The different methods available for measuring water in open channels may be grouped as:
1. Velocity-area methods, and
2. Direct discharge methods.
1. Velocity-Area Methods:
In these methods the velocity of flow in the channel is measured by some means and the discharge is calculated from the velocity and the area of cross-section by using Q=AV.
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The common methods for measuring velocity are:
i. Float Method:
This method is cheap and simple but not very accurate and gives only an approximate measure of the rate of flow. An estimate of the velocity of flow is made by determining the velocity of an object floating with the current. A straight uniform section of the channel about 20 to 25 metres long is selected and marked on the banks.
The time required to traverse the distance by a floating object is measured and the velocity is calculated. Mean velocity in the whole of the cross- section is obtained by applying a coefficient 0.80 to the surface velocity. The area of cross-section (the channel) is calculated from its dimensions and this is multiplied by the average velocity to get the rate of flow.
ii. Current Meter:
The current meter (Fig. 4.2) essentially consists of a wheel having several cups, or wheels, attached to a streamlined weight and the assembly suspended by means of cables or mounted on straight rods. The wheel in the current meter is rotated by the action of flowing water. The rate at which the wheel revolves varies with the velocity of the water. Several devices have been used for determining the speed of the wheel.
The most common one is the mechanism which makes and breaks an electric circuit at each revolution in any selected period of time is counted, the time being determined by means of a stop watch. For each current meter a rating curve is made available by the manufacturer.
This curve shows graphically the relationship between the speed of the wheel and the velocity of water. Knowing the speed of the wheel, the velocity of water can be known from this curve.
Current meters can be used for measuring the velocities in irrigation channels, streams or large rivers. For taking the readings, shallow streams can be waded while large rivers may require the use of a boat or a cable way. An existing bridge across the stream can be conveniently used.
For streams of small widths, one reading at the centre of the stream is sufficient. Otherwise the stream is divided into convenient number of sections (Fig. 4.3) and the velocity is determined for each section.
Again, referring to the vertical sections, where the depth of flow becomes too small to obtain the readings, a single reading taken at 0.6 depth represents the mean velocity. Otherwise two or more readings may be taken at as indicated in Table 4.1 and the average of the velocity is taken. Knowing the average velocity and the area of a particular section the rate of flow is calculated. Adding for all sections the total discharge in a stream or river is obtained.
iii. Tracer Methods:
The principle of these methods is that a substance (referred to as tracer) which dissolves in water but does not react with it is introduced into the flow and its concentration is measured at two points. Assuming that the substance completely mixes with water and that there is no loss of water between the points of injection and’ measurement, the rate of flow can be deduced.
In the salt dilution method a chemical like common salt or a dye is made into a solution of known concentration and introduced into the flow at a known rate. The concentration of the substance is measured downstream.
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If Q is the rate flow, C0 the initial concentration, C1 is the concentration of the solution being added, q1 is the rate at which the solution is added, C2 the concentration measured downstream, the material balance equation can be written as –
Where Q is the rate of flow, F is the counts per unit of radioactivity per unit volume of water per unit of time. A is the total units of radioactivity introduced and N is the total counts. Radioactive materials are hazardous to health and as such special precautions are needed when the radioisotopes are used. The quantities used should be within the safe standards.
2. Direct Discharge Methods:
In these methods, velocity measurements are not involved but rates of flow are measured directly by using certain devices.
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There are many such devices but the most useful on the farm are –
i. Weirs,
ii. Orifices and
iii. Flumes
i. Weirs:
A weir is a notch of regular form through which water flows. The various terms used in connection with weirs as shown in Fig. 4.4.
The weir can be made of wooden planks, sheet metal or concrete and the opening cut on the top edge.
Weirs may be divided into two broad classes as –
(1) Sharp-crested weirs and
(2) Broad crested weirs.
Sharp-crested weirs are generally of three types depending upon the shape of the notch, these are –
(1) Rectangular weir which has a level crest and vertical sides,
(2) Trapezoidal or Cipolleti weir, which has a level crest and sides of the notch sloping outward from the vertical at one horizontal to four vertical and, (3) 90° triangular weir formed by the sides of the notch sloping outward from vertical at 45° angle (Fig. 4.5).
For measuring the rate of flow in a channel, a weir of known dimensions is placed across the channel and the head of flow over the weir is measured.
The basic equation for flow through the weir is:
This formula is applicable for sharp crested rectangular weirs without end contractions, i.e., the length of the crest are equal to the width of the approach channel. In case of weirs with end contractions i.e., when the length of the crest is less than the width of the approach channel the water approaches the notch in a converging manner.
When these distances are large such that the water gets ponded and approaches the weir at negligible velocity, the weir is said to have complete contractions. In such a situation the effective length of the weir crest in the above formula is found from the relation L = L – 0.2H where L is the measured length of the weir crest and H is the head of flow.
The trapezoidal weir with 1 : 4 side slopes is known as a Cipolletti weir. There is no need to apply the correction due to end contractions in this case, as the triangular parts of the notch compensate the decrease in discharge caused by contractions. The formula for discharge to be used with this type of weir is Q = 0.0186 LH3/2 in metric system and Q = 3.367 LH3/2 in the English system.
The triangular or V-notch weir is made with different angles like 45°, 90° and 120°. The 90° V- notch [Fig. 4.5(c)] is the most commonly used. These weirs permit more accurate measurement of lower discharge than horizontal crested weirs because of the fact that the head recorded for the same rate of flow is higher in these weirs compared to other types. The formula to be used in case of 90° triangular weir is Q = 0.0138 H2.48 or nearly 0.0138 H2.50 in metric units and 2.5 H2.5 in English units.
The following general rules should be observed in the construction and installation of weirs so that the above mentioned formulae are applicable for calculating the discharge:
1. The weir should be installed in a straight line at right angles to the direction of flow.
2. The crest and sides of the edge should be sharp edged. The sharp edge should be as shown in Fig. 4.4.
3. The velocity of approach of water to the weir should be negligible.
4. The head over the weir should be measured on the upstream side at a distance of at least 4 times the head of flow from the weir. Again to ensure complete end contraction, the distance between the ends of the notch and the sides of the channel should not be less than 2 times the depth of flow of water over the weir. Similarly the weir crest should be placed no closer than 2H from the bottom of the channel.
5. The falling water surface from the weir should have a free flow i.e., the air should move freely below the nappe and the flow of water should not cling to the weir.
ii. Orifices:
An orifice is an opening with closed perimeter and of a regular shape through which water flows (Fig. 4.6). The orifice acts as a weir when the opening flows only partially. Orifices must have sharp edges or else it is treated as a tube.
The stream of water coming out of the orifice is known as the jet. If this jet discharges into the air the orifice is said to have free discharge and if the discharge is under water it is called a submerged orifice. The depth of water producing the discharge is called the head and in case of submerged orifices the head is the difference in the upstream and the downstream water levels. Submerged orifices can be conveniently used for measuring small discharges. They do not require a fall in the level of the bed of the channel as is required in case of weirs.
Orifices may be circular, square, rectangular, or of any other regular shape. Circular and rectangular orifices are common for the measurement of water on the farm.
The equation of discharge through a sharp edge submerged orifice is given by:
In English units Q is in cu ft per second, a is in sq. ft., H is in ft and g is in ft/s2.
The main advantage of the submerged orifice is the suitability for measuring flows in channels having very small slopes where it is difficult to obtain enough fall for the use of weirs. Submerged orifices have the disadvantage of collecting floating debris and collecting sand and silt above the orifice preventing accurate measurement.
Broad-Crested Weirs:
A broad-crested weir is a broad wall set across the channel bed (Fig. 4.7).
The above discharge formula assumes that the thickness of the sheet over the crest adjusts itself so that the discharge is a maximum, an assumption supported by experimental findings.
Broad crested weirs are not constructed to standard dimensions as readily as the thin plate types. When a broad-crested weir is used, it is normally calibrated in the field by current meter measurements or in the laboratory by model tests which are checked by a small number of gaugings on the field installation.
In many irrigation canals, sediment laden water is a common difficulty. It causes rapid rounding of a sharp weir plate, destroying its ability to throw the nappe clear and altering its discharge characteristics. A flume or a broad-crested weir is preferable in such circumstances.
iii. Flumes:
Flumes are specially shaped and constructed channel sections used to measure the flow of water. The principle of the flumes is based on the concept of specific energy and critical flow in open channels. Specific energy is the total energy at any cross-section with respect to the channel bed. Considering the slope of the channel bed to be small, specific energy E is given by-
When the geometry of the channel section remains constant the cross-sectional area varies with only depth of flow. As such, for a given discharge the specific energy is a function of depth alone.
From Eq. 4.14, plotting E against h for different values of Q/b a family of curves as shown in Fig. 4.8 will be obtained. For any value of Q/b there is a minimum value of E. At this value of E the flow is termed critical and the depth of flow as critical depth and the velocity as critical velocity.
The flow is not physically possible at specific energy values less than the minimum value of E. For any higher value of specific energy, there are two possible depths of flow. At the greater depth the velocity is low and the flow is known as subcritical. At the lesser depth velocity is high and flow is known as supercritical.
Differentiating Eq. 4.13, and equating it to zero for minimum conditions, we have-
The dimensionless quantity, V/√gh is defined as Froude number. For critical flow its value is 1 and it is greater than 1 for supercritical flow and less than 1 for subcritical flow.
For a rectangular channel section, Q is given by Q = b . h . V and substituting in Eq. 4.15, the critical depth is given by-
This property will later be used in the design of gully control structures.
When subcritical flow passes through a structure changing the flow into critical and supercritical state, it is found that the depth of flow upstream of the structure is independent of the downstream depth. In such a situation the discharge is a single valued function of the upstream depth. This principle is used in the flumes used for measuring flow of water.
There are several types of flumes developed and some of the common types used are discussed here:
a. Parshall Flumes:
The parshall flume (Fig. 4.9) consists of three parts:
(1) A converging section on the upstream end,
(2) A constricted section known as throat, and
(3) A diverging section on the downstream side.
The floor of the converging section is level, the floor of the throat inclines downward and the floor of the diverging section slopes upwards.
The size of the Parshall flume is specified by its throat width. The different proportions to be adopted are given in Table 4.2.
For determining the rate of flow the Parshall flume is to be installed in the channel and the depths of flow at two points Ha and Hb as shown in Fig. 4.9 are to be taken. From these readings it can be known whether the discharge through the flume is a free flow or a submerged flow.
The flow is termed as free-flow when the elevation of the water surface near the downstream end of the throat
section is not high enough to cause any retardation of flow due to back water. Free flow conditions can be determined by noting the ratio of Hb/Ha.
The free flow limits of Hb/Ha vary with the widths of the throat and are tabulated as follows:
Under the free flow conditions, the measurement of water level or the head (Ha) in the converging section is sufficient to determine the rate of flow.
When the flow is submerged i.e., when the free flow limits of Hb/Ha are exceeded, it is necessary to take the reading Hb at the lower end of the throat section. In such conditions, a correction has to be applied for free flow discharge upon the percentage of submergence in order to determine the rate of the submerged flow.
The Parshall flume may be constructed of sheet metal, timber, brick masonry or reinforced concrete. Sheet metal flumes are very satisfactory and have the advantage of being portable. Brick masonry and concrete flumes are to be constructed in the channels.
The Parshall flume is very useful equipment for measuring the water flows on the farm. It is self-cleaning, requires only a small amount of drop or head loss in the stream and allows reasonably accurate measurements even when partially submerged.
b. H-Flumes:
These type of flumes are well suited for runoff measurement as they have a high capacity and are accurate at different rates of flow. They are also well suited where sediment sampling of the runoff is done using automatic silt samplers. Fig. 4.10 gives the typical dimensions of H flumes.
The H flume is useful for flows ranging from 0.009 to 0.85 m3/s. For smaller or greater
flows the dimensions of the H flume are modified and are known as HS flumes for smaller flows and HL flumes for larger flows. H flumes are to be calibrated and the rating tables are to be used for measuring discharges.
c. Trapezoidal Flumes:
These are somewhat similar to Parshall flumes but are of different shape. Fig. 4.11 indicates the shape of these flumes.
These were originally developed in Washington State College and as such are referred to as WSC flumes.
Some of the advantages of these flumes are:
(1) A large range of flows can be measured with a comparatively small change in head;
(2) Extreme approach conditions seem to have a minor effect upon head discharge relationship;
(3) Sediment deposits in the approach do not change the head- discharge relationships noticeably;
(4) The flumes will operate under greater submergence than rectangular shaped ones without corrections for submergence;
(5) The trapezoidal shape fits the common channel sections more closely than a rectangular one.
Fig. 4.11 indicates the details of design of the trapezoidal flume. Table 4.3 indicates the standard dimensions. Fig. 4.12 gives a typical rating curve for the trapezoidal flumes to determine the discharge through the flumes.
d. Cut-Throat Flumes:
Cut-throat flumes (Fig. 4.13) have a rectangular cross section, a level floor, a uniformly converging inlet section, and a uniformly expanding outlet section. The throat occurs at the intersection of the inlet and outlet sections.
Because cut-throat flumes have level floors, they are easier and more economical to build and install than Parshall Humes. The level floor also allows them to be placed directly on channel beds and to be installed inside of concrete lined channels.
Similar to Parshall flumes, the flow in the cut-throat flumes could be under free flow or submerged conditions. For the free flow conditions, the following equation is used for determining the rate of flow.
It is desirable that rating tables are developed for the flumes which are used in a given situation.