TIDES Periodic short term changes in the height of the ocean surface at a particular place

TIDAL MARSH

Moon's Gravity Pulls Oceans - Near-side Bulge is Easy to Understand Moon and Earth actually orbit around the Earth-Moon Center of Mass (about 1500 km beneath the surface of the Earth) Motion of Earth Around Center of Mass Creates a Bulge on the Far Side of the Earth

Both Moon and Sun Cause Tides

DEFINITIONS Tidal day Tidal Period Tidal Range Daily inequality 24 hr 50 min Time between successive phases of moon over a given point on the earth Tidal Period Time between two successive high or low tides Tidal Range Difference between highest and lowest tide levels Daily inequality Difference in height  between successive high or low tides

The Tidal Cycle In general, a complete tidal cycle takes 24 hours and 50 minutes. This is the time it takes for the Earth to rotate on its axis back to its original position with respect to the moon, the primary tide-causing force. Because it takes the moon about 27.3 days to complete one orbit around the Earth, the moon moves a little bit further around the Earth each day. Thus, the time of the tides advances about 50 minutes each day.

TIDES Periodic changes in sea level relative to land along a coast Daily or Diurnal Tides One high and one low tide each day Semi-daily or Semidiurnal Tide Two high and two low tides of approximate equal heights occur each day Mixed Tide Two high and two low tides of unequal heights (HHW, LHW, HLW, LLW)

TIDES Many other factors influence the nature and intensity of the tides, including the shape of the ocean basin and the Coriolis effect. These factors create high and low tides. Depending on the position of the Earth with respect to the moon and the sun, differences in the height of sea level during the high and low tides may be great or small

AMPHIDROMIC POINT As the tidal bulge moves across the Atlantic it encounters the American Continents Because the Moon keeps on moving overhead, the tidal bulge gets left behind and the tidal wave is reflected back into the Atlantic The lagging bulge and the reflection of the tidal bulge give rise to different types of tides depending on the dimensions and shapes of the basins.

AMPHIDROMIC POINT As the tidal bulge moves across an ocean and is reflected back from the opposite side, the Coriolis Effect causes the moving water to be deflected. The peak of the tidal bulge moves around the basin rather than just  straight back and forth across it. In an open ocean the crests and troughs of the wave actually rotate around a point near the center of the ocean. This point is called the amphidromic point.

SPRING AND NEAP TIDES Spring Tides Neap Tides Occur at Full and New Moon Sun, Moon and earth in a line Greatest tidal range Neap Tides Occur at the first and third quarter of moon Least tidal range

TIDAL RANGE

The Bay of Fundy Nova Scotia, Canada

In the Bay of Fundy the tidal range can be up to 16m

TIDAL CURRENTS Horizontal water movement caused by tides Tides are like Shallow water waves Orbital motion of water is highly elliptical: can be assumed to be to and from motion Flood tides when water moves in Ebb tide when water moves back

TIDAL BORE

Tidal Friction Rotation and Friction Causes Tides to Lead Moon Bulge Pulls Moon, Throws into Larger Orbit Friction Slows Earth Precambrian (900 m.y.): Year = 500 Days, Day = 18 Hr., Month = 23.4 Days Cambrian (500 m.y.): Year = 400 Days, Day = 22 Hr.

Predicting Tides Predicted tidal heights are those expected under average weather conditions. When weather conditions differ from what is considered average, corresponding differences between predicted levels and those actually observed will occur. Generally, prolonged onshore winds (wind towards the land) or a low barometric pressure can produce higher sea levels than predicted, While offshore winds (wind away from the land) and high barometric pressure can result in lower sea levels than predicted.

The Battle of Tarawa The first amphibious assault in the Pacific during World War II. Sketchy tide data for the island suggested a tidal range of about seven feet. There were puzzling rumors of periods when the tides on Tarawa almost ceased The invasion was set for November 20, 1943 when tide conditions were expected to be favorable. At low tide in the early morning, the bombardment would begin. As the tide rose and water levels in the lagoon reached 1.5 meters (five feet), landing craft would head ashore and by noon, at high tide, heavier craft could come ashore bringing tanks and supplies.

The Battle of Tarawa November 20 was near last-quarter moon, resulting in a neap tide. Military planners did not realize the moon was unusually far from earth. Also, the Earth was only seven weeks from perihelion, meaning solar tides were unusually strong Landing craft hit bottom hundreds of meters offshore and the Marines had to wade ashore under heavy fire. Once ashore, they had to fight without assistance, because supply ships could not come in. For 48 hours, the tidal range was only 60 centimeters (two feet), and it was four days before the tidal range increased to normal. 1027 Marines were killed and 2292 wounded in the battle.

The Grunion Grunion have adapted to tidal cycles in a precise manner Along the Pacific coast of North America the two daily high tides vary in height, and the higher of the two occurs at night during spring and summer months. Spawning must take place soon after the highest tide in a series if the eggs are to have adequate time to develop before the next series of high tides. Looking at the tidal cycle, it becomes apparent that there are only 3 to 4 nights following the highest tide that spawning conditions are right

Tidal Power The potential energy contained in a volume of water is E = hMg where h is the height of the tide, M is the mass of water and g is the acceleration due to gravity. Therefore, a tidal energy generator must be placed in a location with very high-amplitude tides. Suitable locations are found in the former USSR, USA, Canada, Australia, Korea, the UK and other countries

Severn Barrage, UK The Severn Barrage, when built, is projected to produce 8640 MW during flow, or 2000 MW average power. This would provide 17 TWh of power per year (about 6% of UK consumption), equivalent to about 18 million tons of coal or 3 nuclear reactors. The cost in 1989 was calculated to be about £8 billion (£12 billion in 2006 money), and running costs would be £70 million per year. This decreases the output of greenhouse gases into the atmosphere.

Operating Stations The first tidal power station was built over a period of 6 years from 1960 to 1966 at La Rance, France It has 240MW installed capacity. The first (and only) tidal power site in North America is the Annapolis Royal Generating Station, which opened in 1984 on an inlet of the Bay of Fundy. It has 20MW installed capacity. A small project was built by the Soviet Union at Kislaya Guba on the White Sea. It has 0.5MW installed capacity. China has developed several small tidal power projects and one large facility in Jiangxia. Scotland has committed to having 18% of its power from green sources by 2010, including 10% from a tidal generator.