“11·03”金沙江白格堰塞湖溃决洪水沉积物粒度特征

Grain Size Characteristics of Flood Sediment from the“11·03”Jinsha River Baige Barrier Lake Outburst

【目的】探究堰塞湖溃决洪水沉积物粒度特征和理解金沙江溃决洪水泥沙沿程变化特征。【方法】分析了“11·03”金沙江白格堰塞湖溃决洪水沉积物样品颗粒组成,计算了粒度参数,论述了泥沙特征从上到下的变化及其原因,并对比讨论了本次洪水泥沙粒度特征与其他流域洪水及古洪水沉积物泥沙粒度特征。【结果】堰塞湖溃决洪水沉积物类型为粉砂、砂质粉砂、粉砂质砂;泥沙粒径中粉砂占55.18%,砂占32.86%,黏土占11.97%。泥沙中值粒径为41.34μm,平均粒径为31.73μm,两者随离白格堰塞湖距离的增加而逐渐变小;偏度值为0.27,属正偏;峰态值为0.94,属于中等尖锐;分选系数为0.57,分选性好;泥沙的粒度分布曲线双峰占52%,主峰高而窄,峰值为100μm,次峰低而宽;单峰占48%,峰态较窄,峰值为50μm。剖面泥沙粒径组成差别很小,以砂占优,粉砂含量略小于细砂,黏土含量大多在11.0%,中值粒径值大于其平均粒径值,分选系数均小于0.6,偏度为极正偏,峰态为中等。“11·03”溃决洪水与其他洪水相比,泥沙粒度组分较细,以粉砂为主,中值粒径和平均粒径较小,峰态更宽,偏度为正偏,分选性更好。【结论】研究结果对进一步认识堰塞湖溃决洪水的泥沙特征具有重要参考价值。

[Objective]To study particle size characteristics of the burst flood sediment in barrier.[Methods]Composition of flood sediment particle samples from the“11·03”Baige dammed lake outburst flood on the Jinsha River were analyzed,and size parameters of sediment were calculated.Changes in sediment characteristics from upstream to downstream and its causes were analyzed.Then,the granularity characteristics of this flood sediment were compared with those of other primary river basins in China,while sand particle size characteristics of ancient flood sediment were discussed.[Results and Discussions]The sediment types of the Baige barrier lake outburst flood were silt,sandy silt,and silty sand,with 23.65%,64.19%and 12.16%,respectively.All particle sizes of sediment in barrier lake were less than 2.0 mm.The proportion of silt was 55.18%,sand of 32.86%,and clay of 11.97%.The silt content gradually increased,while the sand content gradually decreased from the upstream Maoding village to the downstream Shigu town.Moreover,the clay content always fluctuated around 12%.As the flow rate of the flood was reduced gradually,the ability of hydrodynamic to carry particulate matter was weakened by degrees.The clay was less affected by the hydrodynamic conditions as a result of the small particles and the electric charge,that’s why the clay had a relatively stable proportion during the flood peak process.The median particle size was 41.34μm,and the average particle size was 31.73μm.Both of the value decreased gradually in pace with the increasing distance from the White River barrier lake.The skewness value was 0.27,which is a positive deviation.The peak value was 0.94 and was moderately sharp.The sorting coefficient was 0.57,indicating good sorting performance.Bimodal distribution accounted for 52%of the grain size distribution curve of sediment.The main peak was high and narrow,located at 100μm.And the secondary peak is low and wide,the peak value located at 10μm.The single peak accounted for 48%,with a narrow peak at 50μm.All sediment sizes in the profile were less than 2.0 mm.There were very small differences in composition.The predominant component was the sand.The silty content was slightly less than the fine sand,and the proportion of clay content was approximately 11.0%.The median value of particle size was larger than the average,both have a tendency to increase from the surface layer to the bottom.The sorting coefficient was less than 0.6,with the extremely positive deviation and the moderate peak state.Compared with other floods,the"11·03"outburst flood had a finer sediment particle size of mainly silt,followed by a smaller median and average value of particle size,with wider peak state and positive skewness,indicating better sorting performance by this flood.[Conclusions]The research results have important reference value for understanding the sediment characteristics in barrier lake outburst floods,burst flood hydrodynamic transport process and mechanism in the future.It will promote the development of water conservancy and hydropower projects and ecological civilization construction in river basins.