Research On The Siltation And Erosion Dynamic Characteristics And Ecological Stability Of The Tidal Flat In The Liaohe Estuary Wetland

The estuary wetland tidal flats are an important part of the coastal zone,where are the most frequent area for the exchange of matter,energy and information between land and ocean.The deposition and evolution processes and ecological stability of the tidal flats are critical to the stability of the estuary wetlands.The tidal flats erosion has become the main feature of the important estuaries around the world due to the tidal flats were affected by both natural and human factors.But the both of the siltation and erosion processes and ecological stability processes of the tidal flats are not yet clear.This study conducted in the tidal flat of the Liaohe estuary wetland,and aims to investigate the deposition and evolution dynamics and ecological stability of the Liaohe estuary wetland tidal flat,and to clarify the environmental processes that affected the tidal flat deposition and evolution processes and ecological stability.The isotopic tracer technology and geographic information system was used to analyze the dynamic process of siltation and erosion in horizontal space and vertical of the tidal flat in the Liaohe Estuary wetland in recent 30 years.The annual correspondence between the tidal flat deposition process and the runoff of the Liaohe River was reconstructed based on the dating results,and the basic laws of runoff and sediment discharge and the critical radial flow force conditions for fine particle transportation were clarified.The periodic response was explored between the changes in the tidal flat sediments and runoff based on the spectrum analysis method.The hydrodynamic change process of the tidal flat was analyzed based on the characteristics of sediment particle size distribution.The PSR-Rough Sets and ARIMA models were used to explore the spatial structure ecological stability of the tidal flat in the Liaohe Estuary wetland,and determined the key factors and predicted the development trend of the tidal flat in future.The research results explored the erosion law and ecological stability theory of estuarine wetland tidal flats,which have important guiding significance for the safety of regional estuary wetlands and the sustainable use of tidal flat resources.The main research conclusions are as follows:?1?The average deposition rate of the tidal flat in the Liaohe estuary wetland from 1963to 2017 was 2.24 cm/a,and the vertical deposition rate of the tidal flat was different in different periods.In this study,a combined dating method of ¹³⁷Cs and ²¹⁰Pb was used to determine the average deposition rate from 1986 to 2017 was 2.62 cm/a,and the average deposition rate from 1963 to 1986 was 1.80 cm/a.?2?The tidal flat of the Liaohe estuary wetland generally showed a silting trend in the horizontal space and the shoreline generally moved to the sea from 1985 to 2017.However,the overall area of the tidal flat has a shrinking trend affected by the runoff of the Liaohe River and man-made development activities.And the the artificial shoreline of the tidal flat has increased year by year and the length of the shoreline has shown a decreasing trend.The average siltation and erosion rate in the tidal flat of the Liaohe estuary wetland was342.42hm²/a,and the average net shoreline movement was 36.76m/a from 1985 to 2017.The length of the tidal flat shoreline was reduced by 5.09km.The natural shoreline of the tidal flat is constantly being replaced by artificial shorelines,and the tortuosity of the shoreline have not changed much.?3?The sediments in the tidal flat of the Liaohe estuary wetland follow the land-to-sea direction,the sediment transportation mode changed from uniform suspension to tapered suspension in a horizontal space gradually,and the particle size of the sediments were coarsening.The clay and silt content of the sediment in the tidal flat were decreasing along the land-to-ocean direction,and the sand content were increasing,and the sediment particle size were coarsening,and the main sediment type were sandy silt.?4?The seasonal runoff process of the Liaohe River affects the hydrodynamic balance process of the runoff and tidal interaction zone in the Liaohe estuary wetland,which causing the differences in the spatial distribution of the tidal flat sediments.In the summer,the hydrodynamics of the interaction zone were dominated by the runoff of the Liaohe River,and the sediments were in the refinement stage;On the contrast,in winter and spring,the hydrodynamics were dominated by the the tidal,and the sediments turn to the coarsening stage.There is a clear critical section of hydrodynamic transition along the direction of the Liaohe River in the interaction area of runoff and tidal.Prior to this,the sedimentation process was dominated by runoff,and the sediments particle size of sediments had a tendency to refine.After that,the sedimentation process was dominated by the tides,and the grain size of the sediment showed a coarsening trend.?5?There has obvious periodic response between tidal flat sediment in the Liaohe estuary wetland and the Liaohe runoff.There has obvious sediment discharge power constraints on the fine particulate matter transported to the tidal flat.There was a 14-15-year cycle correspondence between the flow in flood season of the Liaohe River and the median size of the tidal flat sediments,and a 5-6-year cycle correspondence between the mean annual flow and annual sediment discharge of the Liaohe River and the median size of the tidal flat sediments.The total amount of sediment transported by the river to the estuary was directly proportional to the runoff force,and the mass of fine particles was inversely proportional to the runoff force.However,after the runoff force was reduced to a certain extent,the relationship between the fine particle mass and the sediment transport capacity was no longer has an inversely proportional,and there was a significant sediment discharge power limitation.The boundary changed about 400m³/s flow in flood season,mean annual flow of 100m³/s,and annual sediment transport volume of 2 million t/a generally.When they were less than the values,the amount of fine particles carried by the Liaohe runoff was decreased as the runoff transport capacity decreased,and the sediment particle size were coarsened to some extent.When they were larger than the values,the mass of the deposited fine particles decreasesd with the increase of the runoff transport capacity,and the coarsening tendency of the deposited sediments were obvious.?6?The ecological stability of the tidal flat in the Liaohe estuary wetland was the result of the coupling of natural and man-made multi-factors.It has been in a relatively stable state in the past 30 years generally.The current state was unstable in 2017,and the tidal flat will remain in a relatively stable state within the next 10 years.Among them,1986-2000 was in a relatively stable state with an average ecological stability comprehensive index of 0.51.After2001,the tidal flat of the Liaohe Estuary entered an unstable state.By 2017,the cological stability comprehensive index showed a continuous downward trend,reaching a minimum of0.25.The ecological stability of the tidal flat will be in an unstable state during 2018 to 2022;it will develop from instability to a more stable state from 2023 to 2025;it will develop from a relatively stable direction to a stable direction since 2026.The tidal flat ecological stability comprehensive index will reach a maximum of 0.63 in 2027 and the ecological stability of will tend to benign development.?7?The ecological stability of the wetland tidal flat in the Liaohe estuary was more sensitive to the changes of human factors than natural factors.Human disturbance was the key driving force that causes the ecological stability of the tidal flat decreased.Among them,marine aquaculture and traffic road construction were the key driving factors leading to the destabilization and instability of the tidal flat.The annual precipitation,annual sediment discharge,and the flow in flood season were the key guarantee factors for maintaining the material supply of the tidal flat.In summary,the process of tidal flat siltation and erosion and ecological stability in the Liaohe Estuary wetland were the results of the coupling of natural and human factors.The tidal flat has obvious siltation and erosion processes in the horizontal space,and the vertical deposition rate has a stepwise change.With the change of the hydrodynamic conditions of the tidal flat,the sediment transportation mode in the tidal flat changes from uniform suspension to tapered suspension gradually along the land-to-sea direction,and the particle size of the sediments shows a tendency of coarsening.The seasonal runoff process of the Liaohe River affects the hydrodynamic balance process of the estuary runoff and tidal interaction zone.The interannual variation of the Liaohe River runoff has a clear periodic response relationship with the tidal flat deposition process.They caused the siltation and erosion process and distribution characteristics of sediments in horizontal space and vertical respectively.The flow force of the Liaohe River is directly proportional to the total amount of sediments transported to the estuary.The flow force generally inversely proportional to the mass of fine particles transported to the tidal flat and there is a limit condition for critical radial flow force.The driving force of human factors to the ecological stability of the tidal flat is greater than the natural factors,which is the key driving force caused the ecological stability of the tidal flat decreased.In the comprehensive utilization and protection of the tidal flat in the Liaohe estuary wetland,the interference of human activities on the ecological stability of the tidal flat should be rationalized.