Morphodynamic model of the Yichang-Chenglingji Reach: migration of erosion centers downstream of the Three Gorges Dam

Abstract: Many rivers worldwide have suffered great degradation after large reservoirs construction. By investigating the Yichang-Chenglingji reach downstream of the Three Gorges Dam, we identified and analyzed the erosion centers (sub-reach with most severe erosion intensity) which migrated downstream along the river with the rate of 7.5 km/yr. To simulate the phenomenon and study the factors influencing the migration rate of erosion centers, a one-dimensional river morphodynamic model is implemented using field data (including water and sediment regimes and grain size of bed material) of Yichang-Chenglingji reach based on the active layer theory. We set three values for the thickness of active layer and designed four groups of grain size distribution of the sub-layer based on the drill data and the grain size distribution of bed surface material at Yichang station. The simulation results show that the main cause of the erosion centers is bed armoring. A high-speed bed armoring process is instrumental in the formation and migration of erosion centers, as the armoring of bed surface inhibits the further degradation in the upper reach. The thinner the active layer and the coarser the sub-layer, the faster the process of bed armoring. Under the condition that the thickness of the active layer is 1.5m and the sediment of sub-layer is the field data of bed surface material at Yichang station in 2020, the migration rate (13km/yr.) of erosion centers in simulation results are most in agreement with the actual erosion centers. Our results may deepen the understanding of the river evolution after the abrupt sediment reduction.

Key words: Three Gorges Dam; Yichang-Chenglingji Reach; Morphological evolution; Erosion centers; Spatial clustering; Numerical model