北方土石山区植株密度对坡面流粒径分选的影响

Influence of Vegetation Density on Size-selectivity of Sediments Eroded by Overland Flow on Northern Earth-rock Mountainous Area

为研究和阐明植株密度对坡面流粒径分选的影响,通过野外10°草地坡面恒流放水冲刷试验,定量研究3种植株密度(0株/m^2,20株/m^2,60株/m^2)在不同流量下(1m^3/h,2m^3/h,3m^3/h)的坡面流粒径分选性规律,从水动力学和侵蚀方式演变角度分析了植株密度对坡面流粒径分选性的影响。试验结果表明:(1)苜蓿草地侵蚀泥沙中细粉粒组分含量最多,其次是粗粉和细砂组分含量,而粗砂和粘粒组分含量最少。固定流量下,随着植株密度的增加,各粒径组分百分比及平均重量粒径(MWD)随时间的波动性增加,在侵蚀泥沙中细颗粒组分(粘粒、细粉粒、粗粉粒,粒径<50μm)富集比ER相应增加,而粗颗粒组分(细砂、粗砂,粒径>50μm)富集比ER有所减少。固定植株密度条件下,随着流量增加,各粒径组分百分比及MWD随时间的波动性减小,侵蚀泥沙中细颗粒组分富集比ER相应增加,而粗颗粒组分富集比ER增加,细颗粒组分富集比基本大于1,而粗颗粒组分富集比基本小于1。MWD与细颗粒含量呈负相关关系,与粗颗粒含量呈正相关。(2)细沟阶段侵蚀泥沙MWD明显大于片蚀阶段,而粒径分选过程的波动性较小。(3)水流剪切力τ及水流功率ω的增加导致片蚀阶段的缩短和细沟阶段的延长(除植株密度为60株/m^2),水流剪切力τ及水流功率ω的增加导致水流侵蚀力的增加,进而水流分离、输移粗颗粒增加。

In order to study and illustrate the influence of vegetation density on size-selectivity of sediments transported by overland flow,we conducted a constant flow scouring experiment on grassland slope with a gradient of 10°under 3vegetation densities（0individual plant/m^2,20 individual plants/m^2,and 60 individual plants/m^2）to quantitatively investigate the sediment size-selectivity law of overland flow from perspectives of hydrodynamics and erosion pattern evolution.Three overland flow discharge treatments（1 m^3/h,2 m^3/h,and 3m^3/h）were established in this experiment.The results showed that：（1）The proportion of fine silt was the highest in eroded sediments,followed by the proportions of coarse silt and fine sand,and the proportions of coarse sand and clay were the lowest on alfalfa slope.The temporal fluctuation of proportions of different particle-size frac-tions and mean weight diameter（MWD）increased with the increase of vegetation density under the same flow discharge.Meanwhile,the enrichment ratio（ER）of fine particles（clay,fine silt,coarse silt,〈50μm）increased and the ER of coarse particles（fine sand and coarse sand,〈50μm）decreased.Moreover,under the same vegetation density,the temporal fluctuation of proportions of different particlesize fractions and MWD decreased with the increase of flow discharge.At the same time,the enrichment ratio（ER）of fine particles decreased with the increase of flow discharge,which generally exceeded 1,while the ER of coarse particles showed an opposite tendency,which was smaller than 1.There was a negative correlation between MWD and fine particle content,while a posi-tive correlation existed between MWD and coarse particle content.（2）MWD of eroded sediments in rill erosion process was significantly larger than thatin sheet erosion process,while the temporal fluctuation of MWD of eroded sediments showed an opposite tendency.The increases of flow shear stressτand stream powerωshortened the sheet erosion process and prolonged the rill erosion process（except under the vegetation density of 60individ-ual plants/m^2）and led to the increases of flow erosion power and coarse particles transported by flow.