STRESS MEASUREMENT UPSTREAM OF AN OVERFALL

A hot-film anemometer was used to obtain hydraulic shear stress measurements on the boundary of a fixedbed channel. These boundary stresses were measured upstream of an overfall brink for approach flow Froude numbers ranging between 1 and 4. The time-averaged stress was observed to increase as the flow accelerated toward the brink, and the brink stress typically increased as the flow rate and Froude number increased. For a given flow condition, the stress magnitude was a maximum at the brink. This stress decreased to a constant value relatively rapidly with movement away from the brink. The distance required to transition from the peak brink stress to the constant stress value decreased as the Froude number increased. For a Froude number of unity, the brink stress increased by approximately 80% from stress values observed upstream of the overfall. At a Froude number of 4, the brink stress was observed to increase by only 20% above the approach flow stress levels. A relationship was developed and validated that predicts boundary stresses upstream of the overfall brink. These observed variations in stress magnitude are believed to influence gully headcut erosion.