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A '''Parshall flume''' is a fixed [[hydraulic structure]] used in measuring [[volumetric flow rate]] in surface water, wastewater treatment plant, and industrial discharge applications. The Parshall flume accelerates flow though a contraction of both the parallel sidewalls and a drop in the floor at the flume throat. Under free-flow conditions the depth of water at 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).<ref>{{cite web|url=http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/chap08_10.html |title=USBR Water Measurement Manual - Chapter 8 - FLUMES, Section 10. Parshall Flumes |publisher=Usbr.gov |date= |accessdate=2013-04-15}}</ref>

The free-flow discharge can be summarized as

:<math>Q = C H^n</math>

Where
* ''Q'' is flow 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)

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.

A total of 22 standard sizes of Parshall flumes have been developed, covering flow ranges from 0.005 cfs [0.1416 l/s] to 3,280 cfs [92,890 l/s].<ref>{{cite web|url=http://www.openchannelflow.com/products/flumes/parshall |title=Parshall Flumes for Flow Measurement |publisher=Openchannelflow |date= |accessdate=2013-04-15}}</ref>

Submergence transitions for Parshall flumes range from 50% (1”-3” sizes) to 80% (10’-50’ sizes),<ref>{{cite web|url=http://www.openchannelflow.com/products/flumes/parshall/flow-characteristics |title=Flow Characteristics of Parshall Flumes |publisher=Openchannelflow |date= |accessdate=2013-04-15}}</ref> 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. It is important to note that 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. 90% is viewed as the upper limit for which corrections for submerged flow are practical.<ref>{{cite web|url=http://www.openchannelflow.com/products/flumes/parshall/submergence |title=Parshall Flume Submergence |publisher=Openchannelflow |date= |accessdate=2013-04-15}}</ref>

Under laboratory conditions Parshall flumes can be expected to exhibit accuracies to within +/-2%, although field conditions make accuracies better than 5% doubtful.

A wide variety of materials are used to make Parshall flumes, including fiberglass (used in wastewater applications due it corrosion resistance), stainless steel, galvanized steel (common in water rights measurement), concrete, aluminum, wood, and plastic (PVC or Lexan).<ref>{{cite web|url=http://www.fao.org/docrep/T0848E/t0848e-09.htm |title=Field measurement of soil erosion and runoff - Chapter 4 Streamflow |publisher=Fao.org |date= |accessdate=2013-04-15}}</ref> Smaller Parshall flumes tend to be fabricated from fiberglass and galvanized steel (depending upon the application), while larger Parshall flumes tend to fabricated from fiberglass (up to 144" in size) or concrete.

Parshall flumes are available from several commercial manufacturers. These manufacturers typically produce flumes from one type of material only (typically fiberglass/grp or steel). A few manufacturers, such as [[Openchannelflow]], offer Parshall flumes in a variety of materials.

== Development ==

Development of what would become the Parshall flume began in 1915 by Dr. Ralph L. Parshall of the U.S. Soil Conservation Service. In developing the flume, Dr. Parshall sought to overcome limitations of the [[Venturi flume]].

Modifications to the Venturi flume that Dr. Parshall made include:<ref>{{cite book|last=Parshall|first=Ralph|title=The Improved Venturi Flume|year=1928|publisher=Colorado Agricultural College|location=Fort Collins, CO|url=http://digitool.library.colostate.edu///exlibris/dtl/d3_1/apache_media/L2V4bGlicmlzL2R0bC9kM18xL2FwYWNoZV9tZWRpYS8xODU0MA==.pdf}}</ref>
*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

The improved flume was named the Parshall Measuring Flume by the Irrigation Committee of the [[American Society of Civil Engineers]] (ASCE) in recognition of Dr. Parshall's accomplishments. Dr. Parshall was additionally honored as a Life Member of the ASCE.<ref>{{cite web|title=Guide to the Ralph L. Parshall Collection|url=http://lib.colostate.edu/archives/findingaids/water/wrlp.html}}</ref>

== Drawbacks ==

* Parshall flumes require a drop in elevation through the flume. To accommodate the drop in an existing channel either the flume must be raised above the channel floor (raising the upstream water level) or the downstream channel must be modified.
* As with [[weir]]s, [[flume]]s can also have an effect on local fauna. Some species or certain life stages of the same species may be blocked by flumes due to relatively slow swim speeds or behavioral characteristics.
* In earthen channels, upstream bypass and downstream scour may occur.
* Parshall flumes below 3 inches in size should not be used on unscreened sanitary flows, due to the likelihood of clogging.<ref>{{cite web|title=Don't Get Stuck: Sanitary Flows in Flumes|url=http://www.openchannelflow.com/blog/article/dont-get-stuck-sanitary-flows-in-flumes}}</ref>
*The Parshall flume is an empirical device. Interpolation between sizes is not an accurate method of developing intermediate size Parshall flumes as the flumes are not scale models of each other.<ref>{{cite web|title=Water Measurement Manual, Chapter 8, Section 10|url=http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/chap08_10.html}}</ref><ref>{{cite web|title=Parshall Flumes|url=http://www.openchannelflow.com/products/flumes/parshall}}</ref><ref>{{cite journal|title=Recommended Practice for the Use of Parshall Flumes and Palmer-Bowlus Flumes in Wastewater Treatment Plants|date=November 1984|volume=EPA600/2-84-186|pages=5|url=http://nepis.epa.gov/Exe/ZyNET.exe/5000141O.txt?ZyActionD=ZyDocument&Client=EPA&Index=1981%20Thru%201985&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&UseQField=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5CZYFILES%5CINDEX%20DATA%5C81THRU85%5CTXT%5C00000014%5C5000141O.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=p%7Cf&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=1}}</ref> The 30-inch [76.2 cm] and 42-inch [106.7 cm] sizes are examples of intermediate sizes of Parshall flumes that have crept into the marketplace without the backing of published research into their sizing and flow rates.

== Variations ==

Two variations of the Parshall flume have been developed over time: the [[Montana flume]] and the short section Parshall flume.

===Montana Flume===

The Montana flume omits the throat and discharge sections of the Parshall. By omitting these sections, the flume is shortened by more than half, while retaining the free-flow characteristics of the same-size Parshall. With the deletion of the throat and discharge section, the Montana flume has little resistance to submersion and, like the H flume, should be used where free-spilling discharge is present under all flow conditions. The Montana flume is described in The Water Measurement Manual and in technical publications by [[Montana State University]].

===Short Section (USGS or Portable) Parshall Flume===

The short-section Parshall (sometimes referred to as a USGS or Portable Parshall) omits the discharge section of the flume. 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.<ref>http://pubs.usgs.gov/of/1963/0063/report.pdf</ref> This modification - supplied by the USGS Hydrologic Instrumentation Facility - is available in two sizes: the original 3" and the recently added 6".<ref>http://water.usgs.gov/hif/programs/projects/portable_parshallflume_6in/</ref>

Kilpatrick notes that the discharge for this modification of the Parshall flume is slightly greater than for a standard Parshall flume of the same size.<ref>{{cite journal|last=Kilpatrick|coauthors=Schneider|title=Techniques of Water-Resources Investigations|issue=3, Chapter A14|url=http://pubs.usgs.gov/twri/twri3-a14/html/pdf.html}}</ref> This has been attributed to potential manufacturing tolerance variations rather than the actual operation of the flume itself and users are cautioned to verify the flumes dimensions before proceeding with data collection. For those flumes varying from the standard dimensions, it is recommended that the flume be individual rated.

When used for stream gauging, aluminum is the typical material of construction - primarily due to its light weight.

== References ==
{{Reflist|33em}}

== Further reading ==
*ASTM D1941 – 91(2013) Standard Test Method for Open Channel Flow Measurement of Water with the Parshall Flume
*ISO 9826:1992 Measurement of Liquid Flow in Open Channels – Parshall and SANIIRI Flumes
*JIS B7553-1993 Parshall Flume Type Flowmeters
*{{Cite book|last=Bos|first=Marinus|title=Discharge Measurement Structures. Third edition revised. Publication 20|year=1989|location=Oxford, United Kingdom|publisher=International Institute for Land Reclamation and Improvement|isbn=978-9070754150|url=http://content.alterra.wur.nl/Internet/webdocs/ilri-publicaties/publicaties/Pub20/pub20-h1.0.pdf}}

[[Category:Water supply infrastructure]]
[[Category:Fluid mechanics]]
[[Category:Hydraulic structures]]

Homospermidine synthase - Revision history

en>Dcirovic: removed Category:Enzymes; added Category:EC 2.5.1 using HotCat