tfFlood operator applied to the terrain on the right.
tfFlood, Properties tab options menu.
This setting controls the flood height.
Flooding will only occur in areas below this setting.
This function can also be achieved by manually lifting the Y-axis in the viewport.
On the left, the Altitude setting is the default 10cm, raised to 65cm on the right-hand terrain.
Enabling this parameter, will clip the terrain height, instead of filling up (flooding) valleys, to the setting in the Altitude parameter.
Again, this can be done manually in the viewport.