Irrigation engineering -River gauging

module 1. chapter 3.

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.

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

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

9. 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

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

11. 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

12. 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

13. MEASUREMENT OF VELOCITY- VELOCITY ROD

14. 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

15. 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

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

17. By empirical formuale 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

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

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