Tsunamis and coastal defences

Detail

1. What a tsunami is

• Tsunami: a Japanese word meaning "harbour wave". It is a series of long ocean waves, not one single wave. It is also called a seismic sea wave.
• Cause → process → result
• Cause: the sea floor moves suddenly. This can happen because of:
  • an earthquake under the sea (the most common cause)
  • a volcanic eruption under the sea or on the coast
  • a submarine landslide (a large slide of mud and rock on the sea floor, down an underwater slope). NCERT does not list this cause.
• Process: the moving sea floor lifts or drops the whole column of water above it, from the sea bed to the surface. Gravity pulls the water back to its level, and the energy spreads outward as waves.

• Result: a train of waves races across the ocean, many minutes apart, and floods low coasts far from the source.

• Where tsunamis are frequent: around the Pacific Ring of Fire (the belt of volcanoes and earthquake zones around the edges of the Pacific Ocean) and in Southeast and South Asia.

• Why losses are high: coasts are densely populated, with fishing villages, ports, tourist beaches and farmland on low ground.

2. How a tsunami behaves: deep sea compared with the coast

• Wavelength: the distance from one wave crest to the next. Period: the time between two crests passing one point.
• Shallow-water wave: a wave whose wavelength is much longer than the depth of the water. A tsunami behaves like this even in the deep ocean. Its speed therefore depends on water depth. The deeper the water, the faster the wave.

Feature	Deep ocean	Near the coast (shallow water)
Wavelength	Very long (hundreds of km)	Becomes shorter
Period	Stays the same	Stays the same
Height	Small. Ships rise only 1-2 m over several minutes and barely notice	Grows to 15 m or more
Speed	Fastest, about 700-800 km/h (as fast as a jet plane)	Slows down sharply
Danger	Low for ships	Very high. Water floods far inland

• Correction to NCERT: NCERT says waves are faster in shallow water. In fact tsunamis are fastest in the deep ocean and slow down near the coast.
• Why the wave grows at the coast (shoaling: the change in a wave as it moves into shallower water)
• The front of the wave reaches shallow water first and slows down.
• The back of the wave is still in deeper water and moving fast, so it catches up.

• The same energy is squeezed into a shorter, slower wave, so the water piles up into a tall wall.

• Warning sign: sometimes the sea first pulls back a long way from the beach and exposes the sea floor. This happens when the trough (the low part of the wave) arrives before the crest. People should move to high ground at once.

3. The 26 December 2004 Indian Ocean tsunami

• Trigger: a magnitude 9.1 earthquake (estimates range from Mw 9.1 to 9.3) off the north-west coast of Sumatra, Indonesia, on 26 December 2004.
• Mw (moment magnitude): the scale that measures the total energy released by a large earthquake.

• It was the third-largest earthquake in the world since 1900, and the largest since the 1964 Prince William Sound (Alaska) earthquake [5].

• Tectonic setting: the quake struck on the Sunda megathrust. A megathrust is a huge, gently sloping fault where one plate slides under another. Here the Indo-Australian Plate slides under the Burma (Sunda) micro-plate.

• Size of the rupture
• The break in the crust started near northern Sumatra. It ran northward for at least 1,200 km, almost to the coast of Myanmar, in about 8 minutes [6].
• The crust moved by an average of 15 m [6].

• The broken zone passed under the Andaman and Nicobar Islands, which is why the Indian islands were hit within minutes.

• Wave height and reach: waves rose as high as 30 m. They travelled more than 4 km inland in low-lying Aceh Province of Sumatra [6].

• Deaths
• More than 2,30,000 people died in 14 countries around the Indian Ocean (2004) [5]. (NCERT: over 3,00,000.)

• NOAA lists 2,27,898 dead or missing. Indonesia accounts for 1,67,540 of them. More than half of all victims lived in Aceh, and Banda Aceh was the worst-hit city [6].

• India (2004)

• Over 10,000 dead (verify current official figure).
• Tamil Nadu, especially Nagapattinam district, suffered the most deaths on the mainland.
• In the Andaman and Nicobar Islands:
  • Car Nicobar was badly damaged.
  • Land sank at Indira Point (the southern tip of Great Nicobar, India's southernmost point), and part of it went under the sea.

• The kegy106 map (Figure 6.3) shows the areas the tsunami hit.

• Lesson: a tsunami crosses whole oceans. No single country can manage the risk alone. In 2004 the Indian Ocean had no warning system, unlike the Pacific. After the disaster, India joined the international tsunami warning system.

4. Indian Tsunami Early Warning Centre (ITEWC)

• ITEWC: India's national centre that detects tsunamis and issues warnings. It is located at INCOIS (Indian National Centre for Ocean Information Services), Hyderabad, under the Ministry of Earth Sciences.
• India set it up in response to the 2004 tsunami [2].

• It has been working since October 2007.

• How it detects a tsunami

Instrument	What it does
Seismic stations	Record an undersea earthquake and find its location, depth and magnitude within minutes
Bottom pressure recorders (BPRs)	Sit on the deep sea floor. A tsunami passing overhead changes the weight of water above them, and they record this change in pressure
Tide gauges	Measure sea level at the coast and confirm whether a wave has actually arrived

• Speed: it can issue an advisory within about 10 minutes of an earthquake.
• Regional role
• Since 2011 it has been a UNESCO-IOC Regional Tsunami Service Provider (RTSP) for the Indian Ocean. UNESCO-IOC is UNESCO's Intergovernmental Oceanographic Commission.
• It works under the Indian Ocean Tsunami Warning and Mitigation System (IOTWMS).

• It provides tsunami services to 25 Indian Ocean countries [2][3].

• India's two tsunami source zones

Source zone	Sea	Region threatened
Andaman-Sumatra subduction zone	Bay of Bengal / Andaman Sea	Andaman and Nicobar Islands, east coast (Tamil Nadu, Andhra Pradesh, Odisha, West Bengal)
Makran subduction zone (off Pakistan and Iran)	Arabian Sea	Gujarat, Maharashtra and the west coast

• Subduction zone: a place where one tectonic plate sinks below another. Such zones produce the largest earthquakes and most tsunamis.

5. Community preparedness: Tsunami Ready

• Tsunami Ready: a UNESCO-IOC programme that recognises coastal communities that are prepared for a tsunami.

• A community must meet 11 indicators [2]. These include:

  • a community tsunami risk reduction plan
  • mapped tsunami inundation zones (areas the water would flood)
  • public display of tsunami information

• Pilot in Odisha: the Odisha State Disaster Management Authority (OSDMA) ran the programme in 6 villages [2]:

• Boxipalli-Venkatraipur (Ganjam)
• Noliasahi (Jagatsinghpur)
• Keutajanga (Puri)
• Jayadevkasaba Pahi (Balasore)
• Podhuan (Bhadrak)

• Tantiapal Sasan (Kendrapara)

• Recognition

• Venkatraipur (Ganjam) was recognised on 7 August 2020 [4].
• Noliasahi (Jagatsinghpur) was recognised in 2020 [2].
• The village work included inundation and evacuation maps, signs for evacuation routes and assembly points, public education and drills, and an emergency operations plan [4].
• With these two villages, India became the first country in the Indian Ocean region, and Odisha the first state, to get Tsunami Ready recognition [2]. (Verify the current count of recognised villages.)

6. Coastal defence by natural buffers

• Natural buffer: a natural coastal feature that absorbs or breaks wave energy before the wave reaches people.

Buffer	Meaning in simple words	How it protects
Offshore bar	A ridge of sand or shingle in the sea, roughly parallel to the coast	Waves break on it before reaching the shore
Barrier island	An offshore bar that has grown above sea level into a long island	Takes the first blow of the waves and shelters lagoons and the mainland
Beach	A gently sloping strip of sand at the shore	Spreads out wave energy
Dunes	Hills of wind-blown sand behind the beach	Act as a natural wall against flooding
Mangroves	Salt-tolerant trees in the tidal zone, with dense stilt roots	Roots and trunks slow the water and trap debris

• 2004 evidence from mangroves
• Villages behind the Pichavaram mangroves (Cuddalore, Tamil Nadu) and the Muthupet mangroves (Tiruvarur, Tamil Nadu) suffered less damage than open coasts nearby.
• Bhitarkanika (Kendrapara, Odisha) is a similar mangrove shield on the east coast.

• Link: natural-vegetation-forests (mangrove/tidal forests).

• Coastal Regulation Zone (CRZ) Notification 2019

• CRZ: the strip of land along the coast, from the High Tide Line inland, where building is controlled by law under the Environment (Protection) Act, 1986.

• The 2019 notification limits construction near the shore. This keeps homes away from the zone that waves can flood, and protects dunes and mangroves.

• Built defences (sea walls, embankments and coastal shelterbelts of casuarina) add to natural buffers. They do not replace them.

Prelims Hooks

• "Tsunami" is Japanese for "harbour wave". It is also called a seismic sea wave.
• Trap: tsunamis are fastest in the deep ocean (about 700-800 km/h) and slow down near the coast. A statement that they are "faster in shallow water" is wrong.
• As a tsunami moves from deep to shallow water, the period stays the same, the wavelength shortens and the height increases.
• 2004 tsunami: M 9.1, off north-west Sumatra, on the Sunda megathrust. The rupture ran about 1,200 km north towards Myanmar [6]. It was the third-largest earthquake since 1900 [5].
• ITEWC is at INCOIS, Hyderabad, under the Ministry of Earth Sciences. It has worked since October 2007, has been a UNESCO-IOC RTSP since 2011, and serves 25 countries [2][3].
• India's two tsunami source zones are Andaman-Sumatra (Bay of Bengal) and Makran (Arabian Sea).
• First Tsunami Ready communities in the Indian Ocean region: Venkatraipur (Ganjam) and Noliasahi (Jagatsinghpur), both in Odisha (2020) [2][4].
• Mangrove–state pairs: Pichavaram (Tamil Nadu), Muthupet (Tamil Nadu), Bhitarkanika (Odisha).
• A bottom pressure recorder detects a tsunami by the change in water pressure on the deep sea floor. A tide gauge confirms its arrival at the coast.

Mains Points

• Why tsunamis cause heavy losses in India (GS-III)
• Two subduction source zones lie close to India (Andaman-Sumatra and Makran).
• Coasts such as Nagapattinam and the Andaman and Nicobar Islands are low-lying and densely populated.

• In 2004 there was no warning system, so there was no time to evacuate.

• Policy response after 2004

• Detection: ITEWC at INCOIS (2007), with seismic stations, BPRs and tide gauges, issuing advisories in about 10 minutes.
• Regional role: UNESCO-IOC RTSP status (2011), serving 25 Indian Ocean countries [3].
• Community level: Tsunami Ready villages in Odisha (2020) [2][4].

• This chain from detection to last-mile community preparedness matches the Hyogo (2005) and Sendai (2015-2030) frameworks' focus on early warning.

• Ecosystem-based disaster risk reduction

• Mangroves (Pichavaram, Muthupet, Bhitarkanika), dunes and barrier islands absorb wave energy.

• Their 2004 record supports protecting them under the CRZ Notification 2019 instead of relying only on sea walls.

• Limits of the system

• Near-field tsunamis give very little warning. In 2004 the Andaman and Nicobar Islands were hit within minutes.
• Warnings must therefore be backed by drills, evacuation maps, land-use control along the shore, and natural buffers.