Waves, WB #289

[kbowen]

In the "Getting Started in Boats" section I was disappointed not to see a discussion of the difference between fresh and salt-water waves. There are accounts of ocean crews being surprised by the wave heights and steepness they experience on the great lakes and there are ocean-going ships lying on the bottom of the lakes. Can someone direct me to any research on possible differences between fresh and salt-water wave dynamics? I was also sorry that the article didn't close the loop between so called "rogue waves" and wave interference.

(my primary source is "Great Lakes Shipwrecks and Survivals" by William Ratigan)

Ken

[Peerie Maa]

In the "Getting Started in Boats" section I was disappointed not to see a discussion of the difference between fresh and salt-water waves. There are accounts of ocean crews being surprised by the wave heights and steepness they experience on the great lakes and there are ocean-going ships lying on the bottom of the lakes. Can someone direct me to any research on possible differences between fresh and salt-water wave dynamics? I was also sorry that the article didn't close the loop between so called "rogue waves" and wave interference.

(my primary source is "Great Lakes Shipwrecks and Survivals" by William Ratigan)

Ken

Water density is only of relevance to capillary wavelets. Full size wave dynamics are independent on water density, so there is no difference between a wave on a large lake or the ocean.
The topography of the bottom is important, and as you surmise, diffraction/interference.
Some images from Google Earth of wave diffraction

Now where there are a couple of obstacles that my cause several diffraction patterns to overlap, they can augment the wave trains causing localized big seas. It is thought that this may have contributed to the ferry Estonia disaster.

Rouge waves are not "so called" they are a thing.

Rogue Waves

Rogue waves are large, unpredictable, and dangerous. Rogue waves (also called 'extreme storm waves') are those waves which are greater than twice the size of surrounding waves. They often come unexpectedly from directions other than prevailing wind and waves. Many reports of extreme storm waves describe them sudden "walls of water." They are often steep-sided and associated with unusually deep troughs. Some rogue waves are a result of constructive interference of swells traveling at different speeds and directions. As these swells pass through one another, their crests, troughs, and wavelengths sometimes coincide and reinforce each other. This process produces large, towering waves that quickly form and disappear. If the swells are traveling roughly in the same direction, these massive waves may last for several minutes before subsiding. Rogue waves can also form when storm swells move against a strong current, resulting in a shortening of the wavelength and increasing it’s amplitude. Large rouge wave of this kind are frequently experienced in the Gulf Stream and Agulhas currents

https://gotbooks.miracosta.edu/oceans/chapter10.html Rogue waves require high energy wave trains coming together, which can only happen with adequate fetch to build up the wave energy.

[carlg]

You might want to find a copy of "Water Waves" by N. F. Barber. I picked up my copy in London years ago; the copyright is 1969.

[kbowen]

Water density is only of relevance to capillary wavelets. Full size wave dynamics are independent on water density, so there is no difference between a wave on a large lake or the ocean.
.

This blows my mind. Density irrelevant? I know that water waves are unique, but in every other wave I can think of there is a factor akin to density that matters in a big way: sound velocity changes with air pressure and humidity, and is totally different underwater and in a steel rail. The big "c" speed of light, the ultimate constant, only exists in a vacuum, and is slower thru other mediums, electrical signals in a wire are dependent on the impedance of the wire, since the Tacoma Narrows bridge collapsed, they have figured out how to change the resonant frequency of bridges, etc. And whatever other physics may prevail, there remain the historical accounts of proven ocean-going ships failing on the great lakes, with survivors saying they had never seen such conditions. So, what gives??

Ken

[SchoonerRat]

My understanding and experience is that there is great a difference between lake waves and ocean waves. This is not caused by water density, but by fetch. Long rolling ocean waves are generated by wind that's blowing over thousands of miles of water. The short fetch of a lake doesn't give the water enough time to form a rolling wave. The result is steep chop. Add to this the "waves" reflecting off the lake shore and confusing the sea state.

Many moons ago, I spent months getting used to being battered and bruised by Siroccos and Mistrals in the Med. I made my escape through the Straits of Gibraltar in a 20 - 25 knot breeze. The difference was immediate, and extreme. I was no longer being battered by 4 - 6 ft. high, closely spaced chop. I was now surrounded by long rolling ocean waves and wind blown white horses; friends that I've grown up with. I took a couple of extra watches in celebration of looking forward to a couple weeks of uninterrupted "real sailing".

[Ben Fuller]

Check the classic book Waves and Beaches, Willard Bascom, now in a new edition. Mine isn't handy.

[pez_leon]

Hey wait- does this mean that #289 should have been delivered to me by now? Maybe they mixed up and sent me another issue of Professional Dentist again.

I also came here to suggest Waves and Beaches by Bascom. For me it is one of those rare books that teaches its content so well that it transcends the subject matter and becomes a book about learning and teaching. It's also collection of some great stories and hair-raising plates:

[kbowen]

My understanding and experience is that there is great a difference between lake waves and ocean waves. This is not caused by water density, but by fetch. Long rolling ocean waves are generated by wind that's blowing over thousands of miles of water. The short fetch of a lake doesn't give the water enough time to form a rolling wave. The result is steep chop. Add to this the "waves" reflecting off the lake shore and confusing the sea state.
.

I am just checking your perception of fetch, because coastal sailors often mis-judge those little blue blips in the middle of a map of the continental US: Depending on wind direction the fetch in Lake Michigan is between 60 and 300 miles, Superior is 160 miles N/S and 350 E/W. It is not thousands of miles, but it is not a pond. Yes, I should look up some of the technical sources you folks have cited, but I still come back to the personal accounts of ocean going crews that got their feathers clipped on the lakes.

Ken

[Pelirrojo]

I have spent a fair amount of time in the Great Lakes and I would say that in my experience their size is interesting as it is both large enough to generate serious wave conditions but also too small to reliably be able to route fully around weather conditions. The combination of those two factors along with their propensity of built storms fast can lead to folks bei. I’m storms they likely would have been able to avoid on the ocean. 160 miles is not a ton of sea room in a sustained 60 knot blow. Another major factor that is worth considering is the how much more easily fresh water ices up over salt, the propensity of ships building ice is a major factor in winter storms on the lakes. In full disclosure non of my lakes time was on a sea going vessel, and I have never done work on them in winter.
Nicholas

[timo4352]

About fetch:
https://www.naturalnavigator.com/new...what-is-fetch/

[SchoonerRat]

I am just checking your perception of fetch, because coastal sailors often mis-judge those little blue blips in the middle of a map of the continental US: Depending on wind direction the fetch in Lake Michigan is between 60 and 300 miles, Superior is 160 miles N/S and 350 E/W. It is not thousands of miles, but it is not a pond. Yes, I should look up some of the technical sources you folks have cited, but I still come back to the personal accounts of ocean going crews that got their feathers clipped on the lakes.

Ken

Yes, ocean going sailors often get their butts kicked on the Great Lakes, this one included. I did a Chicago Mac Race once when I was still young and sorta able. This trip ranks in the top 3 worst experiences of my 40 years at sea.

I WILL NOT DO ANOTHER!!!

The lakes have enough fetch to generate sizable wave forms, but as you say, 300 miles is not 3000 miles. Waves that travel thousands of miles unimpeded, form into long regular rolling waves... delightful sailing. Waves on the lakes travel till they hit shore where they are reflected back from whence they came. Reflected waves will mix and match, sometimes diminishing and sometimes enhancing height. The result is steep, often high, confused and almost sailable chop.

[robm]

Frank Bethwaite has an excellent section on waves in Lake Ontario, which he studied during the 1976 Olympics. It is in his book "High Performance Sailing". His take on it is the waves are shorter and steeper because of thermoclines. The waves are formed by the wind in the warm upper layer of water, while, the denser, cooler water below acts as if it were the actual bottom.

I have noticed similar effects in Douglas Channel, where the Kitimat River flows in, only in this case, it is a halocline. The waves for the first couple of miles out from the river mouth are really short, steep, and nasty. It is my belief the salt water underlies the fresh, so the waves hit the layer of fresh water on top and break almost as if they had hit a shallow area. In typical summer weather, there is a strong onshore breeze, and many miles of fetch, so the conditions can get a lot uglier than one would expect.

[Autonomous]

The Columbia River's water is thoroughly mixed up by dams, yet it produces epic waves especially in the Gorge.

[SchoonerRat]

The Gorge gets truly epic. But I wouldn't refer to these as waves. I'd call this chop. Epic chop! Sharp peaks and valleys. No rhythm. No form. This is close to what you get on the open ocean generated by local wind.

Waves are well formed. They follow a regular pattern unless disturbed by enough local wind chop.

I know I'm being really nit-picky, but there is a difference between what the wind does to the surface conditions of the open ocean and what the wind does to the surface conditions of an enclosed body of water.

[AJZimm]

Worked Example of Wave Height Differences Fresh vs Seawater

This question had me intrigued. I thought it should be amenable to calculation. I thought that, for the same power input to the water by the wind, it should be possible to calculate the difference in resulting wave height, based on the density difference between sea water and fresh water. I dimly recalled there being a formula for figuring wave power that includes the density. I found it on this web site Wave Energy Calculator:

P = ρ*g2*T*H2/(64*π), [W/m]
H - the vertical distance between crest and trough of the wave, meters;
T - the period of wave, seconds;
g - the acceleration of gravity, m/s2;
ρ - the mass density of sea water, kg/m3;
π - 3.14159

And it has a handy calculator so you don’t have to build a spreadsheet.

In order to use the calculator, you need to know the wave period and the wave height, in addition to the mass density of the medium.
Mass density of seawater, kg/m3: 1020 – 1029
Mass density of fresh water, kg/m3: 997

I went to another online calculator and looked to see what wave height and period would result from arbitrarily chosen values of a 40 knot wind blowing for 8 hours over a 300 kilometer fetch (numbers that wouldn't be unreasonable on a large lake).
Wave Height and Period Calculators:
This was the result (seawater is assumed by calculator):
Significant Wave Height = 4.93 m
Peak Wave Period = 9.06 s
Wave growth limited by: duration

I then plugged those numbers into the wave power calculator, which yielded = 108.0324 W/m

Now, changing density to fresh water, holding wave period constant at 9.06 s and iterating wave height until the power is the same, at 108.0324 W/m, yields a height of 4.999 m.
The difference is 4.999 – 4.93 = 0.069 m or 2.7 inches higher for fresh water waves

This is an oversimplification of course, as the wave length and period might increase with fresh water, but fiddling with those a little yields miniscule differences from the above result.

The conclusion I come to is that while there might be a very small difference in fresh water and seawater wave heights due to density difference between salt and fresh water, most sailors are hardly likely to notice the differences in conditions that would cause them to be concerned about wave heights.

The possible explanation put forward by Robm, of thermoclines acting like a shallow bottom seems more likely to me to account for observed higher and steeper waves in fresh water bodies.

If I have got any of my assumptions wrong, I would be happy to be corrected, and anyone that has a better physics explanation would be welcome, too.

[kbowen]

Wow, The above is a lot to digest. I apologize for being absent for a while: It seemed that the thread had died and coincidentally the weather was lovely so I went sailing instead of computing.. The source cited by Robm is especially interesting to me because it takes it as a "given" that water density changes wave behavior. Pieces of college physics are surfacing concerning the impedance of a medium which transmits waves. In classic wave theory when a wave hits an impedance change (perhaps a thermocline in this case) some wave energy is transmitted but refracted at an angle, and some is reflected. I wonder if there is some explanation hiding there for the observed differenced in fresh vs salt water??

Ken