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, isan effective tool for monitoring the tidal flats. Airborne lightdetection and ranging (LIDAR) or radar interferometry can beutilized to measure precisely the surface topographic change.However, neither the satellite-borne synthetic aperture radar(SAR) nor LIDAR is an effective way to obtain appropriatedata on tidal flats, mainly because of the little opportunity offinding favorable tidal conditions. Therefore, the waterlinemethod is, so far, the only useful approach to the practicalapplication of satellite remote sensing to monitor the tidal flatenvironment. The 'waterline' is defined as the boundarybetween a water body and an exposed land mass in a remotelysensed image. Waterline method has been applied in analyzingthe horizontal evolution of tidal flat, but such approach hasseldom been used for vertical development. Sedimentation rateis an important factor describing the dynamic nature of tidalflat, so waterline method to determine multi-year meansedimentation rate was reported at Chongming DongtanNature Preserve in Shanghai, China in this paper. Thewaterlines were extracted from multi-temporal Landsat TMimages by unsupervised classification method and regiongrowing algorithm. The mean sedimentation rates werecalculated at four transects according to correspondingelevation measurements and waterlines with heights assignedby hydrodynamic model. The results showed that the evolutionof the bed level was changed spatially in the cross-shoreprofiles. The peak of accretion rate occurred at differentelevations of the four profiles. Sediment surface of all profilesshowed a shoreward reduction from the maximum site to thehigh marsh, likely due to the shoreward decrease in waterenergy and submergence time as well as the protective effect ofmarsh vegetation. The offshore decrease may be related tohydrodynamic attenuation by vegetation and settling lag. Apositive relationship was demonstrated between the meanaccretion rates and surface elevation of low marsh and mudflat(R~2=0.8106). Spatialchanges in sedimentation rate were alsostriking at the same elevations of different profiles, whichcould be attributed to differences in sediment distribution andhydrodynamics. The effects of tidal marshes onhydrodynamics and sedimentation were also related to thedegree of shelter from water energy. The relationship betweensedimentation rate and vegetation distribution was alsodiscussed.