Abstract:

[Objective] This study systematically examines the characteristics of sediment deposition and river evolution in Lushui Reservoir over 60 years since its construction and evaluates their comprehensive impacts. [Methods] Based on pre-construction (1959) 1∶10 000 topographic maps and recent (2018) 1∶5 000 maps, sediment volumes and stage-capacity curves were calculated using the cross-section method and 10 m × 10 m grid-based method (combined with Simpson’s 3/8 rule). [Results] The total deposition was 32.397 million m³, accounting for only 4.6% of the total storage below the design flood level, indicating overall light siltation. However, the spatial distribution was highly uneven—74% of the deposition was concentrated below the flood-limit water level, with the Hongshang-Shikeng downstream 11km reach becoming the core deposition zone, accounting for 51% of the total (local maximum deposition thickness up to 11.79 m), while the Hongshang-Langkou reach was likely in a scouring state due to unregulated sand mining and reduced sediment inflow. For channel evolution, the thalweg in the Hongshang-Shikeng reach rose on average 6.27 m, and the shoreline at the 45 m dead water level elevation shrank markedly; maximum thalweg migration reached 660 m (at the Lashupu widening), and the depositional pattern tended toward a deltaic form—with the Hongshang-Shikeng reach dominated by main-channel deposition, and sections above Hongshang mainly showing floodplain deposition. Deposition significantly impaired reservoir functions: (1) For flood control, 6.856 million m³ of flood-control capacity was lost (4.2% loss rate), and the cross-section area in the Hongshang-Shikeng reach was reduced by 26% on average (up to 54%), markedly lowering discharge capacity and raising the regulated flood levels under design floods. (2) For beneficial use, 17.193 million m³ of active storage lost (equal to an annual power loss of 842 000 kW·h), and 12.29 million m³ between the dead water level and flood-limit level was lost (8.1% of mean annual irrigation withdrawal). (3) For navigation, thalweg elevations were generally at or above the 45 m dead water level elevation, severely restricting navigation during low-flow periods. [Conclusion] Although the overall sedimentation rate of Lushui Reservoir is low, local deposition poses a significant threat to flood-control and navigation functions. Mechanical dredging in the key reach (Hongshang-Shikeng) and optimization of sediment-flushing schedules during the flood season are urgently needed to control sediment and maintain effective storage capacity. These findings provide a scientific basis for sediment management in long-operating reservoirs and for the development of “modern reservoir operation and management matrix” proposed by the Ministry of Water Resources.

Key words: Lushui Reservoir, sedimentation, topographic method, river channel evolution, sedimentation impacts