Freeciv

From comments in terrain.ruleset -

; Percentage of "land" tiles required to be adjacent to a water tile before
; it may be "reclaimed" into a land tile (0-101; 0=anywhere, default 101=nowhere)
ocean_reclaim_requirement = 30

; Percentage of "water" tiles required to be adjacent to a land tile before
; it may be "channeled" into a water tile (0-101; 0=anywhere, default 101=nowhere)
land_channel_requirement = 10

If relevant, we should say how many adjacent land/water tiles are required to transform a tile.

Complication - terrain.ruleset file may include a transform from a land to ocean, or from ocean to land,
which then is not possible because the value of land_channel_requirement or ocean_reclaim_requirement
prevents it. If there is such a conflict, the *_requirement wins.