2.2.5. Characteristics of Tides

♤ Tidal range: The difference between the high tide water and the low tide water is called the tidal range. The time between the high tide and low tide, when the water level is falling, is called the ebb. The time between the low tide and high tide, when the tide is rising, is called the flow or flood.

Once in a month, when the moon’s orbit is closest to the earth (perigee), unusually high and low tides occur. During this time the tidal range is greater than normal. Two weeks later, when the moon is farthest from earth (apogee),the moon’s gravitation force is limited and the tidal ranges are less than their average heights.

When the earth is closest to the sun (perihelion), around 3rd January each year, tidal ranges are also much greater, with unusually high and unusually low tides. When the earth is farthest from the sun (aphelion), around 4th July each year, tidal ranges are much less than average.

♤ Tidal current: Tidal currents (a horizontal motion) are a result of the rise and fall of the water level due to tides (a vertical motion). The effects of tidal currents on the movement of water in and out of bays and harbours can be substantial.

The tidal bulges on wide continental shelves, have greater height. When tidal bulge shit the mid-oceanic islands they become low. The shape of bays and estuaries along a coastline can also magnify the intensity of tides. Funnel-shaped bays greatly change tidal magnitudes. The highest tides in the world occur in the Bay of Fundy in Nova Scotia, Canada. The tidal bulge is 15 - 16 m.

♤ Tidal bore: When the tide enters the narrow and shallow estuary of a river, the front of the tidal wave appears to be vertical due to the piling up of the river water against the tidal wave and the friction of the river bed. It looks as if a vertical wall of water is moving upstream. This is called a tidal bore. In India tidal bores are common in the Hugli River.