As a very important component of a coastal system, tidal flats come to be a focus of the studies on land-ocean interaction in the coastal zone because those areas are subjected to intense human activities and are highly sensitive to the global change. The Quanzhou Bay, located along the middle part of Fujian coast of China, covers about 136.4km^2, and the area of coastal wetland in the entire bay from intertidal to subtidal with 6m of water depth accounts for 96% of the total area. Seven short cores were collected and divided in situ with the interval of 5cm on the coastal wetlands of Quanzhou Bay on April 19, 2006. The sediment samples were scattered and the grain sizes were measured by using Mastersizer 2000. Human beings＇ activities on tidal fiat have disturbed the vertical distribution of sediments in stratigraphic sequence and accelerated the sedimentation rates. Grain size analysis results show that the grain size diameters increase and sediment becomes worse sorted towards the sea under the strong human disturbance; Spartina alterniflora can play a role of trapping the fine sediment; but near the bank, the sediment becomes coarse and there are two peak values on frequency curve influenced by the sandpile. The trough formed by human activities along the coastline changes the transport path of water and suspended sediment. The sediments are transported through the trough and deposit in it during the flood; the ebb flow is retarded by the flow output through the adjacent trough, and the deposited sediment can not be re-suspended; then, the sedimentation rate increases. In situ observation show that the sedimentation rate is about 8-10cm/yr.
In order to understand the mechanisms of coastal protection by salt marshes during typhoon events,in situ measurements of water level,tidal current speed and direction,and suspended sediment concen-tration (SSC) were carried out using Electromagnetic Current Meter (EMCM,AEM HR),miniature pres-sure sensor (MkV/D) and Seapoint Turbidity Meter (STM) sensor on a tidal flat in Quanzhou Bay,during the period when the typhoon "KAEMI" was passing through the region. The analysis of the data ob-tained shows that the near-bed current speed within the Spartina alterniflora marsh was generally be-low 5 cm s-1,which was apparently smaller than on the adjacent bare flat (i.e. 5―30 cm s-1). The change in the near-bed current speed in response to the typhoon event was not significant within the S. al-terniflora marsh,but the current direction was influenced by the typhoon. The effect of the typhoon on the SSC was highly significant,with the SSC reaching 13 to 19 times the values on the bare flat or within the marsh under fair weather conditions; the near-bed SSC within the marsh was higher than on the bare flat,after the typhoon landed. The near-bed suspended sediment fluxes within the marsh and on the bare flat during the typhoon event were both enhanced,i.e.,4 times larger than under fair weather conditions. During the ebb,the bottom shear stress on the bare flat exceeded the critical shear stress for sediment motion for most of the ebb duration of the tide and,therefore,the bed sediment was eroding,with the erosion flux after the typhoon landed being around 2 to 3 times the value associated with fair weather conditions. In contrast,within the S. alterniflora marsh,the bottom shear stress was mostly lower than the critical shear stress for sediment motion,or lower than the critical shear stress for the maintenance of suspension; hence,the marsh surface was dominated by settling processes,with a settling flux during the typhoon being 3 to 6 times compared with the fair weather situations. The settling flux during the ebb was
