臺灣北海岸老梅海灘短時距形貌變化之研究

Abstract

海灘是海陸相交的一種海岸線型態，其多變且脆弱，最近的研究發現，風暴和強降雨導致嚴重的海灘侵蝕和沿海地區的變化，進而威脅著我們的生存環境。在臺灣，北海岸同時受到來自北方的中緯度天氣系統 (例如鋒面、冷氣團) 和來自南方的熱帶天氣系統 (例如颱風、熱帶低氣壓) 的影響，而海灘也因天氣的不同變化產生了變化。 為了得知海灘的侵淤變化情形，本研究將嘗試使用無人機 (Unmanned Aerial Vehicle, UAV) 於位於臺灣最北端的老梅海灘獲取高解析度的航拍影像，並且構建出海灘的數值地表模型 (Digital Surface Model, DSM) 以老梅海灘的變化過程，並納入氣象資料做討論，特別是在研究期間內的東北季風吹拂季節，以及三次颱風的侵襲事件中，海灘的變動情形。 本研究於2022/2/11至2023/09/08總共進行了31次的有效航拍作業，並取得航拍影像與數值地表模型，並且在東北季風季節前、後分別於海灘兩側剖線作沉積物的採樣，並進行粒徑分析。 研究結果發現於老梅海灘於31,980平方公尺的測量範圍內，初期呈現持續堆積的現象，在2022年4月7日至8月13日的四個多月期間，堆積量更是達到38341立方公尺，直到颱風軒嵐諾影響下造成海灘短時間內的侵蝕，隨後海灘體積不再有大幅度的變化，僅於颱風卡努的侵襲下導致短時間內的侵蝕。 颱風的影響部分，2022年9月的颱風軒嵐諾造成前灘部分大量的侵蝕，整體侵蝕量達8,522立方公尺；而2023年7月的颱風杜蘇芮對於海灘產生的侵蝕量979立方公尺，堆積量568立方公尺，相較於隨後的卡努颱風侵蝕了4743立方公尺及堆積了975立方公尺，杜蘇芮颱風對海灘產生的改變非常小。 在東北季風影響期間內，沉積物採樣的粒徑分析發現，2023年5月25日採樣的平均粒徑較2022年9月6日大，海灘西側由平均0.384mm提升至0.407mm，而海灘東側由平均0.332mm提升至0.355mm，然而，由於缺乏較為短時距的粒徑分析資料，各採樣點的粒徑產生改變的機制或過程尚不明確，在長時距、季節性觀察到粒徑的變化仍沒有足夠的證據得以解釋或推論粒徑的變化成果。但由採樣點的高程變動發現，在東北季風期間較大的浪高，波浪可沖蝕至距定沙籬42公尺以內的範圍，而非東北季風期間，波浪不易抵達此處。

Beach is a type of coastline which is the intersection of sea and land, it is highly variable and fragile. Recent research has found that storms and heavy rainfall contribute to severe beach erosion and changes in the coastal area, which threaten our living environment. In Taiwan, the North Coast is affected by both mid-latitude weather systems from the north (e.g., fronts, cold air masses) and tropical weather systems from the south (e.g., typhoons, tropical depressions), and the weather events influenced the geomorphologies a lot. In order to investigate the elevation changes of the beach, this study will try to use UAV (unmanned aerial vehicle) Photogrammetry method to obtain high-resolution aerial images of Laomei Beach, located in the northernmost point of Taiwan. Then construct DSMs of the beach to track the change of beach volume under the influence of different weather processes, especially the changes during the northeast monsoon season and the three typhoon events during the study period. A total of 31 valid aerial photographs were taken from 2022/02/11 to 2023/09/08, and aerial images and DSMs were obtained. Before and after the northeast monsoon season, conducted the sample of the beach sediments on both sides of the beach, and analyzed the changes of the particle size. The results found that Laomei Beach had continuous accumulation in an area of 31,980 square meters in the early stage. During more than four months from April 7 to August 13, 2022, the accumulation amount reached 38,341 cubic meters, until the impact of Typhoon Hinnamnor caused the beach to erode in a short period. Then the beach volume no longer changed significantly, and the only erosion of the beach in a short period of time was caused by Typhoon Kanu. As for the impact of typhoons, Typhoon Hinnamnor in September 2022 caused a large amount of erosion on the foreshore, the net erosion amounted to 8,522 cubic meters; and Typhoon Doksuri in July 2023 caused 979 cubic meters net erosion and 568 cubic meters net accumulation on the beach. Compared with the subsequent Typhoon Khanun, which eroded 4743 cubic meters and accumulated 975 cubic meters, the changes are very little. During the period affected by the northeast monsoon, particle size analysis of sediment sampling found that the average particle size sampled on May 2023 was larger than that on September 2022, and the average particle size on the west side of the beach increased from 0.384mm to 0.407mm, while the average particle size on the beach The east side increased from an average of 0.332mm to 0.355mm. However, due to the lack of relatively short-term particle size analysis data, the mechanism or process of particle size changes at each sampling point is not yet clear. There is still insufficient evidence to explain the observed long-term intervals and seasonal particle size changes. or extrapolate inference the results of changes in particle size. However, it was found from the elevation changes of the sampling points that during the northeast monsoon period, when the waves are higher, the waves can erode to within 42 meters of the sand fence, and during the non-northeast monsoon season, it is difficult for the waves to reach there.