Geomorphological Dynamics Of Salt Marsh Fronts In Central Jiangsu Coast,China

As an important part of coastal wetlands,salt marshes play a significant role in protecting the coastal environments and cities,accelerating the expansion of coastal lands and available space,and providing habitats for coastal animals.The increase of carbon burial in salt marshes also contributes to the slowing down of global climate warming.Since there is favorable contuity and preserve potential for sedimentary structures,salt marshes act as a crucial material for researches on paleoenvironments and eustatic cycles.Salt-marsh fronts represent a unique geomorphological unit at the border of marsh vegetation and the intertidal mud flat.Researches on the dynamic changes of salt-marsh fronts will deepen the understanding on coupled marsh-flat system and help to predict the future evolution of salt marshes.The morphology of marsh-edge scarps with their geomorphological characteristics and shore cores on the mud flat in the Yancheng Nature Reserve,central Jiangsu caosts,were obtained in December,2012.Furthermore,the hydrodynamics and suspended sediment concentrations were measured in September,2013.During July,2014,bed elevation across the intertidal flats was measured and the surface substrates were sampled.In addition,the bed elevation from 2008 to 2013 and the remote sensing images from 2005 to 2014 were also collected.Based on the morphological parametes and their relationships of marsh-edge scarps,their morphology cound be divided into two patterns,i.e.,the first pattern with large trough length,width and scarp height but small spatial difference,and the second pattern with small trough length,width and scarp height but large spatial difference,which are related to different development stages of marsh-edge scarps,respectively.The first pattern can be explained by the theory of standing edge waves,but that is not the same case for the second pattern for its unequilibrium.The grain size analysis reveals that the sediments in salt marsh fronts are dominated by silty sand and sandy silt.They can be easily suspendload transported by the hydrodynamics with the dominance of wind-induced waves.The suspended sediment concentrations and the resuspended flux reach 1.0?4.0 g/L and 0.01?0.3 kg/m/s,respectively.Consequently,the variation rate of bed elevations is on the order of-10^-5 m/s,which equals to the erosion rate of 0.2 m/yr.Furthermore,several methods have been applied to quantitively determine the dynamics of marsh fronts,including the discrimination approach of remote sensing images,the comparison of bed elevations,the mathematical method of sediment dynamics and the empirical formula of Schwimmer.Results reveal that the starting time of mud flat eriosion and Spartina marsh-edge retreating are not later than the year of 2010 and 2012,respectively.Their retreating rate could be around 200 m/yr and 20 m/yr,the latter of which is larger than the theoretical value calculated by Schwimmer's formula for its limitations.The analysis results of hydrodynamics and sediment dynamics show that waves are the leading force stiggering the marsh edge retreat.As well the reduction of sediment supply should be mainly responsible for the ersion of mud flats and the retreat of salt-marsh edges,since the expansion of marsh edge had not reached the low limit of Spartina's niche.The persistent erosion of mud flats,caused by the decline of sediment supply,leads to the sharpness of bed slopes between vegetated marshes and mud flats,which results in the enhancement of waves acting on the marsh edges and subsequently the formation of marsh-edge scarps.In this research area,the marsh edges retreat as the development of rhythmic morphology of marsh-edge scarps.Once the erosion of mud flats reach the marsh edges,the bottom of scarps will be undercutted by waves to leave the upper root mat as overhangs,which might collapse onto the mud flats to form notches cutting into the straight line of marsh edges.Then the continuing backward erosion of notch heads gives rise to the formation of troughs and reclic ridges,the latter of which might be cutted by currents or waves,indicating the end of a cycle of the marsh-edge retreating.