(Relationship between wave period and water depth that will cause waves to get steeper)

from PonoBill on Standupzone.com

“The primary reason swells roll under riders (paddlers) is that they are moving too fast and have no definition or steepness on their face. The best riders accelerate their boards with small swells, hop to medium sized ones and then get the biggies. You really can't paddle fast enough to get the big on your own. If there isn't enough of that kind of texture you're not going to make it no matter how good you are.

When swells start to be affected by the bottom they slow down, They contact a sloping shore at the front first, of course, so the energy at the back starts to catch the energy at the front and they stack. If the depth varies, as it does in our runs which are parallel to the shoreline, then the swells stack and unstack–something we've all seen. But nothing makes them speed up again. At Maliko the swell has been traveling hundreds or thousands of miles through very deep water where there's no slowing. Swells would dissipate, but there's steady trades to keep them rolling. So they hit the gulch as big rollers 15 feet high and then slow and stack/ unstack (thank god) over places like Camp One.

The depth of water where they start to be affected is a function of wavelength. From that article above (Sean Collins in Surfology): 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.

Besides the primary swells there's those beneficial smaller, slow swells that we count on to get rolling. They need shallow water to peak up and have a face we can use. Those little texture waves have a very short period–like 1 to 5 seconds, so they won't get peaky in anything much more than 30-50 feet.”