Evolution And Prediction Of The Yangtze Estuary And Its Impact On Storm Flooding

Estuarine delta is a sensitive area,which of the interaction between ocean,river and land.It is also a highly concentrated area of population and economy.So estuary deltas can be highly affected by topography evolution due to human activities and the extreme storm surge caused by global climate change.With the intensification of human activities,global extreme weather and climate events are increasing.It is of great significance for regional security,social and economic development to do researches about evolution and prediction of estuary area,and its impact on storm flooding under the influence of natural conditions and human activities.In this paper,an empirical research was conducted in Yangtze Estuary.From the perspective of natural evolution and human intervention,this study analyzed the topography evolution process over the past 100 years.And then,based on the year of 2010,the Kriged Kalman Filter model was established to predict the topography in the short-term(in the next 10 and 20 years)and the medium-to-long term(in the next 40 years).On this basis,the MIKE 21 hydrodynamic model was used to study the impact of topography evolution of the Yangtze Estuary on the risk of storm flooding.The main research work and conclusions are as follows:(1)Analysis of topography evolution characteristics of the South Branch of the Yangtze Estuary: Based on the bathymetric data of the Yangtze Estuary between 1861 and 2013,a time series of digital elevation models of the submerged South Branch spanning more than one hundred years were established.This research analyzed the general erosion-accretion characteristics of the South Branch,the spatio-temporal variations in different sub-regions,and temporal changes of elevation along the typical cross sections and isobaths.After that,the research focused on the impact of flood,sediment load and human activities,including construction in river basin and estuarine engineering projects,on the evolution of submerged delta by runoff and sediment load data analysis at Datong station.The results showed that: The evolution of the South Branch of the Yangtze Estuary was a product of the coupled nature-human system.In general,the study area was eroded during 1861-2013 when multiple phases of “erosion-deposition” cycles occurred.The sub-regional characteristics of erosion and deposition cycles varied significantly.The contraction and extension of the-10 m isobaths was basically consistent with the overall erosion-accretion of the Yangtze Estuary.The change of the-5m isobaths reflected the formation process of shoals in the South Branch and the four major tidal flats.(2)Prediction of topography evolution trend in the Yangtze Estuary: Based on long-term historical topographic data of the South Branch,the Kriged Kalman Filter model was established to predict the topography in the short-term(in the next 10 and 20 years)and the medium-to-long term(in the next 40 years).In the short term and medium-to-long term,the Yangtze Estuary will show an eroding trend overall,and the accretion quantity will be greater than the erosion quantity on the whole.The accretion areas will be around the shoal and spur dams on both sides of the North Passage;the erosion areas will mainly be the main channel of the river,and the erosion pattern in the front edge of the Qingcaosha reservoir needs to be paid attention to.(3)Storm flooding scenario construction under topography evolution scenarios: Typhoon parameters and typhoon paths was selected and calculated according to the Tropical cyclones best path data sets from CMABST from 1949 to 2018(obtained from the China Meteorological Administration),including central minimum pressure,maximum wind speed radius,near-center maximum wind speed,landing pressure,peripheral air pressure,typhoon path,typhoon landing reference point,duration and so on.Finally,a total of 18 storm surge scenarios in Shanghai were constructed to simulate storm flooding dynamics,considering 12 historical real topographies,3 future predicted topographies,two sets of typhoon paths and parameters.(4)The impact of topography evolution on storm flooding risk: Based on historical topography scenarios,the effects of near-shore seabed evolution on storm surge processes and storm flooding were explored.And then,impacts of topography evolution of the Yangtze Estuary on regional storm flooding risk were discussed.The results show that: the impact of the storm flooding was relatively slight in the year when the Yangtze Estuary was in a pattern of accretion;in the year of erosion,the depth and extent of flooding were increasing.In the future,topography evolution must be fully considered when preventing storm flooding.The extreme water level of the storm surge is increasing in the process of inland propagation.Generally speaking,the variation of the extreme water level change is more gradual than topography change.In the gathering place of shoal,the coefficient of variation of the extreme water level of stations is also larger,because of the remarkable changes of topography.In the short term and medium-to-long term topography scenarios in the future,the topography of the Yangtze Estuary will show an erosion pattern,but changes will not be that much.There will be some impacts on the flooding of the Shanghai area,but the overall risk caused by storm flooding will not be great.