Fetch & wind strength to generate waves

[Doug_Rail]

I'm hoping someone could give some info on what fetch & wind strength is required to generate a ground swell. Any info including web sites would be great [/b]

[thermalben]

There's no single value that can be attributed to these parameters - groundswell is a combination of both fetch and wind strength, but also duration, origin, direction and intensity and proximity to the beach you plan on surfing. Also, the line between groundswell and windswell is a little blurry - groundswell is generally regarded as a swell with a period more than 11 seconds (others call 13 seconds). However, quality is dictated by the proximity of the fetch to your beach, and local weather effects. However, there's a couple of simple rules that increase the prospects of good groundswell.

Fetch: the longer and the wider, the better. A narrow fetch at 40kts can be less effective than a broader fetch at 35kts. But this also depends on the storm's movements - a fast moving front through the Southern Ocean with a broad fetch at 35kts might not have enough time to generate anything significant, however a slow moving East Coast Low or Tropical Cyclone will often pump sustained groundswell out from the same direction.

Wind strength: wave heights increase as wind strength increases, however this is an exponential relationship - there's only a small increase in generated wave heights from a ~20kt fetch to a 25-30kt fetch, but increase the wind speed to 35kts and you'll see a much more noticeable increase. As wind speeds are increased to 40/50/60kts, the generated wave heights is substantially larger (see pic below). The massive swells seen in the northern hemisphere usually result from 60-70kt winds, amongst a variety of other favourable factors.



However, this is also offset to some degree by the direction and origin of the fetch. An optimal setup would have the fetch aimed straight into the coastline (for as long as possible), but with each successive degree out of the swell window the swell size on the beach will be reduced. This is a quick simplification of the process though - there's a lot more to it than this.

In short, for a good 'groundswell' you're looking at at least 30kts (minimum) of fetch aimed into your beach for at least 24-36 hours. Size will vary depending on wind strength, direction and the origin of the storm (some swell windows for WA, SA, Vicco and Tahiti extend up to 8000km, with travel times betwen 8-10 days). However you also have to keep in mind that at any one time, the ocean could be serving up to 4 or 5 swells from different directions (well, not in NSW during the last couple of months anyway). How much each one is worth, their timeline and their strength can be difficult to pin down. Most often, the closest and most significant swells will override the effects of any small distant long-range groundswell.

There's so much more that can be written on this topic - I'm currently working on a variety of quick-info help topics at the moment that should be online in the next month or two (workload permitting).

[hammer time]

okay, heres what i don't get, for the last day or so we've had a small low sitting off the coast , and it looked as though them isobars were directed wind/swell our way. i know it wasn't intense or anything but it looked like something that should have produced some sort of wave.
ive seen cronulla at like 5ft+ with a similar looking synoptic map, but it had a better 'fetch' as someone said.

any tips on this ben?
>good job on swellnet dude.

[hammer time]

actually, hmm..
i think you just answered that before.

[thermalben]

Hammertime - yeah, I kinda mentioned it in another thread:

the low has actually been in the Tasman Sea since around Saturday, it's just quite weak in structure. As of yesterday is was around 330km off the coastline and moving SE'ly; Quikscat picked up a reasonable ESE flow off it's southern flank yesterday afternoon. We're likely to see some refracted swell filter in from this tomorrow, but it'll abate into the end of the week as the fetch slides further south. The only beneficiary from this system looks to the NE coast of Tasmania.

In actual fact, the South Coast and Sydney's exposed beaches might see a small upwards trend later today. But wind strength has only been in the 20-25kt category with isolated areas around 30kts - and with only a small fetch it's not likely to deliver much more than 2-3'+ at super exposed beaches at best. Still, it can only improve what's currently on offer.

[Doug_Rail]

Thanks Ben, really appreciate that info.

Another question: Is there a way of calculating the wind speed by looking at the synoptic chart? ie. distance between isobars/hpa.

[thermalben]

Yes there is, but it's a lot of work - you have to factor in latitude amongst other things, and there's easier methods to work out what the wind is doing. However, try here for some more info on Southern Hemisphere winds and geostrophic flow.

Better still, Quikscat is a useful tool to assess ocean surface winds, by use of a scatterometer aboard a satellite. A wide swath is covered every 11-12 hours or so, and helps to confirm swell generating areas.

[streetdaddy]

keep probing thermalben for those forecasting tips ppl! Soon enough we will know all his secrets, muhahahah

[filthbarrel]

http://buoy.ocens.net/wxnav.jsp?region= ... 10&units=e

Will this horrible emptiness end I hear you all ask,

By looking at the above -not in the next coupla days -interesting jack on 31st, but unimpressive period.

[Doug_Rail]

Thanks again Ben, great links.

[hammer time]

I give it to ya ben, the king of all surf and weather.

learning more every day.
keep the links flowing out. and uh them big informative posts don't hurt either. but when your ready ofcourse. take your time hey.

[dUg]

and it looked as though them isobars were directed wind/swell our way

I thought it might be worth mentioning "Great Circles" here, in that it's a mistake people often make when interpreting swell direction from Isobars on MSLP charts.

Remember the projection of the chart is an issue! If the chart is based on a Mercator projection you cannot "extend" the direction of the isobars using a straight line. If they look like they are pointing right at you on one of these charts, then in reality they are not due to the distortion. This gets more pronounced the further you are away from the source.