CHAPTER 13: THE RESTLESS OCEAN

1. SURFACE CURRENTS
   1. Ocean Circulation Patterns
      1. Surface ocean currents result from frictional transfer of kinetic energy from the wind to the ocean surface.
      2. Gyres are circular patterns of ocean circulation that characterize the North and South Atlantic and North and South Pacific Oceans. They rotate clockwise in the northern hemisphere, counterclockwise in the southern hemisphere.
      3. The Coriolis effect is what makes currents keep turning to the right (clockwise) in the northern hemisphere and to the left (counterclockwise) in the southern hemisphere. It results from rotation of the earth, and variation in surface velocity due to this rotation from one place to another on the surface.
   2. Ocean Currents and Upwelling
      1. Upwelling is the rise of deep water to the surface in areas of wind-induced surface water divergence. It occurs along north-south trending coastlines near the equator, where winds and Coriolis effect move surface water away from shore. Cooler water then moves up from below, bringing with it the nutrients which have collected while the water was below the photic zone.
      2. Most of the oceans' biological productivity is concentrated over upwelling zones.
   
   
2. THE IMPORTANCE OF OCEAN CURRENTS
   1. Navigation: Sail with a current if it flows your way, or avoid it if it flows against you, and save travel time. Knowledge of current patterns also aids greatly in searches for such things as lost vessels, life-rafts, schools of fish.
   2. Climate is strongly influenced by surface currents, which transfer large amounts of heat, and influence temperatures and rainfall in adjacent land areas. For example, the Gulf Stream helps keep London, England warmer in January than New York City (which lies farther south). San Francisco, California has about the same climate year round because of the California current.
   
   
3. DEEP-OCEAN CIRCULATION
   1. Vertical circulation in the ocean is thermohaline, meaning that it is driven by the differences in water density resulting from temperature and salinity variations.
   2. Deep-ocean water moves so much more slowly than surface currents that is considered to be organized into masses rather than currents. Dating studies show that water moved into the deep ocean may take 500 to 2,000 years to make the trip.
   3. Deep ocean circulation is so slow that it seems not to be subject to the Coriolis effect, which causes surface currents to be confined to one side of the equator or the other. For example, North Atlantic Deep Water (SEE Fig, 13.5, p. 352) moves southward from the Arctic all the way to Antarctica, then eastward to loop into the Indian and Pacific Oceans.
   
   
4. TIDES
   1. Causes of Tides: The basic cause of tides is gravitational pull on the earth by the sun and moon. These bodies each tned to pull the earth into a football shape. Oceans, being easier to deform, get pulled more than the solid part of earth, causing bulges which maintain orientation with respect to moon and sun while earth rotates under them.
   2. Spring and Neap Tides: The shape and size of the bulges depends on the positions of the sun and moon in relation to the earth. Spring tides (the highest tidal range) occur when the earth, sun and moon are all in a straight line in space. Neap tides (the lowest tidal) occur when the sun and moon are at right angles with respect to the earth, so that the bulge caused by one is partly cancelled by the other. Tide height is also influenced by the shapes of ocean basins, and varies depending on position in or around an ocean basin.
   3. Types of Tides: The ideal theoretical tide pattern is semidiurnal, with two high tides and two low tides per tidal day (24 hours and 50 minutes). However, tidal patterns are influenced by basin and coastline shape, water depths on the shelf near the shore, and so on. Thus many areas have a diurnal pattern (one high and one low tide per tidal day), or more complex mixed tidal pattern.
   4. Tidal Currents: A rising tide is a flood tide, and a falling tide is an ebb tide. The strength of tidal currents depends on height of tidal range. In some areas they are significant erosional agents. Some coastal areas have wide tidal flats which are exposed at low tide and submerged at high tide. Tidal deltas form from deposition at tidal passes by tidal currents.
   
   
5. WAVES MODIFY THE SHORELINE: Waves are the chief agent of erosion along most shorelines. It is easy to underestimate their power, especially how powerful they can be in storms. Because of the growth of population in coastal areas, the proportion of people living there who are experienced in the ways of waves and the sea has decreased. Thus the potential for more problems, if not more disasters, is also growing.
   
   
6. WAVES
   1. The cause of waves is wind blowing over water, and transmitting kinetic energy to the water by friction. The faster the velocity of the wind, the longer its duration, and the longer the distance (fetch) over which it blows, the bigger the waves that will be formed.
   2. Wave dimensions
      1. Wave height is the vertical distance or elevation difference between crests and troughs of waves.
      2. Wave length is the horizontal distance between crests or troughs.
      3. Wave period is the amount of time it takes for one complete wave to travel past a fixed observation point.
   3. Wave motion
      1. In deep water, the motion of water particles in a wave is circular (a wave of oscillation). The size of the circles decreases downward, and they become negligibly small at a depth equal to half the wave length.
      2. In shallow water (less than one-half wave length deep) the water cannot move freely in circles, thus is constrained to move horizontally back and forth (a wave of translation).
      3. A wave moving toward shore, from deep water to shallow water, will begin to drag bottom so that the part of the wave nearer the surface begins to get ahead of the lower part, and the wave eventually breaks. The zone of wave breakup is the surf zone, and the water motion is completely back-and-forth in the swash-backwash zone.
   
   
7. WAVE EROSION: Waves erode by hydraulic impact, by pressure from water rushing into crevices, and by abrasion using particles they carry in the water. Waves are a powerful erosional agent. In some areas shoreline retreat due to wave erosion is as much as 1 meter per year.
   
   
8. WAVE REFRACTION is caused by irregularities in the shoreline and the sea bottom near the shore. Parts of a wave that reach shallow water in front of headlands drag bottom are slowed first, while other parts of the wave continue to advance unimpeded until they have moved into shallower water. The crest of the wave is thus turned as it seemingly tries to wrap around projections in the shoreline. Refraction of waves thereby causes concentration of wave erosional energy on headlands, as though focused by a lens, and weakening of erosional energy (commonly leading to deposition instead) in bays. Thus wave action tends to straighten irregular shorelines by wearing back headlands and filling in bays with the eroded materials.
   
   
9. MOVING SAND ALONG THE BEACH: When waves strike the shore at oblique angles, they push water along the shore in a longshore current (when they come in at nearly a right angle, they are more likely to produce an undertow or a rip-current). If the swash-backwash is oblique to the shoreline, sediment on the beach is also moved in a fashion along the shore (beach drift). The drift of beach sediment amounts to a "river of sand" that carries 1.5 million tons of sand per year on some Pacific beaches, and hundreds of thousands of tons on average marine beaches.
   
   
10. SHORELINE FEATURES
    1. wave-cut cliffs, terraces, platforms or benches are levelled off by waves during a period of constant sea level with respect to the land: frequently these are the site of a former beach.
    2. sea arches, stacks, erosional remnants of old terraces attacked by waves now operating at a lower level.
    3. spits: beachlike deposits formed where the coastline turns abruptly, and sand can build out from shore in the relatively quite, protected water. A baymouth bar is formed by a spit which closes off a bay from the open sea. Tombolos are like spits which run between islands.
    4. Barrier islands are like large spits or bars that separate the mainland from the open ocean, trapping a relatively shallow lagoon behind them. Several ways to form them have been suggested.
    5. The evolving shore: beeaches and other parts of the coastal system are all balanced in a dynamic equilibrium that gradually changes. The general tendency is for coastlines to straighten through time. but many other geologic events can occur that would redirect the trend.
    
    
11. SHORELINE EROSION PROBLEMS: If the waves are strong and the supply of sand to a beach is cut off (as by damming a river which empties updrift), sand on the beach will disappear quickly. The usual result of interrupting the longshore drift system is that sediment piles up where it is trapped, and the shoreline recedes in the areas where the trapped material was drifting to.
    1. Groins may be used to keep the sand from being washed away in one area, but this usually results in increased beach erosion farther downdrift. Jetties may be built to keep sand from washing into a harbor passageway, but sand accumulates on the updrift side, and erosion is increased on the downdrift side.
    2. Breakwaters and seawalls are often used to create artificial harbors or to lessen waves in natural harbors: the shoreline behind a breakwater frequently becomes a sediment-trap, and as with other drift interruptions, the beaches downdrift are deprived of their sand supply.
    3. Beach nourishment, artificially bringing in sand in place of what was trapped elsewhere, is now necessary in many places. If the material used for such nourishment is much different from what was previously being drifted in naturally, it may have other undesirable effects on the equilibrium or the local ecology.
    4. Abandonment and Relocation is a final option that my prove to be the cheapest and safest.
    5. Contrasting The Gulf/Atlantic and Pacific coasts
       1. The Atlantic and Gulf coasts are both relatively low-lying areas subject to shoreline erosion. The barrier islands along these coasts protect the mainland against marine incursions during storms. Extensive development and urbanization in these areas threatens the integrity of the beach system, and the shoreline is moving inland at a rapid rate in many areas. Beachfront developers should not expect the government to reimburse them for loss due to hurricanes, but government has been slow in restricting such development.
       2. Pacific coastal areas tend to be steeply sloping coasts with narrow beaches. The problems here are that many rivers have been dammed to supply water to residents and industry in coastal cities, and irrigation water for vast agricultural activities. The result is that the sand supply to the beaches is greatly reduced, to the point that some beaches have virtually disappeared, and shoreline erosion and retreat have been correspondingly accelerated.
       3. Sea level is rising as the ice age winds down, probably also as a result of melting of polar ice caps in connection with the atmospheric greenhouse effect. How much of the sea-level rise is natural and how much is due to human activities (in particular the burning of wood, coal, oil and natural gas) is a matter of current debate among scientists. A large rise in sea level, considered possible by some workers, would flood low lying coastal cities. Some researchers are more troubled by possible climatic effects, such as a Florida-like climate in Maine.
       
       
    6. EMERGENT AND SUBMERGENT COASTS - a shore line in which the land is rising (or sea level falling) is said to be emergent, while one in which land is subsiding (or sea level rising) is said to be submergent.
       1. Much of the west coast of the U.S. is emergent. Along the southern California coast, for example, one can find wave-cut terraces which have been raised hundreds of feet above sea level. In general, erosional features such as those listed above tend to be dominant along an emergent coastline.
       2. Much of the east coast of the U.S. is considered to be submergent. Estuaries (drowned river valleys) such as Chesapeake Bay, or the lower Hudson River at New York City are prime evidence for submergence. Overall, depositional features are more common in areas of submergence.
       3. The Texas Gulf Coast is perhaps better described as "depositional" thaneither emergent or submergent. Even though this is a subsiding area, the high rate of sediment supply to the coast here has been enough to keep the Gulf waters from encroaching significantly onto the land, and in the last few tens of millions of years has even been enough to build the shoreline farther out into the Gulf.