| Estuarine delta is the most developed area of human socio-economic activities,which is also the most sensitive and the most vulnerable in the world to global climate change.However,owing to the effects of highly intensive human activities and climate change,estuarine wetlands are facing multiple threats on their survival in future.Hence,it is of great significance to study the evolution of estuarine wetlands under human activities and climate change,supporting the decision-making in sustainable utilization,protection and management.In the present study,a coupled hydrologic-hydrodynamic model was established by integrating a hydrologic model and a hydrodynamic model,to simulate the hydrological and hydrodynamic processes of a large extent of river basin,estuarine wetlands and adjacent coastal ocean.A nested modeling approach was used to downscale the effects of climate change and human activities for the establishment of regional hydrodynamic model of estuarine wetlands.Coupling with remote sensing data,the saltmarsh vegetation spreading model,the marsh equilibrium model and regional hydrodynamic model,eco-geomorphodynamics of estuarine wetlands was simulated to evaluate the evolution of estuarine wetlands in multi-scales.The driving factors cencerning with evolution of estuarine wetlands were detected by a series of future scenarios analysis of climate change and human activities.The major contents of the study are listed as follows:(1)Establishment of the coupled hydrologic-hydrodynamic modelThe coupled hydrologic-hydrodynamic model was established based on the variables infiltration capacity(VIC)model,a semi-distributed large scale hydrologic model,and Delft3D,a hydrodynamic model for estuarine and coastal zone,to simulate freshwater discharge of river basin and tide as well as three-dimensional currents,salinity and suspended sediment concentration(SSC)of estuarine wetlands and adjacent coastal ocean.Comparing with in-situ data,the river discharge was simulated by the VIC model with the Nash-Sutcliffe coefficient of 0.88.In order to validate estuarine hydrodynamics,the simulated and observed tidal level,current speed,current direction,salinity and SSC were compared with the correlation coefficient of 0.96,0.81,0.82,0.76 and 0.70,respectively,and root mean square error of 0.26 m,0.31 m/s,53°,3.86 and 0.48 kg/m~3,respectively.The model validation indicates that the coupled hydrologic-hydrodynamic model has good capacity to simulate the hydrologic and hydrodyamic processes of river,estuarine wetlands and adjacent coastal ocean.(2)Evaluation of hydrodynamics of estuarine wetlands under climate changeAs sea-level rise(SLR),tidal level,current speed and salinity increase significantly with a linear relationship.Result shows that,the 0.5m SLR will lead to increase of salinity,current speed and tidal level with rate of 0.70,0.02 m/s and 0.46 m,respectively.In the future scenarios of RCP 2.6 and RCP 8.5,the simulated river discharge at Datong hydrologic station decreases from 29,193 m~3/s to 25,928 m~3/s and 25,867 m~3/s,respectively.The decrease of river discharge will result in the intensification of effect of ocean on estuarine wetlands.Especially in extreme dry events,salinity and current speed will increase by 200%and 50%,respectively.In the extreme flooding events,current speed will increase by more than 50%.In synergistic analysis of sea level rise and extreme hydrologic events,the proportions of the effect of river discharge on salinity and current speed are about 70%and 67%,respectively.The effects of SLR plays more important role in water level change than river discharge with a proportion of 91%.The synergistic effects of SLR and river discharge on hydrodynamics are higher than the simple sum of each single factor,indicating a non-linear intensification on hydrodynamics of estuarine wetlands.(3)Establishment of the coupled eco-geomorphological model with remote sensingThe eco-geomorphological model was established by coupling remote sensing data,a nested hydrodynamic model,a saltmarsh vegetation spreading model and the marsh equilibrium model to simulate the eco-geomorphological evolution of estuarine wetlands.The nested hydrodynamic model was established based on coupled hydrologic-hydrodynamic model to provide update of regional hydrodynamics of eco-geomorphological model.With the update of salinity,elevation and inundation period by nested hydrodynamic model,the saltmarsh vegetation spreading model could run with not only competition between different species of halophytes,but also the restriction condition of hydrodynamics to improve the accuracy of model result.The case study in Chongming Dongtan wetland showed that saltmarsh vegetation spreading model with updated hydrodynamics has good capacity in simulating the spatial distribution change of vegetation succession on saltmarsh with an accuracy rate of 0.86.Remote sensing data can provide ecological information for model input and calibration for the simulation of dynamics of plant communities to improve the accuracy of simulation result of eco-geomorphological model.(4)Evaluating the effects of SLR and coastal engineering on eco-geomorphology of estuarine wetlandsEco-geomorphological processes can accelerate deposition of sediments on saltmarshes for accretion.Under current sea-level,the deposition rate of saltmarshes and low-marsh in the north of Chongming wetland are 0.3 m/yr and 0.5 m/yr,respectively.Eco-geomorphological processes are sensitive to the fast SLR,which will lead to submerge and death of saltmarsh plant communities.More than 50%of saltmarshes will be submerged under 1 m SLR.And all saltmarshes will disappear under 1.5 or higher SLR,resulting in rapid erosion of wetlands with a rate at 0.5 m/yr.Fast SLR will lead to the conversion of source and sink of sediment on saltmarsh wetlands.Coastal engineering has ability to minimize the bottom erosion of saltmarshes,resulted from intensified hydrodynamics under fast SLR,maintaining the elevation of saltmarshes.The situation of saltmarshes under SLR will be reversed and deposition will be promoted.The difference between sedimentation rate at specific area will reach 1 m/yr.Deposition-promoted engineering will slow down flow and promote deposition of sediments,which lead to accelerated succession of saltmarsh plant communities to landward.It will result in the increase of biomass.bottom friction and accumulation of organic materials,leading to accretion of saltmarshes with a rate at 0.01 m/yr.The results of future scenarios analysis show that,for the future management in estuarine area,the coastal engineering should be constructed properly to maintain the elevation and eco-geomorphological processes of estuarine wetlands based on a conception called build with nature.By that way,an ecological shelter will be built to protect the estuarine security from disasters. |
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