dorking out super hard

[Editor]

Ever wonder what happens to waves in a strong opposing current? It might not be what you think! Our resident ocean physicist and self-proclaimed water loving nerd Patrick Rynne shares some video footage of a unique phenomena that he stumbled upon while cleaning the office.

Think his explanation is correct? Have an alternate theory?

 

[deckersr]

I would be curious to see a view further upwind. It appears that the waves get pushed to the left of the frame until they are no longer in view of the camera.  There is energy in those waves and it had to go somewhere.

 

[5X-MOANA]

The blocking point moves downstream with increasing current, to where the speed of the waves and current match; not really a surprise...
But when observing currents, keep in mind that they are never homogenious and oftentimes heavily 3D. Lots of up/down may be happening and water densities can vary greatly, especially if sea- and freshwater mix. So it's ok to be confused. ;-)

If basic observations of a curious mind are called 'dorking out super hard' what do you call real science?

 

[StraightLeech]

Explanation makes sense to me.  A couple of thoughts.

1. Since different frequencies/wavelengths travel at different speeds, the current will act as a low-pass filter sending the shorter wavelengths/higher frequencies upstream first.  This will possible be cluttered by some local creation of ripples.

2. As deckersr states.  It would be interesting to find the points where wavespeed equates to current-speed.  If we assume that current decreases linearly downstream one could speculate that you would find a distribution of standing waves from long to short wavelengths as you travel down the stream.

3.  If there is plenty of water area upwind from there one can assume that new energy is added all the time.  My guess would be that you will see breaking waves as the energy piles up.  

 

[StraightLeech]

Typo.  1. should be  have been ".......shorter wavelengths/higher frequencies downstream........."

 

[trispirit]

The blocking point moves downstream with increasing current, to where the speed of the waves and current match; not really a surprise...
But when observing currents, keep in mind that they are never homogenious and oftentimes heavily 3D. Lots of up/down may be happening and water densities can vary greatly, especially if sea- and freshwater mix. So it's ok to be confused. ;-)

If basic observations of a curious mind are called 'dorking out super hard' what do you call real science?

Spot on! Seen this happen in the whitsunday Islands  Solway Passage.  Breakers get pushed downstream to where the current equals them where there's normally a large amount of not very nice standing walls of water.  The waves just fall over themselves and depending upon the breeze strength it can get pretty nasty.  

 

[sweetaction1]

We experience this on a smaller scale in the Niagara River. The river moves from south to north. On north wind days winds in the range of 12 -15 knots have no effect on the river for forming waves. The current where we windsurf is approximately  7 knots.  This makes for some awesome flat water blasting.  When the winds increase we get sections of standing waves.

 

[bgytr]

ya i agree there might be quite a bit of turbulent flow going on as the current gets really fast.  Pulled up navionics chart there, and the bottom contours are pretty rugged.  Could be a lot of swirling when the current is moving fast, essentially destroying the surface wind waves. the current rips there, posted as pushing 14 knots on the chart, likely a lot of turbulent flow.

 

[Glenn McCarthy]

Well, if you like this standing wave, try the Pororoca -

The Pororoca  is a tidal bore, with waves up to 4 metres high that travel as much as 800 km inland upstream on the Amazon River and adjacent rivers. Its name comes from the indigenous Tupi language, where it could translate into "great roar".  It occurs at the mouth of the river where its waters meet the Atlantic Ocean.

area, meaning greater tides

During new and full moons, when the ocean tide is highest, water flows in from the Atlantic, rather than the other way around. The Amazon’s flow reverses, the distance of which depends largely on the rainwater-generated outflow of the Amazon, and a water bulge speeds upstream often with great force, forming a tidal bore with an audible noise. The tidal phenomenon is best observed on biannual equinoxes in September and March during a spring tide). On an equinoctial spring tide, the Moon and Sun fall into direct alignment with the Earth, and their gravitational pull is combined, bringing the Pororoca and others around the world to their peak.[2]


Surfing


The wave has become popular with surfers. Since 1999, an annual championship has been held in São Domingos do Capim (on the adjacent Guamá River). However, surfing the Pororoca is especially dangerous, as the water contains a significant amount of debris from the shores of the river (often entire trees), in addition to dangerous fauna. In 2003 the Brazilian Picuruta Salazar won the event with a record ride of 12.5 km lasting 37 minutes.

 

[casc27]

The blocking point moves downstream with increasing current, to where the speed of the waves and current match; not really a surprise...
But when observing currents, keep in mind that they are never homogenious and oftentimes heavily 3D. Lots of up/down may be happening and water densities can vary greatly, especially if sea- and freshwater mix. So it's ok to be confused. ;-)

If basic observations of a curious mind are called 'dorking out super hard' what do you call real science?

Professional dorking out super hard, of course.

 

[Mozzy Sails]

What you are seeing is the Doppler effect, but in water, rather than hearing it through sound. 

Except you don't get the equivalent of a sonic boom because the water waves collapse before they can stack up to create one massive wave. Plus the flow will be pretty messy so it's unlikely to stack in one precise location. The waves don't disappear, and there energy is still there, but is travelling up narrows with the current. Just like the noise from a supersonic jet is still there, but just gets further and further behind the jet. 

Once the waves which have been created in a different current are pushed back up the channel, what you're left with is waves which are typical for the water/ air speed strength (10 knots TWS + 15 knots current = 25knots) and a fetch few hundred meters = hence flat as a lake. 

I wonder when he's releasing his 'Love Fiona' video??

 

[Raked Aft\\]

Similar phenomenon is witnessed by large underwater shock waves,  as in sub surface explosions or earthquakes.

 Anecdotal witnesses attest that the shock waves will temporarily flatten the local surface wave state.

  i suspect that the current turbulence in the narrow, enhanced by the rugged bottom terrain, is creating a similar dynamic.  

 

[Maxx Baqustae]

I'm not sure of the full physics of all of this but we deal with this all the time. In our quite prevalent tidal passages, depending if it's ebb and flood, I feel that there is a certain water level change causing huge current and standing waves. Where it's pretty much calm either side. Mariners with any experience know that you have to be prepared to enter a tidal gate/pass. No more pronounced is the Skookumchuck in the Sunshine Coast of BC. Bin der;dun dat. It's quite an amazing phenomenon:

 

[BboySlug]

I know exactly what's happened here. StraightLeech, you're on the right track, so congrats!

Short version: The opposing current surpassed the wave train's group velocity, and thus the wind generated waves cannot travel faster than the current pushing them back, so they poop out and disappear. 

Long version: Thanks for posting this OP, I love Patrick's videos. I think he may have forgotten his college lecture about a wave's group velocity versus the phase velocity. Or, he may have forgotten his college lab experiments with wave flumes. 

Technical explanation: You may recall a few things: 

1) Deep vs. intermediate vs. shallow water wave condition: For deep, h/L>1/2, for shallow h/L<1/20, intermediate is between these limits. h being the water depth, L being the wavelength.

2) The wave's group and phase velocities (aka Celerity) are different. For deep water: Cp=L/T0, Cg=L0/(2T0), intermediate has longer equations which I don't want to type out, and shallow water is Cg=Cp=sqrt(g*h). Where Cg is the group velocity, Cp is the phase velocity, L is the wavelength, T is the wave period, g is the gravitational constant, and h is the water depth. The subscript 0 is that respective variable for deep water, as these values can change depending on your water condition. 

As one can see, the wave's group velocity is half of the wave's phase velocity in the deep water condition. For intermediate water, the phase velocity is still faster than the group velocity. So, these waves are travelling in either deep or intermediate water depth, and can only travel as fast as the group velocity. Eventually, the opposing current got so fast that it was faster than the group velocity, and the waves  disappeared.

Everyday man's/real life explanation: In university we had a wave flume, but for my readers, let's imagine a swimming pool. If you have your kid cannon ball into the pool's deep end (or, start generating waves with the wave maker in the wave flume), watch the first waves as they travel away from their source. The leading wave crest/trough will disappear, and the second one (if the depth stays the same) will follow suit shortly after. 

This is because the wave train all together can only travel as fast as the wave group. So, the first wave crest/trough will peter out to zero as they overtakes the group velocity. Then, the second wave crest/trough will also do so after it passes. 

This, however, is not true for shallow water wave condition, where the group velocity is equal to the phase velocity.

Now, with the wave disappearing concept in our minds, lets go back to the video.

Given the general topography, and the not fully developed sea state, these waves (especially in the middle of the channel) are likely to be propagating in deep water. Therefore their group velocity (wave train velocity) is higher than their phase velocity. Without the current, if able to propagate infinitely the leading wave phase will disappear as it overtakes the group velocity. 

With the opposing current, it eventually got so fast, that it surpassed the wave train's group velocity. So, in the video with the waves propagating from left to right, the wave phases disappear from right to left with the current as the current is so fast it prevents them from continuing to make "forward" progress. So, they wave group poops out, and gets erased from right to left. 

I understand my "everyday man's" explanation was still pretty technical, but I hope you get my gist. PM me if you would like to ask a question.

Source: My university studies in Ocean Engineering, as well as growing up doing every water sport I can think of minus synchronized swimming and underwater basket weaving/hockey. 

Do I qualify as a mega nerd yet?

 

[Somebody Else]

Great video, Scot! Thanks for posting!

 

[Left Shift]

Have sailed and motored through Seymour Narrows multiple times.  When the waves back up like that they get to the end of the narrows and it widens out into Discovery Passage which starts at Deepwater Bay.   In a northerly against an ebb, you get a nasty, nasty chop all the way up and into Johnstone Strait.  You are still in 3-4 knots of current, but your tacking angles are ridiculously good. 

When still in Seymour, if you get into the current waves, they are square, backless, ugly and up to about 2 meters.  Not fun, but short lived.  

If you want truly unpleasant current waves that go on for miles, head on up to the Nahwitti Bar at the top of the Island on a bad day.  The 300 meter deep Goletas Channel tries to ebb - at 3 knots or so- out into the open Pacific swells by going up and over a 10 meter deep underwater cliff face.  Can be just frickin' scary.