WATER CONSERVATION

Water is a chemical substance that is essential to all known forms of life. It covers 71% of Earth's surface. There are 1.4 billion cubic kilometers (330 million mi³) of it available on Earth. It appears mostly in the oceans (saltwater) and polar ice caps, but it is also present as clouds, rain water, rivers, freshwater aquifers, lakes, airborne vapor and sea ice. Water in these bodies perpetually moves through a cycle of evaporation, precipitation, and runoff to the sea. Clean water is essential to human life. In many parts of the world, it is in short supply. Outside of our planet, a significant quantity of water is thought to exist at the north and south poles of the planet Mars, and on the moons Europa and Enceladus.

Water conservation refers to reducing use of fresh water, through technological or social methods. The goals of water conservation efforts include:

Sustainability - To ensure availability for future generations, the withdrawal of fresh water from an ecosystem should not exceed its natural replacement rate. Energy conservation - Water pumping, delivery and wastewater treatment facilities consume a significant amount of energy. In some regions of the world (for example, California over 15% of total electricity consumption is devoted to water management. Habitat conservation - Minimizing human water use helps to preserve fresh water habitats for local wildlife and migrating waterfowl, as well as reducing the need to build new dams and other water diversion infrastructure.

Social solutions
Water conservation programs are typically initiated at the local level, either by municipal water utilities or regional governments. Common strategies include public education campaigns, tiered water rates (charging progressively higher prices as water use increases), subsidies for showerhead and toilet retrofits, and seasonal restrictions on lawn sprinklers. Cities in dry climates often require or encourage the installation of xeriscaping or natural landscaping in new homes to reduce outdoor water usage. One fundamental conservation goal is universal metering. The prevalence of residential water metering varies significantly worldwide. Recent studies have estimated that water supplies are metered in less than 30% of UK households, and about 57% of urban Canadian homes. Although individual water meters have often been considered impractical in homes with private wells or in multifamily buildings, the U.S. Environmental Protection Agency estimates that metering alone can reduce consumption by 20 to 40 percent. In addition to raising consumer awareness of their water use, metering is also an important way to identify and localize water leaks. Some researchers have suggested that water conservation efforts should be primarily directed at farmers, in light of the fact that crop irrigation accounts for 70% of the world's fresh water use. The agricultural sector of most countries is important both economically and politically, and water subsidies are common. Conservation advocates have urged removal of all subsidies to force farmers to grow more water-efficient crops and adopt less wasteful irrigation techniques.



Technological solutions
Water-saving technology for the home includes: low-flow shower heads (sometimes called energy-efficient shower heads as they also use less energy, due to less water being heated). low-flush toilets, composting toilets and waterless urinals, which can have a dramatic impact in the developed world, as conventional Western toilets use large volumes of water. faucet aerators, which break water flow into fine droplets to maintain "wetting effectiveness" while using less water. wastewater reuse or recycling systems, allowing: reuse of graywater for flushing toilets or for the garden, and recycling of wastewater through purification at a water treatment plant. See also Wastewater - Reuse Overhead irrigation, center pivot designFor crop irrigation, optimal water efficiency means minimizing losses due to evaporation or runoff. An Evaporation pan can be used to determine how much water is required to irrigate the land. Flood irrigation, the oldest and most common type, is often very uneven in distribution, as parts of a field may receive excess water in order to deliver sufficient quantities to other parts. Overhead irrigation, using center-pivot or lateral-moving sprinklers, gives a much more equal and controlled distribution pattern, but in extremely dry conditions much of the water may evaporate before it reaches the ground. Drip irrigation is the most expensive and least-used type, but offers the best results in delivering water to plant roots with minimal losses. As changing irrigation systems can be a costly undertaking, conservation efforts often concentrate on maximizing the efficiency of the existing system. This may include chiseling compacted soils, creating furrow dikes to prevent runoff, and using soil moisture and rainfall sensors to optimize irrigation schedules.

Water catchment management measures include:
recharge pits, which capture rainwater and runoff and use it to recharge groundwater supplies.This helps in the formation of grounwater wells etc. and eventually reduces soil erosion caused due to running water.

Chemical and physical properties of Water:
Information and properties
Systematic name water
Alternative names aqua, dihydrogen monoxide, hydrogen hydroxide
Molecular formula H2O
Molar mass 18.0153 g/mol
Density and phase 0.998 g/cm³ (liquid at 20 °C)
0.92 g/cm³ (solid)
Melting point 0 °C (273.15 K) (32 ºF)
Boiling point 100 °C (373.15 K) (212 ºF)
Specific heat capacity 4184 J/(kg·K) (liquid at 20 °C)
Water (molecule)
Water is the chemical substance with the chemical formula H2O: one molecule of water is composed of two hydrogen atoms covalently bonded to a single oxygen atom. Water is a colorless, tasteless, and odorless liquid at ambient temperature and pressure. It is a very important solvent, capable of dissolving many other tree substances, such as salts, sugars, acids, alkalis, some gases and many organic molecules. Water is unusual in that it is a liquid under normal conditions, when relationships between other analogous hydrides of oxygen's column in the periodic table suggest it should be a gas, as is hydrogen sulfide. If the periodic table is examined, it will be noted that the elements surrounding oxygen are nitrogen, fluorine, phosphorus, sulfur and chlorine. All of these elements combine with hydrogen to produce gases at normal temperature and pressure. The reason that oxygen forms a liquid is that it is more electronegative than all of these elements (other than fluorine). Oxygen pulls on electrons much more strongly than hydrogen, leaving a net positive charge on the hydrogen atoms, and a net negative charge on the oxygen atom. The presence of a charge on each of these atoms gives each water molecule a net dipole moment. Electrical attraction between water molecules due to this dipole pulls individual molecules closer together, making it more difficult to separate the molecules and therefore raising the boiling point. This attraction is known as hydrogen bonding. Water has been referred to as the universal solvent, and is the only real pure substance found naturally on Earth in all three states of matter. It is in dynamic equilibrium between the liquid and solid states at standard temperature and pressure. Water can be described as a polar liquid that dissociates disproportionately into the hydronium ion (H3O+(aq)) and an associated hydroxide ion (OH-(aq)).

Cohesion and adhesion
Water sticks to itself (cohesion) because it is polar. Water has a partial negative charge (s-) near the oxygen atom due the unshared pairs of electrons, and partial positive charges (s+) near the hydrogen atoms. In water, this happens because the oxygen atom is more electronegative than the hydrogen atoms that is, it has a stronger "pulling power" on the molecule's electrons, drawing them closer (along with their negative charge) and making the area around the oxygen atom more negative than the area around both of the hydrogen atoms. Water also has high adhesion properties because of its polar nature.

Surface tension
Surface tension prevents the water from submerging the flower.Water has a high surface tension caused by the strong cohesion between water molecules. This can be seen when small quantities of water are put onto a non-soluble surface such as polythene; the water stays together as drops. On extremely clean/smooth glass the water may form a thin film because the molecular forces between glass and water molecules (adhesive forces) are stronger than the cohesive forces. In biological cells and organelles, water is in contact with membrane and protein surfaces that are hydrophilic; that is, surfaces that have a strong attraction to water. Irving Langmuir observed a strong repulsive force between hydrophilic surfaces. To dehydrate hydrophilic surfaces to remove the strongly held layers of water of hydration requires doing substantial work against these forces, called hydration forces. These forces are very large but decrease rapidly over a nanometer or less. Their importance in biology has been extensively studied by V. Adrian Parsegian of the National Institute of Health. They are particularly important when cells are dehydrated by exposure to dry atmospheres or to extracellular freezing.'

Heat capacity and heat of vaporization
Water has the second highest specific heat capacity of any known chemical compound, after ammonia, as well as a high heat of vaporization (40.65 kJ mol-1), both of which are a result of the extensive hydrogen bonding between its molecules. These two unusual properties allow water to moderate Earth's climate by buffering large fluctuations in temperature.

Freezing point
A simple but environmentally important and unusual property of water is that its common solid form, ice, floats on its liquid form. This solid phase is not as dense as liquid water because of the geometry of the hydrogen bonds which are formed only at lower temperatures. For almost all other substances the solid form has a greater density than the liquid form. Fresh water at standard atmospheric pressure is most dense at 3.98 °C, and will sink by convection as it cools to that temperature, and if it becomes colder it will rise instead. This reversal will cause deep water to remain warmer than shallower freezing water, so that ice in a body of water will form first at the surface and progress downward, while the majority of the water underneath will hold a constant 4 °C. This effectively insulates a lake floor from the cold. The water will freeze at 0°C (32°F, 273 K), however, it can be supercooled in a fluid state down to its crystal homogeneous nucleation at almost 231 K (-42 °C). It also has a number of more exotic states not commonly seen.

Triple point
The triple point of water (the single combination of pressure and temperature at which pure liquid water, ice, and water vapor can coexist in a stable equilibrium) is used to define the kelvin, the SI unit of thermodynamic temperature. As a consequence, water's triple point temperature is an exact value rather than a measured quantity : 273.16 kelvins (0.01 °C) and a pressure of 611.73 pascals (0.0060373 atm).

Electrical conductivity
A common misconception about water is that it is a good conductor of electricity, with risks of electrocution explaining this popular belief. Any electrical properties observable in water are from the ions of mineral salts and carbon dioxide dissolved in it. Water does self-ionize where two water molecules become one hydroxide anion and one hydronium cation, but not enough to carry enough electric current to do any work or harm for most operations. In pure water, sensitive equipment can detect a very slight electrical conductivity of 0.055 µS/cm at 25°C. Pure water can also be electrolyzed into oxygen and hydrogen gases but in the absence of dissolved ions this is a very slow process and thus very little current is conducted.

Forms of water.
Water takes many different forms on Earth: water vapor and clouds in the sky; seawater and icebergs in the ocean; glaciers and rivers in the mountains; and aquifers in the ground, to name but a few. Through evaporation, precipitation, and runoff, water is continuously flowing from one form to another, in what is called the water cycle. Because of the importance of precipitation to agriculture, and to mankind in general, different names are given to its various forms: rain is common in most countries, and hail, snow, fog and dew are other examples. When appropriately lit, water drops in the air can refract sunlight to produce rainbows. Similarly, water runoffs have played major roles in human history as rivers and irrigation brought the water needed for agriculture. Rivers and seas offered opportunity for travel and commerce. Through erosion, runoffs played a major part in shaping the environment providing river valleys and deltas which provide rich soil and level ground for the establishment of population centers. Water also infiltrates the ground and goes into aquifers. This groundwater later flows back to the surface in springs, or more spectacularly in hot springs and geysers. Groundwater is also extracted artificially in wells. Water can dissolve many different substances imparting upon it different tastes and odors. In fact, humans and other animals have developed senses to be able to evaluate the drink-ability of water: animals generally dislike the taste of salty sea water and the putrid swamps and favor the purer water of a mountain spring or aquifer. The taste advertised in spring water or mineral water derives from the minerals dissolved, while pure H2O is tasteless. As such, purity in spring and mineral water refers to purity from toxins, pollutants, and microbes.

Position of the Earth relating to water
Over two thirds of the earth's surface is covered with water, 97.2% of which is contained in the five oceans. The Antarctic ice sheet, containing 90% of all fresh water on the planet, is visible at the bottom. Atmospheric water vapor can be seen as clouds, contributing to the earth's albedo.Scientists theorize that most of the universe's water is produced as a byproduct of star formation. Gary Melnick, a scientist at the Harvard-Smithsonian Center for Astrophysics, explains: "For reasons that aren't entirely understood, when stars are born, their birth is accompanied by a strong outward wind of gas and dust. When this outflowing material eventually impacts the surrounding gas, the shock waves that are created compress and heat the gas. The water we observe is quickly produced in this warm dense gas." The coexistence of the solid, liquid, and gaseous phases of water on Earth is vital to the existence of life on Earth. However, if the Earth's location in the solar system were even marginally closer to or further from the Sun (a million miles or so), the conditions which allow the three forms to be present simultaneously would be far less likely to exist. Earth's mass allows gravity to hold an atmosphere. Water vapor and carbon dioxide in the atmosphere provide a greenhouse effect which helps maintain a relatively steady surface temperature. If Earth were smaller, a thinner atmosphere would cause temperature extremes preventing the accumulation of water except in polar ice caps (as on Mars). It has been proposed that life itself may maintain the conditions that have allowed its continued existence. The surface temperature of Earth has been relatively constant through geologic time despite varying levels of incoming solar radiation (insolation), indicating that a dynamic process governs Earth's temperature via a combination of greenhouse gases and surface or atmospheric albedo. This proposal is known as the Gaia hypothesis.

Health and pollution
Water fit for human consumption is called drinking water or "potable water". Water that is not fit for drinking but is not harmful for humans when used for food preparation is called safe water. This natural resource is becoming scarcer in certain places, and its availability is a major social and economic concern. Currently, about 1 billion people around the world routinely drink unhealthy water. Most countries accepted the goal of halving by 2015 the number of people worldwide who do not have access to safe water and sanitation during the 2003 G8 Evian summit. Even if this difficult goal is met, it will still leave more than an estimated half a billion people without access to safe drinking water supplies and over 1 billion without access to adequate sanitation facilities. Poor water quality and bad sanitation are deadly; some 5 million deaths a year are caused by polluted drinking water. In the developing world, 90% of all wastewater still goes untreated into local rivers and streams. Some 50 countries, with roughly a third of the world’s population, also suffer from medium or high water stress, and 17 of these extract more water annually than is recharged through their natural water cycles[citation needed]. The strain affects surface freshwater bodies like rivers and lakes, but it also degrades groundwater resources.

Human uses
About 70% of the fat free mass of the human body is made of water. To function properly, the body requires between one and seven liters of water per day to avoid dehydration; the precise amount depends on the level of activity, temperature, humidity, and other factors. Most of this is ingested through foods or beverages other than drinking straight water. It is not clear how much water intake is needed by healthy people, though most experts agree that 8-10 glasses of water (approximately 2 liters) daily is the minimum to maintain proper hydration. For those who do not have kidney problems, it is rather difficult to drink too much water, but (especially in warm humid weather and while exercising) it is dangerous to drink too little. People can drink far more water than necessary while exercising, however, putting them at risk of water intoxication, which can be fatal. The "fact" that a person should consume eight glasses of water per day cannot be traced back to a scientific source. There are other myths such as the effect of water on weight loss and constipation that have been dispelled. Original recommendation for water intake in 1945 by the Food and Nutrition Board of the National Research Council read: "An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods." The latest dietary reference intake report by the United States National Research Council in general recommended (including food sources): 2.7 liters of water total for women and 3.7 liters for men. Also noted is that normally, about 20 percent of water intake comes from food, while the rest comes from drinking water and beverages (caffeinated included). Water is lost from the body in urine and feces, through sweating, and by exhalation of water vapor in the breath. With physical exertion and heat exposure, water loss will increase and daily fluid needs may increase as well. Humans require water that does not contain too many impurities. Common impurities include metal salts and/or harmful bacteria, such as Vibrio. Some solutes are acceptable and even desirable for taste enhancement and to provide needed electrolytes. The single largest freshwater resource suitable for drinking is the Lake Baikal in Siberia, which has a very low salt and calcium content and is very clean.

Industrial applications
Pressurized water is used in water blasting and water jet cutters. Also, very high pressure water guns are used for precise cutting. It works very well, is relatively safe, and is not harmful to the environment.

Food Processing
Water plays many critical roles within the field of food science. It is important for a food scientist to understand the roles that water plays within food processing to ensure the success of their products. Solutes such as salts and sugars found in water affect the physical properties of water. The boiling and freezing points of water is affected by solutes. One mole of sucrose (sugar) raises the boiling point of water by 0.52 °C, and one mole of salt raises the boiling point by 1.04 degrees while lowering the freezing point of water in a similar way. Solutes in water also affect water activity which affects many chemical reactions and the growth of microbes in food. Water activity can be described as a ratio of the vapor pressure of water in a solution to the vapor pressure of pure water. Solutes in water lower water activity. This is important to know because most bacterial growth ceases at low levels of water activity.[14] Not only does microbial growth affect the safety of food but also the preservation and shelf life of food. Water hardness is also a critical factor in food processing. It can dramatically affect the quality of a product as well as playing a role in sanitation. Water hardness is classified based on the amounts of removable calcium carbonate salt it contains per gallon. Water hardness is measured in grains; 0.064 g calcium carbonate is equivalent to one grain of hardness.[15] Water is classified as soft if it contains 1 to 4 grains, medium if it contains 5 to 10 grains and hard if it contains 11 to 20 grains.[16] The hardness of water may be altered or treated by using a chemical ion exchange system. The hardness of water also affects its pH balance which plays a critical role in food processing. For example, hard water prevents successful production of clear beverages. Water hardness also affects sanitation; with increasing hardness, there is a loss of effectiveness for its use as a sanitizer.[17]

Power Generation
Hydroelectricity is electricity obtained from hydropower. Hydroelectric power comes from water driving a water turbine connected to a generator. Hydroelectricity is a low-cost, non-polluting, renewable energy source

DAMS:
A dam is a barrier across flowing water that obstructs, directs or slows down the flow, often creating a reservoir, lake or impoundment. In Australian and South African English, the word "dam" can also refer to the reservoir as well as than the structure. Most dams have a section called a spillway or weir over which, or through which, water will flow, either intermittently or continuously. Dams generally serve the primary purpose of retaining water, while other structures such as levees and dikes are used to prevent water flow into specific land regions. Some of the first dams were built in Mesopotamia up to 7,000 years ago. These were used to control the water level, for Mesopotamia's weather affected the Tigris and Euphrates rivers and could be quite unpredictable. The earliest recorded dam is believed to have been on the Nile river at Kosheish, where a 15m high masonry structure was built about 2900 B.C. to supply water to capital of Memphis.

Types of dams
Tehri dam in the state of Uttarakhand in IndiaDams can be formed by human agency, natural causes, or by the intervention of wildlife such as beavers. Man-made dams are typically classified according to their structure, intended purpose or height. Based on structure and material used, dams are classified as timber dams, embankment dams or masonry dams, with several subtypes. Intended purposes include providing water for irrigation or town or city water supply, improving navigation, creating a reservoir of water to supply industrial uses, generating hydroelectric power, creating recreation areas or habitat for fish and wildlife, flood control and containing effluent from industrial sites such as mines or factories. Few dams serve all of these purposes but some multi-purpose dams serve more than one. According to height, a large dam is higher than 15 metres and a major dam is over 150 metres in height. Alternatively, a low dam is less than 30 m high; a medium-height dam is between 30 and 100 m high, and a high dam is over 100 m high. A saddle dam is an auxiliary dam constructed to confine the reservoir created by a primary dam either to permit a higher water elevation and storage or to limit the extent of a reservoir for increased efficiency. An auxiliary dam is constructed in a low spot or saddle through which the reservoir would otherwise escape. On occasion, a reservoir is contained by a similar structure called a dike to prevent inundation of nearby land. Dikes are commonly used for reclamation of arable land from a shallow lake. This is similar to a levee, which is a wall or embankment built along a river or stream to protect adjacent land from flooding. An overflow dam is designed to be overtopped. A weir is a type of small overflow dam that can be used for flow measurement. A check dam is a small dam designed to reduce flow velocity and control soil erosion. Conversely, a wing dam is a structure that only partly restricts a waterway, creating a faster channel that resists the accumulation of sediment. A dry dam is a dam designed to control flooding. It normally holds back no water and allows the channel to flow freely, except during periods of intense flow that would otherwise cause flooding downstream.

Diversionary dams
A diversionary dam is a structure designed to divert all or a portion of the flow of a river from its natural course.

Timber dams
A timber crib dam in Michigan, photographed in 1978.Timber dams were widely used in the early part of the industrial revolution and in frontier areas due to ease and speed of construction. Rarely built in modern times by humans due to relatively short lifespan and limited height to which they can be built, timber dams must be kept constantly wet in order to maintain their water retention properties and limit deterioration by rot, similar to a barrel. The locations where timber dams are most economical to build are those where timber is plentiful, cement is costly or difficult to transport, and either a low head diversion dam is required or longevity is not an issue. Timber dams were once numerous, especially in the North American west, but most have failed, been hidden under earth embankments or been replaced with entirely new structures. Two common variations of timber dams were the crib and the plank. Timber crib dams were erected of heavy timbers or dressed logs in the manner of a log house and the interior filled with earth or rubble. The heavy crib structure supported the dam's face and the weight of the water. Timber plank dams were more elegant structures that employed a variety of construction methods utilizing heavy timbers to support a water retaining arrangement of planks. Very few timber dams are still in use. Timber, in the form of sticks, branches and withes, is the basic material used by beavers, often with the addition of mud or stones.

Embankment dams are made from compacted earth, and have two main types, rock-fill and earth-fill dams. Embankment dams rely on their weight to hold back the force of water, like the gravity dams made from concrete.

Rock-fill dams
A rockfill damRock-fill dams are embankments of compacted free-draining granular earth with an impervious zone. The earth utilized often contains a large percentage of large particles hence the term rock-fill. The impervious zone may be on the upstream face and made of masonry, concrete, plastic membrane, steel sheet piles, timber or other material. The impervious zone may also be within the embankment in which case it is referred to as a core. In the instances where clay is utilized as the impervious material the dam is referred to as a composite dam. To prevent internal erosion of clay into the rock fill due to seepage forces, the core is separated using a filter. Filters are specifically graded soil designed to prevent the migration of fine grain soil particles. When suitable material is at hand, transportation is minimized leading to cost savings during construction. Rock-fill dams are resistant to damage from earthquakes. However, inadequate quality control during construction can lead to poor compaction and sand in the embankment which can lead to liquefaction of the rock-fill during an earthquake. Liquefaction potentical can be reduced by keeping susceptible material from being saturated, and by providing adequate compaction during construction. An example of a rock-fill dam is New Melones Dam in California.

Earth-fill dams, also called earthen, rolled-earth or simply earth dams, are constructed of well compacted earth. A homogeneous rolled-earth dam is entirely constructed of one type of material but may contain a drain layer to collect seep water. A zoned-earth dam has distinct parts or zones of dissimilar material, typically a locally plentiful shell with a watertight clay core. Modern zoned-earth embankments employ filter and drain zones to collect and remove seep water and preserve the integrity of the downstream shell zone. An outdated method of zoned earth dam construction utilized a hydraulic fill to produce a watertight core. Rolled-earth dams may also employ a watertight facing or core in the manner of a rock-fill dam. An interesting type of temporary earth dam occasionally used in high latitudes is the frozen-core dam, in which a coolant is circulated through pipes inside the dam to maintain a watertight region of permafrost within it. Examples of earth-fill dams include Nurek Dam in Tajikistan, the tallest dam in the world, and Oroville Dam, the tallest dam in the United States.

Masonry dams are of either the gravity or the arch type.

Gravity dams
In a gravity dam, stability is secured by making it of such a size and shape that it will resist overturning, sliding and crushing at the toe. The dam will not overturn provided that the moment around the turning point, caused by the water pressure is smaller than the moment caused by the weight of the dam. This is the case if the resultant force of water pressure and weight falls within the base of the dam. However, in order to prevent tensile stress at the upstream face and excessive compressive stress at the downstream face, the dam cross section is usually designed so that the resultant falls within the middle at all elevations of the cross section (the core). For this type of dam, impervious foundations with high bearing strength are essential. The Gilboa Dam in the Catskill Mountains of New York State is an example of a "solid" gravity dam.When situated on a suitable site, a gravity dam inspires more confidence in the layman than any other type; it has mass that lends an atmosphere of permanence, stability, and safety. When built on a carefully studied foundation with stresses calculated from completely evaluated loads, the gravity dam probably represents the best developed example of the art of dam building. This is significant because the fear of flood is a strong motivator in many regions, and has resulted in gravity dams being built in some instances where an arch dam would have been more economical. Gravity dams are classified as "solid" or "hollow." The solid form is the more widely used of the two, though the hollow dam is frequently more economical to construct. Gravity dams can also be classified as "overflow" (spillway) and "non-overflow." Grand Coulee Dam is a solid gravity dam and Itaipu Dam is a hollow gravity dam. With a height of 285m the tallest gravity dam in the world is the Grande Dixence Dam in Switzerland.

Arch dams
In the arch dam, stability is obtained by a combination of arch and gravity action. If the upstream face is vertical the entire weight of the dam must be carried to the foundation by gravity, while the distribution of the normal hydrostatic pressure between vertical cantilever and arch action will depend upon the stiffness of the dam in a vertical and horizontal direction. When the upstream face is sloped the distribution is more complicated. The normal component of the weight of the arch ring may be taken by the arch action, while the normal hydrostatic pressure will be distributed as described above. For this type of dam, firm reliable supports at the abutments (either buttress or canyon side wall) are more important. The most desirable place for an arch dam is a narrow canyon with steep side walls composed of sound rock. The safety of an arch dam is dependent on the strength of the side wall abutments, hence not only should the arch be well seated on the side walls but also the character of the rock should be carefully inspected. The highest arch dam in the world is Inguri Dam in Georgia. It is 272 meters high and it was completed in 1980. Two types of single-arch dams are in use, namely the constant-angle and the constant-radius dam. The constant-radius type employs the same face radius at all elevations of the dam, which means that as the channel grows narrower towards the bottom of the dam the central angle subtended by the face of the dam becomes smaller. Jones Falls Dam, in Canada, is a constant radius dam. In a constant-angle dam, also known as a variable radius dam, this subtended angle is kept a constant and the variation in distance between the abutments at various levels are taken care of by varying the radii. Constant-radius dams are much less common than constant-angle dams. Parker Dam is a constant-angle arch dam. A similar type is the double-curvature or thin-shell dam. Wildhorse Dam near Mountain City, Nevada in the United States is an example of the type. This method of construction minimizes the amount of concrete necessary for construction but transmits large loads to the foundation and abutments. The appearance is similar to a single-arch dam but with a distinct vertical curvature to it as well lending it the vague appearance of a concave lens as viewed from downstream. The multiple-arch dam consists of a number of single-arch dams with concrete buttresses as the supporting abutments. The multiple-arch dam does not require as many buttresses as the hollow gravity type, but requires good rock foundation because the buttress loads are heavy. See Geotechnical engineering.

Steel dams
Intended as permanent structures, steel dams were an (arguably failed) experiment to determine if a construction technique could be devised that was cheaper than masonry, concrete or earthworks, but sturdier than timber crib dams. Only two examples remain in the US.

A cofferdam is a (usually temporary) barrier constructed to exclude water from an area that is normally submerged. Made commonly of wood, concrete or steel sheet piling, cofferdams are used to allow construction on the foundation of permanent dams, bridges, and similar structures. When the project is completed, the cofferdam may be demolished or removed. Common uses for cofferdams include construction and repair of off shore oil platforms. In such cases the cofferdam is fabricated from sheet steel and welded into place under water. Air is pumped into the space, displacing the water allowing a dry work environment below the surface. Upon completion the cofferdam is usually deconstructed unless the area requires continuous maintenance.

Spillways
A spillway is a section of a dam designed to pass water from the upstream side of a dam to the downstream side. Many spillways have floodgates designed to control the flow through the spillway. A service spillway or primary spillway passes normal flow. An auxiliary spillway releases flow in excess of the capacity of the service spillway. An emergency spillway is designed for extreme conditions, such as a serious malfunction of the service spillway. A fuse-plug spillway is a low embankment designed to be overtopped and washed away in the event of a large flood. Any cavitation or turbulence of the water flowing over the spillway slowly erodes the dam's wetted surfaces. To minimize that erosion (especially with maximum water elevation at the crest), the downstream face of the spillway is ordinarily made an ogee curve. It was the inadequate design of the spillway that caused the overtopping of a dam that caused the infamous Johnstown Flood.

Inspirational Posters

Paralumun New Age Village