Spatial variability, mechanisms and propagation of change in an active meandering river

Abstract

This paper addresses questions of the spatial pattern of instability and the mechanisms of change in an active meandering river, particularly whether and how change is propagated. More than 20 years of monitoring of a sequence of nearly 100 bends on one dynamic meandering river, combined with historical data and previous analyses of processes of change, provide a unique insight into the link between annual changes produced by erosion and deposition and the longer-term changes in planform. The study reach of the River Dane in NW England exhibits stable and unstable sections adjacent to one another. Rates of movement range up to 3 m a − 1 , with maxima occurring in high curvature, free bends. Stable reaches are due to factors of gradient, curvature and bank resistance. Analysis of the large amount of data on occurrence of erosion and deposition in each bend each year reveals no definite association of changes in one bend with another. The detailed evidence of the morphological features in the bends shows that changes do not take place by bars moving progressively through reaches. Case studies of bends upstream of constrained, stable reaches indicate an oscillation of widening and narrowing of the channel, over a period of a few years, producing a net rotation of the bend. These areas are zones of stalling of sediment and change takes place by absorption and lateral movement. Overall, changes tend to be localised and fit the bend theory of meanders, but with low sensitivity reaches pinning the planform for longer periods in certain locations.