| The Bohai Sea coastal zone is one of the regions with rapid economic development in China’s coastal zone.Since the 1970s,in order to relieve the contradiction of land supply and demand,large-scale reclamation activities have been carried out along the Bohai Sea coastal area,coupled with the riverway changes of the Yellow River Delta,which lead to significantly changes of coastline and bathymetry in the western and southwestern Bohai Sea,namely the Bohai Bay-Laizhou Bay area.The study on the changes of bay coastline and bathymetry in the past half century and the in-depth discussion on the response mechanism of the bay hydrodynamic environment to its changes of coastline and bathymetry can provide technical support for the reasonable planning of coastal engineering,coastal zone regulation and restoration,and the study of the gulf environmental hydraulic characteristics.In this study,four characteristic years(1975,1990,2005 and 2020)were selected according to the evolution characteristics of coastal development of Bohai Bay-Laizhou Bay and the riverway changes stages of the Yellow River Delta.Based on remote sensing image data from 1975 to 2020,this paper analyzes and compares the spatial and temporal variations of the coastline lengths,coastline structure,bay area and shape,bay centroid displacement,and coastline utilization degree of Bohai Bay-Laizhou Bay.Based on relevant sea chart data of Bohai Bay-Laizhou Bay,DEM models of the four characteristic years were established to analyze and compare the variation characteristics of isobaths,erosion-deposition condition,the area and volume enclosed by eigenvalue water depth and the topography of key areas.MIKE 21 software was used to establish the numerical model of hydrodynamic and water exchange in Bohai Bay-Laizhou Bay for the four characteristic years,and to study the influence of changes in coastlines and bathymetry on tidal current,tidal prism and water turnover capacity.The main conclusions are as follows:(1)Changes of coastline and plane morphology.The length of the coastline increased by 660.53 km(107.01%)for Bohai Bay and 396.33 km(98.92%)for Laizhou Bay from 1975 to 2020.Over the years,coastal development activities such as aquaculture dikes,salt fields and port dam construction have significantly reduced the proportion of natural coastline length and greatly changed the coastline structure in both Bohai Bay and Laizhou Bay.The area of Bohai Bay decreased by about 2200 km~2(17.05%),and that of Laizhou Bay by about 1100 km~2(15.19%).The reclamation activities were the main reasons for the continuous reduction of the bay area in recent years.The shape index of Bohai Bay increased from 1.83 to 3.80,and that of Laizhou Bay increased from 1.73 to 3.24.The centroid of Bohai Bay migrated from southwest to northeast during the period from 1975 to 2005,and turned southeast during the period from 2005 to 2020,while the centroid of Laizhou Bay migrated from southwest to northeast during the period from 1975 to 2020.By 2020,the utilization degree index of Bohai Bay coastline was higher than Laizhou Bay.In the past 45 years,the utilization degree index of Bohai Bay coastline increased by 61%,and that of Laizhou Bay by 84%.The influence of human activities on coastlines of the two bays has gradually increased.(2)Changes of bathymetry and topography.The southern isobaths of the Bohai Bay advanced to the sea greatly from 1975 to 1990.Since the Yellow River changed its riverway to Qingshuigou,the area has been depositing,and the maximum deposition height can reach 10.51 m.From 1990 to 2005,the isobaths near the new estuary advanced to the sea greatly,and the maximum advance of the 2 m isobath was 14.43 km.After the riverway transferred to Qingbacha,there was a deposition trend near the southeast of estuary,and the maximum deposition height reached 11.13 m.From 2005 to 2020,the construction of the port area has significantly changed the nearby isobaths.In the north of Bohai Bay,there is an erosion trend near Caofeidian,a deposition trend in the west and south,a deposition trend in the southwest of Laizhou Bay,and an erosion trend in the east sandy coast.The overall erosion and deposition amplitude of the sea area ranged from-5 m to 5 m.The coastal development activities of Bohai Bay and Laizhou Bay have the greatest influence on the area of the intertidal zone(from coastline to 0 m isobath).In the past 45 years,the intertidal zone area of Bohai Bay and Laizhou Bay has been reduced by about 1600 km~2(38.46%)and 840 km~2(61.76%),respectively.The water volume of Bohai Bay had the largest change range during the period from 1975 to1990,and that of Laizhou Bay had the largest change range during the period from 1990to 2005.Human development activities have a small effect on the offshore underwater topography of Bohai Bay-Laizhou Bay,with the erosion-deposition range from 0 m to 5m,while the natural erosion-deposition range of the Yellow River estuary has a greater effect on the nearby topography,with the erosion-deposition range from 5 m to 11 m.(3)The hydrodynamic effects of coastline and bathymetry changes.In the past 45years,the overall trend of tidal current movement in the Bohai Bay-Laizhou Bay has changed little,and the large changes in the current field are mainly located in the areas where the coastline and bathymetry have changed dramatically.From 1975 to 1990,the current velocity of Bohai Bay,Laizhou Bay and Yellow River Delta was mainly decreased.From 1990 to 2020,the significant changes in velocity were concentrated in the vicinity of the port dike.From 1975 to 2020,the mean tidal prism of spring,middle,neap and whole tides in Bohai Bay decreased by 17.23%,14.43%,12.17%and 14.91%,respectively.The mean tidal prism of spring,middle,neap and whole tides in Laizhou Bay decreased by 5.37%,6.16%,7.92%and 6.31%,respectively.The main reason is that many years of reclamation activities occupy a large amount of sea area and affect the tidal prism of the bay.Bay area,coastline utilization degree and intertidal zone area are the three indexes with the strongest correlation with tidal prism.The overall water turnover capacity of Bohai Bay decreased from 1975 to 1990,and continued to increase from 1990 to 2020.The overall water turnover capacity of Laizhou Bay increased from1975 to 1990,but decreased slightly from 1990 to 2020.The main reasons for the difference between the two trends are related to the process and scope of their respective reclamation.As for the local water turnover capacity,the formation of the encircling harbor was prone to cause the reduction of interior water turnover capacity. |
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