The Response Of Landform And Sedimentary Changes To Reclamation And Aquaculture In Sansha Bay,Fujian

As a key area connecting land and sea,the bay is the area with the most dense population and economic activities.In the past hundred years,with the increase of human production level and demand,strong human activities such as reclamation and aquaculture have become significant factors affecting the natural processes of coastal areas.It is one of the most important scientific problems in earth science and ecological environment science to explore its influence on the natural processes of estuaries,coasts and bays.Sansha Bay is a typical near-closed bay in the East China Sea.In recent decades,it has developed into the world's largest Larimichthys Crocea cage breeding base,and the intensity of reclamation activities is much higher than in other areas.At present,how does large-scale reclamation and aquaculture affect the geomorphic environment in the bay has not received enough attention and systematic investigation and analysis still need to study in-depth.Based on the 80-year nautical chart data of Sansha Bay and the 30-year remote sensing survey,combined with the on-site sediment sampling,shallow formation profile,and hydrological and sediment observations,this paper discusses the impact of reclamation and aquaculture activities on the erosion and deposition environment of Sansha Bay,it provides a typical case and reference for discussing the influence of coastal human activities on the natural process of coastal area.The results show that:(1)From 1934 to 2018,the landform and erosion and deposition of Sansha Bay have undergone significant changes.Over the past 80 years,the total coastline has been reduced from 738.2 km to 695.8 km with a decrease of5.74%.The area above the 0 m isobath changed from 279.1 km~2 to 324.9 km~2,but the area of the tidal flat changed from 279.1 km~2 to 239.7 km~2 with a decrease of 39.2 km~2(22.01%),average annual decrease is 0.48 km~2/a.The scouring and silting generally presents the characteristics of"shallow water siltation,deep water scouring".The siltation and erosion areas are 423.42 km~2 and 372.15 km~2,respectively.The siltation and erosion volumes are 7.61×10~8 m~3 and 23.56×10~8 m~3,respectively,and the vertical deposition rate and vertical erosion rates are 2.2 cm/a and 7.9 cm/a respectively.(2)In the past 100 years,the deposition process in shallow water areas has undergone significant changes.1)The upper and middle tidal flat column samples are relatively uniform in grain size,and the sediments are mainly classified as clayey silt;the low tidal flat column samples have obvious stratification in the vertical direction,with sandy silt in the lower part and clayey silt in the upper part.The sedimentary attribute of the shallow layer of the channel is silty sand;Calculated by the CIC model,the sedimentation rate of the high tidal flat is between 0.96-1.08 cm/a,and the sedimentation rate of the low tidal flat is 1.04 cm/a.2)With 90cm as the boundary,the high tidal flat's shallow bedding is parallel bedding above and below 90cm,the low tidal flat below 90cm changes from parallel bedding to concave bedding gradually,while the shallow bedding above 90cm is convex bedding.3)The average particle size of the sediment measured on the tidal flat is 4.60?m?64.52?m,and the average particle size is coarser in the intertidal zone and finer in the tidal crevices.(3)From 1934 to 2018,the area of reclamation and aquaculture in Sansha Bay has undergone significant changes.Over the past 80 years,the reclamation area has increased from 0 km~2 in 1934 to 28.86 km~2 in 1996,and then to 99.1 km~2 in 2018,the average annual increase is 1.18 km~2/a.The aquaculture area increased from 16.53 km~2in 1995 to 235.1 km~2 in 2020 with an increase of 1322%.The measured data of the aquaculture area and the non-aquaculture area show that show that dominant current in the Sansha Bay is ebb tide,and the change of the suspended sand content is basically controlled by the flow velocity.The characteristics are mainly that the ebb tide duration is greater than the high tide duration;the ebb tide speed is greater than the high tide speed;the ebb tide suspended sand content is greater than the high tide suspended sand content.(4)Sansha Bay reclamation and aquaculture had a significant impact on the landform and sedimentation process.1)The increase in reclamation has led to a decrease in the proportion of natural tidal flats in the bay.The increase in reclamation area is faster than the decrease in natural tidal flat area.Reclamation led to a reduction in the tidal capacity of Sansha Bay from 3.60×10~9 m~3 in 1934 to 3.27×10~9 m~3 in 2015.The decrease in tidal capacity leads to the reduction of the flow velocity in the tidal flat area and the dynamic adjustment of the flow velocity in the deep trough area.2)At the same time,the hydrodynamics adjustment has led to the strengthening of high tidal flat siltation outside the reclamation area,and the siltation zone extends outward;the shallow strata in the area where the beaches and troughs meet have changed from a concave bedding below 90 cm to an upwardly convex bedding above 90 cm;On the profile from the reclamation levee,the grain size becomes coarser gradually during the transition from the fine particles outside the reclamation levee to the intertidal zone.3)The dense aquaculture cages have caused significant changes in the vertical flow field in the bay.The average velocity gradient on the vertical line during the full tide cycle in the non-cultivation area reaches 0.017 and 0.063 respectively,while the vertical velocity gradient in the aquaculture area is only 0.007 and 0.002,resulting in an parabolic-like shape in the vertical velocity profile.The maximum velocity on the vertical moves from the surface down to the subsurface(0.4H,H is the instantaneous water depth),which generally reaches the maximum at about 10 m,and gradually decreases after 0.4H.That is to say,the bottom velocity is basically equal to the vertical average velocity,which increases the power consumption of the water near the bottom,and the resuspension and erosion of the bottom sediment are strengthened,which has become one of the main factors for the changes in the"deep water erosion"of Sansha Bay.