Accordingly, one of the things that we as divers need to consider is the weather and more importantly, the sea conditions (of course I am writing about Australia where 99.99% of dives are done in the ocean or large bays or harbours.

The sea conditions can affect divers whether you are diving from the shore or on a boat. The following is mainly related to boat diving and does not consider the weather conditions (eg wind speed or direction). While some sea heights may not be large, the type of waves can be totally unsuitable to taking a boat out in the open seas or even on the bay or harbour).

Waves can be big or large, rough or smooth. Off Sydney (where I live), the seas can vary from 0 metres to well over 12 metres at times. Of course the large waves are dangerous and would be unsafe for boating or diving. At the same time, small waves of only one metre can be unsafe for boats if the waves are very choppy. Likewise, larger waves of 4 metres can sometimes be safe for boat if the wave is smooth and a long rolling one. However, this wave is probably not suitable for diving, even at depth (which I will show later).

Some stories first. Back on Australia Day 1996 (26 January), I did a dive on the wreck of the Tasman Hauler south of Eden. We had a reasonable dive. The conditions on top were a very small swell and at the bottom (30 metres) it was comfortable. After lunch we came back for a second dive. The conditions on the top appeared similar to the first dive. However, on the bottom the scene was totally different. There was a huge surge, so much so that the visibility had dropped from 5 metres or so to less than a metre and I actually was physically thrown off the wreck and could not find it again.

The difference between the dives was that the period of the wave (the time between the wave peaks) had increased dramatically. My guess is that it had changed from about 10 seconds to about 15 seconds in the five hours between the dives.

Another example is the damage caused to wrecks like the SS Tuggerah which is at over 45 metres. I have dived the wreck a week after huge seas and found pieces of steel twisted by the wave action. Even diving this deep we have at times been pushed 5 or 10 metres backwards and forwards by waves that appeared to be not too big.

HOW DEEP DOES A WAVE GO

How to work this out? Well, there is a formula that can tell you how deep a swell will create surge. It is (WP x WP)/4 where WP is wave period in seconds. The answer is in metres. So, a simple example is if the period is 10 seconds you have (10 x 10)/4 = 100/4 = 25 metres. This is the deepest the swell should be felt, so for 10 second periods, you should be okay at 25 metres but at 15 metres it will be fairly surgy.

WL=1.56xWPxWP where WL is wave length and WP is again wave period. So for 10 seconds, this is WL=1.56 x 10 x 10 = 156 metres. This is the length between each wave peak. For 12 seconds it is WL=1.56 x 12 x 12 = 225.64 metres.

The speed of the wave can be calculated as follows: V=1.56*WP*60*60/1000 where V =velocity in km/h, WP = wave period. So, again for 10 seconds, you have 1.56 x 10 = 15.6 metres per second. Then 15.6 metres/second = 15.6 x 60 x 60 = 56,160 metres per hour or 56.16 km/h. For 12 seconds, you have V=1.56 x 12 x 60 x 60 = 67,392 metres per hour or 67.39 km/h.

Of course, if you are crossing a bar in a boat (especially coming in from the ocean), the longer the period the faster the wave will be going, meaning the faster you will need to go in your boat to stay on the back of the wave (which is the safest spot). However, once the waves get into shallower water, they will actually slow up as the the period will lessen due to the shallowness of the water. I will not go into that formula here.

Well, have a look at the next table.

Wave Period Seconds	Wave Length Metres	Wave Speed Km/h	Depth of surge Metres
8	100	45	16
10	156	56	25
12	225	67	36
14	306	79	49
15	351	84	56
16	399	90	64
17	451	96	72
18	505	101	81

OTHER IMPACTS CAUSED BY PERIODS

The other impact longer wave periods can have is that the longer the period, the more a wave can wrap around a headland or island and then come from a totally different direction to the original swell.

This is caused wholly by the fact that the wave itself is deeper than a short period wave. What happens is that the wave "bottom" hits the ocean floor and this slows the wave up. Meanwhile, the wave further away from the shore in deeper water is now going faster. The slower section of the wave then tends to turn towards this side and if the period is long enough to cause a major interaction with the ocean floor, the swell can change direction by 90 or even almost 180 degrees.

You can see this at any location where waves are hitting a headland or island. As I mentioned, the impact is far worse the longer the period and/or shallower the water where the swell is near a headland or island.

I do not have any formula to help work this out, you need to understand that any wave period over 12 or 13 seconds will probably impact any dive site near enough to the open ocean, even if well inside a bay or harbour.

Boating
If you have access to a wave recording buoy via the internet, then have a look at the graph for the area you are planning to dive and see what the period is. If the seas are over about 1.5 metres and the period under 6 seconds, then I would not take my boat out as it would be far too choppy. Smaller waves with shorter periods would also be unsuitable. See later about diving.

Diving
For diving, the effect is not so obvious . Again, look at a wave recording buoy. If you do not have access to one, then an easy way to work it out if you are out on the water is to count the time between waves reaching you once you are stopped or anchored. Then, use the table above. If you do not have the table, then square the time and divide by four. This gives a rough result as above.

Therefore, if you are planning to dive the SS Myola or SS Birchgrove Park which are both at about 50 metres, then you need the period of the waves to be under about 15 seconds. At 15 seconds you would feel some surge. Under 13 seconds you should feel none.

If you were planning to dive Middle Ground (just over 30 metres), then the 15 second period would be very uncomfortable and 12 seconds would mean you could feel the surge a bit. Ten seconds would have no surge. Also note that there while most wave periods will be constant, there will be some that are longer than the average period which will reach deeper. On 11 May 2011 I dived Middle Ground and there was a period of 8 seconds average but some were 12 seconds. We could occasionally feel the surge.

As to the refraction of waves, again, if it is a long period (and certainly if also a large swell), probably protected sites near the mouth of Sydney Harbour, Botany Bay and Port Hacking will be affected as well as Shelly Beach.

Of course, this is all theory, but for us seems to work.