Tidal Patterns

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

• 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

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.