花蓮七星潭礫灘的地形及沉積特性

Geomorphologic and Sedimentary Features of the Chi-Shing-Tan Gravel Beach, Hualien, Eastern Taiwan

七星潭海岸位於三棧溪與米崙台地之間，有發育良好的礫灘是以往研究重要點。為了探索礫灘的地形和沉積性質，本研究詳細測繪該海剖面分析組織與岩並觀察海灘的側向變化，以探討沉積物來源及搬運機制。七星潭礫北寬南窄最處約160公尺；灘面有平緩的後濱及陡峭前，其上一至三階臺脊。礫淘選良好粒徑由海向陸漸粗，並呈現明顯之疊瓦狀排列朝內陸傾倒示波浪上沖作用主導了灘礫的搬運和堆積。後濱灘礫淘選度較差，雖亦有疊瓦狀組構但排列方向繁亂且常見漂木和雜物成的碎線展現出暴風波浪作用的特徵。七星潭海灘沉積物主要由變質岩碎屑組成，大多來自北方三棧溪及立霧溪的河口堆積物，少部分來自南端米崙台地海崖侵蝕。這些沉受波浪沖刷磨與沿岸搬運，最後被沖積上陸堆成礫灘。

Chi-Shing-Tan, located at northwestern Hualien city of eastern Taiwan, is bordered with a well-developed gravel beach, which has been focused by geomorphologic researches. In an attempt to unravel the characteristics of this gravel beach, this study investigated the beach profiles, gravel texture and composition, and their spatial variations to infer the sources and transport mechanisms of beach sediment. The Chi-Shing-Tan beach is near 160 m wide in the north and about 30 m in the south. The beach has a gentle backshore and a steep foreshore separated with one to three steps of berm crest. The foreshore gravels are generally well-sorted, coarsen landward, and exhibit prominent landward-facing imbrication, which imply that sediment transport and deposition are dominated by strong uprush. In contrast, the backshore gravels are often poorly sorted, randomly imbricated, and featured with a debris line, which can be attributed to stormy waves under extreme storms. The composition of Chi-Shing-Tan beach pebbles are dominated by metamorphic rock clasts, which are mainly derived from the San-Jan River and Li-Wu River in the Central Range. These sediments were constantly longshore transported with increasing abrasion from north to south and then accreted onshore by wave uprush to form gravel beach. In addition, cliff erosion at the northern Milun Tableland also supplies a minor amount of sediment to beach.