Analysis Of Lingding Bay Landform Stage Evolution And Trends

Pearl River Delta region,one of the most significant economic centers,plays an important role in the development of national economy of China.Under the 'Belt and Road Initiative' proposal and Nansha economic free trade construction,Pearl River estuary region is an important geographical location and strategic center.During the recent years,the environment of Pearl River estuary has been intensively impacted by human activity,which resulting in coastal wetland degraded and the waters shrunk.For example,because of the sediment deposition and artificial reclamation,the Lingding Bay area decreased almost by 500km~2 in 2010 which compared to 1980.In order to predict the evolution pattern and life span of Lingding Bay,the historical evolution stages of topography were focused on the basis of sediment source,transport tendency,sedimentation rate,and analysis of natural factors and anthropogenic impact.This study would enrich the theoretical foundation of geomorphology research in the Pearl River Estuary,provide scientific evidences for marine economic development,and environmental protection for this region.The sediment measurement and topography survey data were derived from three voyages during from 2004 to 2006,which supported by '908 special projects'.There are four conclusions based on the comparative analysis of 7 charts of different scales and different periods at the Pearl River Estuary,combined with the quantitative analysis of topography evoluation patterns in the last hundred years.The results were showed below.(1)Compared to the other side of the the Pearl River Estuary sediment,the topography in the east is more stable.Isobath migrated from the northwest to the southeast,the average migration distance is about 2-4km in the past 80 years.A large area of land reclamation reduced the near shore hydrodynamic.Siltation has occurred in the 72.9% of the total area of the Pearl River Estuary in the past 80 years.Except for estuaries,waterways,sea sand mining area and tidal channel and other waters,the rest is silted up.It shows an average annual sedimentation rate of 2.92cm/yr from 1936 to 1997,and that of 3.87 cm/yr from 1997 to 2013.(2)Lingding Bay was affected by human activities severely.From 1936 to 2014,Lingding Bay had been in silting-up condition.Reclamation reached up to 276km~2 and the coastline cumulative growth was 175 kilometers.Shoreline migration responded to the reclamation as the main mean can be divided into three main stages: a stable stage under the development of low intensity(1936-1984),a mutational stage under the exploitation of high intensity(1984-2004),a stage with controlled coastal development(2004-2014).In the inner Lingding Bay the evolution of 0m isobaths was characterized by being stable in the east coast and moving eastward in the west coast.The 5m isobath was gradually stabilized,and the change of 10 m isobath was mostly assosiated with human activities in this area.(3)In the inner Lingding Bay the topography of natural state is that erosion occurs in the east and sedimentation does in the west.With the strengthening of human activity,the inner Lingding Bay landform gradually is dominated by anthropogenic processes.Channel excavation and sand mining activities have become the main factors of seabed erosion.From erosion and siltation volume,inner Lingding Bay is in the state of deposition.The percentage of silting area in three stages—1936-1984?1984-2004?2004-2014—were 66.6%,58.0% and 61.7%,respectively.In the three periods,the average deposition thickness was 2.73 cm,3.40 cm and 8.61 cm.(4)In the nearly 80 years,the evolution of topography in Lingding Bay mainly reflected in the evolution history of ?three shoals and two grooves‘,and it can be divided into two stages(natural sedimentation and interfere of human activities)and further into five steps.In the stage of natural sedimentation,the characters include:(1)The formation of 'three shoals and two grooves': three Longitudinal deep grooves in Lingding Bay,the mid-groove silted up and developed into the mid-shoal,and appeared the embryonic form of 'three shoals and two grooves' in physiognomy generally back to 1936.(2)The 'three shoals and two grooves' were in existence from 1936 to 1984,and the trend is erosion in the east part and sedimentation in the west part.In the stage of interference of human activities,the characters include:(3)the intensify of silted up in mid-shoal: during 1984-1997,because of the sand mining in the upstream,the water split ratio were changed,it lead to increasing of water yield and total sediment momentarily,and the deposit of mid-shoal exacerbated.(4)The mid-shoal: from 1997 to 2004,the sediment amount decreased and the hydrodynamic force intensified,this lead to the mid-shoal unstable.(5)the aggravation of erosion in the mid-shoal: from 2004 to 2014,the west water channel were controlled by sedimentation,but the channel of Lingding Bay and the east-shoal was deepened remarkably because of the sand mining and dredging on the sea bottom.(5)In the future the evolution trend of the Lingding Bay topography would be divided into 4 stages: 'three shoals and three grooves',mid-shoal disappear stage,west-shoal disappear stage and the new 'gate' formation stage.It is predicted that the longevity of the Lingding Bay will be not less than 150 years.