Scour Velocities in Ephemeral Channels

Channel and discharge characteristics from small naturally occuring ephemeral streams in the Southwestern United States were measured and statistically analyzed to determine representative scour velocity values for comparison with accepted maximum noneroding velocity values. Three variables were considered in the analysis: channel velocity, depth of flow, and a general classification of the channel bed material. The latter included sandy-silt, silty-clay, and clay soil conditions. The statistically derived scour velocities associated with flow depths of between 0.5 ft and 5 ft ranged from 1.8 fps to 2.9 fps in sandy-silt and silty-clay soils, and from 2.2 fps to 4.5 fps in clay soils. These values are similar to accepted maximum permissible velocity values for comparable soil conditions, which indicates that published maximum permissible velocity values are appropriate for variable intermittent flow situations as well as long term constant flow conditions.