The Parshall flume is an open channel flow-metering device that was developed to measure the flow of surface water and irrigation flow. The Parshall flume is a modified version of the Venturi flume. Named after its creator, Dr. Ralph L. Parshall of the U.S. 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. The Parshall flume accelerates the flow by contracting both the parallel sidewalls and a drop in the floor at the flume throat. 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. Some states specify the use of Parshall flumes, by law, for certain situations (commonly water rights). 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).
Development Beginning in 1915, Dr. Ralph Parshall of the U.S. Soil Conservation Service altered the subcritical Venturi flume to include a drop in elevation through the throat of the flume. This created a transition from subcritical flow conditions to supercritical flow conditions through the throat of the flume. Modifications to the Venturi flume that Parshall made include:
Decreasing the angle of convergence of the inlet walls Lengthening the throat Decreasing the angle of divergence of the outlet wall Introducing a drop through the throat of the flume In 1930, the improved flume was named the Parshall Measuring Flume by the Irrigation Committee of the American Society of Civil Engineers (ASCE) in recognition of Parshall's accomplishments. Parshall was additionally honored as a Life Member of the ASCE.
Applications Dr. Parshall's initial focus was on the use of his namesake flume to measure flows in irrigation channels and other surface waters. Over time, however, the Parshall flume has proven to be applicable to a wide variety of open channel flows including:
Irrigation channels and ditches Furrows Surface waters (swales, creeks, streams, and rivers) Elevated, above grade piped flows Below grade piped flows (concrete vaults/manholes incorporated into Packaged Metering Manholes)
Construction A wide variety of materials are used to make Parshall flumes, including:
Fiberglass (wastewater applications due to its corrosion resistance) Stainless steel (applications involving high temperatures / corrosive flow streams) Galvanized steel (water rights / irrigation) Concrete (large Parshall throat widths 144 in [3.7 m] and above) Aluminum (portable applications) Wood (temporary flow measurement) Plastic (PVC or polycarbonate / Lexan) (teaching/laboratory investigation) Smaller Parshall flumes tend to be fabricated from fiberglass and galvanized steel (depending upon the application), while larger Parshall flumes tend to be fabricated from fiberglass (sizes up to 144 in) or concrete (160–600 in). By the 1960s, several different companies began to commercially offer Parshall flumes. These manufacturers have typically produced flumes from one type of material only (typically glass-reinforce plastic or steel), although currently a few, offer Parshall flumes in a variety of materials. When used for stream gauging, aluminium is the typical material of construction - primarily due to its light weight. An example can be found via google earth: 50°58'41.34"N, 5°51'36.81"E, eye altitude 200 m. This is in the Geleenbeek, near Geleen in the Netherlands.
Technical details
The design of the Parshall flume is standardized under ASTM D1941, ISO 9826:1992, and JIS B7553-1993. The flumes are not patented, and the discharge tables are not copyright protected. Parshall flumes come in twenty-two standard sizes, spanning flow ranges from 0.005 to 3,280 cubic feet per second (0.142 to 92,900 litres per second). Submergence transitions for Parshall flumes range from 50% (1–3-inch sizes) to 80% (10–50-foot sizes), beyond which point level measurements must be taken at both the primary and secondary points of measurement, and a submergence correction must be applied to the flow equations. 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. Typically, 90% is viewed as the upper limit for which corrections for submerged flow are practical. Under laboratory conditions, Parshall flumes can be expected to exhibit accuracies to within ±2%, although field conditions make accuracies better than 5% doubtful. The free-flow discharge can be summarized in this equation:
Q = C H n {\displaystyle Q=CH^{n}}
Where:
Q is flowing rate C is the free-flow coefficient for the flume H is the head at the primary point of measurement n varies with flume size (e.g. 1.55 for a 1-inch flume) 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
Q = C H n − Q E {\displaystyle Q=CH^{n}-Q_{E}}
Where Q E {\displaystyle Q_{E}} is the "submergence correction" and is found using pre-determined tables for a particular flume geometry.
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