Earth Systems Ch 20

Hydrosphere, lithosphere, and atmosphere meet at

Coastlines

What happens with wave height as they approach shallow water?

Increase

What happens with wavelength as they approach shallow water?

Decrease

What happens with wave speed as they approach shallow water?

Decrease

Where is wave erosion most effective?

At sea level

Why are coral reefs found by nearly all continents and islands in tropical oceans? What conditions has to be present for corals to form?

Clean water

Warm water

Along coastlines

the wind has an even greater influence on topography

Daily tidal fluctuations

move enormous quantities of water

Tectonic events

contribute to water motion

More fundamental are long-term variations in sea or lake level caused by

eustatic sea-level change (sea-level change due to an increase or decrease in the amount of water in the world ocean or a change in the volume of an ocean basin)

However, in terms of geomorphic effects

wind is the most important cause of waves and currents

(In what ways are ocean shoreline-shaping processes different from those shaping lakeshores)

Along lakeshores

the range of tides is so small that they are insignificant to landform development.

(In what ways are ocean shoreline-shaping processes different from those shaping lakeshores)

The causes of sea-level fluctuations are quite different from the

causes of lake-level fluctuations

(In what ways are ocean shoreline-shaping processes different from those shaping lakeshores)

Coral reefs are built in tropical and subtropical oceans only, not in lakes.

Coral reefs are built in tropical and subtropical oceans only, not in lakes.

(In what ways are ocean shoreline-shaping processes different from those shaping lakeshores)

topographic features along seacoasts

re normally larger, more conspicuous, and more distinctive than those along lakeshores

Waves

entail the transfer of energy through a cyclical rising and falling motion in a substance.

Although water waves appear to move horizontally

this appearance is somewhat misleading

Most water waves

are wind generated

Some water waves (called forced waves)

are generated directly by wind stress on the water surface

Water waves become swells when

they travel beyond the location where they were generated by wind

As the wave passes

the water moves upward, producing a wave crest

Crest formation is followed by

a sinking of the surface that creates a wave trough

The height of a wave depends

on wind speed, wind duration, water depth, and fetch

As the wave moves into shallow water, the orbits become more elliptical, the wavelength shortens, and the wave steepens. Eventually, it “breaks” and dissipates its remaining energy as it washes up onto the beach

When the water depth becomes equal to about half the wavelength

he wave motion begins to be affected by frictional drag on the sea bottom

A breaking wave. In shallow water, the ocean bottom impedes oscillation, causing the wave to steepen until it collapses and tumbles forward as a breaker. The surging water then rushes up the beach as swash and then drains off the beach below the waves as backwash.

The breaking wave rushes toward shore or up the beach as

swash

Return flow

backwash

Wave refraction

occurs when a line of waves does not approach exactly parallel to the shore, or where the coastline is uneven or water depth is irregular in the near-shore zone.

The most conspicuous geomorphic result of wave refraction is

the focusing of wave action on headlands

The most notable erosion along coastlines is accomplished by

wave action.

it is often ____ that accomplish most of the erosion along a shorelin

large storm waves

Along a rocky shoreline, wave erosion has another dimension:

air is forced into cracks in the rock as the wave hits the shore.

______________ also plays a part in the denudation of rocks and cliffs because most rocks are to some extent soluble in seawater.

Chemical action

On shorelines made up of cliffs, the most effective erosion takes place at or slightly above

sea level,

(a) Waves pounding an exposed rocky shoreline erode the rock most effectively at water level. As a result, a notch may be cut in the face of the headland. (b) The presence of the notch undermines the higher portion of the headland, causing it to collapse and the cliff face to retreat.

_____ greatly accelerate the erosion of sandy shores; a violent storm can remove an entire beach in just a few hours, cutting it right down to bedrock.

Storms

Difference between a wave of osciliation and wave of translation

In other words, a wave of oscillation is a relatively gentle phenomenon, but a wave of translation can be a powerful force of destruction

Tsunami

are a consequence of abrupt movement along an ocean floor fault—especially from the vertical displacement caused by reverse or thrust faulting along a subduction zone

Tsunami

Tsunami formation

A vertical disruption of the ocean floor, such as from faulting, displaces the entire water column from ocean floor to surface. In the open ocean, the tsunami may be almost indistinguishable because of its great wavelength. Once it reaches shallow water, wave height increases. The tsunami may come onshore as a series of surges and withdrawals.

As a tsunami approaches a coast, it _____ —as do all waves—causing the wavelength to _____ and the wave height to _____

Slows

Decrease

Increase

Tsunami are really destructive

behind the wave crest of a tsunami is an enormous volume of water that can surge great distances inland before receding

The 2004 Sumatra–Andaman Earthquake and Tsunami

9.1 earthquake

Northern coast of Sumatra in Indonesia

The sharp movement of the ocean floor generated a tsunami that spread in all directions

About 28 minutes after the earthquake struck, a wave 24 meters (80 feet) high rushed onshore

Estimates suggest that nearly 228,000 people died

The 2011 Japan Earthquake and Tsunami

Magnitude 9.0 earthquake

The severe ground shaking lasted more than 3 minutes.

Where the Pacific Plate subducts beneath the Okhotsk “microplate

Nearly 16,000 people lost their lives in just the earyhquake

Tsunami warnings were issued within 10 minutes of the earthquake

The height of the tsunami was typically about 10 meters (33 feet), but in some confined harbors the height was as much as 40 meters (130 feet).

Why are tsunami often so much more destructive than even very large storm waves?

Tsunamis are more destructive than storm waves because they carry immense energy throughout the entire water column, have long wavelengths and high speeds, and grow dramatically as they approach shore. They surge far inland, flood large areas, and often strike with little warning, causing widespread and prolonged destruction

In addition to wind-generated waves and tsunami, a variety of other processes also modify coastlines in ways that range from gradual and subtle to sudden and spectacular

Tides

Changes in Sea Level and Lake Level

Global Warming and Sea-Level Change

Ice Push

Organic Secretions

Stream Outflow

Coastal Sediment Transport

Longshore Currents

Beach Drifting

Tides

are significant agents of erosion only in narrow bays, around the margin of shallow seas, and in passages between islands, where they produce currents strong enough to scour the bottom and erode cliffs and shorelines

eustatic sea-level changes

an increase or decrease in the amount of water in the oceans.

As a result of both _____ and _____ changes, many present-day ocean coastlines have been submerged, with a portion of a previous landscape now underwater.

Tectonic

Eustatic sea-level

in 2013 the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) projects

a likely sea-level rise of 0.26–0.55 meter (10.2–21.7 inches) by the end of this century under their lowest-emission scenario, and a rise of 0.52–0.98 meter (20.4–38.6 inches) under their highest-emission scenario

Some scientists consider the IPCC projections to be

too conservative

ice push

the result of the contraction and expansion of ice along a water’s edge.

Ice push is most significant in the ____ and ____, but it can be responsible for erosion along the shores of the Great Lakes and around smaller high-latitude or high-elevation lakes.

Arctic

Antarctic

The source of most sediment deposited in shoreline beaches and other depositional features is the

outflow from streams, although in some locations all or part of the sediment may come directly from the erosion of coastal rocks.

Longshore
currents
• “Along” shore
currents
• Water moves
parallel to shoreline
• Waves striking
coast at an angle
• Waves typically
follow wind
direction

Another significant mechanism of coastal sediment transport involves the

short-distance shifting of sand directly onshore by breaking waves and the retreating water from the beach

beach drifting

a zigzag movement of sediment that results in a general downwind displacement parallel to the coast

Nearly all waves approach the coast _____ rather than at a right angle

Obliquely

The strength, direction, and duration of the wind are the

principal determinants of beach drifting.

Explain how longshore currents and beach drifting transport coastal sediment.

Longshore currents move sediment parallel to the shore, driven by waves hitting the coast at an angle. Beach drifting occurs as waves carry sand up the beach at an angle and backwash it straight down, creating a zigzag movement of sediment along the shore.

Sediment budget

must be in balance if the deposit is to persist: the removal of sand must be offset by the addition of sand.

An idealized beach profile.