Streamflow Measurement Part1

7/25/2019 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

Documents

Streamflow Measurement Part1