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Measurement�o
f�Surface�Flows
ADEQ
�SW�Sho
rt�Cou
rse
June
�13,�2013
Phoe
nix,�AZ
(most�slides�cou
rtesy�of�USG
S)AD
EQ�SW�Sho
rt�Cou
rse
The�University
�of�A
rizon
a1
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
