Measurements of Water Level: Methods, Velocity ,Discharge and Instruments | Fluid Mechanics

Measurements of Water Level: Methods, Velocity ,Discharge and Instruments . In this article we will discuss about the various devices and instruments used in the measurements of water levels, velocity and discharge.

Water Level Measurements:

The gauges used for water level measurements may be classified into:

(i) Non-self-registering gauges, and

(ii) Self-registering gauges.

The non-self-registering gauges need an observer to attend and note the levels of water surface and the times of reading. The self-registering gauges are automatic and they produce a continuous record.

(i) Non-Self-Registering Gauges:

The most common among the non-self-registering guages are:

(a) The Staff Guage:

The staff gauge consists of a graduated board about 150 mm wide firmly fixed in a vertical position. The guage must have a length sufficient to cover the highest and the lowest water levels. The graduations should be bold so that they can be read from a distance. Piers and abutments are excellent supports for the staff gauge. In setting any gauge, the zero division of the gauge may be fixed at a pre-determined level.

(b) The Weight Gauge:

This type of guage is suitable when the staff guage is not found convenient due to inaccessibility for reading. In its simplest form the weight gauge consists of a weight attached to a graduated chain, or tape or wire. The observations are made by lowering the weight so that it touches the water surface, the reading being taken against a fixed mark of known elevation. Alternatively the gauge may take the form shown in Fig. 15.1.

In this gauge, the chain supporting the weight is passed over a pulley and along a graduated board. The chain can be hooked to the board when not in use. The chain is provided with an index. Readings are taken by unhooking the chain, lowering the weight till it touches the water surface and noting the graduation reading corresponding to the index.

The chain must be sufficiently long so that readings can be taken when the water is at the lowest level also. The graduated board need not be inconveniently long as a second or even a third index may be provided to the chain at intervals of say 2.50 metres.

(c) The Float Gauge:

Many types of float gauges are in use two of which are described below:

i. Float Gauge with Pulley:

This gauge consists of a float connected to a wire passed over a pulley. A pointer is attached to the other end of the wire. The pointer directs to a graduated vertical scale. See Fig. 15.3.

ii. Box Gauge:

This gauge is enclosed in a long wooden box about 300 mm square. The bottom of the box is provided with a few holes. The float carries a vertical rod. Either the rod itself is graduated, so that readings are taken against an index fixed in position; or the rod may carry a pointer which can move against a fixed scale. In the former arrangement the readings increase downwards; while in the latter arrangement the readings decrease downwards.

(d) The Hook and Point Gauges:

Fig. 15.5 shows the typical hook and point gauges. These consist of a graduated bar attached to a hook or pointer. The bar can slide up or down a fixed mounting. When a measurement is to be taken the sharp point of the hook or pointer is brought very close to the water surface.

The sliding bar is now clamped and by using a slow motion screw the hook or the pointer is moved slowly. This is continued till the sharp point of the hook or pointer just touches the water surface. Verniers are also provided for accurate measurements. These gauges are generally used in hydraulic laboratories for precise measurements of heads of water over notches, weirs etc.

(ii) Self-Registering Gauges:

There are many types of self-registering gauges in use. The essential components of all these types are similar. A float protected from wind or wave action is attached to a wire which is wound round a wheel. The wire is maintained at constant tension by means of a counter weight or spring.

The vertical displacement of the float transferred through the wheel is reduced in scale by gearing and is finally transmitted to a pencil which traces a curve on a uniformly moving sheet of paper. In most of the types, the paper is mounted upon a drum which is revolved once in 24 hours by clock work.

Self-registering gauges are more advantageous than the non-self-registering gauges. The readings of the self-registering gauges are free from personal errors. These gauges are particularly useful in obtaining gauge readings during floods. The readings are automatically recorded.

Velocity Measurements:

Determination of the direction and velocity of tidal currents are made in connection with coast protection projects and harbours. A very satisfactory method of determining the direction and velocity of a tidal stream over a range, is by immersing floats, which drift along with the current and are observed from time to time.

A float for tidal current observation must satisfy the following requirements:

(i) The float should be carried along by sub-surface current only and so it should present very little of its surface to the action of surface waves and wind.

(ii) The float shall be such that it can be easily identified from a distance. Fig. 15.6 shows a wooden float about a metre in length whose bottom part is weighted by lead shots.

Another type of float is the double float. See Fig. 15.7. The double float consists of a surface float from which a perforated cylinder is suspended. Usually the float and the cylinder are made of sheet iron. The length of wire or chain connecting the two bodies is made adjustable so that the direction and velocity of the water current at any depth can be determined.

Instead of the cylinder shown in the figure, a pail may also be substituted. The pail may be loaded with heavy material like stones until the float is nearly submerged. The flags provided over the floats must be of as small a size as possible. These flags may be of different colours so that they can be easily distinguished.

Float velocities are measured by releasing the floats at the appropriate point upstream and then timing them along the measured distance to a second station downstream.

Hot Wire Anemometer:

This is a very sensitive instrument used to measure the velocity at any point in a moving stream at any instant. This device has a small sensing element (which is a short thin wire of tungsten or platinum) placed at the location where the velocity is to be measured. This sensing element is connected to an electronic circuit.

The electrical resistance of the wire is related to its temperature which depends upon the heat transfer to the surrounding fluid. The rate of heat transfer increases as the velocity of flow past the wire increases.

In the usual type of the instrument, the short wire is maintained at a constant temperature by suitably varying the voltage by which the current through the wire is changed. If the wire tends to cool due to an increase in velocity, there is a balancing device which creates the necessary increase in voltage, thus increasing the current through the wire and heating it suitably to maintain the constant temperature. Then, the voltage becomes a measure of the instantaneous velocity of the fluid at the desired point.

Discharge Measurements:

It becomes necessary to measure the discharge of a stream in connection with the design of water supply, irrigation and power schemes. Since the flow rate in a stream fluctuates from day to day, an elaborate investigation has to be made extending over a considerable period. Such investigations should include the measurements of the maximum and minimum rate of flow and their duration.

The methods of gauging the discharge of a stream at a place may be classified as follows:

(i) By measuring the cross-sectional area of the stream at the place and determining the average velocity of flow.

(ii) By providing a gauging weir across the stream and measuring the head of water.

(iii) By chemical method.

Measurement of Discharge by Area-Velocity Method:

Fig. 15.8 shows the section of a stream. The velocities are different at different points on the section. The velocity varies from point to point depending on a number of factors like the shape of the section, the roughness of the bed of the stream and the depth of flow. Fig. 15.8 shows a typical velocity distribution. It shows curves of equal velocity. The maximum velocity occurs away from the banks and slightly below the surface.

Fig. 15.9 shows a typical velocity distribution along a vertical line. By studying a number of streams it is found that the curve is somewhat a parabola, the axis of which corresponds to the stream line of maximum velocity. The stream line of maximum velocity is situated between the surface and 0.3 of the depth at the vertical, and approaches the water surface for increase in depths.

The mean velocity in a vertical is 0.7 to 0.95 of the surface velocity in the case of moderately smooth channels, the coefficient increasing with increase in depth and velocity. In the case of channels of rough beds or where obstruction due to weeds exists, the ratio is found to be less than 0.70. Generally, the velocity at 0.6 of the depth is the mean velocity. The mean velocity may also be taken as the average of velocities at 0.2 and 0.8 of the depth.

The distribution of surface velocity or of the mean velocity in the verticals from one side to another depends on the shape of the stream section. In the practical cases we come across, the velocity distribution across a stream cannot be predicted satisfactorily.

Hence, for determining the mean velocity throughout the section by floats or current meters, it is necessary to divide the sectional area into a number of strips bounded by verticals and to gauge the mean velocity through each strip.

Floats:

Floats used in stream gauging may be classified into:

(a) Surface floats

(b) Double floats or subsurface floats and

(c) Rod floats.

(a) Surface Floats:

The most commonly adopted floats are the surface floats. These floats should not be used in wind. They are mostly found suitable for approximate determinations and for gauging the stream in high floods, where other methods are difficult to adopt.

In the cases where the route along which the float moves can be easily seen, it is sufficient to use a weighted corked bottle or a flat piece of wood so that the part of the float exposed to wind is minimum. In situations where such objects are not easy to observe, we may use a larger sealed vessel provided with a flag. Surface floats give the velocity of the surface water only. The surface velocity is 0.7 to 0.95 of the mean velocity.

(b) Double Floats:

Double floats are suitable in sea. When used in deep rivers, a thin wire is used in place of the chain in order to reduce resistance and to present minimum surface area to the action of the stream lines of different velocities from those at the depth of the float submerged. These floats are not suitable on small streams.

(c) Rod Floats:

A rod float consists of a wooden rod or a hollow tube weighted at the bottom in order to keep it vertical and so that only a small length of it is exposed above the water surface. The total length of the rod float is such that the clearance or gap between the lower end and the bed of the stream is as small as can be achieved without any fear of the lower and touching the bed. Where depths of flow vary appreciably, a number of rod floats of different lengths are required. A rod float is meant to directly indicate the mean velocity in the vertical.

According to Francis the mean velocity in the vertical, for a rectangular channel, is given by-

v_m = the mean velocity in the vertical of the rod

v_r = the velocity of the rod

c = the clearance between the bottom of the rod and the bed of the stream

d = the depth of the stream

The Current-Meter:

The current-meter is a very effective device for the measurement of velocity of flowing water. There are various forms of current-meters and the most commonly used ones are- (a) The Price current-meter and (b) Propeller type current-meter. All types of the instrument have common features. Any of these instruments, mainly consists of a spindle mounted on a fork and supporting a wheel with cup-shaped or helical vanes and rotated by the dynamic action of the flowing water.

(a) The Price Current-Meter:

The Price current meter is provided with a ring of conical cups or buckets fixed to a vertical spindle which is mounted in a frame. The instrument is suspended by a rope or wire and the weight of the mechanism is properly balanced by a two or four bladed tail, which is helpful in maintaining the instrument facing the current.

A lead weight is provided at the bottom to further stabilize the position of the instrument. Often this weight is made torpedo-shaped and provided with a rear blade. There is also provision to use additional weights when the instrument has to be used in swift currents. The speed of rotation is indicated by a battery operated bell or buzzer or any other signal. The electrical contact is closed at every revolution of the spindle.

Using a stop watch the observer can note the number of revolutions made in a definite interval of time. Thus the rotational speed of the spindle is determined. Corresponding to the speed of the spindle, the velocity of water can be determined by using the rating curve or the calibration chart for the meter.

(b) The Propeller Type Current-Meter:

The propeller type current-meter is provided with 2 or 3 bladed propellers parallel with the direction of flow of the stream. A worm and worm-wheel mechanism actuates a contact and thus transmits audible signals.

Method of Using the Current-Meter:

To use the current-meter for the measurement of discharge, a section of the stream is chosen governed by the following considerations:

(i) The stream should be regular in shape and should be straight up and downstream of the section.

(ii) The stream should be free from obstructions.

(iii) The flow should be as streamlined as possible, as eddy currents are likely to affect the meter.

A rope or wire carrying graduations (or tag marks) is pulled taut across the stream at the section. The rope or wire is divided into equal intercepts from 0.50 metre to 5 metres depending on the width of the river.

Corresponding to each of these points, the current-meter is lowered into the water and the velocity is measured say at 0.2, 0.4, 0.6 and 0.8 of the depth. It is seen that the velocity varies from zero at the bed to a maximum of 1.2 times the mean velocity at 0.2 depth. These velocities may be plotted for each vertical as shown in Fig. 15.9.

By plotting, we note that the mean velocity v_mean occurs at 0.6 depth. Thus, if due to time limitations only one velocity measurement at each vertical can be taken, then this measurement should be taken at 0.6 depth.

Now that we know the mean velocity for each vertical we can determine the mean velocities for the water passing through the trapezoids, into which the section is divided. Thus, by summation, we can determine the discharge from the relation.

Q = Σa.v_mean

Thrupp’s Ripple Method:

This is an approximate but rapid method of determining the surface velocity given by E.C. Thrupp. This method is based on the principle that when a small obstacle is placed in the surface of a stream, ripples are formed when the velocity exceeds 20 cm per sec and as the velocity increases the angle between the diverging lines of ripples becomes more acute. To afford a simple means of measuring the divergence rate, Thrupp used two 7.50 cm wire nails about 3 mm in diameter at a definite distance d apart. See Fig. 15.12.

According to Thrupp, if l is the distance from the base line between the nail and the point of intersection of the last ripples, the velocity of the stream at surface is given by –

V = 0.12 + 0.025l for d = 15 cm.

V = 0.12 + 0.034l for d = 10 cm.

where V = Velocity in metre per sec and l is taken in cm.

Measurement of Discharge by Gauging Weir:

A gauging weir is provided across the stream and the head of water over the weir is measured. The discharge over the weir can now be determined.

Discharge formulae for various weirs are given below:

The Standing Wave Flume:

This is a development of the venturi flume well suited for discharge measurement. Like the weir, a standing wave flume gives continuous recording of flow with only one float mechanism.

The additional advantages are:

(i) It provides little obstruction to floating bodies and hence the debris is not likely to affect the readings.

(ii) The loss of energy head is very small.

Discharge Measurement by Chemical Methods:

(i) The Salt-Velocity Method:

The electrical conductivity of water is increased due to salt in solution. At a fixed distance apart, in a stream two sets of electrodes are established, and are connected to a recording galvanometer which records the changes in electrical conductivity of the stream water with respect to time. In the normal conditions the graph obtained is practically a horizontal straight line.

Suppose a volume of salt solution is injected at upstream electrodes which are in the form of pipes. The graph obtained indicates a rectangular jump. Later on as the salt moves down to the second pair of electrodes, a second jump occurs. The time of transit is taken as the interval of time between the centres of area of the two jumps. Dividing the volume of water between the two stations by this interval of time, the discharge rate is determined.

(ii) The Salt Dilution Method:

At a convenient station a salt solution of known concentration is added to the stream at a constant rate. The subsequent dilution of the solution is determined by analysis. The water samples are taken at some distance below the entry point for thorough mixing to take place. There is no need for either area measurement or distance measurement.

The method is suitable for turbulent mountain streams. The weight of salt that passes in every second at the point where the samples are taken must be equal to sum of the weights of salt normally present and the salt added in solution.