三峡水库不同类型支流河口泥沙淤积成因及趋势

为揭示三峡水库库区不同类型支流河口泥沙淤积的内在机理和变化趋势,本文充分利用水文、泥沙、固定断面和河道地形等原型观测资料,从支流水沙输移规律和河口局部水沙分布特征出发,研究了不同类型支流河口段泥沙淤积规律及主要影响因素的作用机理,探讨其淤积趋势及形成拦门沙的风险。结果表明:三峡水库蓄水后,库区不同类型支流河口普遍淤积,淤积范围及河道形态的变化各有特点;水库蓄水造成水动力条件减弱是河口泥沙淤积的根本原因,淤积幅度和范围主要取决于干支流来沙量和局部河势。在干支流来沙均明显减少的情况下,三峡水库库区支流河口泥沙淤积速度显著下降,形成拦门沙坎的可能性较小。

Sediment deposition of Modao Creek and Xiangxi River mouth section since the impoundment of the Three Gorges Reservoir

There are 66 tributaries in the Three Gorges Reservoir (TGR) area, where most of them witnessed pronounced deposition in mouth reach since the impoundment of the TGR. The lasting sedimentation may lead to the formation of outlet bar at mouth section, which generated adverse effects on storage capacity of the TGR and navigation of tributaries. This paper, taking the Modao Creek and Xiangxi River as examples, examines the characteristics and causes of sediment deposition in their mouth sections based on detailed observation data such as flow, sediment, cross section and channel topography. The results show that: (i) after the impoundment of the TGR, a large amount of deposition occurred in its tributary mouth. The maximum deposition thickness of Modao Creek and Xiangxi River mouth sections exceeded 10 m from 2004 to 2017. Deposition was basically within the range of 15 km of the mouth section, and the strong depositing zone was focused in the reach of about 5 km away from the river mouth. The sediment deposition in the river mouth mainly occurred in the initial operation period of the TGR, and slowed down particularly since 2010 when the cascade hydropower stations in the middle and lower Jinsha River began to operate; (ii) the weakened hydrodynamic condition in dry season at the tributaries mouth, and the significantly reduced sediment-carrying capacity, are the fundamental causes in responsibility to the intense deposition. Sediment deposited at Modao Creek and Xiangxi River mouth sections mainly came from the mainstream in the form of inverted flow, and partially came from their own basin. The deposition range of sediment and its influence on channel morphology were related to local river regime; (iii) Optimized operation of the TGR since 2010 has not affected the sediment deposition in the tributaries mouth. Decreased sediment concentration in mainstream and tributaries and the maintenance the low water level in flood season do not support the formation of outlet bar of tributaries in the TGR.