An open-channel conveyance system is a hydraulic system in which water flows with a free surface exposed to the atmosphere, rather than being fully enclosed under pressure as in a pipe. In stormwater management and hydrology, this term refers to natural or constructed channels that transport runoff, streamflow, or drainage water from one location to another primarily by the force of gravity.
Open-channel conveyance systems include features such as streams, rivers, ditches, swales, canals, and engineered channels. Flow within these systems is driven by the energy gradient, typically the slope of the channel, and is resisted by friction along the channel bed and banks. Because the water surface is open to the air, flow behavior is influenced by gravity, channel geometry, roughness, and depth, rather than internal pressure.
Hydraulically, flow in an open channel is characterized by parameters such as velocity, depth, cross-sectional area, hydraulic radius, and slope, and is commonly analyzed using equations such as Manning’s equation. The interaction between flow and the channel boundary is critical, as it determines shear stress and therefore influences erosion potential and channel stability.
From a stormwater management perspective, open-channel conveyance systems are often preferred over closed systems in certain applications because they can reduce flow velocity, promote infiltration, provide water quality treatment, and offer ecological and aesthetic benefits. For example, vegetated swales are a type of open-channel system designed to both convey and treat runoff.
However, open-channel systems must be carefully designed to prevent issues such as erosion, sedimentation, and flooding. This often involves evaluating flow capacity, permissible shear stress, and channel lining or vegetation to ensure long-term stability.
Basically stated, an open-channel conveyance system is any natural or engineered water conveyance pathway in which flow occurs with a free surface under atmospheric pressure, governed primarily by gravity and channel characteristics.