Chapter 20:  Coastal Processes and Landforms

 

CHAPTER OUTLINE

 

I.        The Impact of Waves and Currents on the Landscape

A.    Coastal processes create a landscape almost completely different from any other on Earth.

1.      Some overlap in the actions of waves and currents, but

a)      Waves are essentially agents of erosion.

b)      Currents are essentially agents of deposition.

2.      Most notable landforms produced by waves: rocky cliffs and headlands.

a)       

b)      Headland — a promontory with a steep cliff face projecting into the sea, created by waves.

B.      Most common landforms produced by currents: beaches and sandbars.

 

II.     Coastal Processes

A.    Coastline represents the interface of hydrosphere, lithosphere, and atmosphere.

1.      Usually an area of ceaseless movement and energy transfer.

B.      Wind action more influential here than in arid areas because can deform body of water, which then impacts landforms.

1.      Wind most influential force in causing water to move.

C.     Other forces affecting coastline topography:  daily tidal fluctuations, diastrophic events like earthquakes, long-term changes in sea or lake level caused by tectonic forces and eustatic forces, and somewhat by actions of continental ice sheets.

1.      Eustatic forces — produce sea-level change entirely by increasing or decreasing the amount of water in world ocean.

D.    Topographic forms on seacoasts similar to those in lakeshores, with three exceptions:

1.      In lakes, tidal changes too small to significantly affect landform development;

2.      Causes in lake-level fluctuations very different from those in sea level.

3.      Reefs do not occur in lakes.

E.      Also, the larger the body of water, the greater the effects of coastal processes, so seacoast topographical features usually larger, more conspicuous, and more distinctive than those on lakeshores.

F.      Shore-Shaping Forces

1.      Seven principal processes that contribute to the shaping of coastal features:

a)      changes in water level

b)      tides

c)      waves

d)     currents

e)      stream outflow

f)       ice push

g)      organic secretions

(1)   Changes in water level

(a)   from uplift/sinking of landmass (tectonic cause)

(b)   increase/decrease in amount to water (eustatic cause).

(2)   Tides have a significantly small effect on shoreline topography.

(a)   Tidal erosion is most effective where tides can produce currents strong enough to scour the bottom and erode cliffs and shorelines:

(i)     Narrow bays; around the margin of shallow seas; and in passages between islands.

(3)   Waves most important wind-driven sculptors of topography.

(a)   Wave action accomplishes most erosion.

(4)   Currents are large volumes of water moving horizontally.

(a)   Longshore current — littoral current — a current in which water moves roughly parallel to the shoreline in a generally downwind direction.

(b)   Most effective type of current in shoreline erosion and deposition.

(5)   Stream outflow is important because streams and lakes feed sediments to oceans and lakes.

(6)   Ice push resulting from the contraction and expansion from freeze–thaw action, so that near-shore ice can deform land in the fashion as a small glacial advance.

(a)   Major alterations in Arctic and Antarctic, minor ones on high-latitude or high-altitude lakes.

(7)   Organic secretions come from primitive aquatic animals and plants producing lime (calcium carbonate), such as coral polyp.

(8)   Chemical action also plays a role in erosion of rocks and cliffs.

G.     Coastal Sediment Transport

1.      Wave action accomplishes nearly all movement of rock debris (mostly sand) on coasts.

2.      Beach drifting — a zigzag movement of sand deposition and erosion in a general downwind direction along the coast.

a)      Creates a downwind displacement of particles parallel to the coast.

b)      Because most waves approach coast obliquely rather than at a right angle, sand and other debris move up beach at oblique angle.

c)      Beach drifting principally determined by strength, direction, and duration of wind.

H.    Coastal Deposition

1.      Often, deposition forms the most conspicuous topographic features of a shoreline.

2.      Maritime deposits usually more ephemeral than noncoastal deposits.

a)      Composition is relatively small particles.

b)      Sand is not stabilized by vegetation cover.

c)      Often constant onslaught of water.

I.        Significance of Recent Sea-Level Fluctuations

1.      A change in sea level is a prominent factor in influencing coastal topography.

a)      Creates areas of submergence and areas of emergence.

(1)   Submergence of shorelines is common, showing up in nearly all the world’s oceanic coastlines because of the melting of Pleistocene ice.

(a)   Estuary — a finger of the sea projecting inland along drowned river valleys.

(b)   Ria shoreline — a coast with numerous estuaries.

(2)   Emergence is usually associated with tectonic uplift.

2.      If global warming is taking place and continues, we can anticipate a period of deglaciation, with melting of ice sheets in Antarctica and Greenland.

a)      Would have a profound effect on all humankind.

(1)   Would inundate many islands and coastal plains.

 

III.   Coastal Landforms

A.     Beaches

1.      Beach — an exposed deposit of loose sediment, normally composed of sand and/or gravel, and occupying the coastal transition zone between land and water.

a)      Most widespread marine depositional feature.

b)      Sediment is usually homogeneous in size, but different beaches or sections of beach can range from fine sand to large cobbles.

2.      Backshore — the upper part of the beach, beyond the reach of ordinary waves.

a)      Covered with waves only during severe storms.

3.      Berm — the relatively flat part of a backshore beach, composed of wave-deposited material.

4.      Foreshore — the lower shore zone of a beach, generally between the levels of high and low tide.

5.      Offshore — that portion of the shore zone seaward from the low-tide line and extending to the area where wave erosion and deposition do not occur.

a)      Permanently underwater, where waves break and where surf action is greatest.

B.      Cliffs/Benches/Terraces

1.      Common landform complex is comprised of wave-cut cliffs, wave-cut benches, and wave-built terraces.

a)      Wave-cut cliff — steep cliffs built as waves erode away at a rocky headland.

b)      Wave-cut bench (wave-cut platform) — a broad erosional platform created by the pounding and abrasion of waves seaward of cliff face, usually slightly below water level.

c)      Wave-built terrace — submarine deposit of sand at the outer margin of an erosional platform or bench.

(1)   Consists of debris eroded from wave-cut cliff and bench, with fallen particles being battered until they are small enough to be transported.

d)     Marine terrace — a platform of marine erosion that has been uplifted above sea level along a tectonically rising coast.

C.     Barrier Islands/Lagoons

1.      Barrier island — a long, narrow sandbar built up in shallow offshore waters.

a)      Distance from shore varies from few hundred yards to often several miles.

b)      Believed to result from debris heaping up when waves break up in shallow waters.

c)      Some larger ones may have been created during Pleistocene, when there was a lowered sea level.

d)     Can become the dominant element of a coastal terrain.

2.      Lagoon — a body of quiet salt or brackish water in an area between a barrier island or a barrier reef and the mainland.

3.      Mudflat — Areas of marshes and eventually meadows created when a lagoon becomes increasingly filled with water-deposited sediment from coastal streams, wind-deposited sand from the barrier island, and tidal deposits if the lagoon has an opening to the sea.

D.    Spits

1.      Spit — a linear deposit of marine sediment that is attached to the land at one or both ends.

2.      Baymouth bar (bay barrier) — a spit that has become extended across the mouth of a bay to connect with a headland on the other side, transforming the bay into a lagoon.

3.      Hook — a curving sandbar at the outer end of a split, produced by conflicting water movements in a bay that guide deposition in a curved fashion toward the coast.

4.      Tombolo — a spit formed by sand deposition of waves converging in two directions on the landward side of a nearshore island, so that a spit connects the island to the land.

a)      Less common feature.

E.      Fjorded Coasts

1.      Fjord — a glacial trough that has been partly drowned by the sea.

2.      Creates most spectacular coastlines, especially when numerous and result in long, narrow fingers reaching more than 100 miles (160 km) inland.

F.      Coral Coasts

1.      Coral reef — a coralline formation that fringes continents and islands in warm-water tropical oceans.

a)      Occurs by organic secretions by coral polyps, tiny marine animals that secrete external skeletons of calcium carbonate, called coral.

2.      Fringing reef — a coral reef built out laterally from the shore, forming a broad bench that is only slightly below sea level, often with the tops of individual coral “heads” exposed to the open air at low tide.

3.      Barrier reef — a prominent ridge of coral that roughly parallels the coastline but lies offshore, with a shallow lagoon between the reefs and the coasts.

4.      Atoll — coral reef in the general shape of a ring or partial ring that encloses a lagoon that had formerly surrounded a volcano, but that volcano has since sunk below surface.

5.      Motus — coral islets that are closely spaced and separated by narrow channels of water and that together form a ring-shaped atoll.

 

IV.  Focus: Waves

A.    Water waves are undulations in the surface layers of a water body.

1.      They do not move water horizontally, except very slightly through the wave shift and except where waves crest and break.

2.      Items in waves only bob up and down without advancing, unless pushed by wind.

B.      Most are wind generated.

1.      Swell — a water wave that has escaped the influence of the generating wind.

a)      Can travel enormous distances away from the source of the disturbance.

b)      Usually produced by stormy conditions, but can also be produced by tidal surge, volcanic activity, or undersea diastrophic movement.

C.     Wave crest — highest point of a wave, occurs when water moves upward in wave.

D.    Wave trough — lowest part of a wave, occurs when water sinks to surface in wave.

E.      Wavelength — the horizontal distance from crest to crest or from trough to trough.

F.      Wave height — the vertical distance from crest to trough.

Height depends on wind speed, wind duration, water depth, and fetch (the area of open water).

G.     Wave amplitude — one-half the wave height; i.e., the vertical distance from still-water level, either upward to the crest or downward to the trough.

H.    Influence of wave diminishes with depth.

I.        Swash — a breaking wave that rushes up the beach, with the cascading forward motion of a breaking wave that rushes up the beach.

J.        Backwash — water moving seaward after the momentum of the wave swash is overcome by gravity and friction.

K.     Wave refraction — phenomenon whereby waves change their direction trend as they approach a shoreline.

L.      Tsunami — seismic sea wave — a very long sea wave generated by submarine earthquake or volcanic eruption.

M.   Waves of oscillation, those passing in open water, are a relatively gentle phenomenon, but waves of translation, such as those hitting the shoreline, can be a powerful force of destruction.