Selection of site for hydro-electric power plant

While selecting a suitable site for a hydro-electric power plant, if a good system of natural storage-lakes at high altitudes and with large catchment areas can be located, the plant will be comparatively economical. Any how the essential characteristics of a good site are large catchment area, high average rain fall and a favorable place for constructing the storage or reservoir. The land should be cheap in cost and rocky in order to withstand the weight of large building and heavy machinery. There should be possibility of providing adequate transportation facilities so that the necessary equipment and machinery could be easily transported.

Discussing the Factors Affecting the Location of Hydroelectric Power Plant Dams:The dam or water reservoir is a crucial part of the hydroelectric power plants. Water stored in the dams is used for generating electricity in hydroelectric power plants. This article discusses factors affecting the location of dams for hydroelectric power plants.

Issues with DamsThe dam also called as water reservoir is the most important part of the hydroelectric power plants. All the water that is used for generation of electricity in the hydroelectric power plants is stored in the dam. Since huge quantities of water are stored in the dam, it is very important that the bed and walls of the dam should be able to sustain all the hydraulic pressures of water. Water has mass and large quantities of water have huge weight which is exerted on the bed and the walls of the dam. If the walls of the dam are not strong enough to sustain the forces of water, the walls will break and water will spread to the surrounding areas producing devastating floods that have potential to cause large scale destruction of human, animal and plant life.

Factors Affecting the Selection of Site for Dams:Apart from the construction of the dam, selecting proper site for the dam is very crucial. Selecting the proper site will help carrying out construction of the strong dam and it will also help reduce risks due to natural disasters like earth quake. Here are some of the important factors to be considered while selecting the site for the dam for hydroelectric power plants:

Factors Affecting the Location of Hydroelectric Power Plant DamsThe dam or water reservoir is a crucial part of the hydroelectric power plants. Water stored in the dams is used for generating electricity in hydroelectric power plants. This article discusses factors affecting the location of dams for hydroelectric power plants.

Issues with DamsThe dam also called as water reservoir is the most important part of the hydroelectric power plants. All the water that is used for generation of electricity in the hydroelectric power plants is stored in the dam. Since huge quantities of water are stored in the dam, it is very important that the bed and walls of the dam should be able to sustain all the hydraulic pressures of water. Water has mass and large quantities of water have huge weight which is exerted on the bed and the

walls of the dam. If the walls of the dam are not strong enough to sustain the forces of water, the walls will break and water will spread to the surrounding areas producing devastating floods that have potential to cause large scale destruction of human, animal and plant life.

Factors Affecting the Selection of Site for DamsApart from the construction of the dam, selecting proper site for the dam is very crucial. Selecting the proper site will help carrying out construction of the strong dam and it will also help reduce risks due to natural disasters like earth quake. Here are some of the important factors to be considered while selecting the site for the dam for hydroelectric power plants:

1) Good topographical location along the path of river : The best location along the path of the river is river canyon or at the location where there is narrowing of the river. If the aim is to store maximum amount of water, then the volume of basin above dam should be calculated so that sufficient quantity of water can be stored in it. The perfect site is one where there is wide and flat valley.

2) Right geological structure: The rock structure on which the dam will be constructed should be strong enough to sustain the weight of dam and water stored in the dam. The rock structure should be able to sustain all the visible and invisible forces. The rock structure should be stable and there should be least occurrence of the earthquakes in the region. The rock structure should not allow the seepage of water and it should be waterproof.3) Sufficient water is available: The flow of water where dam is constructed should be sufficient enough to fill the dam. There is lots of loss of water from dam due to evaporation, the flow of river water should be able accommodate this loss of water without affecting the production of electricity from the hydroelectric power plant.People living around the areas where storage basin is going to be constructed and the areas that will be submerged should be convinced to move from there and they should be given proper compensation and suitable resettlement areas. If this factor is ignored the chances of the success of the hydroelectric power plant will reduced.

So it can be concluded that:Anyhow the essential characteristics of a good site are:

large catchment area, high average rain fall and A favorable place for constructing the storage or reservoir. The land should be cheap in cost and rocky in order to withstand the weight of large

building and heavy machinery. There should be possibility of providing adequate transportation facilities so that the

necessary equipment and machinery could be easily transported.

Hydrology: Hydrological cycle, precipitation

Hydrology:

Water is one of our most important natural resources. Without it, there would be no life on earth. The supply of water available for our use is limited by nature. Although there is plenty of water on earth, it is not always in the right place, at the right time and of the right quality. Adding to the problem is the increasing evidence that chemical wastes improperly discarded yesterday are showing up in our water supplies today. Hydrology has evolved as a science in response to the need to understand the complex water systems of the Earth and help solve water problems. Hydrologists play a vital role in finding solutions to water problems. Hence Hydrology can be defined as the science that encompasses the occurrence, distribution, movement and properties of the waters of the earth and their relationship with the environment within each phase of the hydrologic cycle.

Hydrological cycle:

The water cycle, or hydrologic cycle, is a continuous process by which water is purified by evaporation and transported from the earth's surface (including the oceans) to the atmosphere and back to the land and oceans. All of the physical, chemical and biological processes involving water as it travels its various paths in the atmosphere, over and beneath the earth's surface and through growing plants, are of interest to those who study the hydrologic cycle. There are many pathways the water may take in its continuous cycle of falling as rainfall or snowfall and returning to the atmosphere. It may be captured for millions of years in polar ice caps. It may flow to rivers and finally to the sea. It may soak into the soil to be evaporated directly from the soil surface as it dries or be transpired by growing plants. It may percolate through the soil to ground water reservoirs (aquifers) to be stored or it may flow to wells or springs or back to streams by seepage. They cycle for water may be short, or it may take millions of years. People tap the water cycle for their own uses. Water is diverted temporarily from one part of the cycle by pumping it from the ground or drawing it from a river or lake. It is used for a variety of activities such as households, businesses and industries; for irrigation of farms and parklands; and for production of electric power. After use, water is returned to another part of the cycle: perhaps discharged downstream or allowed to soak into the ground. Used water normally is lower in quality, even after treatment, which often poses a problem for downstream users. The hydrologist studies the fundamental transport processes to be able to describe the quantity and quality of water as it moves through the cycle (evaporation, precipitation, streamflow, infiltration, ground water flow, and other components). The engineering hydrologist, or water resources engineer, is involved in the planning, analysis, design, construction and operation of projects for the control, utilization, and management of water resources.

Description of the Hydrologic Cycle

This is an education module about the movement of water on the planet Earth. The module includes a discussion of water movement in the United States, and it also provides specific information about water movement in Oregon.

The scientific discipline in the field of physical geography that deals with the water cycle is called hydrology. It is concerned with the origin, distribution, and properties of water on the globe. Consequently, the water cycle is also called the hydrologic cycle in many scientific textbooks and educational materials. Most people have heard of the science of meteorology and many also know about the science of oceanography because of the exposure that each discipline has had on television. People watch TV weather personalities nearly every day. Celebrities such as Jacques Cousteau have helped to make oceanography a commonly recognized science. In a broad context, the sciences of meteorology and oceanography describe parts of a series of global physical processes involving water that are also major components of the science of hydrology. Geologists describe another part of the physical processes by addressing groundwater movement within the planet's subterranean features. Hydrologists are interested in obtaining measurable information and knowledge about the water cycle. Also important is the measurement of the amount of water involved in the transitional stages that occur as the water moves from one process within the cycle to other processes. Hydrology, therefore, is a broad science that utilizes information from a wide range of other sciences and integrates them to quantify the movement of water. The fundamental

tools of hydrology are based in supporting scientific techniques that originated in mathematics, physics, engineering, chemistry, geology, and biology. Consequently, hydrology uses developed concepts from the sciences of meteorology, climatology, oceanography, geography, geology, glaciology, limnology (lakes), ecology, biology, agronomy, forestry, and other sciences that specialize in other aspects of the physical, chemical or biological environment. Hydrology, therefore, is one of the inter disciplinary sciences that is the basis for water resources development and water resources management. The global water cycle can be described with nine major physical processes which form a continuum of water movement. Complex pathways include the passage of water from the gaseous envelope around the planet called the atmosphere, through the bodies of water on the surface of earth such as the oceans, glaciers and lakes, and at the same time (or more slowly) passing through the soil and rock layers underground. Later, the water is returned to the atmosphere. A fundamental characteristic of the hydrologic cycle is that it has no beginning and it has no end. It can be studied by starting at any of the following processes: evaporation, condensation, precipitation, interception, infiltration, percolation, transpiration, runoff,andstorage. The information presented below is a greatly simplified description of the major contributing physical processes.

They include:

EVAPORATION

Evaporation occurs when the physical state of water is changed from a liquid state to a gaseous state. A considerable amount of heat, about 600 calories of energy for each gram of water, is exchanged during the change of state. Typically, solar radiation and other factors such as air temperature, vapor pressure, wind, and atmospheric pressure affect the amount of natural evaporation that takes place in any geographic area. Evaporation can occur on raindrops, and on free water surfaces such as seas and lakes. It can even occur from water settled on vegetation, soil, rocks and snow. There is also evaporation caused by human activities. Heated buildings experience evaporation of water settled on its surfaces. Evaporated moisture is lifted into the atmosphere from the ocean, land surfaces, and water bodies as water vapor. Some vapor always exists in the atmosphere.

CONDENSATION

Condensation is the process by which water vapor changes it's physical state from a vapor, most commonly, to a liquid. Water vapor condenses onto small airborne particles to form dew, fog, or clouds. The most active particles that form clouds are sea salts, atmospheric ions caused by lightning,and combustion products containing sulfurous and nitrous acids. Condensation is brought about by cooling of the air or by increasing the amount of vapor in the air to its saturation point. When water vapor condenses back into a liquid state, the same large amount of heat ( 600 calories of energy per gram) that was needed to make it a vapor is released to the environment.

PRECIPITATION

Precipitation is the process that occurs when any and all forms of water particles fall from the atmosphere and reach the ground. There are two sub-processes that cause clouds to release precipitation, the coalescence process and the ice-crystal process. As water drops reach a critical size, the drop is exposed to gravity and frictional drag. A falling drop leaves a turbulent wake behind which allows smaller drops to fall faster and to be overtaken to join and combine with the lead drop. The other sub-process that can occur is the ice-crystal formation process. It occurs when ice develops in cold clouds or in cloud formations high in the atmosphere where freezing temperatures occur. When nearby water droplets approach the crystals some droplets evaporate and condense on the crystals. The crystals grow to a critical size and drop as snow or ice pellets. Sometimes, as the pellets fall through lower elevation air, they melt and change into raindrops.

Precipitated water may fall into a waterbody or it may fall onto land. It is then dispersed several ways. The water can adhere to objects on or near the planet surface or it can be carried over and through the land into stream channels, or it may penetrate into the soil, or it may be intercepted by plants.

When rainfall is small and infrequent, a high percentage of precipitation is returned to the atmosphere by evaporation.

The portion of precipitation that appears in surface streams is called runoff. Runoff may consist of component contributions from such sources as surface runoff, subsurface runoff, or ground water runoff. Surface runoff travels over the ground surface and through surface channels to leave a catchment area called a drainage basin or watershed. The portion of the surface runoff that flows over the land surface towards the stream channels is called overland flow. The total runoff confined in the stream channels is called the stream flow.

INTERCEPTION

Interception is the process of interrupting the movement of water in the chain of transportation events leading to streams. The interception can take place by vegetal cover or depression storage in puddles and in land formations such as rills and furrows.

When rain first begins, the water striking leaves and other organic materials spreads over the surfaces in a thin layer or it collects at points or edges. When the maximum surface storage capability on the surface of the material is exceeded, the material stores additional water in growing drops along its edges. Eventually the weight of the drops exceed the surface tension and water falls to the ground. Wind and the impact of rain drops can also release the water from the organic material. The water layer on organic surfaces and the drops of water along the edges are also freely exposed to evaporation.

Additionally, interception of water on the ground surface during freezing and sub-freezing

conditions can be substantial. The interception of falling snow and ice on vegetation also occurs. The highest level of interception occurs when it snows on conifer forests and hardwood forests that have not yet lost their leaves.

INFILTRATION

Infiltration is the physical process involving movement of water through the boundary area where the atmosphere interfaces with the soil. The surface phenomenon is governed by soil surface conditions. Water transfer is related to the porosity of the soil and the permeability of the soil profile. Typically, the infiltration rate depends on the puddling of the water at the soil surface by the impact of raindrops, the texture and structure of the soil, the initial soil moisture content, the decreasing water concentration as the water moves deeper into the soil filling of the pores in the soil matrices, changes in the soil composition, and to the swelling of the wetted soils that in turn close cracks in the soil.

Water that is infiltrated and stored in the soil can also become the water that later is evapotranspired or becomes subsurface runoff.

PERCOLATION

Percolation is the movement of water though the soil, and it's layers, by gravity and capillary forces. The prime moving force of groundwater is gravity. Water that is in the zone of aeration where air exists is called vadose water. Water that is in the zone of saturation is called groundwater. For all practical purposes, all groundwater originates as surface water. Once underground, the water is moved by gravity. The boundary that separates the vadose and the saturation zones is called the water table. Usually the direction of water movement is changed from downward and a horizontal component to the movement is added that is based on the geologic boundary conditions.

Geologic formations in the earth's crust serve as natural subterranean reservoirs for storing water. Others can also serve as conduits for the movement of water. Essentially, all groundwater is in motion. Some of it, however, moves extremely slowly. A geologic formation which transmits water from one location to another in sufficient quantity for economic development is called an aquifer. The movement of water is possible because of the voids or pores in the geologic formations. Some formations conduct water back to the ground surface. A spring is a place where the water table reaches the ground surface. Stream channels can be in contact with an unconfined aquifer that approach the ground surface. Water may move from the ground into the stream, or visa versa, depending on the relative water level. Groundwater discharges into a stream forms the base flow of the stream during dry periods, especially during droughts. An influent stream supplies water to an aquifer while and effluent stream receives water from the aquifer.

TRANSPIRATIONTranspiration is the biological process that occurs mostly in the day. Water inside of plants is transferred from the plant to the atmosphere as water vapor through numerous individual leave openings. Plants transpire to move nutrients to the upper portion of the plants and to cool the

leaves exposed to the sun. Leaves undergoing rapid transpiration can be significantly cooler than the surrounding air. Transpiration is greatly affected by the species of plants that are in the soil and it is strongly affected by the amount of light to which the plants are exposed. Water can be transpired freely by plants until a water deficit develops in the plant and it water-releasing cells (stomata) begin to close. Transpiration then continues at a must slower rate. Only a small portion of the water that plants absorb are retained in the plants.

Vegetation generally retards evaporation from the soil. Vegetation that is shading the soil, reduces the wind velocity. Also, releasing water vapor to the atmosphere reduces the amount of direct evaporation from the soil or from snow or ice cover. The absorption of water into plant roots, along with interception that occurs on plant surfaces offsets the general effects that vegetation has in retarding evaporation from the soil. The forest vegetation tends to have more moisture than the soil beneath the trees.

RUNOFF

Runoff is flow from a drainage basin or watershed that appears in surface streams. It generally consists of the flow that is unaffected by artificial diversions, storages or other works that society might have on or in a stream channel. The flow is made up partly of precipitation that falls directly on the stream , surface runoff that flows over the land surface and through channels, subsurface runoff that infiltrates the surface soils and moves laterally towards the stream, and groundwater runoff from deep percolation through the soil horizons. Part of the subsurface flow enters the stream quickly, while the remaining portion may take a longer period before joining the water in the stream. When each of the component flows enter the stream, they form the total runoff. The total runoff in the stream channels is called stream flow and it is generally regarded as direct runoff or base flow.

STORAGE

There are three basic locations of water storage that occur in the planetary water cycle. Water is stored in the atmosphere; water is stored on the surface of the earth, and water stored in the ground.

Water stored in the atmosphere can be moved relatively quickly from one part of the planet to another part of the planet. The type of storage that occurs on the land surface and under the ground largely depend on the geologic features related to the types of soil and the types of rocks present at the storage locations. Storage occurs as surface storage in oceans, lakes, reservoirs, and glaciers; underground storage occurs in the soil, in aquifers, and in the crevices of rock formations.

Run-off and its measurement, hydrograph, flow duration and mass curves

The relationship between depth of rainfall and the resulting depth of runoff is mainly dependent on soil infiltration. The type of land use or plant cover, agricultural management, hydrologic conditions, soil type distribution, and soil moisture status all affect the proportion of water running off a given watershed area. Higher amounts of overland flow are usually responsible for undesirable soil detachment, transport, deposition, and flooding. Thus, from an environmental and a social viewpoint, runoff must be adequately evaluated and minimized by soil conservation practices.

RUNOFF

Runoff forms

There are two situations when runoff will occur. If the intensity of rainfall exceeds the infiltration rate at the ground surface, ponding will lead to surface flow. Alternatively, when the soil surface is saturated there will be surface flow when the rainfall intensity exceeds the percolation through the whole soil profile, a combination of downward movement to groundwater and lateral movement to seepage flow.

The rate of runoff is required for the design of drains, canals and other channels, and for the prediction of water levels in streams and rivers. Quantity of runoff is required when storage is involved for irrigation, power generation, river transport etc. Estimates of quantity must include annual or longer term variations, minimum yield and reliability.

Quality of runoff is increasingly a matter of concern; chemical pollution, from health and environmental aspects; sedimentation, because of interference with drainage, land use, irrigation and power generation. The prediction model CREAMS (Chemical Runoff and Erosion from Agricultural Management Systems), currently widely used in the USA, includes estimates of chemical pollution and sediment from agricultural sources.

Runoff Measuring Devices

Unites of measurement of water:

Water is measured under two conditions –at rest and in motion water

Water at rest:

Water in reservoirs ponds soil, tank] it is measured in units of volume such as liter, cubic meters, hectare centimeter and hectare –meter

Water in motion:

[water flowing in rivers canal ,pipelines ,field channels ]it is expressed in rate of flow such as liters per second liters per hour ,cubic meters per second hectare centimeter pre hr and hectare –meter per day                       11) One liter = ---------      cubic meter                   1000

2) 1 ha –cm =100 meter cube   =1, 00, 000 liter

3) 1 ha m =10000 meter cube =10 million liter

Measuring Device:

In field most commonly used devices for measuring water are1. Weirs2. Pre shall flumes3. Orifices4. Meter gates In these devices ,the react of flow is measured directly by making reading on a scale which is part of instrument and computing the discharge rate from standard formulae .choice for the use of one or others devise depends on the expected flow rates site conditions.

Weir:

Weirs are used to measure the flow of runoff; an irrigation channel or discharged of a well or channel outlet at the source.

A weir is a notch of regular form through which the water stream is made to flow. A weir consists of a weir wall of concrete, timber or metal. Weir may be built as stationary structure or they may be made portable. The notch may be rectangular, trapezoidal or triangular.

It is desirable to install the weir at a point where there is dropped in elevation of channel bed.

Terminology:

Weir pond:

Portion of channel immediately upstream from the weir

Weir crest:

Bottom of weir notch

Head:

Depth of water flowing over the weir crest measured at some point in weir pond.

Sharp crested weir:

A weir having thin edged crest such that over flowing sheet of water has the minimum surface contact with crest.

End contraction:

Horizontal distance from end of weir crests to side of weir pond.

Bottom contraction:

Vertical distance from weir crests to bottom of weir pond.

Weir scale or gauge:

The scale fastened on the side of air or a stake in weir pond to measure the head on weir crest.

Nape:

The sheet of water which over flow a weir

Weirs:

1. Sharp Crested:

A. Rectangular weir:B. Cipoletfe weir C.V-notch weir

2. Broad crested.

a. Rectangular Weirs:

It takes its name from the shape of notch. They are used to measure comparatively large discharges.

b .Cipoletfe Weir:

It is contracted trapezoidal weir in which each side of notch has a slope of 1 horizontal to 4 vertical. It is named after its inventor “Cesare Cipolletti” an Italian engineer. It is used to measure medium discharges.

c. V- Notch Weir:

The 90o V- notch weirs are commonly used to measure small and medium size streams. The advantage of V- notch weir is its ability to measure small flows accurately.

Hydrograph:

A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, or other channel or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs).

It can also refer to a graph showing the volume of water reaching a particular outfall, or location in a sewerage network, graphs are commonly used in the design of sewerage, more specifically, the design of surface water sewerage systems and combined sewers.

TerminologyThe discharge is measured at a specific point in a river and is typically time variant.

Rising limb: The rising limb of hydro graph, also known as concentration curve, reflects a prolonged increase in discharge from a catchment area, typically in response to a rainfall event

Recession (or falling) limb: The recession limb extends from the peak flow rate onward. The end of stormflow (aka quickflow or direct runoff) and the return to groundwater-derived flow (base flow) is often taken as the point of inflection of the recession limb. The recession limb represents the withdrawal of water from the storage built up in the basin during the earlier phases of the hydrograph.

Peak discharge: the highest point on the hydro graph when the rate of discharge is greatest

Lag time: the time interval from the center of mass of rainfall excess to the peak of the resulting hydrograph

Time to peak: time interval from the start of the resulting hydro graph Discharge: the rate of flow (volume per unit time) passing a specific location in a river or

other channel

Types of hydrograph can include:

Storm hydrographs Flood hydrographs Annual hydrographs Direct Runoff Hydrograph Effective Runoff Hydrograph Raster Hydrograph

Storage opportunities in the drainage network (e.g., lakes, reservoirs, wetlands, channel and bank storage capacity)

flow duration and mass curves:

Flow-duration curves have been in general use since about 1915; their theory has been discussed by Foster and others. The flow-duration curve is a cumulative frequency curve that shows the percent of time during which specified discharges were equaled or exceeded in a given period. The curve tells us at what percentage if time a certain discharge will be exceeded or equalled. Flow duration curve analysis is a method involving the frequency of historical flow data over a specified period. Typically, low flows (flow during prolonged dry spells)are exceeded a majority of the time, while high

flows, such as those resulting in floods, are exceeded infrequently. Flow duration curve is a useful form to represent the run-off data for the given time. This curve is plotted between flow available during a period versus the fraction of time. The flow duration curve is drawn with the help of hydrograph from the available run-off data and is necessary to find out the time duration for which flows available.

Fig(1)

A basic flow duration curve measures high flows to low flows along the X-axis(Figure 1). The X-axis represents the percentage of time(known as duration or frequency of occurrence) that a particular flow value is equaled or exceeded. The Y-axis represents the quantity of flow at a given time step, e.g.,cubic feetper second(cfs), associated with the duration. Flow duration intervals are expressed as percentage of exceedance, with zero corresponding to the highest stream discharge in the record (i.e., flood conditions) and 100 to the lowest (i.e., drought conditions). For instance, a flow duration interval of 35% associated with a stream discharge of 11 cfs implies that 35%of all observed daily average stream discharge values equal or exceed 11 cfs.

Mass curve is a plot of the cumulative inflows and outflows versus time used for calculating the storage capacity of a hydro unit. The mass curve usually has a wavy configuration in which the steeper segments represent high flow periods and flatter segments represent low flows. Uniform rates of withdrawal (draft) may be represented as tangent lines drawn from high points to intersect the curve at the next wave. The vertical distance between the draft line and the basic curve represents the cumulative difference between regulated outflow and natural inflow, or the required storage. If the draft line does not intersect the mass curve at the end of a year, it means that the reservoir does not refill with that rate of draft and regulation at the proposed draft rate will extend over two years or more. A typical mass curve is shown in the Figure follows:

Estimation of amount stored by a dam across the river, Storage and Pondage

Dams are structures that restrict the flow of water in a river or stream. Both streams and rivers are bodies of flowing surface water driven by gravity that drain water from the continents. Once a body of flowing surface water has been slowed or stopped, a reservoir or lake collects behind the dam. Dams and reservoirs exist in nature, and man-made water control structures are patterned after examples in the natural word. Many lakes are held back by rock dams created by geologic events such as volcanic eruptions, landslides or the upward force of Earth that creates mountains. Humans and beavers alike have discovered how to modify their natural environment to suit their needs by constructing dams and creating artificial lakes.

Dams are classified into four main types: gravity, embankment, buttress, and arch.

• Gravity dams: Gravity dams are massive earth, masonry (brick or stone work), rock fill, or concrete structures that hold back river water with their own weight. They are usually triangular with their point in a narrow gorge (deep ravine). The Grand Dixence dam in the Swiss Alps is the world’s tallest gravity dam.

• Embankment dams: Embankment dams are wide areas of compacted earth or rock fill with a concrete or masonry core that contains a reservoir, while allowing for some saturation and shifting of the earth around the dam, and of the dam within the earth.

• Buttress dams: Buttress dams have supports that reinforce the walls of the dam and can be curved or straight. Buttresses on large modern dams, such as the Itaipu dam in Brazil, are often constructed as a series of arches and are made of concrete reinforced with steel.

• Arch dams: Arch dams are curved dams that depend on the strength of the arch design to hold back water. Like gravity dams, they are most suited to narrow, V-shaped river valleys with solid rock to anchor the structure. Arch dams, however, can be much thinner than gravity dams and use less concrete.

Let's say that there is a small dam in your area that is not used to produce electricity. Maybe the dam is used to provide water to irrigate farmlands or maybe it was built to make a lake for recreation. As we explained above, you need to know two things:

1. How far the water falls. From talking to the person who operates the dam, we learn that the dam is 10 feet high, so the water falls 10 feet.

2. Amount of water flowing in the river. We contact the United States Geological Survey, the agency in the U.S. that measures river flow, and learn that the average amount of water flowing in our river is 500 cubic feet per second.

Now all we need to do is a little mathematics. It has been found that we can calculate the power of a dam using the following formula:

Power = (Height of Dam) x (River Flow) x (Efficiency) / 11.8

Power The electric power in kilowatts (one kilowatt equals 1,000 watts).Height of Dam The distance the water falls measured in feet.River Flow The amount of water flowing in the river measured in cubic feet per second.

Efficiency

How well the turbine and generator convert the power of falling water into electric power. For older, poorly maintained hydroplants this might be 60% (0.60) while for newer, well operated plants this might be as high as 90% (0.90).

11.8 Converts units of feet and seconds into kilowatts.

For the dam in our area, lets say we buy a turbine and generator with an efficiency of 80%.

Then the power for our dam will be:

Power = (10 feet) x (500 cubic feet per second) x (0.80) / 11.8 = 339 kilowatts

To get an idea what 339 kilowatts means, let's see how much electric energy we can make in a year.

Since electric energy is normally measured in kilowatt-hours, we multiply the power from our dam by the number of hours in a year.

Electric Energy = (339 kilowatts) x (24 hours per day) x (365 days per year) = 2,969,000 kilowatt hours.

The average annual residential energy use in the U.S. is about 3,000 kilowatt-hours for each person. So we can figure out how many people our dam could serve by dividing the annual energy production by 3,000.

People Served = 2,969,000 kilowatts-hours / 3,000 kilowatt-hours per person) = 990 people.

So our local irrigation or recreation dam could provide enough renewable energy to meet the residential needs of 990 people if we added a turbine and generator.

Storage and Pondage :

Pondage usually refers to the comparably small water storage behind the weir of a run-of-the-river hydroelectric power plant. Such a power plant has considerably less storage than the reservoirs of large dams and conventional hydroelectric stations which can store water for long periods such as a dry season or year. With pondage, water is usually stored during periods of low electricity demand and days when the power plant is inactive, enabling its use as a peaking power plant in dry seasons and a base load power plant during wet seasons. Ample pondage allows a power plant meet hourly load fluctuations for a period of a week or more.

Turbines: Operational principle of KaplanA turbine,  is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. A turbine is a turbomachine with at least one moving part called a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades so that they move and impart rotational energy to the rotor. Early turbine examples are windmills and waterwheels.

Gas, steam, and water turbines have a casing around the blades that contains and controls the working fluid. Credit for invention of the steam turbine is given both to the British engineer  Sir Charles Parsons (1854–1931), for invention of the reaction turbine and to Swedish engineer Gustaf de Laval (1845–1913), for invention of the impulse turbine. Modern steam turbines frequently employ both reaction and impulse in the same unit, typically varying the degree of reaction and impulse from the blade root to its periphery.

The Kaplan turbine is a propeller-type water turbine which has adjustable blades. It was developed in 1913 by the Austrian professorViktor Kaplan, who combined automatically adjusted propeller blades with automatically adjusted wicket gates to achieve efficiency over a wide range of flow and water level.

The Kaplan turbine was an evolution of the Francis turbine. Its invention allowed efficient power production in low-head applications that was not possible with Francis turbines. The head ranges from 10–70 meters and the output from 5 to 200 MW. Runner diameters are between 2 and 11 meters. The range of the turbine rotation is from 79 to 429 rpm. The Kaplan turbine installation believed to generate the most power from its nominal head of 34.65m is as of 2013 the Tocoma Power Plant (Venezuela) Kaplan turbine generating 235MW with each of ten 4.8m diameter runners.

Kaplan turbines are now widely used throughout the world in high-flow, low-head power production.

Theory of operation

Vertical Kaplan Turbine (courtesy Voith-Siemens).

The Kaplan turbine is an inward flow reaction turbine, which means that the working fluid changes pressure as it moves through the turbine and gives up its energy. Power is recovered from both the hydrostatic head and from the kinetic energy of the flowing water. The design combines features of radial and axial turbines.

The inlet is a scroll-shaped tube that wraps around the turbine's wicket gate. Water is directed tangentially through the wicket gate and spirals on to a propeller shaped runner, causing it to spin.

The outlet is a specially shaped draft tube that helps decelerate the water and recover kinetic energy.

The turbine does not need to be at the lowest point of water flow as long as the draft tube remains full of water. A higher turbine location, however, increases the suction that is imparted on the turbine blades by the draft tube. The resulting pressure drop may lead to cavitation.

Variable geometry of the wicket gate and turbine blades allow efficient operation for a range of flow conditions. Kaplan turbine efficiencies are typically over 90%, but may be lower in very low head applications.

Current areas of research include CFD driven efficiency improvements and new designs that raise survival rates of fish passing through.

Because the propeller blades are rotated on high-pressure hydraulic oil bearings, a critical element of Kaplan design is to maintain a positive seal to prevent emission of oil into the waterway. Discharge of oil into rivers is not desirable because of the waste of resources and resulting ecological damage.

Applications

Viktor Kaplan Turbine Technisches Museum Wien

Kaplan turbines are widely used throughout the world for electrical power production. They cover the lowest head hydro sites and are especially suited for high flow conditions.

Inexpensive micro turbines on the Kaplan turbine model are manufactured for individual power production with as little as two feet of head.

Large Kaplan turbines are individually designed for each site to operate at the highest possible efficiency, typically over 90%. They are very expensive to design, manufacture and install, but operate for decades.

They have recently found a new home in offshore wave energy generation, see Wave Dragon

The Kaplan turbine is the most widely used of the propeller-type turbines, but several other variations exist:

Propeller turbines have non-adjustable propeller vanes. They are used in where the range of flow / power is not large. Commercial products exist for producing several hundred watts from only a few feet of head. Larger propeller turbines produce more than 100 MW. At the La Grande-1 generating stationin northern Quebec, 12 propeller turbines generate 1368 MW.

Bulb or tubular turbines are designed into the water delivery tube. A large bulb is centered in the water pipe which holds the generator, wicket gate and runner. Tubular turbines are a fully axial design, whereas Kaplan turbines have a radial wicket gate.

Pit turbines are bulb turbines with a gear box. This allows for a smaller generator and bulb. Straflo turbines are axial turbines with the generator outside of the water channel, connected

to the periphery of the runner. S-turbines eliminate the need for a bulb housing by placing the generator outside of the water

channel. This is accomplished with a jog in the water channel and a shaft connecting the runner and generator.

The VLH turbine an open flow, very low head "kaplan" turbine slanted at an angle to the water flow. It has a large diameter >3.55m, is low speed using a directly connected shaft mounted permanent magnet alternator with electronic power regulation and is very fish friendly (<5% mortality).

Tyson turbines are a fixed propeller turbine designed to be immersed in a fast flowing river, either permanently anchored in the river bed, or attached to a boat or barge.