======== Newsgroups: alt.surfing Subject: Fetch for wave height From: gleshna@aol.com (Gleshna) Date: 6 Apr 1997 23:59:44 GMT For anyone interested: I have often wondered as a Great Lakes surfer how much fetch waves need for a given height. Today on Wolf Lake in Hammond, Indiana we had 50 to 60 mph winds with a fetch of less than one mile. In those conditions the waves were about 1 to 2 feet. By the way, we were windsurfing. I took the sail off of my board and was able to surf the waves for about 30 yards - extremely lousy shape, but I never thought I would be able to ride waves in such a small lake. So anyway the wave height increased very quickly. Now my next question is when does a wind wave become officially become a swell? When the wind dies? Bob ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: Robert Rainwater Date: Mon, 07 Apr 1997 05:18:11 -0700 Bob wrote: > > Now my next question is when does a wind wave become officially become a > swell? When the wind dies? > Nope, when its period gets beyond 6 seconds or so (exact time debatable, I suppose). ........r ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: John Webster Date: Mon, 7 Apr 1997 22:32:23 GMT Gleshna wrote: > > For anyone interested: > > I have often wondered as a Great Lakes surfer how much fetch waves need > for a given height. Today on Wolf Lake in Hammond, Indiana we had 50 to > 60 mph winds with a fetch of less than one mile. > > Bob While we're on the subject, can anyone define "fetch" in terms of wave prediction? I looked at some of the FAQs and I see it used a lot (as in most wave predictions), but I have yet to see a definition. Thanks, --John W. johnw@cpg.com ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: Robert Rainwater Date: Tue, 08 Apr 1997 05:40:36 -0700 John Webster wrote:> > > While we're on the subject, can anyone define "fetch" in terms of wave > prediction? I looked at some of the FAQs and I see it used a lot (as in > most wave predictions), but I have yet to see a definition. > Fetch is the actual geographic open water area of the wind that creates the waves. The wind waves are choppy within the fetch; 'swell' doesn't develop until the waves get more organized beyond the influence of their creating wind in the fetch. ........r ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: tbmaddux@alumnae.caltech.edu (Timothy B. Maddux) Date: 10 Apr 1997 17:31:13 GMT In article <334975F7.2781@cpg.com>, John Webster wrote: >While we're on the subject, can anyone define "fetch" in terms of wave >prediction? I looked at some of the FAQs and I see it used a lot (as in >most wave predictions), but I have yet to see a definition. The fetch of a wind is the distance over open water that it blows. For fetch-limited seas, waves build, increasing in height and length, until they reach the end of the fetch and then begin to propagate away as swell. If winds blow long enough over a large enough fetch, the seas become 'fully developed'. All the energy being input by wind to the waves as they move along is matched by the energy lost due to breaking (rough seas), and the waves no longer increase in height or length. The original question asked for a direct relationship between fetch and wave height; that can't be given without also knowing the wind velocity and the duration that it blows. See Lud's FAQ. -- .-``'. Timothy B. Maddux, Ocean Engineering Lab, UCSB .` .`~ http://www.engineering.ucsb.edu/~tbmaddux/ _.-' '._ "From the essence of pure stoke springs all creation." ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: "EB" Date: 8 Apr 1997 12:29:10 GMT Fetch is wind speed X time X distance. Fetch does determine the size of a wave. When the distance factor is greater, the wave interval is also longer. Thats why some of the best swells travel long distances and often the wave interval is upwards of the 15 to 20 second period. Gleshna wrote in article <19970406231600.TAA25602@ladder01.news.aol.com>... > For anyone interested: > > I have often wondered as a Great Lakes surfer how much fetch waves need > for a given height. Today on Wolf Lake in Hammond, Indiana we had 50 to > 60 mph winds with a fetch of less than one mile. In those conditions the > waves were about 1 to 2 feet. By the way, we were windsurfing. I took > the sail off of my board and was able to surf the waves for about 30 yards > - extremely lousy shape, but I never thought I would be able to ride waves > in such a small lake. So anyway the wave height increased very quickly. > > Now my next question is when does a wind wave become officially become a > swell? When the wind dies? > > Bob > ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: keensurf@no.spam.cts.com (Thomas Keener) Date: Wed, 09 Apr 1997 07:40:19 GMT gleshna@aol.com (Gleshna) wrote: >I have often wondered as a Great Lakes surfer how much fetch waves need >for a given height. >Now my next question is when does a wind wave become officially become a >swell? When the wind dies? There is a neat little graph in Willie Bascom's Waves and Beaches. (page 9 in my copy.) Across the bottom - wave period in seconds Up the side - relative amount of energy present Across the top- tides ~12hrs (all times approximate) tsunamis +/- 1000 seconds(17 minutes), surf beat 50-100 seconds, swell 8-20 seconds, sea 5-12 seconds chop 2-7 seconds ripples 1 second The graph shows that the most energy is in a swell at 15-20 second wave periods. BTW, (for John Webster,) Fetch is the distance the wind blows. Wave height is: wind speed x fetch x duration. A few years ago, Ricky Grigg wrote an article in one of the surfing mags (I wish I could find a copy,) that summarized his research. He plotted North Pacific storms vs Waimea Bay surf. AIR, he found the optimum wind speed was ~40mph over a 1500 mile fetch for 4 days. When the wind speed gets too high it blows the tops off of the waves. -- TomK keensurf@cts.com ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: Fred Date: Wed, 09 Apr 1997 23:18:17 -0700 Gleshna wrote: > Now my next question is when does a wind wave become officially become a > swell? When the wind dies? If I'm not mistaken, a wave period of 10-12 seconds is the in-between zone...once you get up to 12 seconds, you've got a ground swell. >From what I know about the subject, waves tend to organize themselves and actually become more powerful, even as the swell itself dissipates energy. So a wind swell in the Northern Pacific becomes a ground swell by the time it hits the west coast or Hawaii, with the energy more concentrated...creating nice powerful waves without all the bumpiness in the water. Fred TubeTime - http://www.geocities.com/TheTropics/Shores/2303/TubeTime.html ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: david@crumblingempire.com (david ryan) Date: 10 Apr 1997 14:20:28 GMT In article <334C8615.1836@geocities.com>, freddyv@geocities.com wrote: > Gleshna wrote: > > > Now my next question is when does a wind wave become officially become a > > swell? When the wind dies? > > If I'm not mistaken, a wave period of 10-12 seconds is the in-between > zone...once you get up to 12 seconds, you've got a ground swell. first of all, the surf here sucks. it's cold nasty windblown shit that is product of local storms. that said, let's hear it for wind waves! typical east coast winter scenario: a local brew haha will blow on shore for 12-24 hours with windspeeds of 20-50 knots. on the backside of the blow we'll get offshore winds of15-30 knots. for the six to 12 hours after the wind switches offshore, the surf is great! clean, fast, hollow, and lots of it. of course the water in ~40 degrees, and a 25k offshore makes the spray feel like needles, but their is very little competition (by so. cal standards) for waves either. i've had days this winter in jersey and l.i. that were as good as any at wind n sea or simons. i suspect similiar conditions come into play on the great lake. short fetch shit, with strong offshores to clean it up, but maybe it's just too cold on the upper pen. in january. d.r. ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: gleshna@aol.com (Gleshna) Date: 10 Apr 1997 17:44:09 GMT Why would the period of waves increase as they travel? I think they would start out as sine waves with troughs and crests. I know from windsurfing that gusts come in sets, so I assume this is a big factor in why large waves probably come in sets. Why would the troughs of the waves lenghten with distance traveled? The wind is mostly straight with maybe a slight curving because of the rotation of the earth, but I don't clearly see why this would happen. I have observe the various periods of waves in California and on lakes, but I don't understand why? Bob ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: dagum@barrel.nospam (Leo Dagum) Date: 10 Apr 1997 23:51:20 GMT In article <19970410174400.NAA11045@ladder01.news.aol.com>, gleshna@aol.com (Gleshna) writes: |> |> Why would the period of waves increase as they travel? I think they would |> start out as sine waves with troughs and crests. Yes and no. When formed they have all sorts of different periods, the shorter period stuff has less energy though, so short period waves will dissipate before longer period waves. As the waves travel, the short period stuff dies off, leaving the long period "groundswell". |> I know from windsurfing |> that gusts come in sets, so I assume this is a big factor in why large |> waves probably come in sets. No, not really. Waves come in sets because they travel in groups. The group period is much greater than the individual wave period. It's the principle behind radio and tv (if you want a completely useless non-intuitive analogy :-)). - leo -- Leo Dagum Silicon Graphics, Inc. Mountain View, CA 94043 (415-933-2179) ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: 3343#01A$7F76@discoveryinternational.com (Ric Harwood) Date: Fri, 11 Apr 1997 12:11:04 GMT In alt.surfing, on 10 Apr 1997 17:44:09 GMT gleshna@aol.com (Gleshna), wrote: >Why would the period of waves increase as they travel? I think they would They don't, period remains constant, but mean period increases as short period waves are attenuated more than long period waves. >start out as sine waves with troughs and crests. I know from windsurfing They do approximate sine waves. But less so in the generation system. >that gusts come in sets, so I assume this is a big factor in why large >waves probably come in sets. No. See the sets discussion on my alt.surfing archive. > Why would the troughs of the waves lenghten >with distance traveled? The length of individual waves increases within the generation area. Then they don't change till they reach the shelf, and even then not by much. The average length increases with distance travelled away from generation as the short waves are attenuated. > The wind is mostly straight with maybe a slight The wind is mostly curved. >curving because of the rotation of the earth, but I don't clearly see why >this would happen. The curving is due to Corriolis, [the lower angular velocity of earth at higher latitudes.] It's a long story. There is a good description on the "Bad Science" web page: http://www.ems.psu.edu/~fraser/BadScience.html >I have observe the various periods of waves in California and on lakes, >but I don't understand why? Wave development, ie. height and period is governed in a complicated way by fetch, wind speed, temperature, duration and propogation. To find out all about it read one of the books in the FAQ, but for other specific points please ask. HTH Ric. -- European Surf forecast links and alt.surfing FAQ: http://www.discoveryinternational.com/ric/surf.htm PGP public Key ID: 0766ABE5 ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: gadgetpjt@aol.com Date: 11 Apr 1997 18:13:12 GMT 3343#01A$7F76@discoveryinternational.com (Ric Harwood) writes: < some clever stuff > Like your new sig, Ric. Looks like one of jOCKO's more well thought out posts R#~ards, Gad#*t. ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: tbmaddux@alumnae.caltech.edu (Timothy B. Maddux) Date: 11 Apr 1997 18:32:46 GMT In article <19970410174400.NAA11045@ladder01.news.aol.com>, Gleshna wrote: >Why would the period of waves increase as they travel? Good question. The increase of wave period within a fetch is known as frequency downshifting -- a decrease in frequency is an increase in period. Here's how it works: The wind starts blowing on a flat sea, and short little waves form. They grow in height as high as they can until they can grow no further and begin to break. There's still a bunch of wind energy out there, though, and it can't go into the short waves any more. So, longer waves start forming and they grow until they break, and the process repeats itself. This continues until the waves being formed are so long that they are moving as fast as or even faster than the wind is blowing, at which point the wind cannot push on them to make them larger. Growth stops and you have a "fully developed sea." The waves are as big as they'll ever get for the wind speed that is present. If the fetch is short, the waves never get to be fully developed, and they move away from the wind smaller and shorter than waves from a long fetch. The result is similar for winds that blow for a short period of time. After the storm passes, the waves are free and move across the ocean. The heights of the waves diminish due to directional spreading across the ocean. The longer waves move out faster than the shorter ones, causing the whole wave train to spread, which causes the waves to shrink even more. The shortest-period waves get left behind and become so small that they are undetectable. The result is an increase in the average wave period that we see when they reach us. Take a look at Kinsman's "Wind Waves" (1984, ISBN 0-486-64652-1), for a solid and difficult discussion of all this. Pages 408-409 on Pierson-Neumann forecasting nail it down in plain English pretty well. >I think they would start out as sine waves with troughs >and crests. Yes... in deep water swells are very well-approximated by pure sine waves. As waves come in to shallow water their shape changes to something called a 'cnoid' which has a short, steep crest and a long shallow trough -- those are what we see as lines of corduroy ahoooooooo. -- .-``'. Timothy B. Maddux, Ocean Engineering Lab, UCSB .` .`~ http://www.engineering.ucsb.edu/~tbmaddux/ _.-' '._ "From the essence of pure stoke springs all creation." ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: gleshna@aol.com (Gleshna) Date: 13 Apr 1997 14:31:31 GMT Previous Post: After the storm passes, the waves are free and move across the ocean. The heights of the waves diminish due to directional spreading across the ocean. The longer waves move out faster than the shorter ones, causing the whole wave train to spread, which causes the waves to shrink even more. The shortest-period waves get left behind and become so small that they are undetectable. The result is an increase in the average wave period that we see when they reach us. Response: I have seen a copy of WindWaves and hope to eventually get a copy. Everything in the response made perfect sense; however, the above paragraph gets at the key point. The key area of discussion is: " The result is an increase in the average wave period that we see when they reach us." Okay the slower and smaller waves get left behind. This still does not explain the wave period change. Let's say that we have a set of 5 waves; they are perfect mathematical sine waves traveling at a given rate, they are 6 feet high, and have a period of 5 seconds. Why would the period change? This would mean that the front waves must travel slightly faster than the wave behind it. Why? If it is slightly faster shouldn't it be bigger? My experience has been that the first wave in a set is usually not the biggest? I know of pinwheel spread because the waves are actually moving into the shore at an angle. The other theory, and I think this may be somewhat an interpretation of what is above. The waves are very large at sea and lose size for some reason, for example, slower waves uncombine with them as mentioned above. The wave height decreases, but the period remains the same. Thus we have sine waves with a longer almost horizontal curve This question has all come about because of my experience with very long period s medium to large summer swells in southern California when compared to the similar size medium swells ( offshore wind so I think it gets swell status) but extremely short period waves of Lake Michigan. Bob ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: robt2@ix.netcom.com(Rob(t.) Brannan) Date: 14 Apr 1997 00:45:41 GMT > >reach us." Okay the slower and smaller waves get left behind. This still >does not explain the wave period change. Let's say that we have a set of >5 waves; they are perfect mathematical sine waves traveling at a given >rate, they are 6 feet high, and have a period of 5 seconds. Why would the >period change? This would mean that the front waves must travel slightly >faster than the wave behind it. Why? If it is slightly faster shouldn't >it be bigger? My experience has been that the first wave in a set is >usually not the biggest? > >I know of pinwheel spread because the waves are actually moving into the >shore >at an angle. The other theory, and I think this may be somewhat an >interpretation of what is above. The waves are very large at sea and lose >size for some reason, for example, slower waves uncombine with them as >mentioned above. The wave height decreases, but the period remains the >same. Thus we have sine waves with a longer almost horizontal curve > >This question has all come about because of my experience with very long >period s medium to large summer swells in southern California when >compared to the similar size medium swells ( offshore wind so I think it >gets swell status) but extremely short period waves of Lake Michigan. > >Bob I dont know if my response will match the exact science, but intuitively I have always imagined these things, which I feel are in alignment with scientific fact. As a swell matures or travels, waves combine to spread out the period. ie. 6 waves passing a reference pt. in a 1 minute(10 second interval) might be 4 waves passing another reference pt. 1000/2000 miles later, therefore a 15 second interval. Further as waves combine they combine their undersea force ( that part of the wave that is travelling beneath the water), not their open ocean above water size. Therefore as swells travel and individual waves combine the period becomes longer, and the swell size will not increase but will decline with time/distance travelled. However, for mature swells with a long period, the undersea power of those waves is very high, and as these swells approach shallow water it causes these waves to grow higher and exhibit more strength than shorter period swells. However, the interplay between shallow continental shelfs can actually leach all the power and size out of these swells if the shelf is sufficeintly shallow and long. i.e. the long period swell is dragging on the bottom of the shelf and and contantly losing energy such that at some spots their energy is dissipated and either do not show at the break or show in a greatly detiorated form. Much of Florida exhibits this kind of effect on long period swells as well as some of this occuring in Calfornia. Shorter period swells are less affected by Shallow continental shelfs. Let me explain why: these figures are totally made up but provide for learning. Let say we have a 10 sec period swell. Then the undersea part of the swell is perhaps reaching 200 ft. of depth. Which means that as the swell travels into the coast it will not dissapate or get smaller until it is in 200 ft. or less of water, for most breaks this is very close to shore, a matter of a mile or more. Lets say we have a 17 second swell, and lets say that the undersea part of the swell extends to about 700 ft. depth. depending on the coast we choose 700 ft.depth of water might extend for 20 -40 miles out, depleting these long period waves of alot of their strength, and in some cases making an anticipated swell not even show up except at choice spots that typically handle long period swells due to the bottom configurations that extend out to sea. ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: tbmaddux@alumnae.caltech.edu (Timothy B. Maddux) Date: 16 Apr 1997 18:28:42 GMT In article <5irunl$r4@dfw-ixnews12.ix.netcom.com>, Rob(t.) Brannan wrote: [ someone else asked ] >>Okay the slower and smaller waves get left behind. This still >>does not explain the wave period change. Let's say that we have >>a set of 5 waves ... traveling at a given rate, they are 6 feet >>high, and have a period of 5 seconds. Why would the period change? Because the waves produced by a wind system are not uniform in height and frequency. The period of waves typically reported by buoys is the "significant" or "dominant" wave period, and it is a statistical quantity with several different definitions, see: http://www.ndbc.noaa.gov/waveobs.shtml for an explanation of this and other buoy-reported data. Since the period is statistically determined, loss of energy due to reasons previously stated in the higher frequencies (lower periods) as dispersion and decay occurs results in an increase in the observed period some distance away from the region of wave generation. [ and now Rob Brannan wrote: ] >As a swell matures or travels, waves combine to spread out the period. Not a combination of waves but rather a spreading or dispersion of waves. The many shorter waves simply get left behind because they are moving more slowly. >... for mature swells with a long period, the undersea power of >those waves is very high, and as these swells approach shallow water >it causes these waves to grow higher and exhibit more strength than >shorter period swells. Yes; the energy (or power) of a wave is proportional not only to its height but its period. >Shorter period swells are less affected by shallow continental shelfs. Yes, because again the relative depth is what is important. However, the effects of bottom friction are overstated here; a 17-second period wave train has more than enough additional energy in it as compared to a 10-second period wave train to make up for any bottom friction losses. -- .-``'. Timothy B. Maddux, Ocean Engineering Lab, UCSB .` .`~ http://www.engineering.ucsb.edu/~tbmaddux/ _.-' '._ "From the essence of pure stoke springs all creation." ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: Tauras Date: Sun, 13 Apr 1997 07:54:59 -0700 CC: tbmaddux@alumnae.caltech.edu Timothy B. Maddux wrote: > > In article <19970410174400.NAA11045@ladder01.news.aol.com>, > Gleshna wrote: > >Why would the period of waves increase as they travel? > > Good question. The increase of wave period within > a fetch is known as frequency downshifting -- a > decrease in frequency is an increase in period. > > Here's how it works: The wind starts blowing on a > flat sea, and short little waves form. They grow > in height as high as they can until they can grow > no further and begin to break. > > There's still a bunch of wind energy out there, though, > and it can't go into the short waves any more. So, longer > waves start forming and they grow until they break, and > the process repeats itself. > > This continues until the waves being formed are so long > that they are moving as fast as or even faster than the > wind is blowing, at which point the wind cannot push on > them to make them larger. Growth stops and you have a > "fully developed sea." The waves are as big as they'll > ever get for the wind speed that is present. > > If the fetch is short, the waves never get to be > fully developed, and they move away from the wind > smaller and shorter than waves from a long fetch. > The result is similar for winds that blow for a > short period of time. > > After the storm passes, the waves are free and move > across the ocean. The heights of the waves diminish > due to directional spreading across the ocean. The > longer waves move out faster than the shorter ones, > causing the whole wave train to spread, which causes > the waves to shrink even more. The shortest-period > waves get left behind and become so small that they > are undetectable. The result is an increase in the > average wave period that we see when they reach us. The decay of the wave train intersts me since it seems to be the crux of picking the best wave of the set to ride. Sitting at the line-up and picking usually the second or third from the last wave has been what I like to ride but the second or third waves seem to be the best breaking waves on the sets, if a generality can be made. Not being formally schooled in this topic I find it fasinating when windsurfing waves the rapid speed the shorter period waves decay as they start feeling the bottom. In other words if you pick out the last wave of the set out about a mile from the beach on a short steep swell by the time you reach the beach this wave will be on the front end of the set. But on a long period ground swell there doesn't seem to be as much decay since one can pretty much ride the same wave in, that is in relation to the set. I'm sure there is some formula out there that summerizes this action... ohh well, surfs still small here but we have had some of that south around on incoming tides, I'm sure socal south facing beaches have been fun... -- Tauras Sulaitis Current Project: Bunga USTA Design Inc. ~~~~~~~~~~~~~~ tauras@lietuva.com ~~~~~~~~~~~~~~~~~~~~~ West Coast Sty http://www.slonet.org/~tsulaiti/ East Coast < server down> Sty http://www.minds-online.com/sty/ ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: tbmaddux@gear.ucsb.edu (Timothy B. Maddux) Date: 14 Apr 1997 18:42:39 -0700 In article <3350F3C3.17E0@slonet.org>, Tauras wrote: [ for some reason he included my entire article, it was boring enough having to write it, let alone read it again. snipped it since this is going to be a LONG post anyways. ] >The decay of the wave train intersts me since it seems >to be the crux of picking the best wave of the set to ride... 'Decay' of wave height in the deep ocean is primarily due to lateral spreading and the divergence of wave groups, and secondarily due to still-air resistance. It has little to do with the wave group envelope, which is what makes a set increase in size and then diminish, which Tauras describes as 'decay'. In fact, what Tauras is seeing is not a decay of the wave at all. In fact, it is simply the movement of a wave crest from the back of the set through the largest part of the wave envelope to the front. In relatively deep water, individual waves move at the 'phase speed', while their group envelopes move at HALF that speed. Waves rise up in the back of the group, increasing in size to a maximum in the middle, and then continue to the front where they eventually vanish. As the waves move into relatively shallower waters, the speeds of individual waves in the group become closer to that of the wave envelope, until they match. Shallow water waves do not move through the group and change in height like deep water waves. Note that I said 'relatively'. The 'relative depth' is key. It is the ratio of water depth to wave length: d/L. If you increase L for the same d, then the ratio d/L decreases. This means that longer waves are in 'shallow water' sooner than shorter waves. In fact, tsunami and the tides are so long in wavelength that they are in 'shallow water' even in the deepest portions of the ocean. So, when Tauras windsurfs a short-period swell, the shorter-period waves are not in shallow water a mile out from shore. The waves are still moving through the group. The longer waves, on the other hand, are in relatively shallower water. So they do not move through the group as much, and thus the biggest waves way out to sea are still the biggest waves of the set when they finally break. It's got nothing to do with decaying at all, but it's still cool stuff to think about. I'm gonna have to take up windsurfing just so I can check this out for myself! -- .-``'. Timothy B. Maddux, Ocean Engineering Lab, UCSB .` .`~ Santa Barbara Surfing Web Site _.-' '._ http://www.me.ucsb.edu/~tbmaddux/ ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: Fred Date: Tue, 15 Apr 1997 22:49:35 -0700 Timothy B. Maddux wrote: > 'Decay' of wave height in the deep ocean is primarily due > to lateral spreading and the divergence of wave groups, > and secondarily due to still-air resistance. > > In fact, it is simply the movement of a wave crest from the > back of the set through the largest part of the wave envelope > to the front. > In relatively deep water, individual waves move at the > 'phase speed', while their group envelopes move at HALF that > speed. Waves rise up in the back of the group, increasing in > size to a maximum in the middle, and then continue to the > front where they eventually vanish. > As the waves move into relatively shallower waters, the speeds > of individual waves in the group become closer to that of the > wave envelope, until they match. Shallow water waves do not > move through the group and change in height like deep water waves. > Note that I said 'relatively'. The 'relative depth' is > key. It is the ratio of water depth to wave length: d/L. > If you increase L for the same d, then the ratio d/L decreases. That sound you just heard was my brain cramping up. Apparently Rincon is flat? Fred TubeTime! - http://www.geocities.com/TheTropics/Shores/2303/TubeTime.html ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: gleshna@aol.com (Gleshna) Date: 16 Apr 1997 16:22:40 GMT I may have missed the key point or an important reply in this thread, but I still do not see why waves periods would increase as they travel further. I do see that lateral spreading would dissipate the energy, that still air resistance would decrease the height, and that some sort of a leap frog of sorts is supposedly occuring as a group of waves travel. But why would the wave periods change? Bob ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: tweedt@aol.com Date: 17 Apr 1997 00:32:41 GMT In article <19970416162200.MAA10766@ladder01.news.aol.com>, gleshna@aol.com (Gleshna) writes: > >I may have missed the key point or an important reply in this thread, but >I still do not see why waves periods would increase as they travel >further. I do see that lateral spreading would dissipate the energy, that >still air resistance would decrease the height, and that some sort of a >leap frog of sorts is supposedly occuring as a group of waves travel. But >why would the wave periods change? You forget that the "change" refers to the whole wavetrain or swell, not individual waves, and is relative to an observer (that huge qualifier of all scientific empiricism), seeing the same swell at widely separated times and geographical locations (which is rarely the case in real life). The observer located close to the storm, the point of generation, would perceive (and measure) a much shorter interval due to the mixing and combining of different frequencies, from chop, to large, breaking, crumbling peaks, generated from the strong, gusty local winds. The observer on shore, thousands of miles away, sees a much more homogenous groundswell arrive a week later, of much more uniform (and longer) period, due to the "decay" of the shorter period stuff over the intervening distance, and the fact that the longer period waves travel faster, and arrive first. That's why you see the familiar pattern of a good ground-swell springing up in a matter of hours, peaking quickly, then slowly (over a number of days, usually), increasing in frequency and decreasing in period, until it dies. This seems clear to me, but perhaps it is difficult to express well. I thought T. Maddux did an excellent job of it. Some of the "duuuudes" in the ng were bored by the math, I guess, judging by the followups. -------------------------------------------------------------------------- Tom Tweed La Jolla, CA e-mail: tweedt@ucsd.edu or tweedt@aol.com "Don't let your mouth write no check that your tail can't cash." -- Bo Diddley -------------------------------------------------------------------------- ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: gleshna@aol.com (Gleshna) Date: 17 Apr 1997 23:22:36 GMT The key question was: Why would the wave period increase as they travel? An excerpt from the first response: " the fact that the longer period waves travel faster, and arrive first." This says that longer period waves arrive first, not that the period changes or lengthens. So in summary, can I say that the wave period does not increase as they travel? Again, I am merely noting the difference in southern California large south swells when compared to the north swells. The large south swells as I recall were always much further apart than north swells. Also, I have seen the exact same size and form of waves of up to shoulder high waves on Lake Michigan, but the lake waves are extremely close together, and I am guessing this is become they have just recently come from the source wind.. Bob ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: ric@discoveryinternational.com (Ric Harwood) Date: Tue, 29 Apr 1997 14:28:50 GMT Sorry I'm so late in getting back into this thread that it has probably expired both in your machines and brains, but I just moved house again. In alt.surfing, on 17 Apr 1997 23:22:36 GMT gleshna@aol.com (Gleshna), wrote: >The key question was: Why would the wave period increase as they travel? > >An excerpt from the first response: " the fact that the longer period >waves >travel faster, and arrive first." This says that longer period waves >arrive first, not that the period changes or lengthens. > >So in summary, can I say that the wave period does not increase as they >travel? Yes and No. The period of any individual wave or "wave group" does not change. However the mean period of the swell does increase as the shorter period swells are attenuated. HTH Ric. -- European Surf forecast links and alt.surfing FAQ: http://www.discoveryinternational.com/ric/surf.htm PGP public Key ID: 0766ABE5 ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: Kristof De Backer Date: Fri, 18 Apr 1997 14:30:58 +0200 Hear this story! Yesterday, Daddy Wind did his best to put some of his energy into Mother Ocean, but dispite his efforts, the wavefield he generated was a mess! Quite irregular, a combinations of waves with differnt directions, low periods, but a least, the energy was inside! While travelling towards the coast, the waves lose some of their energy in whitecapping and, near the coast in bottomfriction. The wavefield is also spreadout over a bigger surface... but,...(Nature is good for us, so be good for here)... Ever heard of non-linear wave-wave interactions? While propagting towards your secret surfspot, waves with a strict freqeuncy-relationship interact and creates a new set, with a global lower frequency. This results in a shift of wave energy towards the waves with bigger periods. (yes!) So, out of the messy wavefield, a swellfield is born! And, luckily for us, this change is irriversible! In this story, I'm not talking about single waves.Single waves can also change in period when interacting with currents, but this is quite difficult to explain. Kristof tweedt@aol.com wrote: > > In article <19970416162200.MAA10766@ladder01.news.aol.com>, gleshna@aol.com (Gleshna) writes: > .... ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: gadgetpjt@aol.com Date: 15 Apr 1997 00:04:41 GMT Tim the Mad-Dog writes: >Yes... in deep water swells are very well-approximated >by pure sine waves. As waves come in to shallow water >their shape changes to something called a 'cnoid' which >has a short, steep crest and a long shallow trough -- >those are what we see as lines of corduroy ahoooooooo. That'll piss Tauras off, "riding the 'cnoid", just doesn't have the same ring. Aren't those called something like trapezoidal too? [no... was'nt it, trochioidal?, that seems to ring bells] I can't remember it's a long time since I was doing fluid dynamics [and the lecturer was a real drag... ] -------------------------------------------------------------------------- Gadget [Bude, Cornwall, UK] email: gadgetpjt@aol.com Stupid is Forever. Ignorance can be Fixed. -------------------------------------------------------------------------- ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: Tauras Date: Fri, 18 Apr 1997 07:49:40 -0700 gadgetpjt@aol.com wrote: > > Tim the Mad-Dog writes: > > >Yes... in deep water swells are very well-approximated > >by pure sine waves. As waves come in to shallow water > >their shape changes to something called a 'cnoid' which > >has a short, steep crest and a long shallow trough -- > >those are what we see as lines of corduroy ahoooooooo. > > That'll piss Tauras off, "riding the 'cnoid", just doesn't have > the same ring. Cnoiding chances are weak today on the lost coast. Its looking like England, damp, gloomy and next to flat, hard to believe its CA. Ever since this thread started the cnoids are the size of my atnoids, which fall somewhere between the size of my left hemi and the bunion on my right big toe, time to kill this thread before we jinx the spring season alltogether.. ;) -- Tauras Sulaitis Current Project: Bunga <<>> http://www.slonet.org/~tsulaiti/ ======== Newsgroups: alt.surfing Subject: Re: Fetch for wave height From: tweedt@aol.com Date: 15 Apr 1997 01:11:21 GMT In article <5im04e$7fu@gap.cco.caltech.edu>, tbmaddux@alumnae.caltech.edu (Timothy B. Maddux) writes: > >In article <19970410174400.NAA11045@ladder01.news.aol.com>, >Gleshna wrote: >>Why would the period of waves increase as they travel? > >Good question. The increase of wave period within >a fetch is known as frequency downshifting -- a >decrease in frequency is an increase in period. [snipped lucid and learned explanation] This is the best explanation I've ever seen of this phenomena, Tim, thanks for the posting. I could easily visualize the whole cycle while reading your words, and actually had a quasi- religious experience of insight, imagining a huge storm in the southern hemi working up a batch of "fully developed waves" and sending them our way, watching them (from an enlightened perspective, with time as just another, equally apparent dimension) spread out over the vast ocean and dissipate and resolve over time into "lines of corduroy" striking our coast. What a stoke! Actually, we have had a couple of small souths hit So. Cal already this season, but nothing really big or clean yet. Maybe headhigh on the sets and each time with a WNW swell crossing it up. But it's a good start.... Ric- I hope you archived Tim's whole post for the FAQ- it should be in there. -------------------------------------------------------------------------- Tom Tweed La Jolla, CA e-mail: tweedt@ucsd.edu or tweedt@aol.com "Don't let your mouth write no check that your tail can't cash." -- Bo Diddley --------------------------------------------------------------------------