Chapter 10 Questions
How do wave cause erosion?
9. 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 man made structures.
What is the primary driving force of surface ocean currents? How do the distribution of continents on Earth and the Coriolis effect influence these currents?
1. Wind is the primary driving force of surface currents. The Coriolis effect deflects the currents to the right (Northern Hemisphere) or left (Southern Hemisphere) of their path of motion (the prevailing wind direction). The location of the continents also affects the pattern of surface currents.
What is wave refraction? What is the effect of this process along irregular coastlines?
10. 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.
What is beach drift and how is it related to a longshore current? Why are beaches often called "rivers of sand"?
11. Waves generally hit the beach at an angle, therefore pushing sand and water up the beach face at an angle. However, once on the beach, gravity pulls the water and sand straight down the beach face. This happens thousands of times a day, the water and sand move up the beach face at an angle, then flow straight down the beach, back up at an angle, then straight back down again. Thus the water and sand travels in sort of a zig-zag pattern down the beach and nearshore area. The movement of the sand is called beach drift and the movement of the water generates the longshore current. 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.
Describe the formation of the following shoreline features: wave-cut cliff, wave-cut platform, marine terrace, sea stack, spit, baymouth bar, and tombolo.
12. Wave-cut cliff—a seaward facing cliff along a steep shoreline formed by wave erosion at its base and mass wasting Wave-cut platform—a bench in bedrock at sea level cut by wave erosion Marine terrace—a wave-cut platform that has been uplifted above sea level Sea stack—the result of wave refraction on a headland. When caves on opposite sides of a headland unite, a sea arch is formed. When the arch eventually collapses, it leaves an isolated remnant called a sea stack. Spit—an elongated ridge of sand that projects from the land into the mouth of an adjacent bay, which is formed by beach drift and longshore currents Baymouth bar—a sand bar that completely crosses a bay, sealing it off from the open ocean Tombolo—a sand ridge connecting an island to the mainland or to another island
List three ways that barrier islands may form.
13. 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.
List the types of hard stabilization and describe what each is intended to do. What effect does each one have on the distribution of sand on the beach?
14. 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. 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. Breakwaters are structures designed to protect boats from the force of large breaking waves. However, the quiet water zone behind the breakwater often allows sand to accumulate, thus filling up the boat anchorage.
In what direction is longshore transport occurring in Figure 10.22 (p.298)? Is it toward the top left or toward the bottom right of the photo?
15. Sand is accumulating on the right side of the groins. This means that the longshore current is transporting sand right to left (toward the top left of the figure).
List two alternatives to hard stabilization, indicating potential problems with each one.
16. One alternative to hard stabilization is beach nourishment. This process simply involves the addition of large quantities of sand to the beach system. Beach nourishment is not really a permanent solution because it is often quite expensive, much of the transported sand will be eroded just like the original beach, and sometimes there are environmental effects associated with using different materials. A second alternation is relocation - moving storm-damaged or at-risk buildings and allowing nature to reclaim the beach.
Discuss the oceans tides. Eplain why the Sun's influence on Earth's tides is less than half that of the Moon's, even though the Sun is so much more massive than the moon.
20. Fundamentally, ocean tides are formed 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 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.
How do ocean currents influence climate? Give at least one example.
3. Ocean currents aid in the latitudinal transfer of heat. Warm currents have a definite moderating influence on adjacent land areas, especially during winter. Conversely, cold currents depress the temperatures of nearby coastal areas. Further, the presence of a cold current acts to intensify aridity, and to increase relative humidity and the occurrence of fog. One example of the effect ocean currents have on climate is the comparison of London and St. Johns (located in Newfoundland Canada, see Fig. 10.3 in the text). London is farther north than St. Johns, but London actually has a warmer winter temperatures. This is because the Gulf Stream brings warm water to Western Europe, thus moderating the temperature.
Describe the process of coastal upwelling. Why is an abundance of marine life associated with these areas?
4. Coastal upwelling occurs where winds are blowing toward the equator and parallel to the coast. Because of the Coriolis effect, the surface water moves away from the shore area and is replaced by cold water from below. An example of coastal upwelling is the colder ocean temperatures along the California coast as compared to the east coast of the United States. Upwellings also bring nutrient rich water to the surface thus attracting marine life to the area.
Describe the physical changes that occur to a wave's speed, wavelength, and height as a wave moves into shallow water and breaks onto shore.
8. 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.
