分布於台灣西南部海岸的濱外沙洲島，是海岸防護的天然屏障。本 文藉由航空照片、衛星影像及地面的地形剖面調查等資料，研究台灣西 南部外傘頂洲和網子寮汕這兩個沙洲島近幾十年來的地形變遷，並探討 這些變遷對海岸災害防護的可能衝擊。在雲嘉外海的外傘頂洲整個島往 東南方向遷移，海峽側的濱線後退快速，前列沙丘幾被侵蝕消失，僅在 島的西南段還有一些低矮沙丘殘存。全島高程下降、出露面積也縮小。 而台南外海的網子寮汕，原細長狹窄，但近年卻有多次新潮口的形成與 閉合，尤其北段前沙丘消失，整個高程下降形成一潮汐灘地，而南段的 海岸線也後退百公尺以上，海灘消失，海岸防風林木倒塌，且出現明顯 沙丘崖。兩者均有整個沙洲島沙量收支失衡的問題，追究其原因顯示， 主要和近四分之一世紀以來，鄰近海岸的工程結構物建構直接相關，致 使西南部沙洲島的侵蝕作用速率加快，沙量減損。這些變遷將減損沙洲 對海岸的保護功能，致使未來西南部海岸因全球氣候變遷所引發的各種 海岸災害潛勢增高。

Barrier islands along the southwestern coast of Taiwan usually provide the natural protection to their backbarrier lagoons and mainland shorelines. Analyses of aerial photographs, satellite images and the ground surveys reveal that two barrier islands, the Waisanding Barrier and the Wantzuliao Barrier, have undergone dramatic geomorphic changes in recent decades. The Waisanding Barrier tends to migrate in a southeast direction. The ocean–side shoreline retreats at a very fast rate. The foredunes were washed away, and the elevation of the whole island is lowered. The overall land area has also shrunken. The Wantzuliao Barrier, a long and narrow barrier with lower elevation in the northern part tends to be vulnerable to the storm waves that the island had been breached several times in recent years. The foredunes were disappeared, and the elevation is lowered to form a wide sandy tidal flat. The shoreline has retreated over one hundred meters in the southern reach, and some erosional dune cliffs have formed in front of an artificial forest. All these changes are mainly due to the large negative balance on the sediment budget of the barrier islands. Although both islands remain natural, the geomorphic changes on both islands are enhanced by the human agents. In comparison to the coastal erosion trend in the last century, the human interfer-ence of the coastal processes by the engineering structures in recent couple decades may induce faster shore erosion and affect larger area. Higher vulnerability of the barrier islands may result in more coastal hazards, especially those associated with the future global warming.

沙洲島；地形變遷；工程結構物；氣候變遷；海岸災害

barrier island；geomorphic change；engineering structure；climate change；coastal hazard