Causes of planform stability of a low-energy meandering gravel-bed river (Cher River, France)

Abstract

For at least two centuries, the lateral mobility of the meandering reaches of the Cher River (France) has been very low. This article aims to identify the main causes of this behavior. Two not-mutually exclusive explanatory hypotheses are proposed. Under the first hypothesis, the natural mechanisms of loop migration would have been inhibited or blocked by the presence of bank protections. Under the second hypothesis, a decrease in the frequency and/or intensity of morphogenic hydrological events since the nineteenth century would have reduced the frequency of bedload mobilization and/or reduced the capacity of the river to erode its banks. To test these hypotheses, the diachronic evolution of the planform was reconstituted at different time scales using a GIS and field surveys. Morphological transformations were characterized and quantified (eroded and vegetated areas, length of eroded banks, rates of bank retreat) and the critical discharges of bedload mobilization and of lateral erosion were estimated. Engineering works in the riverbed were identified and, when possible, dated. The results show that meander morphodynamics have been highly constrained and disrupted by engineering works, probably for over a century. However, the meanders still have noticeable potential for bedload mobility and lateral erosion, and hence for self-restoration.