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Water Measurement


 

It is essential that Refuge and Hatchery water use be measured and documented for several reasons.

  • In order to meet the requirements of state water law.

  • To document use for any future changes and/or adjudications.

  • To establish a water availability baseline that will identify and quantify impacts from the use of junior water rights in order to seek remedy from those impacts.

  • To document water requirements in order to apply for new water rights.

  • To provide data for hydrologic water budget studies in order to develop water management plans for the most effective use of dynamic, but usually limited, water supplies.

  • To provide baseline data to use in evaluating the impacts of changes in land use or development of water projects.

The following tools and methods are often used to measure and document water supplies and water use:

Recording Water Levels Volumetric Method
Current Meters Weir Method
Wading Rods Flume Method
Estimating Flows: Timing Floats Pressurized Conduits

RECORDING WATER LEVELS                
Staff Gages are installed either vertically or inclined.  The vertical staff gage can be used in a stilling well or in a stream.   A stilling well is a chamber that is hydraulically connected to the stream through intake pipes.   The stilling well eliminates turbulence that may occur in the stream and the elimination of waves and surges results in more accurate readings.   Vertical staff gages are usually used as reference gages for setting a recording device.   However, in some instances gages are installed without a recording device, when the gages are usually observed at a predetermined frequency.

An inclined staff gage usually consists of a graduated heavy timber or piece of channel-iron which is securely attached to a permanent foundation.  Inclined gages are built flush with the stream bank and are less likely to be damaged by floods, floating ice, or drift material than are projecting vertical staffs.

Stevens A-71 Recorders are instruments for producing a graphic record of the rise and fall of a water surface in a stream, lake, or stilling well.   They consist of time and stage elements which, when operating together, produce a continuous chart record showing elevation fluctuations of the water surface.  The time element is controlled by a spring-driven clock.   The stage element is controlled by a float.

Campbell Scientific CR10 Data Loggers are programmable data logger/controllers with multiple input capability.   Data is stored in the logger and/or in a storage module.   A laptop computer is needed to program the logger.   Data can be downloaded directly from the logger or remotely from the storage module.   Power can be supplied from a 12V battery, usually in conjunction with a solar panel.   Typically, these loggers are used with pressure transducers to determine water levels.

Stevens AXSYS Data Loggers are microprocessor-based, low power instruments with single or dual input capability.   Data is stored in the logger or on a credit-card sized data card.   The data card can be read by personal computers with card-readers, or an auxiliary card reader can be purchased.   The Stevens logger can be programmed using the built-in keypad.   Power can be supplied using a 12V battery and solar panel.   The Stevens data logger can be made to be compatible with Stevens Type A-71 recorders and with float-pulley systems.   They can also be used with pressure transducers.

Current Meters
The Marsh-McBirney Direct-reading Current Meter is an electromagnetic sensor that uses Faradays law of electromagnetic induction to measure water velocity.  This meter is far easier to use than mechanical type meters such as the Price Type AA current meter.  The transducer can be attached to a standard wading rod, and is connected by a cable to the meter.  The meter can be adjusted to give instantaneous or averaged readings to help take turbulence into account.  The readings can also be given in English or metric  units.  This meter does not produce good results in water less than 0.2 feet deep or in very low velocities.

Price Pygmy Current Meter is primarily used for discharge measurements in shallow streams (less than 1-foot depths).  It is similar in construction to the Price AA meter in that both types contain a cup-type bucket wheel that is mounted on a vertical shaft.  However, the Pygmy meter is two-fifths the size of the Price AA meter and has no tail piece.  The contact chamber is an integral part of the yoke and contains a single- revolution contact.  The rotational speed of the Price Pygmy meter bucket wheel is more than twice that of the Price AA meter.  Consequently, use of the Pygmy meter should be limited to velocities of 0.20 to 4 fps or less.

Wading Rods
The two types of wading rods most commonly used in current measurements are the top-setting rod and the round rod.   The top-setting rod is preferred because of the convenience in setting the meter at the proper measuring depth; the person making the measurement can keep their hands dry, and it may be used in making measurements when there is ice in the stream.   This type of measurement is conducted only when the person taking the measurement can safely wade the river.

Estimation of Flows by the Timed Observations of Floats
The float system is not used when a precise measurement of discharge is required, but is useful when conventional flow-measuring equipment is not available; conventional current meters are available but floating ice or other conditions make it difficult or impossible for their use; only an estimate of discharge is required; the velocity is too low to obtain reliable measurements with a current meter, and a rough check of the accuracy of a discharge rating table or flow meter is desired.

Measurement of Flows by the Volumetric Method
The volumetric measurement of discharge is the most accurate method of measuring small flows.   This type of measurement is usually made where the flow is concentrated in a narrow stream, or can be concentrated so that all the flow can be diverted into a container.   The following are examples of sites where volumetric measurements of discharge can be made:

A weir where the flow is sufficiently concentrated so that all of the water passing over it can flow into a container.

Small flows falling over a natural structure such as a large rock or earthen dam which concentrates the flow so that it can be captured in a container.

Measurement of Flows by the Weir Method
A weir is a calibrated structure used to relate water-level (i.e., head) to flow (i.e., discharge) in an open channel.  An open channel is a channel at atmospheric pressure where the only force acting on the water is gravity.  A weir consists of a bulkhead, notch, crest, and gage.  Water flow over a weir creates a weir pond, head, nappe, and flow contractions.  Each of these terms is discussed below.

Bulkhead
The bulkhead is the wood, metal, or concrete portion of the weir which lies perpendicular to the direction of flow and dams the water.

Notch
The notch is the specially-shaped opening in the bulkhead through which the water flows.

Crest
The edge or surface that the water passes over is the crest.  Weirs can be "sharp-crested" or "broad-crested".   A sharp-crested weir has a thin sharp edge on the upstream side so that the water springs clear and does not contact the downstream portion of the crest.   The crest of a broad-crested weir is too thick for the water to spring past the downstream side of the crest.   Only sharp-crested weirs will be discussed herein.

Nappe
The nappe is the sheet of water which passes through the notch and falls over the weir crest.   When the downstream water surface is far enough below the crest to allow air to circulate beneath the nappe, the flow or drop is said to be "free" or "critical."  If air does not freely circulate beneath the nappe, then the flow is "submerged" or "subcritical."  Weirs are calibrated for free-flow conditions and, thus, submerged flow conditions are not desirable and can result in erroneous readings.

Weir Pond
The weir pond is the pooled area upstream of the bulkhead which stills (i.e., quiets) the water flow.

Head
The head is the difference in elevation between the weir crest and the water surface at a specified distance upstream of the bulkhead in the weir pond.   The head is the value that is correlated with discharge.

Staff Gage
A staff gage is a metal, plastic, or fiberglass plate calibrated with incremental lengths, usually expressed in feet, which is used to measure the head.

End Contractions
The horizontal distances from the ends of the weir crest to the side walls of the channel are called the end contractions.   These contractions accelerate the flow and cause the water to spring away from the crest.

Bottom Contraction
This is the vertical distance from the weir crest to the bed of the channel.

For a weir of a given size and shape with the proper flow conditions, only one water level can theoretically exist in the weir pond for a given discharge.   That is, there is always a unique head-to-discharge relationship.   The basic equation for all weirs relate the head above the weir crest to discharge.  Modifications are made to the basic weir equation for the types of weir, the velocity of the water approaching the weir, the degree of contractions, and the degree of submergence.

Weirs are classified based on the shape of the opening through which the water passes.  Types of Weirs include:

V-Notch Weir
The V-notch weir is the most accurate weir to measure small flows, especially those under one cfs.   The V-notch in the bulkhead is usually 90 degrees, that is, each side slopes 45 degrees from the vertical and the crest has a length of zero.

Suppressed Rectangular Weirs
These weirs are less common.   They do not have end contractions and the crest extends across the entire width of the channel.  Consequently, the nappe does not contract from the width of the channel.   For this type system, it is usually necessary to place vents under the nappe on both sides of the weir box in order to obtain adequate aeration under the nappe.

Trapezoidal
The trapezoidal or Cipolletti weir has a trapezoidal-shaped notch.  Standard trapezoidal weirs are sharp-crested.   The crest is level and the sides are inclined outward with a slope of one horizontal unit to four vertical units.

Flash-Board and Stop-Log Structures
Flash-Board and Stop-Log Structures can be used as types of weirs with moveable crests.   The top of the highest log or board determines the current elevation of the crest.   In order to obtain a correct head measurement, one must first obtain the elevation or gage height of the crest by measuring down from the reference mark to the weir crest.   The crest elevation is then subtracted from the water level elevation obtained from the staff gage to obtain the head over the crest.   Discharge can then be derived by referring to the structure's head-discharge rating table

Measurement of Flows by the Flume Method
The following description of a flume and its operational theory is from the ISCO Open-Channel Flow Measurement Handbook, 1989:  

A flume is a specially shaped open channel flow section that restricts the channel area and/or changes the channel slope, resulting in an increased velocity and a change in the level of the liquid flowing through the flume.  Normally, a flume consists of a converging section to restrict the flow, a throat section, and a diverging section to assure that the downstream level is less than the level in the converging section.  The flume restricts the flow then lands it again in a definite fashion.  The flow rate through the flume may be determined by measuring the head on the flume at a single point, usually at some distance downstream from the inlet.  The head-flow rate relationship of a flume may be defined by either test data (calibration curves) or by an empirically derived formula.

The parts of a generalized flume are:

wingwalls
converging section
crest
throat
diverging section
point of measurement of the upstream head, (Ha)

point of measurement of the downstream head, (Hb)

Water is channelized by the wingwalls and enters the upstream portion of the flume known as the converging section.  The point of measurement of the upstream head (Ha) is in the converging section.   The water is constricted at the throat and passes over the crest (i.e., the point where the slope breaks at the beginning of the throat).   The point of measurement of the downstream head (Hb) is in the throat.   Types of flumes include:

Parshall
The most widely known and used flume.   It was designed in the 1920's by R.L. Parshall in order to provide a measuring device which did not have the limitations of weirs and existing flume designs.   Some irrigation canals with flat gradients did not have the head loss (i.e., stream gradient) necessary for proper weir operation.   In addition, many canals had low banks and could not accommodate the weir's required pooling and "stilling" of water.  Parshall flumes can also operate satisfactorily with a percentage of submergence, however two readings must be taken at over 70% submergence, and a correction applied.   There are several firms that manufacture pre-fabricated Parshall flumes.

Cut-throat Flumes
The cutthroat flume was developed by researchers at the Utah Water Research Laboratory in the 1960's.   This is a flat-bottomed flume having only an entrance and an exit section, and no throat section.   These flumes can be used in situations similar to those in which Parshall flumes can be used, however, they are not as widely used, and would probably have to fabricated in the field or at the station.

Long-throated or Ramp Flumes
These flumes were developed by the Bureau of Reclamation to provide flexibility in fitting them to complex channel shapes.   They consist primarily of a trapezoidal channel with an approach ramp transition from the approach channel invert.   The crest drops vertically at the downstream end back to the downstream canal invert.   They have also been called Replogle flumes and modified broad-crested weirs.   They perform well in channels with low gradients and can perform under submerged conditions.   They also have to be fabricated specifically for the site where they will be used.

Measurement of Flows in Pressurized Conduits
The basic types of measurement devices fall into the following categories:

  • Commercial mechanical meters
  • Magnetic meters
  • Sonic meters
  • Constant-area, variable-pressure-drop meters
  • Averaging pitot tubes

All of the described meters, except one type of commercial mechanical meter (a displacement meter), measure velocity.   Flow is the product of velocity and pipe cross-sectional area.   Therefore, each flow meter is manufactured for a specific diameter pipe.   It is important that meters are not reinstalled in larger or smaller pipes.   It is also important that pressurized flow is present to ensure that water is flowing across the full pipe cross-section.   Pressurized flow is liquid flow in a closed conduit where pressure accounts for at least a portion of the energy head.

Both velocity types or displacement types of mechanical meters have an indicator dial mounted on the face of the meter which shows instantaneous discharge as well as a totalizing head that records the total volumetric units of flow, such as acre-feet.  In addition to this indicator dial, meters can also be fitted with an electronic transmitter to remotely indicate flows and volumes either digitally or on chart recorders.

Velocity-type Meters
Velocity-type meters measure the velocity of water flow in a pipe.  The meters are calibrated for a specific pipe size and velocity measurements are converted directly into flow rates (cfs) and volumes (AF) on dials, indicators or transmitters.   The most commonly used velocity meters are of the propeller meter type which utilize a multi-bladed rotor to transmit motion.   This rotor is called either an impeller or a propeller by the various manufacturers.

Displacement-type Meters
Displacement-type meters measure the volume of water flowing in a pipe by repeatedly filling and emptying a space of constant volume.  It utilizes either an oscillating piston or nutating disc to measure this volume.   A nutating disc is one that wobbles around its axis of rotation.   This type of meter is extremely accurate even at very low flows, and unlike propeller meters, they are not as sensitive to flows with uneven velocity profiles.

Magnetic Meters
Magnetic meters operate in accordance with the principle established by Faraday's law which states that an electrical conductor moving through a magnetic field will induce a voltage.  For magnetic meters, the electrical conductor is the fluid flowing through the pipe.   Sensors in the meter create the magnetic field and measure the induced voltage.   The measured voltage is proportional to the velocity of the fluid flow in the pipe and the meters convert this voltage measurement into flow rates.

Ultrasonic Meters
Ultrasonic meters produce a pressure wave to measure water flow.  The term ultrasonic is used because the frequency of the produced pressure waves are above the range audible to human hearing.  Two types of ultrasonic meters are manufactured: those using the Doppler technique, and those using the transit-time technique.   The Doppler technique measures water velocity by emitting an ultrasonic signal and measuring the frequency of the reflected waves.   This type of meter requires the presence of suspended particles or air bubbles to reflect the waves.   The transit time technique measures water velocity by measuring the time it takes the pressure waves to move between two transducers.   A transducer is an electronic device that both sends and receives a pressure wave.   Meters that use the transit-time technique require clear water to operate properly.   Both types of meters use transducers and an electronic processor.

Constant-Area, Variable-Pressure-Drop Meters
Three types of constant-area, variable-pressure-drop meters, also knows as obstruction meters, are available: Venturi meters, flow nozzles, and orifice plates.  The operation of these meters is based on an energy equation resulting from application of the principle of conservation of energy to fluid flow.  The energy equation is known as the Bernoulli theorem which relates the pressure energy to the kinetic energy (velocity).   In these types of meters, water is passed through a constricted section of pipe, increasing its velocity.   The pressure head is measured above (D1) and at (D2) the constriction.  The differential pressure (h) is proportional to the square of the velocity.   Therefore, with the cross-sectional areas of the pipe and constricted section known, the flow rate can be determined.

Venturi Meters
These meters can accurately measure water flow rates.   Because they contain no moving parts, they also require very little maintenance.

Flow Nozzles
Flow Nozzles are, in effect, venturi meters that have been shortened.   The conical entrance section of the Venturi meter has been replaced by a shortened and streamlined entrance and the conical exit section has been omitted.   The absence of the exit section results in substantial turbulence where the water exits the nozzle.   Because of this, the loss of head through the flow nozzle is much greater than that caused by the venturi meter.   However, because of their smaller size, flow nozzles are easier to install and less expensive than Venturi meters.

Orifice Meters
These meters are even simpler than the flow nozzle.   It consists of a thin-plate with a hole in the center inserted in a pipeline.   The upstream piezometer tube is attached one pipe diameter upstream from the plate.   The downstream tube is attached at the vena contracta, the location of highest velocity.   The vena contracta is a natural contraction of the water jet, as opposed to the formed contraction found in the Venturi and flow tubes.  Orifice meters are less expensive than both the flow tube and venturi meter; however, the loss of head is much greater through the orifice meter.


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