複合斷面平衡海灘上之沿岸流

Abstract

自然海灘傾向形成平衡海灘剖面之穩定形狀，此平衡海灘剖面一般在碎 波帶內 較近岸處為均勻坡度而較遠岸處呈現非均勻坡度之下凹形底床， 而此組合之斷 面稱為複合平衡斷面。本文基於此種海灘剖面形式，推導 出複合斷面平衡海灘 上沿岸流之理論解析，文中並對底床剖面形狀，側 向混和應力，剖面交界位置 ，及溯上水平距離做討論。由結果顯示，碎 波點以外底床剖面形狀對沿岸流分 佈影響並不大，但在碎波點內受複合 斷面平衡海灘之組合剖面影響，使得沿岸 流速分佈較為均勻，且若為不 同之底床剖面，則有不同之沿岸流速分佈，對複 合斷面平衡海灘而言， 側向混和應力增加，碎波帶內沿岸流強度在較近岸比遠 岸為大，剖面交 界位置離岸愈遠，也就是均勻坡度灘面寬度增大，沿岸流速會 減小且向 外海移動，至於溯上水平距離的影響，只及於接近岸處流速之高低， 對 剖面交界位置以外之沿岸流速是沒有影響的。 Natural beaches tend to exhibit an equilibrium profile that is planar nearshore and nonplanar, concave-up offshore. Its analytical solution for the longshore current on composite beach profiles was given in this paper. The effects of the shape of bottom beach profile, lateral turbulent mixing stress, the location of the intersection between the planar and nonplanar profiles, and the landward limit of run-up on longshore current inside surf zone are discussed in the present paper. The rusults show that stronger effect of the composite beach profiles acts on longshore current inside surf zone than outside surf zone, and different beach profiles have their own longshore current. For composite beach profiles, the longshore current becomes stronger near the coast than far from the coast when lateral turbulent stresses increases. When the width of the planar beach face increases, the location of the maximum longshore current moves closer to the seaward. The landward limit of run-up has only effects on current near the nearshore but not for the outside of the location of profiles intersection.