The design of open channels consists in designing the cross-section for the given discharge and the velocity of flow. The cross-section should be adequate and the velocity of flow should be permissible for the given material.
The different elements of an open channel as shown in Fig. 14.1 are:
T = Top width,
t = Top width of water surface when water is at depth d,
b = Bottom width,
θ = Angle between the sloping sides and the horizontal,
D = Total depth of the channel,
A = Area of cross-section, and
P = Wetted perimeter.
Velocity of Flow in Open Channels:
The average velocity of flow in open channels can be estimated by using formulae developed based upon a large number of observations.
Two commonly used formulae are described below –
The hydraulic radius has to be calculated depending upon the channel cross-section.
Discharge Capacity of the Channel:
Using the average velocity, the discharge capacity of a channel is calculated as –
The area of cross-section should be so adjusted that for a given discharge the velocity of flow should be within permissible limits in case of earthen channels. Table 14.1 presents the values of permissible velocities for various soil types.
The land slope may be altered using drop structures to reduce flow velocities. Higher velocities are allowed in case of lined channels.
Most Economical Channel Section:
The most economical channel cross-section is the one which gives the maximum rate of discharge with a given cost of excavation or the least amount of construction material used to obtain the given discharge. For a given channel cross-section, the discharge is maximum when the velocity is maximum.
Chezy’s equation shows that the velocity of flow is maximum if the hydraulic mean radius R is maximum, as the values of C and S are generally constant. The area remaining constant, R is maximum if the wetted perimeter P is minimum. The form of the channel which complies with this condition is the semicircular cross-section.
For conditions of maximum discharge the most economical section for different geometries of sections can be derived. An example for a rectangular section is as follows.
The discharge through the channel is given by using Chezy’s formula as –
For the discharge to be maximum, the base width should be determined by the above equation.
The following two more conditions can be derived for the trapezoidal channel to give maximum discharge.
These are:
(1) Hydraulic mean depth = half of depth, and
(2) Perpendiculars drawn from centre of top width on bottom and sloping sides are equal.
Similar expressions can be derived for other geometries such as triangular and circular sections.
Lining Materials:
Different materials like concrete, stone or brick masonry, natural clays of low permeability, baked clay, tiles, bituminous material, and rubber and plastic compounds are used for channel lining. Natural clay is used for lining with varying degree of efficiency, especially when lower initial cost is desirable, and if suitable material is available.
Clay linings can be applied by two methods.
These are:
(1) By placing a blanket of relatively impervious clay 15 to 30 cm thick, well cemented, over or within the permeable channel bed and sides, and
(2) By dispersing clay in the water and having it filter out and seal off the pores in the permeable bed.
Bentonite clay, is noted for its high swelling, and is extensively used for lining reservoirs. Its use for irrigation channels has not yet been perfected. Well mixed and properly made cement concrete lining, and also single-layer bricks or stones laid in cement or lime mortar provide virtually water-proof channel lining.
1. Concrete Lining:
This gives long service. Repair and maintenance costs are minimum. Concrete is used to a much greater extent for lining channels than any other material even though the high initial cost inhibits its extensive adoption at all places.
Concrete mixture usually recommended for lining is 1 : 3 : 4 (Cement: Sand: Gravel or broken stone), and thickness of lining 4 to 5 cm. The side slopes of the channel should not be steeper than 1:1.
The sides and bottom of the channel should be compacted at suitable moisture content. When concrete hardens, it shrinks, and cracking may result. Joints must be provided, at a distance not more than 2 m (6 ft) in order to localise and control the cracks.
There are two other easier methods for using concrete channel lining to convey smaller discharges – One of them is the use of concrete half-pipe, specially made for irrigation use by the commercial manufacturers. When the embankment is built and channel excavated, the pipe sections 1 m or more in length have simply to be laid in place and joints cemented. The work is quickly done but the disadvantage is the freight cost and risk of breakage of the pieces, if brought from a long distance.
A convenient form of lining is the prefabricated concrete channel sections. They can be fabricated by using appropriate molds. Narayan et al. (1990) used M.S. molds for fabricating trapezoidal concrete sections. After filling concrete in the molds, vibratory platform was used for obtaining setting of the concrete and consequently good strength. Manual fabrication of sections is also possible but the procedure will be slow and will not give good strength for the sections.
2. Bituminous Material Lining:
Application of mud plaster containing a smaller percentage of bituminous material to the sides and bottom of an irrigation channel will give a high degree of waterproofing with a small expenditure. Sticky soil is mixed with wheat straw or rice husk at the rate of 3 per cent of the weight of the soil to avoid cracking upon drying, and allowed to rot about 7 days in the presence of excess of water.
The puddle is thoroughly worked, for an hour or two before application of the plaster, a bituminous mixture is added to the puddle at the rate of 1 part for every 15 parts by weight of dry soil used for making the plaster.
The entire mixture is thoroughly mixed up with feet or spade. Before applying the lining, the channel should be cleaned of all weeds, and sides and bottom thoroughly compacted. To prevent growth of weeds in the channel, a suitable weedicide, e.g., Fernoxone mixed with 300 parts of water—is sprinkled in the channel three or four times. Plaster is applied to the wet surface of the channel with a trowel for a thickness of 1 cm.
During hot weather, occasional sprinkling of water on the plaster will minimise cracking. A second coat 3 mm thick is to be applied after 2 to 3 days. Cracks formed in the lining when drying can be filled up by the application of the same mixture.
After the plaster has dried for about a week, the channel is ready for use. Damages by rats or animals can be repaired with the same plaster. The lining is very effective for 2 to 3 years after application. However, it does not prevent weed growth later, and is liable to be damaged by men or animals trampling on it.
3. Soil Cement Lining:
This is made of a mixture of Portland cement and natural soil. For good results, the soil should be mixed well, laid and compacted. It should be cured for seven days with moist soil cover. Soil cement linings offer possibilities in sandy areas, where other suitable materials are not available.
4. Plastic Fabric Lining:
Several experiments have been done in the use of polythene and alkathene as lining material for reservoirs and channels. The sheet for channel lining is placed on the surface with an earth cover, and should be efficiently laid and secured on the sides. These linings are fairly cheap, weeds do not grow on them, but they are subjected to puncture and deterioration due to trampling by animals.
Structures in Conveyance System:
Structures in the irrigation conveyance system are needed to control the flow of water and also distribute the water.
There are a number of such structures used and the more common ones are outlined below:
1. Drop Structures:
These are used in the channel system for conveying water from a higher level to lower level. A series of drops in the channel act as control points and reduce the slope of the channel bed.
The drop structures are constructed either with brick masonry or with precast concrete panels. The structure has a head wall, side walls, stilling basin and end still. The stilling basin helps in the dissipation of the energy of the falling water.
The width of the stilling basin is kept equal to the width of the channel. The length of the stilling basin-should be such that the water jet from the head wall should fall within the stilling basin.
2. Chute Spillways:
Where the irrigation channel has to pass through steep slopes, chute spillways are used to convey water from a higher elevation to a lower elevation. On steep slopes, chutes are more economical than a series of drops. The chute spillway consists of an inlet, conduit and an outlet.
The structure is constructed of concrete, bricks laid in cement mortar or precast concrete sections. The stilling basin helps in the energy dissipation of falling water. A check gate at the inlet helps in controlling the water level in the upstream channel.
For small farm irrigation channels (up to about 0.15 cu. m per second) and up to about 2 m drop, the design of chute spillways is simple and can be based on empirical relations.
3. Structures at Channel Crossings:
When irrigation channels are to cross roads or drainage channels, suitable structures to convey water across them are necessary.
These structures may be classified as –
(i) Bridges,
(ii) Culverts and
(iii) Siphons.
While bridges are used in case of large irrigation canals, culverts and siphons are commonly used in farm irrigation systems. Culverts are used when the irrigation channel is at a lower elevation than the obstruction to be crossed, while siphons are to be used when the channel level is almost same or higher than the obstruction.
The design of these structures usually involves in determining the required pipe diameter and section dimensions, taking into consideration velocity limitations and head losses.
The design of siphon structures is based on the Bernoulli theorem. In case of siphon structures, the same principle is used and the required pipe diameter is selected by trial and error for a given set of conditions. Table 14.2 presents the capacity of siphon structures.
Water Diversion and Control:
Water from the farm irrigation channel is diverted to branch channels or into fields by means of junction boxes, gates or other form of outlets. Metallic gates are used in the junction boxes for diverting the water. The gates are also known as check gates. To insert the check gate in the channel a slot is provided in the channel and a rubber strip is embedded in the slot. The gate is inserted in the groove of this rubber strip. Thus the joint becomes watertight.
To divert the water into the field, check gates are used at intervals along the channel and outlets are provided. The outlets are also provided with check gates to let out water into the field. The rubber sealing strip provided gets deteriorated with the passage of time due to the heat in summer time.
Circular Orifice Panel:
The circular orifice nucca reported being used in Pakistan is a structure for water control. (Nucca is a term used for the gate which allows water to pass through the bank into branch water courses or into the field).
This structure consists of an inclined concrete panel with a circular opening of suitable diameter. Another circular concrete block exactly fits into this opening. The shape of the opening and the lid are such that the joint will be leakproof.
Application of little mud will help in controlling the leakage. Concrete casting of the panel with the opening and the lid is to be carefully done.
It is reported that the panel with the opening itself is used for casting the lid fitting into it. A little application of grease is helpful if the panel is used for casting the lid. The usual care for selecting the materials for the concrete is necessary. Vibrating platforms will produce better quality castings. The lid after the casting is subjected to grinding for getting fine finish and ultimately a watertight joint.
In order to release water from farm irrigation channels into other smaller channels or directly to the field, discharge control devices referred to as field turnouts are used. These could be a fixed opening in the side of the channel or one equipped with check boards or gates to adjust the opening area.
If only a part of the total flow is to be diverted through a given turnout, a more constant discharge is obtained by using an orifice type device rather than a weir type structure. One of the common turnouts is a concrete or metal pipe with a slide gate on the inlet. For unlined ditches the headwall and the slide-gate are usually vertical. The capacity of pipe turnouts can be determined using the equation-
Where, C is usually taken as 0.80, A is the area of cross-section and h is the head of flow. Prefabricated concrete turnouts are convenient to install. The structures need to be well maintained to control leakage which is often a problem in field turnouts.
Appliances for Water Application:
Several appliances are used for water application from the farm irrigation channels to the fields.
The important among them are:
(1) Portable check dams,
(2) Spiles, and
(3) Siphon tubes.
Portable check dams are made of canvas cloth or thin plastic sheets or sheet metal. Canvas or plastic dams are supported on a wooden or pipe cross bar with suitable loops (Fig. 14.10). A loop provide at the bottom helps in anchoring the dam to the channel bed.
In the canvas dam, sometime an opening is provided referred to as sleeve bypass to allow part of the flow to go to downstream side. The sheet metal dams are more durable than the canvas or plastic dams. The canvas or plastic dams can be conveniently used both in the lined or unlined irrigation channels. The sheet metal dams can be used in unlined channels only.
Spiles are short pipes of bamboo, concrete or baked clay having diameters ranging from 2.5 cm to 10 cm or more. They have no gates for water control. Their length is made slightly larger than the width of the field channel. Spiles are convenient in case of irrigation by furrows as one spite can be used for each furrow.
Siphon tubes are used for conveying water from the channel to the field for application. They are conveniently used for furrows also. Plastic, rubber or aluminium tubes are commonly used. Each siphon tube is about a meter long. To use the siphon tubes these are to be primed.
The siphon tube is dipped in water and completely filled. Keeping one end in the water, the other end is tightly closed with hand and is taken out and released at the outlet point. Bigger aluminium siphons use a small hand operated pump for priming.
The discharge of the siphons depends upon their diameter and the difference is elevation between the water surface at the upstream and downstreams ends of the tube. The discharge is given by –
The head causing the flow is estimated depending on the flow conditions at the outlet.
Example:
A siphon tube transmits water from one irrigation ditch to another. The tube has 50 mm diameter and 1.8 m long. The head loss coefficients are 0.8 at entrance, 0.4 at bend and 1.0 at the exit. Calculate the rate of flow through the siphon tube.
Solution:
f value is dependent on Reynolds number (Re), which in turn depends on velocity through the tube. Assuming f value, calculate v from the above equation, calculate Re with the v and check value of f with the Re using Moody’s diagram. The process is repeated until an acceptable value of f is obtained.
Lining of Field Channels in Large Irrigation Systems:
In large surface irrigation systems, the layout of the field channels beyond the outlet depend on the local conditions and is not uniform for all the outlets. Fig. 14.13 shows one such example.
The time for which water flows in these channels is also not same. Considering that the product of length and the time for which water flows in a particular length of the channel will be proportional to the seepage losses, Malhotra (1974) proposed a procedure for deciding the length of the field channel system to be lined.
Considering the layout of the field channels as in Fig. 14.13, a table is prepared as shown in Table 14.3 and a graph is plotted as in Fig. 14.14. It can be seen from the graph by lining a part of the system, seepage losses up to 90% could be controlled. When resources are not available for lining the entire system, this approach could be adopted.
