How come some really big waves are not so deadly and used

for surfing??? I mean I have seen pictures of guys surfing what appears to be at least a 50 ft wave (eg Mavericks). Are these not vicous killers because they are crashing on land used to these waves and there is usually a backdrop of cliffs (probably created by these waves over a long period of time) that shield the rest of the area? Eitehr way, how could such waves be surfed if it only takes 20 foot waves to kill and destroy so much in a single instant?

I initially wondered if the waves were crashing with a 500 MPH velocity, but that theory was debunked earlier today while listening to NPR - their geologist said that the waves go 500 MPH far out at sea, but slow down to a normal speed when they get close to shore.

It will cause massive flooding and then go back to sea... only to come back later.

so it is not unlike the storm surge of a hurricane....except that this can be so much faster and without warning if no system is in place.

certainly not like you see in the movies... it is a tide that goes up really fast, and then goes away just as fast, only to repeat the process until the energy is liberated.

The only warning is that the sea will retreat a few minutes before the water starts rising. In Asia's tsunami, there were places where the water went back up to 500 meters or more.

http://www.geophys.washington.edu/tsunami/general/physics/characteristics.html

"--------------------------------------------------------------------------------
Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterized as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour.
As a result of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets very small. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s/s) and the water depth - let's see what this implies: In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses."

The causes are different, but both the storm surge and the tsunami cause massive flooding inland, which is what causes the death and destruction.

because they see the surf line, and think:

"Gee, if I build my home out there on the reef where the big waves are crashing, the house might not be able to withstand the constant crashing of all that water on the roof"

wind driven waves are surface waves and simply crash somewhere, often on a beach. They pop up and crash down and are no bigger than they look.

Tsunamis are massive amounts of water and start out at the bottom of the sea, as much as a few thousand feet down. All that water moves a huge wave, but an "invisible" one, since it's maybe a hundred miles long and very deep. When it reaches shallower areas, like a continental shelf, the deep water is pushed up, the huge length of the wave is compressed, and a wall of water and something like a tidal/storm surge hits the land.

The water wall, if there is one, is the least of it, since it breaks early like any wave but the surging water, like the surges in a hurricane, cause the real damage. The outflow might be the worst of it, sucking everything back out to sea.

Fill a large basin with water.

Run your hand across the top of the water, you make a big splash but not much water gets out.
That's wind-driven waves.

Run your hand deeper, not much splash, but sloshing a lot more water out of the basin.
That's a tsunami.