| 中文摘要 |
外傘頂洲位於雲林、嘉義外海約10多公里處，主要組成來自濁水溪注入海中的泥沙，由於受到波浪、沿岸流及東北季風等影響，導致沙洲逐年往陸側移動。外傘頂洲因潮汐的變化會產生不同樣貌，具有豐富的生態環境，此外也是雲林、嘉義沿岸的自然防護屏障。但隨著濁水溪注入海中的泥沙減少以及其他自然與人為因素，導致面積逐年縮減。本研究結合Landsat、SPOT系列與Sentinel-2等光學影像以及海潮模式，重建過去近30年的海岸地形模型，並用以監測外傘頂洲的變化。透過自1990年起的多光譜衛星影像計算水體指數以辨別水體出現機率，並使用DTU10的潮汐模型做為高度參考，將淹水機率轉換為實際高程資訊。經與現地單音束測深成果比較後獲得差值均方根（root-mean-square of the difference, RMSD）為36公分。經分析可獲得體積變化量從18.8百萬立方公尺減少為10.3百萬立方公尺，沙洲中於不同剖面求得移動速率為78.7-221.3myr-1，其中沙洲南端移動速度大於北端，且預計2048年前後沙洲本體會向鰲鼓濕地淤積。
The Waisanding sandbar is a major tidal flat in Taiwan that extends about 10 km from Chiayi coastline and its sediment source is mainly from Zhuoshui River. The ebb and flow of this place produce high ecological value and form a natural breakwater to protect mainland coastline. However, the sediment source had been decreased owing to human and natural factors, causing the area of sandbar to become smaller year by year. Therefore, this area is necessary to be monitored for environmental sustainability. In this study, we utilize multiple optical remote sensing satellites in the last three decades, including Landsat/SPOT series and Sentinel-2 imageries, to track the decadal variation of the intertidal zone. We first collect historical cloud-free images since the 1990s and calculate the Modified Normalized Difference Water Index (MNDWI) to identify water pixels. After computing water appearance probability of each pixel, the map is linearly translated into actual elevation by introducing the DTU10 tide model for high tide and low tide boundaries. To validate, we use the ground truth data collected by the single-beam echo sounder and obtain and accuracy of coastal elevation model at 36 cm. In addition, we analyze different variability of Waisanding sandbar. During 1994-2017, the volume decreased from 1.88×107 to 1.03×107 m3 and the rate of moving across the sandbar is 78.7-221.3 m yr-1 . Among all the rates of movement, the south tip is faster than the north tip. Finally, the sandbar is projected to touch Aogu wetland around 2048. |
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