Parshall flume
Soil Conservation Service, the Parshall flume is a fixed hydraulic structure used in measuring volumetric flow rate in surface water, industrial discharges, municipal sewer lines, and influent/effluent flows in wastewater treatment plants.
Under free-flow conditions, the depth of water at a specified location upstream of the flume throat can be converted to a rate of flow.
[1] Differences between the Venturi and Parshall flume include reduction of the inlet converging angle, lengthening the throat section, reduction of the discharge divergence angle, and introducing a drop through the throat (and subsequent partial recovery in the discharge section).
[4] Dr. Parshall's initial focus was on the use of his namesake flume to measure flows in irrigation channels and other surface waters.
When used for stream gauging, aluminium is the typical material of construction - primarily due to its light weight.
Parshall flumes come in twenty-two standard sizes, spanning flow ranges from 0.005 to 3,280 cubic feet per second (cfs) or 0.1416 to 92,890 litres per second (L/s).
The secondary point of measurement (Hb) for a Parshall flume is located in the throat, measuring Hb can be difficult as flow in the throat of the flume is turbulent and prone to fluctuations in the water level.
The free-flow discharge can be summarized in this equation: Where: When the downstream depth is high enough that the transition to subcritical flow advances upstream into the throat and the hydraulic jump disappears, the flume is operating in a "submerged flow" regime, and the discharge is instead given by the function Where
Since E2 is located at the flume crest where there is a steep drop, critical flow conditions occur.
For the Parshall flume equation used to calculate the flow rate, both empirical values C and n are known constants (with various values for each Parshall flume size) leaving Ha (depth upstream) as the only variable needing to be measured.
In natural stream applications, submerged flow is frequently the result of vegetative growth on the downstream channel banks, sedimentation, or subsidence of the flume.
If there is submerged flow, adjustments need to be made in order for the Parshall Flume to work properly.
This equation was derived using the principles of specific energy and is only to serve as an estimate for the actual discharge of the Parshall flume.
When a constriction (decrease in width) happens Between E1 and E2, the q value changes (and becomes the new critical depth), while the energy remains the same.
From this, the principles of conservation of energy are used to develop a set of calculations to predict the flow rate.
[23] By omitting these sections, the flume is shortened by more than half while retaining the free-flow characteristics of the same-size Parshall.
Originally designed by Troxell and Taylor in 1931 and published under "Venturi Flume" as a memorandum from the office of the Ground Water Branch, USGS, the design was again brought to the attention of potential users in Taylors' paper "Portable Venturi Flume for Measuring Small Flows in 1954.
[27] This modification - supplied by the USGS Hydrologic Instrumentation Facility - is available in two sizes: the original 3" and the recently added 6".
