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11 January 2013

Sydney Wave Height, Period & Direction

Manly Hydraulics Laboratory wave rider buoy (Sydney)
See also the link to the animated version.

The plots show the direction from which the waves propagating, so for NSW the wave energy will mostly come from the top right (NE swell) to the bottom (S swell) of the plot. Long period waves are shown towards the outside of the plot, and short period waves show in the centre of the plot.
Directional spectra plots are able to represent all the waves, good and bad.

However, it does express the wave in terms of its power. Wave power is proportional to the wave period and to the square of the wave height. The trouble is that the single measure replaces the more familiar double measure of wave height and period.

But the measure is energy density m2/deg.Hz ((whatthe?)

From Wikipedia:
Wave power formula
Here power is expressed in kilowatts of power potential per meter.
In deep water where the water depth is larger than half the wavelength, the wave energy flux is

with P the wave energy flux per unit of wave-crest length, Hm0 the significant wave height, T the wave period, ρ the water density and g the acceleration by gravity.
When the significant wave height is given in metres, and the wave period in seconds, the result is the wave power in kilowatts (kW) per metre of wavefront length.

Example: Consider moderate ocean swells, in deep water, a few km off a coastline, with a wave height of 3 m and a wave period of 8 seconds. Using the formula to solve for power, we get

meaning there are 36 kilowatts of power potential per meter of wave crest.

In major storms, the largest waves offshore are about 15 meters high and have a period of about 15 seconds. According to the above formula, such waves carry about 1.7 MW of power across each metre of wavefront.

Look, we will sort this out and get back to you. Feel free to comment if you can.


Surf Forecasts

Our favourite forecast for surf in Sydney (and most of Australia) is:

LaJolla is handy for forecasts, you can pick a graphic for anywhere in the world:

There are links to other nice people that provide
this information:

Top view from Queenscliff looking along the beach at Manly NSW ... on Twitpic

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