Streamflow Measurement Part1
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Transcript of Streamflow Measurement Part1
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STREAMFLOW
MEASUREMENT
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Course OutcomesOn completion of this chapter, you will be able to:
perform stage measurement
perform velocity measurement perform river gauging
calculate discharge of streamflow using area-
velocity method
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SURFACE WATER HYDROLOGY
Surface water hydrology deals with the movement of wateralong the earths surface as a result of precipitation andsnow melt.
Streamflow is the occurrence of water in a continuous welldefined flow channel.
Streamflow represents the runoffphase of the water cycle.
The amount ofrunoff is given by the net amount ofinfiltration, evaporation and evapotranspiration.
Streamflow is the only part of the hydrological cycle thatcan be measured accurately.
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Measurement of Stage The stage of a river is defined as its water surface
elevation measured above a datum (e.g. MSL)
Measurement techniques:
Staff gauge
Wire gauge
Automatic stage recorder
MSLdabove MSL
h
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Staff GaugesSimplest stage measurement with a fixed graduated staff.
It is rigidly fixed to a structure, such as pier, wall, abutment, etc.
The staff may be vertical or inclined.
Staff material: durable, low coefficient of expansion with respect totemperature & moisture.
Markings are easy to read from a distance.
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Staff GaugesSometimes it may not be possible to read the entire range of
water surface elevations of a stream by a single gauge.
Sectional gauges are built in sections at different locations.
When installing sectional gauges, care must be taken toprovide an overlap between various gauges and to refer allthe sections to the same common datum.
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Measure the water surfaceelevation from above the surfaceof a structure, such as from abridge.
A weight is lowered by a reeltotouch the water surface.
A mechanical counter measuresthe rotation of the wheel which is
proportional to the length of thewire paid out.
The operating range = 25 m
Wire Gauges
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100 m MSL
Rigid
structure
70 m
MSL
- 10 m MSL
30 m MSL
h = 40 m
Wire Gauges
Elev. of the gauge = 100 m above MSL
Elev. of the bottom = 10 m below MSLLength of the wire paid out = 70 m
Water surface elev. = 30 m above MSL
Water depth = 30 + 10 = 40 m
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Automatic Stage RecorderFloatoperating in a stilling well is balancedby a counterweight over a pulley.
Displacement of the float due torising/lowering of the water surface
elevation causes an angular displacement ofthe pulley.
Mechanical linkages convert the angulardisplacement to the linear displacement of a
pen to record over a drum driven byclockwork that runs for a day, week orforthnight.
A good instrument will have a large-sizefloatand least friction.
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Stilling well
Counter
weight
Float
Pulley
Angular
displacement
Linear displacement
DrumPen
Automatic Stage Recorder
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Stilling well to protect the float from debris, to reduce thewater surface effects on the recording.
The instrument must be properly housedin a suitable enclosureto protect it fromweather element andvandalism.
Located above the highest water level expected in the river toprevent it from getting inundated during floods.
Automatic Stage Recorder
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Stage Data
The stage data is presented in the form of a plot ofstage against chronological time (stage hydrograph).
Stage data is particularly useful for: Determination of stream discharge
Flood warning
Flood protection works
Stage
Time
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Velocity Measurement
Velocity (v) : distance travelled per unit time, or speed.
Unit of velocity is m/s or cm/setc.
Types of velocity measurement devices:
Current meters/ velocimeters Float meters
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To measure the velocity at a point in theflow.
Consists of a rotating element which
rotates due to the reaction of the rivercurrent with an angular velocityproportional to the river velocity.
Its rotation speed varies linearly with theriver velocity (v, m/s).
v = aN+ b
whereN= rev/sec; a, b=constant
Velocimeter
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Velocity Meter Measurements
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Example
A current meter is so designed that it has a relationship of v=0.3N+ 0.05, where v= stream velocity andN= revolutions persecond. The current was used to perform velocity measurement
of a stream. If the reading recorded is 12 revolutions in every 50seconds, what is the velocity of the stream?
Solution:
N= 12 rev / 50 sec = 0.24 rev/sv= 0.3 (0.24) + 0.05 = 0.122 m/s
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Floating object: surface float, canister float, rod float
Procedures:1. Select two sections on a fairly straight reach.
2. Release a large number of floats at fairly uniform spacing on the widthof the stream at an upstream section.
3. Note the time to cross this reach by each float.
4. Calculate the speed of each float by distance/ time.
Float Meters
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Float Meters
Surface floats are affected by surface winds.
To get the average velocity at lower depth, Canister and rodfloats are used.
Application: preliminary survey, small stream with rapidlychanging water surface
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Point Velocity & Average Velocity
Point velocity, v
Velocity that is measured at a certain depth.
Average (mean) velocity, vave For shallow streams
The velocity measured at 60%of the depth of flow below thefree surface, h(single point method).
For deeper streamsThe velocity measured at two points, i.e. 20% and 80%of h(2 points method).
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dQ
V0.2d
V0.8d
Estimated
meanvelocity
Vertical Velocity Profile (Deep Flow)
Point Velocity & Average Velocity