Scattergraph Principles and Practice: Practical Application of the Froude Number to Flow Monitoring

Abstract

Engineers are aware of the instability associated with critical flow conditions and are generally advised to avoid them during sewer design. However, such conditions are often encountered in existing sewers and can impact the accuracy and reliability of flow monitoring data. The Froude number (Fr) is a dimensionless number used to describe flow conditions within a sewer. These conditions can be illustrated on a scattergraph using iso- Froude lines. Certain flow conditions such as hydraulic jumps, sewer bores, and undular jumps are readily identified by evaluating flow monitoring data with respect to iso-Froude lines. The concept of the iso-Froude is presented and developed in this paper. Practical examples from flow monitoring locations throughout the United States are also provided, demonstrating the scattergraph signatures of various transcritical and near-critical flow phenomena. Flow monitors can operate well in sewers under subcritical or supercritical conditions, but accuracy may deteriorate near the transition. Such conditions should be avoided when possible in flow monitoring applications.