Coastal Zone

Coastal Zone

According to Shore Protection Manual, for engineering interest, coastal zone can be classified into six zones:

  1. Surf Zone is zone between waves breaking (start to break) until the beach line
  2. Off-shore Zone is zone between waves breaking until off-shore
  3. Breaking Zone is zone where waves start to break
  4. Beach (shore) is coastal area which is bordered directly with water
  5. Coast is coastal area which is still influenced directly, for instances by tides, sea wind, and coastal ecosystem (mangrove woods, sand dunes)
  6. 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

Tsunami

Tsunamis are long water waves generated by earthquakes, sliding masses, volcanic eruptions, asteroid impacts as well as nuclear explosions. The word tsunami (津波、つなみ) consists of tsu (津) character that means bay or harbor, and nami (波) character that means waves. So literary tsunami means big waves that occur at a bay or an harbor. The Japanese word tsunami has been adopted to replace the expression tidal wave to avoid confusion with the astronomical tides (because they are not in any way related to the tides of the Earth). Tsunamis are sometimes also called as seismic sea waves, although they can be generated by mechanisms other than earthquakes. Because tsunami occur suddenly and often without any warning, tsunamis are extremely dangerous to coastal communities.

Tsunamis are characterized as shallow water waves. These are different from the waves that usually happen on a beach, which are caused by the wind blowing across the ocean’s surface. Wind generated waves usually have period (time between two successive waves) of five to twenty seconds and a wave length of 100 to 200 meters. A tsunami can have a period in the range of ten minutes to two hours and wave lengths greater than 500 km.  A wave is characterized as a shallow water wave when the ratio of the water depth and wave length is very small (d/L ≤ 0.05). The acceleration of a shallow water wave is also equal to the square root of the product of the acceleration of gravity, g, (9.81 m/sec2) and the depth of the water, d.

The rate at which a wave loses its energy is inversely related to its wave length. Since a tsunami has a very large wave length, it will lose little energy as it propagates. Thus, in very deep water, a tsunami will travel at high speeds with little loss of energy. For example, when the ocean is 6100 m deep, a tsunami will travel about 890 km/hr, and thus can travel across the Pacific Ocean in less than one day.

As a tsunami leaves the deep water of the open sea and arrives at the shallow waters near the coast, it undergoes a transformation. Since the velocity of the tsunami is also related to the water depth, as the depth of the water decreases, the velocity of the tsunami decreases. The change of total energy of the tsunami, however, remains constant.

Furthermore, the period of the wave remains the same, and thus more water is forced between the wave crests causing the height of the wave to increase. Because of this “shoaling” effect, a tsunami that was imperceptible in deep water may grow to have wave heights of several meters or more.

If the trough of the tsunami wave reaches the coast first, this causes a phenomenon called drawdown, where it appears that sea level has dropped considerably.  Drawdown is followed immediately by the crest of the wave which can catch people observing the drawdown off guard. When the crest of the wave hits, sea level rises (called run up).  Run up is usually expressed in meters above normal high tide.  Run ups from the same tsunami can be variable because of the influence of the shapes of coastlines.  One coastal area may see no damaging wave activity while in another area destructive waves can be large and violent. The flooding of an area can extend inland by 300 m or more, covering large areas of land with water and debris. Flooding tsunami waves tend to carry loose objects and people out to sea when they retreat. Tsunami may reach a maximum vertical height onshore above sea level, called a run up height, of 30 meters. A notable exception is the landslide generated tsunami in Lituya Bay, Alaska in 1958 which produced a 60 meter high wave.

Because the wave lengths and velocities of tsunami are so large, the period of such waves is also large, and larger than normal ocean waves.  Thus it may take several hours for successive crests to reach the shore.  (For a tsunami with a wavelength of 200 km traveling at 750 km/hr, the wave period is about 16 minutes).  Thus people are not safe after the passage of the first large wave, but must wait several hours for all waves to pass. The first wave may not be the largest in the series of waves. For example, in several different recent tsunamis the first, third, and fifth waves were the largest.

 

source: The Influence of Building’s Height and Lay-out to Tsunami’s Run Up