Stream gauging, also known as stream flow measurement, is the measurement of discharge or stream flow passing through a cross section over a period of time. It is important for several reasons, such as forecasting floods, assessing available irrigation water, and determining seasonal runoff variations. When selecting a gauging site, straight reaches with stable cross sections that can measure the full range of high and low flows are preferred. Common stream flow measurement techniques include the area-velocity method, dilution technique, and use of hydraulic structures like weirs and flumes. Stage is also measured, often using staff gauges, to relate water level to discharge measurements.

1. UNIT-III
Stream Gauging

2. Introduction:
• Stream Gauging also known as stream flow
measurement, which is the measurement of
discharge / stream flow passing through section in a
unit time.
• The necessity for stream gauging arises
because of many reasons; streams provide water
supply for men and animals, irrigation water for
plants, and energy for production of power.

3. • Occurrence of flood can be forecasted.
• Assessment of available water for irrigation planning.
• Variation in seasonal and annual runoff of any
stream can be determined.
• Estimation of water loss at different reaches of
stream can be done.
• Planning of rivers can be made for optimum
utilization of water flowing through them.
• Helpful for construction of river valley project.
Importance of stream gauging:

4. • It should act as permanent control for stream flow
measurement.
• It should be clearly visible and easily assessible.
• Avoid the point where two stream meet because of
back flow of water.
• Site should be free from vegetal growth , boulders or
other obstruction.
Selection of Gauging site (Gauging
point or station):

5. • Gauging site should be straight and uniform.
• Cross - section of gauging site should be firm and
stable.
• The gauging site should be capable to record
highest and lowest gauges corresponding to highest
and lowest flood peaks.

6. • The river stage has been defined as the height
of water surface in the river at a given section
above any arbitrary datum or mean sea level.
• If a relation between stage and discharge can
be established the stream flow can be determined
from a known stage discharge curve.
River stage:

8. A. Manual Gauge
i. Staff gauge
ii. Suspended weight wire gauge
B. Automatic stage recorder
i. Float gauge recorder
Stage measurement methods:

9. Staff Gauge

12. Suspended weight wire gauge

14. Float gauge recorder

16. Various devices used for measuring the
flow velocity are :
A. Surface float
B. Sub- surface float
C. Velocity rods
D. Current meter
Measurement of velocity:

18. Current meter

20. Stream flow measurement techniques can
be broadly classified into two categories as:
A. Direct determination
1. Area- velocity method
2. Dilution technique
3. Electromagnetic method
4. Ultrasonic method
B. Indirect determination
1. Slope Area method
2. Hydraulic structure such as weir , flumes etc.
Measurement of Discharge:

21.  Area- velocity method:

22. Dilution techniques:
In this method discharge is determined
by injecting chemical (normally common salt) in
the stream flow at a known rate and determining
the amount of chemical present in the flow at
downstream point which is located at sufficient
distance from injecting point.

23. Ultrasonic method:

24. Electromagnetic method:

25.  Slope Area method:
In this method stream flow rate is calculated
by multiplying average cross-sectional area of the
stream flow and flow velocity determined with help
of slope of water surface in the stream.
• The computation is carried out in flowing steps;
a. Computation of cross sectional area
b. Computation of wetted perimeter
c. Computation of slope
d. Computation of flow velocity using manning's
and chezy’s formula.

26.  Hydraulic structure such as weir, flumes:
On small streams the flow can be measured
with the help of hydraulic flow measuring
devices such as rectangular, triangular and
trapezoidal weir or flumes like venturi flume and
parshall flume.