Investigations on the Effectiveness of Coastal Vegetation in Tsunami Impact Mitigation

Investigations on the Effectiveness of Coastal Vegetation in Tsunami Impact Mitigation Dr. S.S.L.Hettiarachchi A.H.R.Ratnasooriya Dr. S.P.Samarawickrama University of Moratuwa Acknowledgements- NSF, Sri Lanka WAPMERR, Geneva PARI, Japan Geo Science, Australia University of Arizona

Strategic approach in developing Post tsunami mitigation and Conservation of the coastline ..for protection of lives, coastal ecosystems and infrastructure Multi Hazard Coastal Risk Assessment Framework Multi Hazard approach considers all coastal hazards, each having a frequency of occurrence and potential impact (intensity /spatial distribution) Risk= Hazard x Exposure x Vulnerability x Deficiencies in Preparedness … . towards Disaster Risk Reduction Mitigation Options

Assessment tsunami wave heights and inundation from field studies Galle District Information from investigations on tsunami impact 1

Damage profiles of housing and infrastructure (Galle City) International Cricket stadium. Damaged houses behind Gall bay. Damaged bridge. Damaged houses behind Gall bay.

Damage at the boundary of water bodies penetrated by the tsunami. Rail track severely affected. Damage to protection works and breakwaters in harbours

Impact on vegetation and coral reefs

Erosion and Deposition caused by tsunami waves Long waves of high amplitude Trough Crest

Post Tsunami Scenario
-Damaged infrastructure
-Damaged protection works
Damaged reef and eco-systems
-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

Promote successful evacuation from tsunamis Early Warning and Countermeasures against tsunamis Mitigate the impact of tsunami (Mitigation Options) -Early Warning System (Local and Regional) -Public Warning System -Evacuation Routes & Structures -Community Education, Maps for their benefit and Preparedness - Physical Interventions (Artificial Methods, Natural Methods and Hybrid Methods) Mitigate exposure and vulnerability to tsunami hazard -Land Use Planning -Regulatory interventions such as set back of defense line -Hazard resilient buildings and infrastructure 2

Promote successful evacuation from tsunamis Early Warning and Countermeasures against tsunamis Mitigate the impact of tsunami (Mitigation Options) -Early Warning System (Local and Regional) -Public Warning System -Evacuation Routes & Structures -Community Education, Maps for their benefit and Preparedness - Physical Interventions (Artificial Methods, Natural Methods and Hybrid Methods) Mitigate exposure and vulnerability to tsunami hazard -Land Use Planning -Regulatory interventions such as set back of defense line -Hazard resilient buildings and infrastructure Risk Assessment- Hazard, Exposure and Vulnerability and Capacity Hazard, Vulnerability and Risk Maps

Distribution of epicentres of earthquakes greater than magnitude 5.0 for the period 1976-2000, SE Asia- Indian Ocean (UNDP) Assessment of the Tsunami Hazard and Exposure Tsunami Generation from Earthquake at a Subduction Zone 3 (1) Tsunami Hazard Source (2) Tsunami Hazard Impact on land Exposure

Regional Location Shoreline Geometry Location with respect to the Continental Shelf Exposure at a given location Submarine Geological features Impact of Submarine Geological features, Coastal Processes and Local Geometry on Tsunami Wave Amplification

Tsunami Mitigation using Artificial and Natural Methods Overall Strategic Approach 4 (1) Reduce the impacts of tsunami waves prior to reaching the shoreline (2) Protect the coastal zone thus preventing the inland movement of tsunami waves (3) Mitigate the severe impacts of tsunami waves on entry to the shoreline Energy dissipator/ Partial barrier in coastal waters Full barrier on the coastline Partial barrier on the coastline

Tsunami Mitigation using Artificial and Natural Methods Tsunami Breakwaters Coral Reefs and Sand Bars Revetments,Dikes (High Crest) Sand Dunes Revetments,Dikes (Low Crest) Coastal Vegetation

Natural Methods Coral Reefs & Sand Bars Sand Dunes Coastal Vegetation and Mangrove Forests Hybrid Solutions Combination of Natural /Artificial Methods

Coral Reefs Submerged natural breakwaters d h H i H t L Coral Reefs – act as submerged natural breakwaters Submerged depth (h) Length (L)

Significant length (L) Small submerged depth (h)

Reflection of waves The influence of Wave Reflection from Maldive Islands

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)

2.5 km/hour Current Speed Current Direction Measured MEM CURR Measured currents offshore of Colombo

U C U G H L M y x Wave Parameters U o , λ, a Impact of a gap in the reef Wave Parameters ( U 0 , λ, a) Reef Parameters (M, P, L, H) Reef Gap (  ) Depth of water (H 0 ) Location (x, y, z)  U 0 U C U G z P

Collaborative Research Arizona State University / University of Moratuwa (May/June 2005 and Nov/Dec 2006) PIV method ADV method Simulated reefs Large flume studies

Porosity P = 20% and 50% Amplitude a = 20, 30 and 40 cm for H 0 =30 cm Measurements at z = 5, 10, 15, 20 cm for H =20 cm In the experiments the dependence on are investigated U 0 U C U G

Representation of high dense (20% porosity) and low dense (50% porosity) structures

University of Arizona

(a) 50% porosity (b) 20% porosity Normalized Velocity as a function of normalized height 2a = 30cm U 0 Velocity without the reef U C Velocity behind the reef U G Velocity in the reef gap

Sand Dunes (High Crest Natural Dikes)

Panama – Sand Dunes Breached Depth Safe crest level ? Would vegetation stabilise the dune ?

February 2002 January 2005

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
Coastal Lagoons, Estuaries and Wetlands Human Settlement Tsunamis can cause extensive damage to unique eco-systems Eastern Province, Sri Lanka 2004

Artificial Nourishment of beaches and building of dunes with offshore sand Artificial Nourishment Dynamic behaviour of Sand Dunes

Dune Erosion and Degradation Dune Maintenance

L (Length) Mangrove Forest H 1 H 2 U 1 U 2 Height of mangroves (Hm) Coastal Vegetation (Mangrove Forest)- Partial Barrier Density Plant Characteristics and Resilience Porous Wave Absorber Inundation Height H 1

Height (Hm) Density, Plant Characteristics and Resilience Length (L)

Trough Long waves of high amplitude Crest

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 Classification of Vegetation

Experimental set up for small scale tests The wave in progress through vegetation Spacing s , Diameter D Uniform/Staggered grid c 2 c 1 d 1 d 2 θ L a , ( width = b ) H R Gate (Open)

Simulation of vegetation Types I, II, III and IV for small scale experiment

Experimental set up for large scale tests Wave paddle Slope 1:20 Vegetation 3 4 5 6 7 8 Wave gauges 1 2 Water level 25cm Water level 55cm Water level 35cm 0.8m 1.2m 5.1m 5.4m 4.2m 0.8m 1.2m 1.2m

Type I Resistance provides by stem only Type II Resistance provides by stem and branch structure Simulation of vegetation for experiment

The wave in progress through vegetation

Wave motion through vegetation

Hybrid Solutions Sand dunes and Coastal vegetation WITH VEGETATION IN THE BACKGROUND

Hikkaduwa Train Tragedy

Hikkaduwa road beach washed train railway 64m 188m 188m 228m leaf tarnished Sloping surface towards land Issue-

Source: Port and Airport Research Institute, Japan Experiment conducted with and without the slope

with the slope without the slope Source: Port and Airport Research Institute, Japan

Increase in velocity
20% - 50%
Increase in Impulsive Bore Pressure
100% - 150%
Increase in Sustainable Pressure
more than 100%
Water tends to stay longer
Bore Pressure Sustainable Pressure Impulsive Bore Pressure Maximum Sustainable Pressure

Investigations on the Effectiveness of Coastal Vegetation in Tsunami Impact Mitigation

by Global Risk Forum GRFDavos on Jun 14, 2010

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