Prediction Model Of Decadal Morphological Evolution Of The Yangtze Estuary And Its Applications

Estuarines and deltas worldwide have attracted intensive attentions due to their high ecological and social-economic values.However,these dynamic systems are facing severe challenge under the impacts of global climate change and human activities in the recent decades.Particularly,many deltas are converting from accretional status to regressional status,and the convertion process is accelerated compounded with sea-level rise and land resisdence.Reliable prediction on evolution trends of estuaries and deltas has become the common focus of researchers and managers.The Yangtze Estuary also faces the same problem.Due to upstream dam constructions and soil and water conservation project,the sediment discharge in the rencent decade has decreased to only 30%of the value in 1950s-1960s,resulting in erosion of the part of the submerged delta.Therfore,it is urgently needed to conduct systemetic study on the morphological response to river input changes and estuarine engineering projects.This paper set up a reliable prediction model of decadel morphological evolution based on analysis of the controling factors of the decadel evolution of the whole Yangtze Estuary.The model forecasted the evolution trends under various conditions.The main results and conclusions are described as follows:1.Quantitative analysis on the decadel morphological evolution processes of the whole Yangtze EstuaryA series of navigational charts and bathymetric maps since 1958 has been collected confirming that the bathymetry data covers the whole estuary at least every decade.Using the Surfer software,the DEM is set up to produce the erosion/deposition patterns,sediment volumes,hypsometry curves and typical sections in 1958-2010.The resuts show that bed level changes were remarkable in the study period with strong spatial variations.The inner estuary featured distinct erosion and deposition in the decadal intervals,indicating channel shifting,sand bar accretion and migration,while the mouth bar area was dominated by deposition throughout the study period.Calculation of the sediment volume changes indicated that the study period can be classified into three stages,i.e.rapid accretion stage(1958-1978),decreased accretion stage(1978-1986)and accretion equilibrium stage(1986-1997 and 1997-2010).Net accretion volume of the whole study area decreased from 155 Mm3/yr in the first stage to 39.6Mm3/yr in the second,while the net volumes in the two periods of the third stage were relatively small.The inner estuary converted from accretion to erosion after 1978,while the mouth bar area kept net accretion though the accretion rate decreased.2.Controling factors of decadel morphological evolution of the Yangtze EstuaryFirst,the variation trend of the river sediment supply since 1958 matched the sediment volume changes of the three stages.High sediment input favored deposition in both the inner estuary and mouth bar area,whereas low sediment input induced erosion in the former and decreased accretion in the later.Distinct responses of the inner estuary and mouth bar area to river sediment reduction are determined by the spatial variations of hydrodynamic conditions,sediment properties and transport processes.Sediment discharge reduction is the pcrimary reason for the decreased accretion rate of the whole Yangtze Estuary and deposition/erosion convertion of the inner stuary.Second,erosion rate of the inner estuary in 1986-1997 was higher than that in 1997-2010,and this is inconsistent with the continuous decrease of sediment discharge.This is casued by frequent river floods in 1990s.Peak flood discharge in continuous years of 1990s exceeded 60,000 m3/s,and the river flood enhanced the ebb flow and induced strong erosion compounded with low sediment supply.Third,large-scale estuarie engineering projects exerted profound impacts on morphological evolution of the Yangtze Estuary.Enhanced deposition at the East Hengsha Shoal and the erosion zones at the subaqueous delta were impacted by the construction of the training walls.With the implementation of the estuarine regulation projects,the predominant control over the delta morphodynamics is currently shifting from natural processes to human interferences.3.Numerical model of decadel morphological evolution of the Yangtze EstuaryA numerical model for decadel morphological evolution of the Yangtze Esatury was set up based on the Delft3D model system(2D mode).The model considers seasonal river inputs by schematized the monthly-averaged water and sediment discharge confirming the total amounts of water and sediment.Sediment fractions are considered in the model based on the sediment samples collected in September,2015.The morphological factors(MFs)are determined after sensitivity analysis.Validations on tide,water levels and tidal currents suggested that the model show high accuracy for hydrodynamic modeling.Three peiords(2002-2010?1986-1997 and 1958-1978)with typical evolution features are hindcasted to evaluate the model performance The results of hindcast simulations agree well with the data in terms of deposition/erosion patterns,volume changes and hypsometry curves.The model shows high performance both qualitatively and quantitatively and has the forecasting abilities for futher evolution trends.The model is used to examine the machenism of the controling factors,including the river discharge,cohesive sediment and Deep Navigation Channel project,which further verifies the high mode performance.4.Evolution trend prediction of the Yangtze Estuary and model valuesWe conduct 20-year forecast modeling(2010-2030)according to the "Yangtze Estuary Intergrated Regulation Plan".The scenarios consider predicted water and sediment discharge,relative sea-level rise and panned estuarine engineering projects in 2030.The model prediction shows overall erosion in the deep channels and the eroson zones,and retreat of tidal flats will occur.The inner estuary experiences net erosion,and the mouth bar area shifts from net accretion during 2002-2010 to net erosion by 2030.The effects of the changing water discharge on the morphological evolution of the Yangtze Estuary are limited over the next 2 decades.However,decreasing sediment discharge will accelerate erosion across the entire estuary.The numerical model proposed in this study can be applied to investigate the machenisms of decadal morphological evolution of the Yangtze Estuary,such as physical processes of bed level change during river flooding,the formation of the erosion zone in the mouth bar area and their extreme erosion depth,the tipping point of the deposition/erosion convertion of the entire estuary and so on.Evolution trends of specific areas can provide scientific guidance for integrated estuarine regilation.Therefore,the morphological model show both scientific and practical values.