斜向波浪對離岸堤背後灘線及堆砂效益之影響

Abstract

本文係藉由模型實驗的方式，探討斜向波浪作用下離岸堤背後灘線的變化及堆砂效益。本模型實驗包含了正向及斜向入射之波浪條件。實驗結果顯示，在斜向波浪作用下平衡灘線將形成不對稱之砂舌形狀，隨著波浪入射角度的增加，砂舌頂點自堤中心垂線處逐漸往下游偏移，且灘線到達平衡的時間亦隨之變長。此外，下游平衡灘線之後退量亦隨波浪入射角度的增加而變大。而堤後砂舌面積有隨著波浪入射角度變大而逐漸減少的趨勢，尤其試驗波浪在入射角度為40度時，堤後灘線雖仍具砂舌的形狀，但卻呈現大幅度的縮減而不再突出於初始海岸線，此意味在大角度之斜向波浪入射條件下，離岸堤可能失去堤後堆砂的效果。

In this paper, a series of experiments were conducted to investigate the shoreline changes and sand entrapments behind single offshore breakwater under obliquely incident waves. Three waves of obliquities 150, 300, 400 and normally incident wave were conducted in the experiment. The experimental results indicate that the asymmetric salient was formed and the salient apex drifted downstream behind the offshore breakwater under oblique waves. It also took longer for the shoreline to reach equilibrium as the wave obliquities increasing. When the angle of the incident waves became bigger, the salient area reduced as well. Especially when the incident wave angle was at 400 in the experiments, the equilibrium shoreline still kept the shape of the salient, but it reduced substantially and did not stretch out to the initial shoreline. It implies the offshore breakwater maybe lost the function of sand entrapments and could not protect the shoreline from erosion adequately in the case of the large angle of obliquely incident waves.