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.
2. Gyres are large
circular-moving whorls of water within the ocean basins. Four main currents
exist within each gyre.
North Pacific Gyre—
South
Pacific Gyre—
North
Atlantic Gyre—Gulf Stream, North Atlantic,
South
Atlantic Gyre—
Indian
Gyre—
3. Benjamin Franklin was
interested in surface ocean currents because it took ships carrying mail 2
weeks longer to travel from
4. 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.
5. Coastal upwelling occurs where
winds blow equatorward 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. Coastal upwelling along the
6. Deep-ocean circulation is driven by density
differences between different water masses. The differences in density are the
result of temperature and salinity variations.
7. The
shoreline is the line that marks the contact between land and sea, whereas the
shore is an area that extends between the lowest tide level and the highest
elevation on land that is affected by storm waves. The coast extends inland
from the shore as far as ocean-related features can be found, and the seaward
edge of the coast is the coastline.
8. A
beach is an accumulation of sediment found along the landward margin of the
ocean or a lake. The berm is the relatively flat
platform composed of sand adjacent to dunes or cliffs, and the beach face is the wet sloping
surface that extends from the berm to the shoreline.
Sources of beach sediment include erosion of adjacent cliffs or nearby coastal
mountains and sediment delivered to the coast by rivers.
9. Wind speed, the length of time
the wind has blown, and the fetch (distance the wind has traveled across the
open water) determine the height, length, and period of a wave.
10. Although
the form of the wave moves forward, the water does not. Rather, the water
particle moves in an essentially circular path as a wave passes.
11. 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.
12. Crashing
waves force compressed air and/or pressurized water into cracks and other
openings, expanding them and breaking the material apart. Abrasion results from
the impact of particles on one another, on the bottom, and on bedrock or
manmade structures.
13. 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 slower 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 parallel with the coastline. Over time,
headland erosion and deposition in protected bays and coves tend to even out
irregularities, thus straightening the coastline.
14. Large
quantities of sand move along beaches and just offshore owing to the action of longshore currents and longshore
drift. Thus over time, a flow or stream of sand continuously moves along the
beach and parallel to the beach in the shallow nearshore
waters.
15. 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 formed by beach drift and longshore currents that projects from the land into the
mouth of an adjacent bay,
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
16. 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.
17. Groins
are porous structures built into the surf zone 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.
18. 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 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 alternative is relocation—moving storm-damaged or at-risk
buildings and allowing nature to reclaim the beach.
19. 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.
20. Emergent
coastlines develop as sea level is dropping or when the land is rising faster
than sea level. 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.
21. Estuaries
are present along both coastlines, but the ones along submergent
coastlines are much larger. Estuaries represent the flooded lower portions of
stream and river valleys. Because 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