臺灣東北部福隆海灘類型和裂流分布之長期變動研究

Abstract

巴西南部海灘研究顯示，海灘意外的成因中有90%是來自裂流帶來的危害。鑑於國內海灘意外事件頻傳，裂流既為海灘危險的重要因子，管理當局實有必要針對海灘安全的議題加以探討。過去有關海灘模式研究的理論基礎和分類，主要來自Wright、Short 等多位學者於澳洲海灘進行的研究，其探討海灘營力機制對水下沙洲和裂流變化的影響。福隆海灘有雙溪注入，受到波浪、潮汐等海灘營力的影響，其海灘類型和裂流分布勢必也會受到河口逕流的影響。本研究即以Wright 等人於1984 年發表的海灘模式理論為基礎，透過航照的判讀，來探討福隆海灘長期以來海灘類型和裂流分布的時空間變動，並討論造成其差異的原因。 就時間而論，福隆海灘冬季海灘類型以倒轉沙洲與裂流型（TBR）為主，夏季則以反射型（REF） 至低潮位階地型（LTT）為主。探討冬半年海灘類型較夏半年偏消散之因，為波浪營力的不同所致；河口附近受逕流切穿的影響，其河口海灘類型無法適用於Wright 的海灘模式理論。本研究依河口逕流的流向為指標，對雙溪河口沙洲水下沙洲形態加以分類。結果發現福隆海灘冬季河口海灘類型不僅較偏消散的情形，其河口變化遠大於夏季河口海灘形態的變化。就空間而論，相同拍攝時間內，海域南側海灘類型較北側偏消散；造成其差異的主要成因則是局部礁岩的影響，使得海床粒徑較大，海灘類型因而偏向反射型，裂流頻數也比較低。然而不論任何時間，海域東南端受到河口切穿和人工堤防建造的影響，皆有固定離岸方向水流的存在，對於海灘遊憩者而言，需避免到該地戲水。

According to long term studies from America, Brazil, and Australia, about 90% of drowning incidents were caused by rip currents. It needs researches on the beach safety to investigate the causes. In the past, the beach classification method based on morphodynamics proposed by Wright & Short etc. discussed the influences of process on sand bars and rip currents. Supported by previous studies, we know that the Fulong Beach is not only influenced by waves and tides, but also by the run-off from Shuangsi River. This research is based on a theory developed by Wright & Short in 1984, yet it’s purpose is to investigate the profile of Fulong Beach and distributions of rip current by implementing longitudinal data of aerial photos and digitized locations of rip channel. Another significant purpose is to discuss the elements which cause the time and spatial change. To discuss the time difference, the major types of Fulong Beach were mostly TBR in the winter, and LTT and REF in the summer. The reason Fulong Beach being the dissipative in the second half year was due to the difference of wave energy. The spits of Shuangsi River were influenced by the run-off, indeed the dynamics of the river mouth can not be classified from Wright and Short’s theory. This research is based on the direction of river run-off, to define the classification for the types of sand bar of Shuangsi River. The results show the dissipative beach states and various landforms of the Fulong beach near the river mouth in winter than in summer. To discuss the spatial difference, with the same time period, the southern beach was more dissipative than the one of the north. The reason behind that is the reef and rock which cause greater bed load for reflective conditions. However, cross-shore currents have always existed on the southeastern beach due to river runoff and sea embankments.