terrain.elevation_plainElevation
What it does
Despite the official name, this is really the line where beaches END and normal ground begins. Everything below the line is drawn as flat beach; everything above starts getting the plains texture. Raising it moves both together.
What you seeRaise it and the flat sandy apron around every coast widens while the bumpy ground retreats inland, lower it and terrain starts almost at the water's edge with barely any beach.
Gotcha
The official description says “height of the plains” — it's actually a starting line, not a height. At exactly 0 the beach band disappears entirely.
See it on a real map
Reference seed 12345, size 4500. “Try live” opens the generator with the value applied.
default 0.05
0 — beaches vanish, relief at waterline try live →
0.2 — wide flat coastal aprons try live →
0.5 — huge beaches, plains start late try live →