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The Xiaolangdi Reservoir has been mainly used for sediment trapping and water-sediment regulating in order to reduce sedimentation of the lower Yellow River after operation, which results in lower sediment transport efficiency. At present, a suitable river channel scale of median water with the bankfull discharge capacity of 4 000 m³/s has been formed in the lower Yellow River. Under this riverbed condition, how to maintain the river channel of median water of the lower Yellow River and give full play to the river sediment transport capacity becomes new requirement to the operation of the reservoir. This paper analyzed the sediment transport efficiency at different sediment concentration levels and discharge levels based on the measured data of 306 non-overbank floods in the lower reaches of the Yellow River from 1960 to 2016 and proposed that the downstream river channel had a higher sediment transport efficiency when the large flow with sediment concentration greater than 60 kg/m³, but it was also prone to cause sedimentation in the river channel. The paper further proposed the sediment transport requirement to the operation of the reservoir based on maintaining the river channel of median water of the lower Yellow River. In order to improve the sediment transport capacity of the downstream river, when the bankfull discharge capacity of the downstream river channel was about 4 000 m³/s, the Xiaolangdi Reservoir could discharge large-flow processes with sediment concentration less than 100 kg/m³. When the bankfull discharge capacity of the downstream river channel was greater than 4 500 m³/s, the Xiaolangdi Reservoir could discharge large-flow processes with sediment concentration less than 200 kg/m³. When the bankfull discharge capacity of the downstream river channel was less than 4 000 m³/s in the future, the Xiaolangdi Reservoir would still mainly trap sediment to reinvent the river channel. The calculation results of the mathematical model show that the adjustment of the operation mode of the reservoir can improve the sediment transport capacity of the downstream river, alleviate the sedimentation of the reservoir and the downstream river and maintain the river channel of median water with the bankfull discharge capacity of 4 000 m³/s for a longer period of time. [ABSTRACT FROM AUTHOR]

Fully tapping the regulating potential of reservoir groups in the middle reaches of the Yellow River and improving the sediment transport capacity of the lower reaches of the Yellow River play an important guiding role on river and beach management in the lower reaches of the Yellow River. Through the analysis of measured data and mathematical models, this paper calculated the influence of combined regulation and control of existing reservoir groups under different water and sediment conditions on sediment transport capacity of the lower reaches of the Yellow River, and further to analyzed the change of sediment transport capacity of the lower reaches of the Yellow River after Guxian and Dongzhuang reservoirs come into effect. The results indicate that by optimizing the joint regulation and control mode of the existing reservoir groups, the flow process with large sediment concentration discharged by the reservoir can increase the sediment carrying capacity of the downstream river by about 10%-12%. The sediment discharge into the downstream river during the sediment retaining period after Guxian and Dongzhuang Reservoirs come into effect are reduced, and the sediment transport capacity can be further improved by 11%-12% during the normal operation period. [ABSTRACT FROM AUTHOR]