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Tidal Currents

Flood and Ebb CurrentsAs the tides rise and fall, they create flood and ebb currents.
Tidal currents occur in conjunction with the rise and fall of the tide. The vertical motion of the tides near the shore causes the water to move horizontally, creating currents. When a tidal current moves toward the land and away from the sea, it “floods.” When it moves toward the sea away from the land, it “ebbs.” These tidal currents that ebb and flood in opposite directions are called “rectilinear” or “reversing” currents.

Rectilinear tidal currents, which typically are found in coastal rivers and estuaries, experience a “slack water” period of no velocity as they move from the ebbing to flooding stage, and vice versa. After a brief slack period, which can range from seconds to several minutes and generally coincides with high or low tide, the current switches direction and increases in velocity.

Earth and MoonThe relationship between the masses of the Earth, moon, and sun and their distances to each other play critical roles in affecting tides and the currents they produce.
Tidal currents are the only type of current affected by the interactions of the Earth, sun, and moon. The moon’s force is much greater than that of the sun because it is 389 times closer to the Earth than the sun is. Tidal currents, just like tides, are affected by the different phases of the moon. When the moon is at full or new phases, tidal current velocities are strong and are called “spring currents.” When the moon is at first or third quarter phases, tidal current velocities are weak and are called “neap currents.”

Earth and MoonThe elliptical orbits of the moon around the Earth and the Earth around the sun have substantial effects on the Earth’s tides and the currents they produce.
Also similar to tides, tidal currents are affected by the relative positions of the moon and Earth. When the moon and Earth are positioned nearest to each other (perigee), the currents are stronger than average and are called “perigean currents.” When the moon and Earth are at their farthest distance from each other (apogee), the currents are weaker and are called “apogean currents.”

The shape of bays and estuaries also can magnify the intensity of tides and the currents they produce. Funnel-shaped bays in particular can dramatically alter tidal current magnitude. The Bay of Fundy in Nova Scotia is a classic example of this effect, and has the highest tides in the world - over 15 meters (Thurman, H.V., 1994).

High and Low Tide Move your computer mouse over the image above to see the differences between high and low tides in the Bay of Fundy. Photos © Scott Walking Adventures.

Source: NOAA's Ocean Service

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