Study On The Morphology, Sediment And Spatial Distribution Characteristics Of Scrub Dunes On The East Coast Of Leizhou Peninsula

As a common aeolian landform in sandy coastal zone,nebkha and its formation,development and evolution mechanism are closely related to the coastal vegetation,dynamic conditions and the change of sedimentary environment.The study on coastal nebkhas is not only of geomorphological significance,but also of great significance to reflect the characteristics of coastal aeolian sand environment and ecological environment.In this study,we analyzed the shape,grain size characteristics and spatial distribution of Sesuvium portulacastrum nebkhas and Spinifex littoreus nebkhas on the east coast of Leizhou peninsula.In order to explain the development process and influencing factors of nebkha in tropical coastal areas,the material source,sedimentary environment and dynamic conditions of the evolution process of nebkha were analyzed.The main conclusions of this paper are as follows:(1)The average height and horizontal scale of the Spinifex littoreus nebkhas and Sesuvium portulacastrum nebkhas on Leizhou peninsula are about 0.5m and 4m respectively.The shapes of nebkhas mostly presents as nearly cones,few tadpole shape and irregular shape.The average height of Spinifex littoreus and Sesuvium portulacastrum is about 0.16m and 0.05m respectively.Vegetation height of coastal nebkhas is not the key factor controlling the development of nebkhas.The coverage of plant plays a key role in the shape of nebkhas.The length,width and height of nebkhas are synergistically changing,and the shape of nebkhas is the result of synergistical change.The development stage of nebkhas is related to the vegetation growth.The leeward slopes exert great influence on the length of nebkhas,while the windward slopes contribute more to the height of nebkhas.Field investigation found that the plant grows mainly on the windward slopes.The wind-drift sand jumps and climbs along the windward slope and the sand are intercepted by the grass,which makes the length of the windward slope increase and the height of nebkhas increase.The sand that are not intercepted can pass over the top of nebkhas,and deposit along downwind direction under sedimentation,which increases the leeward slope.(2)Different types of sediments in the three sites are mainly fine sand or medium sand.More than 60%of the sediments of the Sesuvium portulacastrum nebkhas are fine sand,and more than 50%of the sediments of the Spinifex littoreus nebkhas are medium sand.The average particle size of all sediments is 2.12?(fine sand),and the particle size is well sorted,with nearly symmetrical distribution and moderate kurtosis.The average particle size of sediments in different parts of the nebkhas decreases in turn on the interdune,windward slope,dune top and leeward slope,indicating that sediments gradually become thinner along the downwind direction.The grain size characteristics of sediments show that from the seaside to the inland,the average grain size of sediments changes from coarse to fine,and the sorting becomes better,indicating that the sediments of nebkhas mainly comes from the beach.The composition of the sediment in different islands and different parts of nebkhas are different.The grain size characteristics of the sediments from nebkhas and foredunes are highly consistent,for the following two reasons.One is that the nebkhas develop along the backshore continuously,and then splices each other to form a new coastal foredune,so the nebkhas may be the rudiment of the coastal foredunes.The other is that wind-drift sand flows through the nebkhas as a "springboard" and then continuously transported to inland,reaching the foredunes and becoming fixed by the plants on the foredunes.(3)The distribution density of nebkhas on Xinliao Island,Donghai Island and Nansan Island was 111.11,90.9 and 64.58 plots/hm2,respectively.The Spinifex littoreus nebkhas and Sesuvium portulacastrum nebkhas present a discrete or random spatial distribution pattern.The width and slope of the beach determine the extent of the distribution of the nebkhas.The nebkhas are located at the back edge of the beach above the maximum high tide height(2.2m),where seawater is hard to reach.The beach sediments are exposed to the air environment for a long time,so the wind-drift sand is easily formed under the action of the onshore wind.The sand grains are trapped by Spinifex littoreus and Sesuvium portulacastrum that grow in the beach,forming nebkhas.