DISCHARGE MEASUREMENT OF A CANAL/DRAIN
AND CALIBRATION OF HYDRAULIC STRUCTURES
The key to conservation of water is good water management and this cannot be achieved without measurement of surface and drainage flows. The good water measurement practices facilitate accurate and equitable water distribution. The regular discharge measurement can add the following advantages: -
ü Reliable and proper flow measurement data of the system.
ü Transparency in information and data sharing for all stakeholders.
ü Standard and uniform application of flow formulae and discharge coefficients within Irrigation department.
ü Proper water accounting and audit.
ü Capacity building and development of knowledge of the irrigation network for use by the irrigation Engineers.
Canal discharge is the volume of water flowing through a cross section of a waterway per time unit. The unit used to measure canal discharge is usually (ft3/sec-cusecs) or (m3/sec- cumecs). Symbol for canal discharge is Q.
Stream flow records are primarily continuous records of flow passing through a particular section of the stream. Stream flow data are analyzed to determine the magnitude and variability of surface waters. These records are input in planning, design, and operation of surface water projects and are also used in design of bridges and culverts, flood forecasting systems, and flood plain delineation. The sections where canal measurements are carried out are known as canal gauging stations. A network of these stations is established to collect data about surface water resources of a region. There exists a relationship between stage and discharge at a section. This relation is termed as stage-discharge relationship or rating curve.
The international Standard Organization (ISO) and World Meteorological Organization (WMO) have brought out many publication related to stream flow measurement; individual countries have their own standards.
The two most significant flow regimes under which any open channel constriction may operate are free flow and submerged flow. Other terms for free flow are critical depth flow and modular flows, while other terms for submerged flow are drowned flow and non-modular flow. The distinguishing difference between the two flow conditions is the occurrence of critical velocity in the vicinity of the constriction (usually a very short distance upstream of the narrowest portion of the constriction). When this critical flow control occurs, the discharge is uniquely related to the depth or “head” upstream of the critical section. Thus, measurement flow depth at some specified location upstream from the point of the critical condition is all that is necessary to obtain the free flow discharge.
When the flow conditions are such that the downstream depth is raised to the extent that the flow velocity at every point through the constriction becomes less than the critical value, then the constriction is operating under submerged flow conditions.
The following hydraulic formulae are frequently used for computation of discharges at control points:-
Condition of Flow | Formula |
(i). Standing Wave formed and gates are free (Weir Type Free Flow Conditions) | Q = CB (H+Ha)3/2
Where Q = Discharge in Cusecs C = Coefficient of discharge = 3-3.4 for canals and drains = 3.3 to 4.8 for Barrages B = Clear weir width at crest (feet) H = Depth of water over the crest (feet) Ha = Velocity head = V2/2g (feet) V = Average velocity of flow (ft/ sec) g = Acceleration due to gravity = 32.2 ft/sec2 |
(ii). Standing Waves formed when the D/S water level is above crest level (All types of submerged flow conditions) | Q = Cd A (h)1/2 Cd = Coefficient of discharge for submerged flow condition = 5 to 7 A = Flow Area (ft2) h = working head = U/S Water Level – D/S Water Level (feet) |
Head Discharge Relationships
The following hydraulic formulae are frequently used for computation of discharges at control points.
Value of Coefficient and Exponents for Discharge Equations
Constriction Type | Flow Condition | Equation | Coefficient | Exponent | Reference |
Sharp crested weir | Free flow | Q = Ce2/3 bc h1.5 | 0.56-0.78 | 1.5 | Boes et.al 1987 |
Broad Crested Weir | Free Flow | Q = C B (H + ha)1.5 | 3.2 | 1.5 |
|
Open channel constriction | Free flow | | 0.73 | 1.5 | Skogerboe et. Al 1987 |
Open channel constriction | Submerged | |
| nf=1.55 ns=1.0-1.5 | Skogerboe et. Al 1987 |
Orifice | Submerged | Qs=Cd A [2g (hu – hd) ]0.5 | 0.60-0.80 | 0.50 | Skogerboe et. Al 1987 |
Orifice |
| Qs=Cd A [2g (hu – hd) ]0.5 | 0.30-0.90 | 0.50 | Skogerboe et. Al 1987 |
Rectangular control section | Free flow | Q= Cd 2/3 [2/3 g ]0.5 bc H1.5 | 0.1-1.0 | 1.5 | Boes et.al 1987 |
Orifice | Submerged | Q = C A | 0.66 | 0.5 | Boes et.al 1987 |
Source: IWMI (1997).
Channel constriction, broad crested weir and gated orifice are used in Indus Basin System. Value of coefficients and exponent given in above table are theoretical. These may vary due to field conditions variations. Therefore actual head-discharge equations should be developed based on the field measurements.
The steps for the calibration process are the following:-
ü Planning
ü Data collection
ü Procedure equipment and staff
ü Establish field procedures and work program
ü Perform work program
ü Analyze, evaluate and document results
ü Prepare recommendations.
The calibration of an irrigation system is not an undertaking that requires a major effort. It does require a well trained field crew, survey and water measurement equipment and transportation. Even the date collection step can be minimized as it is used more for planning and evaluation than for conducting the actual field work.
Planning
It is a continual process that proceeds as the program advances. Initially it involves the decision of what is to be done, how to do it, what and who to do it, obtaining necessary personnel and equipment including transportation and communication, providing any training or instruction required to make the team more efficient and preparing the work program and making adjustments as required.
Data Collection
Since calibration is the first step of developing a flow monitoring program it is necessary to start at the beginning. Therefore, the first step is to obtain a copy of the following data:
ü System map
ü Schematic diagram locating all structures
ü As-built drawings or latest long-sections
ü List of all sanctioned outlets and RD’s (location)
ü Location and datum for all bench marks along canal/drain.
ü Stage-discharge tables or curves available if any.
Staff and Equipment Requirements
The calibration requires two specialists; a surveyor and a hydrographer. This constitutes the bare minimum and they should really have an assistant to help with all of the work. As it is, the surveyor and hydrographer would have to assist each other as much of the work requires two people.
The size of the crew depends on the speed at which the team should work. A team for man, a 5-man survey crew, a 2-man hydrographer team, a 12-man flume measurement team, driver and a couple of casual laborers could cover one or two canal circles a year. Reductions would obviously decrease the work rate. A 6 to 7-man crew should be adequate at the division level; says foreman, surveyor, hydrographer and 3 to 4 assistants. The number of assistants depends somewhat on the number of portable flumes to be used. A 3-man crew appears to be the minimum requirement.
Transportation and communication are required for two reasons; (1) to move the team and its equipment along the canal and (2) to continuously know what the discharge in the canal is in order to establish water surface elevations at known canal discharges. Calibration should be made throughout the normal range of flows.
A pickup truck would be the most practical transportation unit that could be used. It has the capacity of carrying all the equipment including the portable flumes or full a boat trailer. Other vehicles including vans could be used but the field requirement must be transported carefully.
Field Procedures
A primary objective of this lecture is to make minimum use of existing facilities for water measurement. This will require calibration and rating of various hydraulic structures. The primary problem will be submerged flow conditions. Unfortunately, in many cases, submergence may exceed 95 percent, making measurements less reliable. It is expected that gated and up gated head works for distributaries as well other water control structures will form the basis of measurement discharge stations. The outfall points of sub drains branch drains and main drains would be best measurement sites for drainage discharge.
Under the best of conditions hydraulic structures built for the purpose of measuring the rate of low consist of a converging transition, where subcritical flowing water is accelerated and guided to the crest without flow separation; a crestt where accelerates to supercritical flow so that the discharge is controlled, and a diverging transaction where the flow velocity is gradually reduced to an acceptable subcritical velocity and potential energy is recovered.
Under most field conditions, many hydraulic structures used for the control of flows eliminate or ignore one or more of these parts. The two most common divisions are; first, tail water conditions that submerge the flow through the throat and eliminate modular flow and second, most canal head works are set 90° to the streamlines in the senior canal. The latter often cause non-uniform flow patterns which affect both the water and sediment discharge patterns.
It is assumed that most of the existing stage-discharge relations are based on the engineer’s field observations. In addition, changes in channel control due to sedimentation or other reasons require a periodic check on the stage- discharge relations.
The need for all calibration team members to work in a professional manner cannot be over emphasized. We are referring to the requirement that all work be accomplished in a meticulous manner includes neat and detailed notes, accurate measurements, attention to details, checking and rechecking of measurements, completion of all measurement, data collection and entries on the appropriate forms.
Discharge measurements can be consistently be made in the field with errors on the order of 2 or less if all measurements are carefully made. It is important to know which measurements are the most critical. For instance, an error of 0.010 feet (1/8th inch) in the water surface elevation (the sill reference height) on an outlet results in an error of more than 2 in the measurement of discharge. Fortunately, errors in measurements are not all additive, some compensate each other.
Scale is an important concept to remember. This refers size of the difference with respect to the total. For instance, 2 cfs in 200 cfs versus 2 cfs in 20 cfs or 0.01 inch in 12 inches and 1 inch in 50 feet. The most important dimension is the water surface elevation. The water surface elevation, W.S. Elev., should be read to the closest 1/6th or 1/8th inch depending on canal and water surface movements.
Frequency of Discharge Measurements
Initially the discharge measurements are made with the frequency necessary to define the station rating, as early as possible, over a wide range of stages. Measurements are then made at periodic intervals to verify the rating or to define any changes in the rating caused by changes in the stream channel. Monthly observations of canal/drain discharge are generally sufficient, though daily data are needed to calculate the statistical parameters of the discharge. Manual water discharge observations are still in practice. However, in many countries even paper systems for continuous data recording have been almost entirely replaced by largely automated electronic logging, analysis and data transmission systems.
In general, flows are measured at any point where the conveyance system flows are divided between two or more channels. As an everyday procedure, flows are not directly measured into watercourses. Flows should normally be measured into all canals/drain, although there are cases where it is not practical to measured flows in main or branch canals/drains each time a small distribuaery is measured. The flow in the main canal would be measured at the next downstream control structure and at outfall points in case of drains.
Flow measurements are made in order to account for all water coming into a system as well as its distribution throughout the system. Although flows are not measured in watercourses/sub drains as a routine, they should be measured when calibrating a distributary/main or branch drain.
Discharge measurements by the current-meter method are made from a boat, from a cable or bridge or by wading the stream. In irrigation canals mostly the boat measurements are employed for the purpose. The measurement need not to be made at the exact location of the gauge because the discharge is normally the same as at the gauge throughout a reach of channel in the general vicinity of the gauge. Discharge measurements of the highest accuracy will be obtained at cross-sections having the following characteristics:
ü The velocity at all points is parallel to one another and at right-angles to the cross-section of the stream.
ü The curves of distribution of velocity in the section are regular in the vertical and horizontal planes.
ü The velocity is greater than 0.3-0.5 foot (10-15 cm) per second.
ü The bed of the channel is regular and stable.
ü The stream does not overflow the banks of the channel.
ü There is no aquatic growth.
It is usually not possible to find a site with all of these characteristics. A satisfactory measurement can still be made at most sections by increasing the number of observation points or by using special equipment.
Current meters are velocity measuring devices that sample at a point. Each point velocity measurement is then assigned to a meaningful part of the entire cross section passing flow. The velocity-area principal is used to compute discharge from current-meter data. Total discharge is determined by summation of partial discharges. Data are usually determined over a useful range of total discharges. These discharges are related to measured water surface elevations related to a fixed head measuring device to provide a rating curve. After full confidence in the rating is attained, the calibrated
head measurement device and cross section may be used as a gaging station.
Price Current Meter
Anemometer and propeller current meters are the most common type used for irrigation and watershed measurements. These meters use anemometer cup wheels or propellers to sense velocity. The Price current meter and the smaller pygmy meter modification are the most common current meters in use. These meters are rated by dragging them through tanks of still water at known speeds. The reliability and accuracy of measurement with these meters are easily assessed by checking mechanical parts for damage and using spin-time tests for excess change of bearing friction. This type current meter does not sense direction of velocity, which may cause problems in complicated flow where backflow might not be readily apparent. For irrigation needs, this problem can be avoided by proper gage station or single measurement site selection.
Acoustic Doppler Current Profiler (ADCP)
Doppler type current meters determine velocity by measuring the change of source light or sound frequency from the frequency of reflections from moving particles such as small sediment and air bubbles. Laser light is used with laser Doppler velocimeters (LDV), and sound is used with acoustic Doppler velocimeters (ADV). Acoustic Doppler current profilers (ADCP) have also been developed. These instruments measure average velocities k of cells of selected size in a vertical series. Thus, they measure vertical current profiles. ADCP measurements are becoming more frequent for deep flow in reservoirs, oceans, and large rivers. Most of the meters in this class are multidimensional or can simultaneously measure more than a single directional component of velocity at a time.
The ADCP is a high performance water current profiler. It measures water current with sound using a principle of sound waves. It receives the velocity of water using a physical principle called Doppler shift.
The main external components of an ADCP are transducer assembly and a pressure case. The transducer assembly consists of four transducers that operate at a fixed, ultrasonic frequency, typically 300, 600, or 1200 kilohertz (kHz). The transducers are horizontally spaced 90 degrees apart on the transducer assembly; all transducers have the same fixed angle from the vertical, referred to as a “beam angle,” that is typically 20 or 30 degrees. The transducer assembly may have a convex or concave configuration. The pressure case is attached to the transducer assembly and contains most of the instrument electronics.
When an ADCP is deployed from a moving boat, it is connected by cable to a power source and to a portable microcomputer. The computer is used to program the instrument, monitor its operation, and collect and store the data.
The ADCP measures velocity magnitude and direction using the Doppler shift of acoustic energy reflected by material suspended in the water column. The ADCP transmits pairs of sort acoustic pulses along a narrow beam from each of the four transducers. As the pulses travel through the water column, they strike suspended sediment and organic particles that reflect some of the acoustic energy back to the ADCP. The ADCP receives and records the reflected pulses. The reflected pulses are separated by time differences into successive, uniformly spaced volumes called “depth cells.” The frequency shift (known as the ‘Doppler effect”) and the time-lag change between successive reflected pulses are proportional to the velocity of the scatters relative to the ADCP. The ADCP computes a velocity component along each beam; because the beams are positioned orthogonally to one another and at a known angle from the vertical (usually 20 or 30 degree), trigonometric relations are used to compute three-dimensional water-velocity vectors for each depth cell. Thus, the ADCP produces vertical velocity profiles composed of water speeds and directions at regularly spaced intervals.
ADCP method of discharge measurement of Rivers is being used in almost all the major river basins of the World and its viability and correctness stands verified in research work published for the last ten to twenty five years. ADCP(s) provide the capability to make accurate, rapid and cost-effective measurements of river discharge. This method is recommended to be used for calibration of structures of Indus Basin Irrigation System.
Optical Strobe Velocity Meters
Optical strobe velocity meters developed by the U.S. Geological Survey (USGS) and the California Department of Water Resources use optical methods to determine surface velocities of streams (USGS, 1965). This meter uses the strobe effect. Mirrors are mounted around a polygon drum that can be rotated at precisely controlled speeds. Light coming from the water surface is reflected by the mirrors into a lens system and an eyepiece. The rate of rotation of the mirror drum is varied while viewing the reflected images in the eyepiece. At the proper rotational speed, images become steady and appear as if the surface of the water is still. By reading the rate of rotation of the drum and knowing the distance from the mirrors to the water surface, the velocity of the surface can be determined. The discharge rate of the stream may be estimated by applying the proper coefficient to this surface velocity and multiplying by the cross-sectional area of the flow section.
This velocity meter has several advantages. No parts are immersed in the flowing stream. Moreover, it can be used for high-velocity flows and for flows carrying debris and heavy sediment. The meter can measure large flood flows from bridges. However, the meter measures only the water surface velocity and is very dependent upon the selection of the proper coefficient.
Whenever possible, current-meter gaging stations should be located in straight, uniform stretches of channel having smooth banks and beds of permanent nature. The station should be located far from flow disturbances caused by turnouts and power stations. These flow disturbances will variably affect the relationship of discharge to gage height. In many channels, these conditions are difficult to find, and unusual care must be taken to obtain a satisfactory location.
The changing nature of some rivers and canals may require frequent current-meter measurements. Sand shifts may occur frequently, often daily, and aquatic weeds may continue to grow and increase in area. To obtain the gage-discharge relationship at stations on such streams, current-meter measurements may be necessary two or three times weekly or perhaps daily if the importance of equitable water distribution justifies such action. A rating section consisting of a short-lined section in a straight stretch of channel will ensure a meter station of unvarying dimensions if the sediment problem is not serious.
Figure 1: Different Types of Doppler Current Meters
Pygmy Meters (Vertical Axis)
Pygmy meters are similar to Price meters in that both contain a cup-type wheel mounted on a vertical shaft. The pygmy cup wheel is 2 in diameter, compared with 5 in for conventional Price meters. Thus, the pygmy meter can measure velocities closer to flow boundaries. The contact chamber is an integral part of the yoke and contains a single-revolution contact only. The meter has no tailpiece, and no provision is made for cable suspension. The rotational speed of the pygmy meter cup wheel is more than twice that of Price meters. Consequently, use of the pygmy meter is limited to velocities up to 3 or 4 ft/s. The pygmy meter was specially designed for use in small, shallow streams. The smaller meter was necessary because a standard. Price meter does not perform with sufficient accuracy when it occupies a good share of the available stream depth. The pygmy meter may also be used in large canals where the velocity of flow is low or near the edges of a canal to supplement data taken farther out in the channel with a Price meter.
Propeller Meters (Horizontal Axis)
In special situations, Reclamation has used meters of the propeller type with horizontal axles. Hoff meters, Haskell meters, Ott meters, and Neyrpic "Dumas" meters are examples. An assembly of eight Dumas meters with appropriate handling equipment is shown on next page. In this case, the equipment was mounted on a flatbed truck for positioning. These meters have some advantages compared to the Price meters. They are less sensitive to velocity components not parallel to the meter axis, they are smaller, and they are more suited for mounting in multiple units.
Current meters must receive the best care during transportation and use to ensure accurate velocity measurements. Particular care should be taken when working near bridge piers and abutments, floating debris, and also when measurements are being taken at irregular or unknown sections and the meter is suspended on a measuring line. If the cups or blades become bent or damaged, the results obtained from the rating curve for the meter will be unreliable. After completing the measurements at a rating station, the meter should be carefully cleaned. After each day's use, it should be properly lubricated.
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| Velocity Measurement through Price Type AA Curret Meter |
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| Velocity Measurement through Digital Current Meter |
Propeller Meters (Horizontal Axis)
and
Pygmy Meters (Vertical Axis)
The Price-type meters have special cases for storage when the meter is not in use. For damage protection, the cup wheel should be supported firmly on the resting pin that replaces the needle bearing while the meter is stored or being transported. Meter damage has occurred because of improper packing and careless handling in transportation. The meter case should be substantial and rigid with properly fitted interior supports to prevent movement and damage to the delicate parts.
In-Situ Measurement
Currently there are no cost-effective, reliable options for canal/drain discharge measurement, apart from in-situ methods. The conventional current-meter method is most commonly used in gauging streams and is commonly used world over.
Discharge measurements are made at each gauging station to determine the discharge rating for the site. The discharge rating may be a simple relation between stage and discharge or a more complex relation in which discharge is a function of stage, slope, rate of change of stage, or other factors.
The depth of flow in the cross-section is measured at verticals with a rod or sounding line. As the depth is measured, observations of velocity are obtained with a current meter at one or more points in the vertical. The measured widths, depths, and velocities permit computation of discharge for each segment of the cross-section. The summation of these segment discharges is the total discharge of the site.
Types of Current Meter Measurement
Current meter discharge measurements are classified according to the type of equipment used and the nature of station:
ü Wading measurement
ü Cableway measurement
ü Bridge measurement
ü Boat measurement
Discharge measurement using current meter is accomplished by measuring velocity and area. The depth of flow in the cross section is measured at width stations with a rod or sounding line. As the depth is measured, observations of velocity are obtained with the current meter at one or more points in the vertical. The measured widths, depths and velocities permit computation of discharge for each sub-area of the cross-section. The summation of these sub-areas discharge is the total discharge of channel.
Current meter method can be recommended to measure discharges for calibration of structures of Indus Basin System. As most of the measurements will be done on rivers, main canals and link canals boat measurement will be used.
A current meter is an instrument used to measure the velocity of flowing water. The principle of operation is based on the proportionally between the velocity of the water and the resulting angular velocity of the meter rotor. Velocity of water at that point is determined by placing a current meter at a point in a stream and counting the number of revolutions of the rotor during a measured interval of time.
The number of revolutions of the rotor is obtained by an electrical circuit through the contact chamber of the current meter. Contact points in the chamber are designed to complete an electrical circuit at selected frequencies of revolution. Contact chamber can be selected having contact points that will complete the circuit twice per revolution, once per revolution, or once per five revolutions. The electrical impulse produces an audible click in a headphone or registers a unit on a counting device. The counting intervals are measured by a stopwatch.
Current meters generally can be classified into two main types; those meter having vertical-axis rotors and those having horizontal-axis rotors. The comparative characteristics of these two types are summarized below:
Vertical-axis rotor with cups or vanes
ü Operates in lower velocities than do horizontal-axis meters.
ü Bearings or well-protected from silty water.
ü Rotor is repairable in the field without adversely affecting the rating.
ü Single rotor serves for the entire range of velocities.
Horizontal-axis rotor with vanes
ü Rotor disturbs flow less than-do vertical-axis rotors because of axial symmetry with flow direction.
ü Rotor is less likely to be entangled by debris than are vertical-axis rotors.
ü Bearing friction is less than for vertical-axis rotors because bending moments on the rotor are eliminated.
The vertical-axis current meter, the Price meter, type AA which is extensively used by the US Geological Survey (USGS). The standard price meter has a rotor, 5 inches in diameter and 2 inches high with six cones shaped cups mounted on a stainless steel shaft. A pivot bearing supports the rotor shaft. The contact chamber houses the upper part of the shaft an eccentric contact that wipes a bead of solder on a slender bronze wire (cat’s whisker) attached to the binding post. A separate reduction gear (pent gear), wire and binding post provide a contact each time the rotor makes five revolutions. A tailpiece keeps the meter pointing into the current direction.
In addition to the standard type AA meter for general use, there is a type AA meter for low velocities, no pent gear is provided in it. This modification reduces friction. The shaft usually has two eccentrics making two contacts per revolution. The low-velocity meter is normally rated from 0.2 to 2.5 feet per second (fps) and is used when the mean velocity at a cross-section is less than 1 fps.
In addition to the type AA meters, a Price pygmy meter is used for shallow depths. The Pygmy meter is scaled two fifths as large as the standard meter and has neither a tailpiece nor a pent gear. The contact chamber is an integral part of the yoke of the meter. The pygmy meter makes one contact each revolution and is used for rod suspension.
The detail procedure for current meter discharge measurement is described in the following sections.
Accuracy of Discharge Measurement
Accuracy of a discharge measurement depends, in part, on the number of verticals at which observations of depth and velocity are obtained. In general, the interval between any two verticals should not be greater than 1/20-th of the total width, and the discharge between any two verticals should not be more than 5 percent of the total discharge. Observation verticals should be located so as to best define the variation in elevation of the stream bed and the horizontal variation in velocity. Fewer verticals are required on very small streams, since the intervals between any two verticals must be greater than the diameter of the current-meter propeller. The number of verticals should be increased for the first few measurements at a new state.
Discharge accuracy depends on the reliability of the meter rating, conditions of flow, and number of observations of depth and velocity obtained. New and factory calibrated, or in good condition and well maintained and passed the spin test current meters should be used for flow measurements.
Width Measurement
Channel width and the distance between verticals should be obtained by measuring from a fixed reference point, which should be in the same plane as the cross-section. Normally, the distance between verticals is determined from a wire or graduated tagline temporarily stretched across the stream or from semi-permanent marks painted on a bridge hand-rail or a suspension cable.
Depth Measurement
A graduated rod or a drum-wire-weight system is used for measurement of depth of flow. The effect of drag on a sounding wire may be reduced by using a streamlined weight on the end of a fine wire. If the wire is not normal to the water surface, the angle of departure should be measured with a protractor.
The depth may also be measured with an echo (sonic) sounder. The transducer is usually mounted on the boat, submerged about 1 foot below the water surface, and the depth read from a strip-chart recorder. With previous models of sonic sounders, regular calibrations were required under the same conditions of salt content and water temperature as are encountered in the measurement. However, with the latest versions (such as Raytheon DE-719 B) the instruments are provided with compensatory adjustment devices, thus requiring very calibration.
Depth may be read directly on a graduated rod set on the stream bed if the measurement is done by wading. If the drum-wire-weight system is used for measurement, the meter and weight are lowered until the bottom of the weight just touches the water surface, and the depth dial reading is set at zero. The weight is then lowered until its rests on the stream bed, and the depth is read on the dial. Caution is necessary in alluvial streams to prevent the weight from setting through soft bed material.
In order to increase the accuracy of the depth measurement, the sounding weight may be equipped with an electrical device which signals the moment the weight makes contact with the stream bed. If the weight on the sounding line is not sufficient to keep the line perpendicular to the water surface, the Angle between the line and the vertical should be measured to the nearest degree with a protractor.
The angle should not exceed 30o. Methods of correcting the observed depths for angle of sounding line are available. However, the accuracy of the measurement is increased if sufficient weight can be used to maintain the line in a nearly vertical position.
Two Point Method
In the two-point method of measuring velocities, observations are made in each vertical at 0.2 and 0.8 of the depth below the surface. The average of these two observations is taken as the mean velocity in the vertical. This method is based on many studies of actual observation and mathematical analysis. Experience has shown that this method gives more consistent and accurate results than any of the other methods except the vertical velocity curve method. The two point method is the one generally used.
The two point method is not used at depths less than 2.5 feet (75 cm) because the current meter would be too close to the water surface and to the stream bed to give dependable results.
Six-tenth Depth Method
In six-tenth depth method, an observation of velocity at 0.6 of the depth below the surface in the vertical is used as the mean velocity in the vertical. Actual observations supported by mathematical analyses have shown that 0.6 depth method gives reliable results and is used under the following conditions.
ü Whenever the depth is between 0.3-2.5 feet (75 cm).
ü When large amounts of slush, ice or debris make it impossible to observe the velocity accurately at the 0.2 depth (This condition prevents the use of the two point method).
ü When the meter is placed a distance above the sounding weight which makes it impossible to place the meter at the 0.8 depth. (This also prevents the use of the two-point method).
ü When the stage in a stream is changing rapidly and a measurement must be made quickly.
Computation of Discharge
Discharge is computed either arithmetically or graphical depending upon field procedure used to obtain the observation. There are two arithmetic methods for computing the discharge, i.e. the mean section method and mid-section method. Of these two methods, the mid-section method has less procedural error; therefore, it is used for computing canal discharges.
Boat Measurement
Measurement of discharge from a boat is generally made either on channels that can be readily spanned by a temporary cable of sufficient strength to hold the boat in position while measurements are made, or rivers where a cable cannot be stretched. In such rivers, the boat is equipped with a facility (motor boat engine or anchor) to keep it temporarily positioned in the stream. On channels where cable can be stretched horizontal distances (widths) are measured by affixing graduations on the cable. On channels without cable, the widths are either measured by the observer in the boat or by an observer on the bank. For the purpose, points are fixed on the bank(s) along the line of sight and perpendicular to it.
For actual stream flow measurement by boat, the other equipment requirements are as indicated in the following:
Boat Improvement Set
The boat improvement set consists of two aluminum fabricated parts known as ‘boom’ and ‘horizontal stabilizer’. The boom extends beyond the body of the boat a sufficient distance to eliminate any possible effect of the boat on revolutions of the current meter cups. The boom is fixed by a cross-arm called ’horizontal stabilizer’ which is in turn fastened to the boat sides.
The boom part consists of two structural aluminum channel shapes, one telescoped within the other, to permit adjustment in length. The boom has a pulley at the upstream and to guide the sounding cable and base on the downstream end to the fix the real. The cross-arm called ‘horizontal stabilizer’ constructed of structural aluminum channel, is fixed to the gun-whales of the boat by J-bolts. At both ends of the arm, there are guide sheaves under which the boat tagline is passed. There is also an arrangement to clamp this arm with the stretched tagline to keep the boat stationary at a desired position.
Sounding Reels
A sounding real has a drum for winding the sounding cable, a crank and ratchet assembly and a depth indicator. Five types of reels are available i.e. A, B.C, D and E. All reels are similar in that these are fitted with a depth indicator, a treading sheave for laying the cable smoothly in a single layer on the drum, electrical connections for two conductor cables and a pawl and ratchet which can be used to hold the current meter and weight at any desired elevation. All the reels are made largely of aluminum for lightness and designed to operate under any load to the full strength of the cable ordinarily used. A, B and E type reels have the same spacing of anchor studs, so that they are completely interchangeable. D type reel is larger and is used for heavy weights only.
A type reel, which is smallest of the four, plus a detachable crank handle which is keyed to shaft when in use, has no brake, whereas the other three are fitted with brakes and quick-releasing cranks.
Sounding Weights and Accessories
Columbus-type fish weights suspended with cable are used to sound depth as well as to suspend the current meter in the moving stream. These are commonly known as C-type weights and maintain a steady position in water flowing at high velocities. These are available is sizes of 15, 30, 50, 75, 100, 150, 20 and 300 pounds. The nose of each weight extends beyond the cups of the current meter and hence affords protection against any damage. 15-pound weight is a one-piece casting of gun-metal bronze. All the other sizes are cast from lead and contain removable aluminum-alloy tail vanes. The shape of the slot for the hanger-strap permits it to tip forward 15o and backward 5o from the vertical. This limitation in the angles prevents the weight and current meter from striking each other. Due to some variation in the quality of material used in construction, it is necessary and describes that the weight be balanced under water, regardless of level position it assumes when suspended in air.
The hanger strip is used for carrying the current meter, its lower end is fixed with the sounding weight and the upper end to the cable through a connector. It is 1/8 inch thick, 1-1.5 ft long and made of steel. It contains a hole threaded for a 3/8 inch hole at the opposite end. Three holes, 7/32 inch in diameter, are at 4,9,5,3 and 9.8 inches above the hole for weight suspension. The lower two of these are used for supporting the current meter so that its horizontal axis is 0.5 ft from the bottom of C-15 and C-30 respectively. The third hole is located so that the distance from the horizontal axis of the meter to the bottom of the C-50 or heavier weight is approximately one foot.
Timers and Counting Equipment
In order to determine velocity at a point with a current meter it is necessary to count the resolution of the rotor in a measured interval of time. The equipment required is:
ü Stop watch; and
ü Headphone: Theses convert into individual sound clicks, the electrical impulses resulting from each click (for 1 or 5 revolutions of the bucket wheel as the case may be) in the chamber of the current meter. The resistance of head phone is normally 5-8000 ohms and it operates on 11.5 volt dry cell.
ü Automatic Electric Counter: This counter automatically registers the revolutions up to 999 and has reset button. However, this counter is not recommended with the contact wire chamber because at low velocities the contact-wire wipes irregularly thereby sending several signals to the counter for each revolution.
Float Method
The velocity of flow in a canal or canal/drain, and hence the discharge may be determined approximately by the use of float. A stretch of the canal, straight and uniform in cross-section and grade, and with a minimum of surface waves, should be chosen for this method. Surface velocities only be attempted on windless days to avoid wind-caused deflection of floats. Even for best conditions, surface floats are diverted from a direct course between measuring stations because of surface disturbances and cross-currents. In addition to surface floats, which are immersed one-fourth or less of the flow depth, rod floats, which are submerged more than one-fourth of the depth but not touching the bottom should be used. In general, because a number of other methods are easier and more accurate, this method should be used when other methods are impossible or impracticable. Hence this method is not suitable for calibrating the Indus Systems structures.
Following coefficients are used to convert surface velocity to mean velocity.
Average Depth of water (feet) | Coefficient |
1 | 0.66 |
2 | 0.68 |
3 | 0.70 |
4 | 0.72 |
5 | 0.74 |
6 | 0.76 |
9 | 0.77 |
12 | 0.78 |
15 | 0.79 |
20 ft | 0.80 |
Source: USGS (1960)
Pitot Tube Method
A pitot tube in its simplest form is an instrument consisting of a tube with a right angle bend which, when partly immersed with the bent part under water and pointed directly into the flow, indicates velocity of flow by the height water rises in the vertical stem. The height of rise (h in ft)) of the water column above the water surface, expressed in feet and tenths of feet, equals the velocity head (V2/2g). The velocity of flow (v) in feet per second, may thus be determined from the relation
The simple form of pitot tube has a little practical value for measuring discharges in open channels handling low velocity flows because the height of water rises above the water surface is difficult to measure. Therefore it cannot be used for calibration of Indus System structures.
Dilution Method
The color dilution method consists of injecting a known concentration of a suitable dyne into the stream and measuring its concentration after it has traveled far enough to thoroughly mix with stream. But this method is also not practicable for calibration of structures in the Indus Basin System.
Radioisotope Method
The radio isotopic method is a variation of the dilution method and uses a radioactive material as the tracer substance. A measure of degree of dilution is obtained by counting gamma ray emissions from the concentrated isotope solution, and from diluted solution consisting of the stream plus the tracer, using Geiger counter.
This method is also not suitable for calibration of discharge measurement points of Indus Basin System.
Accuracy of measurement can be maintained by observing the following precautions for Price meters (including the pygmy meter modification of the Price meter):
ü The meter should be spin tested before and after completing measurements to assure that the meter has no error-causing damage. With the shaft in a vertical position and the cups protected from air currents, the cups should be given a quick turn to start them spinning. If the meter is in proper adjustment and the bearings are free from foreign particles, the cups should come to rest in not less than 3 minutes. If the length of spin is only about 12 minutes, but the cup wheel comes to rest gradually, all flows except those of very low velocities may be measured. If the length of spin is only about 1 minute but the cup wheel comes to rest gradually, the meter may still be used to measure velocities above 1 ft/s. If the length of spin is less than 1 minute, the meter should be reconditioned. Under laboratory controlled conditions, rotation should continue for about 4 minutes. The manner in which rotation ceases will help indicate the condition of the meter and should be observed.
ü The cross section of the stream should be divided vertically into 20 or more segments. Very small streams and sections with smooth, firm boundaries are exceptions, and a smaller number of stream cross-section segments would be sufficient. A single vertical reading is used if the distance between verticals is less than 1 ft. Horizontal divisions are generally selected so not more than 10 percent, and preferably not more than 5 percent, of the discharge will occur between any two adjacent verticals.
ü The stopwatch should be checked frequently and kept in good condition.
ü For low and irregular velocities, the period of observation should be lengthened to obtain a more accurate average count.
ü The current meter should be withdrawn from the water between velocity readings to make sure that rotation is not being impeded by debris or any other cause.
ü The meter should be allowed at least 10 to 20 seconds to attain rotation speed before counting commences.
ü The total operation of the meter at each elevation of a vertical should consist of at least two consecutive periods of at least 40 seconds. If significant differences are apparent in each period, more readings should be taken.
ü Measurements while wading should be done facing the bank, standing just downstream from the tag line, and at least 18 in to the side of the meter.
Depth sounding, either with a meter and rod assembly or with a special sounding line and weight, should first be made at each of the permanent measuring points. These depths should be properly recorded. Next, the mean velocity at each of the measuring points should be determined with the current meter by one of the methods listed in the following section. Velocity measurements should be properly recorded.
Errors of velocity measurement will arise if the current meter:
ü Is placed closer to the boundary than 1-2 rotor diameters
ü Is used to measure velocities less than 0.5 ft/s or out of the range of calibration. Overdriving the rotor can damage bearings
ü Is not held steady in one position during the time measurement
ü Is used in significant waves, such as those caused by wind
ü Is used in flow which is not parallel to the axis of the propeller meter or is oblique to the plane of the cup-type meter
ü Is impeded by weeds or debris
The following methods are used to determine mean velocity in a vertical line with a current meter:
ü Two-point method
ü Six-tenths-depth method
ü Vertical velocity-curve method
ü Subsurface method
ü Depth integration method
ü Two-tenths method
ü Three-point method
ü One-point continuous method
The two-point method consists of measuring the velocity at 0.2 and then at 0.8 of the depth from the water surface and using the average of the two measurements. High accuracy is obtainable with this method, and its use is recommended. However, the method should not be used where the depth is less than 2 ft.
The six-tenths-depth method consists of measuring the velocity at 0.6 of the depth from the water surface and is generally used for shallow flows where the two-point method is not applicable. The method gives satisfactory results.
The vertical velocity-curve method consists of measuring the velocities at enough vertical positions so that the velocity profile is defined well enough to calculate a sufficiently accurate mean velocity. The method is very accurate, depending upon the number of data points measured for profile, but is time consuming and costly.
The subsurface method involves measuring the velocity near the water surface and then multiplying it by a coefficient ranging from 0.85 to 0.95, depending on the depth of water, the velocity, and the nature of the stream or canal bed. The difficulty of determining the exact coefficient limits the usefulness and accuracy of this method.
The depth or traveling integration method is performed by observing the velocity along a vertical line by slowly and uniformly lowering and raising the meter throughout the range of water depth two or more times. The method is not accurate and should be used only for comparisons or quick, rough checks.
The two-tenths, three-point, and one-point continuous methods are special procedures based on a relationship previously established for the section between the true discharge and the velocities observed by these methods. These methods are generally reliable for sections which undergo no serious changes because of erosion, sedimentation, or other deformation. They are discussed in detail in USGS (1965) and USGS (1980). Of the methods cited in this section, the two-point method and the six-tenths-depth method are most used in canal work.
The velocity-area principle is used to compute discharge from current-meter data. Total discharge is determined by summation of partial discharges. A partial discharge is the product of an average point or vertical line velocity and its meaningfully associated partial area, expressed as:
The total discharge is then:
The measured and computed variables are as follows:
q = the discharge in cubic feet per second (ft3/s) for a partial area
Q = total discharge
= the mean velocity associated with the partial area
a = partial area of total cross section
L1, L2,... Ln= distance to vertical measurement locations in feet from an initial point to vertical station
L = the distance in feet between consecutive vertical measurement stations
, ,... = the respective mean velocities in feet per second at vertical measurement stations
D1, D2,... Dn= the water depths in feet at verticals
n = the number of verticals related to the partial area
Simple Average Method
Using the simple average of two successive vertical depths, their mean velocity, and the distance between them results in:
The two hyphenated integers as a subscript denote that the partial discharge, q, is for the area between two consecutive vertical measurement points as numbered.
Midsection Method
In the midsection method, the depth and mean velocity are measured for each of a number of verticals along the cross section. The depth at a vertical is multiplied by the width, which extends halfway to the preceding vertical and halfway to the following vertical, to develop a cross-sectional area. The product of this area and the mean velocity at the vertical gives the discharge for the partial section between the two halfway points. A summation of the partial discharges gives the total discharge. The formula for computing the partial discharge using the midsection method is:
The value, n, with plus and minus after it denotes that the partial discharge, q, is for the area between halfway back toward the previous vertical measurement and halfway toward the next forward vertical.
The mean velocities are determined by any one of the methods listed in section 12. For these two methods, the verticals do not need to be equally spaced, but the verticals should be chosen such that:
ü The error of computing the area between the verticals does not exceed 3 percent when the bed is treated as straight lines between the verticals.
ü Except at the banks, the difference between the mean velocities at the verticals does not exceed 20 percent relative to the lower velocity of a pair of verticals.
Simpson's Parabolic Rule
In this method, Simpson's parabolic rule is used twice to compute discharge using the area velocity method. First, the area is computed for three consecutive depths at velocity measuring stations using Simpson's rule. Second, average velocity for the same three verticals is computed by the rule. The discharge between the three verticals is the product of the average velocity and area. Using Simpson's rule assumes both the vertical depths and their corresponding average velocity vary parabolically. Natural riverbeds and older earth-lined canal bottoms follow curved shapes rather than the typical straight line geometry of hard-lined canal designs. Both vertical and horizontal velocity profiles tend to be parabolic in either case. Using Simpson's rule to obtain the area between three equally spaced consecutive verticals or two consecutive partial areas results in:
where is the distance between consecutive vertical velocity measuring stations which are equally spaced across the flow section.
Using Simpson's rule to obtain the mean velocity of three consecutive verticals or over two consecutive partial areas is expressed as:
The product of this velocity and the area from the previous equation results in the relationship for the discharge through the two consecutive partial areas, written as:
Simpson's parabolic rule method is particularly applicable to river channels and old canals that have cross sections conforming in a general way to the arc of a parabola or to a series of arcs of different parabolas. Simpson's method requires equally spaced verticals. The simple average and the midsection methods do not require equally spaced verticals. Thus, these two methods are well suited to computing discharges in canals that conform closely to their original trapezoidal rectangular shapes.
Typical discharge computations obtained by the midsection method, and weir formula are attached in Table 1 and 2.
Table 1: Discharge Measurement at Nawabshah West Main Drain (NWMD) RD 0+00
Weir Formula Free Flow Condition Date of Observation July 06, 2012 Time: 11:25
Q= L * C * H 1.5
Where:
L = Length of weir
C = Constant
H = Head on weir crest
Measured Hydraulic Data
L = 21 ft
C = 3 - 3.4
H = 1.9 ft V = 4 ft per sec
H Total = 1.9 + 0.25 = 2.15 ft
B = 21 ft
C = 3.1
Computer Discharge
Q = 3.1 x 3.15 x 21
= 205 cs.
Total Discharge Measured = 205 cs.
Table 2: Discharge Measurement at Nawabshah East Main Drain (NEMD) RD 0+00
Weir Formula Free Flow Condition Date of Observation July 06, 2012 Time: 11:45
Q = L * C * H 1.5
Where:
L = Length of weir
C = Constant
H = Head on weir crest
Measured Hydraulic Data
L = 12 ft
C = 3 - 3.4
H = 2 ft
V = 3 ft/sec
= 0.14
H+ = 2.0 + 0.25 = 2.25 ft
Computer Discharge
Q = 3.3 x 3.13 x 12
= 3.3 x 3.15 x 12 = 124.74 cs
Discharge measured = 125 cs.
Table 3: Recorded Velocity Measurements Using Flow Probe At Rd 159+700 Of Spinal
Date: 11 Sep 2012 Time of Observation 12:45pm Weather condition: Mild wind opposite to flow direction
S No. | Width (ft) | Depth (ft) | Area (ft2) | Mean Area (ft2) | Velocity (ft/Sec) | Discharge (ft3/Sec) |
1 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
2 | 3.0 | 5.0 | 15 | 7.5 | 1.6 | 12.0 |
3 | 7.0 | 6.5 | 45.5 | 30.25 | 2.2 | 66.6 |
4 | 10.0 | 7.4 | 74 | 59.75 | 2.4 | 143.4 |
5 | 10.0 | 8.9 | 89 | 81.5 | 2.4 | 195.6 |
6 | 10.0 | 10.1 | 101 | 95 | 2.6 | 247.0 |
7 | 10.0 | 10.3 | 103 | 102 | 2.8 | 285.6 |
8 | 10.0 | 11.5 | 115 | 109 | 3.4 | 370.6 |
9 | 10.0 | 10.7 | 107 | 111 | 3.4 | 377.4 |
10 | 10.0 | 11.1 | 111 | 109 | 3.5 | 381.5 |
11 | 10.0 | 10.5 | 105 | 108 | 3.5 | 378.0 |
12 | 10.0 | 10.3 | 103 | 104 | 3.5 | 364.0 |
13 | 10.0 | 9.5 | 95 | 99 | 3.6 | 356.4 |
14 | 10.0 | 10.5 | 105 | 100 | 3.3 | 330.0 |
15 | 10.0 | 10.7 | 107 | 106 | 3.1 | 328.6 |
16 | 10.0 | 10.5 | 105 | 106 | 3.0 | 318.0 |
17 | 10.0 | 9.9 | 99 | 102 | 2.8 | 285.6 |
18 | 10.0 | 9.0 | 90 | 94.5 | 2.8 | 264.6 |
19 | 10.0 | 7.5 | 75 | 82.5 | 2.6 | 214.5 |
20 | 10.0 | 6.2 | 62 | 68.5 | 1.9 | 130.2 |
21 | 10.0 | 2.3 | 23 | 42.5 | 1.3 | 55.3 |
22 | 0.0 | 0.0 | 0 | 11.5 | 0.0 | 0.0 |
Gauge Recorded at Bridge RD 159+700: 9.9 ft KPOD Gauge: 8.8 ft Total Discharge
To rate a flow section, discharge measurements at a current-meter station should be taken over a wide range of canal flows to ensure accuracy in preparing velocity, area, and discharge rating curves (section 1). Water is usually turned into the canals at gradually increasing rates as demand increases during the irrigation season. Thus, measurements for all flow stages in the canal often can be obtained during one season.
The canal bed at a well selected current-meter station is generally permanent in character, and a permanent rating curve could be made if not for sediment accumulations or for growths that occur in the canal during the irrigation season. The sediment and the growths both decrease the discharge capacity of the canal for all flow depths, and the effect is usually most pronounced during the latter part of the irrigation season. This change in flow capacity of the canal for a given depth of flow must be taken into consideration when computing the quantity of water carried by the canal. If the canal is cleaned during the season, the relationship of discharge to gage height is again disturbed. The changing relationship of discharge to gage height in irrigation canals caused by changing boundary conditions is the chief source of error in flow measurements.
Gage Readings
To determine the quantity of water carried by a canal over a period of time, the gage must be read at least twice daily. More than one reading provides a means for checking the readings and also informs the canal attendant of any unexpected changes in canal stage. More frequent readings are needed when changes in stage are suspected or are made in the canal. The readings should be taken by the canal attendant on regular rounds. The gage should be read accurately, generally to the nearest hundredth of a foot. Automatic water-stage recorders eliminate the need for numerous readings and can increase the accuracy of the flow measurements.
Computations of Discharges
Current-meter measurements made at several specific flows can be used to obtain discharge, velocity, and area curves that apply to all inclusive gage heights by plotting the appropriate data on cross-section or graph paper (figure 10-18). Discharges, corresponding mean cross-sectional velocities, and cross-sectional areas are plotted on the horizontal axis. Corresponding gage heights are plotted on the vertical axis. Three separate curves are drawn through these data points.
Figure 5: Typical discharge, mean velocity and area curves for a canal.
The probable area curve is established first by drawing the most probable line through the data points. Using this curve, the accuracy of the area computations and of the flow depth measurements may be checked. Next, the computed mean cross-sectional velocities are plotted, and a curve is drawn through the points. This curve provides a check on the velocity computations and helps detect changes in velocity that may occur in the canal because of changing roughness or silting in the canal.
Finally, the discharge curve is drawn through the computed discharge points. If flow conditions in the channel did not change resistance significantly during the period needed for measurements over the full range of canal flows, the curve will generally be easy to draw. If the relationship of discharge to gage height was affected by growths or sediment deposits, one or more additional discharge curves must be drawn. The number of rating curves required for a cross-section location depends upon the degree of the flow restric- tions encountered and the rate at which the restrictions developed. These curves will generally be parallel to, but slightly displaced from, the curve for the clean canal. For the periods when the change is in progress, discharges may be estimated by proportioning between curves for the clean and restricted conditions on a time basis.
From the rating curve, a rating table may be prepared for each tenth or hundredth of a foot of gage height from zero to the maximum height of water in the canal or stream. For canals affected by growths or sediment, two or more such rating tables will be necessary, one for early in the season when the canal is clean, and the other for late in the season when growths or other restrictions are present. If the canal is cleaned during the irrigation season, operating personnel should be instructed to switch to the curves and tables for the clean canals.
Daily and Monthly Discharges
Discharges in acre-feet may be compiled from the daily gage heights and the rating tables. From these tables, the monthly discharges and the total amount of water delivered by the canal during the irrigation season may be obtained.
