Longshore sediment

NRCEST

Longshore sediment

Chang Hsien Kuo

國立交通大學土木工程學系中華民國 102年 5月 2日

Importance of longshore currentInterception of longshore sediment by groins ( groin effect)

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Observation of longshore current

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Classification of the longshore transport methods

The table is made by Tomasicchio et al. (2013) on Coastal Eng.

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Prediction of longshore sediment transport

Il : Immersed weight transport(kg/yr)Ql : volume transport rate (m3/yr)n: in-place sediment porosity (~0.4)

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Energy flux method

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Energy flux method-K coefficient

K=0.92 (Shore protection manual Is K constant?

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Energy flux method-K coefficientBailard (1981, 1984) developed an energy-based model, which presents K as a function of the breaker angle, and the ratio of the orbital velocity magnitude and the sediment fall speed.

scmu

scmw

mb

b

f

/28333;152.0

;/5.205.2

bmb gdu2

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Energy flux method-K coefficientDel valle, Medina and Losada (1993) presented the K relating to grain size

505.24.1 deK

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Longshore current method

W: the width of surf zone; Vs: the measured longshore current at a point in the surf zone;Y;the distance to the measured current from the shorelineV/V0:Longuet-Higgins dimensionless longhore current velocityCf`~ 0.01

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Practice of longshore sediment transport

Wave breaker from wave transformation where the subcsript refers to offshore conditions

D1=27m;H1=0.93m;α1=33 °;T=8.4sec;

From Airy wave theoryc1=12.2m/c;cg=7.6m/s;

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Practice of longshore sediment transport

Hb=1.2m ;αb=8.8 °;

Percent annual occurrence is 32.7%

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Formulae for longshore transport methods

CERC (1984)

Kamphuis (1991)

Hijum and Pilarczyk (1991)

Van der Meer (1990)

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Formulae for longshore transport methods

Chadwick (1989)

Van der Meer and Veldman (1992)

Virjling et al. (1991)

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Cross-shore distribution of longshore sedimentKana (1978) indicated that for spilling waves, sediment concentration rapidly increased inside the breakingpoint, than remained relatively constant under the bore. For plunging waves, sediment concentration peaked within a few meters of the breakpoint.

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Cross-shore distribution of longshore sediment

Bodge and Dean (1986,1987) showed two cross-shore distribution profiles from field experiments indicating that the presentence of a maxima in the outer surf zone just shoreward of the breaker zone and a second maxima in the swash zone.

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Cross-shore distribution of longshore sediment

For spilling, plunging, and collapsing waves (laboratory study) 1. Error bars reflect the uncertainties in the assessed local magnitude of

cross-shore transport, updrift limit of impoundment, and/or the degree of groin bypassing.

2. The transport distribution is bimodal with peaks at the shoreline and at the mid-outer surf zone.

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Cross-shore distribution of longshore sediment

The relative significant of the peaks was seen to shift from the near breakpoint peak to the near-shoreline peak as the breaking condition varied from spilling to collapsing

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Cross-shore distribution of longshore sediment

1. Longshore transport seaward of the breakingpoint represented about 10 to 20 percent of the total.

2. Swash zone transport accounted for at least 5 to 60 percent of the total for spilling to collapsing conditions, respectively.

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Cross-shore distribution of longshore sediment

1. Significant levels of transport may occur at and above the shoreline.

2. About 10 to 30 percent of the total transport occurs seaward of the breaker line.

3. Maximum local transport has been noted within shoreward half of the surf zone as often as within the seaward half.

4. Greater transport is often associated with shallow depths and breaking waves (i.e., breaking bars and the shoreline)

5. Field measurements demonstrate great variability in the shape of the longshore transport distribution profile across shore.

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Theoretical approach to Cross-shore distribution of longshore sediment

1. Bodge and Dean tested five existing cross-shore distribution relationships, which were concluded to give from fair to poor correlation with measurements.

2. They proposed a relationship for the cross-shore distributin of longshore sediment transport which assumes that sediment is mobilized in proportional to the local rate of wave energy dissipation per unit volume, and transport alongshore by an ongoing current

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Recent study-1

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Recent study-1

The wave height and the set-up are uniform. The wave field is approximately homogeneous for other waves

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Recent study-1Vertical profile over plane beach for regular and irregular waves

regular waves irregular waves

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Recent study-1Vertical profile over bar beach for regular and irregular waves

regular waves irregular waves

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Recent study-2

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Recent study-2

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Recent study-2Duck85 experiment

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Recent study-2SuperDuck experiment

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Recent study-2SandyDuck experiment

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Recent study-2

overestimation

underestimation

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Recent study-21. The VR formula gave the most reliable predictions over the entire range of wvae

conditions (swell and storm) studied, based on criteria involving the scatter, trend, and clustering of predictions around the measurements.

2. The AW formula gave satisfactory results for all conditions, but scatter was marked both swell and storm.

3. Regarding the scatter, the BI formula yielded improved predictions compared to AW, although the transport was systematically overestimated during swell and underetimated during storm.

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Recent study-3

Review of LT methodsEvaluation

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Recent study-3

Database (9 tests)

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Recent study-3

;_23

;_239

;_62

**

**

**

beachSandyforN

beachGravelforN

sbreakwaterBermforN

s

s

s

overestimation

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Recent study-3