I only coverted that
slight detail mere factor of around 22kph compared to 401 kt = 0.51444 ms-1SAsurfa wrote:Yes Darryl, but here is Longy's maths...
Speed = squareroot ( 10x 13 ) = 11.4
Now that's 11.4 m/s
Not 11.4 knots as he has said
11.4 m/s converted to knots is 22knots
See what I'm getting at...
Because I think in mph or kilometres per hour,
I converted 11.4m/s to kph:
11.4x3600=41kph (somehow this turned into 40kph before, just to keep this confusing).
This is similar, converting 11.4 knots to kps:
11.4kt(.51444m/s per knot)x3600=21kph (somehow this turned into 22kph before, just to keep this confusing)
Agreed Craig, they aren't the same, still there's something missing in my thinking, apples and oranges, between converting m/s to knots, and a connection with Longi's last post where he's talking 22knots and 40 knots,
and the surfology quote which is what's below, it's about: swell period, ocean depth and wave height:LONGINUS wrote: Okay maybe this is a better way to explain it. So using the simple shallow water equation that SA has used, you get a 'speed' for a theoretical given 'point' where the depth is uniform for the entire 'wavelength'. The reason you can use this formula for tsunamis in the open ocean is that their wavelengths are so long that everywhere is effectively 'shallow water' for them.
The problem in trying to use a simple formula for a breaking wave is that the wavelength is likely to be laid over a vastly altering depth profile and gradient. Incredibly steep ramp up from the 200m contour to a few feet at Teahupoo, far less gradual in your average beachbreak. Thats why those two waves look very different, the exact same period and height of swell will break several times faster at Teahupoo than it would at Manly.
So in fact, going back to our example at Manly, both answers are correct. somewhere on that wavelength, the 'wave' is travelling at both 22 knots (closer to the shore) and 40 knots (out the back before it has slowed and broken). So we are looking for a simple 'rule of thumb' to give us some guidance here, nothing more.
surfology wrote:# Swell period and ocean depth. The depth at which the waves begin to feel the ocean floor is one-half the wavelength between wave crests. Wavelength and swell period are directly relative, so we can use the swell period to calculate the exact depth at which the waves will begin to feel the ocean floor. The formula is simple: take the number of seconds between swells, square it, and then multiply by 2.56. The result will equal the depth the waves begin to feel the ocean floor. A 20-second swell will begin to feel the ocean floor at 1,024 feet of water (20 x 20 = 400. And then 400 x 2.56 = 1,024 feet deep). In some areas along California, that's almost 10 miles offshore. An 18-second wave will feel the bottom at 829 feet deep; a 16-second wave at 656 feet; a 14-second wave at 502 feet; a 12-second wave at 367 feet; a 10-second wave at 256 feet; an eight-second wave at 164 feet; a six-second wave at 92 feet and so on. As noted above, longer period swells are affected by the ocean floor much more than short-period swells. For that reason, we call long-period swells ground swells (generally 12 seconds or more). We call short-period swells wind swells (11 seconds or less) because they are always generated by local winds and usually can't travel more than a few hundred miles before they decay. Long-period ground swells (especially 16 seconds or greater) have the ability to wrap much more into a surf spot, sometimes 180 degrees, while short-period wind swells wrap very little because they can't feel the bottom until it's too late.
# Shoaling. When waves approach shallower water near shore, their lower reaches begin to drag across the ocean floor, and the friction slows them down. The wave energy below the surface of the ocean is pushed upward, causing the waves to increase in wave height. The longer the swell period, the more energy that is under the water. This means that long-period waves will grow much more than short-period waves. A 3-foot wave with a 10-second swell period may only grow to be a 4-foot breaking wave, while a 3-foot wave with a 20-second swell period can grow to be a 15-foot breaking wave (more than five times its deep-water height depending on the ocean floor bathymetry). As the waves pass into shallower water, they become steeper and unstable as more and more energy is pushed upward, finally to a point where the waves break in water depth at about 1.3 times the wave height. A 6-foot wave will break in about 8 feet of water. A 20-foot wave in about 26 feet of water. A wave traveling over a gradual sloping ocean floor will become a crumbly, slow breaking wave. While a wave traveling over a steep ocean floor, such as a reef, will result in a faster, hollower breaking wave. As the waves move into shallower water, the speed and the wavelength decrease (the waves get slower and move closer together), but the swell period remains the same.
Tomorrow looks good as well, and Wednesday, and.... this spring rocks!!
cnuts 