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!