Exploratory studies of open-channel flow over boundaries of laterally varying roughness

Abstract

An exploratory study was made of open-channel flow over beds consisting either entirely or partially of large granular roughness. Steady, uniform flow was established at various depths and velocities over two types of beds, one rough over the entire width of a laboratory flume, the other rough only over half the width and smooth over the other half. Friction factors were determined for these flows, and detailed velocity distributions were measured in three runs. The friction factors for the entirely rough beds compared closely with those predicted by the Karman-Prandtl equations, and the velocity distributions strongly suggested the existence of secondary circulation of the second kind. Analysis is offered to show that subdivision of the cross section of a turbulent flow by curves normal to the equal velocity curves does not result in hydraulically independent zones of flow, in that there will be turbulent interchange of the longitudinal component of momentum among such zones; other methods of subdivision are considered and none found to be completely satisfactory. The customary side-wall correction method is reviewed and found to have no explicit rational basis, and although it is recognized that the method gives reliable results in the situations to which it is usually applied, its application to widely different situations should be undertaken with caution. Suggestions for needed further research are offered.