前導下沉N型海嘯波之傳遞與溯升

Abstract

本研究中主要利用前導下沉N型波作為海嘯替代模型，並基於N型波造波理論於台灣大學工程科學與海洋工程學系之長20公尺二維斷面水槽進行實驗研究，並探討N型波於斜度1:10之斜坡上溯升過程。 在眾多海嘯資料記載中皆觀察到在海嘯來臨前海岸線會出現海水退位之情形，針對此物理現象我們將以波谷領先之N型波作為海嘯替代模型，並配合適用於推板式造波機之造波理論，以實驗水槽為主OpenFOAM數值造波水槽為輔，對N型海嘯波進行研究。 利用控制變量方法探討不同控制變數對於N型波傳遞之影響。針對N型波與孤立波進行多組溯升實驗，使用均方根誤差方法計算孤立波與N型波之差異並進行歸類，最後比較N型波與孤立波溯升之差異，並得出對於破碎型N型波而言，其波浪溯升會小於對應正振幅之孤立波的結論，其原因在於前導下沉波會造成較強烈的破碎現象。利用影像處理方法求出平均溯升與溯升前線之標準差，以量化溯升線之亂度，最後提出以史托克斯數作為判別波形相似性之方法，當N型波領先波峰之史托克斯數等於1.0時，即定義波峰為遠場孤立波，提供後人一判別與孤立波之波形相似性之方法，最後透過自身擬合之經驗公式，探討遠場孤立波與N型波之溯升差異。

In many recent events, tsunamis have been observed to lead with significant water drawdown. How this wave drawdown affects the tsunami runup requires further investigation. In this study the propagation and runup of leading depression N-waves are investigated using combination of experimental and numerical methods. Attempts to find out the effect of leading depression wave on runup. Based on the theoretical formulation for generation of N-waves, N-waves were generated in the 20 m long wave maker flume of National Taiwan University by using piston type wavemaker and these waves shoal and runup on a 1 on 10 slope. A set of N-wave and solitary wave experiments were carried out and discussed the influence of different control variables on N-wave propagation. By using root-mean-square deviation method to classify N-waves and further discussed the differences between N-waves runup and solitary waves runup. This study found out that for wave breaking case, N-waves runup will be smaller than solitary waves runup because the leading depression waves will cause a stronger breaking waves. At the same time, the average and standard deviation of the runup lines was found by using image processing, in order to compare the uniformity of runup lines between solitary waves and N-waves. In the end, we proposed Stokes number method to quantitatively evaluate the similarity between leading wave crest of N-wave and solitary wave by using open source CFD software OpenFOAM. When the Stokes number of N-wave reach 1.0, the leading wave of N-wave is defined as far-field solitary wave and discuss the runup difference between far-field solitary wave and N-wave by using empirical formula of numerical simulation.