View Full Version : Why do ocean water always seem to flow toward the shore?


jaboo
12-03-10, 05:51 AM
Why do ocean waves always seem to flow toward the shore? Why not in other directions?

Ophiolite
12-03-10, 06:31 AM
Waves are initiated by interaction with the wind. It requires 'reach' for the initially small waves to build up to a detectable level. Therefore if the wind is blowing from the shore to the sea the wave height will be too small to be visually detectable.

Blindman
12-03-10, 06:40 AM
I have been a surfer for many years. Waves come from all directions. They also reflect from the beach and head out to sea. With the right conditions you can ride them out.

When the tide is going out the water flows from the coast. Rips are very strong outgoing flows of water. Off shore winds (wind that come from the land) create waves and currents that travel away from the coast.

spidergoat
12-03-10, 10:13 AM
Blindman is right, but also, if the waves only go in one direction, away from the shore, you wouldn't even see them from the shore.

BenTheMan
12-03-10, 10:24 AM
Waves are the result of currents and wind working over large distances. You can ask, when you see a wave breaking on the shore, where did this wave COME from? Well it started as a ripple somewhere far away, then picked up steam. Conversely, if you're standing at the edge of the water looking out, and the wind is at your back, you probably see a bunch of little ripples heading out to sea....the waves don't just form, you need consistent wind and current, over a long path, to make a wave. These ripples are (if you like) the beginnings of waves.

After having written this, I thought "Jesus, I just wrote a kids book...`The Little Wave that Could' ".

CptBork
12-03-10, 10:50 AM
Waves are the result of currents and wind working over large distances. You can ask, when you see a wave breaking on the shore, where did this wave COME from? Well it started as a ripple somewhere far away, then picked up steam. Conversely, if you're standing at the edge of the water looking out, and the wind is at your back, you probably see a bunch of little ripples heading out to sea....the waves don't just form, you need consistent wind and current, over a long path, to make a wave. These ripples are (if you like) the beginnings of waves.

After having written this, I thought "Jesus, I just wrote a kids book...`The Little Wave that Could' ".

So succinct and nicely-put, I love it! I don't think I would have come up with an explanation like this on my own, I would have probably puzzled over it for a while, given up and looked up the answer from someone like yourself.

RJBeery
12-03-10, 02:48 PM
OMG CptBork to you need a baby wipe for your nose? :)

My first thought on the original question was that the decreasing depth of the water is what makes the wave "expose itself". For example, why don't we see large waves that run parallel to the shore? You could in theory have steady winds that run in that direction for arbitrarily long amounts of time.

Guest254
12-03-10, 03:24 PM
A quick google will give you the obvious answer (i.e. it is due to the sloping of the shore line), but there's a lot of fluff and unconvincing arm waving.

There's a reasonably nice application of the mathematics of linear water waves + ray theory that gives you the result with a certain degree of rigour.

Emil
12-03-10, 03:38 PM
It is an interesting question.I saw the waves coming only to shore.
I would reach the same conclusion as RJBeery, it is related to decreasing depth of the water toward shore.
But I've never been on a small island.I do not think there is valid ie all the waves come to shore.

Read-Only
12-03-10, 03:38 PM
OMG CptBork to you need a baby wipe for your nose? :)

My first thought on the original question was that the decreasing depth of the water is what makes the wave "expose itself". For example, why don't we see large waves that run parallel to the shore? You could in theory have steady winds that run in that direction for arbitrarily long amounts of time.

I've seen plenty of large waves moving parallel to the shore. It all depends on the direction on the wind.

Normally, under usual conditions, the wind blows from the sea onto the land. That's just because the sun warms the land quicker than it does the water, so the warmer air over the land rises and the 'vacancy' is filled by the cooler air arriving from over the sea. But there are times when the wind is parallel to a shoreline - and that's when you'll see those parallel waves being driven by it.

And yes, you're correct, of course, about about the sloping of the beach upwards from the water - that DOES drive the arriving waves into a higher profile.

Emil
12-03-10, 05:34 PM
And at night? There is no sun.

Read-Only
12-03-10, 05:52 PM
And at night? There is no sun.



Not immediately at sunset (the land is still warmer than the water) but much later on the land chills quicker than the water and the wind blows out to sea - and that's the direction the waves take also. Just the reverse of the daytime.

Emil
12-03-10, 06:39 PM
Not immediately at sunset (the land is still warmer than the water) but much later on the land chills quicker than the water and the wind blows out to sea -


Yes, it is called sea breeze. http://upload.wikimedia.org/wikipedia/commons/thumb/9/96/Diagrama_de_formacion_de_la_brisa-breeze.png/320px-Diagrama_de_formacion_de_la_brisa-breeze.png


- and that's the direction the waves take also. Just the reverse of the daytime.

I have never seen waves that start from the shore and go to sea.

Read-Only
12-03-10, 06:59 PM
Yes, it is called sea breeze. http://upload.wikimedia.org/wikipedia/commons/thumb/9/96/Diagrama_de_formacion_de_la_brisa-breeze.png/320px-Diagrama_de_formacion_de_la_brisa-breeze.png



I have never seen waves that start from the shore and go to sea.


That's because they start out VERY small.

Emil
12-03-10, 07:16 PM
That's because they start out VERY small.

I took a bath in the sea at midnight.
Waves were really small but still came to the beach.

AlexG
12-03-10, 09:22 PM
Would that be because you were on the shore? And so would only see any waves coming towards you?

Emil
12-03-10, 09:40 PM
Would that be because you were on the shore? And so would only see any waves coming towards you?
Yes, I saw and I heard the waves come and break to the beach.

Emil
12-03-10, 09:50 PM
I think the answer is somewhere in the place of production of waves and wave propagation mode
and in decreasing depth of the water toward shore.
Waves propagation mode is circular from production center of the waves.
How production center is somewhere in the ocean their propagation is normal to be towards the shore.

Blindman
12-03-10, 11:45 PM
As a surfer we define two types of waves. The swell, and waves.

Swell are long period wave created by wind traveling over a long stretch of the ocean.
Swells will bend toward the coast as they hit shallower water. Surfers are only interested in the swell.

Short period waves are created by local winds. Swell and waves can overlap producing a very bumpy ride.

It is also possible to have two or more swells come in from different directions producing very interesting and in my view the most fun swells to ride.

My favorite local surf beach (Trig Point Western Australia) in the right conditions you will get a swell reflecting from the reef, others coming of the sand bank, main swell coming in from the west, a large wind wave coming in form the south west and a standing wave from the very fast flowing rip along the channel between the reef and sand bank.

Under these conditions a wave can just rise up from nowhere. A 8-12 foot wave can rise from nothing and then travel in a random direction. This is caused by the interference of the many wave sources. You can surf down the face of one of these and then suddenly have another wave rise up in front of you traveling in the opposite direction and send you flying through the air. Great fun.. But only happens once or twice a year and usually in winter with a good low pressure system moving over.

RJBeery
12-04-10, 08:48 AM
As a surfer we define two types of waves. The swell, and waves.

Swell are long period wave created by wind traveling over a long stretch of the ocean.
Swells will bend toward the coast as they hit shallower water. Surfers are only interested in the swell.

Short period waves are created by local winds. Swell and waves can overlap producing a very bumpy ride.
There we go, I figured a surfer would surely have an answer. Swells bending toward the coast makes sense and seems to answer some questions above.

Walter L. Wagner
12-04-10, 12:25 PM
The best swells in Hawaii arise in the South Pacific during the Summer in the northern hemisphere, and travel great distances. They smooth out considerably because of their great distance, and you get excellent, very smooth, long rides such as at Panic Point on Oahu. In the Winter in Hawaii there are few swells from the South Pacific (summertime then), and the large waves come from the North Pacific hitting the North Shore (such as Pipeline, Sunset, Waimea Bay, etc.) They tend to be somewhat choppier because the swells have not travelled as far, but they are much larger than the summertime surf on the south shore beaches.

John99
12-04-10, 12:40 PM
Yes, it is called sea breeze. http://upload.wikimedia.org/wikipedia/commons/thumb/9/96/Diagrama_de_formacion_de_la_brisa-breeze.png/320px-Diagrama_de_formacion_de_la_brisa-breeze.png



I have never seen waves that start from the shore and go to sea.


Never seen a large wave in the middle of the ocean? You have to realize there is land on both sides too so it doesnt matter which way it goes.

Ophiolite
12-06-10, 06:59 AM
So succinct and nicely-put, I love it! I don't think I would have come up with an explanation like this on my own, I would have probably puzzled over it for a while, given up and looked up the answer from someone like yourself.

Sometimes I just wonder why I bother. My explanation in post 2 did not meet those criteria? :rolleyes:

Blindman
12-07-10, 12:22 AM
Never seen a large wave in the middle of the ocean? You have to realize there is land on both sides too so it doesnt matter which way it goes.
I have been at sea with a swell higher then the mast on a 34foot sail boat.

Swells are very powerful and not influenced much by offshore winds.