The Research Of Wave Dissipation On Salt Marsh Tidal Flats In The Southeast Chongming Island

As natural barriers against ocean dynamics,salt marshes can not only protect tidal flat from erosion and have current velocity rapidly slow down,but also damp wave energy greatly.On particularly,salt marshes have a significant role in wave dissipation and current reduction during storm surges.In recent years,coastal cities have suffered from an increasing risk of typhoon attack under the context of sea level rise and extreme weather events.Moreover,many bank revetments located along coastline cannot fully meet the present demands for urban disaster risk management due to early construction with low defense level and aging materials.Therefore,promotion of the significance of the wave attenuation over tidal flats with salt marshes as naturally coastal barriers is of vital importance to support the urban sustainability and protect people's lives and property from storms impacts.Located at the mouth of the Changjiang River,Chongming Island is the largest estuarine fluvial island of the world.As an important ecological barrier of Shanghai,Chongming Island is of great significance to the ecological environment security of the Changjiang River delta region.However,Chongming Island is vulnerable to disasters,such as typhoon storm surge in summer and winter.Although most of the existing seawalls around the island reach new seawall standards,there is serious collapse problem with a mixture of land subsidence and beach erosion,make the seawalls great security risk due to a weak ability to against storm surge.At present,Chongming Island is planned to be a world-class ecological island with the requirements for ecological construction in future by Shanghai government.Therefore,it is of great practical values for ecological seawall construction in Chongming Island to explore the wave attenuation characteristics over coastal salt marshes and reveal their wave attenuation mechanisms in the nearshore zone.In this paper,based on field observation,the spatial distribution characteristics of waves over the salt marshes and tidal flats located at the southeast Chongming are analyzed by mathematical statistical analysis and wave attenuation model.And then the factors affecting wave attenuation are discussed.Moreover,BP neural network model is used to simulate how wave propagates along mudflats and salt marshes area and predict wave dissipation effect of salt marshes in different incident wave heights.Main conclusions are as follows:(1)The slope of salt marshes along the Chongming Island is gentle,the variation of near-shore wave height is greatly affected by water depth,and the wave height is relatively high at deeper water area.Wave height increases obviously due to high energy input and storm surge during typhoon period.(2)The spatial distribution and attenuation law of waves in normal weather and high energy event scenarios are consistent.Wave height gradually decreases as the wave propagates landward,which is more significant in the vegetation area.The wave attenuation effect of salt marshes is stronger during typhoon periods.After passing through the 60 m measuring section,wave height decrease rate in pre-typhoon period is 9% lower than that during typhoon.(3)Wave energy presents different attenuation regularities as the wave propagating landward due to the influence of water depth,incident wave height and drag coefficient.Wave attenuation rate increases with the decrease of water depth,and increases with the increase of incident wave height and drag coefficient.Wave attenuation rate caused by the changes of water depth,incident wave height and drag coefficient is more obvious in vegetation area.(4)At constant water depth,incident wave height ranges between 0.03 m and 0.96 m in pre-typhoon period,more than 90% of which decreases to 20 cm after passing through the 1000 m vegetation area from the salt marshes edge;Incident wave height ranges between 0.10 m and 1.12 m during typhoon period,over 90% of which decreases to 30 cm below after passing through the 1000 m vegetation area.At constant salt marshes distance,significant wave heights of most waves are respectively less than 5cm and less than 10 cm after passing through the 1000 m vegetation area when water depth reduces to 20% of the original value in pre-typhoon period and during typhoon period.(5)A prediction model for waver propagation along mudflat and vegetation area under different incident wave heights is established in terms of a BP neural network using offshore water depth,incident wave height,wave period and wave direction as input factors,and nearshore wave height as output factor through Bayesian regularization algorithm training method.It is shown that wave reduction increases exponentially with the increase of incident wave height under the condition of sufficient vegetation height.