Tsunami Safeguards
Overwhelmed by the devastation wrought by the December 2004 tsunami, coastal dwellers along the Andaman Sea and the Indian Ocean are now focusing attention on safeguards against such dangers. For example, the Bangkok Post (16 October 2005) reported that there has been a boom in construction of seawalls along the Andaman coast.
Concrete seawalls are not only eyesores and environmental nightmares, unless they are several meters high and several meters thick, they may prove totally ineffective against a tsunami of the type that occurred last year. But, are there any alternatives to massive concrete structures?
In fact, there are. The Indonesian Engineers Association has come up with two of them One is a buffer zone to reduce the impact of strong waves and the other is an escape hill to provide a haven for people during earthquakes and tsunamis. Professor Dennes Bergado of the Asian Institute of Technology, Bangkok explains how they are constructed.
Buffer zone
A buffer zone serves to dissipate the impact of strong tsunami waves generated by an earthquake. Buffer zones, therefore, must be located close to the seashore. These zones may consist of a natural barrier such as a mangrove forest or other trees. Where there is no natural barrier, the combination of a man-made coastal road embankment and artificial elevated sand dunes can also function as a buffer zone. These road embankments, which are reinforced and erosion resistant through the reinforcement of geosynthetics (Fig. 1).
Figure 1 Road embankment
Geosynthetics are materials made from various types of polymers (plastics) used in geotechnical and transportation engineering projects . The embankments should be at least 3.0 m high and 6.0 m wide. Between the road and beach, artificial sand dunes can be constructed using, for example, geotubes (Fig. 2) or geotextile sand bags (Fig. 3), both of which are made of geosynthetics.
Figures 2 & 3 Structure of artificial sand dunes using geosynthetics
A preferred arrangement can be seen in Figure 4 where the buffer zone is enhanced by the presence of a mangrove forest between the beach and a highway that fronts and protects a residential area.
Figure 4 Buffer zone for a coastal village with mangrove forest and road embankment. (Indraratna et al,2005)
Escape Hill
A second alternative is the escape hill. This man-made hill can provide a haven for about 3,000 people during a cataclysm like a tsunami or an earthquake. Its construction is similar to that of a pyramid but with geosynthetic reinforcements and erosion protection. An escape hill needs to be from 5.0 m to 10.0 m high; the top of the hill offers a flat rectangular area of about 30.0 m by 15.0 m. Unlike an escape tower with one staircase or ladder, an escape hill allows people to climb up along all four of its sides (Fig. 5).
Figure 5 Diagram of an escape hill
Building an escape hill requires a lot of labor, but it can be carried out with the participation of local villagers in a cash for labor work scheme. With the estimated cost of an escape hill at US $25,000 (1 million Thai baht), this alternative is affordable and the earthquake resistance of such geosynthetic reinforced structures is well documented.
Announcement
An 'International Symposium on Tsunami Reconstruction with Geosynthetics - Protection, Mitigation and Rehabilitation of Coastal and Waterway Erosion Control' will be held at Miracle Grand Hotel in Don Muang Bangkok on 8 to 9 December 2005.
All are welcome to participate in this event. For further information, please contact: Prof. Dennes T. Bergado at email: bergado@ait.ac.th and visit the conference website at