Hydraulic structureFrom Wikipedia, the free encyclopedia

A hydraulic structure is structure submerged or partially submerged in any body of water, which disrupts the natural flow of water. They can be used to divert, disrupt or completely stop the flow. An example of a hydraulic structure would be a dam, which slows the normal flow rate of the river in order to power turbines. A hydraulic structure can be built in rivers, a sea, or any body of water where there is a need for a change in the natural flow of water.[1]

Hydraulic structures may also be used to measure the flow of water. When used to measure the flow of water, hydraulic structures are defined as a class of specially shaped, static devices over or through which water is directed in such a way that under free-flow conditions at a specified location (point of measurement) a known level to flow relationship exists. Hydraulic structures of this type can generally be divided into two categories: flumes and weirs.[2]

Why do we need dams ?

 

In ancient times, dams were built for the single purpose of water supply or irrigation. As civilizations developed, there was a greater need for water supply, irrigation, flood control, navigation, water quality, sediment control and energy. Therefore, dams are constructed for a specific purpose such as water supply, flood control, irrigation, navigation, sedimentation control, and hydropower. A dam is the cornerstone in the development and management of water resources development of a river basin. The multipurpose dam is a very important project for developing countries, because the population receives domestic and economic benefits from a single investment.

Inscriptions in the sluice of the Marib dam, built in 750 BC

Demand for water is steadily increasing throughout the world. There is no life on earth without water, our most important resource apart from air and land. During the past three centuries, the

amount of water withdrawn from freshwater resources has increased by a factor of 35, world population by a factor of 8. With the present world population of 5.6 billion still growing at a rate of about 90 million per year, and with their legitimate expectations of higher standards of living, global water demand is expected to rise by a further 2-3 percent annually in the decades ahead.

 

But freshwater resources are limited and unevenly distributed. In the high-consumption countries with rich resources and a highly developed technical infrastructure, the many ways of conserving, recycling and re-using water may more or less suffice to curb further growth in supply. In many other regions, however, water availability is critical to any further development above the present unsatisfactorily low level, and even to the mere survival of existing communities or to meet the continuously growing demand originating from the rapid increase of their population. In these regions man cannot forego the contribution to be made by dams and reservoirs to the harnessing of water resources.

 

Aerial view of Sayamaike dam built in the 7th century and still in use today

Seasonal variations and climatic irregularities in flow impede the efficient use of river runoff, with flooding and drought causing problems of catastrophic proportions. For almost 5 000 years dams have served to ensure an adequate supply of water by storing water in times of surplus and releasing it in times of scarcity, thus also preventing or mitigating floods

With their present aggregate storage capacity of about 6 000 km3, dams clearly make a significant contribution to the efficient management of finite water resources that are unevenly distributed and subject to large seasonal fluctuations.

The purposes of dams

 

Most of the dams are single-purpose dams, but there is now a growing number of multipurpose dams. Using the most recent publication of the World Register of Dams, irrigation is by far the most common purpose of dams. Among the single purpose dams, 48 % are for irrigation, 17% for hydropower (production of electricity), 13% for water supply , 10% for flood control, 5% for recreation and less than 1% for navigation and fish farming.

 

Irrigation:

Presently, irrigated land covers about 277 million hectares i.e. about 18% of world's arable land but is responsible for around 40% of crop output and employs nearly 30% of population spread over rural areas. With the large population growth expected for the next decades, irrigation must be expanded to increase the food capacity production. It is estimated that 80% of additional food production by the year 2025 will need to come from irrigated land. Even with the widespread measures to conserve water by improvements in irrigation technology, the construction of more reservoir projects will be required.

"Food grows where water flows"

Hydropower:

Generators in the power plant

Hydroelectric power plants generally range in size from several hundred kilowatts to several hundred megawatts, but a few enormous plants have capacities near 10,000 megawatts in order to supply electricity to millions of people. World hydroelectric power plants have a combined capacity of 675,000 megawatts that produces over 2.3 trillion kilowatt-hours of electricity each year; supplying 24 percent of the world's electricity.

In many countries, hydroelectric power provides nearly all of the electrical power. In 1998, the hydroelectric plants of Norway and the Democratic Republic of the Congo (formerly Zaire) provided 99 percent of each country's power; and hydroelectric plants in Brazil provided 91 percent of

total used electricity.

Electricity generated from dams is by very far the largest renewable energy source in the world. More than 90% of the world's renewable electricity comes from dams. Hydropower also offers unique possibilities to manage the power network by its ability to quickly respond to peak demands. Pumping-storage plants, using power produced during the night, while the demand is low, is used to pump water up to the higher reservoir. That water is then used during the peak demand period to produce electricity. This system today constitute the only economic mass storage available for electricity.

 

Water supply for domestic and industrial use:

 

It has been stressed how essential water is for our civilization. It is important to remember that of the total rainfall falling on the earth, most falls on the sea and a large portion of that which falls on earth ends up as runoff. Only 2% of the total is infiltrated to replenish the groundwater. Properly planned, designed and constructed and maintained dams to store water contribute significantly toward fulfilling our water supply requirements. To accommodate the variations in the hydrologic cycle, dams and reservoirs are needed to store water and then provide more consistent supplies during shortages.

 

Industry facilities like this power plant need million of litters per day. A city like Mumbai in India (16 million inhabitants) need 4300 millions of litters per day (lpd) Melbourne in Australia (4million inhabitants) needs around 1000 millions lpd and Paris in France needs some 700 millions lpd. In each of thes examples, water would not be provided without dams.

Inland navigation

 

Natural river conditions, such as changes in the flow rate and river level, ice and changing river channels due to erosion and sedimentation, create major problems and obstacles for inland navigation. The advantages of inland navigation, however, when compared with highway and rail are the large load carrying capacity of each barge, the ability to handle cargo with large-dimensions and fuel savings. Enhanced inland navigation is a result of comprehensive basin planning and development utilizing dams, locks and reservoirs which are regulated to provide a vital role in realizing regional and national economic benefits. In addition to the economic benefits, a river that has been developed with dams and reservoirs for navigation may also

Large shipment of goods move the locks and dams on inland waterways, such as this tow, on the lower part of the picture.

provide additional benefits of flood control, reduced erosion, stabilized groundwater levels throughout the system and recreation.

Flood control

 

Floods can cause major damage to humans lives, property, and livestocks. Cities and towns have been devastated due to flood damage. Lives have bees lost and homes destoyed. Flooding can cause epidemics due to sewer disposal and contamined water supplies. Dams can play a role in limiting the extent of flood damages

Dams and reservoirs can be effectively used to regulate river levels and flooding downstream of the dam by temporarily storing the flood volume and releasing it later. The most effective method of flood control is accomplished by an integrated water management plan for regulating the storage and discharges of each of the main dams located in a river basin. Each dam is operated by a specific water control plan for routing floods through the basin without damage. This means lowering of the reservoir level to create more storage before the rainy season. This strategy eliminates flooding. The number of dams and their water control management plans are established by

comprehensive planning for economic development and with public involvement. Flood control is a significant purpose for many of the existing dams and continues as a main purpose for some of the major dams of the world currently under construction.

eservoir,  an open-air storage area (usually formed by masonry or earthwork) where water is collected and kept in quantity so that it may be drawn off for use.

Changes in weather cause the natural flow of streams and rivers to vary greatly with time. Periods of excess flows and valley flooding may alternate with low flows or droughts. The role of water-storage reservoirs, therefore, is to impound water during periods of higher flows, thus preventing flood disasters, and then permit gradual release of water during periods of lower flows. Simple storage reservoirs were probably created early in human history to provide water for drinking and for irrigation. From southern Asia and northern Africa the use of reservoirs spread to Europe and the other continents.

Reservoirs ordinarily are formed by the construction of dams across rivers, but off-channel reservoirs may be provided by diversion structures and canals or pipelines that convey water from a river to natural or artificial depressions.

When streamflow is impounded in a reservoir, the flow velocity decreases and sediment is deposited. Thus, streams that transport much suspended sediment are poor sites for reservoirs; siltation will rapidly reduce storage capacity and severely shorten the useful life ... (200 of 679 words)

What is the difference between a dam and a reservoir?

The dam is the object made to hold back the water flow. The reservoir is the collected water that is held back by the dam.Both words can carry the same meaning, but the word 'dam' can also mean the blockage that creates a reservoir. A reservoir can be any pool (like 'of knowledge' or 'of talent').

Water Dikes

Dikes used to hold back water are usually made of earth. Sometimes, dikes occur naturally. More often, people construct dikes to prevent flooding. When constructed along river banks, dikes control the flow of water. By preventing flooding, dikes force the river to flow more quickly and with greater force.

The most familiar material used to build or augment dikes is the sandbag. People will fill cloth bags with sand and pile the sandbags along a river bank or lake shore. The cloth and sand absorb the water, letting very little pass through. Sandbags are very heavy and usually stay in place. Dikes made of sandbags can be many meters tall and twice as wide. They can be built quickly, which is why people living near rivers will start sandbagging as soon as heavy rains start to fall.

Enormous construction equipment can also help build dikes. Bulldozers and dredging machines haul in sand and soil from different areas to a specific line along a body of water. This isolates one part of a river, lake, or ocean from the larger body of water. Once the new dike is established, water from the isolated part is drained out of the area. The land on the drained side of the dike is no longer a body of water.

These dikes, which can be hundreds of miles long, are usually used to create farmland or residential space from a lakebed or even the ocean. The nation of the Netherlands has reclaimed more than a thousand hectares of land from the North Sea by constructing dikes along many tidal basins. The Dutch, people from the Netherlands, use the reclaimed land, called polders, for agriculture, residential, and industrial use. The first dikes in the Netherlands were constructed in the 1200s, and the country continues to maintain and expand the dike system today. In fact, dike is a Dutch word that originally

meant the bank of a body of water.

What is a spillway?

by The Brazos River Authority

A spillway part of a dam that is designed to allow water to flow freely over the dam during floods. Spillways may be used on dams with floodgates as an additional means to control release of water during flooding. A spillway may also be used as the main area of water release from a dam, allowing water to flow through the spillway only when the reservoir is full.

SluiceFrom Wikipedia, the free encyclopedia

A sluice gate

Sluice gates near Henley, on the River Thames

A small wooden sluice in Magome, Japan, used to power a waterwheel

Miners working a small sluice on Lucky Gulch, Alaska

A sluice (from the Dutch "sluis") is a water channel controlled at its head by a gate. A mill race, leet, flume, penstock or lade is a sluice channelling water toward a water mill. The terms sluice, sluice gate, knife gate, and slide gate are used interchangeably in the water and wastewater control industry.

A sluice gate is traditionally a wood or metal barrier sliding in grooves that are set in the sides of the waterway. Sluice gates commonly control water levels and flow rates in rivers and canals. They are also used in wastewater treatment plants and to recover minerals in mining operations, and in watermills.

Operation

"Sluice gate" refers to a movable gate allowing water to flow under it. When a sluice is lowered, water may spill over the top, in which case the gate operates as a weir. Usually, a mechanism drives the sluice up or down. This may be a simple, hand-operated, chain pulled/lowered, worm drive or rack-and-pinion drive, or it may be electrically or hydraulically powered.

Types of sluice gates

Flap sluice gate: A fully automatic type, controlled by the pressure head across it; operation is similar to that of a check valve. It is a gate hinged at the top. When pressure is from one side, the gate is kept closed; a pressure from the other side opens the sluice when a threshold pressure is surpassed.

Vertical rising sluice gate: A plate sliding in the vertical direction, which may be controlled by machinery.

Radial sluice gate: A structure, where a small part of a cylindrical surface serves as the gate, supported by radial constructions going through the cylinder's radius. On occasion, a counterweight is provided.

Rising sector sluice gate: Also a part of a cylindrical surface, which rests at the bottom of the channel and rises by rotating around its centre.

Needle sluice: A sluice formed by a number of thin needles held against a solid frame through water pressure as in a needle dam.

The gates of a Guillotine lock work in a way similar to a sluice gate, but most canal lock gates are hinged to swing like doors.

Logging sluicesSee also: Log driving and Timber rafting

In the mountains of the United States, sluices transported logs from steep hillsides to downslope sawmill ponds or yarding areas. Nineteenth-century logging was traditionally a winter activity for men who spent summers working on farms. Where there were freezing nights, water might be applied to logging sluices every night so a fresh coating of slippery ice would reduce friction of logs placed in the sluice the following morning.[1]