Field Measurement And Simulation Of Wave Attenuation Effects In Salt Marshes

Many coastal shores are susceptible to erosion.Global climate change and acceleration of sea-level rise,will magnify erosion risk in coastal areas.Salt marshes in estuarine and coastal zone play a key role on erosion control and coastal protection for they attenuate wave height and dissipate wave energy.The ability of salt marshes has gradually become the focus of domestic and foreign scholars.However,the quantitative research on salt marshes wave attenuation effect pattern and analysis of the factors which affect the salt marshes wave attenuation effect are still insufficient.It is important to explore the wave attenuation characteristics of typical salt marshes in different growth phenophases,and to identify the impact of vegetation density,plant height,plant ground stem,vegetation spatial distribution on the wave attenuation effect.The Changjiang estuary has a moderate tidal intensity,and the wave type is dominated by wind waves,followed by waves.In this study,we selected three typical salt marsh species: Spartina alterniflora,Phragmites australis,Spirpus×mariqueter in Changjiang estuary to measure the capability of attenuate and dissipate wave with self-logging wave sensors and Trimble GPS,using Quadrat method to obtain vegetation growth characteristics data.Six sampling line offshore were set in the vegetation zones of three types of salt marshes(two transects were set up in the lush period and withered period of vegetation,respectively),and three sites on each transect with the pattern of“mudflat-salt marsh frontier-salt marsh” have been set up.Wave attenuation onshore in terms of wave height and wave energy have been analyzed.The main results were as follows:1.The attenuation effect of three types of typical salt marshes were different,while wave attenuation law was similar in different phenological period.The capability of coastal protection in typical salt marshes ranked with an order Spartina alterniflora>Phragmites australis>Spirpus×mariqueter.Spartina alterniflora had the strongest wave attenuation ability,and there was obviously exponential relation between Spartina alterniflora width and wave attenuation.The wave attenuation speed was fast and then slow,when crossed over the Spartina alterniflora 50-m wide,the wave nearlydisappeared.The wave attenuation ability of Phragmites australis was strong,logarithmic relation between Phragmites australis width and wave attenuation.And the wave attenuation speed was fast and then slow,When the wave passed through 60-m wide Phragmites australis,the wave height and wave energy approached to zero.Spirpus×mariqueter has the weakest wave-reducing ability,and there was polynomial relation between Spirpus×mariqueter width and wave attenuation.When the wave passed through a 200-m wide Spirpus×mariqueter,the wave disappeared.2.The spatial distribution width of salt marshes and density were the crucial factors for the wave attenuation effect.A nonlinear positive correlation was both shown between the wave attenuation rate and salt marches width,between the wave attenuation and vegetation density.As the increase in salt marshes width,the wave attenuation effect was enhanced.And for the same type of salt marsh,the greater the vegetation density,the stronger the wave attenuation effect.For instance,the natural factors such as mudflat slope and bed gradient at sample SM1 and SM2 are almost the same.The vegetation density at SM2 is 5.9 times higher than SM1.When the waves crossed over a 100-m wide Spirpus×mariqueter,the attenuation rate of 1/10 wave height in SM2 was 1/4higher than SM1.3.The attenuation effect of salt marshes was affected by plabt height and leaf width.Spirpus×mariqueter is dwarf(<35cm).In different growing seasons,its attenuation effect showed a decreasing trend with the increase of the water depth or the incident wave height.The attenuation effect of Spartina alterniflora differed depending on different growth conditions.Spartina alterniflora which was short(<120cm)and had narrow leaves(<0.80cm)appears to have the similar pattern as Spirpus×mariqueter.In different growing seasons,its attenuation effect decreases with water depth or the height of incident wave increase.In lush period,Spartina alterniflora growed taller(>160cm)and had wider leaves(>1.50cm),its attenuation effect first decreases and then increases with water depth or the height of incident wave increase.While in withered period,due to a slight decrease in plant height and leaf regrowth,the attenuation effect of Spartina alterniflora decreases with the increase of water depth and the incident wave height.The plant height of Phragmites australis is relatively high(180-260cm),and the width of theleaves is wider(0.80-1.60cm).The attenuation effect of Phragmites australis decreases first and then increases(or slows down the decreasing speed)as water depth and the incident wave height increase.4.The attenuation effect of salt marshes also affected by wave intensity,tidal type,wind speed,wind direction,bed gradient and so on.For example,Spirpus×mariqueter area in withered period,the maximum 1/10 wave height at sample SM3 was only 0.19 m,and the maximum 1/10 wave height at sample SM4 was 0.71 m.This is due to the fact that when observed in sample SM3,the wind direction was the offshore,and observed in sample SM4,the wind direction was onshore.There are many issues about the mechanism of the salt marshes attenuation effect that require further study and discussion.