Understanding the key relationships between basin sizes selected for monitoring within a given area and the resultant accuracy of Infiltration and Inflow (I&I) analysis has become a critical factor for…
Category: Scattergraph
Drifting Velocity Sensor
29. Drifting Velocity Sensor
This scattergraph displays data from a flow monitor with a drifting velocity sensor. Note that the reported flow velocity drifts over a wide range without a corresponding change in depth. This flow monitor was equipped with an electromagnetic velocity sensor, which is subject to fouling from grease. The sensor performance worsened over time and eventually caused the flow monitor to report negative velocities. The data from this flow monitor are invalid and should be disregarded.
Drifting Depth Sensor
28. Drifting Depth Sensor
Most flow monitoring data can be described by the Manning Equation using either the Design Method, the Lanfear-Coll Method, or the Stevens-Schutzbach Method. Data that do not lie on a pipe curve generated by one of these methods indicate that either the hydraulics are different or that the flow monitor is not working correctly. This scattergraph displays data from a flow monitor with a drifting pressure depth sensor. Note that the reported flow depth drifts over a wide range without a corresponding change in velocity. In this case, as series of pipe curves are observed at multiple depths and deviate significantly above and below the manual confirmations. The data from this flow monitor are invalid and should be disregarded.
Undular Jump
27. Undular Jump
A hydraulic jump or other transcritical flow condition may reduce the accuracy of a flow monitor. However, this effect is not strictly limited to critical flow (Fr = 1), but may also occur within a wider range of near-critical flow conditions. Research by Chanson, Hager, and others suggests a range of conditions (0.7 < Fr < 1.7) where unstable flow may exist. Pipe shape and material also appear to affect where unstable flow may occur. This scattergraph is believed to result from an undular hydraulic jump and is characterized by a stair step pattern.
Hydraulic Jump
26. Hydraulic Jump
This scattergraph is from a site that experiences a hydraulic jump. This condition occurs when flow transitions from supercritical to subcritical flow. In this example, the hydraulic jump is located upstream from the monitor at lower flow rates, and the subcritical side of the hydraulic jump is observed (Q1). As the flow rate increases, the hydraulic jump is pushedthrough the monitoring location, and the supercritical side of the hydraulic jump is observed (Q2). Flow monitors can operate well in either subcritical or supercritical conditions, but accuracy may deteriorate during the transition. Therefore, a hydraulic jump should be avoided, if possible.
Iso-Froude Lines
25. Iso-Froude Lines
The Froude Number (Fr) is a dimensionless number used to describe flow conditions and is analogous to the Mach Number defining supersonic flight. If the Froude Number is less than one, flow conditions are described as subcritical. If the Froude Number is greater than one, flow conditions are described as supercritical. These conditions are identified on a scattergraph using an iso-Froude, as shown here. Flow conditions such as hydraulic jumps, sewer bores, and standing waves are readily identified by evaluating flow monitoring data with respect to iso-Froude lines.
