Coastal geomorphology: Beach and breakwater

Classical Mechanics Level 1

On a beach, there is a natural, straight shoreline with waves approaching diagonally towards the shore. The arrows indicate the direction of the waves.

A breakwater is placed perpendicular to the shoreline.

After some time, will the shoreline appear more like A or B?

Hint : Waves approaching the shore carry sediment with them.

A B

10 solutions

S I
Apr 11, 2018

This puzzle requires knowledge of what a breakwater is and experience with water and currents. Without such context, this question is impossible to answer logically.

also depends on the speed of the wave and size of the sediment

Aakash Ahamed - 3 years, 1 month ago

I am far from a mathematician however logic would find that a “breakwater” is just exactly what it says it is-it breaks the flow of the water. I also am not versed in waves and currents but having watched shorelines on occasion it was not difficult to figure out in which direction the water would flow.

Joyce Touche - 3 years, 1 month ago

Thanks for the tip!

Half-god Dragon - 3 years ago

Atharva Pathak
Apr 8, 2018

The waves are going to the left (the lines going to the right are the wave fronts). In slow water, sediment settles. The water gets slowed down on the right side of the breakwater, and thus the sediment settles there. This increase of sediment on the right side of the breakwater is shown in A.

There will be a rip current going up the beach the same direction as the waves. This current will be redirected out and partly around the wall, transporting sand around the wall. But also creating a nice sand bank on the same Sade as B, but a bit away from the wall, with possibility for a nice right hand wave :)

That’s what seems to happen in my experience.

Joel Mörtsell - 3 years, 2 months ago

Yes, that's also my thoughts. The water need to be still for the sediment to settle and that is behind the breakwater (shown in A) How can it be that the sediment settles in B where the waves hit the breakwater? If you look at this picture below, it clearly shows where the sand is accumulated, and it's not in front of it. https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&ved=&url=https%3A%2F%2Fblogs.cardiff.ac.uk%2Fshunqipan%2Fresearch%2Fleacoast%2F&psig=AOvVaw0IVntgM_w7aldULZU0yXgc&ust=1523651302598537

Daniel Grafström - 3 years, 2 months ago

I still don't get how

Amelia Hudson - 3 years, 2 months ago

The slower water would be on the left side of the breaker. Think of it as the wing on an airplane. The angle of attack from the water will create a negative pressure on the left and a higher pressure on the right. Also the wave has mass and velocity. The effected of the kinetic energy from this is going to be greater on the right side of the break water creating erosion.

Keith Smith - 3 years, 2 months ago

hold on, the opposite: lower speed means higher pressure; higher speed means lower pressure. So yes in regards to the pressure, (higher on the right), but the slower water is on the right as well. Consequently, the kinetic energy is lower on the right side, which makes it so that the water on that side won’t be able to put in as much work into pushing the sediment further into the shoreline. Meanwhile, the left side and the rest of the waves that have greater velocity than this small section of slowed-down water, will indeed be able to push all the way to the straight line.

Either that or the sediment just accumulates there and gives you that shape. I’m not a harbor engineer...

Erika Giancarlo-Gregg - 3 years, 2 months ago

I disagree. (1) Ocean wave act pretty much light waves, and the wave breaker acts pretty much like a mirror. (2) The wave intensity and water speed would not drop in the right side. you can expect high waves striking the corner and returning fast. (3)the left side, on the other hand, would gain only minor flaw because of this: http://wtamu.edu/~cbaird/sq/2014/02/07/can-light-bend-around-corners/ (4) The right answer is B!

evgeni machavariani - 3 years, 2 months ago

The water on the right side becomes turbulent because it is crashing up against the barrier and the shore. The turbulence will result in slow water, which will then deposit the sediment that it was carrying.

Atharva Pathak - 3 years, 2 months ago

That's not a breakwater, it's a jetty. Breakwaters run parallel to shore. Answer given is only correct in calm waters. In heavy waves, the right side water will have significant turbulence and the leeward side will not, allowing sediment to gather equally on both sides, or even faster on the leeward side (left in the picture), or possibly even taking sediment away from the righthand side. In short, there is insufficient information to answer this question, but across the spectrum of possible conditions, this answer is most likely to be wrong.

Christopher Cornette - 3 years, 2 months ago

Exactly. You said what I was going to say, only better

C Coburn - 3 years, 1 month ago

But what if line is very thin,like a pole?

Satyam singh - 3 years, 1 month ago

thanks for the explanation, it clarified it for me!

Frappa Canu - 3 years, 1 month ago

David Awakim
Apr 10, 2018

My thought process is that as the waves from the right side hit against the breakwater, the water splashes, causing the sediment particles and water to be separated. Some of this sediment lands alongside the edge of the shoreline, where it builds up overtime to develop into shape A, as the waves only hit the right side of the breakwater.

Imagine the edge of the cliff you will find that when waves push toward cliffs near cliffs near cliffs rise seemingly B is right.But actually can also be seen as waves pushing direction of water plane rise.

ZHAO DONG - 3 years, 1 month ago

Paul Duey
Apr 11, 2018

Technically that is a groin, not a breakwater. A breakwater is parallel to the shoreline. The groin traps sediment being deposited by the wave action moving from right to left, so it builds up on the right side pictured in A. The same wave action continues pushing the sediment on the left side of the groin, and since it's not being replaced, it eventually erodes just to the left of the groin.

Yes, it is a groin and not a breakwater.

Florin Zăinescu - 3 years, 2 months ago

You are right, although in this context the word is spelt "groyne", not groin (which is a part of the body) - a breakwater is a structure which lies wholly out at sea. The main part of your answer is also spot on.

Thomas Sutcliffe - 3 years, 1 month ago

Spriha Basir
Apr 9, 2018

As the arrow shows the right...

Dangerkittyii Gugenha
Apr 12, 2018

B can't happen. it breaks the laws of physics, from the fact that it changes direction in the left half before touching anything

Min Jiang
Apr 12, 2018

One way to solve it is that as time goes by, the sediment will settle on the right side of the breakwater.

Another idea is that waves should fill up the area blocked by the breakwater, i.e. it is wierd in picture B.

Donner Tang
Apr 10, 2018

Assumption :The waves drops sediments mostly when the wave is moving back.

Therefore, when the wave is moving diagonally to the right, it would leave pebbles to the left of the breakwater and overtime forming pattern A to happen.

Lamer Zhang
Apr 13, 2018

The one you don't believe is exactly right!!!!

Anas Aboukoura
Apr 11, 2018

Longshore drift.

Coastal geomorphology: Beach and breakwater

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