wave loading & overtoping +measures

Dutch Coast

Wave loading on coastal structuresThe Netherlands

versus

Vietnam

Dutch dike

Wave loading on sea dikes

• Some new developments

(general remarks)

What are under discussion?

• Are storms more frequent and waves larger ?

• Is this all due to climate change and sea-level rise?

• What is the reality?

expected

Sea-level rise

level

subsidence

Time (year)

Historical developments: Dutch history and (possible) future

Sea-level rise

The force of waves

We can not change the water levels but we can reduce the wave height

Wave overtopping and consequences

Haiphong dike

Example from Gold Coast Australia

IJmuiden harbour during storm

Wave run-up and overtopping on dikes

Disaster 1953Actual dikes

Transformation of waves from deep water into shallow water

h

H0

H = (0.5 to 0.6) h

H

After erosion

Original bed

Measuring Stations NL

Petten; measuring instruments and run-up test section

Verification SWAN model

Measurements at Pettemer sea dike

Conclusions for Dutch coast/dikes ?

• Dikes usually older then 30 years• Designed with old knowledge/criteria

• Inventarisation of real state of dikes needed:– Actual and future overtopping of dikes– What is the resistance of inner slope (grass)

against certain overtopping (velocities) ?

• Policy decision needed

Conclusions• Waves became higher not due to

climate change but due to foreshore erosion (deeper water in front of structures)

• Wave have always much destructive effect

• More overtopping is expected during superstorms

• Grassmats not strong enough and reinforcement is needed

Measures for overtopping resistant dikes

Effect of overtopping

Haiphong dike after Typhoon No.2

What about overtopping Vietnamese dikes

Overtopping per wave Average overtopping per length

Vietnam DWL = M.S.L+ Ztide5%+∆Zwindsurge+∆ Ztide : Based on 5% of design frequency and exceedance curve over 19 years of tidal level

observation, the corresponded sea water level takes value of +2.1 to 2.30 (m +MSL).

Tidal level: Ztide=+2.29 m +MSL, is the averaged highest tidal range for the location according

to annual publication of Vietnam Marine Hydro-meteorological Center.

Actual Nam Dinh: Ztide5%=2.1+MSL, Zsurge=0.9m, ∆=0.3, thus design water level is about 3.3m+MSL Wind surge defined for storm with 9B ???

Crest height of the dike

Approach uncleare

Waterlevels

-1

0

1

2

3

4

5

9/24/0512:00

9/25/05 0:00 9/25/0512:00

9/26/05 0:00 9/26/0512:00

9/27/05 0:00 9/27/0512:00

9/28/05 0:00 9/28/0512:00

9/29/05 0:00 9/29/0512:00

time

leve

l

tide

w aterlevel

storm surge

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

9/27/050:00

9/27/052:24

9/27/054:48

9/27/057:12

9/27/059:36

9/27/0512:00

9/27/0514:24

9/27/0516:48

9/27/0519:12

9/27/0521:36

9/28/050:00

storm surge

tide (MSL)

w aterlevel (MSL)

Typhoon N0. 7 Damrey

Storm surge at Hon Dau station = 1 m

Max. water level

Max. storm surge

Hon Dau Station

Nam Dinh ????

(interpolation between stations)

CD

SWL

SWL= CD+1.9m

(or CD+1.86m)

Surge level (cm) 0 - 5050 - 100

100 - 150 150 - 200 200 - 250

Frequency related to number of storms (%)

35 38 17 8 3

Storm surge at Nam Dinh coast[Source: Vietnamese Water Resources Institute]

Typhoon No.7 Damrey:

Station Hon Dau (near Haiphong): reference: MSL=CD+1.86m or 1.90m

HWmeasured = 4.18m+CD

HWtide(tables)= 3.30m+CD

Storm surge at high water: 4.18-3.30= 0.88m; max. surge observed: 1.0m

For Nam Dinh: storm surge about 1.4 to 1.5m (close to 1/20 per year = 5%)

Statistics storm surges Hai Hau

Swedish study, 2004;Institute of Mechanics Hanoi

DesignWater levels : 100 year = +4.56 to +5.06m HD (Nam Dinh)

Tidal levels

Storm surges

Seasonal surges

Sea level rise

Subsidence

+1.6m MSL - 0.14m HD

Wave set-up

+2.5 to 3m (1 in 100 year)

+0.1m

+0.1m

+0.3m (50 years value)

+0.1m (add to water depth)

Judgement Delft HydraulicsVietnam 1996

Typhoon simulation model

HMC

hk [m] q [l/m per s]

2,0 100

2,5 48

3,0 23

0

2

4

6

8

10

12

incoming waves P[%]

U [

m/s

]

hk = 3,0hk = 2,5hk = 2,0

100 50 220 10 5 1 0,5 0,1

m=2

m=3

hk

q l/ms; average discharge

Overtopping Vietnamese dikesHs = 2m

Tp= 8 sec

0

5

10

15

0,010,1110100

incoming waves P [%]V

[m

3 /m] hk = 2,0

hk = 2,5hk = 3,0

Max. volume per wave, l/m

MSL+3.5m

h=4m

Hs,toe=0.5h= 2m

0

2

4

6

8

10

12

incoming waves P[%]

U [

m/s

]

m=2

m=3

100 50 220 10 5 1 0,5 0,1

0

2

4

6

8

10

12

incoming waves P[%]

U [

m/s

]

m=2

m=3

100 50 220 10 5 1 0,5 0,1

0

2

4

6

8

10

12

incoming waves P[%]

U [

m/s

]

m=2

m=3

100 50 220 10 5 1 0,5 0,1

hk= 2

hk=2.5

hk=3

hk

hk [m] q [l/m per s]

2,0 100

2,5 48

3,0 23

qaverage l/ms

hk m

MSL+3.5m

4m1m10

100

1000

(q=10m/ms=max.permissible for grass)

Influence of berm

10,0

100,0

1000,0

0 5 10 15

Bermbreedte

Deb

iet

[lit

er/s

/m]

Periode = 6 sec

Periode = 8 sec

Hmo= 2 m

DWL= +3.5m

Berm at +3.5m

Crest= +5.0m

+3.5m

Runup roughly given by:

Z2% = 8H tan = 8 x 2 x 0.25 = 4m

which supports the previous derivation hk = 4m

Concluded:

Consider and make cost-benefit analyze for:

1) Low-crested dikes (MSL+ 5. to 5.5m) completely protected by revetments against overtopping

2) High-crested dikes (MSL+7.0 to 7.5m) protected only on the seaside (grass on the inner slope)

Use also flume models for studying these alternatives

Good hydraulic boundary conditions (water levels including storm surges and waves) are needed for both alternatives;

For low-crested dikes the good boundary conditions are needed for calculation forces and velocities on the crest and inner slope;

Do not use the criterion 9B knowing that each year you have typhoons stronger than that (it is behind any acceptable logic); it is time to modernize this approach.

Remember:

transitional area drydefence

Example of ComCoast concept NL

transitional area drydefence

Example of ComCoast concept

Alternative concepts

Overtopping resistant dike

Foreshore low-crested sill

for wave breaking

Hydraulic Boundary Conditions

Modeling storm-surge levels

Examples

from measurements

Conclusions

We have to join forces and efforts to minimize effect of typhoons

Look to all failure mechanisms

And design frequency based on cost-benefit analyze

Quality of design and executionSupervision during execution

Compaction

Educate supervisors

No more retreat ?

We can not stop/avoid typhoons

but we can/should minimize

their impacts/effects

New dike Hai Hau Jan 2006

No guarantee

Never safe enough

Alternatives for overtoppingresistant dikes

Compaction is important

(low-cost)

Alternative protections

geotubes

geomattresses

Geobags covered with soil and grass

Thank You