In a lined channel or pipe, what is the lowest part of the internal cross-section called?

The term used to describe the lowest part of the internal cross-section of a lined channel or pipe is "invert." This is significant because it plays a critical role in the design and operation of drainage systems, sewer systems, and any fluid-carrying pipes. The invert is the point where water begins to flow, and understanding its position is essential for ensuring proper drainage and preventing issues such as backups or blockages. In a practical sense, the elevation of the invert can dictate how gravity will influence the flow of water through the system, thus affecting overall efficiency and performance.

The other terms provided, while related to channels and pipes, do not specifically refer to this lowest point. The "flow line" typically refers to the line that represents the water's surface height, not the lowest internal part. "Base" might suggest a foundational component but lacks the precise definition related to fluid flow. "Channel floor" might imply the bottom of a channel, but it is not the specific terminology used in engineering and plumbing contexts when referring to the lowest point of a pipe or lined channel.