應用多期圖資探討臺灣二十世紀以來西北海岸地形與土地覆蓋變遷

Abstract

在全球暖化與海水面上升等潛在威脅的情況下，人為活動造成具有緩衝海岸作用力之地形單元的變化程度，是需要關注的課題。因此，本研究以臺灣西北海岸為研究區，判讀多時期各類圖資所記載的海岸地形訊息，以釐清二十世紀前半期少有人為干預時的海岸地形特徵，並分析近百年來的土地覆蓋變化、現今土地利用狀況及保護措施。本研究所稱的海岸範圍，以二十世紀早期的海岸沙丘與海灘等地形單元為準，不包括海域的部分，也稱為海岸地形緩衝帶 。 本研究採用經由GIS平台整合各式圖資，包含二十世紀初期以來的1:20000或1:25000等高線地形圖、1940年代美軍老航照、1970至1990年代之像片基本圖（1:1000與1:5000）與2009年之彩色正射影像（1:5000）。本研究發現，比對美軍老航照與1920年代地形圖，可以確認西北海岸於二十世紀前半期時，海濱地帶人為結構物極少，雖已有防風定沙工程，但仍屬於近自然的狀態，該時多數海岸即屬「海灘—濱海沙丘」類型，沙丘內緣的位置也較今日偏內陸位置。 海岸緩衝帶內人為土地利用主要為：1970年代開始沿岸港口設施與海岸保護工的興築，造成濱線的侵淤變化；1920年代以來的防風定沙工程，一方面使沙丘與海岸林外緣逐漸向海擴展，另一方面內層林帶的解除，則造成沙丘內緣退縮。鄰近土地的利用強度不斷提高，從沙丘荒地轉為農業用地，再轉移做建築、交通與公共使用。 臺灣西北海岸因各式直接或間接的人為利用方式而漸趨人工化，只是程度有別，經過三十多年的轉變，其中高達七成為港口與堤防等高度人工化海岸，另三成則為受到防風定沙等較低度人工化海岸。而海岸人工化的程度愈高，顯示海岸地形緩衝帶的面積縮減或其可移動的空間減少。考量海水面上升的潛在威脅，則未來的海岸土地管理策略，本研究建議宜盡可能維護海岸地形緩衝帶的原貌，預留空間使其得以變動，並降低或不提高緊鄰海濱帶的土地利用強度。The loss and the level of artificialization of the coastal geomorphic units which consist of the buffer zone, e.g., beach and sandune, has become a great concern under the threat of global warming. By using the northwestern coast in Taiwan as the study area, this research intends (1) to reveal the geomorphic information which is shown or implied in various pictorial materials, (2) to identify the coastal features which represent the natural condition, with limited direct human interference, in the first half of 20th century, (3) to analysis the land cover and land use change since early 20th century. The method is the historical overlap analysis on the GIS platform. The pictorial materials in use include not only historical maps and topographic maps at scale of 1:20000 and 1:25000, but also the aerial photos which were taken in late 1940s and various versions of orthophotos (at scale of 1:1000 and 1:5000). Interpretations of geomorphic units are made by using stereo-pair of historical aerial photos. Then these interpretations are compared with the 1920 topographic map. It is found that the geomorphic features identified from the historical maps in 1920s and aerial photo in 1940s represent the near natural state of the studied coast and can be used as a baseline for the following change... Construction of harbors and breakwaters since 1970s has caused erosion and aggradation along the nearby shorelines. Continuous and widespread windbreak and sand fixation projects since 1920, on the one hand, have moved the outer margin of dunes and coastal forests seaward and, on the other land, subsequent removals of inner forest belt have caused the inner edge of dunes to retreat. The reclaimed coastal dunes have transformed into farmlands and part of them has become construction, transport facilities and public facilities since 1960s. In brief, based on the comparison of multi-temporal maps, aerial photos and orthogonals, the entire northwestern coast in Taiwan during the last eight decades has been artificialized although with various levels. Up to 70% of the coast has been affected directly by harbor and breakwater construction, while the rest, which seemingly rather natural, is actually the result of windbreak and sand fixation projects. Under the threat of sea level rising, it is believed the original extent of the coastal buffer zone and the human-induced change should be taken into account in the national land management.