Discharge Measurement Equipment and Meter Maintenance

September 2011 – Glen Hess ,TESNAR, Oregon (Many slides from USGS Surface Water Field

Techniques May 2008 Class)

Discharge Measurement Techniques

• Discharge measurements may be made using a variety of methods and equipment:

• The current meter is one of the most common tools used to measure discharge.

WadingCablewaysBridgesBoatsWeirs and flumes

VolumetricDye tracersAcoustic Doppler Current ProfilerIndirect measurementEstimates

Instruments and Equipment

• In addition to the current meter, a timer and a means of counting meter revolutions are needed to measure discharge. Additional equipment may be needed, depending on the manner in which the measurement is made – that is, by wading, cableway, bridge, boat, or under ice cover.

• A current meter measures the velocity of flowing water. The principle of operation is based on the proportionality between the velocity of the water and the resulting angular velocity of the meter rotor.

Instruments and Equipment• The number of revolutions of the rotor is obtained by an

electrical circuit through the contact chamber. Contact points in the chamber are designed to complete an electrical circuit at selected frequencies of revolution.

Contact chamber showing contact points and cat whisker.

Instruments and Equipment• The electrical impulse produces an audible click in a

headphone or registers a unit on a counting device. The intervals during which meter revolutions are counted are timed with a stopwatch.

Headset and stopwatch for wading rod.

Instruments and Equipment• The digitizers shown here automatically count current

meter revolutions. Aquacalc by JB Instruments

Instruments and Equipment• Current meters generally can be classified with respect to

two main types: those meters having vertical-axis rotors and those having horizontal-axis rotors.

• The vertical-axis Price-AA current meter is extensively used by the USGS.

Price AA Current Meter• On most meters, both single-count and penta-

count are available.

• A tailpiece issued to align the meter parallel to flow.

• A cat whisker or magnetic head is inside to

measure rotor rotations.

• Should not use meter if depth is less than 1.25 feet.

Price AA Current Meter• Should not use if within 0.5 feet of water surface

or stream bed.

• Raise cups off the pivot when not in use.

• Revolutions can be counted using a headset.

• Can be used with a current-meter digitizer (CMD), AquaCalc or PDA which records both revolutions and time.

Price AA Current Meter

• Available with an Optic head.

• Available with magnetic head for use with electronic counters.

• Don't use for velocities less than 0.2 feet/sec.

• See Office of Surface Water Memorandum 89.07 on procedures to ensure accurate performance of current meters

Price Pygmy Meter• In shallow depths (0.3 to 1.5 ft), a Price pygmy meter,

scaled two-fifths as large as the standard meter is used.

Current Meters

To insure reliable observations of velocity it is

necessary that the current meter be kept in good

condition. Meter repairs by the hydrographer

should be limited to minor damage only. See

Office of Surface Water Memorandum 89.07.

(handout here, field description tomorrow)

Current Meters

To determine the velocity of the water from the

revolutions of the rotor of a conventional current

meter, a relation must be established between the

angular velocity of the rotor and the velocity of

the water that spins the rotor. That relation is

known as the rating of the current meter.

Current Meter Rating Table (1970)

FlowTracker by FlowTracker by wading wading

Advancements in

the technology of

wading discharge

measurements…

Acoustic concepts later by Marc Stewart

What is a FlowTracker?What is a FlowTracker?

Uses acoustic doppler technology to Uses acoustic doppler technology to measure current velocitymeasure current velocity

Can only be used for wading discharge Can only be used for wading discharge measurementsmeasurements

Replaces both the pygmy and Price AAReplaces both the pygmy and Price AA Attaches to standard wading rodAttaches to standard wading rod No moving parts—minimal maintenanceNo moving parts—minimal maintenance Consists of a probe and a handheld controllerConsists of a probe and a handheld controller

• Probe consists of a temperature probe, Probe consists of a temperature probe, acoustic transmitter, and acoustic receiveracoustic transmitter, and acoustic receiver

• Controller records data and diagnostics and Controller records data and diagnostics and calculates discharge (like an AquaCalc) calculates discharge (like an AquaCalc)

Costs $7,000Costs $7,000 Versions of software and firmwareVersions of software and firmware

New (v2.01) Old (v1.30)New (v2.01) Old (v1.30)

AcousticsAcoustics

Measures in water as shallow as 0.1 ft.Measures in water as shallow as 0.1 ft. Measures velocities as low as 0.003 ft/s, Measures velocities as low as 0.003 ft/s,

outperforms pygmy meters in low velocity meas.outperforms pygmy meters in low velocity meas. Never requires calibrationNever requires calibration OSW and HIF approvedOSW and HIF approved Sample volume is .25 cmSample volume is .25 cm22, ,

4 inches from transmitter4 inches from transmitter Boundary ProblemsBoundary Problems Low Backscatter ProblemsLow Backscatter Problems

(<10 SNR) (<10 SNR)

Must always keep FlowTracker perpendicular to tagline not flowMust always keep FlowTracker perpendicular to tagline not flow

Where do you position the FlowTracker?Where do you position the FlowTracker?•With the rod at the station on the tag line?With the rod at the station on the tag line?

•With the rod 2” to the left of the station?With the rod 2” to the left of the station?

?

Rod

FlowTracker Mount

4”

FlowTracker OrientationFlowTracker Orientation

USGS and Marsh McBirney USGS and Marsh McBirney Meters: Comparison tests Meters: Comparison tests show the electromagnetic show the electromagnetic

instrument is not reproducible, instrument is not reproducible, thus USGS does not use themthus USGS does not use them

Sounding Equipment

• Sounding (determination of depth) is usually done mechanically--the exact equipment used depends on the type of measurement being made. Depth and position in the vertical are measured by a rigid rod or by use of a sounding weight suspended from a cable.

• Sounding equipment used by the U.S. Geological Survey includes: wading rods, sounding weights, sounding reels, hand lines, and sonic sounders.

Sounding Equipment

Standard top-setting rod.Sounding reel used to suspend

weightand meter from a bridge board.

Sounding Equipment• The two types of wading rods commonly used are the

top-setting rod and the round rod.

• The top-setting rod is preferredbecause the meter can be convenientlyset to the proper measuring depth. Thehydrographer's hands remain dry whilesetting the observation depth. The rodis placed in the stream so the base platerests on the streambed, and the depth ofwater is read on the graduated main rod.

Sounding Equipment• If a stream is too deep or too swift to wade, the current

meter is suspended in the water by cable from a boat, bridge, or cableway. A sounding weight is suspended below the current meter to keep it stationary in the water. The weight also pre-vents damage to themeter when theassembly is loweredto the streambed.

Sounding Equipment• A sounding reel has a drum for winding the sounding

cable, a crank and ratchet assembly for raising and lowering the weight or holding it in any desired position, and a depth indicator.

• The U.S. Geological Survey has five types or sizes of sounding reels in common use, the choice of reel is dependent on the depth of water to be measured and on the weight required for sounding. A reel, B Reel, E reel are most commonly used reels.

Sounding Equipment

A Reel Sounding reel on bridge board on Pilgrim Creek at Grand Teton National Park, WY.

Width-Measuring Equipment• The distance to any point in a cross section is measured from

an initial point on the bank. Cableways and bridges used regularly for making discharge measurements are commonly marked at 2-, 5-, 10-, or 20-ft (0.61-, 1.52-, 3.05-, or 6.10-m) intervals by paint marks.

Paint marks on the bridge over theSnake River near Jackson, WY

Width-Measuring Equipment• For measurements made by wading, from boats, or from

unmarked bridges, steel or metallic tapes or tag lines are used.

Three commonly used tag-line reels. The yellow reel holdsa kevlar line that will float. The other two reels hold beaded steel line. 

Equipment Assemblies• The cableway provides a track for the operation of a cable car

from which the hydrographer makes a current-meter measurement. Cable cars also support the sounding reel and other necessary equipment. Both sit-down and standup types of cable cars are usedin stream gaging.

Standup cableway on ColoradoRiver at Cisco, UT.

Equipment Assemblies• Sit-down cable cars have a variety of means of supporting the

sounding reel. A-pack and Canfield reels are designed to clamp on the side of the car.Permanent or portablereel seats are attachedto the cable cars for A-pack or larger reels.

Equipment Assemblies• Bank operated cableways are being used more often on smaller

streams (up to 150 ft) for measuring discharge as well as for sediment sampling. These systems are safer than traditional cableways because the hydrographer is not over the water.

Example of bank operated cableway found at the Hydrologic Instrumentation Facility, MS.

Equipment Assemblies• All cableways must be periodically inspected according to

policy established in WRD memorandum 91.07.

Load testing cableway on Provo River at Woodland, UT.

Equipment Assemblies

• When measuring from a bridge, the meter and sounding weight can be supported by a hand line, by a sounding reel mounted on a crane, or by a bridge board.

• All cranes are designed so that the superstructure

can be tilted forward over the bridge rail far

enough for the meter and weight to clear most rails.

Equipment Assemblies

Four-wheel crane being used in a discharge measurement. 

Equipment Assemblies• Bridge boards, usually a plank with a sheave at one end over

which the meter cable passes and a reel seat near the other end, may be used with an A-pack or A-55 sounding reel and with weights up to 100 lb.

Discharge measurement made with a bridge board on Pilgrim Creek at Grand Teton National Park, WY.

Equipment Assemblies• Many special arrangements for measuring from bridges have

been devised to suit particular purposes. Truck-mounted cranes are often used for measuring from bridges over larger rivers. Monorail stream-gaging cars have been developed for large rivers.

Truck-mounted crane.

Next: Measurement Documentation

(Very important to complete)