Daily News: Feature: "07/04/2005 by Prof. S.S.L. Hettiarachchi and Dr. S. P. Samarawickrama, Department of Civil Engineering, University of Moratuwa
Many coastal cities of Sri Lanka were severely affected by the Boxing Day tsunami. One of the principal coastal cities devastated was the historic port city of Galle. Incidentally the first recorded tsunami to have affected Sri Lanka was on 27th August 1883, arising from the eruption of the volcanic island of Krakatoa. On this occasion too, unusually high water levels followed by the receding beach were observed in Galle around 1.30 pm.
The water level fluctuations were not severe and there was no inundation. However, on 26th December 2004, Galle received the severe impact of tsunami waves, their magnitude having increased due to near-shore transformations.
On moving towards land the tsunami wave first interacts with the continental shelf during which process the initial transformation takes place. Depending on the physical characteristics of this shelf, part of the energy is reflected and the rest is transmitted towards land. Any discontinuities in the shelf may lead to complex phenomena. Waves diffracting around the southern parts of the island would be further influenced by the complex wave patterns arising from such discontinuities leading to greater impacts. This is an area which needs further investigation.
On reaching shallow water, the speed of the wave reduces but the energy in the wave remains the same due to minimum energy losses, thus increasing the wave height very rapidly and crashing inland with devastating power and destruction.
The wave height prior to the entry to the shoreline is further increased by the combined influence of near-shore coastal transformation processes of refraction, diffraction, reflection, and energy concentration due to reduced crest width within bays.
The near-shore transformation processes are greatly influenced by the shape of the coastline, geomorphologic features and bottom bathymetry. Depending on these features some coastal cities are more vulnerable than others against coastal hazards.
In the context of tsunamis the location of Galle is extremely vulnerable. It lies besides a wide bay and a natural headland on which is located the historic Galle Fort with very reflective vertical non-porous walls on all sides. Furthermore, there exists the Dutch canal west of the headland, conveying water through the city centre. The waves in the vicinity of Galle, which were increasing in height due to reduced water depths were further subjected to a series of near-shore processes which increased their heights even further. The canal was a facilitator in conveying the massive wave and associated flow towards the heart of the city centre.
In the vicinity of the headland on which the Galle Fort is located, the wave energy concentrates due to refraction. These waves then reflected from the vertical solid walls of the Fort and moved around the headland. Such walls reflect almost all the incident wave energy with very high wave heights at the wall itself.
There is hardly any dissipation. On the west of the headland the waves moved ferociously into the Dutch Canal (as captured by the famous ITN cameraman). On the east it moved along the bay. The wide bay in Galle further contributed to the increase in wave height by modifying the shoaling process via reduced wave crest width to accommodate the bay shape.
The combined effect of this phenomenon and the wave coming around the eastern side of the Fort caused a massive wave of destruction along the Marine Drive. It is certainly not surprising that many survivors referred to a moving large black wall similar to that of the Galle Fort.
The city of Galle is one of the many coastal cities around the world, which remains vulnerable against tsunami waves. The poor drainage only adds to the vulnerability. Planning countermeasures
There are many counter measures that could be adopted in the context of coastal zone management, in planning for a tsunami and other coastal hazards that accompany high waves.
These include engineering interventions such as protection structures, strengthening of natural defences and regulatory interventions in the form of extension of the existing 'setback' defence line. These have to be supplemented with efficient evacuation procedures, incorporating, if necessary, planned evacuation structures that effectively integrate with the overall planning process.
It is important that post disaster planning should be undertaken in the context of overall coastal hazards one of which remains Tsunamis, however remote the chances of an extreme event such as that of the 26th December taking place. It is recognised that a Coastal Hazard Protection Plan for a city that is an integral part of an overall Coastal Zone Management Plan has to be based upon Policy and Management Options.
These options reflect the strategic approach for achieving long term stability in particular for sustaining multiple uses of the coastal zone giving due consideration to the threats and risks of hazards.
Interventions
It is in the above context that attention is focused on three types of interventions for protection, namely those which,
(i) reduce the impacts of tsunami waves prior to reaching the shoreline.
(ii) protect the coastal zone thus preventing the inland movement of tsunami waves.
(iii) mitigate the severe impacts of tsunami waves on entry to the shoreline.
Some of these interventions may be achieved not only by artificial methods via Coastal Engineering Design but also by natural methods.
Typical examples of the first and second types of structures are Tsunami Breakwaters and Tsunami Dikes. Tsunami Breakwaters are constructed offshore to interact with the incoming waves and thereby reducing its energy by efficient dissipation processes. These structures are usually overtopped by tsunami waves but the waves that continue to propagate thereafter have less energy.
It is good engineering practice to encounter tsunami waves in deepwater and reduce its strength before the heights are increased due to complex near-shore processes described earlier. These structures can also be incorporated as part of a coastal development programme such as port development by which means the entire exercise become economically attractive.
Coral reefs depending on their location and geometry can be effective in mitigating the impacts of the tsunami wave prior to reaching the shoreline. The reef system should be of sufficient length to ensure a fair proportion of energy dissipation leading to high hydraulic efficiency. Areas where excessive near-shore coral mining had taken place were severely affected due to the absence of the natural defence system irrespective of its hydraulic efficiency.
Tsunami Dikes are constructed on the shoreline and the structures will prevent the passage of waves and also dissipate energy. In the unlikely event of overtopping of the defences it is equally important to incorporate effective drainage systems without which the catastrophe would be greater.
Natural barriers such as sand dunes have been effective in preventing the inland entry of the tsunami waves. However, if such a system is breached at a weak point there is a high possibility of a progressive collapse of the defence leading to excessive inundation.
Tsunami Dikes of moderate heights will limit the overtopping thus reducing the inundation. Such systems must be able to withstand the overtopping wave forces at crest level and remain stable during the progression of the tsunami.
It also customary to have more robust coast protection structures armoured with concrete armour units which are more stable against massive overtopping waves and efficient in dissipating wave energy. If heavy overtopping is expected it is important that effective drainage is provided for the areas behind the protected area.
Planned growth of Mangroves can be an effective measure in mitigating the impacts of the tsunami wave on entry to the shoreline. A mangrove forest is an efficient natural energy absorber of steady flows and long waves. The natural porous structure of the mangrove forests and their deep roots generate a stable wave absorber.
However it is doubtful whether such a natural system could resist a very large tsunami wave and its effectiveness under such circumstances has to be investigated. The planned growth of mangroves in front of any form of artificial tsunami barriers will absorb part of the wave energy before the waves strike the barrier.
Port of Galle
In 2000, Japanese Port Consultants (JPC) developed a Master Plan for the development of the Port of Galle. In view of environmental issues they recognised that the development should be restricted to a two berth medium size harbour. The Environmental Studies for the project were carried out by the University of Moratuwa.
In order to maintain healthy exchange of tidal flow for the well being of the coral reef system, JPC in consultation with the environmental specialists incorporated an offshore detached breakwater, which coincidentally has all the characteristics of an effective Tsunami Breakwater. It must be admitted tsunamis were furthest in the minds of the engineering and environmental teams at that stage.
By implementing this project with a slightly extended offshore breakwater in the direction of the Galle Fort, the City of Galle will have the benefit of a Tsunami Breakwater as part of a port development project. Once this extension is designed it may be possible to reduce the length of the revetment protruding near the berth. Perhaps a protection wall (tsunami dyke) of modest proportions, along the coastline, can supplement the Tsunami Breakwater.
The design details and the structural configurations can only be determined after carrying out simulation modelling. These are some of the Coastal Engineering mitigation measures, which can be considered in examining the options for the protection of Galle.
These measures will also be effective against potential coastal hazards that have a greater probability of occurrence than a massive tsunami wave.
