Log entry: environmental studies: earth science

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Chapter 13 Landforms Made by Wind and Waves

Visualizing Physical Geography by Timothy Foresman &Alan Strahler

Chapter Overview

Wind Action

Eolian Landforms

The Work of Waves and Tides

Coastal Landforms

Human Interactions with Coastal Processes

Wind Action

Erosion by Wind

Wind performs two types of erosional work:

Deflation

Abrasion

Wind Action

Erosion by Wind

Deflation = The lifting and transport in turbulent suspension by wind of loose particles of soil or regolith from dry ground surfaces

Desert pavement: removal of fine particles by wind leaves coarser particles behind, which make up desert pavement

Wind Action

Erosion by Wind

Deflation produces blowouts

Deflation and water action produce desert pavement

Wind Action

Wind-sculpted rocks

Abrasion: particles carried by wind wear down a surface by impact

Ventifacts: wind-sculpted rocks

Yardang: teardrop-shaped ridge produced by wind erosion

Wind Action

Transportation by Wind

Saltation: process in which sand grains fly in low arcs, from one point to another

Surface creep: high winds drag sand grains across ground surface

Wind Action

Transportation by Wind

Dust storm: dense, high cloud of dust

Saltation working

Abrasion working

Dust cloud extends from surface to heights of several thousand meters

Often accompanies cold front

American Dust Bowl of 1930’s and China today exemplify

Eolian Landforms (shaped by wind)

Sand Dunes

Sand dunes = A hill or ridge of loose, well sorted

sand shaped by wind and usually capable of downwind motion

Dune formation

Dune begins as a sand drift downwind of some obstacle

Obstacle causes wind speed to drop, so saltating sand stops

Sand blows up windward side

Then it moves over top and down leeward side (slip face)

Slip face maintains angle of 33-34o

Eolian Landforms

Sand Dunes

Erg: vast expanse of sand dunes

Reg: Desert pavement surface

Sahara Desert is an example

Eolian Landforms

Sand Dunes

Types of dunes

Barchan

Transverse

Star

Parabolic

Longitudinal

Eolian Landforms

Barchan Dune

Eolian Landforms

Transverse Dune

Eolian Landforms

Star Dune

Eolian Landforms

Parabolic Dune

Eolian Landforms

Longitudinal Dune

Eolian Landforms

Loess = A surface deposit of wind-transported silt; settled out from dust storms over thousands of years

Forms vertical cliffs

Is easily eroded

Is used for cave dwellings

Thickest deposits in China (source: central Asia)

U.S. has thick loess deposits

Forms important agricultural soil

As waves approach shore

Swell waves release energy along continents

Deep-water swell waves shoal (become shallow)

They then become transitional waves, then

Become shallow-water waves

Wave speed decreases

Wavelength decreases

Wave height increases (remaining wave energy used)

Wave steepness increases (due to height increase)

Top of wave topples over base due to decrease in speed caused by friction

The Work of Waves and Tides

Shoaling waves

The Work of Waves and Tides

Waves

Swash

Backwash

Undercurrent (undertow)

Sand movement on the beach

Perpendicular to shoreline (toward and away)

Swash and backwash

Light wave vs. heavy wave activity

Parallel to shoreline (up-coast or down-coast)

Longshore current

Longshore drift

The Work of Waves and Tides

Swash and backwash--general

After wave breaks, uprush of water (swash) on beach

Water from breaking wave

Sediment moved toward land

Backwash

Water returns to ocean

Sediment moved away from shore

The Work of Waves and Tides

Backwash gone wild--rip currents

Backwash usually returns to ocean as sheetflow across ocean bottom

Some flows back to ocean in currents called rip currents

Currents are 15-150 ft wide, 4-5 mph

Can sometimes detect from wave patterns

Relax & swim parallel to shore

The Work of Waves and Tides

Waves

Tsunamis (seismic waves) = A train of sea waves triggered by an earthquake or another seafloor disturbance

2004 earthquake and tsunami in Indonesia and the Indian Ocean was deadliest recorded

Over 265,000 died

Early warning systems installed

The Work of Waves and Tides

Littoral Drift

Littoral drift: the transport of sediment parallel with the shoreline by the combined action of beach drift and longshore current transport; two components are

Beach drift: transport of sediment along the beach by angles of wave approach and resulting backwash

Longshore drift: movement of sand by longshore current in breaker zone

Longshore current

Wave refraction causes water and sand to move parallel to shore

Zigzag motion in surf zone

Longshore current: zigzag movement of water along shore

Longshore transport: movement of sediment caused by the longshore current

The Work of Waves and Tides

Longshore transport

Millions of tons of sediment moved yearly

Direction of transport changes due to wave approach, but

In general, sediment transported southward along Atlantic and Pacific coasts of U.S.; due to direction of storm origin

The Work of Waves and Tides

Refracting waves cause longshore current that moves water, and sand grains, from upstream to downstream

The Work of Waves and Tides

Wave Refraction—process by which waves erode sediment along a shoreline depends upon how much energy the waves have and the resistance of the shore

Headlands: jutting landforms of resistant rock

Wave refraction: wave front wraps around headland

Pocket beaches: crescent shaped

Wave refraction

Some segment of wave approaching land feels bottom first and slows before rest of wave

This slowing causes waves to bend (refract) so that wave crests are nearly parallel to shore

Waves almost always come in straight toward a beach, regardless of their place of origin

Holds for smooth and irregular shorelines

The Work of Waves and Tides

Wave refraction at irregular shoreline

Orthogonals converge in headlands and diverge in bays

Converging means focused energy and erosion

Diverging means deposition

The Work of Waves and Tides

Tides

Ocean tide: rise and fall of water levels in response to gravitational attraction of moon and sun

Causes of tides: inertial force constant, gravitational greatest on near side of Earth; Earth rotates thru bulges

The Work of Waves and Tides

Tides

Spring tide: high tide from alignment of Earth, Moon Sun

Neap tide: lower tide due to right angles between Moon and Sun

Two high tides, called flood tides

Two low tides, called ebb tides

Tidal range

Difference between heights of successive high and low waters

Large range shapes landforms

Bay of Fundy, tide range of 17 m (55.8 feet)

Coastal Landforms

Coastline (coast) = The zone in which coastal processes operate or have a strong influence

Shoreline = The shifting line of contact between water & land

Coastal Landforms

Erosional Coastal Landforms

Sea cliff = A rock cliff shaped and maintained by the weathering and erosion of breaking waves at its base

Coastal Landforms

Erosional Coastal Landforms

Marine terrace = A former shore platform elevated to become a step-like coastal landform

Coastal Landforms

Depositional Coastal Landforms

Beaches = A thick, wedge-shaped deposit of sand, gravel, or larger stones in the zone of breaking waves

Retrogradation: beach sand disappears in winter

Progradation: beach sand restored in summer

Swash and backwash, summer

Light wave activity

Swash dominates

Sediment moved toward shore

Wider beach

Fair weather

Summertime beach

The Work of Waves and Tides

Swash and backwash-winter

Heavy wave activity

Backwash dominates

Sediment moved away from shore

Narrower beach

Sand forms offshore sand bars

Stormy weather

Wintertime beach

The Work of Waves and Tides

Coastal Landforms

Depositional Coastal Landforms

Coastal dunes

Foredunes: where sand is available, dunes form in belt on landward side of beaches

Spits

Baymouth bar

Lagoon

Tombolo

Barrier island near Tom’s River, New Jersey

Long, narrow offshore deposits parallel to shore

Most developed due to rise of sea level about 18,000 years ago

Common in East and Gulf coasts of U.S.

Protect mainland from high wave activity

Barrier islands

Coastal Landforms

Coastlines of Submergence

Ria coast: rise of sea level or sinking of land brings shoreline to rest against the sides of river valleys previously carved by streams

Fjord coast: bays formed in valleys carved by glaciers