Tsunamis are well-known phenomenon for the magnitude of destruction that they represent. In addition to large destruction of property and loss of life, they cause the destruction of the environment. In this post, based on a report done with Anna Bohigas and Èlia Martínez during our Master’s studies, we will talk about the environment destruction due to tsunamis, focusing on coral reefs destruction, specially on the event of 26 December 2004 in the Indian Ocean.
WHAT IS A TSUNAMI?
A tsunami is a natural phenomenon consisting of a series of waves generated by a vertical displacement of the sea surface on a massive scale, caused by an earthquake, underwater volcanic eruption, landslide or other abrupt disturbance. Tsunamis have particularly long wavelengths (100-700 km), travel at high speeds (500-960 km/h) over great distances with losing little energy and have periods of 10-60 minutes. As a tsunami approaches shallow waters, the velocity of the waves decreases with minimal loss of energy, such that the wave increase in height.
WHAT DID IT HAPPEN ON 26 DECEMBER 2004?
On 26 December 2004, a major earthquake with a magnitude between 9.15 and 9.3, originated about 30 km deep within the earth’s crust off Sumatra (Indonesia), and a series of secondary earthquakes took place and ruptured a 1,300 km segment of the Sunda Arc megathrust stretching from Sumatra (3ºN) to the Andaman Islands (14ºN).
The total energy released was more than 1,500 times that of the largest nuclear bomb ever detonated. The vertical seafloor displacement reached its maximum of 15-20 m and caused many simultaneous tsunamis that radiated around the Indian Ocean, displacing more than 30 km3 of seawater. This is known as the Indian Ocean (or Boxing Day) Tsunami. The tsunami arrived within 30-40 minutes, with measured run-up heights exceeding 30 m.
DAMAGE ON CORAL REEFS
The impact of tsunamis on coral reefs has been widely assessed by researchers. There are several factors that influence the vulnerability of coral reefs to damage by tsunamis. These factors are:
- Local bathymetry: coral reefs in shallow waters (less than 10 m) suffer more damage than those in deep water. In addition, the presence of channels on the direction of the striking waves or bays increases the damage.
- Land erosionability: erodible land soil damages the coral reefs by smothering the organisms when tsunami impacts the coast.
- Degree of exposure: damage on sheltered coral reefs is minor than when completely exposed. In addition, the distance from the epicentre seems to be important in some cases.
- Tide status: damage on coral reefs is minor when the tide is low.
- Reef condition: damage is higher on coral reefs previously affected by bleaching (read more about coral bleaching in this post) and/or fractured by destructive fishing (like bomb fishing).
- Coral type: branching corals are more affected than encrusting and massive growth forms.
- Earthquake effects: the tsunami shatters the coral when the earthquake fractures the reef. When the earthquake uplift the coral reef, the tsunami impact is greater.
- Geological composition of the reef: corals growing on loose or rubble are more affected than those growing on carbonate reefs or granitic islands.
In the 26 December 2004 event, the damage to the reefs was relatively modest, with the waves causing far less damage to the reefs than the cumulative direct anthropogenic stresses and the El Niño/La Niña switches of 1998. During the tsunami, there were small areas of coral reef loss and major damage, but the coral reefs in the Indian Ocean will largely recover from the tsunami damage in 5-10 years.
The tsunamis following the earthquakes damaged coral reefs through 3 mechanisms: wave action which dislodged, smashed and moved coral and rubble; smothering of corals by increased sediment movement; and mechanical damage and smothering by debris from the land.
One of the most affected regions in the 2004 tsunami was Indonesia, placed 40 km away from the epicentre. Here, waves had 30 m and flooded villages up to 500 m inland. Before the tsunami, Indonesia had approximately 972.5 km2 of coral reef and more than 590 hard species, with many reefs containing more than 140 species, which were affected by human impacts. After the tsunami, there was 30% of damage on coral reef. In some particular locations, 75% of the reefs were totally destroyed, while where hard coral was present the damage was negligible. In addition, reefs near the epicentre were uplifted out of the water and killed, whereas nearby deeper reefs were apparently unaffected. On other reefs there was substantial mechanical damage, mainly due to debris and sediments washed off the land.
OTHER ENVIRONMENTAL EFFECTS
Other environmental effects of tsunamis are the amount of debris left in land, which may carry hazardous materials, seawage and toxic substances. The salinization of freshwater is another concern. In addition to coral reef loss, other ecosystems might be affected, such as mangroves, coastal areas, wetlands, agricultural fields and forests.
- Foster R, Hagan A, Perera N, Gunawan CA, Silaban I, Yaha Y, Manuputty Y, Hazam I, and Hodgson G (2006). Tsunami and Earthquake Damage to Coral Reefs of Aceh, Indonesia. Reef Check Foundation, Pacific Palisades, California, USA. 33 pp.
- Hettiarachchi S, Samarawickrama S and Wijeratne N (2011). Risk Assessment and Management for Tsunami Hazard Case Study of the Port City of Galle. United Nations Development Programe, Asia-Pacific Regional Centre. Bangkok, Thailand. 40 pp
- Tarbuck E and Lutgens F (2005). Ciencias de la Tierra. Una introducción a la geología física. Pearson Educación, Madrid. 736 pp
- Wilkinson C, Souter D and Goldberg J (2006). Status of coral reefs in tsunami affected countries: 2005. Australian Institute of Marine Science. Australia. 160 pp