Effect of "318" Controlled Operation of Sanmenxia Reservoir on Changes of the Tongguan Elevation

Sanmenxia Reservoir in the middle Yellow River is well-known in China and abroad for its rapid sedimentation and severe backwater impacts, which were present immediately after the construction of the Sanmenxia Dam in 1960. The flood risk at areas impacted by backwater, including the ancient China’s capital Xi’an City, increased dramatically. In the backwater zone, the river is constricted from a width of more than 10 km to less than 1 km at Tongguan, forming a naturally constricted river reach which acts as a hydraulic control section for the reaches of both the Yellow River and its tributary Wei River upstream of Tongguan. Therefore the elevation of Tongguan is extremely critical for limiting the impacts of sedimentation in the farther upstream area of the reservoir. To restrain the elevation at Tongguan from rising, the Sanmenxia Dam was operated to control the pool level to be no greater than 318 m since 2003. This dam operation is generally termed as “318” controlled operation. In this study, the variation characteristics of the Tongguan elevation since the “318” controlled operation of Sanmenxia Reservoir and its delayed response to the water and sediment conditions and the operational water level were analyzed based on the measured data. A formula for calculating the elevation of Tongguan was proposed based on a 5-year superimposed parameter. This formula comprehensively considered the delayed response of Tongguan elevation to the water and sediment conditions and the water level in flood seasons. It can be used to quantitatively estimate the effects of different water and sediment conditions and the water level in flood seasons on the Tongguan elevation. The results showed that the reduction of the elevation of Tongguan since the use of “318” controlled operation was mainly the result of the increase of water volume and the decrease of sediment concentration in the flood season rather than the pool level control.