A two-phase flow regime that occurs in horizontal or slightly inclined pipes and channels where the liquid phase flows as a layer at the bottom of the channel with the gas phase above. This type of flow occurs at low gas velocities where separation of liquid and gas has occurred in which the liquid flows in the lower part of the pipe with the gas above it. At low gas velocities, the liquid–gas interface is smooth without ripples. At higher gas velocities, ripples and waves form on the liquid surface, eventually leading to the breakdown of this flow regime.