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題名 | 人工沙洲形狀對波浪反射率之影響=Bragg Refection of Gravity Waves by Artificial Bars |
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作者 | 張憲國; | 書刊名 | 港灣技術 |
卷期 | 12 1997.01[民86.01] |
頁次 | 頁23-38 |
分類號 | 443.31 |
關鍵詞 | 人工沙洲; 反射率; Artificial bar; Bragg reflection; |
語文 | 中文(Chinese) |
中文摘要 | 本文主要探討三種不同形狀之人工沙洲上波浪之反射率,其反射率之計算式乃引用Miles(1981)[1]公式推導所得。所選擇cosine型、三角型及矩型之三種人工沙洲中,由理論解析發現三者當人工沙洲之間距為波浪波長之半時,均會產生布雷格反射,此時反射率為最大,且人工沙洲之形狀對於波浪反射率略有影響。在三種人工沙洲形狀相同佈置情況,波浪在布雷格反射率產生條件時,其反射率以矩形為最佳,而三角型最差。對cosine型及三角型之人工沙洲而言,其基部與間距比值愈大,波浪反射率愈大,而呈單調增加之趨勢, 但對矩形人工沙洲,波浪反射率最大值發生在B/S=0.5之情況,呈現下凹曲線現象。人工沙洲之個數對波浪反射率有正比之關係,如在2S/L=1條件下,當人工沙洲個數達8個以上時,波浪幾乎可完全反射。人工沙洲應用至現場可能遇上潮位變化及波浪週期不同等因素,改變波長,因而影響波浪反射率。由理論結果發現,因增加水深而改變波浪反射率比水深變淺之影響較為明顯。波浪週期改變在10%以內,反射率即改變20%,如果是週期改變在20%以內,反射率急速降至為0。 |
英文摘要 | The formula for the reflection of a surface wave over artificial bars with different shapes placed discretely on the seabed is derived in the present paper based on Miles's (1981) [1] formula. The chosen three kinds of artificaial bars are rectangular, triangle and sinusoidal. The reflection coefficient displays a resonant Bragg peak centred around the point where the span of the artificial bars is one-half the surface wavelength for three kinds of artificial bras. When Bragg refection occurs, wave reflection by rectangular artificial bars is larger than that by the sinusoidal ones or by triangular ones.For both sinusoidal artificial bars and triangular ones the Bragg reflection monoincreases as the ratio of base to span of bars increases. However, for rectangular artificial bars the Bragg reflection performs a concave downward curve and reaches a maximum value when B/S=0.5. When the number of the sinusoidual artificial bars is more than 8, the surafce wave will be fully reflected by bars near the Bragg-scattering condition. Based on the wave theory, the wave length relates to water depth and wave period. If artificial bars are applied to the coast, the desired reflection will drop down because of the change of tide level and wave period. It is shown that the increase of water depth has more negative effects on wave reflection than the decrease of water depth and that wave reflection is close to zero when wave period deviates from the specified wave period by 20%. |
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