"Swing the sickle for the harvest is ripe..." (Joel 3:13). 14:14-16 sounds like the harvest of God's people yet on earth. "Multitudes, multitudes in the valley of decision..." (Joel 3:14).
14:17-20 is a harvest (destruction of) the disobedient on the earth. 1600 stadia probably is a symbol comprised of 4x10x4x10, meaning: the world times completeness (or complete destruction of the world times complete destruction of the world = total destruction). This is described in more detail in chapters 15-18.