选择性起动条件下砾石沉积物转动角的实验室测量

沉积物颗粒与下覆颗粒间转动角是制约沉积物颗粒起动的一个重要因素。本文样品采集于1987年。作者通过一系列实验探明了沉积物转动角同颗粒形状、相对粒径比以及迭瓦状构造等因素之间的相互关系。在均匀粒径条件下,球状砾石转动角的测量数值同理论数值相吻合,但随粒径的减小而增大。这一结果同前人利用球状砂粒进行实验所获得的结果相一致。利用球状、椭球状及棱角状砾石进行的实验表明,沉积物转动角φ随相对粒径的增大而减小,随颗粒扁平度及颗粒棱角度的增大而增大。扁平砾石的迭瓦状构造将导致转动角的增大,其增大值约等于砾石的迭瓦角。若其它因素相同,沉积物起动的难度自球状、椭球状、棱角状及迭瓦状构造而依次增强。这些结果将有助于对颗粒粒径、形状、构造同沉积物搬运临界状态以及沉积物选择性起动之间的相互关系进行定量分析。

LABORATORY MEASUREMENTS OF GRAVEL PIVOTING ANGLES UNDER SELECTIVE ENTRAINMENT

The pivoting angle of a grain about its contact point with the under-lying grains is very important to grain entrainment. A series of experiments were conducted to investigate how this angle depends on grain shape, relative grain size ratio and factors such as imbrication.The pivoting angles of uniform grain size spheres reasonably agreed with that from a theoretically derived equation, but increased with decreasing grain size. This is consistent with earlier studies limited to sand-size spheres. Experiments with spherical, ellipsoidal and angular pebbles demonstrated that sediment pivoting angles increase with decreasing relative grain size, increasing particle flattness, and angularity. Imbricated arrangement of flat pebbles enlarged greatly their pivoting angles. The increase in pivotal angle approximately equal to the imbrication angle. Other factors being equal, the orders of increasing difficulty of entrainment are spheres, ellipsoidal grains, angular grains and imbricated grains. These results will be helpful for quantitative evaluation of size, shape and structural effects on grain threshold, as well as evaluation of the selective entrainment of grains from a bed of mixed sizes.