Why diurnal and semidiurnal tides

Diurnal tide cycle upper left. An area has a diurnal tidal cycle if it experiences one high and one low tide every lunar day. Many areas in the Gulf of Mexico experience these types of tides. Semidiurnal tide cycle upper right. An area has a semidiurnal tidal cycle if it experiences two high and two low tides of approximately equal size every lunar day. Many areas on the eastern coast of North America experience these tidal cycles. Mixed Semidiurnal tide cycle lower middle.

As the tide rises, water flows into an area, creating a flood current. As the tide falls and water flows out an ebb current is created.

Slack water , or slack tides occur during the transition between incoming high and outgoing low tides, when there is no net water movement.

The strength of a tidal current depends on the volume of water that enters and exits with each tidal cycle the tidal volume or tidal prism , and the area through which the water flows. A large tidal volume moving through a large area may create only a weak tidal current, as the volume is spread over a wide area.

On the other hand, a narrow area may produce a strong tidal current even if the tidal volume is small, as all of the water is forced through a small area. It follows that the strongest tidal currents will result from a large tidal range moving through a narrow area. Tidal bores occur where rivers meet the ocean.

A semidiurnal tidal cycle is a cycle with two nearly equal high tides and low tides every lunar day. In the world map shown above regions experiencing a semidiurnal tidal cycle are marked in red. They have a period of 12 hours and 25 min, and a wavelength of more than half the circumference of Earth [5]. It is also the type of tidal cycle one could expect from a planet covered entirely with water and without any continents obstructing the free motion of water.

By looking at the oceans on Earth we can see that most places experience a semidiurnal tidal cycle. The following diagram shows the sea level change over time for a typical semidiurnal tidal cycle:. A diurnal tidal cycle is a cycle with only one high and low tide each lunar day. Diurnal tidal cycles can be found in the Gulf of Mexico and on the East coast of the Kamchatka Peninsula.