Tides

Although tides are not a factor in sediment transport over much of the marine environment, tides and tidal currents have a dominant role in sediment transport and deposition in some coastal environments. They are important in delta development and in creating patterns of sand ridge distribution on shelves where tidal currents are strong. The persistence of tidal currents can make them effective in sediment transport. Weaker currents combine with wave motion to move sediment at the water-bottom interface and in suspension.

Davis 1964 classified the world's shorelines as either















Tidal ranges vary worldwide as shown in the distribution figure. Hayes summarized the sand body geometries unique to each tidal range . Using the coast of western Europe as an example, he proposed that sandy shorelines could be classified by tidal range. As tidal range increases, barrier islands get progressively shorter and tidal-inlet sand bodies become larger and are oriented perpendicular to shore.

Tide Theory

The theory of tides is presented in many Introductory Oceanography books and is of secondary interest in this presentation, so the following discussion will be brief. The equilibrium theory does not give a complete picture of what creates tides, but it does provide adequate insight. The balance between gravitational and centrifugal forces maintains the solar system of sun, planets and moons. The moon orbits the earth and is held in position by a gravitational force, which seeks to pull the two together, and is balanced by a centrifugal force acting to pull the moon away from the earth. In turn, the earth-moon system is balanced in position in a sun orbit by the same forces. Since the gravitational force is proportional to the square of the distance between two bodies but the tide generating force is proportional to the cube of the distance, distance becomes a more effective variable in developing tides. This causes the moon to have a greater effect on tide generation. The moon's gravitational force is strongest on the side of the earth facing the moon and the centrifugal force is stronger on the opposite side of the earth, resulting in two bulges of water (high tides) on opposite sides of the earth from the moon. Low tides are midway between these positions. As the earth rotates, the position of the moon changes and the tide position follows the earth's rotation, but is modified by the sun's position. The tide generating force of the sun is about half that of the moon and it has an effect on tides as the earth-moon system revolves around the sun.

In opposition with the sun-earth-moon in line, the tide-generating forces of the sun and moon are additive producing maximum tidal ranges (spring tide). With the sun and moon positions at a 90o angle to the earth, the tide-generating forces of the sun are working at right angles to the moon giving a minimum tidal range (neap tides).

Actual tides are at odds with the theoretical because:

If the bulges (wave crests for a tide) are separated by half the earth's circumference (20,000 km), they would move at a speed of 1600 km/hr for the predicted semidiurnal tide (two highs and two lows per lunar day). Since the tides are an extreme example of a shallow water wave, the actual speed over the sea is proportional to the square root of the water depth. In an ocean with an average depth of 3.9 km, the tidal bulges move at speeds slightly under 700 km/hr which is much less than required for two tidal bulges to transit the 20,000 km circumference of the earth in one day. The continents interrupt the free movement of the tidal bulges, and the separate ocean basins develop oscillatory waves whose character modifies the forced astronomical tidal waves.

Tide Classification

We have a variety of tide types that include diurnal (daily), semidiurnal (twice daily) and mixed tides. Diurnal tides of a single high and low water each lunar day are common in the Gulf of Mexico and along the coast of Southeast Asia. The semidiurnal tide of two high and two low waters each lunar day are common along the Atlantic coast of the United States. A mixed tide may have characteristics of both of the other tides. The diurnal inequality is a characteristic of this tide and successive high and low tides will have significantly different heights. They have a semidiurnal tidal period but for a few days of each tidal month will change to diurnal periods. This is the most common tide throughout the world, and it is common on the Pacific coast of the United States.