Flumes
“The most common structures used to measure flow in open channels operate by producing critical flow or flow at critical-depth through a control section of known dimensions. Under this flow condition, the discharge through the critical section is a function of the section shape and the upstream potential energy, as indicated by the water level upstream from the structure. By definition, the presence of critical flow in the control section prevents the downstream water level and flow conditions from affecting the flow through the critical section, and the discharge can be computed as a function of the measured upstream head. Sharp-crested weirs, broad-crested weirs, and a wide variety of flumes are examples of critical-flow devices”.[1]
“Long-throated flumes are generally composed from five primary structural components:
- "An approach channel that is necessary for the development of uniform and symmetrical flow conditions and the establishment of a stable water surface whose elevation can be determined accurately. The approach channel may be lined… or may be the original earthen channel. [1]
- "A converging transition section in which the subcritical approach flow accelerates smoothly toward the throat with no discontinuities or flow separation – the transition may consist of plane surfaces or may be rounded. [1]
- "A throat, or control section, in which the flow passes through critical depth. The throat must be horizontal in the direction of flow, but in the direction perpendicular to the flow any shape may be used. [1]
- "A diverging transition in which the velocity of the supercritical flow exiting the throat section is reduced and energy is dissipated or partially recovered – if energy recovery is not needed, an abrupt transition can be used. [1]
- "A tailwater channel where the water level is a function of the flow rate and the hydraulic properties of the downstream channel and structures. The range of water levels in this channel is fundamentally important to the design of the structure because it determines the elevation and size of the control section needed to maintain critical flow through the flume.[1]
In addition to these five structural components, a gaging station in the approach channel is necessary. At the gaging station, the difference in elevation between the approach water level and the sill or crest of the throat section will be measured. This difference in elevation is known as the sill-referenced head. The flow rate through the flume will be computed as a function of this upstream sill-referenced head”.[1]
Best Practices Resources
Engineering Guidelines for the Evaluation of Hydropower Projects: Chapter 12- Water Conveyance, FERC
Citations:
Revision ID: 7094
Revision Date: 07/11/2023