Paddlewisers, I recently purchased and have been enjoying reading Sea Kayaker Magazine's Handbook of Safety and Rescue by Doug Alderson and Michael Pardy. However, something seemed unusual on page 48 as I read, "The crest will rise and break when the depth of the water is approximately three-quarters of the swell height. A 4-foot swell will break in water 3 feet deep. The height of the wave at the time it breaks will be approximately 1.5 times its original deep-water height. The 4-foot swell will rise to a 6-foot crest before breaking in 3 feet of water." I then checked a couple of other books I've read, and the information conflicted. Derek Hutchinson in The Complete Book of Sea Kayaking wrote, "When the depth is approximately one-and-a-half times the wave height, the wave begins to break." Nigel Foster in Nigel Foster's Surf Kayaking wrote, "The wave will steepen until the water is 1.3 times the height of the wave at that time, when it will break." Foster and Hutchinson's 1.3 to 1.5 times the depth of the water is not a big difference, but Alderson and Pardy's .75 times is a big difference. Maybe the real difference here is that Alderson and Pardy are referring to swells, while Foster and Hutchinson are referring to waves. If that is the case, Alderson and Pardy's number is .5 times when referring to waves and is even a bigger difference. Is there something else going on here that I am missing? Duane Strosaker Southern California www.rollordrown.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
The best reference for waves is "Waves and Beaches" by Willard Bascom. In this book he talks about deep-water and shallow-water waves. Bascom says: "A shallow-water wave is one that is traveling in water whose depth is less than half the wave length; that is, if the depth of water is small compared to the wave length, the effect of the bottom is sufficient to alter substantially the character of the waves." What this says to your question is that you should first look at the wave length, not the height, as it is the wave length that determines if the proximity of the bottom will change the character of the wave from non-breaking to breaking. For example if you have a wave that is 1 foot tall in the middle of the Pacific, but has a wave length of a hundred miles, you should head for the hills, you have a tsunami on the way. But for our purposes, we don't really care what the wave height is out in the middle of the ocean. We are typically interested in waves which are already shallow water. In shallow-water the deep-water wave slows down. As a consequence the wavelength reduces (the faster deep-water wave catches up with the slower shallow-water wave as it approaches shore). As the wavelength decreases, the steepness of the wave increases. As the wave crest enters water that is about twice the wave height, the crest starts rising higher, further increasing the wave steepness. According to Bascom, "Finally, at a depth of water roughly equal to 1.3 times the wave height, the wave becomes unstable." Then it breaks. I read this as the wave height just prior to breaking will be the determining factor, not the original deep-water wave height. I don't think you can predict the breaking depth of a deep-water swell based on it's height alone. A 1' high wave with a wave length of 7 feet will break in about 18" of water, but a 1' high wave with a wave length of 300 feet will break in much deeper water. But if you see a 10' wave coming at you and you are in 13' of water, you might want to move. Nick On Mar 9, 2004, at 10:42 PM, Strosaker wrote: > Paddlewisers, > > I recently purchased and have been enjoying reading Sea Kayaker > Magazine's > Handbook of Safety and Rescue by Doug Alderson and Michael Pardy. > However, > something seemed unusual on page 48 as I read, "The crest will rise and > break when the depth of the water is approximately three-quarters of > the > swell height. A 4-foot swell will break in water 3 feet deep. The > height of > the wave at the time it breaks will be approximately 1.5 times its > original > deep-water height. The 4-foot swell will rise to a 6-foot crest before > breaking in 3 feet of water." > > I then checked a couple of other books I've read, and the information > conflicted. Derek Hutchinson in The Complete Book of Sea Kayaking > wrote, > "When the depth is approximately one-and-a-half times the wave height, > the > wave begins to break." Nigel Foster in Nigel Foster's Surf Kayaking > wrote, > "The wave will steepen until the water is 1.3 times the height of the > wave > at that time, when it will break." > > Foster and Hutchinson's 1.3 to 1.5 times the depth of the water is not > a big > difference, but Alderson and Pardy's .75 times is a big difference. > Maybe > the real difference here is that Alderson and Pardy are referring to > swells, > while Foster and Hutchinson are referring to waves. If that is the > case, > Alderson and Pardy's number is .5 times when referring to waves and is > even > a bigger difference. > > Is there something else going on here that I am missing? > Nick Schade Guillemot Kayaks 824 Thompson St Glastonbury, CT 06033 USA Ph/Fx: (860) 659-8847 http://www.guillemot-kayaks.com/ *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
I've noticed the same thing. I know that the swell interval has as large an effect on how big the shore break is as does swell height. It seems like differences in swell interval may be enough to cause this apparent discrepancy (guess). A 5 foot swell with a 15 second interval will break in much deeper water than a 5 foot swell with a 10 second interval. www.surfline.com used to have a neat section call "waveology" that was the best written explanation of these things I have read. I can't find it on their web site now. Steve Brown -----Original Message----- Paddlewisers, ... A 4-foot swell will break in water 3 feet deep. The height of the wave at the time it breaks will be approximately 1.5 times its original deep-water height. The 4-foot swell will rise to a 6-foot crest before breaking in 3 feet of water." I then checked a couple of other books I've read, and the information Conflicted.... Foster and Hutchinson's 1.3 to 1.5 times the depth of the water is not a big difference, but Alderson and Pardy's .75 times is a big difference. Maybe the real difference here is that Alderson and Pardy are referring to swells, while Foster and Hutchinson are referring to waves. If that is the case, Alderson and Pardy's number is .5 times when referring to waves and is even a bigger difference. Is there something else going on here that I am missing? Duane Strosaker Southern California www.rollordrown.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
> I recently purchased and have been enjoying reading Sea Kayaker Magazine's > Handbook of Safety and Rescue by Doug Alderson and Michael Pardy. However, > something seemed unusual on page 48 as I read, "The crest will rise and > break when the depth of the water is approximately three-quarters of the > swell height. A 4-foot swell will break in water 3 feet deep. The height of > the wave at the time it breaks will be approximately 1.5 times its original > deep-water height. The 4-foot swell will rise to a 6-foot crest before > breaking in 3 feet of water." I don't have the book so I can't verify what it says. However, what it should read is that a wave will break when the depth of the water is approximately four-thirds it's own height (actually when the depth of the water becomes less then 1.28 times the wave height). Your 4-foot wave would actually break in about 5.3 feet of water, a 3-foot wave will break in about 4-feet of water and a 6-foot wave will break in about 8-feet of water. That said I usually use 1.5 when I am discussing this in my classes. It's easier for people to do the numbers in their head while sitting on the beach early in the morning and I feel it is plenty close enough for our purposes. Scott So.Cal. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
I found the link to SEANS' SURFOLOGY 101: http://www.surfline.com/surfline/help/surfology.cfm Pretty interesting and seems to confirm Nicks assertion that different swell intervals will create dramatically different wave heights, but at the end of the day it is that wave height that will determine the depth that a wave breaks at. Steve Brown -----Original Message----- Paddlewisers, ...... Is there something else going on here that I am missing? Duane Strosaker Southern California www.rollordrown.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
> I recently purchased and have been enjoying reading Sea Kayaker Magazine's > Handbook of Safety and Rescue by Doug Alderson and Michael Pardy. However, > something seemed unusual on page 48 as I read, "The crest will rise and > break when the depth of the water is approximately three-quarters of the > swell height. A 4-foot swell will break in water 3 feet deep. The height of > the wave at the time it breaks will be approximately 1.5 times its original > deep-water height. The 4-foot swell will rise to a 6-foot crest before > breaking in 3 feet of water." I tried to send this off yesterday morning but I guess the e-mail got lost in limbo. So let's try this again, shall we? I don't have the book so I can't verify what it says. However, what it should read is that a wave will break when the depth of the water is approximately four-thirds it's own height (actually when the depth of the water becomes less then 1.28 times the wave height), or the waves steepness surpasses 1/7 (the wave height cannot be greater then one seventh of the wave length). Your 4-foot wave would actually break in about 5.3 feet of water, a 3-foot wave will break in about 4-feet of water and a 6-foot wave will break in about 8-feet of water. That said I usually use 1.5 when I am discussing this in my classes. It's easier for people to do the numbers in their head while sitting on the beach early in the morning and I feel it is plenty close enough for our purposes. Scott So.Cal. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Duane asked: Is there something else going on here that I am missing? No matter who is asking the question, the answer to it is always YES! No matter how little you are missing. Matt Broze www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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