Measurement of discharge in channels & Design of lined canal

MEASUREMENT OF DISCHARGE IN CHANNELS

&DESIGN OF LINNED CANALS

Submitted by:C014205, C014215,C014224, C014238.

Measurement of Discharge in Channels

Volume of water passing through a section of a canal, channel or river in a unit time is known as Discharge. It is worked out in cum/sec. or cumec.

Discharge is determined usually for the following objects:• For forecasting the normal and highest

supplied from the irrigation projects• For designing various hydraulic structures,

bridges, cross drainage works, regulators, head works, canals etc.

• For distributing the irrigation water properly among the farmers.

Selection of site for observations• The site should be selected on the straight

reach of the river.• At the site river should not be too wide or

shallow.• In hill streams the site should accessible in all

seasons.• The section line should at right angle to the

direction of flow.• The river should be too tight at the site,

otherwise during floods velocities would be excessively high and dangerous.

For taking discharge observations following points should kept in mind:• The site should be away from hydraulic

structures.• The site should have straight reach not less

than 10 times the width of canal.• At site channel should have silting or scour.• Channel should have stable uniform section at

the site.

Methods of discharge measurements

Following are the common methods used for determining the discharge of river, canal or channel:• Area velocity method • Weir method• Chemical method• Venturi flume method

Area velocity method

• In this method mean velocity of river or canal and cross-section area are determined. The discharge is worked out by multiplying the mean velocity with the cross-sectional area

• In practice as velocity is not uniform throughout the section due to various resistive forces. The cross-section of river or channel is divided into different parts and the area of each is part is determined. The mean velocity of each part is worked out.

Cross-section of a river

Suppose the channel is divided into 8 parts as shown in figure on previous slide.

Let A1, A2 …. A8, be the area of each compartment and V1, V2, V3….. V8 be the mean velocity of each compartment respectively.

Total discharge of river shall be determined by the following:

Q = [A1V1+A2V2+A3V3………+A8V8]In other words Q = ∑A.V

Measurement of Area

For the measurement of Area of each compartment, width and depth at various sections are required. The width and depth of various compartments are determined as in following slide

Width Measurement

For determining the width of open channel having width upto 150m, a wire rope is stretched across the channel and segments are marked on the wire rope by means of pendants. Precautions are taken in marking positions of pendants taking into account the sag correction.

But when the width of the channel is more than 150m, the segments are marked by means of pivot point method which is based on the principle of similar triangles properties.

If the width of river is more than 600m, than 2 pivot points are fixed one on each bank as shown in figure and the width is calculated accordingly.

Measurement of Depth

• By sounding rod: In this method a graduated wooden rod 5-8 cm in diameter, enamelled steel pipes of flat gauge is used. The graduation in these sounding rods are done in m and cm. The bottom of sounded rod is fitted with 10-15cm dia disc, to prevents sinking in bed. The depth is directly by lowering the sounding rod in the channel. It is used for determining depth of shallow channels. The depths measurements should be taken on downstream side.

• By lead line: For deeper channel of channel having high velocity, it is not possible to determine depth by sounding rod, so help of weighted lead line which is essentially consist of copper cores covered with hemp and weight fixed at one end is used. These lines do not shrink, stretch, remain free from knots. The lead weighed attached to it weighs 5-30kg depending on velocity of flow. For starting depth measurement, first a reference point is marked on section line below which depth of water is measured.

• By Echo sounder: This method is based on principle of electricity. Echo sounder transmits sound waves from surface of bed. When these waves reflect back from the bed they are arrested by the transmitter. The time of transmission and time of reception is determined by the echo sounder arrangement. The depth of channel is worked out from these records. This method is usually adopted by ship for determining the depth of channel. This method gives accurate results.

• By Haigh’s depth meter: This method is based on boyle’s law. It consist of coiled tube in cylinder container, figure on next slide shows the Haigh’s depth meter. In this cylinder the water is entrapped under pressure. The compressing volume of air in cylinder, the volume of water is proportional to the Depth upto which depth meter is immersed. The depth of river is worked out from the table along with Haig’s meter.

Haig’s Depth Meter Curve for determining depth

• By Kelvin tube: It consist of glass tube closed at one end open at another end. It is placed in a iron cylinder with holes. Heavy weight is fixed at the bottom of the tube due to which container rest at the bed, when the meter is lowered with wire rope. Depending upon the depth of water which enter in the tube, depth of channel is worked out. Using formula:

D = 10.33 L1/L2

Measurement of velocity

Velocity distribution curve

The velocity of flow can be determined by any of the following methods: • By surface floats• By velocity rods• By double or sub surface floats • By current meter

By Surface floats

• Surface floats are usually wooden disc 7.5 to 15 cm in dia weight and size of these float is designed in such a way that it is least effected by disturbing forces. Three wire ropes are stretch 15m apart across the channel over polls. The width of channel is divided into equal parts by hanging pendants. Floats are released in middle of each section and their timing are recorded.

By surface floats

The surface velocity of the current shall be calculated by the formula:

Velocity = Distance travelled by the float in mtime taken in sec

The velocity determined by the surface float is not mean velocity for obtaining mean velocity it is multiplied by coefficient whose value is 0.8.

By velocity rods

• Velocity rods are wooden or still tubes of 2.5 to 5 cm in dia the length of velocity rod is kept 0.94 D, where D is = depth of channel. Weights are provided at the bottom to maintain it vertical position with 2.5 cm above the water. For visibility flag is provided on top. The velocity rods are released in the same way as the floats, and time taken in travelling is noted and velocities are calculated. This method gives direct mean velocity of channel.

Velocity rod

By double or sub surface floats

• In this method two floats are employed. One is known as surface float and other is sub surface float. The weight of surface float is less than sub surface float

• Both the floats are connected with each other by means of cord. They are so adjusted that sub surface float remains at 0.2 D distance from bed of channel. These floats also give the mean velocity of channel.

Double or sub surface floats

By current meter

Current meter is an instrument used for determining the velocity of channel and the rivers it mainly consist of wheel, contact breaker, tail and the weights. A chart, known as rating chart, is supplied with the current meter, which give relation between the velocity of water and number of revolutions of the wheel per minute.

Component part of a current meter

Method of using current meter

Usually for using the current meter two methods are followed:

• One point method • Two point method

In first method the current meter is positioned at 0.6 D from the top and it gives the mean velocity. And in second method it is placed at two place i.e. at 0.8 D and 0.2 D the mean of these two velocities is the mean velocity of the channel.

The velocity of flow can be easily calculated by the following rating formula of the given current meter:

V = (a + b.N)where, V = velocity of flow in m/sec

N = Number of revolutions made by wheel per sec a and b are the constants, whose value are given by the manufacturer or calculated by the experiments.

Weir Method

When the discharge of an irregular or regular channel is moderate, sharp crested weir may be used for measuring it. Practically it has been observed that weirs with suppressed end contractions give fairly good results. In this method ceppoletti weir having trapezoidal section with 1 : 4 side slopes is used for determining the discharge, figure shows the section of a ceppoletti weir.

Cippoletti weir

In this the increase in the sectional area is equal to the area of contractions. Therefore, ceppoletti weir acts as a rectangular weir having suppressed end contractions.

The discharge is worked out from the Fancis’s formula:

Q=1.85L*H3/2

where Q =discharge in m3/secL = length of the notch in mH = Depth of water over notch still

in m

Venturi flume methodVenturi flume is a hydraulic structure constructed in the channel for measurement of discharge observation. Figure shows the venturi flume. It mainly consist short channel reach for restricting water way. This channel known throat. The in size for normal section to the throat and back to the normal section is done gradually.

To be continued on next page

From pre-slide:

Due to decrease in the water way at the throat the velocity and the discharge per unit increases, due to which the reduction in depth of the flow occurs at the throat. The discharge of the channel is obtained by taking observations of the reading on upstream and inside the throat.

Venturi flume

The principle is the same as of venturi meter, and the discharge of the channel is determined by formula (refer to figure).

Where Q =discharge of channela1 =waterway of the channela2 =waterway at throat sectionh1 =depth of water in upstream sideh2 =depth of water in venturi flume.

Design of Lined Canals •The lined canals are not designed making use of Lacey or Kennedy Theory because the section is rigid

•Generally Manning’s equation is used in design. To carry a certain discharge number of channel sections may be designed with different bed widths and side slopes.

•But it is clear that each section is not equally good for the purpose.

•The section to be adopted should be economical and at the same time it should be functionally efficient

•It has been found that the most suitable cross-section of a lined canal is a circular section with sloping sides. That is, the bed is not flat but it is an arc of a circle. This arc is tangential to the sloping sides

Side Slopes•The side slope is selected in such a way that it nearly equals the angle of repose of the soil in

the subgrade.

•Care is taken to ensure that no earth pressure is exerted on the back of the lining. From the

knowledge of hydraulics it is clear that the section is economical when cross- sectional area

is maximum for minimum wetted perimeter.

•This condition is achieved when the centre of an arc lies at FSL of the canal. This section is also efficient in the sense that as the velocity of flow is higher silt carrying is also higher than a wide and shallow section. Thus the problem of silting is completely eliminated and functioning is efficient

•It may be mentioned here that such section with circular bed may be designed up to a discharge of 85 m3/sec. When the discharge is more than 85 m3/sec the section best suited is one with a flat bed and sloping sides with rounded corners

•This section is certainly better than trapezoidal section, because it is more stable and economical to construct.

The canal sections of this type with two standard side slopes.When r = 3.6 m or less, side slopes may be taken 1: 1When r = 3.6 m side slopes may be taken 1.25: 1

Velocity of Flow:

Mean velocity of flow may be calculated using Manning’s formula.V = 1/N .R2/3. S1/2

where N is coefficient of rugosity and may be taken as 0.018

S is the slope of bed and expressed in fall of bed in m in 10,000 m length

For the given value of N formula may be reduced to

V = 0.556 R2/3. S1/2

where V is velocity of flow in m/sec;

R is hydraulic mean radius in m; and

S is bed slope expressed in metres/metre length,

since (10,000)1/2 is merged in the constant 0.556

Velocity of Flow:

Mean velocity of flow may be calculated from Manning’s formula for value of N = 0.018.

Then V = 0.556 R2/3. S1/2

It may be remembered that lined canal could be given steep slope to achieve recommended velocity of 2 m/sec and value of ‘f’ about 1.2. Critical velocity ratio is not applicable to lined canals. But to avoid possibility of sitting CVR

should be aimed at more than unity

Coefficient of Rugosity (N):

In general practice for lined canal average value of N may be taken as 0.018. For different types

of linings the value of W varies. The values given for straight channels in Indian Standard

4745 are given in Table 10.3. When the alignment is not straight loss of head increases and a small increase in the value of W may be

made to allow for additional loss of energy.

Freeboard:

Freeboard is measured from the (FSL) full supply level to the top of lining. For lined canals having less than 10 cumec discharge 0.6 m free board is recommended. For bigger lined canals

freeboard not less than 0.75 m is generally provided

Bank Widths:

The Indian Standard recommended following values for bank widths for main and branch

canals:

Main canal in cutting and filling = 8.0 m

Branch canal in cutting = 6.5 m

Branch canal in filling, left bank = 6.5 m

Right bank = 5.0 m

The canal sections shown have trapezoidal shape. For circular bottomed canal arrangement will be similar. It may be noted that angle 0 shown in the section varies with the side slopes adopted. 9 for side slope 1: 1 has 45° value and for 1.25: 1 is 38° 40 as shown in Fig. 10.3. ′Maximum height of the spoil bank is limited to 6 m than 10.5 m filling each section shall be designed and tested for stability. when the excavation is done by machinery. In case work is accomplished manually maximum height is to be restricted to 4 m only. Also when a canal section in filling involves more

Problem:

Design an irrigation lined canal to carry a discharge of 34 m3 / sec. The mean diameter of the average soil particles is 0.464 mm. Assume side slopes 1.25 : 1 and width zero.

• Suitable provision of dowel, roadway, catch water drain, under drainage has to be made for every lined canal. Figure 10.4 shows three typical cross-sections of the lined canal in which provision of various components is illustrated.

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