Wave Action

Waves are driven by the wind. It makes the water at or near the surface move in a circular motion. That is, if you watch a floating object move as waves pass ... it would move in tiny circles over and over again. This "circular motion" extends below the surface to a level known as the wavebase. The wavebase is about half the wavelength of the surface waves (distance between wave crests). Click here to see an animation.

This wave action only becomes interesting when the wave approaches the shore. Then the turbulent "circular motion" interacts with the seafloor. This is the area where sand is easily disturbed by the wave and its motion "grinds" everything in its path. It is also where the wave itself gets disturbed and "breaks". Geologists call this area the surf zone.

Image from the movie Cast Away starring Tom Hanks (seeking permission from 20th Century Fox)

As you can see from the image from the movie Cast Away, there are only breaking waves close to the shore. This made it nearly impossible for the character Chuck Noland (played by Tom Hanks) to escape.

The surf zone is where weathering and erosion take place (technically ... the "swash zone" is where water washes up and down the beach ... but don't worry about that) and can alter the landscape in many ways. The physical location of the surf zone may change depending on:

Tide level

There are two high tides and two low tides every day (actually 24 h 52 minutes)
The highest difference between high and low tide occur in the Bay of Fundy, Nova Scotia, Canada where the difference between low tide and high tide can reach about 50 feet.

The Bay of Fundy source unkown (amimation)
Tides are caused by the gravitational pull of the moon on the earth. It is actually a bit more complicated than that, but if you want the whole story, take introductory astronomy.

Global Temperatures

As more ice melts, sea levels rise ... moving the surf zone in.

Uplifts

Plate action may lift the land creating terraces. Each terrace was the former location of a wave-cut beach. If you ever visit the coast of California, you will see many examples of this.

Storms / Hurricanes

Low pressure systems over water will raise the seal level ... creating a "storm surge" (and the strong winds accelerate weathering and erosion)

How do waves effect the coastline?

Beaches need a steady supply of sand to remain stable. The three main sources of this sand are:

Weathering of local rocks

Wave-cut sea arches (who is that dude with the babe?)
Flowerpot Rocks (New Brunswick Canada)

Fossil Cliffs (near Bay of Fundy)
Joggins (Nova Scotia Canada)

Input of sand from rivers

Most healthy beaches are located near the mouth of a river. The river is constantly supplying the beach with a new source of sand. 80-90% of the sand on most beaches are supplied by rivers.
Damming a river will rob the beach of this sand (and cause problems in the dam reservoir itself ... as the sand gets deposited there)

Input from longshore current / beach drift

Waves usually enter the surf zone at an angle. This brings sand "in" at the same angle. However, the sand backwashes "straight back" due to gravity. This continues, slowly moving the sand (and water) down the coastline.
Sand may be transported up to 800 meters in one day!
Man-made obstacles (projections called jetties and groins) tend to restrict this motion of sand ... robbing areas down-current of this resource.

(animation)
Beach Drift

Other ways waves effect the landscape

The action of waves can literally carve a steep cliff into a nearly horizontal platform and create beautiful sea arches (see above).

A phenomenon known a wave refraction results when waves slow down as they approach the surf zone. This is similar to the bending of light in a prism. This means that the wave front changes its direction (coming in more perpendicular) as it hits the shore. Click here to see a diagram of refraction or here.

Sorry .. I can't locate the source for credit

This produces an interesting result. Let's imagine a shoreline which has an irregular shape ... with protruding headlands and inset bays. The animation below shows how wave refraction will attach the headlands (which allows sediments to gradually fill the bays).

How waves attach an irregular coastline (animation)

The net result is that the wave action erodes the headlands and tends to make irregular coastlines straighter.

Links:

http://pubs.usgs.gov/circular/c1075/change.html