Hi ascard,
you just have to ask yourself: what is a beach.
Water comes kind of automatically in Tg, you just add a lake and give it a waterlevel. done.
Then a beach is a layer of sand or gravel which accumulates near the water in flat places. That sounds like a surface layer could be useful, because those can be applied to certain altitude-and slope values.
Basically a beach in terragen is a sandcoloured layer that starts at waterlevel and appears in flat places. You can do a lot to make it more realistic, a wet strip near the water, tiny fakestones, breaking waves (not easy)...
But for a start, thats what you may want.
http://forums.planetside.co.uk/index.php?topic=9306.0keep going!
Best regards,
Jan
By the way, nice low clouds in this one!