TIDES;

any of the cyclic deformations of one astronomical body caused by the gravitational forces exerted by others. The most familiar are the periodic variations in sea level on the Earth that correspond to changes in the relative positions of the Moon and the Sun; the daily fluctuation of elevation of the water surfaces of oceans and seas and the larger lakes of the world.

Tide-generating forces: The forces that cause the tides are called the tide-generating forces. A tide-generating force is the resultant force of the attracting force of the Moon or the Sun and the force of inertia (centrifugal force) that results from the orbital movement of the Earth around the common center of gravity of the Earth-Moon or Earth-Sun system.

At the surface of the Earth the gravitational force of the Moon is about 2.2 times greater than that of the Sun. The tide-producing action of the Moon arises from the variations in its gravitational field over the surface of the Earth as compared with its strength at the Earth's center. The effect is that the water tends to accumulate on the parts of the Earth's surface directly toward and directly opposite the Moon and to be depleted elsewhere. The regions of accumulation move over the surface as the position of the Moon varies relative to the Earth, mainly because of the Earth's rotation but also because of the Moon's orbital motion around the Earth. There are approximately two high and two low tides per day at any given place, but they occur at times that change from day to day; the average interval between consecutive high tides is 12 hours 25 minutes. The effect of the Sun is similar and additive to that of the Moon. Consequently, the tides of largest range or amplitude (spring tides) occur at New Moon, when the Moon and the Sun are in the same direction, and at Full Moon, when they are in opposite directions; the tides of smallest range (neap tides) occur at intermediate phases of the Moon.

Although the observed tides possess the aforementioned broad features, this pattern does not correspond to a pair of bulges that move around the Earth. The inertia of the water, the existence of continents, and effects associated with the water depth result in much more complicated behavior.

For the main oceans, a combination of theory and observation indicates the existence of amphidromic points, at which the tidal rise and fall is zero: patterns of high and low tides rotate around these points (either clockwise or counterclockwise). Amplitudes are typically less than a meter. Tides are most easily observed--and of greatest practical importance--along seacoasts, where the amplitudes are exaggerated. When tidal motions run into the shallow waters of the continental shelf, their rate of advance is reduced, energy accumulates in a smaller volume, and the rise and fall is amplified. The details of tidal motions in coastal waters, particularly in channels, gulfs, and estuaries, depend on the details of coastal geometry and water-depth variation. Tidal amplitudes, the contrast between spring and neap tides, and the variation of times of high and low tide all vary widely from place to place.

For these reasons, purely theoretical calculation of the times and heights of tides at a particular station is quite impossible. Nevertheless, tides are quite successfully predicted on the basis of accumulated observations of the tides at the place concerned. The analysis of the observations relies on the fact that any tidal pattern (in time) is a superposition of variations associated with periodicities in the motions of the Moon and the Sun relative to the Earth. The periods involved are the same everywhere, ranging from about 12 hours to a year or more, but the relative sizes of their contributions are highly variable. Observations over a sufficient time make it possible to calculate which contributions are significant at a particular location and, thus, to forecast tidal times and heights. It is common that 40 components may be significant for practical calculations at one location.

In addition to tides in the oceans (and in large lakes, where similar processes occur with smaller amplitudes), there are analogous gravitational effects on the atmosphere and on the interior of the Earth. Atmospheric tides are detectable meteorological phenomena but are a comparatively minor component in atmospheric motions. An Earth tide (q.v.) differs from oceanic and atmospheric ones in that the response to it is an elastic deformation rather than a flow. Observations of Earth tides contribute to knowledge of the internal structure of the Earth.

Tidal processes can, of course, also occur on other members of the solar system. As just one example, it has been suggested that the volcanic activity of Jupiter's satellite Io is the consequence of internal heating by frictional resistance to tidal deformation.

In contrast to the tides of the Atlantic--which are almost always semi-diurnal (i.e., twice-daily) occurrences--those of the Pacific include many instances of diurnal (daily) and mixed tides. In the diurnal type of tidal oscillation, only a single high water and a single low water occur each tidal day (which lasts for about 24 hours and 50 minutes). Tides of this type occur in the Gulfs of Tonkin and Thailand in Southeast Asia, the Java Sea in Indonesia, and Bismarck and Solomon seas north and east of the island of New Guinea. Mixed tides, in which both diurnal and semidiurnal oscillations appear, are characterized by large inequalities in successive high (or low) water heights. This type of tide is prevalent along much of the Pacific coast.

At certain places in the South Pacific the natural period of oscillation of the sea accentuates the solar tidal oscillation. At these locations the time of the AM (or PM) high (or low) water, instead of getting later each day by about 50 minutes (as is generally the case), occurs at approximately the same time for several days in succession. The tide at Tahiti, for example, follows the Sun and not the Moon--the time of high water occurring, day after day, at about midnight and noon, and that of low water at about 6 AM and 6 PM.

In general, tidal ranges within the Pacific are small. That at Tahiti is about one foot; at Honolulu it is about two feet; at Yokohama, it seldom exceeds five feet; and at Cape Horn it is never more than about six feet. In the upper reaches of the Gulf of California and Korea Bay, however, very large tidal ranges of more than 40 feet are quite common.

Considering the Earth- Moon system, at any time the tide-generating force is directed vertically upward at the two places on the Earth where the Moon is in the vertical (on the same and on the opposite side of the Earth); it is directed vertically downward at all places (forming a circle) where the Moon is in the horizon at that moment. At all other places, the tide-generating force also has a horizontal component. Because this pattern of forces is coupled to the position of the Moon with respect to the Earth and because for any place on the Earth's surface the relative position of the Moon with respect to that place has, on the average, a periodicity of 24 hours 50 minutes, the tide-generating force felt at any place has that same periodicity. When the Moon is in the plane of the equator, the force runs through two identical cycles within this time interval because of the symmetry of the global pattern of forces described above. Consequently, the tidal period is 12 hours 25 minutes in this case; it is the period of the semidiurnal lunar tide. The fact that the Moon is alternately to the north and to the south of the equator causes an inequality of the two successive cycles within the time interval of 24 hours 50 minutes. The effect of this inequality is formally described as the superposition of a partial tide called the diurnal lunar tide, with the period of 24 hours 50 minutes, on the semidiurnal lunar tide.

In the same manner, the Sun causes a semidiurnal solar tide, with a 12-hour period, and a diurnal solar tide, with a 24-hour period. In a complete description of the local variations of the tidal forces, still other partial tides play a role because of further inequalities in the orbital motions of the Moon and the Earth. The interference of the solar-tidal forces with the lunar-tidal forces (the lunar forces are about 2.2 times as strong) causes the regular variation of the tidal range between spring tide, when it has its maximum, and neap tide, when it has its minimum.

Although the tide-generating forces are very small in comparison with the Earth's force of gravity (the lunar tidal force at its maximum being only 1.14 [{times}] 10{sup -7} times the force of gravity), their effects upon the sea are considerable because of their horizontal component. Since the Earth is not surrounded by an uninterrupted envelope of water but rather shows a very irregular alternation of sea and land, the mechanism of the response of the oceans and seas to the tidal forces is extremely complex. A further complication is caused by the deflecting force of the Earth's rotation (the Coriolis force; see above).

In enclosures formed by gulfs and bays, the local tide is generated by interaction with the tides of the adjacent open ocean. Such a tide often takes the form of a running tidal wave that rotates within the confines of the enclosure. In some semi-enclosed seas, such as the Mediterranean, Black, and Baltic seas, a standing wave, or tidal seiche, may be generated by the local tide-raising forces.

In these seas, the tidal range of sea level is only on the order of centimeters. In the open ocean, it generally is on the order of tens of centimeters. In bays and adjacent seas, however, the tidal range may be much greater, because the shape of a bay or adjacent sea may favor the enhancement of the tide inside; in particular, there may be a resonance of the basin concerned with the tide. The largest known tides occur in the Bay of Fundy, where spring tidal ranges up to 15 meters have been measured.

3. TIDES;

4-Understanding tides

The tide is the periodic daily or semidaily fluctuation of the sea surface. Ocean tides occur worldwide, but the degree of fluctuation varies from imperceptible to many meters. The first documented reference to tides was in the fifth century B.C. by the Greek historian, Herodotus, who observed characteristics of the tide in the Red Sea. In the next century, Pytheas noted that the motion of the Moon and the rise and fall of the tide were related. Apparently this observation was an outgrowth of his travels to the British Isles, where the range of tide is many times that of his native Greece.

As human horizons expanded, knowledge of physical sciences and, thus, understanding of tides also increased. From the first, tides have been considered important to navigation. Knowledge of tides was essential for growth and development of coastal communities that flourished as a result of early commerce. Wharves, buildings, and other structures had to be constructed with the ever-changing watr level in mind (figure 1).

Today, it is even more important that complicated but rhythmic tidal motions and their associated forces be understood as we build closer to the waterfront or shore. Bridges and pipelines connect points of land once considered inaccessible. Bays and harbors have to be protected from the forces of the sea, of which the tide is a major contributor. Supertankers, no longer able to enter many existing ports, have to be handled on the continental shelf, requiring deepwater loading facilities in exposed areas. Consequently, we need to understand not only tides in coastal areas, but also those of the open ocean.

fig 2

Figure 2.-The height of the ocean's surface rises and falls with predictable regularity. The means of these periodic high- and low-water conditions are defined as tidal datums. The tidal datums are relative (that is, related to one another) and identified as shown above.

As people seek to better manage the wastes they dump into streams, rivers, and estuaries, they are calling on oceanographers for more information concerning estuarine and coastal circulation. This is essential for establishing intelligent but practical waste management procedures. Tides play an important role in determining rates of dilution, mixing, and flushing of these coastal waters.

Defining seaward boundaries is another issue with relevance for tide knowledge. In the offshore oil industry, for example, state-Federal boundaries must be precisely defined for determining which jurisdiction may claim taxable revenue. Similarly, as in past years along the Oregon coast, private-state boundaries are becoming critical issues. Since the coastline is not static and instead is constantly undergoing change, boundaries are difficult to demarcate. As a result, boundaries are defined in relation to mean tide elevations.

To help keep track of these mean tide elevations and use them, certain standard references have been established. The most effective references are the tidal datums, which are simply fixed references from which we reckon heights or depths. There are a variety of such datums, called by different names, such as mean low water, mean lower low water, mean high water, mean higher high water, and mean sea level.

Each of these tidal datums may be determined in relation to a time period of a specified length, called a tidal epoch. These tidal datums can be located on the ground and mapped .

2 Elementary tidal theory the equilibrium tide

The cause-and-effect relationship between the Moon and tides remained a mystery until 1687, when Isaac Newton published his classic book, Philosophiae naturalis principia mathematica, which stated his laws of gravity.. Newton's work, along with that of Daniel Bernoulli in 1740, led to the equilibrium theory o f tides-a basis for understanding simple tidal generation.

The Moon as primary force. Although a number of forces act to produce tides, for the moment we will consider only the forces caused by the Moon. Newton's law of gravitation states that two bodies are attracted toward each other. The strength of this attraction depends on the mass of the bodies and the distance between them. (Two bodies are attracted directly proportional to the products of their masses and inversely proportional to the square of the distance between them.) In the case of the Earth and the Moon, the gravitational attraction between the two is balanced by an additional force. The balancing force is the centrifugal force caused by the rotation of the Earth and the Moon about the center of mass of the Earth-Moon system.

On the side of the Earth nearer the Moon, the gravitational attraction between the Earth and the Moon is greater than the centrifugal force. On the side of the Earth farther from the Moon, the centrifugal force is greater than the gravitational attraction between Earth and Moon.

Thus, the tide-generating forces try to create two tidal "bulges" on opposite sides of the Earth along a line connecting the Earth's center and the Moon's center.. Because there are two bulges,, there are generally two tides per lunar day (figure 3).

The Sun as secondary tidal force. Heavenly bodies other than the Moon cause tide-generating forces, but the only other body of significance is the Sun. Although it has far greater mass than the Moon, the Sun is much farther from the Earth than the Moon. Consequently, the Sun's tide-generating force on Earth is only about 46 percent as great as that of the Moon. ,

To understand the variations in tides as they occur over extended periods of time, consider the constantly changing relationship of'the Earth, the Moon, and the Sun. Remember that the Moon orbits about the rotating Earth, and both the Earth and the Moon orbit about the Sun. In addition, remember that they do' so not in perfect circles, but in ellipses, so that distances one from the other are constantly-and predictably changing.

Remember that the Earth's axis is tilted with respect to its orbit about the Sun, and the Moon's orbit is also at an angle to the Earth's orbit. Therefore, the angular relationships between the Earth and the Moon, and the Earth and the Sun, are constantly-and predictably changing. Now let's look at the effects of all these dynamic relationships.

Figure 3.-Two tidal "bulges" are present on opposite sides of the Earth, formed by the difference between the gravitational forces and the centrifugal force caused by the Earth's revolution around the center of mass of the Earth-Moon system. The Earth makes one complete rotation relative to the moon every 24 hours and 50 minutes. Thus, a location on a coast moves through each of the ocean bulges in a lunar day, and there are two tides a day along most coasts.

The Moon and the Sun interact. Anyone who has observed tides or studied a tide table has noted that the difference between a high tide and a low tide may be greater at one time of the month than at another. The range of tide, or difference between successive high and low waters, varies primarily as a result of the changing positions of the Sun and Moon with respect to the Earth.

Figure 4 reminds us that as the Moon rotates about the Earth approximately once a month, it is aligned with the Sun twice a month and it is at right angles (quadrature) at two other times during the month.

When the Moon is on a line connecting the Earth and the Sun, we have either a new Moon or a full Moon. At this time the attractive forces of the Sun and the Moon are aligned and reinforce each other, increasing the tidal bulge. When this occurs semimonthly, the range is increased, with the high tides being higher and low tides being lower than average. These are called spring tides (this name implies no reference to the season of the year).

When the Moon is at quadrature, we have either a first-quarter or a third-quarter Moon. At this time the attractive forces of the Sun and the Moon are at right angles and tend to counteract one another, resulting in a decreased tidal range. .high tides are lower and low tides are higher than average. These are called neap tides.

Figure 4a.-During times of full and new Moon, the Earth, Sun, and Moon are in a line; and spring tides occur.

Figure 4b.-When the Moon is at first and third quarter, the Moon and Sun form a right angle with the Earth; neap tides now occur.

Effects of elliptical orbits. As the Moon moves through its elliptical orbit about the Earth approximately once each month, it passes through points nearest and farthest from the Earth. Figure 5 illustrates this phenomenon. The point nearest the Earth is called perigee; that farthest from the Earth, apogee. Tide range is increased when the Moon passes through perigee. The tide range is decreased at apogee.

As the Earth moves about the Sun, a similar situation occurs. The point when the Earth is nearest the Sun is perihelion; farthest from the Sun, aphelion. The effect of the Earth's passing through perihelion and aphelion is less pronounced than the counterparts of the Moon's motion but is of the same sort. And, of course, it occurs on a yearly basis instead of monthly.

The angular relationship. As noted previously, we observe a changing angular relationship between the Earth and the Moon, and the Earth and the Sun. The angular distance north or south of the equator is called declination. The changing declination of the Moon and the Sun also play an important role in modifying tides.

The Moon's declination completes a full cycle approximately every 27 1/3 days. In completing this cycle, it can reach maximum values of nearly 28.6⁰ north and south of the equator.

As the Moon approaches its maximum declination (once north and once south each cycle), its attractive force is unevenly distributed with respect to the equator, as shown in figure 8. The effect is to cause a difference in the heights of succeeding high waters and succeeding low waters in the same day. The difference between high waters and between low waters is known as diurnal inequality (diurnal means "daily").

Diurnal inequality is generally at a maximum when maximum declination occurs, producing what are called tropic tides. Diurnal inequality is at a minimum when the Moon is over the equator, causing equatorial tides. As one would expect, tropic and equatorial tides each occur twice every cycle of 271/3 days.

Interaction. Of course, all of these astronomic movements go on simultaneously in cycles whose lengths vary one from the other. Thus, their combined effects may be to enhance or nullify one another. In a later section, we will note how all combine to affect clamming tides in Oregon.

The real tide varies from theory.

In discussing equilibrium theory, we assumed the Earth was a smooth surface completely covered by a fluid in equilibrium with the tide generating forces. We ignored the effects of friction in the movement of fluid, inertia, depth of the ocean, presence of continents, and rotation of the Earth. Of course, all of these factors must be considered when we study the tides as they really are. If equilibrium assumptions were valid, tidal response would be simultaneous with the tide-producing forces.

Because this is not the case, the time of high tide varies considerably throughout the world's oceans in relation

to when the Moon passes over the local meridian. (A meridian is a great circle of the Earth passing through the poles and any given point on the Earth's surface.) The height of tide also cannot be explained entirely by the simplified theory. Consequently, equilibrium theory does not fully account for the observed tidal phenomena. Instead it only gives us insight into the basic causes and fluctuations.

Predicting tides. When we deal with nature, one of our prime objectives is to predict future events. Tides are no exception. In predicting the behavior of the ocean, we generally can predict tides better than any other natural phenomenon, at least in coastal areas where knowledge of the tides is most essential.

Our ability to predict tides is good not because we understand the theory of tides better than that of other oceanic events, but because the tide is determined by the Sun and Moon, movements of which are well-ordered in time and space.

The marigram, or graphic record of the rise and fall of the tide, at a given location, is a continuous function that is periodic, readily lending itself to a curve-fitting procedure and thus a forecast of tidal heights.

Types of tide

A marigram is distinctive for a specific location, but there are general characteristics of the ties throughout the world that permit us to establish a classification system. Figure 7 shows examples of marigrams for diurnal, semidiurnal, and mixed types.

Diurnal. A tide is diurnal if, during the period of a lunar day (of 24 hours and 50 minutes), there occurs only one high water and one low water. Diurnal tides are primarily caused by the changing declination of the Moon and are most pronounced at the times of maximum declination (figure 6). These tides are found in the northern Gulf of Mexico and in southeast Asia.

Semidiurnal. The semidiurnal tide is that which is most commonly found throughout the world. It is characterized by two high waters and two low waters in the lunar day. The elevations of succeeding high waters and succeeding low waters are nearly the same. A semidiurnal tide is found on the East Coast of the United States, for example.

Mixed. Just as with a semidiurnal tide, the mixed tide is marked by two high waters and two low waters in a lunar dav. Succeeding high waters, low waters, or both are generally different in height, however. These differences are known as diurnal inequality. Remember the inequality is caused by the changing declination of the Moon. Mixed tides are common to the West Coast of the continental United States, Alaska, and Hawaii.

Phenomena associated with tides

Marigrams may show changes in water level that are not due solely to tidal movement caused by heavenly bodies. A nong these tidal phenomena are meteorological effects. Also, in any discussion of tides the related horizontal movement of water, or tidal currents, should be mentioned.

The meteorological effects. Water responds to external forces applied to it. Two forces always at work in varying degrees on the water surface are wind and direct barometric pressure. They combine to effect a change in the elevation of the water surface known as wind setup or storm surge.

In coastal areas where the water is shallow, wind interacts with water at the surface and as a result moves the water from one area to another. It is not easy to say exactly how a water body will respond because the effects are determined by wind speed, duration, and distance over which the wind blows (fetch), as well as by such other complicating factors as topography and stage of the tide.

It is generally true in coastal areas, however, that the water surface will respond directly to the wind. Thus, a wind blowing toward shore will tend

to raise the water level on the coast and wind blowing away from shore, to decrease it.

As barometric pressure increases, the elevation of the surface of the water tends to be depressed and, conversely, as the pressure decreases, the elevation tends to be increased. As with wind, the amount of displacement is difficult to predict and depends on the intensity of the barometric disturbance and the speed with which it moves, in addition to the characteristics of the body of water.

The meteorological effects are particularly noticeable with the passage of large storms such as hurricanes. Strong winds and low pressure can raise the water level along a coast considerably. If this resultant storm surge is superimposed on an unusually high tide, occurring normally at that time, extreme flooding is often the result.

To some extent, the meteorological effects are predictable with change of seasons and can be estimated for predicting water level. Random disturbances raise havoc in predicting water level, however. If the astronomic tide is small, then random meteorological disturbances often represent a significant portion of the total change in water level-making accurate predictions extremely difficult.

Tide predictions generally do not include provisions for the contributions to the change in water level for other than the astronomic tide. In some cases, however, the seasonal fluctuations are included. Thus, when actual water level does not agree with predicted tide elevations, it is not the result of poor

tide predictions, but rather the influence of random meteorological effects.

Seiches. A seiche (pronounced SAYsh) is a stationary wave oscillation the period of which depends on the dimensions of the local semienclosed body of water. You create a simple miniature seiche when you tilt a rectangular dishpan of water. The maximum change in water elevation occurs at the ends of the pan while no change in water level occurs in the middle. Figure 8 shows the oscillations of a seiche with ape period of about an hour imposed on the tidal fluctuation at San Pedro, California.

A seiche is generated by an external force, often one of the same forces that generate a storm surge. After the force has been removed, the body of water responds by oscillating at its natural frequency.

As the period of the seiche approaches that of the tide, it is possible that the range of the tide can be considerably affected. Some of the great tidal ranges in the world can be attributed to this interaction of the tide of the open ocean and the seiche of a semienclosed body of water. For example, the mean range of tide at Burntcoat Head, which is in the Minas basin of the Bay of Fundy, is 11.69 meters.

Tidal currents. Oceanographers usually define current as a horizontal flow of water. A tidal current is a horizontal flow of water generated by the tide-producing forces .

Tidal currents, like the tides themselves, are periodic, and they can be analyzed and predicted. Tides, as one-dimensional phenomena, are easy to monitor. Tidal currents, on the other hand, are more complicated because they are two-dimensional (speed and direction). There are some cases, however, in which the movement of the waters is confined to one dimension, so the direction of flow reverses as the water flows alternately toward (flood current) and away from (ebb current) the land.

Oregon tides

Taking a look at tides along the Oregon coast offers an opportunity to apply our understanding of tidal phenomena.

What kind of tides does Oregon have? A look at the sample marigram for Newport, Oregon, in figure 9 reveals that there are two low waters and two high waters each day. In addition, there is an inequality both in the low waters and in the high waters. From this, we conclude that tides along Oregon are mixed tides.

The average difference in the elevation of the two low waters each day is 0.67 meter. The average difference in the elevation of the two high waters each day is only 0.43 meter. Thus, the low water inequality is about 1.5 times larger than the high water inequality for the Oregon coast.

What is the range o f tides in Oregon? 'The range of tide varies along the coast, but it is generally less in the south. Table 1 shows tide ranges from 1.80 meters at Brighton and Yaquina Bay to 1.52 meters at Taft.

When do tides occur in Oregon? As a standard, the time of high water and low water for a given location is described in relation to the Moon's passage over the Greenwich (England) meridian. The time between the Moon's transit over the Greenwich meridian and the succeeding high or low water at a certain location is known' as the high water interval or low water interval. Along the Oregon coast, these intervals generally increase from south to north, with the time of both high water and low water occurring about an hour sooner in the south than in the north.

When do Oregon's highest and lowest tides occur? Each year, the largest diurnal inequality occurs in June and July, and in December and January. These times are associated with solstices in June and December. (A solstice is either of the two times a year when the Sun is at its greatest distance from the equator: about June 21, when the Sun reaches its northernmost point on the celestial sphere, or about December 22, when it reaches its southernmost point.) Mean sea level, which varies annually because of the meteorological effects and runoff from winter rains and lower water temperatures, reaches its highest level in December on the Oregon coast.

As a consequence of the greater diurnal inequality and high sea level, the highest predicted tide usually occurs during December or January. Often winter storms further raise the water level above the normal winter high tides, resulting in local flooding.

Location	Mean range (meters)
Columbia River entrance (N. jetty)	1.70
Brighton	1.80
Barview	1.74
Taft	1.52
Yaquina Bay entrance	1.80
Waldport	1.77
Umpqua River jetty	1.55
Coos Bay entrance	1.58
Bandon	1.58
Port Orford	1.61
Brookings	1.55

Table 1.-Mean range of tide at selected tides locations along the Oregon coast

The lowest tides of the year generally occur in summer as a result of the increased inequality in the low waters combined with the lowered sea level.

Why are good clam tides in the evening in December and in the morning in June? This is a result of the concurrence of extreme astronomical phenomena:

In December

-when there is a full Moon, resulting in spring tides,

-the Moon is near north declination, increasing diurnal inequality.

In June

-when there is a new Moon, resulting in spring tides,

-the Moon is near north declination, increasing diurnal inequality.

The times of the lows are related to the passage of the Moon over the Oregon coast. Full Moon crosses overhead at midnight, while new Moon crosses overhead at noon. The difference in time between transit of the Moon over the Oregon coastal area and the following lower low water is about 18 hours (Low water interval, LWI, for Oregon referred to Greenwich meridian is about two hours. The Moon passed over Oregon's local meridian some 16 hours earlier, however; 16 hours plus 2 hours makes the difference, 18 hours.) Thus, the good clamming tides follow the transit of the full Moon in December by 18 hours, and occur at about 6 p.m., and in June by about 18 hours after the transit of the new Moon, or near 6 a.m.

How do tides vary in the estuaries? The time and range of tide in the estuaries vary considerably from the time and range of tide along the coast. Each estuary has its own individual characteristics. In the case of the Columbia River, the periodic tide can be detected up to Bonneville Dam. The tide progresses up the river so that the mean range decreases to only 0.3 meter at Ellsworth, Washington, 182 kilometers from the river's mouth. At Ellsworth, high water occurs about six hours after that at Astoria; low water, nearly eight hours after that at Astoria.

Fundy’s Tremendous Tides

An incredible one hundred billion tons of briny water swirl their way up the shores of the Bay of Fundy twice each day as the tide comes in. Every 12 hours and 30 minutes this spectacular feat occurs along the southern coast of New Brunswick as water from the open Atlantic Ocean surges through the rocky portals of the bay. The onslaught is estimated to nearly equal the 24-hour flow of all the rivers in the world.

At the bay's eastern extremity spectators have seen the highest tides ever recorded anywhere. The tide there has been measured at 16 m (52 1/2 ft), the equivalent height of a four-story building or nearly the distance from a pitcher's mound to home plate. And that's not the distance traveled up the beach- it is the height the water has risen!

These tides not only have left their indelible mark on the people who inhabit Fundy coastal and island communities but also reveal the geological history of the awesome power manifested in forming the region. Down the eons of time they have battered the cliffs of Grand Marian, sculpted the flowerpot rocks at Hopewell Cape, quarried the echo caves at St. Martins and created the world's second largest whirlpool, Old Sow, off Deer Island.

On its swift way to shore the tide climbs the barnacle crusted legs of wooden wharves along the jagged perimeter of the bay. It swells tiny creeks and submerges reefs, sandbars, beaches and mud flats on its way to program the regular arrival of the Tidal Bore in the Petitcodiac River.

And to show off a rather distorted sense of humor, twice each day the tides charge the mouth of the mighty Saint John River so violently that the rapids there retreat in disorder, reversing direction and forcing the river to flow upstream. Their effect can be measured upriver as far as Fredericton.

At Alma the water creeps quietly and inexorably up the wide, flat stretches of beach and mud from the distant low tide fine. It rises as much as a foot in seven minutes, to waist height in half an hour and often to a depth of 13 m (43 ft.) in six hours and 13 minutes. Then it turns and just as silently retreats back to the Atlantic leaving the land that has been the bottom of the sea uncovered for visitors to see and explore before the next tidal assault occurs.

Along the bay where there are rocks and cliffs in place of sloping beaches, a very different kind of sea life is revealed when the tide recedes. Instead of the constantly scoured, soft sand or mud where creatures can burrow but plants cannot cling, layers of rock are gradually exposed showing the varied sea life that thrives at different water levels and leaving protected, calm tidal pools that are replenished twice daily.

WHY ARE THE FUNDY TIDES SO HIGH?
HOW Do THEY WORK?

Fundy tides are the highest in the world because of the shape and dimensions of the bay.

When the tidewater enters the Bay of Fundy at its widest point, it comes in just as it does elsewhere in a world. But the farther it travels the more it changes. The water literally piles up as it moves up the funnel-shaped bay. It is, in effect, squeezed by the ever- narrowing sides and the constant shallowing of the bottom, forcing the water higher up the shore.

The length of the bay is another reason the Fundy tides are so high. When the low or ebb tide runs out of the bay it collides with the new, incoming high tide combining forces to make an even higher wave coming in. The combination of wave forces is called resonance. The length and depth of the basin determine its particular rhythm. e water of the Bay of Fundy rocks from one end to the other in time with the water in the Atlantic Ocean.

The mechanics of tides is complex, but it's the pull of the moon that does most of the work. Sometimes the sun ves a hand when the moon and sun are in phase, or lineup, with the earth. This occurs about twice a month, once at the new moon again when the moon is full. The resulting tides are some 20 per cent higher than normal. These are called spring tides not for the season of the year but simply meaning `when waters spring up from the sea!" When the pull of the sun and moon are at right angles to the earth during the first and third quarters of the moon, the tides are lower and are called neap from the Anglo-Saxon word meaning sparse or scanty.

The sea and its tides have always played a major role in the development of New Brunswick, providing her people an industry, a playground and a source of tribulation. The tide governs the times for shipping and fishing and influences the weather. By pushing depth chilled water to the top where it collides with sun-warmed air, it air-conditions the summers and hatches fog banks.

The constant, forceful motion of the water is instrumental in providing enough food for the amazing range of birds, fish and marine mammals that make the bay at least a temporary home. Whether raising nutrients from the deep, uncovering feeding grounds or increasing the hunting area, the richness of the supply keeps nature's circle of small devoured by larger rolling horn microscopic organism to whale size.

The Bay of Fundy is an important stop on the route of countless migratory birds because it can supply them with enough food - some birds double their weight at this provision stop - to fuel them for the long Journey that may be as far as the Arctic or South America. The impressive sea-life population is the subject of study at a federal Fisheries Research Branch in St. Andrews. Adjacent to the research station is an aquarium where some of the large number of species found in the bay may be seen or even touched in the Please Touch Tank. Displays also help explain the tide.

Dulse, pronounced to rhyme with pulse, is an edible seaweed, a nutritious and tasty product harvested during low tide when its reddish color can be seen above the water line. Experienced pickers find the best, leanest, long-stranded dulse between the rocks rather than on top. It is then dried in the sun so that its crisp saltiness provides a tang of the sea in all seasons. Most of North America's duke comes from Grand Marian Island, located 19 km (12 mi.) off New Brunswick.

Herring fishing along the Fundy Coast is also dependent on the tides. Many fishermen employ a unique technique using weirs (locally pronounced wares). Long poles are placed in the water on the floor of the bay and then wrapped with a net Into a rou$lily heart like shape A school of herring swimming with the ale becomes caught in the pattern of the net and cannot escape. The fishermen then seine ^or7the weir.

There have been a number of off-again schemes to utilize the tide of Fundy. A young engineer Cooper, a native of New York, used to spend his summers on Campobello Island. In 1919 he had a vision that, if implemented would have harnessed the tides and supplied cheap electrical energy.

A Saint John engineer Wallace R. Tumbull, inventor of the first variable-pitch airplane propellor was equally convinced that utilizing the tidal movements of Passamaquoddy Bay on the Maine-New Brunswick border held the answer to all the power needs of the Atlantic provinces and New England.

These ingenious men weren't the first to seriously consider tidal energy. A mill partially powered by tidal force was built under the instruction of Sicur de Poutrincourt at Port Royal in 1609, five years after Champlain established Canada's first settlement on an island in the St. Croix River. Two tidal energized grist mills were operated on Passamaquoddy Bay before 1800 and it is known that there were at least two tide mills in the Saint John area.

The closest New Brunswick has ever come to witnessing a commercial-scale Fundy tidal power reality was in the mid1930s when under the Quoddy Plan, authorized by executive order of U.S. President Roosevelt, a two-dam system in Passamaquoddy Bay near Eastport, Maine, was started. Roosevelt had been a summer resident of Campobello and knew the tides from his boyhood days. The project was discontinued in 1936.

While mankind presses its apprehensive face against the window-pane of tomorrow, the flowing treasure known as the tides of Fundy continues moving in a mighty swell in and out, back and forth, in an eternal rhythm orchestrated by the moon.

Marine Science Reading Program 4 Name____________________________________pd

11. Tide readings Name……………………………..pd……..

1. What is a tide?

2. What is a tide generating force

3. How much stronger is the gravitational force of the moon on the earth than the sun on the earth?

4. What is the average interval between tides?

5. What is a amphidromic point?

6. How does shallow water effect tides?

7. How are tides predicted?

8. Where do diurnal tides occur?

9. How are tides in the South Pacific different from other world tides?

10. Why is there an inequality of two successive tidal cycles?

11. What is a tidal seiche?

12. What is a cause that effects the range of tides?

12 Understanding Tides
1. Name 2 people that documented tides and the places they observed these.
2. Give 3 reasons knowledge of tides is important today.
3. What is meant by the term tidal datum?
4. Who was responsible for the formation of the equilibrium theory of tides and when?
5. Explain how the moon affects the tides on earth.
6. What force is the sun on tides compared to that of the moon?? Why is it different?
7. What is meant by the range of the tide and what causes the differences?
8. What is responsible for spring tides? Neap tides?
9. Explain perigee and apogee…how do these affect tides?
10. Explain perihelion and aphelion…how often do these occur?
11. How long does the moon take to complete its declination cycle? How far north/south of the equator does it travel?
12. When is diurnal inequality at its Maximum? Minimum?
13. Why does the real tide vary from theory?
14. What factors allows tide prediction to be accurate?
15. Explain the 3 types of tides.
16. What are some meteorological effects on tides and explain how each happens.
17. When actual water level does not agree with the predicted tide elevations, what went wrong?
18. Describe a seiche.
19. What causes a tidal current?
20. What is meant by the terms flood and ebb?
21. What kind of tides does Oregon have?
22. What is the tidal range in Oregon?
23. What is meant by the solstice and what effect does it have on tides?
24. Explain why there are good clam tides in the evening in December and in the morning in June?
25. Do tide times vary in estuaries?

Types of tide

Phenomena associated with tides

Fundy’s Tremendous Tides

Marine Science Reading Program 4 Name__________________________________________pd______

Marine Science Reading Program 4 Name____________________________________pd