wave loading & overtoping +measures
DESCRIPTION
wave loading on dikes ; comparison Netherlands and VietnamTRANSCRIPT
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