Download CHAPTER 14 - WAVES, BEACHES AND COASTS

CHAPTER 18 - COASTS AND OCEANS
Overview
Wind energy is transferred to water surface and forms waves. Wave height
(distance between crest and trough) is determined by the amount of energy
transferred. Wave length is the distance between crests. As a wave passes,
water molecules describe circular orbits, equal to wave height at the surface,
but decreasing with depth until there is essentially no motion at a depth equal
to half the wavelength. Waves break because friction with the sea floor causes
the top of the wave to move faster and it eventually collapses producing surf.
As waves approach shore, they change direction as the leading edges are
slowed by contact with the bottom and the waves become more parallel to
shore. This wave refraction produces longshore currents that flow parallel to
the shore. Rip currents are localized narrow currents that flow perpendicular to
shore in areas with low wave height. They may either be in fixed positions
over channels or shift because of wave interference.
Tides involve the regular rising and falling of the sea surface and are caused
by the gravitational attraction between the Earth, the sun and the moon. Two
bulges are produced on the surface of the oceans that move around the Earth
and cause water levels to rise (flood tide) or fall (ebb tide), usually on a
semidiurnal (twice a day) cycle. Tidal range is the difference in water level
between high and low tide and can range from less than 1 m to over 16 m.
Areas of the shoreline exposed between high and low tides form an intertidal
zone.
Beaches are strips of sand or gravel along which waves break and extend
inland from the low water line to a cliff or vegetated area. Beaches can be
subdivided into an offshore marine terrace, a beach face, and an upper berm.
The form and dimensions of beaches change seasonally, with winter beaches
being narrow, and summer beaches wider. Tremendous amounts of sediment
are transported along the beach face by swash and backwash and immediately
off shore by longshore drift. Spits, baymouth bars and tombolos are the
natural products of longshore drift. Jetties and groins constructed by humans
can also interrupt sand transport along a beach. The bulk of beach sand is
transported to the coast by rivers and streams. Damming this source cuts off
sand supply and promotes beach erosion.
Erosional coastlines exhibit features such as sea cliffs, arches, stacks, wavecut platforms and undergo progressive coastal straightening. Depositional
coastlines are characterized by barrier islands, deltas, and tidal deltas.
Drowned coastlines are submergent and identified by estuaries or fiords.
Uplifted coastlines are emergent and marked by uplifted marine terraces.
Coastlines can also be shaped by organisms such as reefs and mangroves.
The ocean floor consists of the gently sloping continental shelf, the steeply
sloping continental slope and the deep ocean floor. Passive continental
margins are those in areas of little tectonic activity, such as the eastern coast
of the US, while active continental margins are associated with volcanic and
earthquake activity, such as the margins of the Pacific Ocean. Continental
shelves lie on the edge of continents and are mostly covered with land-derived
sediment, although in areas of little sediment input, reefs may form.
Continental slopes link continental shelves to the deep ocean and are
characterized by steep slopes and turbidite deposition. Submarine canyons
cut into the edge of continental shelves and the upper part of continental
slopes and allow the transport of large amounts of sediment by turbidity and
bottom currents to be deposited on submarine fans on the continental rise at
the base of the slope. Turbidity currents are sediment-laden current that
move rapidly downslope under the influence of gravity. Contour currents are
slow moving and flow along (parallel to) the slope. Abyssal plains are flat
areas at the base of the continental rise, although some areas of the deep
ocean floor are characterized by rugged topography. Sediments accumulating
on the sea floor are either terrigenous (derived from land) or pelagic (settle
from suspension. Pelagic sediments include clays and the skeletons of
microscopic organisms.
Learning Objectives
1. Wind energy is transferred to water surfaces to form waves. Wave height
(distance from crest to trough) reflects wind speed, duration and distance.
Wavelength is the distance between crests. Waves passing a point move water
particles in a circular orbit. At the surface, diameter of the orbit equals wave
height. At depth, effects of wave passage are lost below depths equal to half
the wavelength.
2. Wave refraction is the change in direction along a wave crest as it comes in
contact with the bottom while approaching shore. Wave refraction helps
develop longshore currents that are flow parallel to the shoreline and transport
considerable amounts of sediment in the surf zone. Rip currents flow
perpendicular to shore and carry fine sediment offshore.
3. Tides are caused by changes in the elevation of the sea surface, usually on
a semidiurnal cycle. Tides are caused by a force resulting from the
gravitational attraction between the Earth, sun and moon.
4. Beaches are strips of sand or gravel that extend from marine terraces
offshore to cliffs or permanent vegetation zones onshore. The beach face is the
steepest part and is exposed to wave action. The berm extends landward from
the beach face.
5. Summer beaches have wide berms, while winter beaches have narrow
berms and sandbars offshore. Most sediment on beaches was brought to the
coast by rivers and streams. Damming free flowing rivers reduces sediment
supply to beaches and promotes beach erosion.
6. Longshore drift is the movement of sediment parallel to shoreline by a
combination of swash and backwash along the beach face, and longshore
currents. Spits are fingerlike ridges of sediment deposited into open water.
Baymouth bars extend from headland to headland cutting off bays from the
ocean. Tombolos connect offshore islands to the mainland. All three
depositional features are the result of longshore drift processes. Jetties and
groins are structures made by humans to interrupt sand being transported
along the shoreline by longshore drift.
7. Coastal areas are classified as erosional, depositional, drowned, uplifted, or
shaped by organisms. Erosional coasts are subject to coastal straightening
(erosion on headlands and deposition in bays), and exhibit sea cliffs, wave-cut
platforms, stacks and arches.
8. Depositional coasts have barrier islands, deltas, tidal deltas, and may
preserve glacial deposits such as moraines. Fiords and estuaries are typical of
drowned (submergent) coasts. Uplifted coasts exhibit uplifted marine terraces.
Reefs and mangroves may also shape coasts.
9. Beyond the shoreline, the ocean floor consists of a continental shelf,
continental slope and abyssal plain. Passive continental margins are those
formed as rifted margins, whereas active continental margins are those along
which tectonic activity occurs.
10. Continental shelves are usually covered with sediment derived from land
and moved by currents and waves. Reefs are resistant bodies formed of coral
and other organisms and develop in areas where sediment supply is low.
Continental slopes are steeply sloping and link shelves with the deeper ocean.
Submarine canyons are erosional features that cut into the outer part of
shelves and the upper slope and transport sediment by turbidity currents. The
continental rise is a wedge of sediment at the base of the slope and passes
into the flat abyssal plain.
11. Ocean crust on the ocean floor is covered with either terrestrial sediment
derived from land or pelagic sediment consisting of clay and microscopic
organisms.
Boxes
18.1 ENVIRONMENTAL GEOLOGY – COMMUNITIES AND COASTAL EROSION
Communities built on shorelines often face severe problems with coastal erosion,
particularly when shorelines consist of loose and easily eroded sediments. Erosion
rates are particularly high in areas of Atlantic Canada and along the Pacific coast of
British Columbia and have resulted in damage to docks, roads, homes and
lighthouses. Coastal erosion is not restricted to marine areas and shorelines of the
Great Lakes also experience high erosion rates. Canadian Arctic coasts are often
protected from wave erosion by sea ice although damage can be considerable during
years of sparse sea ice development.
18.2 ENVIRONMENTAL GEOLOGY - THE EFFECTS OF RISING SEA LEVEL
Long term sea level rise is caused by melting glaciers, while short term sea
level rise is caused by storm surge. Sea level has risen 130 meters during the
past 15,000 years. Initial sea level rise was 1.3 meters per 100 years, but
declined to 4 centimeters per 100 years. Since 1930, however, the rate of sea
level rise has increased six-fold to 24 cm per century. This rapid rise caused
barrier island migration, increased erosion and allowed storm waves access
further inland than before. Even small increases in sea level have the potential
for catastrophic flooding of large portions of the coastal United States because
these areas are so flat and low. Melting all glacial ice would raise sea level by
60 meters drowning significant coastal areas. Short-term sea level rise occurs
from storm surges caused by hurricanes that produce a doming of sea level as
they pass. This situation is compounded by high winds that drive the seawater
onto shore. Storm surges can exceed 5 meters and have caused considerable
damage and loss of life in coastal areas. A storm surge of 8 meters struck
Galveston, Texas, in 1900, killing 6,000 people. A 5 meter storm surge
produced by Hurricane Hugo in 1989 caused $10 billion damage to South
Carolina.
Short Discussion/Essay
1. How much would sea level rise if 1) the north polar ice cap melted; 2) the
south polar ice cap melted; 3) all glacial on earth melted?
2. How do tides affect the operation of coastal processes?
3. Explain the relationship between wave refraction, longshore currents and
longshore drift.
4. Why is beach erosion such a problem along the Atlantic coast?
5. Explain how jetties and groins work.
Longer Discussion/Essay
1. If sea level is actually rising, how can both emergent and submergent
coasts be found around the world?
2. How might a geologist use the concept of effective wave base to
differentiate between rocks deposited in "shallow" and "deep" settings?
3. Why would beach dimensions change with season?
4. Explain the influence of waves and wind on the character of beach sand (i.e.
sorting, roundness, size differences).
5. Why are beach replenishment programs ultimately doomed to failure?
Selected Readings
Businger, Steven. 1991. "Arctic hurricanes", American Scientist 79 (1): 18-33.
Davis, R.E. and Dolan, R. 1993. "Nor'easters," American Scientist 81 (5): 428439.
Horton, T. 1993. "Hanging in the balance: Chesapeake Bay" National
Geographic 183: 3- 35.
Pilkey, O. H. 1989. "The engineering of sand. Journal of Geological Education
37: 308 -311.
Richardson, P.L. 1993. "Tracking ocean eddies," American Scientist 81 (3):
261-272.
Schlee, J.S., Karl, H.A. and Torresoan, M.W.. 1995. "Imaging the Sea Floor"
U.S. Geological Survey Bulletin 2079.
Schneider, D. 1997. "The rising seas," Scientific America 276: 112-117.
Trenhaile, A.S., 1998. Geomorphology: A Canadian Perspective. Oxford
University Press Canada, 340pp
Walden, D. 1990. "Raising Galveston." American Heritage of Invention and
Technology (winter): 8-18.