Study Of Short-term Morphodynamic Process In The Typical Coast Of Upper Jiuduansha In Yangtze River Estuary

The Jiuduansha is located between the south and north Passage of Changjiang River estuary, which shows the three grades furcation and fore mouths going into the sea.The Jiuduansha is the only region that has been kept in the original state of tidal-flat in the big estuary.The study area is beyond the Upper Jiuduansh dock.The short-term morphodynamic process are studied in this paper.The observed data includes the bank and surface sediment data from June 2006 to June 2007 at nine fixed sampling sites of two profiles along Upper Jiuduansha and suspended dediment was observed at one fixed site of dock.At the same time,the depth,velocity,current direction and sediment concentration was observed in the dry or flood season of 2006.Methods of hydrology and sediment transportation and statistics are used to analyze the evolution characters of the bank,surface sediment and suspended dediment,and to discuss the morphodynamic processes and the evolution mechanism of the typical bank near the Upper Jiuduansha dock over a year period.The tidal-flat shows the obvious seasonal erosion-deposition cycle,which is "deposition in flood season and erosion in dry season".The tidal flat sediment grain sizes also show the significant seasonal changes.The sediment sizes are freer during the tidal flat accretion period in flood season,and are coarser during the tidal flat erosion in dry season.It shows the obvious longitudinal and lateral variation in space.The reason is that the construction of artificial riprap-embankment changed the way of sediment sources and hydrodynamic conditions.During the typhoon "Bilis" in 2006,the wave was as high as 2.56m,which just increased more than 1m.The strong wave going with the increased water led to an average 6.4cm and 20.2cm maximum depth of erosion.The erosion during the typhoon accounts for almost half of the total erosion.The storm waves have an obvious change for the bank and tidal flat sediment during the Typhoon "Billis" period.The range during the Typhoon is bigger with 1～2 levels.The storm wave is the important dynamical factor for the break-change.The reasons for the short-term variety is the joint operation of the sands transported by tidal current and the sands lifted by waves.The erosion-deposition and rate for the artificial profile is smaller than the natural one,while the variation of linear regression correlation coefficient between tidal flat vertical relative elevation and suspended sediment concentrations is larger.This is because that artificial riprap has the function of flat-protection,but it alters the nature process and evolution.The phase of tidal current isn't consistent with that of tide height,which makes a larger sand amount of flux tide to shoal but less sand amount of ebb-tide to the channel.The tidal flat eroded by waves covers the silt formed by the sands transported through the flux tides.As a whole,the tidal flat shows the erosion.In a word,the short-term erosion-deposition characters for tidal flat and temporal and spatial characteristic of sediment and suspended dediment are combined effects of storm and runoff from the basin on the basis of human factor.The study results can offer scientific reference for estuarine wetland protection and construction.