Mechanism Study Of Extreme Storm Surge Dynamic Attenuation Under Coastal Flexible Vegetation

In the face of extremely complex and severe situation of natural hazards,such as storm surge and waves,it is still necessary to further strengthen the ability for disaster prevention and mitigation.Main issues,as well as new trends and requirements,are calling for investigating and constructing ecological vegetation revetment in order to enhance the ability of disaster prevention and mitigation,promote ecological carrying capacity and build ecological security shelters.In order to ensure the effectiveness of artificial construction of ecological vegetation revetment,it is essential to carry on more profound research in the force and motion of ecological vegetation as well as its principle of energy dissipation.Meanwhile,to provide technical support for studying disaster mitigation of submerged vegetation during storm surge process,accurate knowledge of extreme water level is needed.Therefore,this research has important scientific significance and application value.Wave and wave setup attenuation under the influence of flexible vegetation are investigated in terms of two primary coastal dynamics in estuaries including water level and wave.The study first conducts the research on accurately predict extreme water levels in an estuary where river discharge plays an important role.Then a coupled flexible vegetation and wave interaction model is developed.Through applying this coupled model,vegetation-induced wave and wave setup attenuation are analyzed.Based on theoretical analysis,numerical simulation and physical model experiment,related technical issues of extreme storm surge dynamic attenuation under coastal flexible vegetation are investigated.The main contents and conclusions of the study are as follows:(1)For the estuary region where river discharge plays an essential role in extreme water level,the variable of river discharge has been taken into consideration,main variables influencing storm surge are fit by conventional probability distribution function.Meanwhile,hydrodynamic numerical model forced by multiple factors around Yangtze Estuary is set up based on Delft3 D orthogonal curvilinear grids and domain decomposition modeling approach.Then a new method using the improved JPM-OS-Q model to predict extreme water levels in Yangtze Estuary is proposed.This method indicates promising results when compared to existing results for predicting extreme sea levels at selected stations.Finally,the response of estimation results to sea level rise is also investigated based on the improved JPM-OS-Q model.Simulated results indicate that sea level rise has noticeable nonlinear effects on extreme water levels.(2)A mechanical model for simulating flexible vegetation is proposed.Then a flexible vegetation dynamic model under the effect of waves is established.Computing method of flexible vegetation forces in XBeach phase-resolving and phase-averaged wave models are proposed.Subsequently,improvements and secondary development are carried out for these two wave models.Meanwhile,a wave flume experiment is conducted,and the effectiveness of above model is verified based on the results of the wave flume experiment and some supplementary data of literature.Wave flume experiment and numerical simulation studies all indicate that flexible vegetation has the ability to attenuate wave height and wave setup.Attenuation effect of flexible vegetation on storm surge dynamic can not be ignored.(3)Force and motion characteristics of single flexible vegetation induced by wave has been indicated.Results show that relative submergence,wave height,wave period,drag coefficient and elasticity modulus have significant influences on the force and motion of single flexible vegetation,corresponding influence mechanism is also analyzed.Then,the response of vegetation-induced wave attenuation to the main influence factors are analyzed.Importance of forces of single flexible vegetation on wave attenuation is proposed.Finally,based on the coupled flexible vegetation and wave interaction model,response of wave attenuation to vegetation distribution density,length of vegetation area and elasticity modulus are investigated.