Study On The Interaction Between Seeds And Plants Of Salt Marsh Plant And Water-sediment Movement

Estuarine tidal flats salt marshes are located at the junction of lands,rivers and oceans,and the area has a variety of economic and ecological functions,including storm surge protection,pollution purification,and providing suitable habitats for endangered species and so on.In recent years,tidal flat restoration projects based on natural ecology have attracted more and more attention.As the main body of restoration projects,the growth history of salt marsh plants in tidal flat hydrodynamics and sediment environment runs through the development of estuary tidal flats.Therefore,a full study of the interaction between the key stages of the growth history of salt marsh plants and the hydrodynamic and sediment environment is not only the basis for understanding the growth history of salt marsh plants,but also the theoretical basis for the restoration of natural ecological tidal flats.In this paper,the local dominant pioneer species of Chongming Dongtan Scirpus mariqueter is taken as the research object to study the process and mechanism of its two important development stages,namely,ungerminated seeds and plants with stems,roots and leaves,interacting with hydrodynamic forces(wave and current)and sediment.Smoothed Particle Hydrodynamics model is used to model the seeds and plants with flexible stems of Scirpus mariqueter in hydrodynamic and sediment environments.The characteristics of horizontal and vertical dispersal of Scirpus mariqueter seeds in wave environment and wave-sediment environment are summarized,and the reasons influencing the dispersal of Scirpus mariqueter seeds are analyzed,and the suggested burial depth of Scirpus mariqueter seeds in sediment is given.Effects of plants on the hydrodynamic environment of flexible salt marsh plants under the combined action of waves and currents are analyzed.It provides not only a basic understanding of the interaction between the various stages of the development of salt marsh plants and the hydrodynamic environment and sediment environment,but also a theoretical basis for the tidal flat restoration project based on the natural environment.The core insights obtained are as follows:1.Clarify the ungerminated Scirpus mariqueter seeds dispersal distances in the offshore and alongshore direction highly positively correlated with the wave height.The ungerminated Scirpus mariqueter seeds dispersal distances are negatively correlated with the water depth and the elevation of the seeds.The ungerminated Scirpus mariqueter seeds dispersal distances change abruptly in both distances under the wave-induced shear 0.26 Pa.The effect of wave height on the ungerminated Scirpus mariqueter seeds is studied.It is found that the seed dispersal distances increase with the increase of the wave height.When the wave-induced shear stress reaches 0.26 Pa,the seed dispersal distances increase abruptly,and about 70% of the seed dispersal distances occur under this waveinduced shear stress;for the effect of water depth on ungerminated Scirpus mariqueter seeds,the seed dispersal distances first increase slightly and then decrease with the increase of water depth,which may be related to the relative water depth of seeds and water depth.For the effect of the elevation of ungerminated Scirpus mariqueter provenance,the seed dispersal distances generally decrease with the increase of the seed elevation,and fluctuate slightly when the seed elevation is 2.3 m.At different provenance elevations,the seed dispersal distances of Scirpus mariqueter are affected by the changes of wave height and water depth.2.Determine the law of movemet of ungerminated Scirpus mariqueter seeds buried in the sediments in the wave-sediment environment.Confirm the depth(5?10 cm)of ungerminated Scirpus mariqueter seeds in the sediments under certain conditions,which is most conducive to seeds dispersal.When the ungerminated seeds of Scirpus mariqueter are on the sediment surface,the mutual movement of the two phases can be roughly divided into three processes:the transient upward movement of seeds;in the process of floating,after the seeds float completely on the surface of the sediment,they quickly fall into the sediment under the action of gravity;with the change of sediments motion state,namely changing from bed load to suspended load,the seeds of Scirpus mariqueter begin to disperse due to the gravity of sediments and seeds and the turbulent diffusion between them.When the ungerminated seeds of Scirpus mariqueter are completely buried in the sediment,the mutual movement of two phases shows two characteristics of "clumps" and "instability".In addition,trying to confirm the condition that ungerminated Scirpus mariqueter seeds are most on the surface of sediments,a small number of seeds buried in sediment(buried depth is 5?10 cm),that is,most of ungerminated Scirpus mariqueter seeds that retain on the tidal flat surface after primary dispersal,is conducive to the dispersal of ungerminated Scirpus mariqueter seeds in the sediments.3.It is revealed that with the increase of wave height by 80%,the turbulence intensities increase at water surface and rough height of plant,reaching 23.2%and 16.3%,respectively,and the horizontal turbulence intensity decreased by 2.5times in plant area.The front of vegetation is the area to slow down water flow.Vortex appear at about one spacing between adjacent stems before the submerged vegetation.The water flow resistance caused the front of vegetation is at least 4 times that caused by plant in and behind of the vegetation.With the increase of wave height and hydrodynamic force,the bending of flexible salt marsh plant becomes larger,the lodging point in the direction of plant height and the redirection of water flow is closer to the ground,which decreases from 70% to 30% of plant height.The turbulence intensities of water surface and rough height of plant are increased by 23.2% and 16.3%,respectively,and the displacement of plant top is periodic.With the increase of velocity,the phenomenon of backwater in front of the vegetation is not obvious to emergent vegetation.When vegetation is emergent,Reynolds shear stress changes greatly in low flow velocity,and when vegetation is submerged,Reynolds shear stress varies greatly in high flow velocity.In the submerged vegetation,“upsurge” appeared and with the increase of wave height,the size of vortex increases by 60% of plant height and the place of vortex moves down by 20% of plant height within the distance of one spacing between adjacent stem in front of the vegetation.