考慮潮汐與波浪溯升因素以衛星影像監測灘線變遷之可行性研究

Abstract

台灣海岸因大波浪及海岸結構物等因素常有海岸變遷問題。對海岸變化特性瞭解後 有助於海岸侵淤問題之解決，台灣海岸變遷之研究對國土保護是相當重。往昔研究海灘 地形變化的現場調查方法，當在近灘線或有結構物處有不易測量之技術問題及現場調查 的高經費的困擾下，本研究為克服此問題而提出三年之研究計畫，主要建立以新科技之 衛星影像的擷取灘線技術及因潮汐高低及波浪溯升而移動的修正的方法，本研究結果可 做為未來海岸變遷研究所需灘線位置的基本資料。 在第一年研究，主要提出修正潮汐所造成的海灘線位置方法。申請人以前已提出將 原本擷取衛星影像的灘線，修正因潮汐影響至平均潮位的兩種方法(張與陳，2005)。然 而，此方法在兩張衛星影像之時間間距長及灘線變化不是穩定時就有誤差，本研究參考 Gardel and Gratiot (2005) 求出海灘坡度的方法來降低潮汐對辨識灘線位置的影響。第二 年計畫為提出修正波浪溯升所造成海灘線位置差異的方法。計畫中利用衛星影像中的海 面資訊，以衛星影像拍攝時間，先判斷波浪波向及計算波浪大小後，以波浪理論估算波 浪的溯升，進而修正海灘線位置。第三年之計畫主要先統計台灣目前較常使用的SPOT 以及FORMOSA2 等光學衛星的雲覆蓋率，主要當地表雲量大的時候，會遮蔽地表進行 光反射，造成地表資訊無法判別，故限制了光學衛星影像的使用。本研究即欲提出解決 此問題之統計方法，能正確的決定灘線位置。

Some coasts around Taiwan have been suffering from beach erosion or accretion due to large wave actions or marine structures. Investigating on bathymetrical variations around Taiwan becomes important for protecting coastal land of Taiwan. An early coastline surveying by an altazimuth or real-time kinematic GPS is a factitious and tedious work that needs high manpower and cost. Some problems by traditional measurements sometimes happen at the places near the shoreline and marine structures. A three-year project is proposed to solve the problems. This project aims at establishing techniques of extracting the shoreline from satellite imagines and of correcting the originally extracted shoreline to the position at a datum of mean water level. The proposed method of determining shorelines from satellite images provides a set of shorelines of different time for future study on beach evolution. In the first year, a correction on shoreline position due to tidal variation is considered. Chang and Chen (2005), the applicant of this project, proposed two ways of correcting shoreline position from satellite images due to tidal variations.The results showed that large discrepancy between the extracted shorelines and measured ones occurs when the beach slope is very mild or beach evolution varies with a stable trend. The Gardel and Gratiot’s (2005) method determining the beach slope will be suggested to modify the originally proposed method to accurately detect the shoreline position. Run-up of wave’s swashing on the beach is also a key factor of effecting the position of shoreline. A correction of wave’s run-up on the shoreline position will be investigated in the project of second year. The wave pattern of satellite images can offers the wave direction and marine data base of satellite information includes wave height. Both wave direction and wave height obtained can be used to compute the run-up. Spot and Formosa 2 are two commonly used satellites in Taiwan. Some satellite images including most clouds are common. Clouds in satellite image reflecting sun’s light cover the land. Therefore, determining the shoreline position in this case becomes difficult. A statistical analysis on the rate of clouds covering the land will be carried out to accurately determine the shorelines in the project of third year after.