This can be used to determine what throat width should be used for a given Parshall flume application. The last column in the spreadsheet shows a maximum recommended flow rate for each throat width. The image only shows throat widths from 1" to 9", but the spreadsheet that can be downloaded through the link below has throat widths up to 50'.
Then the Excel formulas will calculate the flow rate for free flow through the flume, Qfree. The user simply needs to enter the measured head, Ha, in the row for the correct flume throat width. The image at the right shows an Excel spreadsheet for calculating the flow rate through a Parshall flume under free flow conditions. Throat widths ranging from one inch to fifty feet. The image at the left gives the constants C and n in the Parshall flume equations for standard C and n are constants for a given Parshall flume throat width, W.Ha is the head measured at the correct point in the converging section of the Parshall flume as described in the previous section, in ft.Qfree is the open channel flow rate through the Parshall flume under free flow conditions, in cfs.The Parshall flume equation for free flow conditions is Qfree = C Han, where Excel Formulas for Free Flow Parshall Flume Calculations Within this article there are Excel spreadsheets that can be downloaded to calculate the flow rate through a Parshall flume for both free flow and submerged conditions with either U.S. If the submergence ratio is greater than the values given above for any given throat width, then the flow through the Parshall flume is submerged, and both of the head measurements, Ha and Hb, are needed for the Parshall flume equations and calculations. for 1' for 6" for 1" The introductory Parshall flume article mentioned above has diagrams and specifications showing the specified locations in the Parshall flume for measurement of Ha and Hb.Ī summary of the conditions required for free flow through a Parshall flume are as follows:
The submergence ratio, S, is defined as Hb/Ha, where Ha is the head measurement needed for free flow calculations, measured at a point 2/3 of the length of the straight-sided converging section upstream from the beginning of the Parshall flume throat, and Hb is the head measured at the throat. The conditions for free flow through a Parshall flume are specified in the form of a maximum submergence ratio for each of several ranges of throat widths. Image Credit: Montezuma Valley Irrigation Company In the past, however, it was common to design Parshall flumes for 90% submergence, so there are still many flumes operating for open channel flow measurement under submerged conditions. Good practice calls for design of Parshall flumes to operate under free flow conditions because the flow rate can be measured more accurately and only one head measurement is needed.
When the downstream flow “backs up” into the flume, obscuring the hydraulic jump, the flow is said to be submerged. Of the Parshall flume during free flow conditions. A hydraulic jump will be visible in the throat When the flow through a Parshall flume is unaffected by flow conditions downstream of the flume, there is said to be “free flow” through the flume. For more background about the Parshall flume for open channel flow measurement and specifications for its dimensions, see the article, “ Open Channel Flow Measurement 2: the Parshall Flume.” Free Flow and Submerged Flow Through Parshall Flumes Using the Excel formulas in the spreadsheets that can be downloaded from links later in this article, the flow rate through a Parshall flume can be calculated for both free flow and submerged flow conditions, in either U.S. Parshall flumes also have less head loss than weirs for open channel flow measurement. The Parshall flume is an open channel flow measurement device that is well suited for flows containing suspended solids because it tends to be self-flushing.