Open Channel Flow Measurement 1: Introduction to the Weir and Flume

Written by: Harlan Bengtson • Edited by: Lamar Stonecypher
Updated Jul 9, 2010

Open channel flow measurement is usually done with a weir or a flume. The weir or flume causes a change in water depth that correlates with water flow rate. Common open channel flow meters are the sharp crested weir (v notch weir and rectangular weir), broad crested weir, and Parshall flume.

Introduction

Open channel flow occurs with a free surface open to the atmosphere, thus meters using change in pressure cannot be used as in pipe flow measurement. For open channel flow measurement, a change in depth of flow at some point is typically measured and correlated with water flow rate. The most common methods of measuring open channel flow rate are with a weir or a flume. The sharp crested weir (v notch weir and rectangular weir), broad crested weir, and Parshall flume are introduced in the following sections.

Weir Basics

A weir is basically an obstruction in the flow path in an open channel. The weir will cause an increase in the water depth as the water flows over the weir. In general, the greater the flow rate, the greater will be the increase in depth of flow, The height of water above the top of the weir is the measurement usually used to correlate with flow rate.

click to enlarge

The two major types of weir are sharp crested weir and broad crested weir. The crest is the term used for the top of the weir, where the water flows over it. The two diagrams here show a sharp crested weir and a broad crested weir. As you can see by the diagrams, the names are very descriptive. The sharp crested weir has a sharp surface at the crest or top, where the water flows over it, and the broad crested weir has a broad flat surface at the top.

The height of the water above the crest of the weir is called the head over the weir and is shown as H in both of the diagrams. It is the parameter that is measured and used to determine the flowrate. There are equations available to calculate flow rate, Q, over the weir for given head over the weir, H. Also for a given weir, Q can be experimentally correlated with H. See the article, The Broad Crested Weir for Open Channel Flow Measurement for more detailed information on the broad crested weir.

Types of Sharp Crested Weir

Two common shapes for a sharp crested weir are the v notch weir and the

click to enlarge

rectangular weir. A rectangular weir (horizontal across the top) may go all the way across the channel (called a suppressed rectangular weir). If it is inset at the sides as shown in the diagram, then it is called a contracted rectangular weir. Three common types of sharp crested weir are shown in the diagram at the right. Another type of contracted, sharp crested weir is the cipolletti weir. It has a trapezoidal opening for the water to go through,

The Flume For Flow Rate Measurement

Instead of putting in a vertical obstruction, as with a weir, a flume consists of a constriction in the channel width. This constriction causes

click to enlarge

the water level to change, and that water level in the constriction can be correlated with flow rate. The diagram at the right shows a top view of a venturi flume and a top view and side view of a Parshall Flume. A venturi flume has no bumps or dips in the floor of the channel. The Parshall Flume has the specific shape shown for its floor. The Parshall Flume has very specific dimensions for any channel width. There are equations and tables available, giving flow rate through the flume as a function of head (water height) above the bottom of the flume at a specific location. Parshall Flumes are widely used to measure wastewater flow rates in wastewater treatment plants. See the article, The Parshall Flume for Open Channel Flow Measurement for more detailed information about the Parshall flume.

Harlan Bengtson Jan 9, 2011 2:25 PM

Parshall Flume Approach Length

Here is a set of recommendations from the U.S. Department of the Interior, Bureau of Reclamation, Water Measurement Manual (ref #3 for the second article in this series: "Open Channel Flow Measurement: Parshall Flumes." Note that the 'control width" below is the Parshall flume throat width.

Bos et al. (1991) gives approach length requirements stated in various terms of flow depths, head readings, and widths as follows:

(1) If the control width is greater than 50 percent of the approach channel, then 10 average approach flow widths of straight unobstructed approach are required.

(2) If the control width is less than 50 percent, then 20 control widths of straight unobstructed approach are required.

(3) If upstream flow exceeds critical velocity, a jump should be forced to occur. In this case, 30 measuring heads of straight unobstructed approach after the jump is completed should be provided.

(4) If baffles are used to correct and smooth approach flow, then 10 measuring heads should be placed between the baffles and the measuring station.

BANU ERKUL Jan 9, 2011 1:37 PM

parshall flume

Dear Sir,

I want to learn what must be the channel length before parshall flume .It must be 5 times bigger than the throat width or the inlet with of parshall flume ??

thanks

rana m akmal Jul 30, 2010 5:43 AM

v-notch

v good

Sam Ancil Francis Jul 12, 2010 3:24 AM

Flume Construction

Hi,
we constructed a flume recently in the middle east and there were some design related issue. The panels were precast of 2m width and was jointed by using sealant material. there is a 10cm blinding concrete behind both the sides of the slabs and at the bottom we provided counter weight concrete slabs of minimum thickness 50cm. the total inner height of the slab is 1.6m and the water table stands at 1.2m. now the problem is that the sand fines are settling and we are having cracks in the sealant and the side concrete layers are shifting outward.
anybody has comments about this please reply. for any further details please email me.
any design related suggestions please mail me.

Harlan Bengtson Jun 29, 2010 11:41 AM

flume identification

The flume you describe is similar to a cut throat flume, because it has only a converging and a diverging section with no extended throat, however the dimensions don't match those for a 2" cut throat flume. I'm not familiar with any standard type of flume with the proportions you've given.

Jason van Breda Jun 29, 2010 9:19 AM

Flume identification

HI. I came across a flume with an unknown identity. I thought it might be a cut throat but the dimensions are not right. Will you help. The dimensions are as follows: Throat width 5cm, inlet width 13cm, outlet width 20cm. Inlet length 11cm, outlet length 40 cm. Total length 51cm.

Harlan Bengtson Mar 23, 2010 11:13 PM

RE: Open Channel Flow Measurement 1: Introduction to the Weir and Flume

Hello Mr. Craig,

Yes a rectangular weir that goes all the way across the channel is called a suppressed rectangular weir and the length of the weir crest is equal to the channel width.

Also, see the article, 'Open Channel Flow Measurement 5: the Rectangular Weir', at: http://www.brighthub.com/engineering/civil/articles/65880.aspx, for more details about rectangular weirs.

Harlan Bengtson

Tony Craig Mar 23, 2010 6:57 PM

Rectangular

Hello Dr Harlan, I am a unversity student in the UK on a master degree course. I have found your website very useful . I have a question you said a "A rectangular weir (horizontal across the top) may go all the way across the channel " dose that means that the length of weir is equal to the width of the weir?

Harlan Bengtson Feb 25, 2010 9:50 AM

equation for Parshall flume

An equation for the Parshall flume can be found in my civil engineering article: 'The Parshall Flume for Open Channel Flow Measurement', at www.brighthub.com.

Ritul Dobariya Feb 25, 2010 12:40 AM

equation of parshall flume

i have no equation for parshall flume for open channel

Open Channel Flow Measurement 1: Introduction to the Weir and Flume

Introduction

Weir Basics

Types of Sharp Crested Weir

The Flume For Flow Rate Measurement

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