Determination of sedimentation rate of tidal flats at the Yangtze Estuary,China,using multi-temporal Landsat TM images

摘要

Remote sensing, combined with in situ surveying, is an effective tool for monitoring the tidal flats. Airborne light detection and ranging (LIDAR) or radar interferometry can be utilized to measure precisely the surface topographic change. However, neither the satellite-borne synthetic aperture radar (SAR) nor LIDAR is an effective way to obtain appropriate data on tidal flats, mainly because of the little opportunity of finding favorable tidal conditions. Therefore, the waterline method is, so far, the only useful approach to the practical application of satellite remote sensing to monitor the tidal flat environment. The ‘waterline’ is defined as the boundary between a water body and an exposed land mass in a remotely sensed image. Waterline method has been applied in analyzing the horizontal evolution of tidal flat, but such approach has seldom been used for vertical development. Sedimentation rate is an important factor describing the dynamic nature of tidal flat, so waterline method to determine multi-year mean sedimentation rate was reported at Chongming Dongtan Nature Preserve in Shanghai, China in this paper. The waterlines were extracted from multi-temporal Landsat TM images by unsupervised classification method and region growing algorithm. The mean sedimentation rates were calculated at four transects according to corresponding elevation measurements and waterlines with heights assigned by hydrodynamic model. The results showed that the evolution of the bed level was changed spatially in the cross-shore profiles. The peak of accretion rate occurred at different elevations of the four profiles. Sediment surface of all profiles showed a shoreward reduction from the maximum site to the high marsh, likely due to the shoreward decrease in water energy and submergence time as well as the protective effect of marsh vegetation. The offshore decrease may be related to hydrodynamic attenuation by vegetation and settling lag. A positive relationship was demonstrated between the mean accretion rates and surface elevation of low marsh and mudflat (R2=0.8106). Spatial changes in sedimentation rate were also striking at the same elevations of different profiles, which could be attributed to differences in sediment distribution and hydrodynamics. The effects of tidal marshes on hydrodynamics and sedimentation were also related to the degree of shelter from water energy. The relationship between sedimentation rate and vegetation distribution was also discussed.