wave loading & overtoping +measures ext+ ppt
TRANSCRIPT
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Wave loading on sea dikes
• Some new developments
(general remarks)
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Developments
• Last decades:increase economical values, population (coastal and riverine areas)
• Large Disasters (Floods, Traffic, Planes, Fire, Earthquakes, Mud flow, etc.)
• Future developments: climate change, sea-level rising, soil subsidence, air/water pollution
Bas Jonkman
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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?
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expected
Sea-level rise
level
subsidence
Time (year)
Historical developments: Dutch history and (possible) future
Sea-level rise
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The force of waves
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We can not change the water levels but we can reduce the wave height
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Wave overtopping and consequences
Haiphong dike
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Example from Gold Coast Australia
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IJmuiden harbour during storm
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Wave run-up and overtopping on dikes
Disaster 1953Actual dikes
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Transformation of waves from deep water into shallow water
h
H0
H = (0.5 to 0.6) h
H
After erosion
Original bed
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Measuring Stations NL
Petten; measuring instruments and run-up test section
Verification SWAN model
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Measurements at Pettemer sea dike
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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
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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
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Measures for overtopping resistant dikes
Snelkoppeling naar Video1Kesz.lnk
•Video1Kesz.mpg
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Video overflow dikes
• Video1Kesz.mpg
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Effect of overtopping
Haiphong dike after Typhoon No.2
What about overtopping Vietnamese dikes
Overtopping per wave Average overtopping per length
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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
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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)
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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%)
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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
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Extreme water levels (extra 60cm sealevel rise dashed line)
0
1
2
3
4
5
6
1 10 100
Return period (years)
Red River
Mekong
Possible design waterlevels Vietnam ???(judgement by Delft Hydraulics)
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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
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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)
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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:
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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
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Hydraulic Boundary Conditions
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Modeling storm-surge levels
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Examples
from measurements
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Conclusions
We have to join forces and efforts to minimize effect of typhoons
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Look to all failure mechanisms
And design frequency based on cost-benefit analyze
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Quality of design and executionSupervision during execution
Compaction in 2005 not better than in 1995; no proper supervision
Educate supervisors
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No more retreat ?
We can not stop/avoid typhoons
but we can/should minimize
their impacts/effects
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New dike Hai Hau Jan 2006
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No guarantee
Never safe enough
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Alternatives for overtoppingresistant dikes
Compaction is important
(low-cost)
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Alternative protections
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geotubes
geomattresses
Geobags covered with soil and grass
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MARD/PolicyDike Department/DDMFC
Provincial MARD/PDDMFCDesign Office/Consultants
Supervision/QualityUpgrading knowledge
Institutional reform
More decentralisation
More local involvement
Shearing responsibilities
Databank
Institutional remarks
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Safety versus design standard: 5% = 1/20 years ??????????
For 5% the dike crest is about MSL+7mActual Vietnamese dikes probably no more than:
Crest of dikes 20% = 1/5 years (MSL+5m)
(frequent overtopped dikes)
However, the strength of revetments, including crest and inner slope, should be adequate to resist at least the attack of typhoons such as
No.2 and No.7.Therefore, the loading by these typhoons should be
studied; why MARD/DDMFC is not willing to do that???? (unbelievable)
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Co-operation needed
• Within MARD/DDMFC
• Between ministries
• Between Institutes
• Between Universities
• International
• Upgrading documents
• Unification of Polices
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Thank You