Geomorphological changes after river-meander surgery

Abstract

A meandering river, upon preliminary inspection, looks like a very inefficient system, in need of serious correction. That the contrary is the case is shown by geomorphological changes which occur when sets of river meanders are cut off and replaced by straight or curved channel sections. Subsequent to the meander cutting, the potentially available energy per metre of channel increases dramatically and the water velocity increases. The excess energy causes erosion of bed and banks, while eroded material is deposited in the downstream channel. Before equilibrium can be reached again, a river bed must incise deeply for considerable distance upstream. Thereupon the river will start to make a new set of meanders not unlike the old meander pattern. The one-dimensional open-channel flow equation. when applied to river erosion (triggered by meander cutting), shows that the 'half life' of such man-made disturbances ranges from hundreds to a thousand years. Consequently, man-made interference with natural rivers represents a costly capital operation, which requires a long term (50-100 years) commitment for up-keep, improvement and replacement, if not in the short run, then in the long run (next 500 years).