River gauging is the process of measuring river discharge and characteristics over time. It is important for distributing irrigation water, designing irrigation structures, flood warnings, and determining dependable water supplies. The ideal site for river gauging is on a straight river section at right angles to flow, without structures or risk of silting. Discharge can be measured directly using weirs and flumes, or indirectly using area-velocity methods where cross-sectional area and velocity are measured and multiplied. Velocity is typically measured using floats, velocity rods, or current meters. Empirical formulas can also estimate maximum flood discharge based on catchment area.

1. RIVER GAUGING

2. DEFINITION
The process of gauging a river flow is called river gauging.
It helps in determining characteristics of a stream flow at different times during a year
It is a technique used to measure the discharge or the volume of water moving through a
channel per unit time of a stream
The height of water in a stream channel is called stage or gage height

3. IMPORTANCE
● To distribute uniformly the irrigation water among the cultivators
● To determine the coefficient of roughness of lined and unlined canals and to find the
amount of absorption losses
● To forecast the normal and highest water supplies for designing the various irrigation
structures
● Flood warnings can be issued at times of excess flow in the rivers
● To know the dependable supplies in a river

4. SITE SELECTION
● The river should have a straight reach length of which should not be less than 20 times
the width of the water
● The river should not be wide and shallow
● No hydraulic structure should exist near the gauging site
● The river section line should be at right angles to the flow of the river
● There should not be any chance for silting and scouring at the gauging site

5. OPEN GAUGE WELL
● Gauges fixed in open areas and kept exposed, water level cannot be read with accuracy as the
currents against the gauge piller affect the water levels due to waves and surgings which
cause constant variations in depth.
● A gauge well is a well sunk on the margin of the river and provided with a gauge to show the
level of water inside the gauge well
● One or more pipes or openings or narrow slits in the side connect the well with the water of
the stream and thus the level of water in the well is kept at the true level of the surface of the
stream
● Wherever A gauge well is installed there must be two gauges with their zero at the same
level one in the well and the other in the stream and readings of both gauges must be
recorded.
● Pipes connecting gauge well with the stream must be more than 10 cms in diameter and each
pipe should be without bend

6. ● The lowest pipe must be fully submerged below the level stage of flow to be recorded.

8. Gauging of river
1. By actually measuring the discharge at the outlet of the catchment basin
2. By determining the area and velocity of a river

9. 1. By actual measurement
1. For a rectangular weir
L length of weir
H head of water over crest

10. 2. For a triangular notch fixed over small rivers and channels
H head of water over sill of notch
Cd co efficient of discharge of the notch

11. For a broad crested weir
L length of weir
H head of water over crest

12. 2. By area velocity method
● It involves the measuring area and average velocity of flow
● As it is difficult to correctly measure the average velocity the stream or river should be
divided into sections
● If V is the velocity in m/sec and A is the sectional area of the stream then volume of
water through stream Q= AxV
● In this method both quantities are measured at a suitable site when the discharge is
steady
● Area is the product of width and depth
● Flow section is divided into no pf equal compartments
● The depth of water is measured in the middle of each compartment whose area is then
calculated by multiplying the depth with its width
● The total cross sectional area of flow is the sum of areas of all compartments

14. MEASUREMENT OF VELOCITY- SURFACE FLOATS
● The floats are pieces of wood or hollow metallic cylinders or bottles etc
● Good for measuring small discharges of stream with an uneven bed
● can be used in absence of current meters and velocity rods
● The velocity is obtained by dividing the length of run in meters by the time in seconds
● Floats are easily affected by winds
● Mean velocity Vm= 0.85 Vs
Vs observed velocity

16. MEASUREMENT OF VELOCITY- VELOCITY ROD

17. ● A circular wooden rode of 5cm diameter with a hook at the topand of length that it
floats only a few cms above the bed with its lower end at 0.94d from the top
● It is weighed at the bottom by piecesof metal to keep it in vertical position while floating
● Not affected by winds
● Not suited for measuring velocity in a stream with weed growth

18. MEASUREMENT OF VELOCITY- CURRENT METER
● Accurate results
● Operate by a dry battery cell
● Two types cup meters and propeller meters
● In cup meters 6or 8 cups rotate on a vertical axis due to the velocity of flow when the
meter is immersed in water
● In propeller meters there are vanes revolving round a horizontal axis
●

20. MAXIMUM FLOOD DISCHARGE
The maximum flood discharge is also called as maximum rate of run off during a peak flow
or maximum flood
1. By indication of past floods
2. By empirical formulae
3. By unit hydrograph method
4. By flood frequency analysis

21. By empirical formuale
1. RYVE’S FORMULA
FOR SOUTH INDIAN CATCHMENTS
Q= CM2/3
Q MAX FLOOD DISCHARGE IN CUMECS
M IS CATCHMENT AREA IN SQ.KM
C RYVE’S COEFFICIENT

22. 2. Dicken’s formula
Q= CM3/4
Q MAX FLOOD DISCHARGE IN CUMECS
M IS THE CATCHMENT AREA IN SQ.KM
C DICKENS COEFFICIENT

23. INGLIS FORMULA
Q= 124A
√(A+10.4)
Q MAXIMUM FLOOD DISCHARGE CUMECS
A CATCHMENT AREA IN SQ.KMS