STRESS MEASUREMENT UPSTREAM OF AN OVERFALL

A hot-film anemometer was used to obtain hydraulic shear stress measur ements on the boundary of a fixed-bed channel. These boundary stresses were measured upstream of an overfall brink for approach pow Froude n umbers ranging between 1 and 4. The time-averaged stress was observed to increase as the flow accelerated toward the brink, and the brink st ress typically increased as the flow rate and Froude number increased For a given pow condition, the stress magnitude was a maximum at the b rink. This stress decreased to a constant value relatively rapidly wit h movement away from the brink. The distance required to transition fr om the peak brink stress to the constant stress value decreased as the Froude number increased. For a Froude number of unity, the brink stre ss increased by approximately 80% from stress values observed upstream of the overfall. At a Froude number of 4, the brink stress was observ ed to increase by only 20% above the approach pow stress levels. A rel ationship was developed and validated that predicts boundary stresses upstream of the overfall brink These observed variations in stress mag nitude are believed to influence gully headcut erosion.