during this process and these are deposited as a lag deposit across the wandering surface. Intermediate floods infill this material with fines allowing areas to vegetate and further stabilise. Movement of this material is severely restricted and appears confined to rare local extreme flood concentration, generating scour and new channel development with internal channel switching more generally controlled by legacy palaeo-channel routes. These tend to be infilled by finer material characteristic of the contemporary sediment transport regime. As such low flow channel configuration flips between old channel routes rather than developing significant new channels through lateral movement and cutoff or headcut processes. Volumetric change and individual clast movement monitoring using sUAV based photogrammetry supports the above process inferences with larger clast stability noted in the data and patchy sediment movement evident in repeat topographic survey.

It is suggested here that coarse sediment wandering system form and dynamics are strongly controlled by an inability to transport larger bed material restricting internal channel development to palaeo-route switching and encouraging lateral extension through erosion of weaker unconsolidated mixed legacy fluvio-glacial material, which is also supplying largely immobile larger clasts to the active channel. Evidence of past wandering is common in valley bottom sediments in upland areas of England and Scotland and it is suggested that with reduced channel management these diverse systems will develop along more fluvio-glacial influenced watercourses.