Bottom Outlet for Sewers

Abstract

The bottom opening is a standard structure in storm sewers, provided the approach flow is supercritical. Based on extended experimentation and by using the governing flow equations, the hydraulic characteristics of bottom openings were derived. These include the approach reach, the end depth ratio, the average discharge coefficient, the tailwater depth, and the hydraulic loss across the outlet. Further, velocity distributions in the outlet zone are discussed. The results of this study reveal that the approach Froude number has a significant effect on the flow mechanism. General expressions are derived for the lower and upper nappes of the flow across the bottom opening, and a hydraulic design procedure is described. A generalized streamwise coordinate is also introduced that allows description of supercritical sewer flow. It is demonstrated that the excess discharge to the treatment station under design discharge is within the requirements of European standards.