| Mangrove has important ecosystem services for wave attenuation,and it is a natural barrier against waves.The nonlinear characteristics and laws of wave attenuation about nearshore mangroves are the hot topics of current research,which have important reference value and practical significance for exploring the wave abatement mechanism of mangroves and building a natural wave abatement ecosystem.In this study,Xiaoguansha mangrove forest in Beihai,Guangxi was taken as a case,and four transects for wave monitoring were designed in the direction of coastal waves passing through the mangrove forest.For each transect,six monitoring points with an interval of 20 meters were designed from the edge of the sea forest to the forest for wave monitoring.The data of water depth and wave height were collected at each monitoring point based on wave recorders,and the ecological characteristics such as tree height and density were measured in a total of 20 mangrove quadrats.Focusing on the construction of the nonlinear wave attenuation model of mangroves integrated into the ecological characteristics of mangroves and water depth,and the scientific assessment of the wave attenuation function of mangroves,the following three aspects of research are carried out:(1)Nonlinear characteristics of wave attenuation by mangroves.(2)Establishment of prediction model of mangrove wave attenuation.(3)Scientific assessment of mangrove wave attenuation.There are four conclusions in this study:(1)All of four transects showed a significant exponential attenuation with the distance across the forest area,but the wave attenuation coefficient of the same transect varied with the water depth of the tidal cycle,which was attributed to the fact that the main structure organization of the resistance and wave breaking effect of mangroves in different water depths was different.At the same water depth,the wave attenuation coefficients of the four transects are also different,which is attributed to the differences in the landscape ecological structure of the different transects.(2)Based on the analysis of the nonlinear characteristics of mangrove wave attenuation,this study constructed a mangrove wave attenuation model integrating water depth and landscape ecological structure through mathematical modeling.The model prediction and fitting results can better approach the actual measurement results,with great simulation accuracy(R~2=0.9093)and broad application value.(3)Based on the establishment of the mangrove wave attenuation model,combined with the previous wave attenuation model of bare beach,this study puts forward the idea and estimation model of scientific evaluation of the function of mangrove wave attenuation.It is necessary to simultaneously consider the wave energy of the bare beach in the mangrove forest area(assuming that there is no mangrove forest)reaching the breakwater after the attenuation of wave energy,the wave energy of the mangrove reaching the breakwater after the attenuation of wave energy,and the actual wave stress pressure and intensity borne by the mangrove in the annual cycle.The calculation results of Xiaoguansha mangrove forest show that: In a year,waves can only reach the edge of the mangrove forest in 1/3 of the time,and only reach the edge of the mangrove forest in 1/10 of the time after attenuating through the bare beach in front of the forest,exceeding the control value of the "safe wave height" of the protection bank(0.3 meters).That is,the time frequency of this mangrove forest actually performs the function of wave attenuation in a year is only 1/10.However,in the time when the wave height is over the control value,after the mangrove attenuation,there is only less than 1/100 of the time in a year when the wave height reaches the breakwater and exceeds the "safe wave height" control value of the protective bank,which shows that the effect of mangrove wave attenuation is obvious. |
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