Mitigation of the Tsunami Hazard

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Mitigation of the Tsunami Hazard
Dr. S.S.L.Hettiarachchi Dr. S.P.Samarawickrama Uni
versity of Moratuwa Acknowledgements- USAID/IOT
WS Project NSF, Sri Lanka WAPMERR, Geneva PARI,
Japan University of Arizona
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Approach towards Mitigation

Multi Hazard Coastal Risk Assessment Framework
. towards Disaster Risk Reduction
Risk Hazard x Exposure x Vulnerability x
Deficiencies in Preparedness
Mitigation Options
Multi Hazard approach considers all coastal
hazards, each having
a frequency of occurrence and potential impact
(intensity /spatial distribution)
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• Post Tsunami Scenario
• Damaged reef and eco-systems
• -Damaged protection works
• -Increased bathymetry and changes in near-shore
  areas

-Waves of greater height close to the
shoreline -Increase in coastal erosion due to the
changes in the bottom bathymetry
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Early Warning and Countermeasures against
tsunamis
Promote successful evacuation from tsunamis
Mitigate tsunamis (Mitigation Options)

• Early Warning System
• (Local and Regional)
• Public Warning System
• Hazard, Vulnerability and Risk Maps
• Set Back
• Evacuation Routes Structures

• Physical Interventions (Artificial Methods,
  Natural Methods and Hybrid Methods)
• Design Guidelines for exposed infrastructure

Risk Assessment- Hazard, Exposure ,Vulnerability
and Deficiencies in Preparedness
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Tsunami Mitigation using Artificial and Natural
Methods

Overall Strategic Approach
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Tsunami Mitigation using Artificial and Natural
Methods
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Natural Methods
Coral Reefs Sand Bars
Sand Dunes
Coastal Vegetation and Mangrove Forests
Hybrid Solutions Combination of Natural
/Artificial Methods
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Coral Reefs Submerged natural
breakwaters
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Small submerged depth (h)
Significant length (L)
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The influence of Wave Reflection from Maldive
Islands
Reflection of waves

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Coral reefs were severely affected and damaged by
the debris and sand transported during the inland
and shoreward movement of the tsunami wave.
(Source-Prof. H Fernando)
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Measured
Measured currents offshore of Colombo
MEM
CURR
Current Speed
2.5 km/hour
Current Direction
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Impact of a gap in the reef
Wave Parameters (U0 , ?, a) Reef Parameters
(M, P, L, H) Reef Gap (?) Depth of water
(H0) Location (x, y, z)
U0 UC UG
UC
UG
M
L
y
x
H
?
Wave Parameters Uo , ?, a
Collaborative Research Arizona State University /
University of Moratuwa
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Representation of high dense (20 porosity) and
low dense (50 porosity) structures
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University of Arizona
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(a) 50 porosity (b) 20 porosity Normalized
Velocity as a function of normalized height 2a
30cm
U0 Velocity without the reef UC Velocity behind
the reef UG Velocity in the reef gap
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Sand Dunes (High Crest Natural Dikes)
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Panama Sand Dunes
Would vegetation stabilise the dune ?
Safe crest level ?
Breached Depth
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February 2002
January 2005
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Human Settlement
Coastal Lagoons, Estuaries and Wetlands
Tsunamis can cause extensive damage to unique
eco-systems
Eastern Province, Sri Lanka 2004

• Sand Dunes can be used effectively to protect
  land, life, ecosystems and infrastructure from
  excessive overtopping and damage
• Dynamic behaviour of sand dunes (Dune
  Erosion/Degradation)
• Dune Rehabilitation, Construction and Maintenance

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Building of dunes thru Artificial Nourishment
with offshore sand
Dune Erosion and Degradation
Dune Maintenance
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Coastal Vegetation (Mangrove Forest)- Partial
Barrier
Height of mangroves (Hm)
Mangrove Forest
H1
U1
H2
U2
L (Length)
Density
Porous Wave Absorber
Plant Characteristics and Resilience
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Height (Hm)
Length (L)
Density and Plant Characteristics and Resilience
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Classification of Vegetation
Type I Resistance provides by stem only Type
II Resistance provides by stem and branch
structure Type III Resistance provides by stem
and aerial roots structure Type IV Resistance
provides by stem, branch structure and aerial
roots structure
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Experimental set up for small scale tests
The wave in progress through vegetation
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Simulation of vegetation Types
I, II, III and IV for
small scale experiment
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Experimental set up for large scale tests
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Simulation of vegetation for experiment
Type I Resistance provides by stem only Type
II Resistance provides by stem and branch
structure
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The wave in progress through vegetation
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The wave in progress through vegetation
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M
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M
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Wave motion through vegetation
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Hybrid Solutions
Sand dunes and Coastal vegetation
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