A Laboratory Study of Rill Hydraulics: I. Velocity Relationships

Abstract

ABSTRACT RILLS caused by runoff over an erodible soil are usually very irregular in profile and cross section and may not approximate any regular geometrical shape. Therefore, hydraulics of flow in rills may differ greatly from the hydraulics of flow in larger channels. A full-scale fiberglass, fixed bed, replica of a rill formed on an erosion plot by runoff from simulated rainfall was constructed and used in laboratory studies of flow hydraulics in a rill. Average velocity at a section was very nonuniform along the rill because of nonuniform slope and cross sections along the rill. The distribution of these velocities along the rill was described by a normal distribution. Mean velocity for the rill varied with the product of discharge to the 0.28 power and average slope for the rill to the 0.48 power. Even though flow was fully turbulent, the friction factor decreased with increasing Reynolds numbers, a laminar-like behavior, especially for the lower discharge rates. The reason for the decrease was likely caused by flow more closely following the waviness of form roughness in the rill as discharge increased. Point velocities along vertical profiles in the flow in the middle of the rill were described by a logarithmic function.