lined canals

Lined Canals: 

Lined Canals Definition : The impervious layer which protects the bed and sides of the canal is called lining. 1. This layer must be of sufficient strength in order to resist water pressure. 2. In canal system 50 % of water is lost during conveyance and on account of losses due to seepage, evaporation etc. 3. It is very essential to provide impervious layer on the bed and sides of the canal in order to minimize these losses. Advantages of lining: To save water for extension of irrigation To minimize percolation losses. To improve duty To minimize cost of maintenance To carry water at higher velocities To prevent growth of weeds To prevent canal breaches and maintenance.

Slide 2: 

SELECTING THE TYPE OF LINING Suitability of Lining : The following properties have to be considered in determining the suitability of lining: 1. Permeability : If the surface is impermeable, then seepage losses will be less and more water will be saved. 2. Durability: It is the property which provides resistance to various disintegrating forces such as weathering, chemical attack and wearing. 3. Structural suitability: Reinforcement Thickness of lining Strength of lining Earth backing 4. Coefficient of rugosity : It determines carrying capacity of canals. 5. Initial cost and subsequent maintenance 6. Permissible velocities

Types of canal linings: 

Types of canal linings The most commonly used types of lining are shown in Figure 40, and include: Cement or lime concrete lining Brick lining Cement mortar lining Shot Crete lining Stone masonry lining Sodium carbonate lining Precast concrete block lining Asphaltic lining The choice of lining material depends primarily on: • local costs; • availability of materials; and • availability of local skills (local craftsmen). If cement, gravel and sand are relatively cheap and locally available, concrete lining is generally a good choice. Although the initial investment in concrete lining is generally high, if it is properly constructed and maintained it could last for many years, which thus offsets the high initial cost.

Slide 4: 

If a local fired brick industry produces cheap bricks or if construction stone or precast concrete slabs are locally available, brick or stone masonry or a concrete slab can be considered. Large amounts of cement are required for mortar and plastering. The construction of this type of lining requires more labour than other methods, thus its use tends to be limited to where labour is abundant and the material cost is relatively low. If a sufficient volume of heavy clay is available near the irrigation scheme, a clay lining could be considered. Lining canals with clay is rather labour intensive, and so the costs of labour should be taken into account when comparing costs and benefits. The use of clay can reduce seepage losses and improve the smoothness of the canal surface, but does not stop weed growth and possible erosion. If coarse aggregates are not available and cement is relatively cheap, soil (sand) cement lining could be considered.

Slide 5: 

Types of canal linings &their methods of construction Concrete lining Concrete lining can be placed in many ways, including: hand placing by plastering on sides and bed using forms and pouring alternate panels and using prefabricated concrete elements When the concrete lining is hand placed, attention has to be paid to the concrete mix. The concrete must not be very fluid to avoid it creeping downward from the sides. On steep side slopes, formwork is necessary to hold the concrete in place until it sets. When the lining is placed using the alternate panel method, guide forms are used. Sections are poured alternately, with the finished sections being used as forms for the sections in between. Small openings or expansion joints spaced at intervals of 1.5 to 3 m are needed for the expansion and contraction of non-reinforced concrete. These joints are filled with flexible, asphaltic material to prevent water leakage. For small canals, prefabricated concrete elements can also be used , although the prefabricated elements in Figure 43 are provided with gates.

Slide 8: 

Concrete block, brick or stone masonry lining The concrete blocks, bricks or stones are laid flat on the compacted sides and bed of the trapezoidal canal. The joints are filled with cement mortar, which should have a cement- tosand ratio of 1:3 to 1:4 (one part of cement to 3-4 parts sand, by volume). A rectangular canal can be constructed with a concrete or masonry bed and vertical masonry walls. See Figures 44-A to 44-D. Figure 44-A shows the destruction of the old unlined canal bed. The foundation for the concrete block lining is in preparation. The block in the foreground will be used as a reference level. The blocks in Figure 44-C need to be plastered. Usually the water side of the masonry structure is plastered, particularly if the bricks are not of good quality.

Slide 10: 

Compacted clay or plastic lining One of the oldest methods for reducing seepage losses and improving canals is to remove the porous earth and replace it with clay material. The clay is moistened and placed in layers on the bed and sides of the canal. Each layer should be compacted. Canals can also be lined with plastic or asphalt. These materials can be covered with earth or gravel to protect them from weathering and mechanical damage. However weed growth and soil erosion could continue on such cover. See Figure 45.

Slide 11: 

Other types of linings Cement mortar lining :- In this type of lining it is very essential to have well graded sand. The sand should range from fine to coarse to meet the requirements of durability and appearance the amount of cement required is more , hence the cost is also more . the thickness of this type of lining may very from .5 to 4cm. Soil cement lining:- sometimes cement may fixed with water and locally available soil. The water , soil and cement is mixed to get a workable mixture on the sub grade it is compacted. Asphalt lining:- In this type a flexible membrane impregnated with tar or bitumen is spread over the sub grade. The thickness may vary from 3 to 4 mm. Sodium carbonate lining:- It consists of local salty soil which contains 10 % clay and 6 % sodium carbonate. This type of lining is not durable. This type of lining is restricted to small canals and water courses . The reason being this type of lining is not very durable .in ordinary cases it lasts for about 3 to 4 years.

Economics of canal lining: 

Economics of canal lining Lined canal costs 2 to 2.5 times as much as an unlined canal. It is very essential to make sure before lining the canal that the cost incurred on lining is recoverable during the life time of the canal. If the value of annual benefits exceeds the annual cost of the lining then construction of canal lining is considered to be good investment. Annual cost of lining includes: Annual interest charges Annual depreciation charges . P = First cost of lining per KM R = Rate of interest N= Estimated useful life of canal in years D= Annual depreciation = P / N I= Average annual interest charge = P . R /2 Average annual cost of lining per KM = D + I = P/N + P. R / 2 Average annual benefits derived by lining the canal include : Saving in seepage loss per Km (A) Saving in maintenance and operation cost (B) per KM Other additional benefits.(C) Total benefits = A + B + C

Slide 13: 

Maintenance of lined canals:- Causes of failures which are most common occurrence are weed growth , silting of canals ,weak banks, overflowing, canals breaches. In order to maintain the stability of irrigation canals the following methods should be adopted: 1. Controlling weed growth: Increasing velocity of flow Adopting rush rotation method Removing plants 2. Removing silt deposits from the canal: When the silt deposits on the bed and sides it reduces the capacity of canal section. The silt load is prevented from entering the canal at its head. But 100 % can not prevented. The deposited silt should be removed as it deforms the shape of the channel and changes the flow conditions. This deposited silt may be washed off through the openings provided in the sides of the channel section. Silt ejectors and silt escapes:- The channel may be silt cleared by actual excavation. This is done when canal is dry. Some times dredgers may be used for clearing silt. Pits are dug in the canal bed to trap the silt load. In course of time pits get filled up. The silt may be taken out from the pits and disposed off suitably. 3. Strengthen the canal banks: Internal silting method: When canal runs in a filling reach, the banks are subjected to water pressure. In such conditions it is very essential to provide extra strength to the banks by increasing the section of the bank. To keep the cost of construction low, additional soil for increasing the bank section may be derived from the natural process of silting internally. It is called internal silting method.

Slide 14: 

External silting method: To accomplish silting of banks externally it is very essential to bring flow of water out of the channel section. To hold the water externally in a confined portion subsidiary banks are required. Thus subsidiary banks are constructed which run parallel to canal main banks. Cross embankments are constructed with in the two parallel banks to form compartments or silting tanks. Each silting tank has an inlet from the canal and outlet back to the canal. 4. Closing the canal breaches: The gap created in the bank of the canal is called a breach. The occurrence of breaches should be prevented. Breaches are caused due to Faulty construction of canal banks Leakage Piping due to excess supply Internal cut by farmers. Remedies: By filling sand in the holes created by rats or insects By patrolling the banks at night By taking legal action against those farmers who create intentional cuts.

Measurement of discharge in canals: 

Measurement of discharge in canals Area Velocity method : This method consists of measuring cross sectional area of flow and mean velocity of flow separately. Then the product of two gives the discharge. Measurement of velocity: Floats Surface floats Sub surface floats Twin floats Velocity rod method Pitot tube Current meter Colour method Measurement of area of flow: Measurement of width Measureemnt of depth Sounding rod Lead line Haigh’s depth meter Echo sounder

Weir Method: 

Weir Method Weirs are sharp-crested, overflow structures that are built across open canals. They are easy to construct and can measure the discharge accurately when correctly installed. However, it is important that the water level downstream is always below the weir crest, otherwise the discharge reading will be incorrect. Three well-known weir types are illustrated: the Rectangular weir ,Cipoletti trapezoidal weir ,and V-notch weir Rectangular weir has a rectangular opening. The Cipoletti trapezoidal weir is in fact an improved rectangular weir, with a slightly higher capacity for the same crest length. Its opening is trapezoidal with the sides inclining at a slope of 4 (vertical) to 1 (horizontal). The V-notch weir has a triangular opening, and this type is well suited to measuring small flows with high accuracy. VENTURI FLUMR or METER FLUME: It consists of short channel reach in which water way is made narrower than its normal waterway for some distance and then again it is made normal. The restricted portion is called throat. The velocity of flow in restricted area is increases to obey continuity equation. Q = 1.84 L .H 3/2

Methods of providing drainage behind lining: 

Methods of providing drainage behind lining Lining fails due to inadequate drainage facilities. The soil behind lining is clay or any other type which is having low permeability, it gets saturated due to rain water. This water develops a pressure on the lining from beneath. The increases in back pressure due to high water table or due to saturation of back fill may be controlled by providing some pressure relief arrangements. Continuous inverted filter behind the lining. System of drains behind the lining System of vertical relief pipes. By disposing storm water safely For lined canals berms should not be provided. The banks should be given outward slope. At the outward toe of the bank open drains should be provided to take away the storm water.

Slide 18: 

Design of Lined Channels Mannings equation is used in design. (V = 0.556. R 2/3 . S 1/2 ) Most suitable section for lined canal is circular section with side slopes up to a discharge of 85 cum/sec. The bed is not flat but it is an arc of a circle. When the radius is = 3.6 m or less side slopes may be taken as 1:1. When radius is greater than 3.6 m, side slopes may be taken as 1.25: 1 When the discharge is more than this, canal with flat bed and sloping sides with rounded corners is best suitable. This section is better than trapezoidal section because it is more stable and economical to construct. Cofficient of rugosity (N): In general practice for lined canals, average value of N may be taken as 0.018. CIRCULAR SECTION (Design) Sectional data Side slope 1:1 Side slope 1.25: 1 Sectional area 1.785 r 2 1.925 r 2 Wetted perimeter 3.57 r 3.85 r Hydraulic mean depth 0.5 r 0.5 r Discharge (Q= AV) 0.624 r 8/3 . S 1/2 0.674 r 8/3 . S 1/2 Lacey’s silt factor F = 2.46 V 2 / R 0.603 r 1/3 . S 0.603 . r 1/3. S

Canal with flat bed: 

Canal with flat bed Sectional data Side slope 1:1 Side slope 1.25:1 Sectional area (K + 1.785)D 2 (K + 1.925)D 2 Wetted perimeter C = R/D (K + 3.57)D (K + 3.85)D Hydraulic mean depth C.D C.D Discharge (Q= AV) (K + 1.785)x 0.556 C 2/3 . D 8/3 .S 1/2 (K + 1.925)x 0.556 C 2/3 . D 8/3 .S 1/2 Lacey’s silt factor F = 2.46 V 2 / R K 0.763.C 1/3 . D 1/3 .S B/D 0.763.C 1/3 . D 1/3 .S B/D