青海共和盆地不同发育阶段风蚀坑表面气流场与形态反馈研究

风蚀坑是沙质草原沙漠化的主要地貌响应和驱动,但目前我们对其形成演化的动力机制知之甚少。利用二维超声风速仪和积沙仪观测了共和盆地不同发育阶段风蚀坑表面气流和风沙流特征,研究其形态—动力反馈过程。结果表明:(1)风蚀斑与碗状坑内气流沿主风向先减速后加速,槽形坑内气流则先辐散减速—中部气流加速—积沙体迎风坡风速降低—背风坡风速有所恢复;且发育初期风蚀坑内风速与风速变异系数和风向稳定系数均呈负相关,而发育中期坑内风速与前者呈正相关,与后者呈负相关。(2)受坑体内的涡流影响,槽形坑内风速廓线不符合对数分布规律。(3)槽形坑内不同部位的输沙率随高度均呈指数式递减,但受气流—形态间的反馈作用,各部位输沙通量差异明显,坑底最低、积沙体迎风坡前端最大。风蚀坑内气流场与形态间存在反馈关系,坑体越大反馈效果越明显。

Form-flow Feedback within Blowouts at Different Developing Stages in the Gonghe Basin,Qinghai Province

Blowouts are the primary geomorphologic manifestation and driving force of sandy grassland desertification in the Gonghe Basin.However,their feedback mechanism between the flow dynamics and geomorphology is unclear.Two-dimensional ultrasonic anemometers and gradient sand traps were used in this study to measure the characteristics of wind flows and sediment transport at different blowouts of different developing stages in the Gonghe Basin.The feedback between the morphology-dynamic processes of the blowouts was discussed.Results show as follows.(1)After entering the sand patch and small bowl blowout along the prevailing wind direction,air flow expanded and decelerated,and then accelerated until going outside the blowout;when entering a trough blowout of a small or medium size,it expanded and decelerated at the headwall,accelerated at the bottom of blowout,decelerated at the windward slope of the depositional lobe,and then recovered somewhat at the leeside slope of the depositional lobe.Besides,the wind speed was negatively correlated with steadiness of flow and directional steadiness in the early stage of blowout,but was positively correlated with the steadiness of flow and negatively correlated with the directional steadiness in the middle stage of blowout.(2)Due to the rotating vortices in the blowout,the wind speed profiles in the trough blowout displayed a nonlogarithmic behavior.(3)The measured sand flux density at different stations decreased exponentially with height.However,due to the feedback effect between flow dynamics and morphology,the sediment transport fluxes at different positions were obviously different,with the lowest at the bottom of the blowout and the largest in front of the windward slope of the deposition lobe.In conclusion,there is a form-flow feedback in the blowout,and the bigger the blowout is,the more obvious the feedback effect is.