Embed Size (px)
Citation preview
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
