| Summary: | 碩士 === 國立中央大學 === 水文與海洋科學研究所 === 98 === Estuaries are the most productive areas in the coastal environment and also most susceptible to human disturbances. When a river runs into the coastal zone, it is affected by the adjacent ocean circulation that influences the dynamic process of the suspended material dispersion. Therefore, to understand the characteristics of a river plume and the major factors controlling its distribution patterns is helpful to delineate the biogeochemical pathways and ecological feature in the coastal areas. For this study, we derived the distribution patterns of the Lanyang River plume from SPOT satellite images; we also collected data on the river discharge, satellite-observed two-dimensional sea surface wind field, and model results of the tidal motion. Using these data, we tried to understand the size and variation of the Lanyang River plume. This study is the first to use remote sensing images to investigate spatial and temporal distribution of river plume and its relationship to environmental factors.
The foremost task of this study is to use spectral image classification to objectively generate the Lanyang River plume distribution. Image classification techniques are typically applied to land-based data sets, but rarely used for oceanographic application. In this study, we applied supervised maximum likelihood classification, to classify surface water off the Lanyang River mouth using SPOT data from the following three bands: XS1(545 nm), XS2(645 nm), and XS3(840 nm). The retrieved images were used to investigate river plume behaviors. The results show that Lanyang River plume distribution patterns were influenced by more than one factor. The tidal effect could be important. In 8 out of 14 cases, the plume distribution was consistent with the tidal current, namely: the plume to the north of the river mouth during ebbing tides, and to the south during flood tides. The wind effect would be more obvious if the tidal current was weak. However, other factors, such as the mean coastal current and the offshore flow field, could also play important role in controlling the river plume distribution and warrant further study. Satellite-measured river plume area was correlated well with the river discharge rate with a 1-day time lag, i.e., the plume size is well correlated with the discharge on the previous day. The correlation would be even better, if the cumulative discharge of the previous 3 days are used. The offshore distance of the river plume appears to be affected by the magnitude of river discharge, and also by wind direction.
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