Research On The Morphological Characteristics And Evolution Of Tidal Creeks On The Shore Of The Radiation Sandbar In Jiangsu

Jiangsu Province is a province with large tidal flat resources in China,with huge radial sand ridges developing off the coast.Due to the abundant material sources and strong hydrodynamic conditions,the tidal trenches on the inner shore tidal flats sheltered by the radiation sandbanks are widely developed.Under the influence of many human activities and natural factors such as tidal flat reclamation,sea aquaculture,storm surge disasters and sea level rise,the shape of tidal trenches is sensitive and changeable,which will cause certain harm to the development of coastal tidal flats and the ecological environment of tidal flats.Therefore,exploring the number,length,morphological change characteristics and dynamic evolution of tidal trenches,analyzing the response of tidal trenches to the erosion and deposition changes in the intertidal zone,and simulating the development trend of tidal trenches can provide data support and decision-making reference for the reasonable management and control of the coastal zone in Jiangsu Province.Accordingly,according to the hydrodynamic conditions,topography and geomorphology and sedimentary environment characteristics of the central coast of Jiangsu,this paper divides the radiant sandbank into four areas:the north wing,the inner edge area,the middle and the south wing.Analysis of the evolution of tidal creeks in six periods in 2002,2006,2013,and 2018,studied the morphological characteristics and dynamic evolution of the tidal creeks on the shore of Jiangsu Radiation Sandbank under the reclamation regulation policy.Specific work includes:extracting tidal trenches using multi-spectral remote sensing image data,grading tidal trenches combined with the natural discontinuity method,analyzing the difference in the number and length of tidal trenches and the reasons;selecting tidal trench frequency,density,curvature,fractal dimension value and score The five morphological indexes of the dilatation rate quantitatively analyze the morphological changes of the tidal channel in the past 30 years,and analyze the correlation of the morphological index in the evolution process;further extract the corresponding coastline of each study period,and explore the influence of the tidal morphological index on the erosion and deposition changes of the intertidal zone By simulating the change trend of the tidal trench length under different intertidal area change rates,the development of the tidal trench in 2025,2030 and 2035 is predicted and analyzed.The main research conclusions of the thesis are as follows:(1)The number and length of tidal creeks on the shores of radiation sandbanks have been decreasing due to the influence of human activities,and the tidal creeks on the whole have been degraded.From 1988 to 2018,the number and length of tidal creeks on the north flank of the radial sandbar have decreased.Among them,the number of tidal creeks of level 4 decreased the most,with a decrease of 25%.The length of first-level tidal trenches has the largest decrease of 66%,and some first-level tidal trenches are transformed into second-level tidal trenches;the number of tidal trenches in the inner marginal area fluctuates up,but the length fluctuates down.The average length of tidal trenches is greatly reduced.The number of tidal trenches in the central area increased first and then decreased.The length fluctuated and decreased with the passage of time.The length of each level was the same as the total length.The number of tidal trenches in the south wing had the largest decrease,at 67%.The total number and total length fluctuated with each other.Years increased and fluctuated.From 2006 to 2013,due to the construction of large-scale reclamation projects in the Tongzhou Bay,the average annual tidal channel length decreased the fastest;the four study areas showed certain differences in the distribution of tidal channels,according to the total number of tidal channels.The order of quantity and total length from largest to smallest is:central>north wing>south wing>inner edge area.The average length of the four grades of tidal trenches in the south wing is the shortest,which is related to the source of sediment and hydrodynamic environment of the tidal flat.(2)Under the influence of human activities in the past 30 years,the shape of the tidal channel has become simpler and the development status has deteriorated.According to the tidal gully morphology index,the study on the morphological changes and correlation of tidal gullies found that since most of the tidal flat reclamation areas are located in the supra-tidal zone,the density of tidal gullies generally shows an increasing trend.Comparing different regions,the density of tidal channels in the south is the highest,and the density of tidal channels in the north is the smallest.The growth rate of tidal channel density slowed down and decreased with time,reflecting the restraint of tidal flat reclamation on the development of tidal channels;the frequency of tidal trenches has generally increased.From a regional perspective,the frequency of tidal trenches in the northern and inner rims is relatively small,and the frequency of tidal trenches in the central and southern wing is relatively high;the overall change trend of tidal ditches is the same as the density.The highest area is the central part of the radial sandbank,and the lowest area is the north wing;the north flank has the largest curvature and the inner edge region has the smallest curvature.This is related to the hydrodynamic environment of the tidal flat.The curvature of the tidal gully in the high tide zone is greater than that of the low tide zone.The fractal dimension of the tidal trench fluctuates as a whole.The fractal dimension of the tidal trench in the middle is the largest and the tidal trench in the south is the smallest.There is also a certain correlation between the morphological indicators of the tidal channel.When the length of the tidal channel increases,the value of the tidal channel fractal dimension will increase;the tidal channel branching rate increases with the increase of the tidal channel density.(3)Coastline changes have reduced the area of the intertidal zone,which has restricted the development of tidal creeks.Taking the area of the intertidal zone in1988 as the benchmark,the area changes between the intertidal zone in the remaining years and 1988 were calculated.It was found that the area of the intertidal zone on the radial sandbank first increased and then decreased during these 30 years.The decreasing area was mainly concentrated in Dafeng Port to Yaowang Port section.When the area of the intertidal zone decreases,the length of the tidal channel decreases basically at a uniform rate;the density of the tidal channel first increases and then decreases,which is related to the distribution of the density of the tidal channel;the response of tidal ditches to the erosion and deposition of intertidal zones is similar to the density.As the area of the coastal zone decreases,it first increases and then decreases,and the number of intersections of the tidal ditches continues to decrease;the fractal dimension value of the tidal channel shows a trend of declining fluctuation,indicating that the complexity of the tidal channel is reduced.(4)The development of tidal creeks on the shore of Radiation Sandbanks in Jiangsu Province was predicted in time and space.The results showed that the development of the north wing tidal creeks is restricted when the area of the intertidal zone decreases.When the growth rate of the intertidal zone is 2.8km~2/a,by 2035,the tidal channel will basically develop to the state in 1988;when the area of the inner and central intertidal zone increases,the development of the tidal channel will be better.As the area of the intertidal zone continues to decrease,the tidal creek begins to degenerate;after 2018,the development of the tidal creek in the south wing first becomes better and then worsens with the increase in the area of the intertidal zone.When the area deceleration of the intertidal zone reaches 3 km~2/a,7km~2/a and 12km~2/a,the tidal channel will have disappeared before 2025.