Chapter 12 Wind, Waves & Shoreline Currents BFRB Pages &

Erosion By Wind Dry ClimatesDry Climates (soil made of dry & loose sand, silt, & clay which are easily carried by wind) ABRASIONMechanical weathering by ABRASION (wind carries sediments and blows them against rocks – like sandpaper) –VENTIFACTS –VENTIFACTS – “dents” in rocks from sand –Side of the rocks facing the wind are usually worn smooth over time

VIF!!!!!

DEFLATION Wind removes the top layer of soil, causing the ground surface to lower over time –The soil “DEFLATES” (like a bicycle tire) and there are small depressions or “pits”

Deflation “pits”

SAND DUNES Large piles of sand deposited by wind –Usually start piling up against rocks, bushes, clumps of grass, etc… (WINDWARD SIDE)Side of the dune facing the wind (WINDWARD SIDE) is a long gentle slope (LEEWARD SIDE)Side opposite (LEEWARD SIDE) is a short, steep slope – also known as the “slip face” (like if you tried to walk down it you’d slip because it’s steep…)

windward leeward Remember Mrs. Frisby & the rats of Nimh? (Secret of Nimh) The Rats of Nimh were asked to move Mrs. Frisby’s house to the LEEWARD side of the hill so it would be protected!

Barchan Dunes - are crescent-shaped dunes with the points of the crescents pointing in the downwind direction, and a curved slip face on the downwind side of the dune.

Transverse (Longitudinal) Dunes - are large fields of dunes that resemble sand ripples on a large scale.

Parabolic (also called blowout) Dunes - "U" shaped dunes with an open end facing upwind. They are common in coastal areas.

Star Dunes - are dunes with several arms and variable slip face directions that form in areas where there is abundant sand and variable wind directions.

Waves & Currents

Causes of Waves Wind blowing the surface of the water causing it to “pile up” HEIGHT depends on 2 things: –1 – length of time the wind blows FETCH –2 – FETCH – distance the wind blows over the open water

Wave Features Waves cause surface water to rise & fall in an orbital motion CRESTTop of the wave = CREST TROUGHBottom of wave = TROUGH WAVE HEIGHTWAVE HEIGHT is the difference between the crest and trough WAVE LENGTHWAVE LENGTH is the distance from one crest to the next crest

PERIODPERIOD – the time it takes one complete wavelength to pass a point SPEEDSPEED = wavelength (dist) / period (time) Units are m/sec or ft/sec Wave Features

Waves in Shallow Water BREAKERSBREAKERS – waves that enter shallow water, slow down & “break” –The bottom of the wave “scrapes” along the bottom of ocean & slows down due to friction, this reduces its wavelength. –The reduction in wavelength (horizontal) increases the wave height (vertical) – (Conservation of energy.) appears –The crest of the wave continues forward at the same speed & falls over onto the beach. (The crest is not moving faster, it just appears to move faster because the bottom part of the wave is moving slower!!!)

As waves enter shallow water, their length decreases which causes their height to increase!

Waves in Shallow Water Waves approach the shoreline at an angle In shallow water, the part of the wave that “hits” the shallow shoreline first, slows down (due to friction) and the part of the wave that isn’t hitting the shoreline keeps moving at the same speed. The wave crest direction “bends” due to the difference in speed. REFRACT almostShoreline waves REFRACT & hit the shore almost parallel to shore BUT NOT PARALLEL!!!!!

CURRENTS SWASHSWASH – water moving ONTO the beach BACKWASHBACKWASH – water moving back off beach –Pulls sand from the beach into the water –UNDERTOW –UNDERTOW – a strong backwash – very dangerous for small children! Ocean

SWASH BACKWASH

VIF!!! LONGSHORE CURRENTSVIF!!! LONGSHORE CURRENTS – move parallel to shoreline & carry sand down the beach (erosion) –Ever notice how when you get out of the ocean after swimming you’re farther down the beach from your towel?!?! RIP CURRENTSRIP CURRENTS – very strong currents that move OUTWARD from the shore –Extremely dangerous – can pull you from shore! The WORST thing to do if caught in a riptide is to panic & swim directly to shore! The current is too strong & you will fatigue and drown! Swim diagonally across the current to safety! CURRENTS

Longshore Current: transportation of sediments parallel to the beach…this is the current that can transport you parallel to the beach as well!

SHORELINE FEATURES

Waves easily erode soft rocks found along deep water shorelinesWaves easily erode soft rocks found along deep water shorelines Sea Cliffs, Sea Arches, Sea Caves, and Sea Stacks are commonly found off the coast of Maine, California, Oregon, Hawaii, Canada and Australia.Sea Cliffs, Sea Arches, Sea Caves, and Sea Stacks are commonly found off the coast of Maine, California, Oregon, Hawaii, Canada and Australia.

Haystack Rock, OR

Longshore currents deposit large amounts of sediment over time.Longshore currents deposit large amounts of sediment over time. A SANDBAR is formed when ocean currents deposit sand near the shore. Sandbars may take many different forms…A SANDBAR is formed when ocean currents deposit sand near the shore. Sandbars may take many different forms… 1) BARRIER ISLANDS are sandbars parallel to shore (Fire Island, NY & the Outer Banks, NC)1) BARRIER ISLANDS are sandbars parallel to shore (Fire Island, NY & the Outer Banks, NC) –Barrier islands protect the shoreline from the full strength of waves –These shoreline features are NOT permanent! They are constantly being built up and destroyed by waves! –Most of the Eastern Coast of the USA has Barrier Islands and flat sandy beaches.

Barrier Islands… think Fire Island, Long Island and Outer Banks, No. Carolina

Bay: indentation of shoreline Think – San Francisco Bay…

Spit: sandbar that is connected to land on one side of a bay

Baymouth Bar – a sandbar that completely closes access to a bay, thus closing it off from the main body of water

– a shallow, protected area of water located landward from a sandbar. Lagoons – a shallow, protected area of water located landward from a sandbar.

Jetties and Groins