Nearshore circulation system

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Presentation transcript:

Nearshore circulation system Chang Hsien Kuo 國立交通大學土木工程學系中華民國102年4月18日

Surf zone (type of breakers)

Movement of sand on the beach Movement parallel (↔) to shoreline Caused by wave refraction (bending) Each wave transports sand either upcoast or downcoast Huge volumes of sand are moved within the surf zone The beach resembles a “river of sand”

Wave set-up Cross-shore balance of momentum

Wave run-up Regular wave (Miche, 1951) Irregular wave (Mase, 1989) Surf Parameter ξ 0 = 𝒕𝒂𝒏β 𝑯 𝟎 / 𝑳 𝟎 :

Longshore current and longshore drift Longshore current = zigzag movement of water in the surf zone Longshore drift = movement of sediment caused by longshore current

Nearshore circulation system Longshore current and Rip current Depth-averaged momentum equations and continuity Eq.

Wave induced longshore current Longuet-Higgins (1970) Assumption: 1. Longshore homogeneity (𝑼=𝟎,𝝏𝒚=𝟎) 2.No wind forcing (Rs=0) 3. Uniform sloping bottom (d=x*tanβ)

Wave induced longshore current Wave forcing Bottom friction 紊流渦動滯度模式 (eddy viscosity model), Reynolds stress P: lateral mixing coefficient

longshore current Wave forcing Slope-type

longshore current Normalized variables and equations Special case: p=1 (linear ratio of eddy viscosity to offshore difference) ; q=1 (uniform slope)

Solution of Euler equations Particular solution Eigen value

Boundary conditions Four conditions Solution

Boundary conditions Chang, H.K. and H.W. Hsu (1996), Longshore current on an equilibrium beach,” China Ocean Engineering, 10(2), 181-192.

Mean longshore current longshore current at mid-surf zone is regarded as mean longshore current Komar and Inman, 1970 um : orbital velocity Komar, 1979 Hbr : root-mean-square wave breaker height Hbs : significant wave breaker height Shore protection manual, 1984 tanβ : beach slope

Observed longshore current Wave height and current at surf zone

Observed cross-shore current Four conditions

Recent study-1 Hasaki coast of Japan

Recent study-1 Marine climate: height and direction of wave and wind Longshore current

Recent study-1 Longshore current in northward or southward directions

Recent study-1 shear stress at the bottom Wind induced shear stress at the surface

Recent study-1 Radiation stress And shear stress at the bottom

Recent study-1 Longshore current of different season

Recent study-2

Recent study-2 Wave height variations by seven models

Recent study-2 Longshore current by seven models

Recent study-2 Comparison with measured longshore current

Recent study-2 Comparison with measured longshore current for spilling breaker

Recent study-2 Comparison with measured longshore current for plunging breaker

Recent study-2 Comparison with measured longshore current for some cases of Leadbetter

Recent study-2 Comparison with measured longshore current for some cases of DELII

Recent study-2 Comparison with measured longshore current for some cases of HORS

Recent study-2 Suggested parameters