Velocity of flow in mains is usually expressed in terms of...

The velocity of flow in mains is typically expressed in feet per second because this unit provides a clear and straightforward measurement of how fast water is moving through a pipe or conduit. This measurement is critical for engineers and water resource managers to evaluate the efficiency and effectiveness of water distribution systems. It allows for the analysis of flow rates, pressure losses, and the design of components to ensure adequate water transport.

The other options do not provide an appropriate measure of flow speed. Gallons per minute is a volumetric flow rate, which indicates how much water is being moved over time rather than the speed of the water in a specific section of pipe. Liters per foot is not a standard measure used in hydrology as it mixes units inappropriately, while milligrams per liter refers to concentration and is used in water quality analysis, not flow velocity.