库布齐沙漠南缘抛物线形沙丘表面风沙流结构变异

对库布齐沙漠南缘抛物线形沙丘表面气流和输沙率的野外观测和分析结果表明,沙丘表面约90%的风沙输移集中在距沙面0.10 m高度范围内,输沙率随高度递减的形式在沙丘各部位因风速、下垫面状况和坡面形态不同而发生变异。沙丘迎风坡坡脚因出露坚硬、含砾石地表,颗粒跃移高度大,风沙流上层相对输沙率大;迎风坡沙粒沿坡向上运动,颗粒跃移高度减小,风沙流中近地表相对输沙率大;沙丘背风坡沙粒沿坡向下运动,加之来自丘顶变型跃移物质的影响,风沙流上层相对输沙率较大;脊线受迎风坡各个断面地形差异的影响,各观测点间风沙流结构差异显著。风沙流结构在迎风坡和丘顶均遵循指数递减规律(Q=aexp(-z/b)),其中,指数函数拟合中系数a与输沙率具有良好的幂函数关系,随风速增加而增加,但二者关系较弱;b与二者无相关性。背风坡风沙流结构具有明显的分段现象,以0.10 m高度为界,下层符合指数函数,上层符合幂函数。

Variation of aeolian sediment flux profiles on a parabolic dune at the southern fringe of the Hobq Desert

The airflow and sediment transport rate on the parabolic dune and the aeolian sediment flux profiles were examined in details in the southern Hobq desert in late April 2010 when the wind was strong and sediment movement was intense. The analyses show that 71%~91% sediments moved in the space between 0 and 0.10 m above surface. This confirms the results of previous research that blown sand activity occurs near bed. Due to the difference of wind velocity, underlying surface conditions, and slope profiles, sediment transport rate and relative sediment transport rate vary, so do the aeolian sediment flux profiles. At the measurement spots where wind velocity and sediment transport rate are small, such as the toe and the lower part of the windward slope, the aeolian sediment flux profiles vary considerably. Underlying surface condition is a vital factor influencing aeolian sediment flux profile. At the windward toe that suffers deflation, the gravelly surface is hard. This contributes to sediments’ saltaltion, so sediment transport rate is relatively high at the upper layer of the aeolian sediment flux profile. Along the windward slope, sediments move upward and have a lower saltation height. Therefore sediment transport is relatively high near surface. In contrast, sediments move downward at brink-line on the leeward slope with higher height and the trajectories deform, so the saltation height is higher. Affected by the difference of windward slope profile shape, the aeolian sediment flux profiles at the crest vary markedly among the measurement spots. In spite of the variation of the aeolian sediment flux profiles, they can still be well described by the exponential decay law at the windward slope and crest. In the equation Q=aexp(-z/b), the coefficient a has an apparent power function correlation with sediment transport rate and increases simultaneously with wind velocity, but there is no clear correlation between them. The coefficient b has no significant correlation with these two factors. At the brink-line, the profiles were segmented at the 0.10 m height: the profiles of near surface follow exponential decay while the upper layers obey power function.