床沙粒径及级配对非均匀沙起动的影响分析

Effects on the Incipient Motion of Non- Uniform Sediment of Size and Gradation of Bed Materials

对Wu W.等的荫暴系数计算方法进行了修正,根据滚动起动模型建立了以荫暴系数为主要参数的起动底流速公式,在此基础上理论探讨了暴露程度、床沙各分组泥沙粒径及其质量比对泥沙起动的影响,并论述了非均匀沙起动中完全暴露、完全荫蔽和极限跃移3种极限起动情况。结果表明:1非均匀沙起动底流速随着荫暴系数的增大而减小,荫暴系数越大,泥沙越容易起动;2在由两种粒径泥沙组成的非均匀沙中,粗颗粒的荫暴系数在定义域上均随着细颗粒床沙比例和粗细颗粒比例的增大而增大,即细颗粒比例越大,粗细颗粒间粒径差异越大,粗颗粒越容易起动;细颗粒的荫暴系数在定义域上随着细颗粒床沙比例的增大而增大、粗细颗粒比例的增大而减小,即细颗粒的比例愈大,细颗粒愈容易起动,且起动规律渐趋于和均匀沙的相仿,粗细颗粒粒径相差愈大,细颗粒受到的荫蔽作用愈显著,其起动流速也愈大。

A modification of Wu’s correction factor was proposed to reveal the exposure and hidden effects of non-uniform sediment parti-cles. After that,an incipient near-bed velocity formula with the modified correction factor as a parameter was established using the rolling model of sediment incipient motion. The formula was applied in the analysis of the effects on the incipient motion of non-uniform sediment of size and gradation of bed materials and the exposure degree. Three cases of position were discussed,including the completely exposed,to-tally sheltered and extremely saltation,to demonstrate the inception characteristic of broadly graded non-uniform sediment. Results show that the grains with significant exposure degree are easy to start moving. For particles with two kinds of size,the near bed incipient velocity of coarse grain decreases with the proportion of fine grain and the difference in size between coarse and fine grain. The near bed incipient ve-locity of fine grain decreases with the proportion of fine grain and increases with the difference in size between coarse and fine grain. In other words,the greater proportion of fine grain,easier fine grains start moving,which share almost the same rule of uniform sediment. The greater difference in size between coarse grain and fine grain,more significant shelter effect act on fine grains,which incipient velocity is larger.