Are We Over-designing or Under-designing Coastal Defence ...

Are We Over-designing or Under-designing Coastal Defence:

Interaction between Coastline and Wave Angle

Keming Hu, Greg Guthrie, Katarzyna Bozek

28 March 2017

Are We Over-designing or Under-designing Coastal Defence: Interaction between

Coastline and Wave Angle | 28 March 2017

Oblique waves

Estimating wave overtopping is one of the key processes in coastal flood risk

assessment and design of coastal flood defence schemes.

Most wave overtopping estimation tools were designed for shore-normal attacking

waves.

There is a common opinion that the assumption of shore-normal waves would give

conservative estimation.

This opinion is supported by EurOtop.

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What is the concern?

Reduction factor could be higher, over-design?

Concern in potential over-design of some sea defences where dominant waves are oblique. In some situations, over-design may cause flood defence scheme rejected due to financial or other reasons

Oblique waves may increase overtopping, under-design?

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Where do oblique waves occur?

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Oblique waves are

often seen in man-

made environment:

Dredged bathymetry

Hard defence



Numerical experiment using AMAZON

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AMAZON is a numerical engine solving the two-dimensional Non-Linear Shallow Water

Equations (NLSWE)

It is capable of simulating super-critical flow and capturing hydraulic jump, and

numerically stable

Its ability of capturing hydraulic jump and numerical stability leads to its use in

modelling wave overtopping.

It has been extensively used by Royal HaskoningDHV and its collaborators in

estimating wave overtopping.

AMAZON 2D is computational expensive but this constraint is being lessened

benefitting from improved computer power.



Numerical Experiment

Uniform waves along inlet

boundary

Slope wall

Vertical wall (single)

Vertical walls (double)

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Slope wall

Hs: 0.4m

Tp: 6s

JONSWAP spectrum

Water level: 2.76m

Crest: 3.54m

Number of waves: 1000

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Slope wall

Left 1/5 defence

receives more

overtopping than

predicted by 1D

simulation

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Slope wall

Left 1/3 receives more

overtopping than

predicted by 1D

simulation

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Slope wall

Increased part moved

towards centre.

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Amazon 2D animation

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Example

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Slope wall Total overtopping volume reduced; but

Spatially varied overtopping with

higher overtopping in some locations;

Effect of rolling waves.

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30deg 45deg 60deg

Points 1-2 3.66 1.98 2.04

Points 3-4 3.52 2.51 2.13

Points 5-6 1.17 2.51 2.33

Points 7-8 0.05 0.95 2.56

Points 9-10 0.00 0.06 1.81

Points 11-12 0.00 0.00 0.47

Points 13-14 0.00 0.00 0.01

Points 15-16 0.00 0.00 0.00

Points 16-17 0.00 0.00 0.00

Points 17-18 0.00 0.00 0.00

Points 19-20 0.00 0.00 0.00

Average 0.76 0.73 1.03

Reduction (%) 63% 65% 50%

2.08

2D1D




Hs: 0.4m

Tp: 6s

JONSWAP spectrum

Water level: 2.76m

Crest: 3.54m

Number of waves: 1000

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Left 1/4 defence

receives more

overtopping than

predicted by 1D

simulation

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Left 1/3 defence

receives more

overtopping than

predicted by 1D

simulation

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Left half receives

slightly higher

overtopping than

predicted by 1D

simulation

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Amazon 2D animation

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Vertical wall (single) Total overtopping volume reduced; but

Spatially varied overtopping with

higher overtopping in some locations;

Overall, less reduction than slope wall

due to reduced wave reflection effect

in addition to rolling wave effect

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30deg 45deg 60deg

Points 1-2 7.27 6.06 6.07

Points 3-4 7.66 6.54 6.00

Points 5-6 7.29 7.99 6.55

Points 7-8 5.07 7.16 6.72

Points 9-10 3.94 5.45 7.39

Points 11-12 3.74 3.04 7.02

Points 13-14 3.97 1.18 5.39

Points 15-16 3.98 0.91 2.98

Points 16-17 3.84 1.05 1.76

Points 17-18 3.63 1.24 0.91

Points 19-20 3.30 1.54 0.45

Average 4.88 3.83 4.66

Reduction (%) 17% 35% 21%

5.91

1D2D



Vertical walls (double)

Hs: 0.3m

Tp: 6s

JONSWAP spectrum

Water level: 3.76m

Crest (1st wall): 3.7m

Crest (2nd wall): 3.9m

Wave angle: 60º

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Vertical walls (double) – first wall

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Vertical walls (double) – second wall

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Summary

Interaction between coastline and wave angles is complex;

Overtopping reduction is not uniform along coastline and may be not as high as people

would expect due to effects of rolling and reflected waves;

Overall, total overtopping volume over simulated coastline is less comparing oblique

waves with shore-normal waves; but

With varied overtopping along coastline, overtopping can be higher at some locations

than the situation under shore-normal attacking waves;

Need further investigation into input waves, particularly, wave crest length and its

alignment along model boundary.

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Interaction between coastline and wave angle is

complex, another example.

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Questions?

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Are We Over-designing or Under-designing Coastal Defence ...

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