Geography — World, India and Andhra Pradesh for Group-I · Chapter 5

Oceans and Hydrological Hazards

What to remember

Ocean currents are driven by planetary winds, the Coriolis force and differences in temperature and salinity; they move in gyres, clockwise in the northern hemisphere and anticlockwise in the southern.
Tides come from the pull of the Moon and Sun; spring tides occur at new and full moon, neap tides at the quarter moons. Tsunamis come from sudden sea-floor movement, not from wind.
Floods, cyclone storm surges and tsunamis are the main water-related hazards of the AP coast; the sea also gives fish, oil and gas, salt, minerals and energy.

Ocean floor and water properties

Continental shelf: The shallow, gently sloping edge of a continent. It is rich in fish, oil and gas.
Continental slope: A steeper drop that follows the shelf.
Abyssal plain: The deep, flat sea floor.
Trenches: The deepest parts, mostly near island arcs and the Pacific rim. The Mariana Trench holds the deepest point (Challenger Deep).
Mid-ocean ridges: Under-water mountain chains, formed where plates move apart.
Temperature: Surface water is warmest near the equator and cooler towards the poles. Temperature falls quickly in the thermocline layer below the surface.
Salinity: Salt in grams per 1,000 grams of water, shown in parts per thousand (ppt). Average ocean salinity is about 35 ppt. It is higher where evaporation is high and fresh water input is low (Dead Sea, Red Sea). It is lower near big river mouths and in cold, ice-melting seas (Baltic Sea).

Ocean currents

Meaning: A current is a large stream of water that flows in a fixed direction through the ocean.
Causes: Prevailing winds, the Coriolis force (deflects right in the north and left in the south), differences in temperature and salinity (density), the shape of the coast and the rotation of the Earth.
Gyres: Circular patterns of currents. They move clockwise in the northern hemisphere and anticlockwise in the southern.
Warm and cold currents: Warm currents flow from the equator towards the poles. Cold currents flow from the poles towards the equator, and also along the west coasts of continents in the tropics.

Ocean	Warm currents	Cold currents
Atlantic	Gulf Stream, North Atlantic Drift, Brazil	Labrador, Canary, Benguela
Pacific	Kuroshio, North Pacific Drift	Oyashio, California, Humboldt (Peru)
Indian	Agulhas, Mozambique	West Australian

Indian Ocean monsoon drift: The currents of the north Indian Ocean change direction with the monsoon. They flow clockwise in the south-west monsoon and anticlockwise in the north-east monsoon. This is a special feature of the Indian Ocean.
Effects of currents:
Warm currents raise coastal temperature and rain (North Atlantic Drift keeps north-west Europe ports ice-free).
Cold currents lower temperature and cause fog and desert coasts (Atacama and Namib).
Where warm and cold currents meet, plankton grows and fish are plentiful. Examples are the Grand Banks near Newfoundland, the seas near Japan and the North Sea.
Currents help ships save fuel and time when they follow them.

Tides

Tide: The regular rise and fall of the sea level. It is mainly due to the gravitational pull of the Moon, with a smaller pull from the Sun.
Frequency: Most places have two high tides and two low tides in a little more than a day (the tidal day is about 24 hours and 50 minutes).
Spring tide: The Sun, Moon and Earth are in a line at new moon and full moon. The tide range is the greatest.
Neap tide: The Sun and Moon are at right angles at the first and last quarters. The tide range is the least.
Perigee and apogee: The Moon is nearest to the Earth at perigee (higher tides) and farthest at apogee (lower tides).
Use: Tides help ships enter ports, help in fishing and salt-making and can give electricity (tidal power). Gulf of Khambhat and Gulf of Kutch have large tidal ranges and suit tidal power.
Tidal bore: A wave of tidal water moving up a river mouth. The Hooghly is known for it.
Highest tide: The Bay of Fundy in Canada has one of the highest tides in the world.

Hydrological hazards

Hazard	Cause	Main effect
Flood	Heavy rain, river overflow, dam break, cyclone surge	Loss of crops, houses, life, disease
Flash flood	Sudden heavy rain on steep, small basin	Fast rise, very little warning
Storm surge	Cyclone winds push sea water inland	Coastal flooding, salt in fields
Tsunami	Sea-floor earthquake, landslide or volcano	Giant waves, coastal destruction

Floods

Causes: Heavy and long rain, cyclones, silt-filled river beds, blocked drains, breach of embankments, deforestation, unplanned building on flood plains.
Types: Riverine flood, flash flood, urban flood and coastal flood.
Regions in India: The Brahmaputra and Ganga basins are the most flood-prone. In the east-coast deltas, the Mahanadi, Godavari and Krishna also flood in the monsoon.
In Andhra Pradesh: The Godavari and Krishna deltas and the Vamsadhara and Nagavali rivers are at risk. Cyclones add to the risk on the coast.
Management: Reservoirs and dams, embankments, channel improvement, flood-plain zoning, afforestation in catchments, flood forecasting and early warning. The Central Water Commission issues flood forecasts. The National Disaster Management Authority (NDMA) and the National Disaster Response Force (NDRF) lead preparedness and relief.

Tsunami

The word is Japanese; it means "harbour wave".
Cause: A sudden vertical movement of the sea floor, mostly in undersea earthquakes at subduction zones. Landslides and volcanic eruptions can also cause it.
Features: In the deep ocean the wave is low and fast (it may move at speed like a jet plane). Near the shore it slows down and rises high.
Distribution: Most tsunamis occur around the Pacific Ocean (the Ring of Fire).
Indian Ocean tsunami of December 2004: It hit the east coast of India, including the AP coast, after a very large earthquake near Sumatra. Afterwards India built a warning system. The Indian Tsunami Early Warning Centre works at INCOIS, Hyderabad, using sea-level gauges, buoys and earthquake data.
Safety: Move to high ground, follow warnings, protect mangroves and coastal shelter belts.

Marine resources

Fisheries: Shelf waters give the main catch. India's Exclusive Economic Zone (EEZ) extends up to 200 nautical miles from the coast. Territorial waters extend up to 12 nautical miles.
AP: The state has one of the longest coastlines in India (third after Gujarat and Tamil Nadu by the 2024 re-measurement; second by the older 974 km figure). Fishing harbours and ports include Visakhapatnam, Kakinada and Machilipatnam. Aquaculture (shrimp and fish ponds) is large in the Godavari and Krishna delta areas.
Mangroves and lagoons: Coringa mangroves (Godavari delta) and the Krishna delta mangroves protect the coast and support fish breeding. Pulicat lake is a large coastal lagoon.
Minerals and energy: Offshore oil and gas (the Krishna-Godavari basin), sand and beach minerals, common salt, polymetallic nodules in the deep sea. Future sources are tidal, wave and ocean thermal energy.
Blue economy: Sustainable use of ocean resources for fishing, shipping, ports and tourism, with care for marine life.

Waves and other water movements

Waves: They are caused mainly by wind blowing over the sea surface. The water moves in small circles; only the wave shape travels forward. A wave breaks when it reaches shallow water.
Wave parts: Crest is the highest point, trough is the lowest point, wave height is the vertical gap between them and wavelength is the distance between two crests.
Upwelling: Cold, nutrient-rich water rises from below to replace surface water pushed away by winds. It happens along west coasts, such as off Peru, and gives good fishing.
Coastal landforms: Waves build beaches, spits and lagoons and cut cliffs and sea caves. A spit that closes a bay forms a lagoon, as at Pulicat.
Coastal protection: Mangroves, shelterbelt plantations, sea walls and early warning reduce damage from storm surge and erosion.

Exam traps

Gyres move clockwise in the northern hemisphere and anticlockwise in the southern. Do not swap them.
Labrador and Canary currents are cold. The Gulf Stream and the Kuroshio are warm.
Humboldt is the cold current off Peru; Benguela is the cold current off south-west Africa.
Spring tide has nothing to do with the spring season; it is the highest tide at new and full moon.
Neap tide happens at the quarter moons, not at the full moon.
Tsunami waves are not caused by wind or tides.
Territorial waters (12 nm) are not the same as the EEZ (200 nm).
Salinity is high where evaporation is high, and low near river mouths.

One-liners

1. The Mariana Trench has the deepest point of the oceans.
2. Average ocean salinity is about 35 ppt.
3. The Gulf Stream is a warm current of the Atlantic.
4. The Humboldt current is a cold current off Peru.
5. The currents of the north Indian Ocean reverse with the monsoon.
6. Spring tides occur at new moon and full moon.
7. The Bay of Fundy has among the highest tides in the world.
8. Tsunami is a Japanese word meaning harbour wave.
9. The Indian Tsunami Early Warning Centre is at INCOIS, Hyderabad.
10. India's EEZ extends up to 200 nautical miles.
11. Coringa has mangroves in the Godavari delta.
12. Gujarat has the longest coastline in India; by the 2024 re-measurement Tamil Nadu is second and AP third (older figures put AP second).

Practice questions

The deepest point of the world's oceans lies in the
1. Atacama Trench
2. Java Trench
3. Puerto Rico Trench
4. Mariana Trench
Answer
D. Mariana Trench
Challenger Deep is in the Mariana Trench.
The average salinity of ocean water is about
1. 3.5 ppt
2. 350 ppt
3. 0.35 ppt
4. 35 ppt
Answer
D. 35 ppt
Oceans average about 35 parts per thousand.
Ocean currents in the northern hemisphere move in gyres that are
1. anticlockwise
2. straight
3. zig-zag only
4. clockwise
Answer
D. clockwise
The Coriolis force deflects water to the right in the north.
The Gulf Stream is a
1. warm current of the Pacific
2. warm current of the Atlantic
3. cold current of the Indian Ocean
4. cold current of the Atlantic
Answer
B. warm current of the Atlantic
It carries warm water from the Gulf of Mexico north-east.
The Humboldt (Peru) current is a
1. cold current of the Indian Ocean
2. warm current of the Pacific
3. warm current of the Atlantic
4. cold current of the Pacific
Answer
D. cold current of the Pacific
It flows along the west coast of South America.
Which of the following is a cold current?
1. Kuroshio
2. Gulf Stream
3. Labrador current
4. Brazil current
Answer
C. Labrador current
Labrador is cold; the others listed are warm.
Spring tides occur when
1. the Moon is at right angles to the Sun
2. the Sun, Moon and Earth are in a line
3. only the Sun pulls the Earth
4. it is the spring season
Answer
B. the Sun, Moon and Earth are in a line
At new moon and full moon the pulls add together.
The word 'tsunami' comes from which language?
1. Hindi
2. Chinese
3. Arabic
4. Japanese
Answer
D. Japanese
It means harbour wave.
The Indian Tsunami Early Warning Centre is at
1. INCOIS, Hyderabad
2. ISRO, Bengaluru
3. IMD, Delhi
4. CWC, Vijayawada
Answer
A. INCOIS, Hyderabad
It was set up after the 2004 tsunami.
India's Exclusive Economic Zone extends up to
1. 350 nautical miles
2. 200 nautical miles
3. 12 nautical miles
4. 24 nautical miles
Answer
B. 200 nautical miles
EEZ is 200 nm; territorial waters 12 nm.
Which state has the longest coastline in India?
1. Odisha
2. Tamil Nadu
3. Gujarat
4. Andhra Pradesh
Answer
C. Gujarat
AP is second.
The Coringa mangroves lie in the delta of the
1. Penna
2. Vamsadhara
3. Tungabhadra
4. Godavari
Answer
D. Godavari
Coringa is a Godavari delta mangrove forest.
The shallow gently sloping edge of a continent under the sea is the
1. continental shelf
2. mid-ocean ridge
3. oceanic trench
4. abyssal plain
Answer
A. continental shelf
Shelf waters are rich in fish.
The Pulicat is an example of a
1. glacial lake
2. coastal lagoon
3. crater lake
4. oxbow lake
Answer
B. coastal lagoon
Pulicat is separated from the sea by a sand spit.
The neap tide is the tide that
1. has the highest range
2. occurs only at full moon
3. occurs only at the equator
4. has the least range, at the quarter moons
Answer
D. has the least range, at the quarter moons
Sun and Moon pull at right angles.
Which cold current flows off the south-west coast of Africa?
1. Agulhas
2. Kuroshio
3. Benguela
4. Brazil
Answer
C. Benguela
The Benguela is cold and helps form the Namib coast.
Which is a major cause of storm surge on the AP coast?
1. Glacier melt
2. Tidal bore
3. Volcano
4. Cyclone winds pushing sea water ashore
Answer
D. Cyclone winds pushing sea water ashore
Cyclones raise sea level near the coast.
Warm and cold currents meeting give rich fishing grounds because
1. there are no waves
2. the water is still
3. plankton grows well there
4. there is no salt
Answer
C. plankton grows well there
Mixing brings nutrients that support plankton and fish.
Currents of the north Indian Ocean reverse with the seasons because of
1. tides
2. monsoon winds
3. earthquakes
4. El Nino only
Answer
B. monsoon winds
They are clockwise in the south-west monsoon and anticlockwise in winter.
Surface salinity is lower near the mouths of big rivers mainly because
1. fresh water dilutes sea water
2. there is little rain
3. the water is cold
4. evaporation is very high
Answer
A. fresh water dilutes sea water
River water adds fresh water.
Which pair is correctly matched?
1. Canary - warm Atlantic current
2. Oyashio - warm Pacific current
3. Kuroshio - warm Pacific current
4. Agulhas - cold Indian Ocean current
Answer
C. Kuroshio - warm Pacific current
Oyashio is cold, Canary is cold, Agulhas is warm.
The Hooghly river is known for
1. tidal bore
2. tsunami
3. storm surge only
4. glaciers
Answer
A. tidal bore
Tidal bores move upriver from the sea.
Fog off Newfoundland is caused by
1. monsoon winds
2. tsunami waves
3. the Sahara
4. meeting of the cold Labrador and warm Gulf Stream waters
Answer
D. meeting of the cold Labrador and warm Gulf Stream waters
Warm moist air cooled over the cold current forms fog.
Most tsunamis occur around the Pacific Ocean because
1. it has the lowest salinity
2. it has many subduction zones and earthquakes
3. it has no currents
4. it is the shallowest ocean
Answer
B. it has many subduction zones and earthquakes
The Ring of Fire has frequent undersea earthquakes.
Which of these is the most common cause of a tsunami?
1. Undersea earthquake with vertical movement of the sea floor
2. Monsoon winds
3. Strong tidal pull
4. Heavy rain
Answer
A. Undersea earthquake with vertical movement of the sea floor
Sudden sea-floor movement displaces a huge water column.
Which flood management method is non-structural?
1. Channel straightening
2. Flood-plain zoning and early warning
3. Reservoir
4. Embankment
Answer
B. Flood-plain zoning and early warning
Zoning and warnings do not need building works on the river.
Which river basin system is the most flood-prone in India?
1. Narmada
2. Cauvery
3. Tapi
4. Brahmaputra and Ganga
Answer
D. Brahmaputra and Ganga
Heavy rain and silt make these basins flood-prone.
The Gulf of Khambhat and the Gulf of Kutch are suited to tidal power because
1. they have low salinity
2. they lie on the equator
3. they have large tidal ranges
4. they have no tides
Answer
C. they have large tidal ranges
A large range gives more energy.
Cold currents along tropical west coasts often cause
1. desert-like coasts with fog
2. frequent cyclones
3. tidal bores
4. heavy rain
Answer
A. desert-like coasts with fog
Cold currents cool the air and reduce rain, as at Atacama.
Which is an offshore resource in the Krishna-Godavari region?
1. Bauxite
2. Oil and gas
3. Iron ore
4. Coal
Answer
B. Oil and gas
The offshore KG basin holds natural gas and oil.
Territorial waters of a country extend up to
1. 200 nautical miles
2. 24 nautical miles
3. 350 nautical miles
4. 12 nautical miles
Answer
D. 12 nautical miles
This is the sovereign zone, smaller than the EEZ.
As a tsunami wave approaches the shore, its
1. speed rises and height falls
2. speed falls and height rises
3. speed and height both fall
4. speed and height are unchanged
Answer
B. speed falls and height rises
Shallow water slows the wave and piles it up.
The tidal range is largest at
1. first quarter, with the Moon at apogee
2. last quarter, with the Moon at perigee
3. new moon and full moon, when the Moon is near perigee
4. any time when it is windy
Answer
C. new moon and full moon, when the Moon is near perigee
Perigee adds to the spring tide effect.
Which statement correctly explains the gyre direction in the southern hemisphere?
1. Water is deflected to the right, giving clockwise gyres
2. Water is not deflected, giving straight flows
3. Water is deflected to the left, giving anticlockwise gyres
4. Water is deflected up, giving upwelling only
Answer
C. Water is deflected to the left, giving anticlockwise gyres
The Coriolis deflection is left in the south.
A sea has very high salinity when
1. evaporation is high and fresh water inflow is low
2. ice melts into it
3. many rivers enter it
4. rainfall is very high
Answer
A. evaporation is high and fresh water inflow is low
This is the case for the Red Sea and the Dead Sea.
Consider the statements. 1. Spring tides occur at new moon and full moon. 2. Neap tides occur when the Sun, Moon and Earth are in a line. Which of the statements is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Neap tides occur at right angles, so only 1 is correct.
Consider the statements. 1. Gulf Stream is a warm current. 2. Canary current is a cold current. Which of the statements is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are correct.
Consider the statements. 1. Tsunami waves are caused mainly by strong winds. 2. Tsunamis can be caused by undersea landslides. Which of the statements is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Tsunamis are not wind waves; only 2 is correct.
Consider the statements. 1. India's EEZ extends up to 200 nautical miles. 2. Territorial waters extend up to 12 nautical miles. Which of the statements is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both limits are standard.
Consider the statements. 1. The Mariana Trench is in the Pacific Ocean. 2. Mid-ocean ridges are formed where plates move apart. Which of the statements is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both statements are correct.
Consider the statements. 1. Warm currents flow from the equator towards the poles. 2. Cold currents never occur along the west coasts of continents. Which of the statements is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Cold currents often flow along west coasts of continents in the tropics, so only 1 is correct.
Consider the statements. 1. The Godavari and Krishna deltas are at risk of floods and cyclone surges. 2. Mangroves protect the coast. Which of the statements is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are correct.
Consider the statements. 1. Currents in the north Indian Ocean change direction with the monsoon. 2. The Mozambique current is a cold current. Which of the statements is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
The Mozambique current is warm, so only 1 is correct.
Consider the statements. 1. The Hooghly has tidal bores. 2. Tides are mostly caused by the pull of the Sun alone. Which of the statements is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Tides are mainly caused by the Moon, so only 1 is correct.
Consider the statements. 1. The National Disaster Response Force assists in flood relief. 2. Flood-plain zoning increases flood damage. Which of the statements is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Zoning reduces damage, so only 1 is correct.

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1.Andaman and Nicobar Islands are located in which sea/ocean?
2.Since 2011 the supreme court banned the manufacture, sale and use of which pesticide that cause hazardous effects to human life as well as the environment? (RRB NTPC CBT-I (17/02/2021) Shift 1)
3.The provision of ‘Prohibition of Employment of Children in Hazardous Jobs’ is reflected in which of the following Fundamental Rights of the Indian Constitution? (RPF SI CBT-I 2024 (13/12/2024) Shift 1)
4.The trench of Japan is located in which ocean? (RPF SI 2018 (10/01/2019) Shift 3)
5.The telegraph cable was laid across the Atlantic Ocean in 1958 for rapid communication between North America and __________. (RRB NTPC GRADUATE CBT 1 (27/03/2026) Shift 1)
6.The Indian Ocean is named after India because:
7.The mysterious Bermuda triangle is located in which of the following ocean? (RPF Constable 2018 (03/02/2019) Shift 2)
8.The ocean with the highest depth is _________. (RPF Constable 2018 (14/02/2019))
9.The ocean covers ________ percentage of Earth’s surface. (RRB JE EE (19/09/2019) Shift 2 CBT 2)
10.ISRO'S communication satellite GSAT-29 carried a high resolution camera named as ______ that is capable of tracking enemy ships in the Indian Ocean. (RRB NTPC CBT-I (17/02/2021) Shift 2)

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