Impact of elevation at Tongguan on flood defence in the lower Weihe River

The Weihe River is the largest tributary of the Yellow River which discharges into the Yellow River at Tongguan. The lower Weihe River is in the backwater region of the Sanmenxia Reservoir, the first large‐scale hydraulic power project in the Yellow River. Severe sedimentation has occurred in the lower Weihe River since impoundment of the Sanmenxia Reservoir in 1960. The so‐called elevation at Tongguan occurs at the stage while the discharge is 1,000 m3/s at the Tongguan hydrologic station, with this being the datum plane of the bed profile of the Weihe River and varies with the reservoir sedimentation and erosion levees. By analyzing the sedimentation and erosion during different periods, this paper studies the impact of the elevation at Tongguan on the flood stages in the lower Weihe River. The sedimentation in the lower Weihe River is closely related to the rise in the elevation at Tongguan, which has been propagating upstream in a series of retrogressive waves at a rate of about 10 km per year. Erosion has taken place in the main channel and sedimentation has occurred over the floodplains during the descending and ascending periods of the elevation at Tongguan. In the lower Weihe River, the water stage has changed with the elevation at Tongguan. The scenario of a 2‐m fall in the elevation at Tongguan is studied in terms of the influence on flood stage reduction in the lower Weihe River. If the elevation at Tongguan is reduced by 2‐m, then the stages of mid‐ and small‐scale floods will be lowered, but the stage for over bank floods will not change significantly. The bankfull discharge will increase, hence, mid‐ and small‐scale floods can be kept within the channel. However, the risk of a large flood is still high because the stage will be slightly higher if over‐bank floods occur. Therefore, a powerful flood defence system is still necessary for mitigation of major flood disasters.