Lowland gravel-bed river recovery through former mining reaches, the key role of sand

Whereas the geomorphic effects of in-channel mining have been widely documented, very few studies have focused on recovery trajectories after mining ended. This paper describes and quantifies the channel adjustment of extended and over-widened mining pits located in a low energy gravel-bed river, and identifies the factors that control their recovery rate. The study was based on aerial photographs and LiDAR analysis, grain-size sampling, bedload computation and geophysical measurements made during ground penetrating radar (GPR) and electrical resistivity tomography (ERT) surveys. We describe the spatio-temporal changes in the morphology of former pits as well as the current surficial longitudinal grain-size pattern in pits and in neighbouring upstream and downstream reaches. Our results show a wide range of geomorphic readjustments. Between 23 and 47 yr after the end of mining, the mining reaches have still not yet fully recovered. The planimetric recovery rate is closely correlated with the ratio of the volume deposited in the pits to their original volume. Finally, the infilled material is only partly composed of bedload and the mainly sandy fine sediments play a primary role in the recovery process. The abundance of sand, controlled by the lithology of the watershed, explains the unexpectedly high recovery rates of some pits despite the low energy of the river. (C) 2020 Elsevier B.V. All rights reserved.