Storm surge

Also called: Surge · Topic: Natural Hazards and Disaster Management in India · NCERT: Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"; Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"; Class 11, Ch 13 "Movements of Ocean Water"

Meaning

A storm surge is an abnormal rise of the sea above the normal astronomical tide (the regular tide caused by the pull of the Moon and Sun). A tropical cyclone causes it: its strong onshore winds and very low pressure push this extra water onto the coast.

It matters because the surge, not the wind, causes most cyclone deaths. It also floods farmland, and the salt it leaves behind harms crops for years.

Explanation

How a storm surge forms

Three forces work together: the air, the sea and the land.

• Wind pushes the water
• Strong onshore winds (winds blowing from the sea towards the land) pile seawater against the coast.

• NCERT gives an average wind speed of about 180 km/h on the coast.

• Low pressure lifts the water

• Air pressure at the centre of a cyclone is very low.

• With less air pressing down, the sea surface bulges upward under the storm.

• Rain blocks the rivers

• Heavy cyclone rain swells the rivers.

• The rivers cannot drain into a sea that is already raised, so the water backs up and floods the land.

• Result: a wall of water moves inland when the cyclone makes landfall (crosses the coast).

Why the cyclone has so much power

• A tropical cyclone is a very strong low-pressure system (an area where air pressure is lower than all around it). Winds spiral into its centre.
• Its energy comes from latent heat of condensation. This is the heat stored in water vapour. It is released when the vapour turns back into water droplets.
• Warm sea water evaporates, and the moist air rises.
• The vapour condenses into clouds and releases latent heat.

• The heat makes the air rise faster, so surface pressure drops further. A deeper low makes a bigger bulge in the sea.

• The strongest winds are in the eyewall (the ring of tall clouds around the calm eye). So the strongest push on the sea comes when the eyewall reaches the coast.

Conditions that make a surge worse

• Shallow sea floor: the water cannot spread downward, so it piles up higher.
• Gently sloping coast: the water travels further inland.
• Funnel-shaped coast: the coast narrows, squeezes the water and raises it even more.
• Low, crowded deltas: many people live in weak houses only a little above sea level.

What a surge does

• floods settlements and farmland
• makes soil salty (salinisation), which harms crops for years
• knocks down buildings and other structures
• causes most cyclone deaths, more than the wind does

Where it occurs

• Head of the Bay of Bengal: shallow and funnel-shaped. It has some of the highest surges in the world.
• Sundarbans delta (West Bengal and Bangladesh), Bangladesh and Odisha: the coasts that suffer most.
• East coast of India (Odisha, Andhra Pradesh, Tamil Nadu, West Bengal):
• The Bay of Bengal gets about 4 times as many cyclones as the Arabian Sea.

• Bay storms peak in October-November and move west or north-west, straight onto this coast.

• West coast (Gujarat, Maharashtra): the risk is growing because Arabian Sea storms are getting stronger, for example Tauktae (2021) and Biparjoy (2023).

• Recorded and forecast surges:
• Phailin (2013), Ganjam district, Odisha: 2-2.5 m above the tide in low areas. Salt water reached about 1 km inland. [9]
• Fani (2019), Odisha: a surge of about 1.5 m above the tide was forecast for low-lying districts. [10]

• Amphan (2020), Sundarbans: up to 4-5 m above the tide was forecast for West Bengal and 2.5-3.5 m for Bangladesh. [7]

• Who gives warnings: RSMC New Delhi (Regional Specialised Meteorological Centre, run by IMD) sends cyclone and storm-surge warnings to 13 member countries of the WMO/ESCAP Panel on Tropical Cyclones. [4][5]

Don't confuse with

• Astronomical tide: the Moon and Sun cause it. It comes every day and can be predicted. A storm surge is an extra rise above this tide, and it comes only with a cyclone.
• Tsunami: an undersea earthquake or similar shock moves the sea floor and causes it, so it has nothing to do with weather. A storm surge is caused by the weather: cyclone winds and low pressure.
• Cyclone wind damage: the strongest winds are in the eyewall, and they knock down trees and power lines. But the surge, not the wind, causes most cyclone deaths.
• River flood from heavy rain: fresh water overflows river banks. A storm surge brings salt water from the sea and leaves salinised land behind. Rain makes a surge worse because swollen rivers cannot drain into the raised sea.

Prelims Hooks

• Storm surge, not wind, causes most cyclone deaths.
• Surges are worst on shallow, gently sloping, funnel-shaped coasts, for example the head of the Bay of Bengal, the Sundarbans, Bangladesh and Odisha.
• Three causes of a surge: onshore wind, very low central pressure and heavy rain that blocks river drainage.
• Pairs: Phailin (2013) — Ganjam, Odisha, 2-2.5 m surge [9]; Amphan (2020) — Sundarbans, forecast up to 4-5 m in West Bengal [7]; Fani (2019) — Puri, Odisha, about 1.5 m forecast. [10]
• RSMC New Delhi gives storm-surge warnings to 13 countries. [5]
• Trap: a tsunami comes from an undersea earthquake. A storm surge comes from a cyclone. Both flood the coast, but the causes are different.

Mains Points

• Why the east coast suffers the most surge damage (GS-I):
• The Bay of Bengal is warm and gets about 4 times as many cyclones as the Arabian Sea.
• Its head is shallow and funnel-shaped, which builds high surges.

• Low, crowded deltas (Mahanadi, Godavari-Krishna, Ganga-Brahmaputra/Sundarbans) put many poor people in the path of the water.

• Surge mitigation (GS-III):

• Hard measures: embankments and dykes (raised earth walls that keep the sea out) and saline embankments.
• Natural barriers: shelterbelts (rows of trees along the coast that slow the wind and the surge) and restoring the mangroves lost in the Sundarbans.

• People-centred measures: IMD surge forecasts, multipurpose cyclone shelters and early evacuation under NCRMP, which is run by NDMA with World Bank loans. [2][3]

• From mass deaths to preparedness:

• About 10,000 people died in the 1999 Odisha super cyclone. [6]
• About 45 died in Phailin (2013), even though it was as strong as the 1999 storm. [6][8]
• Accurate surge warnings and OSDMA's (Odisha State Disaster Management Authority's) "zero casualty" evacuation made the difference.
• Problems that remain: salinised farmland, lost livelihoods and weak coastal houses.

Related concepts

• Cyclone hazard mitigation
• National Cyclone Risk Mitigation Project

Read more

• Tropical cyclones and storm surges