Drainage and sewer network runs mostly under free surface flow condition. Among the characteristics which are important for practitioners are the flow velocity and water surface angle. The computation of these parameters in partially full pipes using Manning equation is implicit and requires iterative and laborious calculation methods. The goal of this paper is to provide a new method, where the exact computation of the flow velocity and water surface angle in partially filled pipe becomes easy, direct and simple using a reference pipe with known characteristics.
Keywords: water surface angle, free surface flow, uniform and steady flow, circular pipe, Manning equation