Beach Aeolian Sand Transport Influenced By Tide

The study of sediment transport,crossfeed and geomorphological evolusion of dune-beach system in the environment of sea,land and air interaction has been one of the widely concerned research hotspots of governments and coastal scientists all over the world.Based on the precious aeolian geomorphological environment of Fujian Pingtan island,the microcosmic transport process of beach aeolian sand influenced by tide and the macroscopic evolusion process of coastal dune and beach in the environment of sea,land and air interaction were discussed by the way of the combination of field observation and indoor analysis,in order to clarify the intrinsic interaction relationship between tide,beach and dune.The main conclusions are as follows:(1)Temporal and spatial variabilities of beach surface moisture influenced by tideThe surface moisture content of intertidal zone declines from sea to land in general.Affected by the beach microtopography,especially the local uplift area,there is a noticeable decrease in surface moisture.The fixed surface moisture is asymmetrical,as the moisture content rises abruptly and drops off slowly compared to the near-sinusoidal tidal process.The above changing process of surface moisture in the tidal fluctuation cycle mainly reflects the interaction between air-drying and capillary action of groundwater.Besides,different values of "surface"thickness have a significant impact on the observed values of surface moisture content.Compared to"2cm","2mm"can better reflect the gradual drying of beach surface and its effect on the entrainment and transport of aeolian sand.(2)Several characteristics of beach aeolian sand transport influenced by tideThe aeolian sand transport in the tidal fluctuation cycle shows remarkable stage characteristics,with the activity of high tide significantly higher than that of low tide.The vertical distribution of wind-sand flow in intertidal zone is almost 100%in the range of 0?30cm,which is more fastened on the ground than that of inland dry surface.The total sediment concentration of wind-sand flow shows an exponential declining distribution with height,while the distribution function varies with different particle sizes.Under the premise of same particle size,the threshold velocity of beach intertidal zone is significantly higher than that of inland desert,and the amount of increase varies with different modes of entrainment.The changing process of threshold velocity in the tidal fluctuation cycle mainly reflects the interaction between average surface moisture and fetch length.The aeolian sand transport of beach intertidal zone can be divided into two basic modes of "wet sand entrainment + wet surface transmission" and "dry sand entrainment + wet surface transmission".Different patterns shows different forms of transport.(3)Geomorphological evolusion of coastal dunes in the ordinary wind regime and typhoon extreme environmentDifferent types of coastal dunes respond differently to the same typhoon event:taking typhoon "Nesat" and "Haitang" for example,transverse dune is in silting state,while coastal foredune(shadow dune)erosion.Taking the observation year(2017)as an example,the transverse dune of Changjiangao coast has always been in a state of siltation during the winter and summer alternation,while the shadow dune of Yuandangao coast shows different depositional conditions during different periods of winter and summer alternate:silting in winter and spring,erosion in summer.Overall,transverse dune and coastal foredune(shadow dune)will be fully developed under the premise of no dune collision or overwash.(4)Tide,beach and dune interaction--take P:ingtan Island as an exampleUnder the action of waves and tides,sediments from rivers,coastal loose deposits and bedrock headlands continuously transport and accumulate in intertidal zones to form broad sandy beaches.Under the action of strong prevailing winds in winter and spring(especially in winter),beach sands continuously transport and accumulate shoreward.Under the environment of alternate wetting and drying induced by tide,the vertical distribution,entrainment and transport characteristics of beach aeolian sand are significantly different from that of inland desert.After a whole winter's and spring's supplement and nourishment from beach aeolian sand,the supratidal zone and coastal dune are generally in a state of siltation.Affected by the summer and autumn typhoons,coastal dune and beach can be eroded or silted.Three types of beach-dune storm response mode were initially discussed and summarized,according to the relative position of tide level,dune ridge and the base of dune.