The Oceans Exam 3

Capillary Wave

Disturbing Force: usually wind

Restoring Force: cohesion of water (surface tension)

Typical Wavelength: up to 1.73 cm (0.68 in)

Wind Wave

Disturbing Force: wind over ocean

Restoring Force: gravity

Typical Wavelength: 60-150m (200-500 ft)

Seiche

Disturbing Force: change in atmospheric pressure

Restoring Force: gravity

Typical Wavelength: large, variable; set by basin size

Tsunami (Seismic Sea Wave)

Disturbing Force: faulting of the seafloor, volcanic eruption, landslide

Restoring Force: gravity

Typical Wavelength: about 200km (125 mi)

Tide

Disturbing Force: gravity of the Moon and Sun, rotation of Earth

Restoring Force: gravity

Typical Wavelength: half of Earth’s circumference

What builds wave energy

wind speed, duration, fetch

wind speed

how fast the wind blows

duration

amount of time that wind blows in one direction

fetch

the distance over which wind blows in a single direction

Fully-developed sea

When the maximum fetch and duration are achieved for a given speed

Wave motion

The transmission of energy through cyclic movement of matter.

• The wave carries energy, but the water itself generally doesn’t travel with the wave, except in shallow water.

crest

the top of a wave

trough

the bottom of a wave

wavelength (L)

the horizontal distance between corresponding points on successive waves

wave height (H)

The vertical distance between crest and trough

Deep-water waves

d > L/2
Not affected by the ocean floor

Transitional Waves

L/20 < d < L/2
Begin to “feel the bottom” and steepen into peaks

Shallow-water waves

d < L/20
The wave becomes unstable

Plunging breakers

Curling crest that moves over an air pocket.

• Curling particles outrun the wave, typically occurs on steep beach slopes
  
  

Spilling breakers

More common type of breaker on gentle slope beaches

• last longer because energy is removed more slowly

Constructive wave interference

In phase: crest meets crest, trough meets trough.
Waves add up and get bigger.

Destructive wave interference

Out of phase: crest meets trough.

Waves cancel out and get smaller.

Mixed wave intereference

A combination of both, with an irregular surface.

The most common*

What triggers tsunamis

Earthquake, volcanic eruption. seafloor shift, landslide

How tsunamis form

1. seafloor moves suddenly and pushes the entire column of water above it.

2. The waves spread out fast in deep water, up to about 700km/h but are only about a meter tall.

3. It reaches shallow water and slows down; its wavelength shortens

4. Height builds at the shore as the water piles up into a huge wave.

How storm surges form

• Low air pressure: the low pressure at the storm’s center lets the sea surface rise into a dome of water beneath it.

• Wind: strong winds push that dome of water towards the shore, where it piles up → bomb cyclone

Tides

caused by a combination of the gravitational forces of the moon and the sun and the motion of the Earth.

Moon’s gravity

the strongest tide-making force: the moon is close to Earth

Sun’s gravity

Also pulls on the oceans, but less, because the sun is much farther away.

Earth’s motion

The moon orbits Earth, the pair orbits the Sun, and Earth spins under the bulges.

Gravity and Inertia

are not parallel

What makes tides

Gravity, Inertia, Not on the same plane, two bulges, Earth spins under the bulges

Gravity (Centripetal Force)

pulls toward the center of the moon

Inertia (Centrifugal force)

acts to pull the moon away from the Earth and send it spinning out into space (salad spinner)

Not on the same plane

the two forces only balance exactly at the center of the Earth

Two bulges

one toward the moon, one on the opposite side

Earth spins under the bulges

So a spot on the coast passes through high and low tide

Spring tides

The sun, Moon, and Earth are in line (full and new moon)

Neap tides

The moon is at right angles to the Earth and Sun (first and third quarter moon)

Coral spawning

Many corals spawn all at once 1-2 days after a full moon in the spring months

Spring-tide egg laying

Grunions, sea turtles, and horseshoe crabs lay eggs high on the beach at high spring tides, out of reach of water predators

Intertidal zones

organisms sort into zones by how much air exposure vs. submergence they can tolerate

Tidal mirgration

Some species move in and out with the tides

Tidal patterns: in theory

Most areas should get two high tides and two low tides each lunar day, a semidiurnal tide.

Tidal patterns: in practice

the depths, sizes, and shapes of ocean basins modify the tides into three patterns

Semidiurnal Tide

two highs and two lows per lunar day, about equal in height

Diurnal Tide

One high and low per lunar day

Mixed tide

two highs and two lows of unequal height

Tidal Bores

A TRUE tidal wave, a steep front, a sudden rise

A TRUE Tidal Wave

a wall of water that moves up low-lying rivers due to an incoming tide

A steep front

the incoming tide develops a steep forward slope because the river’s flow resists the advance of the tide

A sudden rise

produces a rapid change in water level after the wave passes

Rip currents (not a tide at all)

Rip tides or rip currents (undertows) are narrow, fast currents that move directly offshore, produced by wave action and seafloor dynamics

Emerging shorelines: moving above sea level

due to an uplift of the continent, a lowering of sea level, or both.

Emerging shorelines: what to look for

stranded beach deposits or marine terraces

→ Pacific Coast of the U.S.

Submerging shorelines: sinking below sea level

due to subsidence of the continent, an increase in sea level, or both.

submerging shorelines: what to look for

Drowned beaches and submerged dune topography

→ Atlantic coast of the U.S.

Backshore

Above the high tide shoreline; covered only during storms

Foreshore

Intertidal or littoral; exposed at low tide

Shoreline

migrates back and forth with the tide; the water’s edge

Nearshore

low-tide shoreline to low-tide breaker line; never exposed; affected by waves

Offshore

Deep water; waves don’t feel the bottom

Beach composition

Beaches are composed of whatever material is locally available (sediment from beach cliffs, rivers, shell fragments, lavas)

Beach composition and slope

Always in transit: beach material keeps moving (very dynamic environments)

Sediment moves

sediment eroded from the beach may be carried along the shore and deposited where wave energy is low

Erosional Shores

steep cliffs, in areas where tectonic uplift occurs, e.g., the U.S. Pacific coast

Depositional Shores

U.S. East and Gulf Coasts: sandy beaches and barrier islands, BUT storm erosion can still be a problem.

Erosion can produce

Wave cut cliffs → sea caves → sea arches → sea stacks

The rate of wave erosion is determined by

Degree of exposure of the shore, tidal range, composition of the coastal bedrock

Depositional-Type Shores

Spit, Barrier Islands, Tombolo

Spit

Linear ridge of sediment extending alongshore from land into deeper water; the end usually curves inland

Barrier Islands

extensive offshore sand deposits parallel to the coast; may have formed as sea level rose when glaciers melted 18,000 years ago (old sand bars)

Tombolo

sand ridge connecting an island or sea stack to the mainland, or connecting two islands.

Groin

Wall perpendicular to the shore. Traps sand on the up-drift side but starves the beach down-drift

Seawall

Wall parallel to the shore. Protects buildings but can speed up erosion of the beach in front of it.

Renourishment

Sand trucked or pumped back onto the beach. Works, but is temporary and has to be repeated.

Types of Coastal Wetlands

salt marsh, mangroves, freshwater marsh

Nursery Habitat

A refuge for many species

Buffer zone

Filters pollution before it reaches the sea

Intertidal

Flooded and drained by the tides

Wetland Loss

a major ongoing problem

Estuary

a partially enclosed area of the coast where salt water is diluted by freshwater from land

Point source pollution

one identifiable outlet, e.g. a factory or sewage pipe

Non-point source pollution

Diffuse runoff, e.g., fertilizer from farms and lawns.

Eutrophication

The excessive nutrient enrichment of water leading to algal blooms and ecological imbalance

Process: Nutrients runoff land via rivers → algae bloom → algae die; bacteria decompose them -→ oxygen used up → dead zone

Euphotic

Enough light for photosynthesis (down to about 1% of surface light)

• most marine life lives here

Disphotic

Twilight zone: some light, not enough for photosynthesis

Aphotic

completely dark

Primary production

Photosynthesis makes glucose and O2. Cellular respiration runs it in reverse. Production rises and falls with seasonal light.