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Measurement�o

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ADEQ

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June

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Phoe

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The�University

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Outline

• Measurement�of�streamflow– Velocity�Area– Velocity�measurement– Selection�of�cross�section

• Measurement�of�stage– Use�of�controls

• Gaging�stations�and�recorders– Staff�gages– Water�level�sensors�and�Transducers

ADEQ�SW�Short�Course The�University�of�Arizona 2

Bottom�line:�one�page�summary

• Discharge�is�Area�x�Velocity�(ft3/sec�=�cfs)• Method�to�measure�Velocity�depends�on�flow• Area�must�be�subdivided�into�>25�subsections• Stage�is�water�level�above�a�datum• “Controls”�are�necessary�to�control�x�section• Method�to�measure�Stage�depends�on�site

ADEQ�SW�Short�Course The�University�of�Arizona 3

water.usgs.gov

ADEQ�SW�Short�Course The�University�of�Arizona 4

USGS�Stream�Gauge�Network

ADEQ�SW�Short�Course The�University�of�Arizona 5

http://waterwatch.usgs.gov/?id=ww_current

NWS�– River�Forecast�Ctr View

ADEQ�SW�Short�Course The�University�of�Arizona 6

ADEQ�SW�Short�Course The�University�of�Arizona 7

What�is�the�origin�of�this�data?

ADEQ�SW�Short�Course The�University�of�Arizona 8

U.S. Department of the InteriorU.S. Geological Survey

Measurement of Streamflow

Definition of Streamflow

� Streamflow, or discharge, is defined as the volumetric rate of flow of water (volume per unit time) in an open channel

� Streamflow in the USGS is usually expressed in English dimensions of cubic feet per second (ft3/s) or CFS.

Velocity-Area Method

Discharge = (Mean water velocity) x (Cross-section area of water)

orDischarge = (Water velocity) x ((Width) x (Depth))

Velocity-Area Method

Discharge =

Cross Section Area

X Mean Water Velocity

The Current Meter

Price AA PricePygmy

Acoustic Current Meter

� SonTek/YSIFlowTracker

ADCPs

ADCPs The Current Meter Method

Miscellaneous Measurement Methods

Miscellaneous Measurement Methods

Measuring High Flow Measuring High Flow

Conventional Current Meter-Method

• The USGS measures water depth and velocity in at least 20 locations or verticals (generally >25 verticals) across the channel.

• Velocity is measured at each location at water depths that best represent average velocity at that location.

Conventional Current Meter-Method

� The places where depth and velocity are measured (verticals) define subsections of the channel cross section.

� Discharge is determined by summing discharge in these subsections using the “mid-section method”

The Midsection Method Measurement of Velocity

� Current-meter measures velocity at a point� Discharge measurements require determination of

the mean velocity in each of the selected verticals� Mean velocity in a vertical can be approximated by

making velocity observations and using a known relation between those velocities and the mean in the vertical.

Velocity Curve Two-Point Method

� Generally used for depths greater than or equal to 2.5 ft

� Velocity observations are made at the 0.2 and 0.8 depth below the surface.

� The average of the two observations is used as the mean velocity in the vertical

Surface-Velocity Method

• In a natural channel a surface-velocity coefficient of 0.85 or 0.86 is used to compute mean velocity

• In a smooth artificial channel a surface velocity coefficient of 0.90 is used

Selection of Measurement Cross Section� Cross section lies within a strait reach, and

streamlines are parallel to each other� Velocities are greater than 0.5 ft/s and depths are

greater than 0.5 ft� Streambed is relatively uniform and free of numerous

boulders and heavy aquatic growth� Measurement section is relatively close to gaging-

station control to avoid the effect of tributary inflow, and storage during periods of rapidly changing stage

Selection of Measurement Cross Section Spacing of Observation Verticals

� Spacing of verticals is determined to provide about 25 to 30 subsections or more

� No subsection should have more than 10 percent of the total discharge

� Ideal measurement has no subsection more than 5 percent of the total discharge

� Spacing between verticals should be closer in areas of the cross section that have greater depths and velocities

� Note: Ideal measurements are seldom achieved with 25 subsections

Indirect Measurement of Discharge

1

10

100

1 10 100 1000 10000 100000Discharge (cfs)

Stag

e (ft

)

Rating Curve

Discharge Measurements

Discharge measurements are used to develop rating curves

U.S. Department of the InteriorU.S. Geological Survey

Measurement of Stage

Primary Data Sources

Primary Data Sources Measurement of Stage

Explanation of Stage

� The stage of a stream or lake is the height or elevation of the water surface above an established datum plane.

� Gage height – the height of the water surface above the gage datum.

� Gage height is often used interchangeably with the more general term stage.

� Stage or gage height is usually expressed in feet and hundredths of a foot.

Gage Datum at a station

Uses of Stage

� Stage records along with stream discharge measurements are used to develop stage-discharge relations

� Stage records can be used to design structures affected by stream elevation and planning of floodplain use

� Stage records can be used to provide an index of surface area and volume of a water body

Controls

� The conversion of a record of stage to a record of discharge is made by the use of a stage-discharge relation.

� The physical element or combination of elements that controls the relation is known as a control.

� There are artificial and natural controls

Artificial Controls Artificial Control

Artificial Controls Artificial Controls

Natural Controls Natural Controls

Natural Controls Point of Zero Flow

1

10

100

1 10 100 1000 10000 100000Discharge (cfs)

Stag

e (ft

)

Rating Curve

Discharge Measurements

Discharge measurements are used to develop rating curves Datum of Gage

� Gage datum – The zero elevation reference surface at a gaging station to which all gages are set

� Datum could be of known elevation or an arbitrary elevation

� A permanent datum should be maintained for the life of the gage

Reference mark

� A permanent marker, installed in the ground or on a structure, whose elevation above a set datum is known.

� Used to check and make sure that all gages and reference points are properly set to gage datum.

Reference Points

� Objects, often bolts or screws that are assigned an elevation in the gage datum.

� Used to obtain gage heights when necessary by measuring their distance to the water surface.

U.S. Department of the InteriorU.S. Geological Survey

Gaging Stations and Recorders

Primary Data Source

Stage Data Collection

� Stage data is collected through the use of non-recording and recording gages.

� Stage data is collected continuously through the use of recording gages

Nonrecording Gages

Can be used as an auxiliary or reference gage to indicate the water-surface elevation in a� Stream or reservoir� Stilling well

Can also be used as a temporary substitute for the recorder when the intakes are plugged or there is an equipment failure.

Vertical Staff Inclined Staff

Wire Weight Gage Wire Weight Gage

Float-Tape Gage Electric Tape Gages

Water-level Sensors

� A device that automatically determines, or senses, the vertical position of the water surface.

Float and Shaft Encoder

Submersible Pressure Transducers Noncontact Water-Level Sensors

Typical Gaging-Station Instrumentation Configurations

Typical Gaging-Station Instrumentation Configurations

Crest Stage Gage Crest-Stage Gage