A Quantitative Evaluation of Landsat for Monitoring Suspended Sediments in a Fluvial Channel.

Since the launch of Landsat-1 in July, 1972, the orbital remote sensing system has made significant contribution to earth science research. Landsat's repetitive "earth observations" are of great value for monitoring changes in earth surface conditions through time. Investigation of suspended sediment concentrations in natural water bodies is one of the major areas of Landsat applications. An attempt has been made to evaluate the utility of Landsat Multispectral Scanner (MSS) digital data for monitoring suspended sediment concentrations in a natural river channel using Tarbert Landing, Mississippi on the lower Mississippi River as the test site. Specific purposes of the study were: 1) to investigate vertical dis­ tribution of suspended sediment concentrations at the cross section of the river channel; 2) to develop a method of eliminating environ­ mental effects from the MSS digital data obtained during successive Landsat overpasses; 3) to evaluate the statistical properties of MSS digital data as related to suspended sediment concentrations in the surface layers of a natural river; and 4) to evaluate the feasibility of estimating suspended sediment concentrations in entire depth of the river channel via Landsat MSS digital data. The hydrological data utilized in this study were collected by the U.S. Corps of Engineers, New Orleans District, at Tarbert Landing during 1974-75 and included discharge, velocity, suspended sediment concentrations, and water sampling depth. Analyses of the data for R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. the vertical distribution of suspended sediments employed both the diffusion model and the linear model. Analyses of the hydrological data indicated that the diffusion model does not adequately describe the relationship between suspended sediment concentrations in surface and subsurface layers of the river. Poor performance of the diffusion model is attributable to the over­ simplifying assumptions applied to the model, notably the twodimensional flow. The full rank linear model was better than the diffusion model in accurately estimating sediment concentrations, particularly the silt-clay fraction. However, the linear model suffered from a large sample variance. Computer Compatible Tapes (CCT's) from three cloud-free Landsat overpasses (July 11, 1974; December 2, 1974; April 16, 1975) were obtained and processed at the NASA's Earth Resources Laboratory in Slidell, Louisiana. MSS 4-channel radiance data from the CCT's were analysed using the Corps of Engineers' suspended sediment data as the surface truth and the linear model. Analyses of the MSS radiance data and the surface truth showed that the actual suspended sediment concentrations in the surface layers of a natural river can be estimated with better than 80% accuracy when using all four MSS channels in the linear model. Results from the analyses also indicated that most of the information on suspended sediments is contained in the MSS channels 4 and 5. The negative relationship found between channel 5 and suspended sediments was not as anticipated, however, a complete explanation of this phenomenon was not available. xi R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The transformation method developed in this study to eliminate the environmental effects should prove to be very useful. The basic principles involved in the transformation should be readily applicable to any other type of investigations that deal with monitoring surface physical phenomena through successive Landsat overpasses.