According to Shore Protection Manual, for engineering interest, coastal zone can be classified into six zones:
- Surf Zone is zone between waves breaking (start to break) until the beach line
- Off-shore Zone is zone between waves breaking until off-shore
- Breaking Zone is zone where waves start to break
- Beach (shore) is coastal area which is bordered directly with water
- Coast is coastal area which is still influenced directly, for instances by tides, sea wind, and coastal ecosystem (mangrove woods, sand dunes)
- Coastal Area is coastal wet or dry area up to 100 or 150 meters depth
While according to Hendratmo (2004) has classified coastal zone into four zones: sandy coast & sand dunes, rocky coast, coastal wetland & estuaries, and coral reef.
Mitigation of Risks and Hazards in Caostal Zone
The main damage from tsunami comes from the destructive nature of the waves themselves. Secondary effects include the debris acting as projectiles which then run into other objects, erosion that can undermine the foundations of structures built along coastlines, and fires that result from disruption of gas and electrical lines. Tertiary effects include loss of crops and water and electrical systems which can lead to famine and disease.
However, disaster cannot be vanished due to its big power and destructive force. Hence, there is a need to have a disaster prevention which is called a mitigation. Mitigation is a process of efforts with various preventive actions, in order to minimize negative impacts of a disaster which will happen in the future at certain area, and so it is a long term investment for the sake of people welfare (Diposaptono, 2005). Within the last century, up until the December 2004 tsunami, there were 94 destructive tsunami which resulted in 51,000 deaths. Despite the fact that tsunami warning systems have been in place in the Pacific Ocean basin since 1950, deaths still result from tsunami, especially when the source of the earthquake is so close to a coast that there is little time for a warning, or when people do not heed the warning or follow instructions associated with the warning. These factors point out the inadequacy of the world in not having a tsunami warning system in place in the Indian Ocean, where in one event, the death toll from tsunami was increased by a factor of 5 over all previous events.
In order to overcome tsunami disasters, there are some mitigation approaches which can be done:
1. Tsunamis Reduction
This method can be done by making a natural coastal barrier such as mangrove tree plantation. According to Hirashi and Takemura (2000), mangrove plantation in a coastal area can reduce a tsunami’s height up to 29% and tsunami’s force up to 27%. How big is this reduction of course will depend on the thickness, type, density, and height of the woods. Tsunamis reduction also can be done by the application of civil engineering structure, that is by standing up sea walls along the coast line. However, this method is less popular due to its expensive costs.
2. Early Warning
With regard to early warning, there are some certain things that should be prepared in a mass evacuation such as a good early warning system, a good evacuation access planning, accessible evacuation spots, high rise building planning, public facilities, tsunami aid devices, and related acts which regulate coastal zone management.
3. Learning on tsunami disasters and other natural disasters that need evacuation process
The first possible strategy is by giving a sufficient knowledge for people so that they will ready if a disaster emerges.
4. Zone Management
As one of the most effective mitigation method, zone management will cover many problems, such as green tracks determination, accesible roads determination, and evacuation spots determination.
source: The Influence of Building’s Height and Lay-out to Tsunami’s Run Up