The USGS Portable Parshall flume is a modification of the standard Parshall flume where the discharge section has been removed. The purpose of the modification is to reduce the weight of the flume and to make it easier to install.
This modified design was originally developed by Troxell (USGS) and Taylor (USDA) for use in southern California. Originally described as "Venturi Flume" in a March 10th, 1932 memorandum from the office of the Ground Water Branch, USGS, the flume was little used until Taylors' paper "Portable Venturi Flume for Measuring Small Flows was published in May of 1954 that the flume again came to the attention of its potential users.
The original design of the 3-inch flume had a reduced sidewall height of 6-inches [15.24 cm] but was later increased to 9-inches [22.86 cm] to provide additonal free-board in the flume. The intiai rating of the flume took place at Colorado State University Hydraulics Laboratory in 1931, while the redesigned flume was checked at the Hydrologic Laboratory of the USGS in Denver, Colorado.
With the discharge section removed, the USGS Portable Parshall flume should not be used for submergence ratios greater than 0.5 - in fact, Kilpatrick and Schneider recommend the flume be used only where free-fall conditions from the throat of the flume will occur. Should the flume become submerged, there is no correction available.
As portability is of primary concern, USGS Portable Parshall Flumes are commonly constructed of 1/8-inch [3.17 mm] thick sheet aluminum. In a 3-inch size, the flume weighs approximately 12 lbs. [5.443 kg].
Johnson provides the discharge equation for the flume as:
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