The Relationship Between Parshall Flume Converging Section Length and Flow Rate

parshall flume field installation In his original developed of what would become the Parshall flume, Dr. Parshall arbitrarily defined the length of the converging, inlet section of his flume as:

L = W / 2 + 4

W = flume throat width, in feet, from 1 to 8-feet

For throat widths greater than 8 feet, this relationship was found to be unsatisfactory, and the converging section length was considerably lengthened for flumes 10-feet and larger.

While the flow rate was know to be a function of the throat width, Dr. Parshall gave little consideration to the influence of the converging section length.

Investigation

Laboratory investigations into the impact that the converging section length had upon a 1-foot Parshall flume were conducted at the University of California (Davis) in 1959. In all, six converging section lengths were investigated:

L/W = 10.0, 8.0, 5.5, 4.41 (standard), 2.45, and 1.14

holding

H_max / W = 1.0, where H_max = depth at the primary point of measurement

Results

The investigation showed that for L/W greater than or equal to 4.4, the length of the converging section had very little effect on the free-flow discharge for the Parshall flume.

For values of L/W less than 4.4, the length of the converging section influences the free flow discharge, and as L/W becomes smaller and smaller, the effect on the equation becomes larger and larger.

For a 1-foot Parshall flume, the standard depth equation (for L/W = 4.41) is:

Q = c H ⁿ = c H ^1.56

As L/W decreased, the value of the depth exponent, n, increased, to 1.57 at L/W = 2.45 and ultimately to 1.67 at L/W = 1.14.

Also, as L/W decreased below 4.41, turbulence at the entrace of the flume increased.

Source: Relation Between the Flow-Rate Equation and Length of Converging Section of Parshall Flume