A geometric property of a pipe or open channel that determines its flow efficiency and resistance. Despite the name, it is not half the diameter; for a full circular pipe, it equals one-quarter of the diameter. Engineers use this metric in the Manning equation to calculate flow velocity and shear stress along the channel walls.
Heard on the job
The hydraulic radius drops significantly when the culvert is only running at ten percent capacity.
Insider tip
A larger hydraulic radius means less frictional resistance per unit of fluid volume, which translates to higher flow velocity.