Numerical Simulation Of Hydrodynamic Characteristics In Coastal Waters With Vegetation Effects

The tidal flows,river flows and particularly extreme nature hazards(tsunami waves,storm surges)propagate to near shore regions where they not only change offshore water environment,but also affect sediment transport and cause bed deformation.When they spread further to intensive production and living areas,massive destruction appears in engineering structures and coastal ecosystems,even life and property of the inhabitants.As an integral part of the aquatic ecosystem,the plants,such as Mangrove,Pandanus odoratissimus(P.odoratissimus),Suaeda heteroptera(S.heteroptera)and Phragmites australis(P.australis),are widely distributed in tidal flats,lakes and natural rivers.The presence of these aquatic vegetation increases the resistance of water body,and thus plays an important role in dissipating wave energy,weakening flow velocity,protecting coastline from erosion,and mitigating extreme nature hazards(tsunami waves,storm surges).In summary,the researches of hydrodynamic characteristics in coastal waters with vegetation effects can provide theoretical guidances and technical supports for the establishment of ecological buffer zones such as coastal wetland and forest.Based on the shallow water equations of mass and momentum conservation,an explicit depth-averaged 2D shallow model considering vegetation effects is established in this study.The finite volume method(FVM)is applied to solve the equations,and the Roe approximate Riemann solver coupled with drying-wetting boundary technique are proposed to evaluate the interface fluxes and track the moving shoreline caused by the evolution of tidal currents,the land intrusions of tsunamis as well as storm surges.A vegetation drag force term is added into momentum equations to express vegetation resistance effects on the flows(e.g.,tidal currents,tsunamis,and storm surges).Firstly,a higher calculation accuracy of the numerical scheme are validated by a series of classic experimental cases,the hydrodynamic model can simulate well the propagations of flows without and with vegetation.Then the established model is applied to simulate characteristics of tidal currents in Liao River Estuary Wetlands(LREW),the type of vegetation(S.heteroptera and P.australis)and their distributions are cost-effectively acquired by Satellite Remote sensing techniques,and the characteristics(drag force coefficient CD,diameter,density and height)of plants are obtained according the field survey as well as the relevant reference.The applicability of the model in the real-scale estuary is validated by measured water surface elevations,flow velocities as well as directions.The validated model is applied to simulate the characteristics of tidal currents and flow structures in Liao River Estuary(LRE)during the spring and neap tides,the variations of tidal flows in Pink Beach wetlands(PBW)under different conditions(without and with vegetation,variable drag force coefficient and vegetation density)are also emphatically analyzed to reveal the interaction mechanism of flow-vegetation.Tsunami waves,generated by impulsive disturbance(e.g.,submarine earthquakes,underwater landslides,and volcanic activity),propagate from open sea to near shore regions where they cause massive destruction,thus it is considered as one of the most destructive ocean disasters.According field surveys,P.odoratissimus,Casuarina equisetifolia(C.equisetifolia),and Mangroves have obvious attenuation effects on the run-up and land inundation of tsunamis.Based on the depth-averaged 2D shallow model,we firstly simulated the propagation of tsunamis on bared and vegetated beach,and the resistance of vegetation on the run-up and land inundation of tsunamis.Then the run-up and land inundation of tsunamis on vegetated beach with variable conditions of vegetation parameters(density,width and arrangement)and tsunami height are emphatically investigated for revealing the interaction mechanism of tsunami-vegetation.The established model is used to explore the land intrusion degrees for storm surges and the interactions of S.heteroptera vegetation and storm surges in PBW.Jelesnianski 65 typhoon model is adopted to simulate wind and pressure fields by comparing with predicted results of other typhoon model.To give a reasonable wind field far away from the typhoon centre(distance > 500 km)where Jelesnianski 65 typhoon model has terrible application,the background wind field should be included in the total wind field.In this paper,APDRC(Asia-Pacific Data-Research Center,http://apdrc.soest.hawaii.edu/)wind data is used to acquire background wind data.Before the applications of the model in researching storm surge of North Yellow Sea and Bohai Sea,the model is validated by measured data such as wind speeds,tidal levels,storm surges and flow velocities.Based on the depth-averaged 2D shallow model considering wind and pressure effects,the wind and pressure fields in North Yellow Sea and Bohai Sea are firstly predicted during 9711 Typhoon "Winnie" and 0509 Typhoon "Matsa",and the typhoon effects on and sea level are also investigated.Then the land intrusion degrees of storm surges,variations of tidal levels,and the characteristics of tidal flows in PBW are emphatically explored to reveal the effects of 9711 Typhoon "Winnie" on tidal flows.