Abstract: To study the distribution of erosions and deposition in Fengxian tidal flat, the elevation of tidal flat were measured in 9 years The elevation data in 1999 and 1998 were digitized to get point distribution map The Beijing Projection was used Interpolating elevation and slope analysis to point elevation were finished by using the Grid and Tin (Triangular Irregular Network) modulus of Vertical Mapper1 5 software, which generated the topographic and slope maps of 1999 and 1998 The project and cell length of the two maps identical Length of per grid is 0 05 km In the environment of Vertical Mapper1 5, the 1998’s topographic grid subtracted the 1990’s topographic grid to acquire the grid and contour maps of erosion and disposition between 1990 to 1998 Then overlaying the erosion disposition grid map and slope grid with the point distribution map, profile location map and standard tidal level location map, the erosion disposition amount and slope value of point measured, the data of profile and the shoreline change map were acquired Based on the analyses of the erosion deposition amount, slope value, coastal profile, erosion deposition distribution, shoreline change, the erosion deposition features of Fengxian tidal flat are classified as follows: 1 Three belts from north to south are divided, that is, slight eroding belt of tidal flat, severe eroding belt with steep slope and slight deposited belt between -7 and -9 5 Due to eroding, the elevation of high tide will decrease and even disappear, and the steep slope is approaching toward the shoreline The landward retreat of MLTL is at a rate of 31 m/a The feature of erosion deposition distribution from west to east is that the tidal flat of the east and the central sections are eroded more severely than the west At present, there is no high tidal flat in the east 2 On the tidal flat above MTL(the mean tide level), there are erosion area and deposition area, but are mainly erosion areas The mean erosion velocity of this area is about 2 8 cm/a Most of the tidal flat between MTL and MLTL is erosion area The mean erosion velocity is about 6 5 cm/a The steep bank slope between MLTL and -7 m contour is the most severely eroded area The mean erosion velocity is about 17 9 cm/a The deep bank slope between -7 m and -9 5 m contour is depositional area The mean deposition velocity is about 5 3 cm/a 3 The amount of erosion deposition is related to the slope The regression equation is given below:Y=0 062-0 12X Y is the velocity of erosion deposition The unit is m/a X is the slope of tidal flat The depositional areas are distributed on the high flat and the deep water bank, the slope of which is less than 0 1 The most severe erosion is distributed on the steep bank slope According to the relationship between erosion deposition and slope, we can foresee that the erosion on the slope between MLTL and -7 m contour will be more severe because the slope of this area is increasing 4 The sediments were mainly eroded from tidal flat and the steep slope between MLTL and -7 m contour and deposited on the deep bank slope below -7 m contour The closure depth is about -9 5 m

Key words: GIS, spatial analysis, DEM, tidal flat, erosion and deposition