Tides

Notes

Tides are the periodic rise and fall of sea level, caused chiefly by the gravitational pull of the Moon (and, less so, the Sun) and the centrifugal force of the Earth-Moon system. The Moon is closer, so its pull is more than twice that of the Sun. A tidal bulge occurs on the side facing the Moon (gravity) and the opposite side (centrifugal force).

Types

Spring tides: at full moon and new moon, when the Sun, Moon and Earth are in a line; the highest high tides and lowest low tides (greatest range); twice a month.
Neap tides: at first and last quarter, when the Sun and Moon are at right angles; the lowest range; about a week after a spring tide.
By frequency: semi-diurnal (two highs and two lows a day; the Atlantic coasts), diurnal (one high and one low; the Gulf of Mexico) and mixed.
Successive high tides are about 12 hours 26 minutes apart, since the lunar day is about 24 hours 52 minutes.
Perigee (Moon closest) produces higher tides; apogee, lower tides. Perihelion (Earth closest to the Sun, about 3 January) also raises tides. Southampton has double high tides.

Tidal features

Tidal bore: a wave that moves up a funnel-shaped estuary against the river current: the Hooghly, Amazon (Pororoca) and Qiantang (China).
Tidal range: the highest in the world is in the Bay of Fundy (Canada), about 15-16 m. In India, the Gulf of Khambhat (Cambay) and the Gulf of Kutch (Gujarat) have large ranges (about 6-7 m or more) and the Sundarbans also has tidal potential.
Tidal currents (flood and ebb).

Significance

Navigation and ports: tidal ports and harbours (Kolkata uses Hooghly tides; Kandla).
Tidal energy: La Rance (France), Kislaya Guba (Russia), Annapolis (Canada); India's potential in Khambhat, Kutch and the Sundarbans.
Fishing, flushing of harbours and estuaries, salt production, and coastal erosion and flooding (storm surges at high tide).

Key Points

Moon is the main cause; Sun's pull about half of the Moon's.
Spring tides at full and new moon; neap tides at quarter moons.
Tidal bore: Hooghly and Amazon; highest tidal range: Bay of Fundy.
India's tidal potential: Khambhat, Kutch, Sundarbans.
Tidal energy: La Rance (France).

MCQs

Q1. Spring tides occur:

(a) At the quarter moons
(b) At full moon and new moon
(c) Only at new moon
(d) Only at full moon
Ans: (b).

Q2. The highest tidal range in the world is found in the:

(a) Gulf of Khambhat
(b) Bay of Fundy
(c) Gulf of Mexico
(d) North Sea
Ans: (b).

Q3. A tidal bore is seen in the:

(a) Ganga
(b) Hooghly
(c) Narmada
(d) Mahanadi
Ans: (b). Also in the Amazon (Pororoca).