Most of the beaches on Earth experience rising and falling sea levels due to gravity orbiting the moon, which is called tides. The difference between high and low tide levels varies from day to day and from place to place. As the tides rise and fall they carry the water on the shores in strong currents, first one way and then another.
The effect of the moon
As the moon orbits the earth, its gravitational pull pulls the water towards it. But the earth also orbits the moon very slightly, and this water turns the other side of the earth and creates another tide. As the Earth rotates, most shores move in and out of both bubbles daily, resulting in two high tides and two low tides.
Spring tides
The Moon’s gravity is the main cause of the tide, but the Sun’s gravity also affects it. When the sun and moon are in line, their gravitational pulls combine to form an extra large tide. This is called spring tide, and it occurs twice a month, on the full moon and the new moon. During a bi-monthly crescent, the sun’s gravity pulls in a different direction, causing very small ocean currents.
Ebb and flow
As the tide rises, it moves along the long shores of the tidal stream. During rising tides the currents flow on one side and during falling tides they flow on the other side. This makes the water journey difficult, as the boat crossing the tide can be carried sideways as fast as it moves forward. To compensate, he has to move in a different direction.
Local tides
The size of the beach can make a big difference in tidal rise and fall. In some places, such as here in Cornwall, England, it acts like a funnel, concentrating tidal currents so they create very large tidal currents, They go too high and fall too low. But on the other shores there are very small tidal queues and on some there is only one tide a day.
Race and Whirlpool
On some shores, tides and currents can create dangerously fast local currents. This usually happens when the tidal current has to be forcibly diverted through a narrow valley or around a famous headland, creating intense, dangerous waves of tidal race. Sometimes the flow can return to itself and become a whirlpool.
Rotating water
Whirlpools form when opposing currents come together. The large, powerful whirlpool is known as the Maelstroms.
Tidal Rivers
Tides also force water to flow into river basins. This stops the flow of the river and it can reverse. As the river water stops flowing, the suspended particles settle on the river bed and form mudflats that open up several river basins at low tide. In some places, very strong rising tides can bring water waves into tidal boreholes. It can be high enough to surf on and can reach speeds of up to 15 mph (25 km / h).
Gleaming mud
As the tide level in the mouth of this river decreases, the water flowing down the winding creek exposes the tidal mud.
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