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Water Resources and Irrigation Engineering

by Engineering KingsEngineering Kings
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Engineering Kings
Downloaded CE6703 WATER RESOURCES AND IRRIGATION ENGINEERING UNIT I WATER RESOURCES Water resources survey – Water resources of India and Tamilnadu – Description of water resources planning – Estimation of water requirements for irrigation and drinking- Single and multipurpose reservoir – Multi objective - Fixation of Storage capacity -Strategies for reservoir operation - Design flood-levees and flood walls. INTRODUCTION Primary categories of water 1. Saline – predominantly seawater 2. Freshwater. Global Water Distribution: S.NO 1 2 3 4 5 6 PLACE OF OCCURENCE Oceans Snow and ice Groundwater Atmosphere Lakes & rivers Soil moisture NATURE OF WATER Saline Freshwater Freshwater Freshwater Freshwater Freshwater QUANTITY,km3 1,350,000,000 27,500,000 8,200,000 460,000 207,000 70,000 PERCENTAGE % 97.37% 1.98 0.59 0.033 0.015 0.005 HYDROLOGIC CYCLE The hydrologic cycle consists of four key components 1. Precipitation 2. Runoff 3. Storage 4. Evapo-transpiration 1. Precipitation Precipitation occurs when atmospheric moisture becomes too great to remain suspended in clouds. It denotes all forms of water that reach the earth from the atmosphere, the usual forms being rainfall, snowfall, hail, frost and dew. Once it reaches the earth’s surface, precipitation can become surface water runoff, surface water storage, glacial ice, water for plants, groundwater, or may evaporate and return immediately to the atmosphere. Ocean evaporation is the greatest source (about 90%) of precipitation. Rainfall is the predominant form of precipitation and its distribution over the world and within a country. Downloaded
Downloaded India has a typical monsoon climate. At this time, the surface winds undergo a complete reversal from January to July, and cause two types of monsoon. In winter dry and cold air from land in the northern latitudes flows southwest (northeast monsoon), while in summer warm and humid air originates over the ocean and flows in the opposite direction (southwest monsoon), accounting for some 70 to 95 percent of the annual rainfall. The average annual rainfall is estimated as 1170 mm over the country, but varies significantly from place to place. In the northwest desert of Rajasthan, the average annual rainfall is lower than 150 mm/year. In the broad belt extending from Madhya Pradesh up to Tamil Nadu, through Maharastra, parts of Andhra Pradesh and Karnataka, the average annual rainfall is generally lower than 500 mm/year. At the other extreme, more than 10000 mm of rainfall occurs in some portion of the Khasi Hills in the northeast of the country in a short period of four months. In other parts of the northeast (Assam, Arunachal Pradesh, Mizoram, etc.,) west coast and in subHimalayan West Bengal the average annual rainfall is about 2500 mm. Except in the northwest of India, inter annual variability of rainfall in relatively low. The main areas affected by severe droughts are Rajasthan, Gujarat (Kutch and Saurashtra). The year can be divided into four seasons: • The winter or northeast monsoon season from January to February. • The hot season from March to May. • The summer or south west monsoon from June to September. • The post – monsoon season from October to December. The monsoon winds advance over the country either from the Arabian Sea or from the Bay of Bengal. In India, the south-west monsoon is the principal rainy season, which contributes over 75% of the annual rainfall received over a major portion of the country. 2. Runoff  Runoff is the water that flows across the land surface after a storm event. As rain falls over land, part of that gets infiltrated the surface as overland flow. As the flow bears down, it notches out rills and gullies which combine to form channels. These combine further to form streams and rivers.  The geographical area which contributes to the flow of a river is called a river or a watershed. The following are the major river basins of our country 1. Indus 2. Ganges 3. Brahmaputra 4. Krishna 5. Godavari 6. Mahanandi 7. Sabarmati 8. Tapti 9. Brahmani-Baitarani 10. Narmada 11. Pennar 12. Mahi 3. Storage  Portion of the precipitation falling on land surface which does not flow out as runoff gets stored as either as surface water bodies like Lakes, Reservoirs and Wetlands or as subsurface water body, usually called Ground water.  Ground water storage is the water infiltrating through the soil cover of a land surface and traveling further to reach the huge body of water underground.  The following definitions may be useful: Lakes: Large, naturally occurring inland body of water Reservoirs: Artificial or natural inland body of water used to store water to meet various demands. Wet Lands: Natural or artificial areas of shallow water or saturated soils that contain or could support water–loving plants. Downloaded
Downloaded 4. Evapotranspiration  Evapotranspiration is actually the combination of two terms – evaporation and transpiration. The first of these, that is, evaporation is the process of liquid converting into vapour, through wind action and solar radiation and returning to the atmosphere.  Evaporation is the cause of loss of water from open bodies of water, such as lakes, rivers, the oceans and the land surface. It is interesting to note that ocean evaporation provides approximately 90 percent of the earth’s precipitation.  Transpiration is the process by which water molecules leaves the body of a living plant and escapes to the atmosphere. The water is drawn up by the plant root system and part of that is lost through the tissues of plant leaf (through the stomata).  In areas of abundant rainfall, transpiration is fairly constant with variations occurring primarily in the length of each plants growing season. However, transpiration in dry areas varies greatly with the root depth.  Evapotranspiration, therefore, includes all evaporation from water and land surfaces, as well as transpiration from plants. WATER RESOURCES SURVEY The term water resources refer to the exploitable surface water and groundwater in a defined water management unit (e.g. catchment basin). All Water Resources projects have to be cost evaluated. This is an essential part of planning. Since, generally, such projects would be funded by the respective State Governments, in which the project would be coming up it would be helpful for the State planners to collect the desired amount of money, like by issuing bonds to the public, taking loans from a bank, etc. Since a project involves money, it is essential that the minimum amount is spent, under the given constraints of project construction. Hence, a few feasible alternatives for a project are usually worked out. For example, a project involving a storage dam has to be located on a map of the river valley at more than one possible location, if the terrain permits. In this instance, the dam would generally be located at the narrowest part of the river valley to reduce cost of dam construction, but also a couple of more alternatives would be selected since there would be other features of a dam whose cost would dictate the total cost of the project. For example, the foundation could be weak for the first alternative and consequently require costly found treatment, raising thereby the total project cost. At times, an economically lucrative project site may be causing submergence of a costly property, say an industry, whose relocation cost would offset the benefit of the alternative. Downloaded
Downloaded On the other hand, the beneficial returns may also vary. For example, the volume of water stored behind a dam for one alternative of layout may not be the same as that behind another. Hence, what is required is to evaluate the so-called Benefit-Cost Ratio defined as below: The annual cost and benefits are worked out as under. Annual Cost(C): The investment for a project is done in the initial years during construction and then on operation and maintenance during the project's lifetime. The initial cost may be met by certain sources like borrowing, etc. but has to be repaid over a certain number of years, usually with an interest, to the lender. This is called the Annual Recovery Cost, which, together with the yearly maintenance cost would give the total Annual Costs. It must be noted that there are many non - tangible costs, which arise due to the effect of the project on the environment that has to be quantified properly and included in the annual costs. WATER RESOURCES OF INDIA AND TAMILNADU The following are the major river basins of our country, and the corresponding figures, as obtained from the web-site of the Ministry of Water Resources; Government of India is mentioned alongside each. 1. Indus 2. Ganges 3. Brahmaputra 4. Krishna 5. Godavari 6. Mahanandi 7. Sabarmati 8. Tapi 9. Brahmani-Baitarani 10. Narmada 11. Pennar 12. Mahi Surface water potential: The average annual surface water flows in India has been estimated as 1869 cubic km. This is the utilizable surface water potential in India. But the amount of water that can be actually put to beneficial use is much less due to severe limitations posed by Physiography, topography, inter-state issues and the present state of technology to harness water resources economically. The recent estimates made by the Central Water Commission, indicate that the water resources is utilizable through construction of structures is about 690 cubic km (about 36% of the total). One reason for this vast difference is that not only does the whole rainfall occur in about four months a year but the spatial and temporal distribution of rainfall is too uneven due to which the annual average has very little significance for all practical purposes Monsoon rain is the main source of fresh water with 76% of the rainfall occurring between June and September under the influence of the southwest monsoon. Downloaded

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