4 Water Measurement Techniques-I

Ranjana Ray Chaudhuri

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Objectives

 

  • To understand why water needs to be measured
  • To understand different techniques of water measurement for open channels (including streams and rivers)
  • To understand measurement techniques available for groundwater assessment and rainfall assessment To be able to apply water measurement technique

 

Introduction

 

Water was always a precious resource, today it is a scarce resource as well. With rising development, the demand for water is increasing. Water is needed for agriculture, industries, municipalities, commercial use, recreation, environmental flow in rivers amongst many others. Water allocation to various sectors would involve estimation of water requirements at storage facilities, estimation of losses incurred in conveyance and actual water consumption. The measurement techniques that are discussed in these modules are for open channel flow measurement and pipe flow measurement.

 

Good water measurement techniques lead to detection of losses in the urban water distribution system and in water delivery in the agriculture sector. In case of urban areas, the pipe distribution network is monitored for water losses (water losses are dependent on the age of pipes amongst other defects) so that the water utility provider can determine the amount of water lost in supply and make reassessments. On the consumer side when individual water use is accounted for, together with water pricing policies excess water use can be brought under control. The flow measuring devices are used to measure wastewater flow too in urban areas. This is very important considering the fact that both dry weather flow and mixing of sewage effluent with storm water flow during rainy season determine the load on the waste water treatment system.

 

In case of irrigation networks, the canal system is monitored for water losses. The canal lining and canal age plays an important role in water losses. If measurement techniques are in place then water supply assessment can be made, while if assessment is carried out in agriculture fields, the water usage at field level is quantified. Amongst other benefits of water measurement in field level, one major benefit is lessening of polluted water reaching ground water, as excess runoff without drainage facilities leads chemicals and pesticides to pollute ground water.

 

The modules discuss various water measurement techniques commonly used in canals, stream flow, and pipes. The modules do not discuss advanced water measurement techniques. The water measurement devices are explained in a simple manner so that the application is easily understood. To understand water flow measurement devices, it is important to understand various kinds of flow. Flow is classified in many ways but for explaining measurement techniques, we will use the classification of pipe flow and open channel flow. A pipe is a closed conduit and water flows under pressure. The rate of flow occurs in a pipe due to development of pressure gradient, the flow is dependent on factors like friction losses, the cross-section area of the pipe, presence of bends, etc. In case of open channel flow, the rate of flow depends on the channel gradient, cross sectional area of the channel and the surface roughness of the open channel.

 

In case of open channel flow or stream flow, both direct and indirect measurement techniques are used. The direct methods of measurement of stream discharge include:

 

a.      Area velocity methods

 

b.      Dilution techniques

 

c.       Electromagnetic method

 

d.      Ultrasonic method

 

Measuring stream discharge continuously is generally not practiced; normally stage discharge (curves) relationship is established. For this the discharge in a given stream is related to the water elevation in the stream (stage) through a series of measurements. In order to measure stage (depth of water in the stream), we use staff gauge (vertical single staff gauge and sectional staff gauge). Stage measurement is carried out using staff gauge (manual gauge). The manual gauge is fixed to a structure like bridge abutment, pier as shown below. It should be vertical with no inclination; the markings should be clear. There should be no hindrance in taking the reading, the water surface elevation of the stream should be clearly visible. When the depth of the stream is large, there are large fluctuations, sectional gauges are used. They are also manual gauges which can measure large variation in water depth. Care should be taken to see that there is an overlap between the gauge readings of different staves as shown in the figure 1.

 

Sometimes wire gauges are used to measure depth of a stream, by hanging a wire with attached weights from a bridge or other structure and depth is measured.

 

Automatic stage recorders are now used more often than manual stage recorders and the reading may be recorded continuously. A float gauge recorder is typically housed in a stilling well as shown in the diagram below. The float is balanced with a counter weight over a pulley system which is attached to a recorder. The displacement of the float due to change in water surface elevation causes an angular displacement in the pulley, which causes movement in the recorder. The recorder records stage versus time data continuously. In the latest system stage versus time data recorded is transmitted through the satellite so that instantaneous records are available.

 

The stilling well allows water to come from the stream through intake pipes, so that the float and recorder are protected from debris and even when flood inundation takes place. Care should be taken to see that the stage recorder is placed above the highest water level that may be reached at that site. The arrangement of the stilling well installation is such that in functions in all weather conditions. Many times a bubble gauge type recorder is used in which case compressed air or gas bubble is released through an outlet placed at the bottom of the stream. The advantage of such recorders is that it does not need a stilling well arrangement and the recorder assembly can be placed far away from the stream. The other important characteristic of direct determination of discharge is the estimation of velocity. Velocity is measured by many methods. The first is as follows:

 

Current Meters

 

The current meters can be both vertical axis and horizontal axis current meters. The current meter is used to measure velocity at a point across the cross section of the stream. The vertical axis current meter consists of a series of conical cups mounted on a vertical axis as shown in the figure below. The conical cups fill with water and start rotating, this helps in the stream velocity measurement.

The normal range of velocities recorded is between 0.15-4.0m/s. The current meter is designed in such a manner that the number of revolutions of the meter is related to the linear velocity as follows:

 

v=aNs+b

 

where v is the stream velocity at the instrument location in m/s,

Ns is the number of revolutions per section of the current meter and a,b are constants.

 

The vertical current meters are easy to use, however, whenever there is appreciable velocity difference with depth, they are not that accurate, under such circumstances the horizontal axis current meter may be used. The cups are of standard diameter 12.5cm and the constant values of a and b are at a=0.65 and b=0.03. In case of broad streams, current meters are carried in boats.

 

Calibration is carried out to determined values of a and b in a special tank (towing tank) which contains still water. The instrument is run over the entire range of velocities (each time the speed is kept constant at a certain value, the number of revolutions per second, Ns is measured). Over the entire range of velocities and revolutions, Ns, graph is plotted linearly, and equation constants a and b are determined.

 

In the field, velocity is measured at a chosen cross section where there is least resistance to flow. Depending on the depth of the stream, since stream velocity is subjected to friction resistance from stream bed, side slopes and air friction at surface, in order to get representative velocities, it is measured at the following depths.

  • In case of shallow streams of depth up to 3.0m, the velocity is measured at 0.6 times the depth of flow below the water surface.
  • In case of moderately deep streams the velocity is measured at 0.2 times the depth of flow below the water surface and 0.8 times the depth of flow below the water surface. The average of these two values gives the average velocity of the stream at that section.
  • When streams are in flood, the surface flow velocity is measured (up to a maximum depth of 0.5m from the surface). The average flow velocity is then measured as v=K*Vs, where Vs is the surface flow velocity and K is the reduction factor. The reduction factor varies from 0.85-0.95.

 

Stream velocity can also be measured with the help of floats also, these are generally used for preliminary investigations, in case of small streams, and streams in which there is rapid change in water surface. A simple floating object is measured for the distance travelled over a period of time. This gives the surface velocity, many times a series of floats are released along the width of the stream a regular interval, so that an average velocity can be determined.

 

Area velocity method

 

The area velocity method is used to determine the discharge of the stream at the selected cross-sectional area otherwise called the gauging site. The stream is divided into sections as shown in the figure 4, at each of these sub sections the average velocity is measured, the area of the sub section is determined by measuring the depth and the width of the subsection. The discharge at each of these subsections is the cross-sectional area multiplied by the average velocity at the sub section (the velocity being measured by the current meter method as explained above). The discharge estimation at the gauging site is the summation of all these individual discharges. Discharge is the volume of water flowing through the stream or river at a certain time, so it is measured as

Q= AV               where Q is discharge in m3/s (cumec), A is cross sectional area measured in m2 and velocity V is measured in m/s.

 

The area velocity method is thus the summation of individual discharges as measured below

 

∑Q =∑AV,

 

A rating curve is drawn plotting the discharge measured by area velocity method on the y axis and the measured stage depth at the gauging station on the x axis. The more the number of measurements of discharge and corresponding stage height, better is the representative plotted curve. The best fit curve gives an equation of the relationship between the stage and discharge, the diagram below shows a typical rating curve. While using the area velocity method, care should be taken to see that the cross section is accessible throughout the year, the site is within a stable stretch of the stream, there are not much erosion or sedimentation and the gauging site is free from back water effects.

 

The dilution technique of stream flow uses tracers which may be chemicals, fluorescent dyes or radioactive materials. An initial high quantity of tracer is mixed in a small discharge and then the concentration of the diluted quantity is measured at another section using steady state continuity equation. Care should be taken to see that the tracer is not absorbed by the sediment, channel boundary and vegetation. It should not evaporated easily, should be non toxic and should be detectable even in small quantities and be relatively inexpensive. The method is particularly recommended for small turbulent mountainous streams and it is also used to check calibration of stage discharge curves obtained by velocity area method.

 

Similarly, the electromagnetic flow meter is used to measure flows in tidal streams/channels where is fluctuation in quantity and direction of flow. The minimum detectable velocity in this case is 0.005m/s. They can also be used for cross sections which are disturbed by weed growth and sedimentation.

 

Ultrasonic method of velocity measurement of flow uses two transducers fixed at two banks of the river or channel, receive and send ultrasonic signals. The time taken from one end of the bank to the other is different for the left and right bank. In one case the component of flow velocity in the direction of the sound waves is added to the velocity of sound, while in case of the other bank the flow velocity component is subtracted. The difference in the two velocity components since it can be measured, and the width of the stream is known, the velocity of flow can be determined. This method is recommended for unstable cross sections, which have fluctuating weed growth sections, high suspended solids in flow and there is rapid change in magnitude and direction of flow. The systems are used for channel widths of upto 500m.

 

Measuring stream flow using weirs, this forms a part of indirect measurement of flow in open channels.

 

The field measurements involve only measurement of depth at specified locations. There exists a predetermined relationship between discharge and depth. The two most common types of weirs are rectangular weir and V notch type weir. Typically, a weir is a small wall constructed across the width of the stream to create an obstruction to flow to measure discharge. The wall can be made of concrete or thin steel plate as well, the material should be erosion resistant.

 

 

Rectangular weir

 

Q  = 1.84 (b-0.2h)h2/3, for measurements in meters

Where Q is discharge measured in m3/s, b is the width of the weir measured in metres, h is the head drop or depth measured at the weir section in metres.

 

90° V-notch weir

 

Q  = 1.379h5/2, for measurements in meters, where θ=90o,

 

Where Q is discharge measured in m3/s, h is the head drop or depth measured at the weir section in metres.

 

Measurement of groundwater depth and water level

 

Groundwater levels are measured along with ground water sampling (to determine the water quality of ground water) to establish potentiometric surface of the hydraulic line of ground water. This is important to understand the direction of ground water flow and gradient. The difference in ground water level due to draw down is also needed to understand the amount of water drawn out of the well. The ground water depth measurements are usually done keeping a reference point in the well. In case of a PVC pipe we can use a marker pen to mark the reference point, in case of steel pipes we a notch to determine the reference point. The equipment used is either an electronic water level indicator or a steel measuring tape (used with a weight or buoy). Ground water levels in aquifers is measured by Central Ground Water Board (CGWB) in India. They measure ground water levels in pre-monsoon, monsoon and post monsoon seasons.

 

 Measurement of Rainfall

 

Rainfall is measured with the help of rain gauges, it is sometimes called a pluviometer as well. It is used by hydrologists and meteorologists to measure depth of rainfall over a given period (usually 24 hours). The Symons non-recording type of gauge is one such rain gauge, measures total rainfall depth. Some instruments are used to measure rainfall on hourly basis as well, also called recording type of rain gauge (tipping bucket type, which tips after the cylinder or bucket is full). The measurement is in terms of depth of rainfall (mm or cm), the tipping bucket type measures intensity of rainfall (mm/hr, cm/hr).

In India, the India meteorological department (IMD) is responsible for rainfall measurement, they have many weather stations including automatic weather stations which monitor rainfall and keep records which then they share with all.

 

Summary

 

Measurement of discharge using velocity area method gives an estimate of stream velocity at various points along the cross section however, the assumption is that the cross section remains constant. While in case of weirs, though they may be more reliable in terms of channel section or site consistency (parameters not changing with time) but since the formulae are empirically derived using the coefficient of discharge, the variability may arise due to these constants.

 

So, the selection of instrumentation depends on the purpose of measurement, desired accuracy, range of discharge measurement to be undertaken, operation and, maintenance standards desired, life of the instrument, impact of instrumentation on the surrounding environment, ability to adapt to local site conditions, user friendly devices are preferred. In conclusion we may say that there is no instrument which can measure both large and small flows with equal accuracy. So, the range of measurement required is very important to determine, also the accuracy desired (mostly it is in the range of 3-5%).Typically broad crested weirs, are used for measuring large flows while V notch triangular notches are used for measuring small flows. In case of weirs large drop in head is essential for accurate measurement but in case of relatively flat slope gradient like in irrigation channels it is a challenge. They are still preferred as they are relatively inexpensive, however, sophisticated flow meters require very little head difference, though they may be expensive. Similarly, sophisticated instruments cannot sustain contaminants in flow, acidity or alkalinity and may become inoperative over time. Measurement of discharge is extremely important in hydraulics and hydrologic studies so World Meteorology Organization recommends a minimum number of hygrometry stations in every country depending on the geographical locations. An assessment of water flow helps to accurately determine the working of the water system in an area.

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References

 

  • Bos, M.G., (ed.) (1989), Discharge Measurement Structures, third revised edition, International Institute for Land Reclamation and Improvement, Publication No. 20, Wageningen, The Netherlands.
  • Chow,V.T. (1964),Handbook of Applied Hydrology, Mc Graw Hill, New York, USA.
  • Fetter, C.W. (2001), Applied Hydrogeology. 4th ed. New Jersey: Prentice Hall.
  • Subramanya K.(2008), Engineering Hydrology, third edition, Tata McGraw Hill Publishing Co. , New Delhi, India
  • Subramanya K. (1997), Flow in open channels, second edition, Tata McGraw Hill Publishing Co, New Delhi, India.
  • Weight, W.D. and Sonderegger, J.L. (2001), Manual of Applied Field Hydrology. New York: McGraw Hill Publishing Co