Storm sewers are closed conduits but usually maintain a free surface and therefore are considered open-channel flow.
In order to conceptualize head along the course of flow within a pipe, diagrams often contain a hydraulic grade line (HGL).
Pipe flow is subject to frictional losses as defined by the Darcy-Weisbach formula.
Depending on the effect of viscosity relative to inertia, as represented by the Reynolds number, the flow can be either laminar or turbulent.
For circular pipes of different surface roughness, at a Reynolds number below the critical value of approximately 2000[2] pipe flow will ultimately be laminar, whereas above the critical value turbulent flow can persist, as shown in Moody chart.
[3] Earlier transition to turbulence, happening at Reynolds number one order of magnitude smaller, i.e.