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11.1: Tides

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    10320
    • Contributed by Miracosta Oceanography 101
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    Tides are one of the most reliable and predictable phenomena in the world.

    What are tides and how they are created?

    Tides are cause by the gravitation pull of extraterrestrial objects, the Sun and Moon being the most significant tidal forces on planet Earth. Tidal forces can affect crustal rocks and especially water (oceans and great lakes). Water will flow in the direction of gravitational pull. However, because the earth is rotating, this gravitational pull is constantly changing causing daily tide cycles.

    Tides are very long-period waves that move through the oceans in response to gravitational forces exerted on the oceans by the Moon and Sun. Both the solid Earth and the oceans are impacted by tidal forces, but oceans can move because they are fluid. Tidal forces create bulges on the ocean surface (Figure 11.1). The largest tidal effect is from the Moon due to its proximity to Earth; a smaller tidal effect is from the Sun. The sun's gravitational pull on the Earth is about half (~44%) of the moon's gravitational pull.

    Tidal bulge from the gravitaional attraction of Earth, Moon, and Sun
    Figure 11.1. Tidal bulge from the gravitational attraction of Earth, Moon, and Sun

    Tides are consistently predictable because the rotation of the Earth is a consistent 24 hours (a solar day). Tides are influenced by a lunar day (a consistent 24 hours 50 minutes). Tides advance 50 minutes each day. This is because the Moon rises 50 minutes later each day.

    Tides arise in the oceans and move toward the coastlines where they appear as the daily rise and fall of the ocean surface. Large lakes can have tides, but they are small because of the comparatively small volume of water.

    A tidal range is the difference in height between the highest high water (HHW) and the lowest low water (LLW) (Figure 11.2). Tidal ranges vary from region to region, influenced by the geography of coastlines.

    Tidal Ranges
    Figure 11.2. Tidal range is the distance between average highest and lowest tides.

    A tidal current is a horizontal flow of water that accompanies the rising and falling of the tides. Tidal currents can be strong on shallow continental shelves and coastlines with restricting geography (such as in bays, inlets, narrow straits, lagoons, and estuaries). Tidal currents are relatively weak in the open ocean.

    Tidal Currents

    An incoming tide along a coast is called a flood current; an outgoing tide is called an ebb current. The strongest currents usually occur near the time of the highest and lowest tides. The tidal currents are typically weakest midway between the flood and ebb currents and are called slack tides.

    Tides at Mont St.-Michel
    Figure 11.3. Tides and tidal flats at Mont Saint-Michel, France, a region with a high tidal range.

    Daily tides move vast quantities of water along coastlines, filling in and emptying coastal bays and estuaries, flushing out stagnant waters, and moving nutrients in and out. The ebb and flood tides cause rivers in delta regions to reverse their flow directions and bring in seawater to mix with freshwater (creating brackish waters).

    The speed of tidal currents can reach up to several miles per hour.

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