Swell Directions

philosurphizingkerching's picture
philosurphizing... started the topic in Tuesday, 7 Jun 2011 at 7:11am

One aspect that is crucial to surfing reef designs is swell direction.

We need to understand swell direction angles and how the different angles react to a surfing reefs orientation.

What is the difference in angle between South, Southeast, East and Northeast swell directions.

Here is a scenario.
A long east facing beach.
An east swell approaches at zero degrees, parrallel to the beach.
What angle does a SE swell aproach at.
What angle does a South swell approach at.
And what angle does a NE swell approach at.

Anyone got any opinions or theories on this subject?


bob_s's picture
bob_s's picture
bob_s Wednesday, 8 Jun 2011 at 6:58am

this guy has a series of videos on youtube explaining coastal process's
Its a good place to start and your own interest will have you looking further.

go here for coastal process explanations

fredie-c's picture
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fredie-c Thursday, 9 Jun 2011 at 8:18am

The better way to deal with all the variants is to build an adjustable reef structure, it will add maybe 30% to the cost, but will create at least 3 times more surf time AND, save a heap of study time and guesswork not to mention dealing with differences of opinions.

The 3 main reasons would be swell direction, tides and swell size. the mechanism for swell direction could be self adjusting as in the mechanism for tidal action, but to deal with swell size needs special sensors incorporated with the tide adjusting mechanism.

On headland modifications,, does anyone think permission would ever be granted for such works? There is a design for a parallel reef on the sand that would not disturb the original break. I think there lies the most cost effective and rewarding works, but a beach A frame will be first tried .

beavis's picture
beavis's picture
beavis Friday, 10 Jun 2011 at 6:13am


The way in which different waves propagate, refract and dissapate in response to nearshore bathymetric features can all be modelled using wave modelling programs such as SWAN, SWANSURF and others. These account for different swell sizes and directions and different bathymetric features. Where the data is available, the modelling is accurate and comprehensive.

An adjustable reef idea is interesting but sounds expensive and more prone to breakage. To create a reef large enough to produce a decent wave and be adjustable sounds like a mammoth task.

philosurphizingkerching's picture
philosurphizingkerching's picture
philosurphizing... Friday, 10 Jun 2011 at 6:54am

I agree, an adjustable up and down reef is not beyond the realms of possibility.
Adjusting sideways to suit swell direction would be much more complex.
I wonder if somehow the power of tide movement could be utilised to raise and lower a reef.
After all isn't tidal power used to turn turbines to generate electricity.
So many possibilities.
Working out swell direction angles doesn't have to be guess work, but surfing reef designers need to come up with some ball park numbers.
Google earth shows up the swell angles off our beaches.
One of the characteristics of existing reef breaks is that each work on a variety of swell directions.
Some places work on only one direction, some on two directions, some on three directions and places like the Superbank where there is south swell refraction work on all directions.

If for instance I throw up a hypothetical number and say there is a 5 degree difference between a south swell and a southeast swell, and a 5 degree difference between a SE and East, and a 5 degree difference between East and Northeast.
This would mean a 15 degree difference between a south and a northeast swell.
This would mean completly different wave peeling characteristics on a reef.
On one direction you might have a fast peeling wall and on the other direction a shouldery ordinary slow fat wave.
Therefore the direction a surfing reef is orientated is crucial but can be worked out before building.

Regarding headland modification, headlands or groynes that stick out a long way are the canvases where surfing reef designers will create masterpieces.
Start off small with an A frame reef and learn from feedback before going big.

philosurphizingkerching's picture
philosurphizingkerching's picture
philosurphizing... Friday, 10 Jun 2011 at 7:02am

Gidday beavis,
Do you know of a source where there are some actual numbers that could be projected onto the scenario I proposed?

beavis's picture
beavis's picture
beavis Sunday, 19 Jun 2011 at 12:05pm

Hey philosurphizingkerching,

to run one of the equations you need the bathymetric data and to learn how to run the program. You can download the SWAn program online fro there site, but it is very complex and operates in code. So Maybe you'll have moe luck than I did!

Coastalwath has a page with a SWAN diagram on their site which updates with the conditions, worth looking at

philosurphizingkerching's picture
philosurphizingkerching's picture
philosurphizing... Monday, 20 Jun 2011 at 6:24am

I tried typing in the link you posted but the address is wrong.
Check out my post on how to make a link active.
It's fairly easy to do.