IRRIGATION ENGINEERING: IRRIGATION ENGINEERING Advantages of well irrigation: Advantages of well irrigation Well irrigation system may be introduced at any time when the cultivator feels it necessary Rain fall does not affect tube well discharge immediately. Water is more economically used. Patches of land which are not commanded by canal, can be easily brought under irrigation by sinking wells. Water losses in transit and handling are less Duty of well water is higher than canal water Water logging is reduced. Water is warm in winter and cool in summer, this condition is more suitable for crop growth. Disadvantages of well irrigation : Working expenses are high Machinery is used to lift the water, it is always liable to give troubles. Water is free from silt. The well water lacks the manurial property due to lack of silt load. Life of tube well is shorter than flow irrigation. Tube well irrigation: Tube well irrigation Water well : It is a vertical, excavated in the earth for bringing ground water to the surface. Open wells : These are open masonry wells, having comparatively bigger diameter ( 2- 9 meters) and are suitable for low discharges (1-5 lit per second). Discharge is limited because can be excavated up to a limited depth. Shallow depth open wells : It rests on pervious stratum and draws its supply from the surrounding material Deep open well: It rests on an impervious mota layer and draws its supply from the pervious formation laying below the mota layer. Tube well: It is a long pipe or a tube is bored or drilled deep into the ground, intercepting one or more water bearing stratum. A tube well is a type of water well in which a long 100–200 mm (5 to 8 inch) wide stainless steel tube or pipe is bored into the underground aquifer. The lower end is fitted with a strainer, and an electric pump at the top lifts water for irrigation. Deep tube well : These are 70 to 300 m deep and tap water from more than one aquifer. Shallow deep well : These are 20 to 70 m deep and tap water from only one aquifer. Types of tube well:- 1.Strainer type 2. Cavity type 3. Slotted type 1. Strainer type tube well: Types of tube well:- 1. Strainer type 2. Cavity type 3. Slotted type 1. Strainer type tube well Slide 5: Strainer type tube well:- It essentially consists of a perforated or a slotted pipe with a wire mesh wrapped around the pipe with smaller annular space between the two. In India, whenever we refer to a tube well, generally it means a strainer type of a tube well . In this type of tube well a screen is placed against the water bearing stratum. The wire screen prevents sand particles from entering the well pipe through the fine mesh (screen) and the sand particles of size larger than the size of mesh, are kept away from entering the pipe. This reduces the danger of sand removal and hence, larger flow velocities can be obtained. In the strainer type tube wells drilling is continued through different layers and after ascertaining the water bearing strata, strainers are located opposite these strata to allow the water to come into the tube well. Strainers are pipes with fine perforations specially constructed for the purpose. Plain pipes are to be located against those layers which are not water bearing. This is done by joining the whole length of strainers and the plain pipes in the same length and order in which they are to be lowered in the bore. For the strainer type wells drilling is to be started with pipes of larger diameter (known as casing of pipes) than the strainer and these pipes are to be extract after the strainers are installed. Slide 6: Types of strainers: To exclude average soil particles from entering the bore hole strainers are used. Common types of strainers are Cook strainer : It consists of a solid drawn brass tube. The wedges shaped slots are cut on the surface of the tube. These slots are horizontal. Tej strainer : Only difference between the Tej and Cook strainer is in former the brass sheet is first slotted and then bent to form a tube. The tube is then brazed. As there are joints this strainer is weaker. Brownlie strainer : It is made of a convoluted tube of steel. There are perforations in the tube. The wire mesh of copper surrounds the steel tube. Ashfold strainer : It consists of a perforated tube with a wire mesh surrounding it. Legger strainer : In this type cutter are provided in the tube. By operating cutters from the ground surface perforations in the tube can be cleaned. Phoneix strainer : It is made up of mild steel. To avoid corrosion of tube it is plated with cadmium. The slits are made from inside. Slotted Type Tube Wells : Slotted Type Tube Wells It uses a slotted pipe without being covered by any wire mesh. If proper depth of water bearing strata is not available even at deep depths of 85 to 100 m, so as to obtain the required discharge from a strainer well. After placing the assembly of plain and slotted pipes in the bore hole, a mixture of gravel and bajri is poured into the bore hole between the well pipe assembly and the casing pipe. Gravel pack is highly perforated for deep wells and tap more than one aquifer. Therefore most of state tube well departments prefers this type. Slide 12: Cavity Type Tube Wells A cavity type tube well draws water from bottom of the well and not from the sides. The difference in flow pattern of a screen well and cavity well is that whereas in a strainer well, the flow is radial, the flow in a cavity well is spherical. In a strainer well, the area of flow is increased by increasing the length of strainer pipe, while in a cavity wall the area of flow is increased by enlarging the size of cavity. The cavity formed with a certain discharge enlarges in size if an increased discharge is pumped out. The cavity type well do not have a strainer and draw water from one stratum only. Drilling is started with the correct size of pipes which are left to serve as tube for the well. The cavity is developed by drawing out a part of the sand from this layer so that such a cavity act as storage reservoir for pumping. The cavity well requires a strong and dependable stratum immediately on top of the water bearing stratum. Slide 14: Aquifer An aquifer is a saturated formation of earth material, which not only stores water but yields it in sufficient quantity. Thus, an aquifer transmits water relatively easily due to its high permeability. Unconsolidated deposits of sand and gravel form good aquifers. Porosity: It is a measure of voids present in the soil mass. It is expressed as a percentage of volume of voids and total volume of the mass. More than 20 % - porous Uniformity coefficient(Cu ): It is D 60 / D 10 of any given soil sample. D10 is 90% of the grains retained on sieve D60 is 40 % of the grains retained on sieve. Cu < 4 poorly graded soil Cu > 4 well graded material. Specific yield :- This term provides a measures of productivity of a well . It is defined as the ratio of the volume of water that can be obtained from an aquifer and total volume of the mass. Sy = Wy /y x 100 where Sc is specific yield Wy Volume of the water obtained Y Volume of the soil mass. Slide 15: Specific retention( Sr ): Ratio of volume of water retained by saturated soil mass after drainage and total volume of the soil mass. Sr = Wr / Y x 100 Wr Volume of water retained, Y – volume of soil Coefficient of permeability(K ):- It is defined as the velocity of flow of water through a porous medium under a unit hydraulic gradient . It indicates the ease with which water can flow through soil mass . Mathematically coefficient of permeability K= Q/A Q = pumping rate, A –Area, L – lengh , h1, h2 are heights (h1-h2)/L Coefficient of transmissibility (T):- It is the rate of flow through the entire thickness of a saturated aquifer of unit width under unit hydraulic gradient. T = b.k b saturated thickness of aquifer, K is the coefficient of permeability. Dupuit’s theory of unconfined aquifer: Dupuit’s theory of unconfined aquifer Confined aquifer: It is the aquifer which is confined by an impervious stratum at the top. Other words water bearing layer is between two impervious layers. Therefore the water confined in aquifer is at a pressure more than atmospheric pressure. Unconfined aquifer : It is that aquifer in which water table forms the upper limit of saturation. The pressure at the upper surface of the aquifer is atmospheric. Assumptions for yield of tube well :- Water table is horizontal and at rest. The well penetrates homogeneous isotropic aquifer . The bottom of the strainer reaches a horizontal impervious layer. The inclination of free surface is equal to hydraulic gradient and it does not change with depth There is no flow in aquifer other than that towards the well . That is the flow is radial . It is also assumed that the flow is in horizontal direction . Total discharge remains constant across the surface of a series of concentric cylinders . The flow through soil is assumed to be steady and laminar . Slide 17: Yield of Tube Wells in unconfined aquifer: Q = total discharge K = Coefficient of permeability R = Radius of circle influence, m, H1= Height of static water table above impervious layer in m H2= Height of water level in the tube well in meters after depression. r = Radius of tube well, m, Q= 2∏ K ( (H 1 2 – H 2 2 ) 2.303 log10( R/r) Discharge through confined aquifer: Discharge through confined aquifer Yield of Tube Wells in unconfined aquifer: Q = total discharge K = Coefficient of permeability R = Radius of circle influence, m, H1= Height of static water table above impervious layer in m H2= Height of water level in the tube well in meters after depression. r = Radius of tube well, m, m = Thickness of confined aquifer Q= 2∏ k.m ( (H 1 – H 2 ) 2.303 log10( R/r) Slide 19: Duty in wells :- In well irrigation water is lifted intermittently . Therefore duty can not be expressed in hectares /meter cube / sec for a base period of a crop the duty of a well is the area of land in hectares . The duty of well is the area of land in hectares, that well can irrigate annually and fully to bring a crop to maturity, grown on that land. The duty of wells differ from well to well because of the following reason :- Some wells are too deep and therefore choice of lifting device is limited .It increases cost of lifting water . Duty depends upon methods of irrigation adopted by cultivators . Duty will be more if a cultivator puts the well water to maximum use. Duty depends upon type of grown . Rainfall also affects duty of well water . Slide 20: Delta in well irrigation :- Delta in irrigation is the total depth of water required to mature a crop .It can be measured in meters or centimeters . The amount of water required from wells to mature a similar crop is less than that required in canal irrigation . It is seen that delta of well water is about 66 percent that of canal water . The well water delta is lower because of the following reason :- Well is situated in a field itself and therefore conveyance losses are minimum Well water is used economically as energy is required to lift it up to ground level . Well water is used at right time in right quantity . Slide 21: During the last ten years much research has been conducted by KIWA, in close cooperation with various waterworks, into the rehabilitation of clogged production wells. During this research it became apparent that various types of clogging could be distinguished. One type of clogging is caused by the mixing of groundwater's with incompatible chemical compositions in the well itself, due to the vertical chemical heterogeneity of groundwater in the aquifer. Another type of clogging is caused by the increased velocity of groundwater flow near the borehole, which increases the intensity of various biological and chemical processes. The selection of the correct method of rehabilitation, the rehabilitation results, and the costs, all depend on the type of cloging that has occurred. Slide 22: Interference of wells :- The wells should be located in such a way that their circles of influence . Under maximum draw down conditions do not cross the neighboring property line, it is likely to damage the structure above. Two wells of any area should be spaced in such a way that their cones of depression or circles of influence do not cross each other . Mutual interference reduces the dependable ground water storage of each well . In turn it reduces the discharge of wells . Sufficient spacing should be kept between the wells to avoid mutual interference . Slide 26: Investigation and preparation of irrigation projects :- Classification of projects : Irrigation projects can be classified in number of ways. Classification based on type of flow : Gravity canal system or flow irrigation system Pumped canal system or lift irrigation system Classification based on function served : Irrigation scheme Drainage scheme Reclamation scheme Classification based on scope of project : Major irrigation project (C.C.A. >10,000Ha.) Medium irrigation project (2000Ha<C.C.A. <10,000Ha.) Minor irrigation project (C.C.A. <2000Ha.) Classification based on purpose of the project : single purpose project . Multipurpose project . Slide 27: Project preparation: Although the details and design widely differ for each irrigation project , in principle all the projects require same steps to be taken in planning and preparation . Thus in preparation of a sound an economical irrigation project following steps are taken in order of sequence Investigation Design of works Drawings Estimates and financial analysis Documentation of project report Investigation:- Following methodical investigations are generally carried out in preparation of an irrigation project : Reconnaissance: once it is decided to prepare an irrigation scheme for any region first question is which should be the site . There may be localities where the scheme can be implemented . By a very rough type of survey , which is also known as reconnaissance , it is actually possible to find out the no. of sites where the project can be started . In reconnaissance the inspection of the region is done physically , without using any of the precise surveying instruments. Slide 28: 2. Preliminary or feasibility investigations:- the next step is to study the merits and demerits of each alternative in the light of following considerations : - Necessity for irrigation in that area Availability of adequate water supply Topography of the area Culture practice of the tract Adequacy of existing irrigation system if any Possibility of growing cash crops or other valuable crops after provision of irrigation water . Facilities regarding accessibility to the site and transportation of construction materials. Economical justification for implementing the irrigation scheme. 3. Detailed and precise investigations : after selecting the alternative detailed surveys are conducted to decide following aspects : Selection of site for a reservoir and a dam Selection of site for locating head-works Alignment for field channels and irrigation chaks Slide 29: Design of works :- While conducting detailed surveys data should be collected which will be useful in design of various structure . Generally following works are to be designed completely : Storage dam Head works Irrigation canal system Falls Bridges Cross drainage works Outlets Other regulatory hydraulic structures (cross regulators , structure for silt control , etc.) Drawings After designing the structure mentioned above their detailed working drawings are prepared to facilitate construction . In addition to these drawings following plans are generally required : Slide 30: Index or key plan : scale used may be 1:50,000. General plan of catchment area : scale used may be 1:50,000. Contoured plan of a reservoir basin : scale used may be 1:10,000 to 1:15,000. Land plan : it shows the land to be acquired temporarily and permanently for construction of works . Scale used may be 1:25,000. Plan of the area to be irrigated : this plan shows the villages , roads ,other works together with the boundaries of cultivable tracts . Longitudinal section of canal Cross section of a canals at 100 m interval specially for maximum filling sections. Estimates and financial analysis A estimates is a statement which forecasts the expenditure likely to be done on the project . It consists measurement of works, specifications and rates of various items . The total cost of the project includes expenditure on various items from planning to completion of the project . Various items considered are following : Cost of preliminary and precise survey and investigations. Cost of land acquisition , resettlement and environment protection measures. Slide 31: Cost of various structures Cost of earthwork and lining for canal system Allowances made for foreseen and unforeseen contingencies . When allowance is made for foreseen expenditure the circumstances should be fully explained in the report. Maintenance of facilities during construction Cost of communication system Cost of establishments during construction . Proper allowance have also to be made for miscellaneous items . It includes leave an pensionary charges etc. Cost of tool and plants. The estimates should be prepared correctly . All calculations of quantities and costs are checked carefully to avoid calculational Slide 32: Documentation of project report : It is a comprehensive statement which discusses all the steps taken in planning and preparation of an irrigation project . In general the report includes discussions of the following heads : Introduction :- it includes aim and history of the project, location of project area , accessibility , population interstate and international aspects , stages of development etc. Physical features Surveys and investigation : it includes surveys and investigations carried out for various alternative to justify final choice of location and type of structures Hydrology : it includes the details of data collected and various studies made Design features and drawings Irrigation planning Command area development Flood control and drainage Benefits expected from project Construction programme and manpower and plant planning Schedule of rates Detailed estimate of various structure . Economic justification for taking up the project Slide 33: Concept of Multi purpose projects : some major and medium projects provide other benefits in addition to irrigation. They are called multipurpose projects. Other benefits are hydro power , flood control , domestic and industrial water supply , navigation etc. Major medium and minor projects : Previously when the total expenditure on a project was more than Rs. 5 crores it used to be called major project . Those costing between Rs . 25 lakhs and 5 crores medium and rest minor . Owing abnormal price rise this classification is no more valid . Now the projects are termed according to the benefits provided by them . A small scheme benefiting less than 2000 ha. C.C.A is called minor scheme . It includes tube well as well as surface schemes . Similarly project having more than 10,000 Ha C.C.A. is termed major irrigation project . Rest are called medium irrigation schemes.