| Summary: | 碩士 === 國立臺灣海洋大學 === 海洋環境資訊學系 === 95 === The objective of this study was to obtain sediment size information around the Pratas (Dongsa) Island. Sediment samples were taken from coastal ocean sites surrounding the island, sites in the lagoon which was located in the center of the island, and sites from the beach of the island during November, 2005 (fall) and march 2006 (spring). Results of sediment size analysis showed that:
(1) The median diameter of the sediment in the surrounding sea ranged from 200 to 1700 □m. Essentially they are all sand sized.
(2) In the fall season, the median diameter for the sediments from the coastal sea north of the island ranged from 200 to 600 □m, while those from the southern coastal sea from 500 to 1300 □m. In the spring season, the sizes for the northern coastal sea were between 800 and 1700 □m, and between 200 and 400 □m for the southern coastal sea.
(3) The sediment sizes varied with the monsoon direction. With northerly monsoon in the winter, the northern coastal sea in the spring had the coarser sediment. Conversely, the southern coastal sea in the fall had the coarser sediment due to the southerly monsoon in the summer.
(4) Sizes of sediments in most of the lagoon also changed with season. At the entrance region, particles were coarser in the fall season. At the center locations of the lagoon, particles were coarser in the spring. However, sediment size did not change with the season for those located in the innermost part of the lagoon. This implied that sediment transport during these two seasons only occurred up to the center section of the lagoon.
(5) Sizes of sediment from the surrounding coastal sea did not show a clear relationship with water depth.
(6) Sizes of sediment from the beaches surrounding the island did not change with the season. The sediment sizes were between 250 and 1000 □m.
(7) Sediment sizes at stations near the southeast corner of the island had the most dramatic changes of all stations with the monsoon direction.
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