Review Questions and Answers; Shorelines

Waves are generated by winds blowing across the surface of the water. The main factors that determine wave characteristics are the wind speed, the length of time that the wind blows, and the expanse of water (fetch) affected. The longer period waves arriving at a beach were generated in distant storms at sea. Shorter period, "choppy" waves move at slower speeds and are dissipated at sea through wave interference.

2. Describe the motion of a water particle as a wave passes (Figure 14.2).

Water particles follow prograde, circular paths as a deep-water wave passes overhead (Figs. 14.2 & 14.3). The circular paths gradually decrease in diameter downward, shrinking to a point at a depth equal to one-half the wavelength; water below this depth does not move in response to the passing wave. At depths less than one-half the wavelength, the waves become shallow water waves. The water particles follow prograde, elliptical paths, the longer axis being oriented horizontally. The ellipses also decrease in size downward and degenerate to short, horizontal lines (linear paths) that represent the back-and-forth sloshing motion of the water just above the bottom.

3. Explain what happens when a wave breaks.

Drag with the bottom slows an incoming wave; wave height increases and wavelength (distance between adjacent crests) decreases. As the water depth decreases, bottom drag increases; thus the top part of the wave moves forward faster than the base, causing the wave to collapse as a breaker or plunger. Water flowing back to the sea from previously breaking waves increases drag on incoming waves.

4. Describe two ways in which waves cause erosion.

Crashing waves force compressed air and/or pressurized water into cracks and other openings, expanding them and breaking the material apart. Abrasion results from particles impacting one another, the bottom, and bedrock or manmade structures.

5. What is wave refraction? What is the effect of this process along irregular coastlines (Figure 14.8)?

In deeper water offshore, incoming waves move at constant speed, but they slow down in shallower waters. As an incoming wave approaches the shoreline at an oblique angle, the part of the wave in shallower water will have a lower speed than the part in deeper water. These different speeds for different parts of the same wave cause the wave to refract (bend). In general, wave refraction rotates obliquely incoming waves toward parallelism with the coastline. Over time, headland erosion and deposition in protected bays and coves tend to even out irregularities, thus straightening the coastline.

6. Why are beaches often called "rivers of sand"?

Large quantities of sand move along beaches and just offshore due to the action of longshore currents and longshore drift. Thus over time, a flow or stream of sand is continuously moving along the beach and parallel to the beach in the shallow, nearshore waters.

7. How has the construction of artificial levees and dams on the Mississippi River and its tributaries contributed to a shrinking of the Mississippi's delta and its extensive wetlands? (Box 14.1)

Dams trap sediment and reduce the stream's load below the dam. For a delta to grow or maintain a stable size, especially during periods of rising sea level, enough sediment must be added to counter losses from coastal wave erosion and from land subsidence due to compaction. Deprived of its natural sediment supply, a delta will slowly decrease in size. For example, since the Aswan High dam was completed, the Nile delta of Egypt has been shrinking and salty waters are steadily encroaching into formerly brackish and freshwater areas. The delta shrinking process is accelerated by rising sea level.

Dams on major tributaries to the Mississippi (such as the Missouri) have reduced the total volume of sediment moving to the delta region. Artificial levees and engineering structures to improve navigation below Head of Passes prevent water and sediment from spreading over the low lying delta swamps and marshlands. Instead, the sediments are carried directly into the deep waters at the extremities of the lowermost, bird-foot delta distributaries and deposited well below sea level (Fig. 10.20).

Sediment deposition no longer counteracts the combined effects of rising sea level, land subsidence, and coastal erosion. Thus the delta wetlands are shrinking and seawater is moving into formerly brackish water marshes and freshwater wetlands.

8. Describe the formation of the following features: wave-cut cliff, wave-cut platform, sea stack, spit, baymouth bar, and tombolo.

The first three are erosional features; the others are depositional. As sea level slowly rises in areas with bedrock hills or bluffs along the coast, storm waves break against the base of the hill or bluff. Erosive forces are intense and a sea level notch is cut, undermining the higher parts of the hill or bluff (Figs. 14.5 & 14.6). The undermined areas fail by mass wasting, producing a steep slope called a wave-cut cliff which migrates landward as sea level rises; the relatively smooth, erosional surface formed as the cliff retreats is a wave-cut platform (Fig. 14.10). Resistant rock masses are occasionally bypassed and left as isolated peaks and pinnacles rising above the water; these features are called sea stacks (Fig. 14.11).

Spits are straight to slightly curving, low, linear sand bars that extend partway across the mouths of bays or inlets (Fig. 14.12). The sand is carried parallel to the coast by longshore currents. Baymouth bars begin as spits; they eventually reach the opposite side of the inlet and isolate the bay (now a lagoon) from the open sea. Tombolos are narrow, often curving, sand bars oriented at sharp angles to the mainland coast; they develop in the lee of actively eroding islands where sand, produced by headland erosion, is deposited. If such deposits build far enough landward to connect with the mainland or with another island, a tombolo is formed.

9. List three possible ways in which barrier islands originate.

Barrier islands are long, narrow, offshore sand bars that face the open ocean on one side and a lagoon, estuary, bay, or sound on the landward side. Barrier islands rim much of the Atlantic coastline from New Jersey to Florida and most of the Gulf coast. They seem to develop during times of rising sea level and migrate toward the mainland.

Barrier islands may evolve from old sand dunes, sand ridges, or topographic escarpments formed on the continental shelves at times when sea level was lower. As sea level rises, these act as sand traps and build to sea level or just above. With continued sea level rise, the newly built barrier island migrates landward as sand is slowly moved from the seaward to the landward side by wind and overwashing storm waves. Thus previously formed sand deposits such as spits, offshore bars, baymouth bars, or coastal dunes could act as nuclei around which a barrier island system could later develop when sea level rises.

10. Hurricanes Andrew (1992) and Hugo (1989) were very costly natural disasters. How did each storm cause most of its property damage? (Box 14.2)

Most property damage from hurricanes and typhoons (western Pacific) is caused by storm surge, high winds, and flooding due to heavy rains. Storm surges are restricted to immediate coastal areas, but wind damage and flooding can effect areas far inland from the storm's landfall point.

As a hurricane churns across the open water, a broad, low mound of water is generated under the eye and leading quadrant of the storm. This mound is dynamically sustained by the storm's rotational (counterclockwise) wind circulation and extremely low atmospheric pressures in the eye zone. As the eye makes landfall, the mound impacts on the coast, raising water levels, causing coastal flooding, and allowing storm waves to impact against natural and manmade features that normally are well above sea level. Most property damage and loss of life in coastal regions are due to storm surge, although these same areas are also subjected to the storm's strongest eyewall winds. Storm surge effects are maximized if the surge arrives at the same time as the high astronomical tides, and the surge height is added to the high-tide water level.

As it moves inland, the storm system can spawn tornadoes, powerful thunderstorms, and heavy rains. These can cause severe wind damage and dangerous flash flooding, especially in mountainous areas such as the Blue Ridge and Appalachian Mountain areas. For example, forest devastation, downed trees, and wind damage to structures from hurricane Hugo extended from Charleston, SC, (landfall point) to the Charlotte, NC, area.

Wind damage associated with hurricane Andrew was unusually severe and widespread, especially in the vicinity of Homestead, FL, and surrounding communities. Unusually strong, gusty winds along the storm's eyewall, poor construction practices, and substandard building materials have all been blamed.

However, in retrospect, numerous broken windows and exterior glass panels exposed interior areas of buildings to the full fury of the storm, resulting in far more severe damage than would have been sustained had the glass panels not been broken. The glass breakage was caused by natural and manmade objects entrained in the wind. At velocities of 100 mi/hr and greater, these acted as high-speed projectiles that smashed glass on impact. Relatively inexpensive metal window shutters and stronger glass are now available for homes and commercial buildings; these should lessen the overall structural damage should a similar storm strike the area in the future.

11. For what purpose is a groin built? Why might the building of one groin lead to the building of others?

Groins are porous structures built into the surf zone in order to slow longshore currents and promote sand deposition on the upcurrent side. However, having been deprived of its sediment load, the current speeds up again after passing the groin; thus beach erosion intensifies on the downcurrent side. An owner may build a new groin to stop erosion of one property, but the new groin will accelerate erosion elsewhere on the beach. Thus each new groin tends to require another to mitigate the adverse effects of the earlier one (Fig. 14.17).

12. How might a seawall lead to increased beach erosion?

Seawalls reflect wave energy and breaking waves directly out to sea, thus increasing erosion immediately in front of the seawall. For this reason, seawalls are often undercut and destroyed, and the intensified erosion steepens and narrows the beach. When sea level is rising, as at the present time, other parts of the beach slowly retreat, leaving the seawall outflanked and vulnerable to erosional attack from the rear.

13. What are the drawbacks of beach nourishment?

Methods to combat beach erosion can be classed as hard and soft. Hard methods include structures like seawalls, jetties, and groins. They may stabilize an area for a few years, but severe erosional problems will probably develop at nearby locations. Soft methods include adding sand to the beach (beach nourishment) and a "move back from the beach" relocation policy based on the fact that sea level is slowly rising and the beach will gradually migrate landward in the future.

Soft methods are the least expensive and least damaging environmentally. Beach nourishment is effective but eventually (usually within five or ten years), the new sand is eroded away; thus the process must be repeated. Maintenance costs are fairly high, and an environmentally acceptable, low cost source of sand must be available. In the long run, prohibiting beach-front development and moving existing structures away from the beach will minimize costs associated with beach erosion and storm damage, which, incidentally, are largely charged to the taxpaying public, not to beachfront land owners or developers.

14. What is the basis for the predictions that global air temperatures will rise? How can a warmer atmosphere lead to a rise in sea level? (Box 14.3)

The global-warming predictions are based on increasing concentrations of the so-called greenhouse gases (carbon dioxide, methane, etc.) in the atmosphere. Warmer conditions will cause faster melting of polar ice sheets, adding liquid water to the oceans; warmer air temperatures will also cause a gradual warming of the oceans, resulting in a slight thermal expansion of the water and an additional, small rise in sea level. Complete melting of the Antarctic ice sheet would raise sea level about 70 meters, more than enough to "drown" low lying coastal areas in this and other countries.

15. Relate the damming of rivers to the shrinking of beaches at many locations along the West Coast of the United States. Why do narrower beaches lead to accelerated sea cliff retreat?

Along the West Coast, much of the sand on beaches originates as clastic sediment in streams and rivers that discharge into the sea. Damming these streams traps the sand behind the dam and reduces the input of new sand to the beach system. With reduced input, not enough of the sand lost to offshore areas is being replaced; thus the beach is starved and narrowed by erosion. Narrowed beaches allow storm waves to directly impact a sea cliff with minimal loss of energy, thus accelerating its erosional retreat.

16. What observable features would lead you to classify a coastal area as emergent?

Emergent coastlines develop as sea level is dropping or when the land is rising faster than sea level. Since sea level has been steadily rising for at least the past 30,000 years, present-day, emergent coastlines only develop where coastal lands are being tectonically uplifted; a good example would be the coast of California. Such areas typically have higher elevations, higher relief, and steeper, more narrow, river and steam valleys than tectonically stable continental margins with wide coastal plains and continental shelves.

Emergent coastlines feature landforms of marine depositional and erosional origin that have been elevated above sea level. Old, wave-cut cliffs and platforms (now steep slopes and terraces) are common. The terraces typically have thin covers of very young marine sediments and depositional or erosional features such as sands bars, coral limestone, and old sea stacks.

17. Are estuaries associated with submergent or emergent coasts? Why?

Estuaries are present along both coastlines, but the ones along submergent coastlines are much larger in size. Estuaries represent the flooded, lower portions of stream and river valleys. Since sea level has been rising steadily, large estuaries and estuarine systems have developed along tectonically stable continental margins with wide continental shelves and coastal plains; the Atlantic and Gulf coasts of the United States are good examples. Along wide coastal plains, each incremental rise in sea level inundates much larger areas than along tectonically rising coasts where elevations and relief are higher and stream valleys are more likely to be steep-sided and narrow.

18. Discuss the origin of ocean tides.

Fundamentally, ocean tides are forced by gravitational and rotational forces exerted in the Sun-Moon-Earth system. These forces deform the ocean surface from a sphere to an ellipse, producing two bulges with their apices lying along the lines of action of the resultant forces; the gravitationally dominated bulge points toward the Moon-Sun system and a rotationally dominated bulge of equal size points in the opposite direction. As the Earth rotates, these two bulges act as whole-ocean waves, sloshing back and forth to produce the tides.

The amplitude (height) of the daily tidal variation is strongly dependent on shoreline and nearshore depth configurations. Very high tides develop only in certain bays or estuaries with tapered shapes and special depth configurations that amplify the rising (incoming) tide.

19. Explain why an observer can experience two unequal high tides during one day (Figure 14.22).

The Earth's axis is inclined (not perpendicular) to the equatorial plane of the Sun (the plane of the ecliptic). In general, a plane through the crests of the tidal bulges is inclined to the Earth's equatorial plane. Rotation through the tidal bulges thus results in unequal amplitudes for the two, daily, high tides at a specific coastal location. Only at times when the resultant of the tide-causing forces is parallel to the Earth's equatorial plane would the daily high tides have equal amplitudes; this would occur twice each month about midway between the dates of the spring and neap tides.

20. How does the Sun influence tides?

The Sun has a less important effect on the tides than the Moon. Although far more massive than the Moon, the Sun is so much farther away that its gravitational force is only about one-half (1/2) that exerted by the Moon, and the vector can be assumed to always lie in the plane of the ecliptic. The highest monthly tides (spring tides) occur when the Sun, Moon, and Earth are in a straight line; thus their gravitational pulls are aligned. The lowest tides of the month (neap tides) occur when the Sun, Moon, and Earth form a right angle, and the resultant gravitational pull is minimized. Thus day-to-day tidal variations are mainly due to changes in the Moon's position.

21. Distinguish between flood current and ebb current.

These terms describe currents associated with rising and falling tides (sea level). A flood current describes a tidal current moving into an estuary on a rising sea level (incoming tide), and an ebb current describes a tidal current moving from an estuary into the open sea as the tide (sea level) is falling.

22. How have tides affected Earth's rotation? How did geologists substantiate this idea?

Water and earth tides convert gravitational energy into mechanical energy and heat, gradually lowering the total kinetic energy of the Earth-Moon system. The daily energy loss is infinitesimally small; but, integrated over geologic time, the loss is finite and measurable.

Some marine organisms show daily and monthly cycles in the rates at which calcium carbonate is added to their shells. These cycles result in delicate, patterned layering in the shells, somewhat like seasonal growth rings in trees. By carefully studying fossilized, Paleozoic shell material, the number of days (Earth's rotations) per year (one complete, seasonal cycle corresponds to one revolution around the Sun and is assumed to have been constant, i.e., equivalent in hours to the length of a present-day year) can be determined at the time the organism was growing. Such studies have documented a progressive slowing in Earth's rotation since the Cambrian period, when a single rotation (one Cambrian day) took only 21 hours.