Project Report on Irrigation Canals

A project report on Irrigation Canal. This report will help you to learn about:- 1. Meaning of Irrigation Canal 2. Design of Stable Irrigation Canals 3. Alignment 4. Full Supply Discharge 5. Longitudinal Section 6. Cross-Section.

Contents:

1. Project Report on the Meaning of Irrigation Canals
2. Project Report on the Design of Stable Irrigation Canals
3. Project Report on the Alignment of Irrigation Canals
4. Project Report on the Full Supply Discharge of Irrigation Canals
5. Project Report on the Longitudinal Section of Irrigation Canals
6. Project Report on the Cross-Section of Irrigation Canals

Project Report # 1. Meaning of Irrigation Canal:

An irrigation canal carries water from its source to agricultural fields. Canals used for transport of goods are known as navigation canals. Power canals are used to carry water for generation of hydroelectricity.

A feeder canal feeds one or more canals. A link canal links the two canals so that, if required, water of one canal can be diverted to the other canal through the link canal. A given canal can serve more than one function.

Based on the nature of source of supply, a canal can be either a permanent or an inundation canal. A permanent canal has a continuous source of water supply. Such canals are also called perennial canals.

An inundation canal (or non-perennial canal) draws its supply from a river only during the high stages of the river. Such canals do not have any head-works for diversion of river water to the canal, but are provided with a canal head regulator. An irrigation canal system consists of canals of different sizes and capacities.

Accordingly, the irrigation canals are further classified as:

(i) Main canal,

(ii) Branch canal,

(iii) Major distributary,

(iv) Minor distributary, and

(v) Watercourse (or field canal).

The main canal takes its supplies directly from the river through the head regulator and acts as a feeder canal supplying water to branch canals and major distributaries. Usually, direct irrigation is not carried out from the main canal.

Branch canal (also called ‘branches’) takes its supplies from the main canal. Branch canals generally carry a discharge higher than 5 m³/s and act as feeder canals for major and minor distributaries. Large branches are rarely used for direct irrigation. However, outlets are provided on smaller branches for direct irrigation.

Major distributaries (also called distributaries or rajbaha) carry 0.25 to 5 m³/s of discharge. These distributaries take their supplies generally from the branch canal and sometimes from the main canal. The major distributaries feed either watercourses through outlets or minor distributaries.

Minor distributaries (also called ‘minors’) are small canals which carry a discharge less than 0.25 m³/s, and feed the watercourses for irrigation. They generally take their supplies from major distributaries or branch canals and rarely from the main canals. A watercourse is a small canal which takes its supplies from an irrigation channel (generally distributaries) through an outlet, and carries water to different parts of the area to be irrigated through the outlet.

Project Report # 2. Design of Stable Irrigation Canals:

Irrigation canals generally have alluvial boundaries and carry sediment-laden water. A hydraulic engineer is concerned with the design, construction, operation, maintenance and improvement of irrigation canals. Irrigation canals should be stable over a period of their life span.

According to Lane, a stable channel (or canal) is an unlined earth channel:

(a) Which carries water,

(b) The bed and banks of which are not scoured objectionably by the flowing water, and

(c) In which objectionable deposits of sediments do not occur. Sediment is a loose non-cohesive material through which a river or channel flows. In other words, sediment is the fragmental material transported by, suspended in, or deposited by water or accumulated in the bed of a river.

This means that silting and scouring in a stable channel should balance each other over a reasonable period so that the bed and banks of the channel remain unaltered. The cross-section of a stable alluvial channel would depend on the flow rate, sediment transport rate and the sediment size. Regime methods are commonly used for the design of alluvial channels carrying sediment-laden water.

(i) Regime Methods:

Regime methods for the design of stable channels were first developed by the British engineers working for canal irrigation in India in the 19th century. At that time, the problem of sediment deposition was one of the major problems of channel design in India.

In order to find a solution for this problem, some of the British engineers studied the behaviour of such stretches of the existing canals where the bed was in a state of stable equilibrium. These stable reaches had not required any sediment clearance for several years of the canal opera­tion. Such channels were called regime channels.

These channels generally carried a sediment load, usually measured in terms of concentration defined as the ratio of weight (or volume) of solids and weight (or volume) of sediment-water mixture expressed either in percentage or parts per million (ppm), smaller than 500 ppm. Suitable relationships for the velocity of flow in regime channels were evolved.

These relationships are now known as regime equations which find acceptance in other parts of the world as well. The regime relations do not account for the sediment load and, hence, should be considered valid when the sediment load is not large. Most commonly used regime methods are Kennedy’s method and Lacey’s method.

(ii) Kennedy’s Method:

Kennedy collected data from 22 channels of the Upper Bari Doab canal system in Punjab. His observations on this canal system led him to conclude that the sediment supporting power of a channel is proportional to its width (and not wetted perimeter).

On plotting the observed data, Kennedy obtained the relation, known as Kennedy’s equation,

U₀ = 0.55 h^0.64 (5.1)

Kennedy termed U₀ as the critical velocity (in m/s) (defined as the mean velocity which will not allow scour or silting) in a channel having depth of flow equal to h in metres. This critical velocity should be distinguished from the critical velocity of flow in open channels corresponding to Froude number equal to unity.

Equation 5.1 is obviously applicable to such channels which have the same type of sediment as was presented in the Upper Bari Doab canal system. On recognising the effect of the sediment size on the critical velocity, Kennedy modified Eq. 5.1 to

U = 0.55 m h^0.64 (5.2)

in which m is the critical velocity ratio and is equal to U/U₀. Here, the velocity U is the critical velocity for all sizes of sediment, whereas U_o is the critical velocity for Upper Bari Doab sediment only. This means that the value of m is unity for sediment of the size of Upper Bari Doab sediment. For sediment coarser than Upper Bari Doab sediment, m is greater than 1, while for sediment finer than Upper Bari Doab sediment, m is less than 1.

Kennedy did not try to establish tiny other relationship for the slope of regime channels in terms of either the critical velocity or the depth of flow. He suggested the use of the Kutter’s equation along with the Manning’s roughness coefficient. The final results do not differ much if one uses the Manning’s equation instead of the Kutter’s.

U = 0.55 mh^0’64 (5.2)

Q = AU (5.3)

Thus, the equations enable one to determine the unknowns B (i.e. bed width), h and U for given discharge Q, Manning’s roughness coefficient n and m if the longitudinal slope S is specified. In Eqs. 5.3 and 5.4, A and R are, respectively, area of flow cross-section and hydraulic radius (i.e. A/P where P is the wetted perimeter).

The longitudinal slope S is decided mainly on the basis of ground considera­tions. Such considerations limit the range of slope. However, within this range of slope, one can obtain different combinations of B and h satisfying Eqs. 5.2 to 5.4.

The resulting channel sections can vary from very narrow to very wide. While all these channel sections would be able to carry the given discharge, not all of them would behave satisfactorily. Table 5.1 gives values of recom­mended width-depth ratio, i.e., B/h for stable channels.

Several investigations carried out on similar lines indicated that the constant C’ and the exponent x in Kennedy’s equation, U = C’. mh^x are different for different canal systems. Table 5.2 gives the values of C’ and x in Kennedy’s equation for some regions.

The design procedure based on Kennedy’s theory involves trial. For known Q, n, m and S, assume a trial value of h and obtain the critical velocity U from Kennedy’s equation, Eq. 5.2. From the continuity equation, Eq. 5.3, calculate area of cross-section A and, hence, the value of B for the assumed value of h and known channel geometer trapezoidal section with side slope of 0.5H:1V), is known.

Using these values of B and h, compute the mean velocity from the Manning’s equation, Eq. 5.4. If this value of mean velocity matches with the value of the critical velocity obtained earlier, the assumed value of h and the computed value of B provide channel dimensions. If the two velocities do not match, assume another value of h and repeat the calculations.

One can, alternatively, use Garrett’s diagrams (Fig. 5.1), which are sort of nomograms. For this purpose draw a vertical line through the intersection of the relevant discharge curve and horizontal line representing the desired value of S. The vertical line would intersect several bed width curves.

A horizontal line drawn through each of such points of intersection would enable determination of the depth of flow h and critical velocity U₀ for the chosen width B corresponding to the selected point of intersection.

Using the Manning’s equation, Eq. 5.4, the velocity of flow U is estimated and U/U₀ (= m) is thus determined. If this value of m matches with the given value of m, the selected value of B and the corresponding depth of flow h are accept­able. Otherwise, repeat the procedure after selecting another point of intersec­tion of the vertical line and the bed width curves.

It should be noted that the Garrett’s diagrams have been drawn for Manning’s n = 0.0225. In case the value of Manning’s n is different than 0.0225, then the vertical line (drawn through the intersection of the discharge curve and the slope-line) will be shifted towards either left (if n < 0.0225) or right (if n > 0.0225) by a small distance which is obtained from the small nomogram shown at the top of the Garrett’s diagram.

This distance is equal to the distance between the point marked with an arrow (and which corresponds to n = 0.0225) on the nomogram and the point on the same nomogram where the value of n equals the given value. Remaining procedure of determining B and h remains the same as explained earlier for n = 0.0225.

Example 5.1:

Design a channel carrying a discharge of 30 m³/s with critical velocity ratio and the Manning’s n equal to 1.0 and 0.0225, respectively. Assume that the bed slope is equal to 1 in 5000.

Solution:

Since the velocities obtained from the Kennedy’s equation and the Manning’s equation are appreciably different, assume h = 2.25 m and repeat the above steps.

Since the two values of the velocities are matching, the depth of flow can be taken as equal to 2.25 m and the width of trapezoidal channel = 13.31 m.

(iii) Lacey’s Method:

Lacey stated that the width, depth and slope of a regime channel to carry a given water discharge loaded with a given sediment discharge are all fixed by nature.

According to him, the fundamental requirements for a channel to be in regime are as follows:

(i) The channel flows uniformly in an incoherent alluvium. Incoherent alluvium is the loose granular material which can scour or deposit with the same ease. The material may range from very fine sand to gravel, pebbles and boulders of small size.

(ii) The characteristics and the discharge of the sediment are constant.

(iii) The water discharge in the channel is constant.

The perfect ‘regime’ conditions rarely exist. The channels which have lateral restraint (because of rigid banks) or imposed slope are not considered as regime channels.

For example, an artificial channel, excavated with width and longitudinal slope smaller than the required, will tend to widen its width and steepen its slope if the banks and bed are of incoherent alluvium and non-rigid. In case of rigid banks, the width is not widened but the slope becomes steeper. Lacey termed this regime as initial regime.

A channel in initial regime is narrower than what it would have been if the banks were not rigid. This channel has attained working stability. If the continued flow of water overcomes the resistance to bank erosion so that the channel now has freedom to adjust its perimeter, slope and depth in accordance with the discharge, the channel is likely to attain what Lacey termed as the final regime.

The river bed material may not be active at low stages of the river, particularly if the bed is composed of coarse sand and boulders. However, at higher stages, the bed material becomes active, i.e., it starts moving. As such, it is only during the high stages that the river may achieve regime conditions. This fact is utilised in solving problems related to floods in river channels.

Based on the analysis of data Lacey finally gave the following regime relations which can be used to obtain channel dimensions, channel slope S and the flow velocity U for given discharge Q and sediment size d in an alluvial channel carrying sediment-laden water:

Here,

F₁ = the silt factor,

d = the sediment size in mm, and

q = the discharge per unit width of channel.

Equations 5.5 to 5.12 are all dimensional equations in which units of metre and second have been used for length and time dimensions except in case of d which is in mm.

Lacey’s regime equation, Eq. 5.5 is of considerable use in estimating mean velocity of flood flow and hence, flood discharge in an alluvial stream. Equation 5.6 is very useful in fixing clear waterways for structures such as bridges and weirs.

Equation 5.7 relates the discharge per unit width of a regime channel with the total discharge flowing in the channel. For wide channels, the hydraulic radius is almost equal to the depth of flow h. Hence, Eq. 5.9 reduces to which can be utilised to estimate the depth of flow in a river during flood.

Equation 5.13, therefore, gives the depth of scour below high flood level. This helps in estimating the levels of foundations, vertical cutoffs and lengths of launching aprons of a structure constructed across a river. Compared to Kennedy’s equations, Lacey’s equations yield explicit solution for the design of stable channels.

Nevertheless, Lacey’s regime diagrams, Fig. 5.2, are also available to obtain the design perameters graphically. The use of these charts involves obtaining the intersection of the relevant fi curve with the relevant discharge curve and then read the values of the bed width B and depth of flow h from the abscissa and ordinate. Figure 5.2 also enables determination of the slope S for known Q and f.

The regime equations are purely empirical relations lacking any sound theoretical basis. These equations consider only sediment size to be important and do not take into account the sediment load. Neverthe­less, the regime equations are generally used on the plea that structures designed using regime concepts have stood the test of time and worked satisfactorily.

It should be noted that the regime charts (i.e., Garrett’s and Lacey’s diagrams) were very useful in times when calculators and computers were not available. However, in the present times, one would prefer to compute the design parameters directly from Lacey’s equation using calculators rather than use the Lacey’s diagrams.

Similarly, one can prepare a computer programme for Kennedy’s method and obtain design parameters and thus avoid the cumbersome task of manual trial or making interpolation errors in reading the Garrett’s diagrams.

Example 5.2:

Design a stable channel for carrying a discharge of 30 m³/s using Lacey’s method assuming silt factor equal to 1.0.

Project Report # 3. Alignment of Irrigation Canal:

The layout and alignment of an irrigation canal should be such that it ensures equitable distribution of water with minimum expenses. In plains, it is ad­vantageous to align a canal on the watershed; see Fig. 5.3.

Watershed is the dividing line between the parts of two catchment areas (from where rain water flows into a drain or stream) of two adjacent streams and is obtained by joining the points of highest elevation on successive cross-sections taken between the two streams or drains. Such an alignment will

(i) Ensure gravity flow irrigation on either side of the channel, and also

(ii) Minimize the expenses on cross-drainage structures.

Since the main canal takes off its water from a river (which is at the lowest point in the cross-section), the main canal necessarily crosses some streams before it mounts the watershed.

Once the canal mounts the watershed, it is always kept aligned along the watershed as far as possible. Sometimes the canal has, however, to leave watershed in order to either keep its alignment straight (such as at R in Fig. 5.3) or bypass a township or for some such reasons.

In hilly areas, however, contour alignment (Fig. 5.4) is followed. After taking off from a river in a valley, a channel follows contours maintaining its required slope. As river slopes are much steeper than the required channel bed slope, the channel encompasses more and more area between itself and the river.

Whenever curves or bends are unavoidable in canal alignment, it is advisable to provide curves of large radius of curvature to minimise the adverse effects of cross currents which develop in the reach of a curved channel. The radius of curvature may be taken as more than 60 times the width of channel for large channels, and more than 45 times the width of channel for smaller channels.

Project Report # 4. Full Supply Discharge of Irrigation Canal:

Part of canal water gets lost due to evaporation and seepage. These losses are termed conveyance loss and may be of the order of about 10 to 40% of the discharge at the head of the channel. Usually, these losses are calculated at the rate of about 3 m³/s per million sq m of the exposed water surface area.

The maximum discharge to be carried by an irrigation channel at its head to satisfy the irrigation requirements for its command area under the worst conditions during any part of a year is said to be the designed full supply discharge (or capacity) of the channel. The water level in the channel when it is carrying its full supply discharge is termed full supply level (FSL), and the corresponding depth of flow is called full supply depth.

Project Report # 5. Longitudinal Section of Irrigation Canal:

The full supply level of a canal aligned on the watershed need to be only about 10-30 cm higher than the adjacent ground level to ensure gravity flow irriga­tion in its command area. Too high an FSL would result in

(i) Uneconomical canal cross-section of higher banks,

(ii) Increased seepage loss (and, therefore, increased chances of waterlog­ging) due to higher head, and

(iii) Wasteful use of water by the farmers.

The FSL of an off-taking canal at its head should always be kept at least 15 cm lower than the water level of the parent (i.e., supply) channel so as to account for

(i) The loss in the regulator;

(ii) The possibility of the off-taking canal bed getting silted up in its head reaches, and

(iii) The possibility of increase in the withdrawal discharge sometime in future.

If the designed slope of a canal is steeper than the available ground slope, the canal is laid at a slope equal to the ground slope, and adequate measures are taken at the head regulator to prevent or minimise the entry of coarse sediment into the canal so that there would not occur any silting problem in the canal due to its flatter slope.

On the other hand, if the designed slope of a canal is flatter than the available ground slope, the canal is laid at the designed slope. Such a canal would soon have its entire section above the ground surface and its FSL would also be at too high level above the ground. There­fore, vertical fails (or drops) are provided at intervals in the canal. For economic reasons, a fall can be combined with a regulator or a bridge.

Such falls can also be used for generation of power. One may provide either a larger number of smaller falls (i.e., falls with smaller drop in the channel bed elevation) or smaller number of larger falls.

Relative economy of these alter­natives will decide the location of falls which also depends on other considera­tions such as balanced earthwork and existence of another off-taking channel. If the off-taking channel is located immediately upstream of a fall, it may not be able to withdraw its designed supply on account of lowered water surface due to drawdown effects of the fall.

Payment of earthwork for canal construction is made on the basis of either the excavation or the filling in embankments, whichever is more. Therefore, it will be economical to keep excavation and filling reasonably balanced which requires that the amount of the excavated soil is fully utilized in fillings.

If the amount of earth required for filling exceeds the amount of the excavated earth, this excess requirement of filling is met by digging earth material from suitable places known as borrow pits. These borrow pits can be made in the bed of the channel; Fig. 5.5.

Such borrow pits will get silted up in due course of time when the canal carries sediment-laden water. If still more material is need for filling, the borrow pits can be located about 5 to 10 m away from the toe of the canal bank. These should not be deeper than 0.3 m, and should always be connected to a drain. Surplus excavated earth material can be utilized

i) To widen or raise canal banks, and

ii) Fill up local depressions in the vicinity. Remaining surplus excavated earth has to be deposited in spoil banks (Fig. 5.5) on one or both sides of the canal. The spoil banks should be discontinuous to allow cross drainage between them.

Freeboard:

Freeboard is the vertical distance from the full supply level to the top of the canal bank. It provides the margin of safety against overtopping of the banks due to sudden rise in the water surface of the canal on account of

(i) Improper operation of gates at the head regulator,

(ii) Accidents in operations,

(iii) Wave action,

(iv) Land-slides, and

(v) Inflow during heavy rainfall.

Freeboard in unlined canals may vary from 0.3 m in small canals to about 2 m in large canals. Freeboard can also be estimated by adding 0.3 m to one quarter of the flow depth in metres.

Project Report # 6. Cross-Section of Irrigation Canal:

When the full supply level of an irrigation canal is lower than the surrounding ground level (Fig. 5.6 (a)), the canal is said to be in cutting. If the canal bed is at or higher than the surrounding ground level the canal is in filling. When the surrounding ground level is in between the full supply level and the bed level of the canal (Fig. 5.6 (c)), the canal is partly in cutting and partly in filling and such a canal, usually, results in balanced earthwork.

For the purpose of proper inspection and maintenance of irrigation canals, service roads are provided on one or both sides of the canal. These service roads can be part of the bank embankment as shown in Fig. 5.6.

These service roads are about 6 m wide and may or may not be metalled. If these service roads are likely to meet the communication needs of the local people, they will be much wider and also metalled. Between the canal road and the canal, a ‘dowla’ of height 0.5 m and top width 0.5 m is also provided.

To prevent sloughing of inner surface and canal banks, a narrow horizon­tal strip is provided on the inner sloping surface of the bank. This strip is known as berm. For stability of banks, side slopes of an unlined channel should, obviously, be flatter than the angle of repose of the saturated bank soil.

Generally, it varies from 1:1 to about 2(H): 1(V). Further, the bank dimensions should be such that there is a minimum cover of 0.5 m above the saturation line which, for small embankments, may be approximated by a straight line drawn at a slope ranging from 4H: 1V (for relatively impermeable material) to 10H: 1V (for porous sand and gravel) from the point where full supply level meets the bank.

Cover over saturation line can be increased by providing a horizontal strip similar to berm on the outer slope of bank in which case this strips is known as counter berm (Fig. 5.6(b)). It also helps in collecting rain water and disposing of the same without letting rainwater make a continuous gully on the outer bank.

The calculations for the design of an irrigation canal are carried out in a tabular form and the table of these computations is referred to as the ‘schedule of area statistics and channel dimensions’. The computations start from the tail end and the design is usually carried out at every kilometer of the channel downstream of the head of the channel.