When does depth affect ocean swells?

[newt]

140 feet? Yeah, that makes sense to me.

[mrohr]

Originally Posted by zeehag

less than 100 ft has chop from rebound coastal waters---i try to sail out about 40- 60 miles off coast...i like 100 mi best, but isnt always practical.

It ain't necessarily so. Inshore waters indeed will have reflected waves resulting in confused sea conditions close in ,if the existing seas are breaking against a steep-to headland,but waves breaking on a sandy beach, or any gradually shelving bottom,mangroves,etc. do not reflect back out into the existing waves to cause "chop".In these cases the wave energy is dissipated on the beach.
Of course there are plenty of other reason to avoid shallow waters if possible.

[DumnMad]

Ocean swell waves with longer periods even bounce off beaches. Gotta be a pretty long flat beach to absorb all the energy.

[Cheechako]

Originally Posted by newt

140 feet? Yeah, that makes sense to me.

Yeah, wish I had a chart to see what that looks like distance wise. He was an older guy/couple who made the trip almost yearly in a Cal 2-46.

[barnakiel]

Well, one can feel the swell changes as soon as it runs over the shelf!

So to say, it affects early, but it takes pretty shallow water for the swell to break. Many sources quote this happens @ water's depth 1.3 of wave's height. However - it breaks earlier on ebb later on flood, etc. etc..

! You do not need a breaking wave to toss you about - enough if the waves are steep enough and this happens much earlier than at the a/m 1.3 ratio.

Just keep off shallows when in doubt.

b.

[newt]

Originally Posted by Cheechako

Yeah, wish I had a chart to see what that looks like distance wise. He was an older guy/couple who made the trip almost yearly in a Cal 2-46.

I charted a rough course keeping it 100 plus feet. Comes out about 120 kn miles. I will tell you how it goes...

[Cheechako]

wow, further out than I would have thought! sure you werent looking at 100 fathoms?!

[Jim Cate]

G'Day All,

I'm getting the feeling that folks are worrying too much about this effect. It is common practice here on the east coast of Australia when heading north (against the prevailing southerly flow of the East Australia Current) to sail with "one foot on the beach". For us, this has meant in depths of 40-60 feet, and something like 1/4 to 1/2 mile off the beach. The coast is usually subject to long wave length swells from the southern ocean, yet we don't experience any severe sea states as a result.

There are a couple of places where there are long, steep-to cliffs along the shore, notably near Jervis Bay and the Sydney heads. In those areas there are indeed reflected wave patterns that cause chaotic chop and an uncomfortable ride, and this effect extends out several miles... but not tens of miles. The majority of the coast is comprised of the usual mixture of beaches and broken rocky bits, and we do not notice any peculiar wave action in these parts, even quite close in to shore.

Obviously, where there are rivers or estuaries with outflow patterns there are local issues, but they are well defined and easy to avoid. We don't have anything like the Columbia to deal with, though!

From memory, going north along the California coast (SD to SF primarily) we found much the same sort of conditions, but that was a long time ago and I could have distorted the facts in my mind.

So, I wonder if there are some particular conditions in the PNW that make for worse sea states, or if we're making too big a deal out of this? Any thoughts?

Cheers,

Jim

[newt]

Jim,
A couple of things make this different. I have sailed a few oceans, and this one has a large robust fetch across from Japan and The Gulf of Alaska. That makes the typical waves here over my head in the cockpit. That combined with a very irregular underwater shelf, and Massive rivers flowing out of the continent (and out behind Vancouver Island) and the frequent gales- none of which I noticed while in Queensland. Yeah it really is like sailing on another planet.
The shallow water further mixes things up, and you get what they call "the washing machine". I must say I thought I was pretty good coming up here from the Gulf stream and the Atlantic. I have taken a little humble pie....

[GordMay]

Originally Posted by barnakiel

Well, one can feel the swell changes as soon as it runs over the shelf!

So to say, it affects early, but it takes pretty shallow water for the swell to break. Many sources quote this happens @ water's depth 1.3 of wave's height. However - it breaks earlier on ebb later on flood, etc. etc..

Indeed.

As waves enter shallow water, they slow down, grow taller and change shape.
At a depth of half its wave length, the rounded waves start to rise and their crests become shorter while their troughs lengthen. Although their period (frequency) stays the same, the waves slow down and their overall wave length shortens.
The 'bumps' gradually steepen and finally break in the surf when depth becomes less than 1.3 times their height.
Note that waves change shape in depths depending on their wave length, but break in shallows relating to their height.

The following is excerpted from ➥ SURFOLOGY

Swell period and ocean depth:
The depth at which the waves begin to feel the ocean floor is one-half the wavelength between wave crests. Wavelength and swell period are directly relative, so we can use the swell period to calculate the exact depth at which the waves will begin to feel the ocean floor. The formula is simple: take the number of seconds between swells, square it, and then multiply by 2.56. The result will equal the depth the waves begin to feel the ocean floor.

A 20-second swell will begin to feel the ocean floor at 1,024 feet of water (20 x 20 = 400. And then 400 x 2.56 = 1,024 feet deep). In some areas along California, that's almost 10 miles offshore. An 18-second wave will feel the bottom at 829 feet deep; a 16-second wave at 656 feet; a 14-second wave at 502 feet; a 12-second wave at 367 feet; a 10-second wave at 256 feet; an eight-second wave at 164 feet; a six-second wave at 92 feet and so on.

As noted above, longer period swells are affected by the ocean floor much more than short-period swells. For that reason, we call long-period swells ground swells (generally 12 seconds or more). We call short-period swells wind swells (11 seconds or less) because they are always generated by local winds and usually can't travel more than a few hundred miles before they decay. Long-period ground swells (especially 16 seconds or greater) have the ability to wrap much more into a surf spot, sometimes 180 degrees, while short-period wind swells wrap very little because they can't feel the bottom until it's too late.

Shoaling:
When waves approach shallower water near shore, their lower reaches begin to drag across the ocean floor, and the friction slows them down. The wave energy below the surface of the ocean is pushed upward, causing the waves to increase in wave height. The longer the swell period, the more energy that is under the water. This means that long-period waves will grow much more than short-period waves. A 3-foot wave with a 10-second swell period may only grow to be a 4-foot breaking wave, while a 3-foot wave with a 20-second swell period can grow to be a 15-foot breaking wave (more than five times its deep-water height depending on the ocean floor bathymetry).

As the waves pass into shallower water, they become steeper and unstable as more and more energy is pushed upward, finally to a point where the waves break in water depth at about 1.3 times the wave height.

A 6-foot wave will break in about 8 feet of water. A 20-foot wave in about 26 feet of water. A wave traveling over a gradual sloping ocean floor will become a crumbly, slow breaking wave. While a wave traveling over a steep ocean floor, such as a reef, will result in a faster, hollower breaking wave. As the waves move into shallower water, the speed and the wavelength decrease (the waves get slower and move closer together), but the swell period remains the same.

Refraction:
Waves focus most of their energy toward shallower water. When a wave drags its bottom over an uneven ocean floor, the portion of the wave dragging over shallower water slows down while the portion wave passing over deeper water maintains its speed. The part of the wave over deeper water begins to wrap or bend in toward the shallower water -- much the same as how waves wrap and bend around a point like Rincon or Malibu. This process is called refraction.

Deep-water canyons can greatly increase the size of waves as the portion of the swell moving faster over deep water bends in and converges with the portion of the swell over shallower water. This multiplies the energy in that part of the wave, causing it to grow into a larger breaking wave as it nears shore. The effects of a deep-water canyon just offshore is often why we see huge waves along one stretch of beach, while maybe just a few hundred yards down the beach the waves are considerably smaller. This happens at spots such as Black's and El Porto in Southern California, and Maverick's in Northern California.

Remember, the longer the swell period, the more the waves will be affected by the ocean floor bathymetry, the more they will wrap into a spot and the more the waves will grow out of deep water.

[webejammin]

Great artical Gordmay . So if the waves are 5 ft and the period is 15 at what depth do you get the smoothest sailing ?

[capn_billl]

Originally Posted by Dockhead

This question was a puzzle in a recent edition of Yachting Monthly, the excellent UK sailing mag. The problem -- a real life one -- had someone sailing in South Brittany in brisk conditions - a F7, I think -- down a piece of water between some islands and the mainland, where the water gets narrower and much shallower -- I think it was just 4 or 5 meters. They guy was afraid to do it, and the puzzle was -- would you dare?

My answer was -- hell no! Wind gets funneled in spots like that, and I can just imagine what kind of sea state you might get when 3 or 4 knots of tidal current heaves up over that bank, with 30 knots of wind blowing.

But I was wrong -- the answer to the problem was -- ask local people -- that bit of water is navigable in those conditions. The guy in the puzzle got through it without any difficulty.

A pass inbetween an island and the mainland is by definition a barrier island, and it is safe to be behind as the sea shallows at the barrier, then gets slightly deeper behind them attenuating the waves instead of forcing waves to break.

That is a special case. Not something I would make a rule out of.

[sarafina]

Originally Posted by Jim Cate

I'm getting the feeling that folks are worrying too much about this effect.
So, I wonder if there are some particular conditions in the PNW that make for worse sea states, or if we're making too big a deal out of this? Any thoughts?

I think this is definitely a case of "horses for courses". While some shallow areas may be very forgiving, others are not at all.

And the North American Pacific coast is right up there with the best of 'em as far as unforgiving goes. We are pretty much a rocky lee shore all the way up. Added into that our weather patterns and lack of harbour access for fair stretches and you will get an idea of why we call it the Baja Bash when sailing north from the tip of Cabo.

You want offshore estimates for safe smart sailing? Try Hawai'i as your first tack. What's that? Say 2500 miles? In COLD water.

; -D

[barnakiel]

I think it is prudent to stay well clear of any shallows unless one has personal local knowledge that would suggest otherwise.

Too many boats were badly bashed over shallows to make any guesses.

When in doubt ...

b.

[AuroraGH]

Originally Posted by mrohr

It ain't necessarily so. Inshore waters indeed will have reflected waves resulting in confused sea conditions close in ,if the existing seas are breaking against a steep-to headland.

Of course, that pretty well describes the entire Pacific coast.