福建海坛岛海岸沙地风蚀坑形态动力学与形态演化特征

Morphodynamics and morphological evolution characteristics of blowouts dunefields:A case study in Haitan Island,Fujian

风蚀坑作为一种沙地负地貌形态一直是风沙地貌学关注的焦点,对风蚀坑的研究有利于更好理解其形成演化及其动力机制。选择海岸风蚀坑发育具有典型代表性的海坛岛沿岸沙丘区,以碟形、半碗形、槽形风蚀坑为研究对象,利用RTK GPS技术及二维超声风速仪对风蚀坑形态重复测量与气流观测,采用“空代时”方法探讨了风蚀坑演化模式及其形态与气流反馈机制。结果表明:①碟形风蚀坑形态沿盛行风向向侵蚀盆后部扩大加深并形成陡坎,入口则沿盛行风向呈后退趋势;半碗形风蚀坑形态沿盛行风向扩展并突入侵蚀壁形成破口,不断扩大加深;槽形风蚀坑侵蚀盆沿盛行风向向积沙区扩展且东侧壁形成分叉并逐渐向后部延伸。②来流风向与风蚀坑形态影响坑体内气流场分布,而坑体内部不同部位流速差异则促使不同部位发生侵蚀或堆积。③风蚀坑演化模式为沙斑或缺口的产生→碟形风蚀坑→半碗形风蚀坑→槽形风蚀坑扩大并消亡四个阶段,地表植被在整个演化过程中起关键作用。其中,受形态与深度限制促使半碗形风蚀坑向槽形风蚀坑转变,槽形风蚀坑则不断扩大形成分叉逐渐走向消亡。

Blowouts,developed over the dunefield,as a type of coastal aeolian landforms,have been the focus of aeolian geomorphology research.Meanwhile,the core issue of studying blowout is the model of development and evolution of blowout and feedback mechanism between blowout aerodynamic and morphodynamics.Choosing the typical area of blowouts developing in Haitan Island,we investigated the morphodynamics and aerodynamics of the saucer,half-bowl and trough blowouts by conducting field monitoring.Real Time Kinematic GPS was used to topography survey and 2D ultrasound anemometers were to explore wind flow dynamics."Space for time substitution"has been used to explore the evolution models of blowouts and feedback mechanism between aeolian process and blowouts morphological development.The results show that:(1)The trailing edge of saucer blowout enlarges and erosional basin gets steeper till a scarp occurs along prevailing wind direction.The entrance tends to recede as wind flow approaching.Half-bowl blowout expands and is deepened and the erosional wall is broken into the outlet.The erosional basin of trough blowout enlarges into deflation zone,the east-hand erosional wall is broken and extends the trailing edge.(2)The airflow distribution pattern within the blowout is influenced by wind direction and blowout topography,and the flow velocity fluctuation within blowout determine the erosion and deposition at different spots.(3)The evolution model for blowout can be concluded that erosion notches are initiated,and developed into a saucer blowout,till enlarged into the halfbowl shape blowout,turn into the trough blowout and die out.The vegetation cover plays an important role in the entire evolution cycles,especially the half-bowl shape blowout transfers into trough blowout,and trough blowout enlarges until it dies out,as a result of topography influence and depth limits.Therefore,by analyzing and exploring the morphological changes,morphology and dynamics mechanisms,and evolution patterns of blowouts in the degenerated area of Haitan Island,this study will provide an in-depth understanding of the morphologydynamics feedback mechanisms,and evolution patterns in the degradation process of coastal blowouts,and will be conducive to providing better scientific guidance for the prevention and treatment of coastal blowouts,so as to better cope with global climate change in terms of the destruction of coastal littoral habitats,erosion of foredunes and shoreline retreat,and the human properties lost,which is of great scientific significance and application value.