A Laboratory Study of Rill Hydraulics: II. Shear Stress Relationships

Abstract

ABSTRACT SURFACE runoff over erodible soil erodes small channels, rills, having features and erosion rates that depend on the hydraulics of the flow in the rills. Shear stress is frequently used as a measure of the erosivity of flow in rills, but it is highly nonuniform around the wetted perimeter and along most rills. Since very few hydraulic data are available for rills, we conducted a laboratoy study using a full scale fiberglass, fixed bed, replica of a rill on an eroded field soil to study characteristics of shear stress of flow in a rill. A flush-mounted hot-film sensor and anemometer were used to measure shear stress attributed to grain roughness. This shear stress around a cross section was highly irregular in contrast to the smooth distributions for shear stress around prismatic and semicircular channels. As expected, the shear stress was highly variable in time because the flow was turbulent. The ratio of maximum to minimum section average shear stress from grain roughness along a 3-m reach of the rill was 2.4 to 1. Average shear stress from grain roughness for the rill varied with discharge rate to the 0.63 power and slope to the 1.38 power. The probability distribution of instantaneous shear stress from grain roughness was described either by a Pearson Type III or a log-normal distribution.