Wind and Waves, Part II

Where do those waves come from, then, that aren't caused by the current wind conditions?  Those waves are called swells, and are generally the remnants of far-off storms, with periods exceeding 10 seconds.  Apparently, waves with short periods tend to dissipate within a couple thousand miles, but waves with longer periods can propagate halfway around the Earth, or even farther, as long as they don't run into a continent.  This is where the really good surfing waves come from, and helps to explain why the West Coast and Hawaii have better surfing than the East Coast.  This is why I bothered listing wave period on my earlier tables (don't worry, I haven't forgotten to post the rest of them).  And that's about where the college intro level of research ends.

To find out how fast waves dissipate and how far they propagate required digging in pretty deep.  This is the "professional" level of research (so my way of thinking goes).  It's also pretty much the "in over my head" level of research.  To get any kind of idea how to work up some game-able numbers, I had to spend several hours finding the exact combination of Google search terms that eventually led to the 1966 edition of thePhilosophical Transactions of the Royal Society of London, specifically the paper Propagation of Ocean Swell across the Pacific, by F. E. Snodgrass, G. W. Groves, K. F. Hasselmann, G. R. Miller, W. H. Munk, and W. H. Powers.  I then spent several more hours poring over 65 pages or so of measurements and equations that were absolutely too complex to use in game preparation, except for the most obsessed, masochistic gamemasters.  Eventually, I happened upon a statement that short period waves decay by about a decibel for every degree of great circle they travel, and later found a series of measurements that gave me some long period decay rates.  Interpolating to find approximate decay rates for wave periods that weren't specifically listed, and reading up on how decibels apply to ocean waves as opposed to noise levels, I've been able to come up with a short table that now shows a game-able approximation of the end of the life cycle of waves.

 

Period	Wave height as a proportion of original height
	90.00%	80.00%	70.00%	60.00%	50.00%	40.00%	30.00%	20.00%	10.00%	1.00%
<10	55	116	186	266	361	478	627	839	1200	2400
10	73	155	248	355	481	637	837	1118	1600	3200
11	110	233	372	532	722	955	1255	1678	2400	4800
12	137	291	465	666	903	1194	1569	2097	3000	6000
13	183	388	620	887	1204	1592	2092	2796	4000	8000
14	275	581	929	1331	1806	2388	3137	4194	6000	12000
15	366	775	1239	1775	2408	3184	4183	5592	8000	16000
16	549	1163	1859	2662	3612	4775	6275	8388	12000	24000

So, a wave with a height of 28 feet and a period of 8 seconds will have a height of 14 feet after 360 miles.  A wave of equal height but a period of 12 seconds will still be at about 75% of its original height (21 feet) after 360 miles, and will take another 540 miles (total of 900) to decay to 14 feet.

Ah heck, here's the same table, but instead of miles, it's in 24 mile hexes.  I didn't do 6 mile hexes like the last table because most maps don't cover hundreds of miles of ocean 6 miles at a time:

Period	Wave height as a proportion of original height
	90.00%	80.00%	70.00%	60.00%	50.00%	40.00%	30.00%	20.00%	10.00%	1.00%
<10	2	5	8	11	15	20	26	35	50	100
10	3	6	10	15	20	27	35	47	67	133
11	5	10	16	22	30	40	52	70	100	200
12	6	12	19	28	38	50	65	87	125	250
13	8	16	26	37	50	66	87	117	167	333
14	11	24	39	55	75	100	131	175	250	500
15	15	32	52	74	100	133	174	233	333	667
16	23	48	77	111	151	199	261	350	500	1000

I, of course, embarked on a much more detailed, time-intensive course of number crunching before I realized that this short version would probably work more easily.  And thinking about it now, it's probably better to just randomly determine swell on a given day with a 2-3d6 table.  But I think that table will be easier to come up with now that I have a better handle on what it's supposed to represent.