- Reduction of Wave Overtopping by Vetiver grass -

1Introduction Objectives Model Set-up Result Case study Conclusions

Reduction of wave overtoppingby Vetiver Grass

Presentation: Vu Minh Anh

- Reduction of Wave Overtopping by Vetiver grass -

2Introduction Objectives Model Set-up Result Case study Conclusions

Contents of presentation

Introduction

Physical model and model set-up

Results and Case study

Conclusions & Recommendations

Questions and Discussion

- Reduction of Wave Overtopping by Vetiver grass -

3Introduction Objectives Model Set-up Result Case study Conclusions

Violent wave overtopping at the sea dikes

- Reduction of Wave Overtopping by Vetiver grass -

4Introduction Objectives Model Set-up Result Case study Conclusions

Wave overtopping at the sea dikes

Dike

Wave Runup

Wave Overtopping

Waves

Initial profile Erosion profile on outer slope

Erosion profile on inner slope

Revetment

- Reduction of Wave Overtopping by Vetiver grass -

5Introduction Objectives Model Set-up Result Case study Conclusions

Design Sea Dike

Reduce wave overtoppig

Traditional method:

‘’Hard’’ revetments like concrete blocks, big rocks, glacial stones

- Reduction of Wave Overtopping by Vetiver grass -

6Introduction Objectives Model Set-up Result Case study Conclusions

‘’Hard’’ Revetments

- Reduction of Wave Overtopping by Vetiver grass -

7Introduction Objectives Model Set-up Result Case study Conclusions

Design Sea DikeReduce wave

overtopping height

Traditional method:

‘’Hard’’ revetments like concrete blocks, big rock, glacial stone

Methods which are low-cost and readily available

Combination between ‘’Hard’’ revetments with Vegetations

or Bioengineering methods.

- Reduction of Wave Overtopping by Vetiver grass -

8Introduction Objectives Model Set-up Result Case study Conclusions

‘’Soft’’ Solutions

- Reduction of Wave Overtopping by Vetiver grass -

9Introduction Objectives Model Set-up Result Case study Conclusions

Vetiver grassIn a number of tropical countries Vetiver grass is well-known bioengineering. Vertiver grasses grow naturally in clump with thin, long, and erect leaves.

- Reduction of Wave Overtopping by Vetiver grass -

10Introduction Objectives Model Set-up Result Case study Conclusions

Vetiver grassVetiver grasses as sea dike revetments

Lack of basic understanding for processes and propertiesLack of quantitative and qualitative knowledge of the protection on the outer slope of the sea dike by Vetiver

Addressing previous information: reduce wave overtopping by Vetiver grasses

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Objectives

The hydraulics of flow with Vetiver grass

The interaction between flow velocity and flow depth in cases of Vetiver hedge in relation to the reduction of wave overtopping

Improving the guidelines in designing sea dike dimensions

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Model set-up

R 1

B

ChB

vB

vc

h c

R 2

Full scale

Vetiver grass

Wave parameters in front of Vetiver hedge.

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Model set-up

Overview of experimental set-up

Vetiver hedge

Reservoir

Slope

Flume

Gate

Root

Leaves

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Model set-upLocations and instruments

EM S8 EM S12

S lope 1:30

W ater system

Reservo ir

G 27 G 26 G 25 G 24

Gate

Electromagnetic Flow Velocity Meter - EMSWave Gauge Height Meter - GHM

15

- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Model set-up

CaseDensity of grass

(Stem/m2)Water level inside reservoir

hr (cm)

Case 1 Without grass 50, 45, 40, 35

Case2 530 50, 45, 40, 35

Case 3 265 50, 45, 40, 35

Case 4 160 50, 45, 40, 35

The experiment scenarios

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

ResultsThe interactions

hr Vetiver hedges

Stem density

Individual stem strength

Stem moment of inertia

Modules of elasticity

hi

ui

In front of the

Vetiver hedges

ub

Behind the

Vetiver hedges

hb

Energy losses

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Results

Flow through Vetiver hedges: Manning factor

Overtopping discharge

Practical application: Nam Dinh-Vietnam

Reduction of wave run-up

Guideline for designing dams and reduction of the cost for upgrading of the present sea dike

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

ObservationsOverview (film)

EMS8 EMS12

Slope 1:30

Water system

Reservoir

G27 G26 G25 G24

Gate

Water

system

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Wave Flow though the Vetiver hedgeObservations

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Results (1)Flow through Vetiver hedge

Water level in front of Vetiver hedge and Grass density

Water level in front of grass ~ grass density

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0 100 200 300 400 500 600

Grass density (stem/m2)

Wat

er le

vel i

n fr

ont o

f gra

sses

(m

)

h=35cm h=40cm h=45cm h=50cm

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Results (1)Flow through Vetiver hedge

Variation of roughness coefficient with flow depth through Vetiver hedge

23

1

1 . .fn S hu

=0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26

Average Water Depth (m)

Man

ning

's n

Case 4 - 160 stems Case 3 - 265 stems Case 2 - 530 stems Case 1 - no grass

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Results (2)Overtopping Discharge

Reduction of overtopping discharge ~ Density

0

10

20

30

40

50

60

70

80

0 160 265 530

Grass density (stem/m2)

Red

uctio

n of

ove

rtop

ping

dis

char

ge

ΔV/

V [-

]

Reduction of overtopping

0

10

20

30

40

50

60

70

80

30 35 40 45 50 55

Water level inside reservoir hr (cm)

Dis

chag

re re

duct

ion Δ

Q/Q

(%)

Case 1 - w ithout grass Case 4 - 160 stem/m2Case 3 - 265 stem/m2 Case 2 - 530 stem/m2

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Practical Application The reduction of wave run-up

Ru1 Ru2

Calculate wave run-up on the outer slope.

Re-calculate wave run-up in case of Vetiver grass are planted on the outer slope.

Find the reduction of wave run-up.

Define the influence factor for roughness of Vetiver grasses.

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Practical Application - Results (4)The reduction of wave run-up

Nam Dinh province

Location

72km length of coastal line

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Practical Application - Results (4)The reduction of wave run-up

Hs=(0.8-2.0)mT=(8-10)s

1:3

MSL

+5

Hs = 0.8-2mT = 8-10s

m:(3-5)

1:100 - 1:300

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Practical Application The reduction of wave run-up

UnitRealvalue

Case1

Case4

Case3

Case2

Grass density stem/m2 - 0 160 256 530

Ration of wave height - - 0.886 0.501 0.422 0.406

Ru2% m 3.26 2.49 1.41 1.19 1.14

Reduction of wave run-up % - 23.73 56.85 63.64 65.1

γf - 0.95 0.75 0.410 0.345 0.332

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Practical Application The reduction of wave run-up

Relationship between grass density and wave run-up

10

20

30

40

50

60

70

80

0 160 265 530

Grass density (stem/m2)

Red

uctio

n of

wav

e ru

-up

(%)

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Practical Application - Case StudyUse the previous influence factor for roughness of Vetiver

grasses ( Result 4)

Allowed discharge of overtopping q=0.0001m2/s

(Dutch Guideline for the design dam )

Use Van der Meer formula (2001)

Calculation the total cost for upgrading the present sea dike

030 00

0, 06 1. . .exp 4, 7. .. . .tan.

cb

m b fm

RQHg H β

γ ξξ γ γ γα

⎛ ⎞= −⎜ ⎟⎜ ⎟

⎝ ⎠

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Case StudyCrest height of sea dike

No Hedge One Hedge Two Hedges Three Hedges

6.606.706.806.907.007.107.207.307.407.507.607.70

1 2 3 4

Cres

t hei

ght (

m)

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Case StudyTotal cost for one meter length

1:3

+7.6

MSLClay

Rock

+7.11

1:3

MSL

Vetiver hedges

Rock

Clay

Without Vetiver grass: $147,5

Two Vetiver hedges: $128,96

Reduction 12,6% total costs

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Conclusions

The resistance of the slope with Vetiver grass is 2.5 times larger in comparison with the slope without grass.

The Vetiver hedges have ability to withstand flow which reaches depths up to 40cm.

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- Reduction of Wave Overtopping by Vetiver grass -

Introduction Objectives Model Set-up Result Case study Conclusions

Conclusions

The wave overtopping reduces with 45% in case of 200 stem/m2.

The influence factor for roughness of Vetiver grass varies from 0.33 to 0.41.

For upgrading sea dikes, the crest level would reduce 0.49m, and the total costs 12.6% if two Vetiver hedges are planted.

- Reduction of Wave Overtopping by Vetiver grass -

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RecommendationsThe influence factor of berm and angle of wave attack

Further research which includes these factors.

The accurate velocities in the middle of grass need more studies and investigations.

The living condition of Vetiver on the outer slope under saline condition.

A problem could appear because of grass’s roots.

- Reduction of Wave Overtopping by Vetiver grass -

34

Thanks for your attention