NOAA, significant wave heights and what it means to us

[Phil Allen]

It's been a while since we've had a discussion of wave heights, periods and the like.

http://www.mxak.org/weather/pdfs/waves.pdf

is a nice short, clear (IMO) article about waves with a good explanation of why, even though the significant wave height forecast might be X, you're sure you got hit by a wave 2X big. Enjoy.

Phil

[Ann Shepardson]

Thanks, Phil. I can't say I understand it completely, but this was a good place for me to start.

Ann

[leong]

It's been a while since we've had a discussion of wave heights, periods and the like.

http://www.mxak.org/weather/pdfs/waves.pdf

is a nice short, clear (IMO) article about waves with a good explanation of why, even though the significant wave height forecast might be X, you're sure you got hit by a wave 2X big. Enjoy.

Phil

Nice article, Phil.

I’m not certain, but I think NOAA’s predictions only apply to deep water (that’s where the Raleigh distribution function for wave height works well). But for paddling in shallow waters near shore (where the wave height is between than 5% and 50% of the wavelength) the Raleigh distribution doesn’t apply very well. In that depth region (and below) the maximum wave height can be a lot more than twice the significant wave height.

The table here shows that for sustained wind-speeds of 23 MPH, the average wavelength of wind generated waves is 111 feet. So in water depths between 5.5 feet and 55 feet, NOAA’s rule of thumb for maximum wave height doesn’t work well (unless, of course, they model shallow water too and then I’m all wet, pun intended).

One of my favorite deep-water paddles is along a straight line from Plum Island Sound to Lanes Cove; the depth there is mostly less than forty feet.

-Leon (who could be wrong)

[Inverseyourself]

Great article! One little glitch if I'm not completely off: Under Wave Dimensions, the author mentions the 3 wave components height, wave length and period. Here, he puts "or frequency" in parentheses after "period". He later states that, as waves leave their area of generation, "length and period increase". He concludes that periods of greater than 10-12 seconds are termed swell.

However, if you equate frequency and period, as the author seems to be doing here, clearly unintentionally, an increase in distance of waves from their birthplace, combined with the statement that " length and period increase" with said difference, would result in frequency increasing over distance, i.e. waves following each other at smaller time-intervals, which is clearly not the case. Period has an inverse relation to frequency. Therefore, putting "or frequency" in parenthesis after "period" could generate the utmost confusion.

For engineers and physicists this is trivial, not for simple folk like me. Not trying to be a stickler but Devil's in the details. Just sayin'.

Leon, that Plum Island Sound-Lane's Cove axis sure sounds scary.

[Pintail]

...and remember, Inverseyourself: when the learned gentleman writes "Plum Island Sound to Lane's Cove" (as above), he does <not> mean simply paddling one-way! ;^)

[Phil Allen]

Leon and Andy,

A couple of points of clarification (which I'm sure Leon knows). There's a wave forecast and actual wave measurements from a buoy or satellite. In the absence of a buoy or satellite measurement, Leon is correct in that the wave forecast is based on the wind speed over deep water and the probability distribution of wave heights is presumed to be somewhat of a bell curve. As swell and wind waves interact with the bottom, things change significantly. Those that like to surf enjoy that change.

The article also uses some common nomenclature and often forgets to add the word distribution to frequency. The frequency and period are just the inverse of each other, but the frequency distribution describes all the wave-lets, short period and long period, that interact to give the final result on the water. Unlike the plot of distribution of sizes versus number, where increases on the X axis indicate bigger numbers, when it comes to frequency, longer periods (with often bigger waves) come closer to 0.

best

Phil

[leong]

...and remember, Inverseyourself: when the learned gentleman writes "Plum Island Sound to Lane's Cove" (as above), he does <not> mean simply paddling one-way! ;^)

Sir Christopher,

Actually, sometimes, it’s just one-way* from Lanes to PIS, er, Plum Island Sound … Lisa doesn’t like my abbreviation; she said “I don't think I like the abbreviation "PIS". Not easy to pronounce :-) I mean, sometimes that's what we do the minute we get there, but no need to drive the point home.”

* In the occasional one-way trip from Lanes Cove, I go to the west end of Plum Island and then do a clock-wise circumnavigation around the island and end the trip when I get back to Little Neck (where a second car is parked). It’s just about 25 miles.

-Leon

PS

I’m hoping my racing partner is in a good mood when she reads this, or else, OUCH!

Lisa, if you can find it would you please link the following GPS track: You, Shari and I went from Lanes to Plum Island. Then we started to go through Fox Creek to the inside of Castle Neck. But we then aborted that due to shallow water (and getting lost too). Finally, we circled back to PI Sound and went straight back to Lanes Cove. That’s the trip where Shari and I were huffing and puffing to keep up with you, you machine!

Andy,

I concur with Phil’s response to you.

-Leon

[Inverseyourself]

...and remember, Inverseyourself: when the learned gentleman writes "Plum Island Sound to Lane's Cove" (as above), he does <not> mean simply paddling one-way! ;^)

I suspected as much

Andy

[MPARSONS]

Thanks, Phil. That was a really good read over lunch.

The most interesting takeaway for me is that if the forecast is X, you will encounter a few up to 1.6X. Exceeding X by a meaningful amount should be expected and planned for.

[Phil Allen]

Yeah, it doesn't take long to get that 1 in 100 wave. Common periods around here for swell are in the 9-10 second range (very different in the Pacific). At 10 second period, it only takes about 20 minutes to have 100 waves pass by the boat.

best

Phil

[JohnHuth]

Here's a link to an article on shallow wave conditions - for the expert only:

http://journals.tdl.org/icce/index.php/icce/article/viewFile/1140/pdf_38

I've become interested in the tradition of wave piloting in the Marshall Islands and am slowly teaching myself a bit about this. The idea is that waves will refract and reflect on encountering shallow waters. The indigenous navigators in the Marshall Islands could read the patterns of interactions of waves with land and find distant islands.

I'm only beginning my own practice of wave piloting, but it's interesting - I often find that there are very distinct wave patterns depending on where I'm paddling.

A curious feature is that a sloping shore will reflect low frequency waves much more than high frequency waves.

Relating one wave piloting experience - I was paddling up the Sheepscott River. At the entrance there were swells from both the SE and South. As I went up the entrance, where it starts to neck down, the SE swell was extinguished by land and the S swell was amplified a bit by estuary becoming narrower.

Right now, I'm playing around with modeling wave interactions with land - there are some nice computer packages out there, but it's not clear that they get reflections right.

[lhunt]

...but we then aborted that due to shallow water (and getting lost too). Finally, we circled back to PI Sound and went straight back to Lanes Cove...

Maybe you were lost, you old sea dog, but I knew exactly where we were (by GPS), and Shari did too (by just knowing). I only passed the turn initially because it had no water in it, which was rather disappointing - I was hoping I was wrong. (Shallow water my eye, that thing was dry as a bone with probably another 8 or 12" to go). I can't seem to get to the historical tide levels, but as I remember we were within 1 or 2 hours of high tide and we were surprised to find no exit. Oh, well. It's a nice challenge trying to do a 180 in an 18-footer in those little channels.

Pix here (many of these are Shari's and Leon's).

I think the real hero of the day was Shari - she volunteered not to wait for the tide but go back the long way and finished it strongly. I mean we two had been training for the Blackburn but she was coming into it with less preparation.

Sorry to be so late about this - I missed the postings the first time around.

Lisa

Edited May 31, 2014 by LHuntington

[leong]

Maybe you were lost, you old sea dog, but I knew exactly where we were (by GPS), and Shari did too (by just knowing).

OUCH, sorry, partner! In fairness, Shari said to me “Leon, looks like you’re hoping we’re lost”

-Leon

PS

I think the Picasa pix are not there now.

[lhunt]

Pix. Picasa playing games with me (again)!

[leong]

Here's a link to an article on shallow wave conditions - for the expert only:

http://journals.tdl.org/icce/index.php/icce/article/viewFile/1140/pdf_38

John,

Good article, not that I’m an expert.

Then I noticed that I ought to be ashamed of myself for misspelling Rayleigh as Raleigh (like in Sir Walter), especially since I once worked on estimating radar cross section of a target using Rayleigh distributions.

I just ran the MS spell checker on the above. Whew, it’s not me; it’s the spell checker!

Christopher, why aren’t you on the job when we need you?