回涡沙丘的形态特征与表面物质组成

在沙丘分类系统中,回涡沙丘是一种重要的有障沙丘形态,系运移风沙流在障碍物前遇阻堆积而成。野外测量表明,回涡沙丘具有与新月形沙丘相似的外观,其迎风坡坡度一般在2°～18°之间,背风坡坡度较陡为21°～31°。回涡沙丘高度及其与障碍物距离受障碍物高度的控制,障碍物越高则回涡沙丘也越高,沙丘离障碍物也越远,沙丘高度、沙丘与障碍物距离分别与障碍物高度线性相关。粒度特征反映了局地沙源特征,粗细颗粒均可在风力作用下形成障前堆积形态。同一地区回涡沙丘与新月形沙丘表面物质的粒度参数对比表明,由于障碍物前回旋涡流的影响,回涡沙丘表面物质经历了更为强烈的风力分选过程,物质组成更细,分选更好,更趋于正偏和窄峰态。

Morphology and Grain Size Characteristics of Echo Dunes

Echo dunes are one of the important anchored dune types and they can form in front of various obstacles due to the deposition of blowing sands.Our field investigations illustrate that the echo dunes show similar shapes to barchan dunes,their stoss slopes range from 2° to 18° while the steep lee slopes reach 21°-31°.Both the dune height and the sand-free distance(i.e.the distance from lee slope base to the obstacle) are affected by the height of obstacle,the higher the obstacle means the higher dune and the longer sand-free distance.The relationships can be described with linear functions.The grain size characteristics strongly imply that the dune sands are composed of local deposit.The saltating particles and even the granular materials could be transported and deposited in front of obstacle to form an echo dune.The comparisons of grain size and sorting parameters between echo and barchan dunes indicate that the surface sands over a echo dune experience more intensive winnowing processes.The finer and better sorted as well as the more positive skewness and leptokurtic distribution of the sediments over an echo dune reflect the influences of reversing airflow formed in front of obstacle.