Natural shield necessary to protect RI coastline
Bambang M, Contributor, Jakarta
A lot can be learned from the massive earthquake and tsunami that devastated part of Nanggroe Aceh Darussalam and North Sumatra provinces, on Dec. 26.
Among other things, this natural disaster has reminded the public of the need to have a natural shield along the coast to withstand the devastating blow of a tsunami.
If only the coastal area of Banda Aceh had had such a shield, fewer people would have been killed in the disaster and much of the destruction of Banda Aceh would have been prevented.
A natural shield might take the form of a mangrove forest, a green belt or a coastal forest comprising various species of suitable trees.
In addition, concrete fortifications could also be built along the coast to withstand a powerful tsunami. Such a concrete structure, however, would be costly to build and would not be as strong as a natural shield.
Tectonic plates
As Indonesia lies precisely over the meeting point of the Indo-Australian and Eurasian plates, the country is close to many underwater epicenters that might cause a tsunami.
Geological disaster expert Mas Atje Purbawinata said that based on the history of earthquakes and their setting, Indonesia had 28 areas prone to tsunamis, such as the coastal areas in Aceh, the western part of North Sumatra, West Sumatra, Bengkulu, South Lampung, west and south Banten, the southern part of Central Java, North, Central and South Sulawesi, north and south Maluku, Biak-Yapen, Fak-Fak and Balikpapan.
Before the tsunami hit Aceh and North Sumatra on Dec. 26, 2004, 16 tsunamis had occurred across the country, with the worst three occurring in Flores in 1992, Banyuwangi in 1994 and Biak in 1996.
A shield would be particularly appropriate for a coastal area that faces an epicenter," said Eko Teguh Paripurno, a lecturer at the school of geology, National Development University, Yogyakarta.
Coastal areas that need such protection have shores that shelve steeply into the sea, like the beach around Parangtritis. "In Yogyakarta's deep waters, a tsunami moves fast," said Eko, who is also an environmentalist.
A dense mangrove forest in a coastal area would be able to withstand a tsunami. That is why one of the items on the working agenda of the forestry ministry's Disaster Management Coordination Team in Aceh province and North Sumatra is the rehabilitation of the mangrove forests in these provinces (visit www. Dephut.go.id).
"One of the functions of a mangrove forest is to stabilize the land," said Erny Poedjirahajoe, a mangrove expert from the forestry school of Gajah Mada University (UGM). A mangrove forest protects the land from abrasion, seawater intrusion, wind from the sea and the tsunami itself.
How mangroves protect the coast
When the tsunami that followed the earthquake on Dec. 26, 2004, destroyed coastal areas in Aceh, North Sumatra and a few other countries in South Asia plus Somalia in Africa, Bangladesh saw the smallest number of disaster victims.
One of the reasons is that Bangladesh still has a vast mangrove forest area," said Rudhy Pribadi, a mangrove expert from Diponegoro University, Semarang. He said that the mangrove forests in Bangladesh were properly conserved and served to protect the country's coastline, which is buffeted by 40 storms every year.
In his 2003 thesis for his master's degree titled Mangrove Forests as Protectors of Coasts Against Tsunami Runup (a physics model simulation) in the postgraduate program at UGM, Karuniadi Satrijo Utomo indicates that a mangrove forest may considerably reduce the energy and runup distance of a tsunami.
A mangrove forest with 5 percent density, a height of five meters and a breadth of fifty meters will be able to cut 52 percent of the height of the tsunami, 38 percent of its energy and 14 percent to 22 percent of its runup distance on the coast with slopes of five, 10 and 15 degrees.
Meanwhile, in his paper written for the school of fisheries and maritime sciences of Diponegoro University (2003), titled The Influence of Density and Breadth of Clumps of Mangrove in Withstanding the Tsunami (a physics model simulation), M. Qodri Agus writes about the important role of a mangrove forest in reducing the destructive power of a tsunami.
In this thesis he argues that the density of the mangrove forest plays a bigger role in reducing the power of the tsunami than its breadth.
Natural protection
Indonesia has the world's largest mangrove forests, with 8.6 million hectares. About 3.8 million hectares are found in forest areas while the remaining 4.8 million hectares grow in coastal areas.
Some of the mangrove species found in Indonesia are bakau (Rhizopora spp), apiapi (Avicenia spp), pedada (Sonnerata spp) and (Xylocarnus spp).
Unfortunately, much of this natural shield has been destroyed by man. Many mangrove forest areas along the country's coast have been converted into fish farms.
According to 1999 data of the directorate general of rehabilitation of land and social forestry of the Ministry of Forestry, about 68 percent (some 5.9 million hectares) of mangrove forest in Indonesia have disappeared.
As many areas in Indonesia are prone to tsunamis, it is imperative that mangrove forest areas are rehabilitated.
Fishermen wishing to boost their income by establishing fish hatcheries should avoid clearing mangrove forests.
In fact, there is a mutually beneficial relationship between a mangrove forest area and a fish farming pond. Mangrove leaves that fall into the pond become a food source for fish and shrimps.
"The research that I have made on the northern coastal area of Java shows that a fish farm made in a mangrove forest will yield better results than a pond with no mangrove surrounding it," said Erny, who has planted mangroves on the northern coast of Java for a decade.
Care needed when planting mangroves
Although mangroves are effective in withstanding a tsunami, Erny has reminded the government not to be careless when cultivating mangroves on the coast.
Not all coastal areas are suitable for mangroves. The area around the estuary of the Bogowonto River on the Congot coast, Yogyakarta, is prone to tsunamis but the planting of mangroves there has failed.
"I wish to call on policy makers to carry out research before deciding to plant mangroves in a particular area,"Erny said. In this research, it is necessary to ascertain soil conditions, salinity and the time when there will be strong winds. "If a mangrove is planted when there is a strong wind, the seeds will be blown away by the wind," she added.
In her experience, mangrove planting will fail if it is done only for the sake of carrying out a project. If little is done to take care after it is planted, only a small number of the mangrove seeds will grow.
Therefore, the coastal community must be directly involved in mangrove planting. They must be assisted so that they will really understand the benefit of such activities.
Other plants are effective, too As not all coastal areas are good for mangroves, Erny said that a non-mangrove forest area can be established on the coast to withstand a tsunami.
Baron Beach in Yogyakarta is a good example. Several species of trees suitable for this purpose are, for example, cemara udang (Casuarina sp), sengon, maleleuca and ketepeng. If these non- mangrove forest areas are managed in a sustainable manner, people living on the coast can also boost their income and make use of the wood," Erny added.
Aside from coastal forest areas, green belts must also be established on coastal areas where mangroves are unsuitable. These green belts, which can comprise a variety of large, strong trees, are necessary for the southern coastal areas of Java as the area is unsuitable for mangroves.
In their study titled Post-Tsunami Mitigation Investigation in South Banyuwangi, East Java in 1995, Kris Budiono and colleagues from the Center for Maritime Geology in Bandung, under the auspices of the directorate general of geology and mineral resources of the Ministry of Mining and Energy, noted that the territorial waters in the south of Java are the most active areas in Indonesia due to insertion of the Indo-Australian plate under the Eurasian plate.
In the last hundred years, there have been about 20 earthquakes of 6 to 7.7 magnitude on the Richter scale under the sea.
An example of a green belt that can withstand a tsunami can be found in the Air Hitam coastal area and the Pantai Panjang tourist beach in Bengkulu, Sumatra.
These areas sustained seven tsunamis between 1770 and 1958. According to Nur Adi Kristanto, in a report titled Natural Disaster Mitigation Investigation in Bengkulu and its Surroundings (directorate general of geology and mineral resources, Ministry of Mining and Energy, 1990), green belts are multifunctional as they can stop floating objects like wood and vessels, reduce water speed, save drowning people and reduce winds that carry fine sand so that the sand can form dunes, which is also another type of shield able to reduce the impact of a powerful tsunami.
It is now time for the Indonesian government and people to learn from the natural disaster that hit Aceh and North Sumatra.
Natural shields must immediately be established in tsunami- prone coastal areas. Erny is right when saying that to prevent a natural disaster we must make use of the forces of nature, such as mangrove forests, coastal forests and green belts.
Unless this measure is taken, we could suffer a repeat of the tragedy in Aceh.