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Tide Patterns and Currents

The content and activities in this topic will work towards building an understanding of how the earth, the sun and the moon interact to influence the daily pattern of tides and the generation of tidal currents.

Tidal Patterns

Tides occur in characteristic patterns along the coastlines of different regions of the earth. Tides are classified into three common types.

<p><strong>Fig. 6.16.</strong> Types of tidal cycles: (<strong>A</strong>) semidiurnal, (<strong>B</strong>) mixed, and (<strong>C</strong>) diurnal</p> <p><strong>Fig. 6.17.</strong> Different types of tides in the world ocean</p>


  • A semidiurnal tidal cycle is characterized by two high tides daily of about equal heights occurring about 12 hours and 25 minutes apart (Fig. 6.16 A). The east coast of the United States usually experiences semidiurnal tides (Fig. 6.17).
  • In a mixed tidal cycle, the tides also occur twice daily, but the two high tides and two low tides are unequal in height (Fig. 6.16 B). Mixed tides occur on the west coast of the continental United States and in Alaska and Hawai‘i (Fig. 6.17).
  • A diurnal tidal cycle is characterized by a single high tide every 24 hours and 50 minutes (Fig. 6.16 C). Diurnal tides typically occur in partially enclosed basins, such as the Gulf of Mexico (Fig. 6.17).

 

Different places in the world have a variety of tides throughout the tidal month. For example, the tide may be a semi-diurnal for a few days, gradually change to a mixed tide, and finally show signs of being a diurnal tide before gradually changing back to a semi-diurnal cycle.

 

Tidal Currents

<p><strong>Fig. 6.18.</strong> Tide height and tidal current change direction and strength throughout the day in one location. Longer arrows indicate stronger tidal currents</p><br />

Tidal currents are produced by the large quantities of water moving toward or away from shore as the tides change. During a flood tide, when the water level is rising between low and high tides, the tidal current flows toward shore. During an ebb tide, when the water level is falling between high and low tides, the tidal current moves away from shore (Fig. 6.18). The greatest tidal currents occur midway between high and low tide. A slack tide is when there is no current. In most areas of the world, slack tides occur near high and low tide when the flow of water is changing direction.

 

In addition to moving water toward and away from shore, tidal currents often cause a parallel flow of water along the shoreline. For example, in waters off Honolulu, Hawai‘i, the rising tide tends to cause coastal currents to flow west, and falling tides tend to cause coastal currents to flow east.

 

Tidal currents are usually the strongest currents in coastal regions. Experienced coastal sailors know that they must be prepared for both tides and tidal currents when navigating sailboats and other crafts into and out of coastal inlets. They must make sure that hulls of their crafts do not run aground during low tide and that they have enough power to advance against strong tidal currents.

 

Climate Connection: Tidal Power

Activity

Activity: Tidal Patterns Across the Globe

Compare tide records in two parts of the world.

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Representative Image: 
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.