水沙过程对道人矶-杨林岩顺直分汊河段洲滩演变影响初步研究

Preliminary study of impact of water and sediment transport process on bars in Daorenji-Yanglinyan straight braided reach

基于实测资料分析,对道人矶-杨林岩顺直分汊段的洲滩演变特征进行了研究,并进一步探索了其与水沙过程的响应关系。结果表明,洲滩演变特征与水沙过程的时间尺度特征相对应:年际间特大洪水易切割边滩,年内主流线摆动导致洲滩冲淤交替,数天、月的次生流输沙造成局部倒套和串沟形成;洲滩变形特征响应于洪季特征流量级的持续时间,蓄水后大于35 000 m3/s流量级持续时间的减少是近年来丁家洲边滩洪季冲刷幅度较小,年际间微淤的原因,大于等于25 000 m3/s流量级持续时间达4个多月且变化不大是南阳洲中水位以上部位剧烈冲刷的原因;洲滩的几何要素与断面平均含沙量具有较好的增减对应关系,但滞后于含沙量的变化;蓄水后在中小水小沙组合条件下,丁家洲边滩将呈现微淤演变趋势,而南阳洲呈现冲刷演变的趋势,其中洲长变形速度最大,洲宽其次,洲面积变形最慢。

Based on the measured data,the evolutional characters of bars（point bar and mid-channel bar） in Daorenji-Yanglinyan straight braided reach are investigated,and the relationship between fluvial processes and water and sediment transport process is explored.The result shows that the evolutional characters of bars respond to water and sediment transport process at different time scales.The extra flood is prone to cut the point bar,and the mainstream swing leads to alternate erosion and deposition of bars,and the sediment transport by secondary stream causes partial reversed stream and string groove formation in a few days or several months.The bars deformation characteristics respond to the duration of characteristic discharge in flood seasons and the reduce of duration of discharges larger than 3 5000 m3/s after the commissioning of Three Gorges Project results in the small erosion in flood seasons and annual slight deposition of Dingjia point bar,and the four months′duration of discharges larger than 2 5000 m3/s leads to the great erosion of the part above median water level of Nanyang mid-channel bar.Geometric elements of bars increase or decrease with the cross-sectional average sediment concentration,but lag behind the changes in sediment concentration.Under conditions of small water-sediment combination after the commissioning of Three Gorges Project,the Dingjia point bar will present micro sedimentation evolution trend.In contrast,the Nanyang mid-channel bar will present erosion evolution trend,among which the length changes most rapidly,followed by the largest width,and the area changes the slowest.