Flow measurement in stream, Lecture notes of Groundwater Hydrology

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STREAM FLOW MEASUREMENT

MOHIT MAYOOR

CUJ/I/2013/IWEM/

CENTRAL UNIVERSITY OF

JHARKHAND

INTRODUCTION

• As we know that the Precipitation, Evaporation and Evapotranspiration are all difficult to measure exactly but the Stream flow is the only part of the hydrologic cycle that can be measured accurately.
• A stream is defined as a flow channel into which the surface runoff from specified basin drains.
• Stream flow is measured in units of discharge(m 3 /s).

DIRECT METHOD

• Direct determination of stream discharge measurement includes :- (a) Area velocity method (b) Dilution techniques (c) Electromagnetic method (d) Ultrasonic method

INDIRECT METHOD

• Indirect method of stream discharge measurement includes :- (a) Slope area method (b) Hydraulic structures

MEASUREMENT OF STAGE

• The stage of a river is defined as its water surface elevation measured above a datum(Mean Sea Level or any datum connected independently to MSL)
• For the measurement of stage we have:- (1). Manual gauges (2). Automatic stage recorder
• Under Manual gauges we have (a). Staff gauge (b). Wire gauge

STAGE DATA

• The stage data is often represented in the form of a plot of stage against chronological time.
• This is popularly known as stage hydrograph
• Stage data is of utmost importance in design of hydraulic structures, flood warning and flood protection work. Abscissa- Time Ordinate- Stage

• Robert Hooke(1663 ) invented a propeller type current meter for traversing the distance covered by ship.
• Later on it was Henry(1868) who invented present day cup- type instrument and the electrical make-and-break mechanism.

TYPES OF CURRENT METER

• VERTICAL AXIS METERS

• HORIZONTAL AXIS METERS

CALIBRATION EQUATION

• Now we need to find out the relation between the stream velocity and revolutions per second of the meters which is nothing but the calibration equation.
• The calibration equation is unique to each instrument.
• It is determined by towing the instrument in a special tank
• A towing tank is a long channel containing still water with arrangements for moving a carriage longitudnally over its surface at constant speed.

• The instrument to be calibrated is mounted on the carriage with the rotating element immersed to a specified depth in the water body in the tank.
• The carriage is then towed at a predetermined constant speed(v) and the corresponding avg value of revolutions per second is determined.
• In India we have an excellent towing tank facilities for calibration of current meters at CWPRS(Central Water And Power Research Station) and IIT MADRAS.

• So certain specified procedure have been evolved.
• For the streams of depth 3. 0 m the velocity measured at
  1. 6 times the depth of flow below the water surface is taken as the average velocity.(single point observation model) V(avg)= v 0. 6
• For the depth between 3. 0 m to 6. 0 m we have V(avg)= (v 0. 2 +v 0. 8 )/ 2 For the river having flood flow we have v(avg)= kvs where vs( surface velocity) and k( reduction coefficient) Value of k lies between 0. 85 to 0. 95

SOUNDING WEIGHT

• Current meter is weighted down by lead weight called sounding weight.
• It is connected to the current meter with a hangar bar and pin assembly.
• These weights are of steramlined shapes with a fin in the rear
• The minimum weight of sounding weight is estimated as w= 50 v avg d Where d= depth of flow at vertical v avg = avg velocity

HORIZONTAL AXIS METERS

• This instrument consist of a propeller mounted at the end of horizontal shaft.
• The propeller diameter is in the range of 6 to 12cm
• It can register velocities from 0.155m/s to 4.0m/s.
• This meter is not affected by oblique flows of as much as 15 0 .
• Ott, Neyrtec, Watt type meters are typical instruments under this kind.

• This method of discharge measurement consists essentially of the area of cross section of the river at a selected section called the gauging site and measuring the velocity of flow through the cross sectional area.
• The gauging site must be selected with care to ensure that the stage discharge curve is reasonably constant over a long period over a few years.

AREA VELOCITY METHOD