Post on 22-Jun-2020
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Quantitative Elements of Physical Hydrology
Measuring Stream Discharge (Q)Measuring Stream Discharge (Q)
© John F. HermanceJanuary 30, 2007
Contact information:Jack HermanceEnvironmental Geophysics/HydrologyDepartment of Geological SciencesBrown UniversityProvidence, RI 02912-1846Tel: 401-863-3830e-mail: John_Hermance@Brown.Edu
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End of section. (Review)
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Parameterizing Stream Discharge:Lateral & Vertical Velocity Profiles
© John F. HermanceJanuary 30, 2007
Measuring Streamflow
© John F. HermanceJanuary 30, 2007
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Stream Gauging
Current meters
Rotating wheels, propellers, or cups electrically connected to recording device; resistance wires or films; electromagnetic induction devices; ping-pong balls.
Stages
Vertical rulers of measuring stakes set in stream to measure elevation of water surface.
Culverts
Employs theory of flow in a partially filled pipe.
Weirs
Dam w/ spillway in a stream — temporary or permanent.
© John F. Hermance
Stream Gauging
Weirs
Dam w/ spillway in a stream — temporary or permanent.
© John F. Hermance
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Formulae for flow over a weir:Rectangular weir w/ end constrictions:
Q = 3.33 [L - 0.2H] H3/2
Q is discharge in ft3/s.L is the horizontal cross-sectional length in ft.L should be 4 - 8 times H.
H is the differential height between the lowest point of the spillway (weir crest) and the height of the level, undisturbed water surface behind (upstream from) the weir (>2H).
Alternatively,
90o V-Notch weir:
Q = 2.5 H5/2
© John F. Hermance
The Moshassuck River (looking downstream).
Standard USGS Procedures forMeasuring Stream Discharge.
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The Moshassuck River (looking downstream).
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The Moshassuck River (looking upstream).
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USGS Standard Stream Gage Station: 01114000.
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The Host: Jim Campbell.
© John F. Hermance
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The Class.
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Lance Ramsby.
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Rotating Cup Streamflow Meter.
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Rotating Cup Streamflow Meter.
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Begin documentation.
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Entering water.
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Preliminary inspection (and maintenance).
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Extending measuring tape.
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Update notebook.
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Begin first measurement.
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Begin first measurement (view from above).
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Second measurement.
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Continuing profile.
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Continuing profile.
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Inside recording shed.
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Compressed air supply.
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How a gas bubbler operates.
How a gas bubbler operates.Pressure increases
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How a gas bubbler operates.Pressure decreases
Data acquisition unit continuously records air pressure in the line.
© John F. Hermance
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© John F. Hermance
Standby for extreme flows.
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The velocity profile of streamflow transverse tothe axis of the stream.
© John F. Hermance
Ideally, the velocity profile hasthe mathematical form
of a parabola.
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The velocity also varies“parabolically” with depth.
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(Finished !)
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The concept of "equivalent uniform flow".
© John F. Hermance
To Review:
Pose the question: What is the "average"discharge velocity for the entire stream
section, relative to the maximum velocity at the surface?
© John F. Hermance
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The average velocity is 2/3 Vmax.
© John F. Hermance
Because the velocity at the surface is usually disturbed by breezes or winds,
we need a more robust measure.
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Pose the question: What is the depth at which the actual velocity at that point
equals the "average" discharge velocity for the entire section?
© John F. Hermance
Answer: At 0.58 d.
© John F. Hermance
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The implications of the concept of
equivalent uniform flow.
© John F. Hermance
© John F. Hermance
The implications of the concept of
equivalent uniform flow.
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In practice, the operator constantly adjusts the depth of the cups to be at 0.6 of the total depth.
© John F. Hermance
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The velocity profile in 3D.The velocity profile in 3D.
© John F. Hermance
Putting it together: A stream rating curve.
© John F. Hermance
The operation you have just witnessed provides one (1) of the points on this curve. Many profiles on different days and for different flow conditions are needed to produce a rating curve for each streamflow station.
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End of Presentation(Measuring Stream Discharge)
End of Presentation(Measuring Stream Discharge)
Quantitative Elements of Physical Hydrology
© John F. HermanceJanuary 30, 2007