Abstract: Quantitative research on the reservoir sediment trapping effect is key in revealing the influence of cascade reservoirs on sediment transport. This paper estimates the trapping efficiency through an analysis on the variations in runoff and sediment load in the middle and lower Beijiang recorded at the hydrologic stations on the mainstream and tributaries in the past 60 years. We modify the formula of reservoir trapping efficiency using G. M. Brune’s trapping efficiency curves, and apply it to evaluating the influence of cascade reservoirs on sediment transport. The results show that (1) from the period of 1956-1999 to 2000-2005, Shijiao station manifested a reduction of 39.17% in sediment discharge while a reduction of only 6.16% in annual runoff. In 2000-2005, the total sediment trapped in the reservoirs was about 1.43 million tons per year, or 52.3% of the reduction at Shijiao. To the total sediment reduction, the contribution was 18.83% by the engineering measures for water and soil conservation, and 5.07% by runoff drop. (2) By the modified formula, the sediment trapping efficiency of cascade reservoirs in the 2012-2016 conditions is about 12.81%, which agrees well with the measurement. (3) Since 2006, the amounts of sedimentation and sand mining have nearly balanced in the cascade reservoir area, and we predict that the sediment trapping efficiency of reservoirs will be maintained at the existing level for long in future.

Key words: river sediment engineering, Beijiang River, reservoir sediment trapping efficiency, sediment deposition