Gravel excavation and geomorphic evolution of the mining affected river in the upstream reach of the Yangtze River,China

As economic development upstream in the Yangtze River basin has progressed in recent decades,the demand for sediment has rapidly increased and contributed to an expansion in sediment excavation that may affect the river's stability and navigation safety.In the current study,the distribution of gravel mining in the upstream reach of the Yangtze River was investigated using field measurements obtained from2008 to 2017.An experimental investigation was then done to analyze the bed load behavior in a typical mined channel using a physical river model.Finally,a two-dimensional hydrodynamic-sediment transport numerical model was developed to predict the evolution of the mined channel.The results indicate that gravel mining was mainly distributed among the following nine river sections:Tongluoxia-Yibin,Erlong-Xianglutan,Naxi-Jinkou,Binpangqi-Hongyanqi,Chayuqi-Shenbeizui,Yangshipan-Shangbaisha,Dongxikou,Baisha-Wenzhongba,and Jiangjin-Shenzhongba.The gravel shoals near the riverbank have been destroyed,and the bed topography has been altered by up to 1.65 × 10～9 m3 over the past 10 years.A bed load velocity formula was proposed to describe bed load transport in the mined channel based on dimensional analysis.The recovery process of the excavation pits was slow in both the experiments and simulations,with only 0.12% and 1.39%,respectively,of the mined amounts of gravel being restored.Finally,the recovery rate of the mining pits gradually slowed as the sediment supply from upstream decreased,suggesting that the destroyed gravel shoals rarely returned to their original forms due to the combined operation of hydro-projects in upstream areas and limited sediment storage in the channel.