VERIFICATION OF THE MANNING FLOW EQUATION IN THE ILLINOIS BAYOU DRAINAGE BASIN

The Illinois Bayou drains a large portion of the southern Boston Mountains in Arkansas, north of the Russellville area. The Manning Flow Velocity Equation has been one of the standard equations used in geomorphological investigations to predict flow velocities in streams that are not gauged. Velocities are predicted by using the channel slope, hydraulic radius of the wetted stream, and the Manning roughness coefficient. The roughness coefficient changes depending on the material that constitutes the channel bed. Although the equation is widely used, it is not always clear whether the theoretical velocities are accurate. The focus of this study is to compare gauged velocities with the Manning velocity in streams at varying flow stages. Calculated Manning velocities in the Illinois Bayou range from 1.94 ft/sec to 4.33 ft/sec. The measured velocities for these streams range from 0.26 ft/sec to 1.01 ft/sec. Most of the measurements result in over a 100% difference in measured and calculated flow velocities. The large difference may be caused by the roughness coefficient not being sufficient to account for the materials that make up the stream beds. Further comparison should allow for either verification of the Manning equation, or for possible adjustment to the equation or roughness coefficient for the Illinois Bayou Basin.