桐花树红树林潮滩近底层悬沙浓度垂向剖面变化特征分析

Analysis of the variations of suspended sediment concentration in vertical profile near the bed of Aegiceras corniculatum tidal flat

观测红树林潮滩在波浪和潮流作用下的近底层垂向剖面悬沙浓度变化过程,对理解海岸带植被的消能促淤机制和滨海湿地生态修复工程有着重要作用。本文以北部湾七星岛岛尾桐花树红树林潮滩为例,基于剖面流速仪HR、声学多普勒单点流速仪ADV、浪潮仪T-wave及剖面浊度仪ASM,获取了研究区域2019年夏季大潮连续3天的水文数据,同时结合桐花树典型植株实测参数,分析了潮周期内红树林潮滩近底层垂向剖面悬沙响应波浪、潮流作用及桐花树空间结构的运动过程。结果表明:1)桐花树潮滩近底层悬沙浓度和悬沙通量具有涨潮明显大于落潮的潮汐不对称现象,剖面垂向高悬沙浓度区域在涨潮初期—涨急由距底部0.1~0.37m转变为距底部0.5~0.67m,落急—落潮末期则由上部转变为下部;2)潮周期内悬沙起动和再悬浮阶段发生在以波浪作用主导的涨潮初期和落潮末期,平流和沉降发生在以潮流作用为主的涨急至落急整个阶段;3)涨潮阶段桐花树冠层的茂密枝叶通过减缓流速拦截多于冠层上部40%以上的悬沙,落潮水体则挟沙自陆向海经过桐花树群落,使得悬沙浓度下降超过71%。该不对称涨、落潮动力沉积机制有利于悬沙向岸输运,促进潮滩扩张过程。

Suspended sediment concentration(SSC)observation in vertical profile near the bed of mangrove tidal flat under the waves and tidal currents,is important to understand the mechanisms of energy dissipation and deposition promotion by vegetations,and the ecological restoration in coastal zone.In this study,we measured the hydrological data using HR,ADV,Twave and ASM and the typical plants’parameters of Aegiceras corniculatum for 3 consecutive days of flood tides in the summer of 2019.Then the process of the near-bed vertical SSC of mangrove tidal flat was analyzed in response to waves,tidal currents and spatial structure of Aegiceras corniculatum.The results showed that:(1)the variations of suspended sediment flux and SSC near bed in Aegiceras corniculatum tidal flat had the characteristic of tidal asymmetry,the SSC of flood tide was significantly greater than that of ebb tide.The vertical high SSC area of the profile changed from 0.1~0.37 m to 0.5-0.67 m from the bottom in the early flood tide to flood maximum,and from the upper to the bottom in ebb maximum to end.(2)In the intra-tide,the suspension and resuspension stages occurred at the early flood and latest ebb that were dominated by waves.The advection and deposition stages occurred at the whole period from flood maximum to ebb maximum dominated by tidal currents.(3)Dense branches and leaves in the canopy of Aegiceras corniculatum intercepted more than 40%of the suspended sediment in the upper canopy during flood tide.The SSC decreased by more than 71%when the flow during ebb tide.This dynamical deposition mechanism of asymmetrical flood and ebb tide was conducive to sediment transportation landward which then led to flat expansion.