## Surfing Vancouver Island## Lud's Wave Prediction FAQ |

cold, 20 knot offshore, stormy, overhead, awesome March day at Spot C

G'day all, For those of you wanting to try your hand at surf forecasting, and gain a greater insight into waves as an extra benefit, all the info is out there in various meteorology and oceanography books. Could be something to do during those long flat spells :-). Some of the books I've come across that have usefull chart's and tables are: Oceanography, second edition Dietrich, Kalle, Krauss, Siedler. Prentice Hall. (Good chart which shows how far a swell has travelled, how long it has travelled, and what the conditions were in the swell generating area, ie wave size and wind strength, from the size and period of the swell at your location! Great for trying to correlate swell conditions today, with the weather maps a number of day's back, ie identifying weather features which may have generated the swell. GOOD) Introduction to physical oceanography Knauss Prentice Hall (Chart showing wave height vs windspeed in the generating area, also fetch lenth and min. wind duration's required) Estuary and Coastline Hydrodynamics Engineering Societies monographs Idden, Arthur T. ed. McGraw Hill (Wave generation and decay chart's, GOOD) Physical Oceanography of coastal waters K.F. Rowden Ellis Horwood Ltd. Halsted Press (Wave generation chart for shallow coastal waters) Proceedings of the International Synposium on Ocean Wave Measurement and Analysis, Volumes 1 and 2 Published by the American Society of Civil Engineers (All sorts of general papers on ocean waves etc) After reading a great pile of these, (only the relavent stuff mind you, the maths and other junk gets a bit heavy in some of them, but they all seem to have snippet's of usefull information buried away including all sort's of usefull chart's, tables etc.), I was suprised by how relatively clearcut the whole thing of swell prediction, and wave statistic's, sizes etc is, even though it's a little to complicated to describe in one post. There seem to be a couple of different methods used, and both give slightly different answers, both methods seem to be a mixture of theory, tempered/modified by actual wave statistic's. (They also tell you how to measure the size of a wave :-), and, it's not from the back :-), but these guy's are just oceanographer's, so what would they know :-), sorry, just couldn't resist stirring the frontside vs backside wave measurement argument again :-) ) Anyhow, here is a distillation of some of what I've gleaned, and how I use it in my attempts at predicting surf. With no background in meteorology, or oceangraphy, apart from a lot of reading, and bobbing around on surfboard, off and on for quarter of a century, I can't really vouch for the accuracy of what follow's, so if you adopt a similar procedure, and after a three hour drive, your favourite spot is flat, instead of the predicted double overhead, I don't want to hear about it :-). Simplified description of steps required to predict the surf are: (For all the fine detail check the book's, and references within, listed above). 1/ From a weather map that cover's your area of ocean, locate an area on the ocean that is capable of producing swell, ie any area of ocean, where there are winds of reasonable strength, blowing for a reasonable duration and distance. ie, Isobars close together, for a reasonable distance, hundreds of Km. minimum, but better if Km's are in the 000's.) This could be anything from a cold front to a hurricane to a prevailing wind etc., probably one particular weather pattern will be the main swell producer in your area, and that will probably vary throughout the year. Obviously the swell/wind must be heading in your direction, though you can make a rough allowance for swells not heading directly towards you, say within 30 degrees. (Reduce predicted swell size by cos * angle for this case) To estimate the fetch length (Fetch is the distance the wind is blowing over, at about the same strength and direction), do a scale measurement off the weathermap, of the distance where the isobars and wind are running roughly in your direction (within, say, 30 degrees). 2/ From the wind strength, duration, and fetch length, one can calculate, or use a chart/table to estimate the size of the sea's at that point, and also the period of the waves, and hence their speed, and therefore swell arrival time !. The main problem is to get the windstrength. For the wind direction, I make the assumption that it is parallell to the isobar's, and the usual direction, clockwise around the low's, anticlockwise around the highs for the southern hemisphere, and vice versa up north. Our weathermaps don't show the windstrength over the ocean, so what I do is use a formula that gives an approximation of the windspeed, based on the spacing of the isobars, ie air pressure difference per km, and the latitude: Geostrophic Wind = (delta P/ delta Km) / ( 2 * R * p * sin(L) ) where windspeed is in M/second, (Multiply by two to give Knots) Delta P is the difference in airpressure, in mBars/10 , across the widthof the fetch, and Delta Km is the width of the fetch in Km. -5 R is the earths rotation, 7.3 * 10 rads/sec 3 p is the air density, 1.3 kg/m L is the latitude All this simplifies down to : Windspeed in knots = (( mb per km * 10000)/(19 * Sin Latitude)) * 2 To get the windspeed at sea level, this windspeed must be reduced further, multiply it by 0.7 for example, fetch width=1000 km, air pressure on high pressure side =1020 mb, on low pressure side =990mb, therefore mb per km =0.03, latitude =45 degrees. so windspeed in knots = ((.03*10000)/(19*sin(45)))*2 = 44.6 knots multiply by .7 for sea level windspeed = 31.26 knots From the Table below, as long as the wind duration has been at least 24 hrs, and the fetch > 290 Nm, the mean wave height would be 4.5 metres at the end of the fetch. The significant period is 12.4 seconds Here are some typical figures for wave generation in "deep water": Wind Min. Min. Sig. Wave H1/10 Hmean Speed Dur. Fetch Period Length Height Height Knots Hrs. Nm Seconds Metres Metres Metres 11-16 5 24 3.9 47 1.12 .55 17-21 9 65 7.7 93 2.7 1.3 22-27 15 140 9.9 153 5.2 2.5 28-33 24 290 12.4 240 8.8 4.5 34-40 37 510 14.9 345 14.2 7.0 41-47 52 960 17.7 490 22.2 11.0 48-55 73 1510 20.8 675 32 15.8 56-63 101 2500 24 1060 45 22.2 H1/10 is the is the average height of the 10% of the highest waves, and roughly correlates with the max size of a wave group (set) that can be expected. Hmean is the mean hight of all the waves. The period and wavelength are those for the significant wave (H1/3) which is the average height of the 1/3 highest waves, which is roughly where the majority of wave energy is, and is approximately what an observer would call the wave height of a particular swell. Wave height is measured from TROUGH to CREST. The calculation's, chart's etc. are different for swell's generated by localised high intensity winds with short fetches, ie hurricanes, intense low pressure systems, vs long duration/fetches ie cold front's. There are formulae for hurricane wave height's. Swells generated in shallow water, (continental shelf ?) require different chart's than swells generated in deep water. 3/ From the above figures you can now (more charts :-) ) obtain a decay factor to indicate the final swell size as it approaches your break. Here are some typical figure's, but you really need the charts to be more accurate, but these should serve as a rough guide. Again, these figures are for "deep water". Swell Decay Decay Swell Decay Decay Height Dist. Factor Height Dist. Factor Feet Nm Feet Nm 5 50 .73 10 50 .75 5 100 .6 10 100 .62 5 200 .47 10 200 .5 5 500 .35 10 500 .36 5 1000 .25 10 1000 .27 5 2000 .125 10 2000 .18 5 4000 .1 10 4000 .12 5 10000 .05 10 10000 .055 Swell Decay Decay Swell Decay Decay Height Dist. Factor Height Dist. Factor Feet Nm Feet Nm 20 50 .77 40 50 .78 20 100 .64 40 100 .65 20 200 .525 40 200 .54 20 500 .38 40 500 .41 20 1000 .285 40 1000 .305 20 2000 .165 40 2000 .2 20 4000 .14 40 4000 .15 20 10000 .07 40 10000 .08 Note the swell size and final wave size will not be the same, as the swell moves into shallow water, it first decreases in size, and then increases in size, by a factor of 1.5 to 2 before it break's, so to get the final wave size in the surf zone, multiply the decayed wave size by 1.5 to 2. The wave height is still from trough to crest. An interesting point regarding the wave shape in shallow and deep water, In deep water the swell shape is similar to a sine wave, in that the crest elevation, and trough depression are more or less symetricaly displaced with respect to the mean water level. But as the waves move into shallow water, ie surf zone, the crest's actually increase in height, and the wave troughs flatten out, so that what you have in effect, is an area of relatively flat water, between the crests, with a series of solitary, much more peaky crest's comprising the set waves. The majority of the height increase as the waves move into shallow water, comes from the crest elevation above the mean water level, eg, for a 20 ft wave, (trough to crest), 17 to 18 feet would be crest elevation above the mean water level, and 2 to 3 feet would be the trough below the mean water level. The height increase comes about, because the waves bunch up, ie the wave speed and wavelength decrease, but the period remains the same when the waves enter the shallower water, that same wave energy is now contained in a shorter distance, and hence the height increases. Swell direction and local geography, bottom topography etc. will also alter the final wave size, and also if you get any of the swell at all (swell windows). Wave refraction must also be taken into consideration, but it gets too complex, it's easier to incorporate a local "fudge factor" to allow for these differences. 4/ Swell Travel time. From the period of the wave you can determine the swell travel time, from fetch to your location. Note that the speed of a wave group (set) is half the speed of individual waves (for deep water) ie the swell itself progresses at half the wave speed. You can see this by watching a set of waves in deep water from a high vantage point, if you watch the first wave of a group, you will see it dissapear !, only to pop up at the back of the set, this keeps repeating until waves get to shallow water. It's sort of like taking two steps forward, and one step back. The group speed (in Km/hr.) = 2.8 times the period (seconds) So to predict the travel time of the swell, multiply the wave period from the above chart by 2.8, which will give you the group speed in km/hr, divide that by how far away the fetch is from your location (Km) and you have an approximate travel time for the swell. (For deep water conditions only) For shallow water the wave group speed, and the wave speed are the same, see books for formulae. 4/Period Increase, as the waves move out of the generating area and start to decay (swell), the period also increases. Here are some typical values, again original chart is required for accurate figures. Wave Decay Period Period Dist. Increase (sec) Nm factor 10 100 1.14 10 500 1.23 10 1000 1.33 10 2000 1.38 10 5000 1.45 10 10000 1.49 12 100 1.15 12 500 1.29 12 1000 1.36 12 2000 1.42 12 5000 1.5 12 10000 1.54 14 100 1.15 14 500 1.31 14 1000 1.4 14 2000 1.45 14 5000 1.54 14 10000 1.57 Note, the definition of "shallow" or "deep" water in wave terms, is defined as the water depth relative to the wavelength of the swell, if the depth is greater than 0.5 times the wavelength its deep water as far as the wave is concerned, and the wave does not feel the bottom, and if the depth is less than 1/20 the wavelength, its shallow water. Inbetween these two values it's intermediate, a different set of formulae apply in each of the 3 case's for wave speed etc etc. The period (time between waves) is important because in conjunction with the size of the waves, it can tell you how from far away the swell originated, how long the swell has travelled for, and what the swell size, (more accurately the Sea size, as swell is defined as waves that have moved out of the generating area) and wind strength were at the swell's origin !, pretty amazing I think. (From a chart in oceanography book listed above, too difficult to tabulate) To measure the period, it's best to average out the duration of a set, ie number of waves in the set, less one, divided by the time taken to pass some reference point, could be a rock, end of pier, or even the beach itself. It doesnt matter what reference point you use to time the passing of the waves, as the period remains constant, regardless of water depth, though the wave speed and wavelength do vary. There will be some variation in the period from set to set, but drastic differences could indicate two different swell trains. The weather maps ideally need to cover a reasonable section of the ocean, otherwise swell's can arrive from outside that area, which obviously can't be predicted, but it's nice to be suprised by a decent groundswell coming from many thousand's of miles away anyhow :-). That's it! :-), Iv'e run out of patience, RSI has set in to my two typing finger's , I'm going for a surf, there's a few more things to say but that will have to wait...... some-one else will have to proof read this, predicting head high, 15 sec period and offshore winds :-)........ Cheer's, keep surfing, (or predicting if it's flat :-) ) Lud. -- \|/ ^o^ --O-- \______ \\______ \\\______ \\\\______ ^o^ ______// /|\ ^o^ Ludwig Omachen ludwig@deakin.edu.au Deakin University 3217 Geelong, Victoria, Australia. _ Phone: BH 61 52 272879 .' `. AH 61 52 612061 .-. / (`.\ ,-. / \ / `.` ~~~~~~~~^~~~~~~~~~-' `~~~~~~~~~-' `~~~~~~-' ` -~~~~~~ >';))) =-{ _____________________,================ ______________________________/ Celerity tells no lies ........Copyright (C) 1996, Ludwig S. Omachen, All rights reserved.