2010 Montana

Association of Dam and

Canal Systems

Workshop

Billings, Montana

Water Measurement

A Basic Introduction to

Surface Water Measurement Devices

October 2010

Larry A. Schock DNRC MRO

Civil Engineering Specialist

• Headgates

• Types of flow

• Flow measurement basics

• Rated devices

• Flumes and Weirs

• Manual measurements

Water Measurement

Water MeasurementHeadgates

Water Measurement

• Selecting the right measuring device for the situation

• Proper installation

• Maintenance

Rated and standard devices:

• staff gages

• flumes

• Weirs

Manual measurement:

• float-area method

• current meters

Water Measurement

Water MeasurementTypes of Flow

Open Channel Flow Closed Conduit Flow

Water MeasurementOpen Channel Flow

• Occurs whenever the flowing stream has a free or

unconstrained surface that is open to the atmosphere

• The force that causes flow is the force of gravity on the fluid

• Canals and streams

Water MeasurementClosed Conduit Flow

• Occurs when the conveyance conduit carries water under

pressure

• No free surface open to the atmosphere

• Pipelines

Water Measurement

• Most devices measure flow indirectly

• Classified into those that measure velocity and those that

measure pressure or head

• All measurement devices only provide an estimate of the flow

• Some are more accurate than other

Water Measurement

• Flow Rate or discharge is the volume of water passing a flow

section per unit time

• The most common flow rate units are either cubic feet per

second (cfs), or gallons per minute (gpm), and occasionally

Miners Inch (MI).

Water Measurement

Flow rate (discharge) units

1 cubic foot per second (cfs) is equivalent to:

448.8 gallons per minute (gpm)

40 miner’s inches (MI)

1 Montana miners inch (MI) is equivalent to:

11.2 gallons per minute (gpm)

40 MI equals 1 cubic foot per second (cfs)

Water MeasurementVolume Units

Standard unit of volume is acre-feet (ac-ft)

An ac-ft is equivalent to a 1 foot of water on one acre

OR

325, 851 gallons

1 cfs produces a volume of 1.98 ac-ft per day

Water MeasurementRated Devices

• Staff Gages

• Flumes

• Weirs

Water MeasurementStaff Gages

Water MeasurementFlumes and Weirs

Flume

An open-channel flow section that

forces flow to accelerate through a

known channel shape. A minimum

head of 0.2 feet is needed.

Weir

An overflow structure built

perpendicular to an open channel, for

use on slopes > 0.5%.

Water MeasurementFlume Classes

Long-Throated

Controls discharge rate in a throat

that is long enough to cause nearly

parallel flow lines. Ex. Ramp Flume

Short-Throated

Controls discharge in a region

that produces curvilinear flow.

Ex. Parshall Flume

Water MeasurementParshall Flumes

Advantages

• low head loss requirement

• allows debris passage

• wide range of sizes and flows

Disadvantages

• expensive to buy

• difficult to build

• installation accuracy critical

Water MeasurementMontana Flume (short parshall)

Advantages

• low head loss

• conveys sediment and debris

• measures a wide range of flows

• easy to build

Disadvantages

• will not measure when submerged

Installation Requirements:

• A straight clean section of ditch, clear of obstructions

• Must be level lengthwise and cross wise

• Flume floor must be set above the elevation of the ditch bottom

• Staff gage set at floor of converging section (crest)

Water MeasurementShort -Throated Flumes

Problem??

Water MeasurementLong-Throated Flumes

Long-Throated flume (Ramp or Replogle) under construction

Water MeasurementLong-Throated Flumes

• Long-throated flumes control discharge rate in a throat that is

long enough to cause nearly parallel flow lines in the region of

flow control

• Long-throated flumes are more accurate, less expensive, have

better technical performance, and can be computer designed

and calibrated.

Water MeasurementLong-Throated Flumes

Water MeasurementLong-Throated Flumes

Advantages

• Provided that critical flow occurs in the throat, a rating table can be

calculated with an error less than 2%

• Long-throated flumes can have nearly any desired cross-sectional shape

and can be custom fitted into most canal-site geometries

• Because of their gradual converging transition, these flumes have few

problems with floating debris and sediment

Water MeasurementWeirs

A weir is an overflow structure built perpendicular to an

open channel axis to measure the rate of flow of water.

Water MeasurementWeir Classifications

Sharp-crested

A sharp-crested weir has a notch plate that is mounted on bulkhead such that water does not contact or cling to the downstream weir plate or bulkhead, but springs clear.

Broad-crested

A broad-crested weir is a raised overflow crest, commonly a flat block.

bulkhead

approach velocity

crest

notch

nappe

notch plate or

metal strip

Water MeasurementSharp-Crested Weir

• Contracted Rectangular

• Suppressed Rectangular

• Cipolletti Contracted

• Contracted Triangular or V-Notch

Standard Types

Water MeasurementSharp-Crested Weir

Contracted Rectangular

Water MeasurementSharp-Crested Weir

Cipolletti Contracted

Trapezoidal in shape with sides that incline outwardly at a slope of 4:1

Water MeasurementSharp-Crested Weir

Contracted Triangular or V-Notch

Water MeasurementSharp-Crested Weir

Installation Requirements for all Sharp-Crested Weirs

• Weir should be installed in straight section of ditch/canal.

•Approach velocity should be <= 0.5 feet/second (appear still).

• The weir should be perpendicular to the channel.

• All weir blades should have the same thickness for the entire boundary of the overflow crest.

• The upstream edges of the weir plates must be straight and sharp.

• The entire crest should be plumb and level.

Water MeasurementBroad-Crested Weir

• A broad-crested weir is a raised overflow crest, commonly a

flat block.

• No clear-cut classification distinction or hydraulic difference

exists between broad-crested weirs and long-throated flumes.

Water MeasurementWeirs vs. Flumes

• Weirs do not work well on flat slopes, flumes do.

• Weirs have approach velocity requirement, flumes do not.

• Flumes can often be expensive and difficult to build, weirs can be easily built.

• Weirs can collect sediment and debris, flumes usually do not.

Water MeasurementManual Measurements

• Float-area method

• Current meters

Water Measurement Float-Area Method

Advantages

• better than a guess

Disadvantages

• difficulty in determining average cross section

• susceptible to wind currents, surface disturbances, and cross currents

• least accurate of all other methods, not applicable for enforcement

Water MeasurementCurrent Meters

Water MeasurementCurrent Meters

Types of current meters

Anemometer and propeller velocity meters

• use anemometer cup wheels to sense velocity

– Price AA

– Pygmy

Electromagnetic meters

• Electromagnetic current meters produce voltage proportional to the

velocity

– Marsh-McBirney

Water MeasurementCurrent Meters

Types of Current Meter Measurements

Wading Cable supported Bridge

Water MeasurementRequirements

• Selecting the right measuring device for the situation

• Proper installation

• Maintenance

Questions?

Larry A. Schock

(406) 542-5885

lschock@mt.gov