While the Parshall flume addresses many water measurement applications well, there are times when it just isn’t quite for the application. At those times a modification of the Parshall may be a better fit. Usually these modifications are driven by cost, portability, convenience, or the need to accommodate free-spilling discharge.
To fit these needs, two modifications to the Parshall have been developed:
Both modifications start with the Parshall flume but then remove certain elements of the standard design to obtain the desired set of features.
USGS Portable Parshall Flume
Of the two modifications, the USGS Portable Parshall flume is the more mild. The flume consists only of the converging and throat sections; omitting the diverging section. The modified Parshall flume was originally described by H.C. Troxell and C.A. Taylor in a 1932 memorandum from the Office of the Ground Water Branch, USGS.
It should be noted that the original USGS Portable Parshall flume was a 3-inch unit, although recently the USGS Hydrological Facility has expanded this to include a 6-inch model. There has been no published research on size larger or smaller than these and as such, the operator should be aware that there may be differences in operation that are not anticipated.
Johnson provides the discharge equation for the flume as:
This equation differs slightly from the standard discharge equations for the 3-inch and 6-inch Parshall and user of the flume should endeavor to ensure that they are using the published table if strict accuracy is to be mainted.
Both Johnson and Kilpatrick / Schneider recommend that the USGS Portable Parshall flume not be used on applications where the submergence ratio is greater tha 0.5 Ha. Shoud the flume become submerged, no correction is avaialble.
For applications where free-spilling discharge can be guaranteed under all flow conditons, the Montana flume is the preferred modified Parshall of choice. The Montana flume deletes not only the discharge seciton (as does the USGS Portable Parshall), but it also deletes the throat. What is left is the flat-floored converging seciton. With the throat and discharge sections omitted, the narrow portion of what would have been the begining of the throat in a standard Parshall now becomes the discharge of the flume.
The Montana flume is defined in both the US Bureau of Reclamation's Water Measurement Manual and in Montana State University technical documents MT 9127 (AG) and MT 9128 9AG).
The Montana flume is both shorter and lighter than the USGS Portable Parshall flume, but does require free-spilling discharge. Like a weir (or H flume) there is no resistance to the effects of submergence.
Unlike the USGS Portable Parshall flume, the Montana flume utilizes the same discharge equations as the same size Parshall flume and there are no stated restrictions in flume size - althoug in practice it is unsual to see Montana flumes larger than 24-inches in size.
Sources: Johnson, A., Modified Parshall Flume, U.S. Geological Survey, 1963, Kilpatrick, F., Schneider, V., Use of Flumes in Measuring Discharge, U.S. Geological Survey, Techniques of Water-Resources Investigations, Book 3, Chaper A14, MT 9127 (AG) Montana (Short Parshall) Flume (Part 1), Montana State University, MT 9128 (AG) Montana (Short Parshall) Flume (Part 2), Montana State University