Methods for measuring water in open channels can be grouped into two methods: 1. Velocity-Area Methods 2. Direct Discharge Methods.
Method # 1. Velocity-Area Methods:
In this method, velocity of flow in the channel is measured by some means and the discharge is calculated from the area of cross-section using the equation Q = A × V
i. Float Methods:
A float is any piece of material that floats on water (cork, wooden piece etc). A straight uniform section of the channel of about 20 to 25 m long is selected and marked on the banks. The time required to traverse the distance by a floating object is measured and the velocity calculated. Mean velocity in the whole of the cross-section is obtained by applying a coefficient (0.80 to 0.85) to the surface velocity. The area of the channel is calculated from its dimensions and this is multiplied by the average velocity to get the rate of flow.
where, Q = Discharge (cm3 s-1)
a = Channel width at flow surface (cm)
b = Channel width at the bottom (cm)
H = Flow depth in the channel (cm)
V = Flow velocity in the channel (cm s-1)
This method is simple and cheap but not accurate. This method is restricted to straight rivers and channels having almost uniform cross-section throughout. It can be used when the depth of water is less than 1.5 m.
ii. Current Meters:
Various types of current meters are in use these days, but the commonly used in our country is Price’s Current Meter. It consists of a horizontal wheel carrying a series of cups that rotate on a vertical axis. There is a tail-vane and a counter weight at the bottom to balance the meter and to keep it steady (Fig 9.2).
When the current meter is suspended in water, the velocity of flow causes the wheel to rotate. In this type of meter, the difference in pressure produces the rotation. Hence, it is also called as differential meter. The current meter is fitted with a device for recording the number of revolutions of the horizontal wheel due to the velocity of flow.
In the modern current meters, electro-magnetic counters are provided for recording the number of revolutions. Knowing the number of revolutions per second made by the wheel, the velocity of flow can be calculated using the formula for the given current meter.
The rating formula is, generally, in the form:
V = (a + b N)
where, V = Velocity of flow (m s-1)
N = Number of revolutions per second
a and b = Constants given by the manufacturer or determined by experiments.
iii. Tracer Methods:
This method is also called dilution method. The principle underlying this method is that when a chemical solution or tracer is injected into the flow, it will get completely and uniformly mixed with the flow and that the diluted concentration downstream will decrease due to increased discharge of the mix. Chemical concentrations are measured by titration or colourimetric methods. Radioactive tracers, when used, can be measured for radioactivity by Geiger counter.
If Q is the rate of flow, C0 initial concentration, C1 is the concentration of the chemical solution being added, q1 is the rate at which the solution is added, C2 the concentration measured downstream, the equation can be:
Q C0 + q1 C1 = (Q + q1) C2
From this, the equation for the rate of flow (Q) is obtained as:
In radioactive method, if F is the counts per unit radioactivity per unit volume water per unit time, A is the total units of radioactivity introduced and N is the total counts, the flow is given by:
The usual tracers used are chemicals like sodium chloride, sodium dichromate, manganese sulphate and sodium nitrate; fluorescent dyes like sodium fluoroscein and rhodamine-WT and radioactive isotopes like bromine-82, sodium-24 and lindane-132.
iv. Velocity Head Rod Method:
Velocity head rod is used to measure the velocity of water in a ditch and is relatively inexpensive and fairly accurate. The rod is, in actuality, a ruler used to measure the depth of the water. Water height is first measured with the sharp edge of the ruler parallel with the flow and the again with the ruler turned 90 degrees (Fig 9.3).
Difference in the height of water is the head differential and using Table 9.2, an estimate of the velocity (feet per second) can be made. From there, follow the same formula as with the float or tracer method, i.e. multiply the velocity by the cross sectional area of the ditch to get cubic feet per second. The velocity head rod method works only for velocities greater than 1.5 ft s-1 and less than about 10 ft s-1.
The procedure is:
1. Place the rod with the sharp edge upstream. Record the depth of the water (normal depth)
2. Place the rod sideways. This will cause some turbulence and the water level will “jump” (inches) causing the water level to rise. Record the level again (turbulent depth)
3. Subtract the normal depth from the turbulent depth and this will be the jump height
4. Find the corresponding velocity from Table 9.2
5. Multiply the velocity by the cross sectional area of the ditch to get the flow rate (cfs).
Method # 2. Direct Discharge Methods:
In these methods, velocity measurements are not involved, but rates of flow are directly measured by certain devises. There are many such devises, but the most useful on the farm are weirs, flumes and orifices.
i. Weirs:
Weir is a notch through which water is made to flow. A weir consists of a weir wall of concrete, timber or metal with a sheet metal plate fixed to it. Weirs may be built as stationary structures or may be made portable. Weirs are divided into two broad groups: sharp-crested weirs and broad-crested weirs.
Sharp-crested weirs are, generally, of three types, depending on the shape of the notch: rectangular weirs with a level crest and vertical sides, trapezoidal or cipoletti weirs, which has a level crest and sides of notch sloping outwards from the vertical at one horizontal to four vertical and 90° triangular weir (V-notch) formed by the sides of notch sloping outwards from vertical at 45° angle (Fig 9.4). Rectangular and 90° V-notch weirs are commonly used on the farms. Discharge through a weir is proportional to the head on the crest.
The basic formula for calculating discharge through a weir is:
Q = C × L × Hm
where, Q = Discharge
C = Coefficient, depending on the nature of crest and approach conditions
L = Length of weir crest
H = Head on the crest
m = Exponent depending upon weir opening.
Rectangular weirs:
They have horizontal crest and vertical sides. They may be either contracted rectangular weirs or suppressed rectangular weirs. They have a sharp crest and are beveled on the downstream side only. The sides are not beveled.
Discharge is computed by the formula: Suppressed rectangular weir:
Q = 0.0184 x L x H3/2
where, Q = Discharge (lps)
L = Length of crest (cm)
H = Head over the weir (cm).
Contracted rectangular weir (end contraction at both sides):
Q = 0.0184 × (L – 0.2 × H) × H3/2
Cipoletti weir:
It is a contracted trapezoidal weir in which each side of the notch has a slope of 1 horizontal to 4 vertical. It is named after its inventor Cesare Cipoletti. It has sharp crest and sharp sides, beveled from the downstream side only. It commonly used to measure medium discharges. The main advantage of trapezoidal notch is that as the flow passes over the notch, the end contraction either get eliminated or considerably reduced.
Discharge is computed with the formula:
Q = 0.0186 × L × H3/2
V-notch weir:
The 90° V-notch weir is commonly used to measure small and medium streams up to 1.25 cumecs. It would be better if this notch is used when the head is more than 6 cm. Table 9.3 presents the values of discharge through under the operating heads commonly available on the farms. The advantage is its ability to measure small streams accurately.
The discharge is calculated with the formula:
Q = 0.0138 × H5/2
Limitations in the Use of Weirs:
Weirs are easy to construct and convenient to use.
There are certain limitations for their use:
1. They are not accurate in the absence of proper conditions for their measurement
2. They require considerable head loss, often not available in channels on flat grades
3. They are not easily combined with turnout structures
4. They are not ideal for water carrying silt, which deposits in the approach channel and destroys proper conditions for weir measurement.
ii. Flumes:
A flume is a structure, constructed across a stream; by which critical depth can be produced by changing a subcritical channel flow into a supercritical flow and vice versa. Such an arrangement, thus involves formation of a standing wave or hydraulic jump. This can be achieved either by raising the bottom of the channel or by fluming the width of the channel.
The throat of the section is made rectangular or trapezoidal. The floor of the throat is almost level, whereas the floor of expanding outlet is given a steep slope, sufficient to cause the water to leave the throat at supercritical velocity and that to ensure the presence of critical depth at some point in the throat.
Hence, a control meter, if properly designed, will be associated with the phenomenon of hydraulic jump. The discharge can be determined by applying the relationship between the discharge (Q) and specific energy of the critical depth. Thus, the principal of flume is based on the concept of specific energy and critical flow in open channel.
The commonly used flumes are shown in Fig 9.5:
Parshall flume:
Rate of flow can be determined by taking the depth of flow at two Ha and Hb. Discharge through flume (free or submerged) can be known from these readings. When the elevation of water surface near the downstream end of throat is not high enough to cause retardation of flow due to backwater, it is termed free flow.
Conditions of free flow can be determined by noting the ratio of Hb/Ha. Free flow limits of Hb/Ha vary with throat width. Under free flow conditions, measurement of water level or Ha in the converging section is sufficient to determine the rate of flow from standard tables. When the flow is submerged, Hb reading at lower end of throat section is necessary.
In such condition, a correction has to be applied for free flow discharge, depending upon the percentage of submergence for determining the rate of submerged flow.
Free-flow discharge values are presented in Table 9.4:
Cutthroat flume:
It has flat bottom and vertical walls with a rectangular cross-section. As the flume has no throat section, it is named as cutthroat flume. It can operate either as a free or submerged flow structure. Under free flow conditions, critical depth occurs in the vicinity of minimum depth, w, which is called the flume throat or the flume neck.
The attainment of critical depth makes it possible to determine the flow rate, knowing only an upstream depth, ha. This is possible because whenever critical depth occurs in the flume the upper stream depth, ha, is not affected by the changes in the downstream depth. For free flow, the ratio of the inlet flow depth ha to flume length should be preferably less than 0.4. Rating curves can be used for determining flow rates.
The relationship between flow rate Q and upstream depth of flow ha in a cutthroat flume under free flow conditions is given by the relationship:
Q – C1 × han1
where, Q = Flow rate
C1 = Free flow coefficient, which is the value of Q when ha is 1.0 foot, which is the slope of the free flow rating curve when plotted on logarithmic paper
n1 = Exponent, whose value depends only on the flume length L.
H-flumes:
They are better suited for runoff measurements and where sediment sampling of runoff is done using automatic still samplers. They are used for flows ranging from 0.009 to 0.85 m3 s-1. For smaller or greater flows, the dimensions of H flumes are modified and are known as HS flumes for smaller flows and HL flumes for larger flows. Rating tables are used for measuring the discharges.
Trapezoidal flumes:
They are developed at Washington State College, also referred to as WSC flumes are available in three sizes and the smallest one is ideal to measurement of furrow streams. The flume consists of an entrance section upstream, a converging section leading to throat and a diverging section downstream. Rating curves are available to determine the discharge.
These flumes have several advantages:
1. Simplicity of construction and low cost
2. Large range of flows can be measured with a small change in head
3. Sediment deposits does not change the head-discharge relationships appreciably
4. These flumes will operate under greater submergence than rectangular shaped ones without corrections for submergence
6. Ease of installation in common channel sections
7. Very low head losses.
Merits and Drawbacks of Flumes:
Merits:
1. Flumes are preferred to weirs for large flows and for silt-laiden situations
2. Debris damage a sharp-crested weir, while there is no such damage in flumes
Drawbacks:
1. The flumes should be used only when the head is more than 6 cm
2. Minimum width of flume should be 9 cm.
Orifices:
Orifice is an opening with closed perimeter and of a regular shape through which water flows. Generally, rectangular or circular orifices are constructed, which may be of three types (Fig 9.6).
1. Orifice having inlet at higher level than the outlet
2. Orifice having inlet at lower level than the outlet
3. Submerged orifice.
If the stream of water coming out of orifice discharge into air, the orifice is said to have free flow and if the discharge is under water, it is called submerged orifice. The depth of water producing discharge is called the head.
Free flow orifice:
It can be used to measure relatively small streams like the flow into border strips, furrows or check basins.
The discharge is calculated by the formula:
where, Q = Discharge through orifice (lps)
a = Cross-sectional area of orifice (cm2)
g = Acceleration due to gravity (981 cm s2)
H = Head of the water causing the flow (cm).
iii. Submerged Orifice:
Submerged orifices may be divided into two types: those having orifices of fixed dimensions and those in which the height of opening may be varied. A standard submerged orifice has fixed dimensions. The opening is sharp edged and usually rectangular, with the width being 2 to 6 times the height. Discharge through a standard orifice may be obtained using equation given under cutthroat flume.
Submerged orifice 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 the case of weirs. Submerged orifices have the disadvantage of collecting floating debris, sand and silt above the orifice, preventing accurate measurements.