Discharge Measurment

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USGS Home Contact USGS Search USGS Water Science for Schools Search Links Glossary Site map Help <Contact Home Water Basics Earth's Water Water Cycle Special Topics Water Use Acti vity Center Wa ter Q&A Pictures Back to previous page Printer-friendly version of this complete article (PDF). How Streamflow is Measured If you're a teenager, I imagine your favorite activity is to sit with your parents on a quiet river bank, drink your glass of lemonade, and ponde r the complexities of life. Probably the first question you ask is "How much water is flowing in this river?" You've come to the right place for an answer. The U.S. Geological Survey has been measuring streamflow on thousands of rivers and streams for many decades and by reading this set of Web pages you can find out how the whole streamflow-measurement process works. Often during a large rainstorm you can hear an announcement on the radio like "Peachtree Creek is expected to crest later today at 14.5 feet." The 14.5 feet the announcer is referring to is the stream stage. Stream stage is important in that it can be

Transcript of Discharge Measurment

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How Streamflow is Measured

If you're a teenager, I imagine your favorite activity is to sit with your parents on a quiet river bank, drink your glass of 

lemonade, and ponder the complexities of life. Probably the first question you ask is

"How much water is flowing in this river?" You've come to the right place for an answer.

The U.S. Geological Survey has been measuring streamflow on thousands of rivers andstreams for many decades and by reading this set of Web pages you can find out how the

whole streamflow-measurement process works.

Often during a large rainstorm you can hear an announcement on the radio like"Peachtree Creek is expected to crest later today at 14.5 feet." The 14.5 feet the

announcer is referring to is the stream stage. Stream stage is important in that it can be

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used (after a complex process described below) to compute streamflow, or how much

water is flowing in the stream at any instant.

Stream stage (also called stage or gage height) is the height of the water surface, in feet,above an established altitude where the stage is zero. The zero level is arbitrary, but is

often close to the streambed. You can get an idea of what stream stage is by looking atthis picture of a common staff gage, which is used to make a visual reading of stream

stage. The gage is marked in 1/100th and 1/10th foot intervals.

Introduction to USGS Streamgaging

The U.S. Geological Survey (USGS) started its first streamgage in 1889 on the Rio

Grande River in New Mexico to help determine if there was adequate water for irrigation

 purposes to encourage new development and western expansion. The USGS operatesover 7,000 streamgages nationwide. These streamgages provide streamflow information

for a wide variety of uses including flood prediction, water management and allocation,

engineering design, research, operation of locks and dams, and recreational safety andenjoyment.

Streamgaging generally involves 4 steps. Click on the links below to explore each topic.

1. Measuring stream stage —obtaining a continuous record of stage—the height of 

the water surface at a location along a stream or river 2. The discharge measurement —obtaining periodic measurements of discharge (the

quantity of water passing a location along a stream)

3. The stage-discharge relation —defining the natural but often changing relation

 between the stage and discharge, and

4. Converting stage information to streamflow information —using the stage-discharge relation developed in step 3 to convert the continuously measured stage

into estimates of streamflow or discharge

Streamflow summary

Streamgaging involves obtaining a continuous record of stage, making periodic discharge

measurements, establishing and maintaining a relation between the stage and discharge,

and applying the stage-discharge relation to the stage record to obtain a continuous recordof discharge. The USGS has provided the Nation with consistent, reliable streamflow

information for over 115 years. USGS streamflow information is critical for supportingwater management, hazard management, environmental research, and infrastructuredesign. For more information on USGS streamgaging, go to the USGS Web site at

http://water.usgs.gov. The National Streamflow Information Program offers more

information on this topic., Go to the USGS Office of Surface Water Web site for moreinformation on surface-water activities, and the USGS WaterWatch site gives you current

streamflow conditions nationwide or in your area.

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Printer-friendly version of this complete article (PDF).

Sources and more information

Discharge measurements at gaging stations, USGS Techniques manual• A Day in the Life of a USGS Water Scientist

How the stream height (stage) relates to the amount of water flowing in a stream

Real-time USGS streamflow data 

The water cycle: Streamflow 

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U.S. Department of the Interior  | U.S. Geological Survey

URL: http://ga.water.usgs.gov/edu/measureflow.html

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Method of Measuring River Disc 

• The first step in making a conventional current-meter measur

discharge is to select a measurement cross section of desirabl

stream can be waded, the hydrographer looks for a cross sectimeeting as many of the following criteria as possible:

1. Cross section lies within a straight reach, and streamlines are

other 

2. Velocities are greater than 0.5 ft/s (0.15 m/s) and depths are g(0.15 m).

3. Streambed is relatively uniform and free of numerous boulde

aquatic growth.4. Flow is relatively uniform and free of eddies, slack water, and

turbulence.

5. Measurement section is relatively close to the gauging-station

the effect of tributary inflow between the measurement sectioto avoid the effect of storage between the measurement sectio

during periods of rapidly changing stage.

An ideal cross section for a discharge

measurement.

 

• After the cross section has beenselected, the width of the stream is

determined. A tag line or measuring

tape is strung at right angles across themeasurement section for measurements

made by wading, from a boat, or from

an unmarked bridge.

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•  Next the spacing of the verticals is

determined to provide about 25 to 30subsections. The verticals should be so

spaced that no subsection has more

than 10 percent (ideally 5 percent) of 

the total discharge.

• After all items listed above have been done, the measurementrecording observations are prepared. The following informati

recorded for each discharge measurement:

1. Name of stream and location to

correctly identify the establishedgauging station; or name of stream and

exact location of site for a

miscellaneous measurement.2. Date, party, type of meter suspension,

and meter number.3. Time measurement was started using

military time (24-hr clock system)

4. Bank of stream that was the starting

 point.

5. Control conditions.6. Gauge heights and corresponding

times.

7. Water temperature.

8. Other pertinent information regarding

the accuracy of the dischargemeasurement and conditions which

might affect the stage-dischargerelation.

• After the note sheet is readied, the meter assembly is checked

should balance on the hanger and should spin freely; the electthrough the meter should operate satisfactorily; and the stopw

check satisfactorily in a comparison with the hydrographer's w

recording on the note sheet the station (distance from initial pof water, the actual measurement is ready to be started.

• Depth (if any) at the edge of water is measured and recorded.

determines the method of velocity measurement to be used, n point method or the 0.6-depth method (See below).

• After the meter is placed at the proper depth and pointed into

rotation of the rotor is permitted to become adjusted to the sp

 before the velocity observation is started.

• Then the number of revolutions made by the rotor is counted to 70 s. The stopwatch is started simultaneously with the first

which is counted as "zero," and not "one."

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• The stopwatch is stopped on a convenient number coinciding

given in the column headings of the meter rating table and is

nearest second. The number of seconds and the number of revrecorded.

• The hydrographer moves to each of the observation verticals

above procedure until the entire cross section has been traverCurrent-meter measurements by wading

• Current-meter measurements are best made by wading, if con

Wading measurements have a distinct advantage over measur

from cableways or bridges in that it is usually possible to seleseveral available cross sections for the measurement.

• If depths or velocities under natural conditions are too low fo

current-meter measurement, the cross section should be modito provide acceptable conditions, for example by building tem

 by removing rocks and debris.

• The hydrographer should stand in a

 position that least affects the velocityof the water passing the current meter.

That position is usually obtained by

facing the bank so that the water flowsagainst the side of the leg. The wading

rod is held at the tag line by the

hydrographer who stands about 3 in(0.07 m) downstream from the tag line

and at least 1.5 ft (0.46 m) from the

wading rod.

• The wading rod should be held in a vertical position with the

the direction of flow while the velocity is being observed.

• When measuring streams having shifting beds, the soundings

 be affected by the scoured depressions left by the hydrographstreams, the meter should be placed ahead of and upstream fr

Current-meter measurements from bridges

• Bridges are often used for making discharge measurements o

cannot be waded. Such measurements are often satisfactory, bsections are usually superior.

• Either the upstream or downstream side of the bridge can be u

discharge measurement. The advantages of the upstream side

1. Hydraulic characteristics at the upstream side of bridge openmore favorable.

2. Approaching drift can be seen and thus can be more easily av

3. The streambed at the upstream side of the bridge is not likely

 badly as the downstream side.

• The advantages of using the downstream side of the bridge ar

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1. Vertical angles are more easily measured because the soundi

away from the bridge.

2. The flow lines of the stream may be straightened by passing opening with piers.

• Whether to use the upstream side or the downstream side of a

current-meter measurement should be decided individually foafter considering the above factors.

• Footbridges are sometimes used for 

measuring the discharge of canals,tailraces, and small streams. Usually

rod suspensions are used for the meter,

 but hand lines, bridge cranes, and bridge boards can also be used.

Measurement of Velocity• The current meter measures velocity at a point. The method o

discharge measurements at a cross section requires determina

velocity in each of the selected verticals. The mean velocity i

obtained from velocity observations at many points in that ve be approximated by making a few velocity observations and u

relation between those velocities and the mean in the vertical

methods of determining mean

vertical velocity are described below. Flow velocity in naturagenerally pulsates. Velocity is therefore measured for at least

effort to better represent average velocity at a point.

Two-point method

• When velocity profiles are relatively normal, it has been foun

velocity can be adequately estimated by averaging velocities

depth below the water surface.

• The vertical-velocity curve will be distorted by overhanging v

in contact with the water or by submerged objects, if those ele

to the vertical in which velocity is being measured. In this ca

velocity observation at 0.6 of the depth should be made.

• When a Price type AA (or type A) current meter is used, the t

cannot be applied unless depths are greater than 2.5 ft (0.76 m

Six-tenths-depth method

• In the 0.6-depth method, an observation of velocity made in tof the depth below the surface is used as the mean velocity in

depth gives the nearest velocity to the mean velocity (see tabl

Rantz).

 

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7. Head Measurements

The head is usually sensed either in the channel itself or in a stilling well located to one

side of the channel. The stilling well is connected by a small pipe to the channel. Many

methods can be used to detect the water surface in a stilling well or in the flume channel.

Some methods exploit the electrical conductance of water and capacitance of immersedinsulated wires. Sonic sensors depend on timing sound pulses reflected from the water 

surface. Measurement heads can also be determined with a variety of pressure sensing

devices. The most frequently used methods are wall-mounted staff gages in the entrancesection of the flume or in a stilling well or float-operated recorders placed in a stilling

well.

(a) Location for Head Measurement 

The measuring station for short-form flumes must be installed as specified to matchclosely the location used when the flume was empirically calibrated. For example, the

measuring station of a Parshall flume is in the convergence water surface drawdown. For long-throated measurement structures, the gaging or head-measuring station should belocated sufficiently far upstream to avoid detectable water surface drawdown, but close

enough for the energy losses between the gaging station and approach section to be

negligible. This placement is particularly critical if the ratings are based on coefficient

values in a discharge equation as discussed in Bos (1989). In the computer derivedratings, drawdown and friction losses caused by the gage location are an integral part of 

the calculation. Therefore, the gage should be located as indicated in the precomputed

long-throated structure design and selected tables.

(b) Selection of Head-Measurement Device 

The success or failure of the structure and the value of the collected data depend closely

on the proper selection of a suitable head measurement device. The three most important

factors that influence this selection are: (1) frequency of discharge measurement, (2)allowable error in the head detection, and (3) type of measurement structure under 

consideration.

The usual expected reading errors in the sill-referenced head are listed in table 8-1 for 

some common head measurement devices. The listed errors are higher than the expectedrandom errors, partly to compensate for the effects of several systematic errors, such as

zero-setting, instrument lag, reading error, temperature, and stilling well leakage. If no

device with sufficient accuracy is found from this procedure, two choices are available:(1) allow greater error in the measured discharge for the minimum required head loss or 

(2) redesign the structure with a narrower bottom width, resulting in a higher value of 

minimum measurement head.

(c) Staff Gages 

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Periodic readings on a calibrated staff gage may serve adequately when continuous

information on the flow rate is not required. Examples are canals where the flow changes

are gradual. The gage should be placed so that the water level can be read from the canal bank. The gage should be easy to clean.

Table 8-1. Common reading errors in flat crest reference head as detected byvarious devices

Device

Reading error  h1, ft

 If head detection is in 

Remarks

Open channel Stilling well

Point gage Not applicable 0.0015Commonly used for research

Dipstick Not applicable 0.003 Good for research/field use

Staff gage

0.013

0.023> 0.050

0.013

0.0160.023

1 <0.1

1 = 0.2

1 = 0.5

Pressure bulb +

recorder 0.066 Not required

Suitable for temporary

installations (Error = +2%h1max).

Bubble gage +

recorder 0.033 Not required

Stilling well is not required

 but can be used

Float-operated

recorder   Not applicable 0.016 Stilling well is required

Float totalizer attached to recorder 

- -Some additional randomand systematic error is

 possible

For concrete-lined canals, the gage can be mounted directly on the canal wall. The value

for measuring head on the sloping walls of trapezoidal-shaped canals must beappropriately converted to vertical head values before entering the discharge tables.

These tables are usually made for stilling well use or vertical gage applications. The

sloping gage can be marked to read direct values or equivalent values of vertical head.Sometimes, sloping staffs are marked to display discharge directly, but the discharge

gradations are not equally spaced. The gage may be mounted onto a vertical support for 

unlined canals.

Most permanent gages are enameled steel, cast aluminum, or some type of plastic resin.Enameled linear scales marked in metric or English units are available from commercial

sources. An example staff gage is shown on figure 8-4. Important flow rates can be noted

on these scales by separate markings, allowing convenient adjustment of control gates todesired discharges without requiring tables. For convenience, the gages can be marked

directly in discharge units rather than in measuring head units.

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Figure 8-4 -- Typical staff gage for measuring head or water stage.

(d) Stilling Wells 

For accurate discharge measurements, the effective head in flumes, accurately referenced

to a known elevation in the flume, must be measured. Head readings on staff gagesattached directly to the inside channel walls may be only estimates because of waves and

turbulent fluctuations on the scale face. Thus, stilling wells are connected by holes and

 pipes to the body of water at the measuring station to translocate head and dampen water surface fluctuations by throttling, which increases head measurement accuracy.

The pipe connecting the stilling well to the flume/canal should be large enough to allow

the stilling well to respond quickly to water level changes. Usually, this pipe diameter is

about one-tenth the diameter of the stilling well. However, special cases may requiremore dampening using smaller connecting pipe diameters.

The pipe connection to the stilling well should be perpendicular and carefully cut flush

with both the canal and the stilling well walls. Otherwise, the translocated water surface

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elevation in the well can deviate considerably from the actual elevation in the flume

 because of flow velocity impact or aspiration. Connections that are not flush and/or have

rough edges have different head losses depending on direction of flow in the connecting piping. This causes buildup or reduction of head in stilling wells compared to the actual

head in the measurement device.

The size of the stilling well depends on the method used to measure the head. The

diameter, if circular shaped, ranges from a recommended minimum size of 4 in for hand-inserted dipsticks to 18 in to accommodate larger diameter floats. Wells may be much

larger to provide access for cleaning or to make the reading of wall attached staff gages at

sight angles at least as flat as 30 degrees. It is recommended that well walls have a 2-inclearance from floats and weights used with recorders.

A stilling well may need to house the float and recorder system or other surface detecting

equipment. The wells may need to be tall enough to provide convenient access to

recorders for reference setting and maintenance. The wells may also need to be tall

enough to keep counterbalance weights from interfering with float movement.

Before making a measurement, the wells should be flushed with fresh water to be sure

they are free of sediment, foreign material, or blockages, which could cause erroneous

head readings. Recording equipment should be checked and serviced regularly. Cross-checks should be made between the staff gages, hook gages, plumb bobs, recorder values,

and any other discharge indicators to expose system errors. Thus, even when using

stilling wells, staff gages should still be used on the insidewalls of flumes for cross-checking. Further details on stilling wells can be found in chapter 6 and Bos (1989), Bos

et al. (1991), and Brakensiek et al. (1979).

(e) Gage Installation and Zero Setting 

The most important factor in obtaining accurate discharge measurements is the accuratedetermination of the sill-referenced head, h1. The upstream sill-referenced head can be

measured by a gage or recorder only if the observed water level is known with respect to

the weir sill (or flume crest) level at the control section. The method used to set (zero

register) the gage, recorder, etc., depends on the structure size, the flow rate in thechannel during the setting procedure, and available equipment. Standard surveying

techniques are practical for accurate setting of most wall or staff gages.

The canal side slopes usually only approximate the intended slope. Mounting sloped

gages so that a selected scale reading in the most frequently used range of the gagecoincides with the corresponding elevation for that reading will partially compensate for 

deviation from design slope. Thus, the greatest reading errors will occur in the flow

ranges that are seldom used. If this procedure causes the zero end of the scale to bedisplaced by more than about 0.015 ft, the actual side slope should be determined for 

adjustments to the calibration. This determination also should be made if accuracy over 

the full flow range is required.

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Several methods can be used to zero a water level recorder; three are particularly suitable.

The recorder can be set: (1) when the canal is dry, (2) when water is ponded over the

flume, or (3) when water is flowing through the flume. These zero-setting methodsassume that the sill-referenced elevation can be determined during the procedure. This

determination is not always possible, especially on wide structures. A stable and

 permanent surveying bench mark, such as a bronze cap placed in concrete, should beadded in an acceptable location near the measuring structure. Its elevation should have

 been previously established relative to the sill elevation. More detailed information on

zero-setting procedures is presented in Bos et al. (1991).

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