利用沿岸流推估沿岸漂沙量之研究

Abstract

本文首先提出利用沿岸流來推估漂沙量的模式。本文乃引用Longuet-Higgins (1970)的沿岸流理論來計算沿岸流的分佈，藉由沙粒起動剪應力、Shields數等條件，來決定沙粒的起動速度，及界定沿岸漂沙的範圍。另外，聯合Bagnold (1956)的底床載載推估方式及Van Rijn (1984)的懸浮載推估方式，本文推導出沿岸漂沙總輸沙量的公式並探討底床載及懸浮載佔沿岸總輸砂量的比例。當總沿岸漂沙量可表示為與波浪能通率成正比關係時，其比值為 值，而與底床坡度、波浪尖銳度及相對粒徑有關，而並非如Komar 和Inman (1970)和CERC (1977)所提的常數，但本文的平均 值介於兩者之間。本文進一步分析 值與底床坡度、波浪尖銳度及相對粒徑成冪次方正比，此結果與Kamphuis (1990)結論相似。本文所提出沿岸漂量的公式，相當實用而精確的估算沿岸漂量，並可提供工程計算或數值模式的參考。

A new evaluation of longshore sediment transport resulted from threshold longshore was proposed in this paper. Longuet-Higgins’ (1970) theory of longshore current was chosen to calculate velocity distribution due to its explicit form. The threshold velocity of longshore current to start to move sands was decided by one-equation Shields parameter that was extended to the case of a sloping bed. The amount of longshore sediment was classified into bed-load and suspended load in this paper. The amount of bed-load and suspended load was estimated by the formula proposed by Bagnold (1956) and Van Rijn (1984), respectively. If the amount of littoral sand drift the was related with wave energy flux and a coefficient K, it is found that coefficient K depends on the beach slope, the wave steepness, and the sediment grain size. The average value of K is 0.53 and lies between those of Komar and Inmans’ (1970) and CERC’s (1977). Coefficient K was represented by a power law in terms of dimensionless environmental factors which are relative grain diameter, beach slope, and wave steepness. The value of K computed from the regressed formula is fairly close to the value of original K by error deviation within 27%. It is easy and accurate to obtain longshore sediment transport by the present formula when the offshore wave condition, sand size, and bottom slope is given.