Experimental Study On Wave Attenuation Due To Flexible Intertidal Vegetation

Intertidal vegetation grows in the transition region between land and sea,providing habitat for coastal organisms and impacting on coastal ecological structures and food chains.It not only plays an important role in communicating with the land and sea ecosystems,but also can resist and reduce damages due to waves to the shoreline.Another physical effect of intertidal vegetation is it changes transport and diffusion of sediment and marine pollutants along the coast,which depends significantly on velocity distribution inside and outside of vegetation region.At present,there are many researches on rigid intertidal vegetations.However,the distribution of them is regional and limited,and flexible intertidal vegetation are more widely distributed around the world.Scirpus mariqueter,a native vegetation in China,is also a kind of flexible coastal vegetation which has been greatly reduced due to the biological invasion.It has been strongly supported for conservation and recommended as restoration material of coastal vegetation by the government of China.In the present study,the physical effect of flexible vegetation on wave decay was investigated by using vegetation model in an experimental flume.The experiment in this paper is divided into two parts.The first part is wave decay measurements on vegetation models for different water depth,vegetation density and vegetation flexibility.The free surface elevation at three wave gauges for every case was measured and the wave decay ratio between at the front of vegetation model and the back of it was calculated.The vegetation model used in the experiment is made by a3-D printer.In order to find the difference between flexible and rigid vegetation for wave decay,the further experiment was performed.The wave decay ratio was measuredfor flexible and rigid vegetation model under the conditions of the same wave,vegetation density and vegetation belt length.The second part of the experiment is velocity fields measurement on vegetation models for different flexibility by using Particle Image Velocimetry(PIV).Under the wave action,the inside and outside flow structure of nearshore vegetations is closely related to the distribution,diffusion and transport of microbial nutrient,contaminants and bottom sediments along the coast.The velocity field distribution of that is the basis for further study on the turbulence characteristics,vorticity field characteristics and transportation characteristics.However,the domestic research on this aspect has been making slow progress for a long time,especially the difference between the velocity fields with flexible and rigid vegetations is very little studied.Therefore,in the second part of the experiment,the velocity fields under the flexible and rigid vegetation models and the velocity fields without the interference of vegetation models are measured respectively under the same water depth.The results of the experiment are shown as follow:(1)Under conditions of the same vegetation density and water depth,wave height decreases along the horizontal distance and significantly decreases in region of vegetation model.Under conditions of the same vegetation density,the decay ratio of wave height decreases with increase of water depth,showing approximate linear changes.Higher vegetation density gives larger absolute value of the slope of linear functions.Under conditions of the same water depth,wave decay ratio increases with vegetation density.The linear fitting shows that the decay ratio variation with density is larger for the smaller water depth.(2)Both the flexible and rigid vegetation model have some effects on wave decay.When the water depth is smaller than the height of model,decay ratio of rigid model is higher than that of the flexible model.When the water depth exceeds the height of model,the decay ratio of rigid model decreases a lot and is smaller than that of the flexible model under the same water depth.(3)In the case of no vegetation model,the velocity field is very consistent and can be clearly seen that all the velocity vectors are in the same direction.In the cases with rigid and flexible vegetation models,the velocity fields are disturbed by vegetation.Moreover,there are mixing layers around the interface between the canopy region and open water region in both cases of rigid and flexible vegetation,and the boundary layer on the vegetation is clearer with the rigid vegetation model.(4)At wave crest,the horizontal averaged velocity on flexible vegetation model can be slightly smaller than the case of rigid vegetation which is much smaller than that for no vegetation model.However,the magnitude of it is larger at wave trough.The variation of averaged velocity distribution is large on the flexible vegetation model both at the wave crest and trough phases,which means flexible vegetation may affect the velocity fields more than the rigid vegetation.