Grain-size selective longshore transport at a headland-bay beach: insights from a sediment

tracer experiment

M. Ribeiro, R. Taborda, J. Duarte and A. Rodrigues

5as Jornadas de Engenharia Hidrográfica19th June 2018

Motivation and Goal

Sediment characteristics:• size• shape• density

beach slopebreaking type

Longshore Sediment Transport (LST)

↓

site specific factors Understand and quantify the effects on

LST rates

→ wave power

Study Site

W

WE

E

Cabo da Roca

Praia Grande

Tracer Preparation

1.7 tons of tracer

Tracer Method

Continuous injection or dilution methodDuane, D.B. and James, W.R. (1980). Littoral transport in the surf zone elucidated by an Eulerian sediment tracer

experiment. Journal of Sedimentary Petrology, 50(3): 929–942.

𝑸 =𝒒

𝒄𝒆𝒒

𝑞 =𝑞𝑑𝑚

1 − 𝑝)𝜌𝑠

Longshore drift direction

Constant injection rate

𝑞𝑑𝑚Elapsed time

Conce

ntr

ation

Elapsed time

Conce

ntr

ation

𝒄𝒆𝒒

Tracer Experiment

Oceanographic conditions

𝐻𝑠 0.7 m

𝑇𝑧 6 s

𝐷𝑖𝑟 W and NW

𝑣𝑙0.6 m.s-1

North-directed

8th September 2014

Tracer Experiment

Sediment sampling(1 cm thickness)

Tracer injection16 kg.min-1

Tracer injection

Sampling

Data Processing and Analysis

SedPhoto2 toolbox - MATLAB

• Grain-size

• Tracer dilution

Results

Results

00:00 01:00 02:00 03:00 04:000

0.005

0.01

0.015

0.02Overall particles

00:00 01:00 02:00 03:00 04:000

1

2

3

4

5x 10

-4 Medium gravel

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5

2x 10

-3 Fine gravel

00:00 01:00 02:00 03:00 04:000

0.002

0.004

0.006

0.008

0.01Very coarse sand

00:00 01:00 02:00 03:00 04:000

1

2

3

4

5

6x 10

-3 Coarse sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-3 Medium sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-4 Fine sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-4 Fine sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-3 Medium sand

00:00 01:00 02:00 03:00 04:000

1

2

3

4

5

6x 10

-3 Coarse sand

00:00 01:00 02:00 03:00 04:000

0.002

0.004

0.006

0.008

0.01Very coarse sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5

2x 10

-3 Fine gravel

00:00 01:00 02:00 03:00 04:000

1

2

3

4

5x 10

-4 Medium gravel

00:00 01:00 02:00 03:00 04:000

0.005

0.01

0.015

0.02Overall particles

00:00 01:00 02:00 03:00 04:000

0.005

0.01

0.015

0.02Overall particles

00:00 01:00 02:00 03:00 04:000

1

2

3

4

5x 10

-4 Medium gravel

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5

2x 10

-3 Fine gravel

00:00 01:00 02:00 03:00 04:000

0.002

0.004

0.006

0.008

0.01Very coarse sand

00:00 01:00 02:00 03:00 04:000

1

2

3

4

5

6x 10

-3 Coarse sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-3 Medium sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-4 Fine sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-4 Fine sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-3 Medium sand

00:00 01:00 02:00 03:00 04:000

1

2

3

4

5

6x 10

-3 Coarse sand

00:00 01:00 02:00 03:00 04:000

0.002

0.004

0.006

0.008

0.01Very coarse sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5

2x 10

-3 Fine gravel

00:00 01:00 02:00 03:00 04:000

1

2

3

4

5x 10

-4 Medium gravel

00:00 01:00 02:00 03:00 04:000

0.005

0.01

0.015

0.02Overall particles

00:00 01:00 02:00 03:00 04:000

0.005

0.01

0.015

0.02Overall particles

00:00 01:00 02:00 03:00 04:000

1

2

3

4

5x 10

-4 Medium gravel

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5

2x 10

-3 Fine gravel

00:00 01:00 02:00 03:00 04:000

0.002

0.004

0.006

0.008

0.01Very coarse sand

00:00 01:00 02:00 03:00 04:000

1

2

3

4

5

6x 10

-3 Coarse sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-3 Medium sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-4 Fine sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-4 Fine sand

-25 m

25 m

50 m

75 m

100 m

125 m

150 m

175 m

200 m

225 m

250 m

275 m

300 m

325 m

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-4 Fine sand

-25 m

25 m

50 m

75 m

100 m

125 m

150 m

175 m

200 m

225 m

250 m

275 m

300 m

325 m

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-4 Fine sand

-25 m

25 m

50 m

75 m

100 m

125 m

150 m

175 m

200 m

225 m

250 m

275 m

300 m

325 m

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-4 Fine sand

-25 m

25 m

50 m

75 m

100 m

125 m

150 m

175 m

200 m

225 m

250 m

275 m

300 m

325 m

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-4 Fine sand

-25 m

25 m

50 m

75 m

100 m

125 m

150 m

175 m

200 m

225 m

250 m

275 m

300 m

325 m

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-4 Fine sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-3 Medium sand

00:00 01:00 02:00 03:00 04:000

1

2

3

4

5

6x 10

-3 Coarse sand

00:00 01:00 02:00 03:00 04:000

0.002

0.004

0.006

0.008

0.01Very coarse sand

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5

2x 10

-3 Fine gravel

00:00 01:00 02:00 03:00 04:000

1

2

3

4

5x 10

-4 Medium gravel

00:00 01:00 02:00 03:00 04:000

0.005

0.01

0.015

0.02Overall particles

Conce

ntr

ation

Elapsed time

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-4 Fine sand

-25 m

25 m

50 m

75 m

100 m

125 m

150 m

175 m

200 m

225 m

250 m

275 m

300 m

325 m

00:00 01:00 02:00 03:00 04:000

0.5

1

1.5x 10

-4 Fine sand

-25 m

25 m

50 m

75 m

100 m

125 m

150 m

175 m

200 m

225 m

250 m

275 m

300 m

325 m

Discussion

Selective transport

Longshore drift direction

Finer sediments travel faster alongshore

Discussion

𝑄 = 𝑣𝑎 × 𝐴

𝑄𝑠

𝑄=

𝑣𝑎𝑠

𝑣𝑎

Analytical solution of advection-diffusion equation

𝑐 𝑥, 𝑡 =𝑐02𝑒𝑟𝑓𝑐

𝑥 − 𝑣𝑎𝑡

4𝐷𝑡

𝑸 ??

Discussion

00:00 03:00 06:00 09:000

0.2

0.4

0.6

0.8

1x 10

-4 100

Tra

cer

dilu

tion

Elapsed time

data

AD model

ceq

asymptote

injection end

00:00 03:00 06:00 09:000

0.2

0.4

0.6

0.8

1x 10

-4 125

Tra

cer

dilu

tion

Elapsed time

data

AD model

ceq

asymptote

injection end

00:00 03:00 06:00 09:000

0.2

0.4

0.6

0.8

1x 10

-3 100

Tra

cer

dilu

tion

Elapsed time

data

AD model

ceq

asymptote

injection end

00:00 03:00 06:00 09:000

0.2

0.4

0.6

0.8

1x 10

-3 125

Tra

cer

dilu

tion

Elapsed time

data

AD model

ceq

asymptote

injection end

00:00 03:00 06:00 09:000

1

2

3

4x 10

-3 100

Tra

cer

dilu

tion

Elapsed time

data

AD model

ceq

asymptote

injection end

00:00 03:00 06:00 09:000

1

2

3

4x 10

-3 125

Tra

cer

dilu

tion

Elapsed time

data

AD model

ceq

asymptote

injection end

00:00 03:00 06:00 09:000

1

2

3

4x 10

-3 100

Tra

cer

dilu

tion

Elapsed time

data

AD model

ceq

asymptote

injection end

00:00 03:00 06:00 09:000

1

2

3

4x 10

-3 125

Tra

cer

dilu

tion

Elapsed time

data

AD model

ceq

asymptote

injection end

00:00 03:00 06:00 09:000

0.002

0.004

0.006

0.008

0.01

100

Tra

cer

dilu

tion

Elapsed time

data

AD model

ceq

asymptote

injection end

00:00 03:00 06:00 09:000

0.002

0.004

0.006

0.008

0.01

125

Tra

cer

dilu

tion

Elapsed time

data

AD model

ceq

asymptote

injection end

Overall particles Fine sandMedium sandCoarse sandVery coarse sand

Concentr

atio

nC

oncentr

atio

n

Conclusions

del Valle, R., Medina, R., Losada, M. (1993). Dependence of coefficient K on grain size. Journal of Waterway, Port, Coastal and Ocean Engineering, 119: 568-574.

Komar, P.D. (1988). Environmental controls on littoral sand transport. Proceedings of the 21st International Conference on Coastal Engineering, 1238-1252, ASCE.

This experiment allowed to estimate the effects of grain-size on longshore drift rates.

The results show the transport rate decreases with the increase of grain-size, a result in agreement with previous works (e.g. Komar, 1988; del Valle et al., 1993).

AcknowledgementsThis work was supported by a PhD grant (SFRH/BD/79126/2011) and by the project Beach to Canyon (PTDC/MAR/114674/2009), both

funded by Fundação para a Ciência e Tecnologia.

Muito obrigada!