21.1: Water in Motion
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Waves and water movement
Waves are undulations in the surface of a water body. Most waves are created when kinetic energy is transferred to water by the frictional stress of wind blowing over it. The resulting transfer causes a rise in water level producing a wave crest, followed by the sinking of the surface creating a wave trough. The wave length is the distance between successive crests. The time required for successive crests to pass a point is the wave period. The wave height is the distance between the crest of the wave and the still water level. Wave height is determined by (1) wind velocity, (2) duration of the wind, and (3) the fetch. The fetch is the distance of uninterrupted flow over an open water surface. An increase in any these factors will increase wave height and length. [See these effects using the "Savage Seas" Wave Machine. ]
The rise and fall of oscillatory waves in an open water reflects the circular motion of water particles. There is relatively little forward motion by a water particle as a wave passes. It is simply the wave form and its energy that is transmitted across the ocean surface. Water particles move in circular orbits that diminish with depth. The radius of the circular path is greatest at the surface and decreases with toward the bottom of the wave. Larger waves exhibit larger orbital radii and extend to a greater depth than smaller waves. At some point in deep water, the wave has no effect on the motion of the water. Thus a zone of no wave motion exists from the base of the wave to the ocean floor. "Observe an animation of wave motion" (Courtesy NSF/TERC/McDougall Littell)
Swells are smooth, rounded waves that travel outward from a storm center or continue as broad undulations of the ocean surface after the wind dies down. The wave slope is expressed as the ratio of the wave height to wave length, ranging from 1:25 to 1:50. A wave will become unstable at slopes greater than 1:7 and will fall over itself, or break.
As a wave approaches the coast, a depth is reached offshore where the wave touches the ocean floor. The tug of the ocean floor changes the circular wave motion into an elliptical one; the water moves back and forth over the bottom as each wave passes. The friction imparted from the floor slows the wave base. At a depth of 1.3 times the wave length, the drag causes the top of the wave to rush forward, become unstable and break. Water in the breaking wave is transported toward shore as a wave of translation.
Nearly all marine coastlines experience the rhythmic rise and fall of sea level called tides. The daily oscillation in ocean level is a product of the gravitational attraction of the Moon and Sun on Earth's oceans and varies in degree worldwide. Tidal action is an important force behind coastal erosion and deposition as the shoreline migrates landward and seaward.
Causes of Tides
The gravitational attraction of the Sun is about half that of the of the Moon on the Earth. Gravitational attraction is a function of both the mass of the objects and the distance between them. Even though the Moon is much smaller in mass than the Sun it is closer and thus has a greater influence on the Earth than does the Sun. The gravitational pull of the Moon and the Sun stretches both solid and fluid surfaces of the Earth. This creates a tidal bulge in the atmosphere, the oceans and to a very slight extent the Earth's crust.
Gravity is not the only force responsible for a tidal bulge. Inertia, the tendency of moving objects to continue moving in a straight line or stay motionless, also affects the tidal bulge. As the gravitational force draws water closer to the Moon the inertial force tries to keep it in place. The tidal bulge forms as the gravitation force exceeds the inertial force on the near side. The gravitational force of the far (opposite) side is less because it is farther away from the Moon. On this side, the inertial force exceeds the gravitational force. Here the water attempts to keep going in a straight line, moving away from the Earth, creating another, smaller bulge. Thus tidal bulge, is greatest on the side of the Earth facing the Moon or Sun ("near side") simply because it's closer than the "far side" of the Earth.
Watching the tide "come in" one gets the impression that ocean water is moving in and out along the shoreline. The landward and seaward movement are a result the Earth rotating into and out of a semi-fixed tidal bulge as it changes its position relative to the Sun and Moon. Any point on Earth rotates through two bulges every 24 hours and 50 minutes producing two high tides and two low tides called each day. The difference in height between consecutive high and low tides is the tidal range. During a high tide water moves landward as a flood current. During low tide water recedes seaward as an ebb current. The two high tides and the two low tides do not have to be of equal height because the angle between the Moon and Earth changes each day. The tidal range is the difference in height between high and low tide.
Spring and Neap Tides
The Sun and Moon are said to be in conjunction with they are aligned with the Earth. The highest tides, and greatest gravitational attraction, occur when the Sun and Moon are on the same side of the Earth creating a spring tide. This occurs during the new moon phase. A second lower spring tide occurs when the Sun and Moon are on opposite sides of the Earth during the full Moon phase. A neap tide results when the Sun and moon are at right angles to the Earth. There are two neap tides, at the first-quarter and second quarter phases of the Moon.
Located on the northeast end of the Gulf of Maine between New Brunswick and Nova Scotia, the Bay of Fundy is known for its high tidal range, as much as 48 feet (14 meters). Over a 100 billions tons of water passes in and out of the bay every 12 1/2 hours each day. Watch this amazing process occur below.
Video: Bay of Fundy Tides - Halls Harbour, Nova Scotia timelapse (Courtesy: https://www.bayoffundy.com)