前導下沉N型海嘯波於不同傳遞距離之溯升

Abstract

臺灣四面環海且位於環太平洋地震帶上，海嘯一直都是潛在的天然災害之一，在早期的海嘯研究當中，大多以孤立波作為海嘯波替代模型進行研究，後來有學者發現前導下沉N型波因具有一領先波谷，能夠模擬海嘯來臨前的海水退位現象，因此被認為比起孤立波更適合當作海嘯波替代模型。本研究在臺灣大學工程科學及海洋工程學系總長30公尺的二維斷面水槽進行實驗，基於N型波造波理論於1:10斜坡進行溯升實驗，探討孤立波與N型波之間的溯升差異。 本研究共蒐集了24組孤立波和88組N型波溯升結果以及波破碎情形，並探討斜板置於距離造波板13.5公尺以及7公尺之結果比較，而根據實驗結果，可將N型波溯升表現分類為：一般N型波、段波狀N型波、陡峭型N型波以及強烈破碎N型波四種類別，對於一般非破碎N型波其溯升值會與孤立波相當，而有破碎之一般N型波溯升值則會小於孤立波，這是受到前導下沉波谷的影響使得N型波產生較強烈的破碎導致能量耗散。而段波狀N型波大部分的溯升值會大於孤立波，只有少數段波特徵不明顯或是沒有前導下沉波谷導致波形變成數個分離孤立波，其溯升值會小於或等於孤立波。陡峭形N型波在非破碎的情況下溯升值會大於孤立波，而強烈破碎N型波因破碎運動強烈使溯升前線混亂，進而導致溯升值無統一趨勢。綜觀13.5公尺以及7公尺的溯升結果，排除因傳遞距離過短受到反射波影響溯升結果之後皆可得到相似的分類，可以認定無論傳播距離遠近，皆能透過N型波上斜坡前的最終波形來判斷與孤立波溯升值之間的關係。

Taiwan is an island located on the Pacific Ring of Fire, and tsunamis have always been one of the potential natural disasters. In the early studies of tsunami, solitary waves were commonly used as an alternative model. However, some scholars found that the leading depression of N-waves have a leading wave trough, which can simulate the water drawdown before the arrival of a tsunami. Therefore, N-waves are considered more suitable alternative model for tsunami waves than solitary waves. This research investigates the propagation and runup of N-waves using experimental methods. The experiments are conducted in a two-dimensional wave maker flume with a total length of 30 meters at the Department of Engineering Science and Ocean Engineering, National Taiwan University. Based on the theoretical formulation for the generation of N-waves on a 1 on 10 slope to investigate the differences between solitary waves and N-waves. This study collected 24 sets of solitary wave data and 88 sets of N-wave data, including runup and wave breaking conditions, to compare the results of placing the plate at distances of 13.5 meters and 7 meters from the wave generator. According to the experimental results, the N-wave runup performance can be classified into four categories: general N-waves, bore-like N-waves, steep N-waves and strong breaking N-waves. For general non-breaking N-waves, their runup values are comparable to solitary waves. However, for breaking general N-waves, their runup values are smaller than solitary waves. This is due to the influence of the leading depression wave trough, which leads to more intense wave breaking and energy dissipation in N-wave. For most of the bore-like N-waves, their runup values are greater than solitary waves. Only a few have less bore characteristic or lacking a leading depression wave trough that causes the waveform to become several separated solitary waves, with their runup values being less than or equal to solitary waves. Steep N-waves exhibit higher runup values than solitary waves in the non-breaking condition, while strong breaking N-waves due to strong breaking, which lead to uneven runup lines and inconsistent trends in runup values. By observing the runup results at distances of 13.5 meters and 7 meters, after excluding the influence of reflected waves due to short propagation distances, similar classifications can be obtained. It can be concluded that regardless of the propagation distance, the relationship between runup values and solitary waves can be determined by examining the final wave shape in front of the slope in N-waves.