Development Process And Environmental Significance Of Coastal Dunes In Changli,Hebei Province

The coastal dunes in the eastern coastal area of Changli,Hebei province are typical coastal dunes in China.In this paper,coast dunes in Changli gold coast are selected as our research object.Based on the field investigation,the regional variation characteristics of wind and resultant drift potential were analyzed with the observation data of national meteorological station,and the relationship between spatiotemporal variation characteristics of coastal aeolian environment and the distribution pattern of coastal dunes was discussed by combining the comparative analysis of multi-period high-resolution satellite images.The age of the coastal aeolian sediment came out by OSL dating,and the grain size parameter is choosed to reconstruct the evolution history of coastal environment in the past three hundred years.With the interpretation of typical GPR profile image of dunes,field geomorphology,coastal aeolian environment and regional historical documents were investigated to go deeply into the formation and development process of coastal dune in Changli coast.The results show that:(1)Based on the analysis of observation data of Funing,Changli and Laoting national meteorological stations,the sand-driving wind is strong and high in frequency during spring and winter;From north to south,the resultant drift direction of the three stations are WNW,NW and ENE.From the morphological distribution of aeolian dunes and their internal bedding tendency in study area,Laoting station can better reflect the overall characteristics of wind regime in the study area among the three stations.Based on the analysis of regional wind regime,it can be concluded that the wind regime in study area belong to bimodal type,and the reason why the transverse dunes(transverse sand ridges and barchans)intersects at acute angles are mainly affected by the angle between the bimodal wind directions,the availability of sand source and the trend of coastline.The formation of barchan dunes are mainly influenced by the prevailing wind direction and the relative lack of sand source,while transverse ridges are formed by a condition of abundant sand source and prevailing wind direction,but the coastline trend limits it's space distribution.(2)According to the analysis of observation data from three meteorological stations from 2008 to 2018,the average sand-driving wind speed and the the resultant drift potential showed a declining trend on the whole,the the resultant drift direction was stable,and the annual precipitation fluctuates but showed no significant change trend.However,multi-phase of Google Earth high-resolution satellite images show that the coastal dunes in Changli have been suffer from wind erosion over the past decade.According to the regional wind speed and precipitation records in the past 50 years,both wind speed and precipitation show a trend of decreasing,their effects on weakening or promoting coastal aeolian activities roughly offset each other.Therefore,the impact of climate change on the reduction of coastal dunes is relatively limited.However,for more than half a century,the reduction of sand source caused by artificial afforestation and wind-protection and sand-fixing activities should be the main driving factor of wind erosion reduction of coastal dunes in study area.(3)According to the OSL dating results,the oldest sediment of profile on transverse ridge and foredune were respectively deposited in 231±306 and 324±88 years ago,which means foredune began to deposit as early as 300 years ago,while the main part of the transverse sand ridge began to develop about 230 years ago.In the past decade,the average annual wind erosion rate of the barchan dune could reach about 0.18m/a.And the OSL dating shows that the bottom sediment in the pit with a depth of 1m at the bottom of the groove on the barchan was deposited in about 100 years ago.The sediment at the bottom of the interdune pit which located between the transverse sand ridge and the barchan were deposited in the past 100 years.(4)The coastal barrier began to develop after the transgression event at about 2000 years ago.Since then,the sea level fell,and the barrier gradually exposed,which became the basis for the development of coast dunes.Subsequently,vegetation grew,intercepting the sand that migrating to the shore.About 1500?850 a B.P.,the sea level rose volatily,the surface of barrier was still dominated by low and vegetated dunes.Until about400?500 years ago,the climate became colder,the sea level fell,the exposed area of the barrier expanded and the sand-land transport was strengthened,the vegetated dunes connected with each other and turned into a transverse ridge.Meanwhile,a new vegetated dunes were formed near the coast.Subsequent sand accretion heightened the transverse ridge,the sand source of inland side of the transverse ridge became less,so the barchan formed.After that,the barchan chain developed,and then eventually evolved into the present coastal geomorphology.(5)The main driving factors for the development of coastal dunes are available sand supply,strong onshore wind,wide flat topography and human activities.From the shore to the inland,the driving factors of sand deposition in different geomorphic units appear different.The development of foredune is greatly influenced by the location of coastline and high-energy events(storm surge,etc.).The formation of transverse sand ridges and barchan is mainly influenced by the prevailing wind direction(resultant drift potential),coastline and sand supply.Due to the vegetation development,the sand deposition in the interdune is influenced by the climatic conditions.While in the past 100 years,the influence of human activities on the development of coastal dunes in the study area has been increased significantly.