The Parshall flume is a fixed hydraulic structure originally developed to measure surface water and irrigation flows. The Parshall flume is now frequently used to measure industrial discharges, municipal sewer flows, and influent / effluent at wastewater treatment plants. Of all the flumes, the Parshall flume is the most recognized and commonly used.
Development of the Parshall Flume
Development of the Parshall flume began in 1915 by Dr. Ralph L. Parshall of the U.S. Soil Conservation Service. Using the sub-critical Venturi flume as his starting point, Dr. Parshall introduced several radical modifications with his Improved Venturi Flume – the most greatest of which was a drop in elevation through the throat of the flume. The drop produced supercritical flow through the throat of the flume. With supercritical flow, only one head measurement is necessary to determinee the flow rate, greatly simplifying the use of the flume.
Design of the Parshall Flume
The design of the Parshall flume consists of a uniformly converging upstream section, a short parallel throat section (the width of which determines the flume size), and a uniformly diverging downstream section. The floor of the flume is flat in the upstream section, slopes downward in the throat, and then rises in the downstream section; ending with a downstream elevation below that of the upstream elevation. The need to accommodate the required drop in the floor elevation, either by either modifying the downstream channel or by raising the elevation of the flume above the channel floor, may eliminate the flume from some applications.
Although the basic shape of all Parshall flumes is the same, the flumes are not scale models of each other, so that the discharge equation for each flume had to be obtained by direct calibration. It is important, therefore, that Parshall flumes are constructed to the dimensions specified. Interpolation between sizes to obtain intermediate flumes (e.g. 30" and 42") is not recommended.
22 sizes of Parshall flumes have been developed, covering flows from 0.005 cfs [0.1416 l/s] to 3,280 cfs [92,890 l/s]. Throat sizes range from 1” to 50’.
Parshall Flume Applications
Discharge and Submergence
Discharge flows from a Parshall flume can either reenter the channel at the same elevation as the discharge section or free-fall off the end of the flume. When free-fall conditions exist for all flows, the throat and downstream diverging sections of the flume may be left off. This simplification results in a modified style of Parshall flume called the Montana flume. Parshall and Montana flume of the same throat width use the same discharge tables and equations – there is no difference so long as the Montana flume operates under free-fall conditions under all flows.
Transition submergence, st, values for Parshall flumes range from 50% to 80%, increasing as the flume size increases. When used in submerged flow applications, two head measurements are made: one in the converging section of the flume, the other in the throat of the flume. Due to the added instrumentation costs and operational complexity of operating under submerged flow conditions, Openchannelflow recommends operating flumes under free-flow conditions whenever possible.
Parshall Flume Accessories
Openchannelflow offers full and custom height (half or extended – in select sizes) Parshall flumes, nested (smaller flumes nested in larger flumes), and “magic bottoms” (replacement floors for existing Parshall flumes to raise the zero flow elevation).
Half height flumes provide a convenient solution to applications where the anticipated flows do not warrant a larger flume, but where the flow channel is wider than one in which a smaller flume can easily be installed.
Extended height flumes offer higher flow capacities than standard height flumes, but in a smaller footprint than a larger flume would occupy. The developmental literature for the Parshall flume leads Openchannelflow to note that the flow equations are believed to be accurate for extended height flumes, but are not guaranteed to be so.
Openchannelflow Parshall flumes are available in a variety of materials, including:
- Fiberglass (FRP) (standard)
- Galvanized steel
- Lexan (small sizes)
- PVC (small sizes)
- Stainless steel
Parshall flumes (in fiberglass) range in price from $635 (1-inch) to $40,900 (144-inch). Contact Openchannelflow for detailed pricing.
Source: Sewer Flow Measurement A State-of-the Art Assessment, U.S. Environmental Protection Agency, Municipal Environmental Research Laboratory, EPA/600/2-75-027, November 1975.