Study On Wave Attenuation Under Vegetation Based On Physical Experiment And XBeach Model

For nearly a century,the ability of coastal wetlands to stabilize coastlines and protect coastal areas has been recognized and received wide attention.Wetland vegetation can provide better protection for the coast when coastal disasters such as storm surge occur,effectively reduce wave height,coastal erosion and loss of life and property.Wetland vegetation has been regarded as a cost-effective alternative to defend storm surge,but its interaction with wave involves complex hydrodynamic processes,the quantitative relationship of wave attenuation under vegetation is still unclear.Therefore,it is of great theoretical value and practical significance to carry out relevant research.Combined with physical model experiment and XBeach numerical simulation,this paper quantitatively studies the attenuation ability of vegetation on wave height,analyzes the influence parameters of wave height attenuation under vegetation,and establishes the semi-empirical formulas of drag coefficient under the action of rigid and flexible vegetation,respectively.The main research work and results are as follows:(1)Based on the physical wave flume,the wave attenuation experiments of three kinds of vegetation and the control experiments without vegetation under 13 wave conditions were carried out.The time series wave heights at free surface of flume are analyzed by using zero-crossing method,and the average wave heights at seven measuring points are determined.By reproducing the layout and wave-making sequences of the wave flume,the XBeach numerical model is established and the key parameters of the wave-dissipation module under vegetation are calibrated.The verification results indicated that the XBeach model could reproduce the experiment of wave flume well.(2)The parameters affecting the wave dissipation under rigid vegetation are divided into wave-related parameters and vegetation-related parameters.Using the calibrated XBeach model,the effects of incident wave height,wave period and vegetation belt length,height,density,and diameter are analyzed item by item.The influence parameters are integrated into four main parameters: wave steepness coefficient,vegetation relative height coefficient,and vegetation belt length and vegetation coefficient.The ability of wave attenuation under vegetation increases with the increase of wave steepness coefficient,vegetation relative height coefficient,vegetation belt length and the vegetation coefficient.(3)Through the iterative value-taking process,the drag coefficient of rigid vegetation under different wave conditions are determined according to the best fitting between the calculated values of numerical model and the measured values of physical experiment.Based on the correlation between the drag coefficient and Keulegan-Carpenter number,the influences of the vegetation relative height coefficient and the wave steepness coefficient on the drag coefficient for rigid vegetation are further considered,and the improved formula of the drag coefficient for rigid vegetation is improved.The fitting coefficients of the improved formula in this paper are obtained by using the measured data,to obtain more accurate predicted value of the drag coefficient for rigid vegetation.(4)Considering the characteristic parameters of vegetation stem flexibility,the factors affecting drag coefficient of flexible vegetation are analyzed.The elastic modulus and material density of flexible vegetation are selected as the main parameters for the wave attenuation under flexible vegetation.According to the fitting value of equivalent drag coefficient for flexible vegetation,two dimensionless parameters,stiffness and vibration coefficient,are obtained through literature analysis.The semi-empirical formula for stiffness,vibration coefficient and drag coefficient of flexible vegetation is established and fitted,to extend and improve the simulation accuracy of XBeach model for wave attenuation under flexible vegetation.