單音束測深資料應用在海岸地形變遷的研究-以竹南海岸區域為例

Abstract

在近岸區域淺水環境主要使用單音束測深進行水深調查，使用單音束資料做地形變遷調查，要充分了解測深的誤差量及其來源，首先必須進行誤差分析，了解誤差量的大小，適當的定義地形變化數值上限為多少，誤差量越小解析度越高，所能夠展示的地形變化量越精準，其中最需要分析的是各年次測量成果是否發生SETTLEMENT效應，所謂SETTLEMENT指的是在測量航行中，音鼓入水深度有長週期的改變，其因素包括頂浪、順浪的行進效應，船身重量的變化以及音鼓進入水中的數值裝置誤差等，造成地形變遷出現和測線方向平行的地形變化量，是不合理的，通常SETTLEMENT效應一致時，測深誤差並不會展現在平常誤差分析結果上，所以在不同年份(次)所進行的測深成果，必須透過剖面來做比較和分析，以消除所謂SETTLEMENT效應。我們依照72條測線各自相差的平均值調整2019年資料，參考誤差分析的成果，假設兩年資料地形變化量在±0.3M為沒有地形變化，繪製了地形變化圖，海岸地形在離岸區域地形穩定，除非遇到極端事件(颱風)，否則都不會有短周期的變化，長時間下也沒有明顯的淤積和侵蝕現象，近濱區地形隨著水流和沉積物供應量變動而產生地形變化，在特殊環境(港口堤防)之外，地形變化圖上交錯出現且平行海岸的侵蝕淤積帶，可能一個沙堤的移動、兩個沙堤移動或是沙堤消失變成平坦地形，需透過觀察資料剖面才能知道變化細節。

Single beam echo-sounding is commonly used to investigate coastal change in shallow water. In order to avoid recognizing bathymetric change caused by surveying error, error analysis is essential. The most important part of using single beam echo-sounding data is to check if there is settlement effect during the measurement. The settlement effect is relatively position change of transducer depth, which may be caused by the followings: assemble error, draft change, sail following or against waves. Error analysis may not discover the settlement effect. Therefore, we check the error from settlement effect by comparing topographical profiles obtained in different survey period. Base on the result of error analysis, topographical difference of less than 0.3 meters is assumed there is no topographical change in the calculated period of time. Offshore Zhunan coast, results indicate the topography of offshore area is stable. Nearshore area is mainly affected by longshore current. Current direction and sediment supply would lead seafloor to change. Sand bars would become bigger or smaller and have some movement in the direction vertical to shoreline