IRRIGATION ENGINEERING: IRRIGATION ENGINEERING Slide 2: Chapter 2 Canal Irrigation Canal irrigation in is one of the principal methods used for improving the growth of the crops. After wells and tube wells, canal irrigation is the second most important irrigation source. However, this method is only extended to those areas that are large level plains of deep fertile soil and are drained by well distributed perennial rivers. Classification of canal:- Classification based on the type of soil alluvial canals Non alluvial canals Classification based on the nature of supply perennial or permanent canals Non- perennial or inundation canals Classification based on the funds available protective canals Productive canals Classification according to the nature of source of supply: Permanent canals: A canal is said to be permanent when its source of supply is sufficiently assured to warrant construction of protective and regulatory works with a regular, permanent and graded channel. Inundation canals: Supply of water to the canal depends upon periodical rise in the water level of a river from which the canal takes off. Construction of permanent regulatory work is very rarely done in this case. Slide 3: Canal Alignment: Irrigation water in flow type should reach the fields by gravity. To accomplish this requirement irrigation canal is always aligned in such a way that the water gets proper command over the whole irrigable area. Various types of canal alignments: Alignment of contour channels Alignment of ridge or watershed channels Alignment of side slope channels Points to be considered while aligning a canal It should be aligned on the ridge to get maximum benefits . As far as possible canal alignment should be kept in the centre of the commanded area. Length of canal should be as minimum as possible Alignment should avoid inhabited places, roads, railways, peoperties, places of worship etc. Canal should be taken through area where subsoil formation is favorable. Alignment should be straight as far as possible. Excessive cuttings and fillings should be avoided. While aligning canal cost of acquisition should be taken in mind. It should cross minimum no of drinages Inundation Canals: Inundation Canals This canal takes off directly from a canal. The canal is taken in the direction of general slope of the area. As no head work is provided supply of water to the canal wholly depends on the stage of the river. When there is a flood in the river, water level remains high, flood water flows in the canal from the river. This type of canal is in use during period of rainfall and subsequent runoff in the streams and rivers. Once the canal alignment is fixed the canal is excavated to accommodate anticipated quantity of water. Selection of site for takeoff: River reach from where inundation canal is to take off should be straight. As far as possible there should be a shoal in front of take off point. Presence of shoal creates a still pond in the pocket, thus silt entry in the canal is restricted. River bank should be high and stable at the site. BANDHARA IRRIGATION SYSTEM: BANDHARA IRRIGATION SYSTEM This is a type of flow irrigation system. Extensively used in some parts of Maharastra. This method consists in building a small barrier across a stream or a river to raise water level on the up stream of the barrier. Then the water is diverted in a canal through a head regulator for irrigation purposes. After raising the water level to a required level excess water flow over the barrier. Spilling water flows down which can be further checked by constructing another barrier . This type of system is suitable in places where there are number of small rivers and streams. Selection of site for Bhandhara: There should be sufficient supply of water at site. There should be a good foundation available at moderate depth in the bed of the river. The site should not be far away from the irrigable land. The banks of the river at the site should be high to avoid flooding. The section of the river at the site should be tight. Advantages: : Advantages: The area irrigated is small and close to source. Naturally the duty is high and irrigation is of intensive type. As the conveyance system is of small length losses in handling and conveyance of water are less. The cost of system is low. The small quantity of water flowing in river which would have gone waste, is used up to a maximum extent in this system. Disadvantages : If the stream or river is non perennial then the supply of water is uncertain. As even small portion of water is dammed, population below the site of bandhara do not get water supply for domestic uses in dry periods. Irrigable area for one bandhara is fixed and even if more quantity of water is available it is of no use in that system. Regime Channel : The channel which neither silting nor scouring takes place is called regime or stable channel. Slide 7: Design by Kennedy’s theory Kennedy, an executive engineer of Punjab PWD, did pioneering research for obtaining a stable non-silting, non- scouring irrigation canal system. On the basis of observations made in certain reaches of Upper Ban Doab Canal system in Punjab, which were found to be fairly stable, i.e., required no silt clearance for about three decades, Kennedy concluded that, 1. The flowing water is to counteract friction against the bed of the channel resulting in generation of vertical eddies rising up gently to the water surface and work up against the depth of channel. These eddies keep most of the silt in suspension. 2. The silt supporting power of a channel cross section is mainly dependent on these eddies. Some of the eddies may start from the sides of the channel but these are for most of their part horizontal and as such of no silt supporting power. The silt supporting power of the channel is, therefore, proportional to the bed width, 3. A velocity sufficient to generate these eddies keeps the sediment in suspension thereby avoiding silting up of the channel. He designated it as critical velocity (Vo) defined as the mean velocity which just keeps the channel free from silting or scouring. The silt supporting power of a channel is proportional to V0, The amount of silt held in suspension is proportional to the upward acting force of vertical eddies and varies as bed width and some power of the velocity of flow in the channel, Design of Channel by Kennedy’s Theory: Design of Channel by Kennedy’s Theory Requirements: Requirements The channel flows uniformly in 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. The characteristics and the discharge of the sediment are constant . The characteristics discharge of the sediment are constant . The water discharge in the channel is constant . the perfect regime condition rarely exist . Design procedure : Design procedure Q and m are initially known Calculate the silt factor “f” Compute V from Lacey’s equation Compute A from continuity equation Compute P & S from Lacey’s equations Slide 11: Silt factor = Where, m = mean particle size, mm Draw backs in Lacey’s theory: : Draw backs in Lacey’s theory: The concept of true regime is only theoretical and cannot be achieved practically. The various equations are derived by considering the silt Factor of which is not at all constant. The concentration of silt is not taken into account. The silt grade and silt charge are not clearly defined. The equations are empirical and based on the available data from a particular type of channel. The characteristics of regime of channel may not be same for all cases. Slide 15: Kennedy theory Lacey’s theory 1.It states that the silt carried by the following water is kept in suspension by the vertical component of eddies which are generated from the bed of the channel. 1.It states that the silt carried by the following water is kept in suspension by the vertical component of eddies which are generated from the entire wetted perimeter of the channel. 2. Relation between ‘V’ & ‘D’. 2. Relation between ‘V’ & ‘R’. 3. Critical velocity ratio ‘m’ is introduced to make the equation applicable to diff. channels with diff. silt grades. 3. Silt factor ‘f’ is introduced to make the equation applicable to diff. channels with diff. silt grades. 4., kutter’s equation is used for finding the mean velocity. 4. This theory given an equation for finding the mean velocity. 5. This theory gives no equation for bed slope. 5. This theory gives an equation for bed slope. 6.In this theory, the design is based on trial and error method. 6. This theory does not in valve trial and error method. Slide 16: . Channel Pattern is a function of:: Channel Pattern is a function of: Discharge the volume of water that passes a given location within a given period of time. Usually expressed in cubic feet per second (cfs) Load The sediments and dissolved ions carried by the stream are collectively the stream's load. Divided into three parts: Dissolved load Suspended load Bed load Bed load Primarily from groundwater seepage into the stream. Also come from the weathering and dissolution of materials that line the channel. Dissolved (solution) Load Slide 18: Design based on lacey’s theories Lacey attempted to remove the deficiencies in various formulae developed earlier by Kennedy and Lindley which did not take into account the physical dimensions and properties of the channel such as slope, size and bed material. Lacey on the analysis of data from a large number of natural drainages and canals running for long put forth his theory, the salient features of which are ( i ) If a channel runs indefinitely with constant discharge and sediment charge rates, it will attain a definite stable section having a definite slope, (ii) He used Kennedy data and substituted silt factor f for m and stated that required slope and channel dimensions are dependent on the silt factor f, (iii) Lacey considered that the silt is kept in suspension by the vertical components of eddies generated at all points by forces normal to the wetted perimeter of the channel and accordingly adopted wetted perimeter and hydraulic mean depth as the flow parameters instead of the surface width and depth of flow assumed by Kennedy, (iv) There is only one channel section and one longitudinal slope for a given discharge and particular silt factor, (v) Lacey stated that rugosity coefficient of the channel solely depends on the size of the boundary material forming the bed and sides and is independent of channel condition. Regime channel has constant rugosity coefficient for given size of silt grade, Slide 19: (vi) Lacey’s standard silt is that which connotes a coefficient of rugosity of 0.0225 when hydraulic mean depth is 1.0 m, (vii) Lacey assumed channel section as semi-elliptical, (viii) Ratio of bed width to depth effects channel capacity, i.e., shape of channel for a given discharge is a function of silt grade, channel in finer material being narrow and deeper (Fig. 5.5), (ix) Lacey considered hydraulic mean depth as more relevant variable. In elliptical or semi-circular channel section assumed by Lacey there is no side in true sense and hence hydraulic mean depth is the correct basis. According to him concept of ‘R’ in place of ‘D’ is because eddies which keep the silt in suspension are generated from bed and sides both normal to the surface of generation and not from bed only as stated by Kennedy, (x) Channels designed with Lacey’s theory forf= 0.8 were smaller in section than those designed with Kensiedy’s N = 0.0225 for the same discharge and slope, (xi) Lacey considered that his formulae are applicable to channels of all sizes, and (xii) Lacey enunciated that a channel flowing in its own silt, if not interfered, reaches final stability and in case the discharge and silt remain constant final regime is ultimately attained. He thus propounded regime theory of channels. Suspended Load: Suspended Load Sediment suspended and transported through the stream. Turbulent flow suspends clay and silt in the stream. Suspended load comes from material eroded from the surface bordering the channel and deposited in the stream, as well as, erosion of the channel itself. Bed Load: Bed Load Is moved across the bed of the channel. Bed load is transported in two ways, Traction, which is a scooting and rolling of particles along the bed. Saltation, which occurs when particles are suspended in the stream for a short distance after which they fall to the bed, dislodging particles from the bed. The dislodged particles move downstream a short distance where they fall to the bed, again dislodging particles upon impact.