應用無人載具於現地波浪溯升的初步研究

Abstract

波浪溯升是近岸海灘漂沙的重要影響因子。因為現地自然海灘溯升相當複雜及觀測儀器的限制往昔在此方面的研究相當少。本文利用無人載具搭載全片幅單眼相機於現地的拍攝，紀錄波浪溯升的運動過程，並透過影像解算至實際座標的統計分析，探討現地波浪溯升的特性。系統化的影像前置處理包括，影像灰階化、影像強化、形態學及Canny法，及能剔除可能誤判水線的離群值的分段線性迴歸法，諸技巧可擷取每張影像清楚的連續平滑水線。本文特別提出符合現地海灘底床的雙線形底床坡度法，以取代往昔的等效坡度法。 由本方法在新月沙灣所獲得的實測R_(2%)溯升與往昔自然海岸的經驗式比較，實測值與Mase (1989)公式的計算值較接近，二者溯升的均方根誤差為0.096m，平均偏差為-0.043m，相對RMSE及Bias分別為27.1%及12.1%。 新月沙灣有範圍不大且高度小的潛沒沙洲。由長波理論計算長波通過潛沒沙洲的透過率約為0.93，影響溯升不大。冬天北向入射的波浪，有南向的速度分量，導致北側溯升的水體會進入潛沒沙洲的谷處，水體的累積增大了溯升，可解釋本次觀測波浪溯升特性。現地的自然沙灘，地形複雜，造成波浪在不同位置，有不同的溯升高度及延時的現象，自然沙灘的波浪溯升是值得再進一步研究的學術課題及有工程應用的價值。

Runup is an important factor in the coastal sediments. Few previous studies on in-site wave runup were fulfilled due to complexity of runup in a natural beach and limited for in-site observation. In this study an unmanned aerial system with a full-chip camera was set up to have living recording the whole process of wave runup. Through bilinear transformation between image coordinates and world coordinates for each picture instant wave runup along the shoreline was computed to explore its characteristics. Some techniques of image processing were used to obtain clear waterline of each picture, including the image binarization, enhancement, morphological and the Canny algorithms, and piecewise linear regression. An applicable method of bilinear beach slopes instead of the equivalent slope method was proposed to closely represent the actual beach profile. A comparison of R_(2%) between the measured and estimated ones by empirical formula of Mase (1989) shows less difference by the mean squared error of 0.096m and bias of -0.043 m, indicating relative values of 27.1% and 12.1%, respectively. The new-moon beach as the present research base has a submerged small bar. According to the long wave theory, the transmitted coefficient of a long wave over the submerged bar is about 0.93, which has slight effect on runup. Northern waves during the winter time, have southward velocity component to push water to the trough of the submerged bar and to increase runup at this portion. Wave runup on Topography of the natural beach is commonly complex so that wave runup on the natural beach are different and have time delay in different positions. This is an interesting academic topic and the results are valuable to coastal engineering.