Read interesting facts about water, matching questions with the answers.

ПРЕДИСЛОВИЕ

Пособие предназначено для студентов, специализирующихся в области гидротехнического строительства и имеющих базовую начальную подготовку по английскому языку: знают фонологическую систему, знакомы с основными грамматическими категориями и владеют определенным объемом лексических единиц и речевых моделей. Пособие подготовлено в соответствии с требованиями типовой программы по иностранным языкам для высших учебных заведений (2008г.). Целью пособия является совершенствование и систематизация знаний и умений студентов, обогащение их словарного запаса в пределах предлагаемой тематики, формирование навыков правильного понимания, перевода и реферирования текстов по специальности, дальнейшее развитие разговорных навыков на основе письменного текста.

Пособие состоит из девяти разделов. Материалом пособия послужили оригинальные тексты. Наряду с текстами в каждом разделе предлагаются разнообразные языковые и речевые упражнения. Языковые упражнения используются для обучения студентов структурным элементам языка и их систематизации, активному усвоению профессиональной лексики и повторению некоторых аспектов грамматики. Большое внимание уделяется работе с профессиональной лексикой. Используются задания на понимание смысла через толкование соответствующих терминов, нахождение синонимов и антонимов, словообразование. Речевые упражнения позволяют проверить общее понимание прочитанного, закрепить приобретенные лексические навыки и предполагают обучение говорению на базе прочитанного текста. Предлагаемые для перевода мини-тексты позволяют практиковаться в корректном употреблении терминов. В конце каждого раздела предусмотрены задания на развитие навыков монологической речи с привлечением дополнительной информации, что, с одной стороны, стимулирует научный поиск, а с другой – формирует у студентов навыки общения с аудиторией и ведения дискуссии на английском языке.

 

UNIT I WATER ON THE EARTH

Warming-up

Read interesting facts about water, matching questions with the answers.

 

1) How much water is on the earth?

2) How much of the earth's water is fresh?

The largest single use of water is by industry. It takes about 300 liters of water to make the paper for one Sunday newspaper, and about 20 gallons of water per pound (170 liters per kilogram) of steel produced.

On the av­erage, each person in the United States uses more than 380 liters of water a day in the home.

 

 

3) How much water do living things contain?

4) How much water does a person take in over a lifetime?

All living things consist mostly of water. For example, the body of a human being is about 65 % water. An elephant is about 70 % water. A potato is about 80 %.

 

There are about 1.4 billion cubic kilometers of water.

 

5) What are the different forms of water?

6) How much water does a person use every day?

On the average, a person takes in about 60,600 litres of water during his or her life.

Only about 3 % of the earth's water. About three-fourths of the earth's fresh water is frozen in icecaps and other glaciers.

 

 

Water is the only substance on earth that is naturally present in three different forms—as a liquid, a solid (ice), and a gas (water vapour).

7) What is the largest single use of water?

Water is used and reused over and over again—it is never used up. Every glass of water you drink contains molecules of water that have been used count­less times before.

8) Can water ever be used up?

Reading Task: A

Read the text carefully paying attention to the terms in italics. Answer the following question.

 

Why is water never used up?

Text A Water

 

Water is the most common substance on earth. It covers more than 70 per cent of the earth's surface. It fills the oceans, rivers, and lakes, and is in the ground and in the air we breathe. Water is everywhere.

Without water, there can be no life. In fact, every living thing consists mostly of water. Your body is about two-thirds water. A chicken is about three-fourths water, and a pineapple is about four-fifths water. Most scientists believe that life itself began in water—in the salty water of the sea.

Water resources on the Earth

Water in Hydrosphere 1 386 000 000 km^3 100%

Fresh Water 35 000 000 km^3 2.5%

Salt Water 1 351 000 000 km^3 97.5%

Permafrost, ice, etc. 24 300 000 km^3 69.4%

Liquid 10 700 000 km^3 30.6 %

Ever since the world began, water has been shaping the earth. Rain hammers at the land and washes soil into rivers. The oceans pound against the shores, chiselling cliffs and carrying away land. Rivers knife through rock, carve canyons, and build up land where they empty into the sea. Glaciers plough valleys and cut down mountains.

Water helps keep the earth's climate from getting too hot or too cold. Land absorbs and releases heat from the sun quickly. But the oceans absorb and release the sun's heat slowly. So breezes from the oceans bring warmth to the land in winter and coolness in summer.

Throughout history, water has been people's slave— and their master. Great civilizations have risen where water supplies were plentiful. They have fallen when these supplies failed. People have killed one another for a muddy water hole. They have worshiped rain gods and prayed for rain. Often, when rains have failed to come, crops have withered and starvation has spread across a land. Sometimes the rains have fallen too heavily and too suddenly. Then rivers have overflowed their banks, drowning large numbers of people and causing enormous destruction of property.

Today, more than ever, water is both slave and master to people. We use water in our homes for cleaning, cooking, bathing, and carrying away wastes. We use water to irrigate dry farm lands so we can grow more food. Our factories use more water than any other mate­rial. We use the water in rushing rivers and thundering waterfalls to produce electricity.

Our demand for water is constantly increasing. Every year, there are more people in the world. Factories turn out more and more products, and need more and more water. We live in a world of water. But almost all of it— about 97.5 per cent—is in the oceans. This water is too salty to be used for drinking, farming, and manufacturing. Only about 2.5 per cent of the world's water is fresh (unsalty). Most of this water is not easily available to people because it is locked in icecaps and other glaciers.

There is as much water on earth today as there ever was—or ever will be. Almost every drop of water we use finds its way to the oceans. There, it is evaporated by the sun. It then falls back to the earth as rain. Water is used and reused over and over again. It is never used up.

Although the world as a whole has plenty of fresh water, some regions have a water shortage. Rain does not fall evenly over the earth. Some regions are always too dry, and others too wet.

Some regions have a water shortage because the people have managed their supply poorly. People settle where water is plentiful—near lakes and rivers. Cities grow, and factories spring up. The cities and factories dump their wastes into the lakes and rivers, polluting them. Then the people look for new sources of water. Shortages also occur because some cities do not make full use of their supply. They have plenty of water but not enough storage tanks, treatment plants, and distribution pipes to meet the people's needs.

As our demand for water grows and grows, we will have to make better and better use of our supply. The more we learn about water, the better we will be able to meet this challenge.

Comprehension Check

Fill in the tables.

verb	noun
to control	a control
	utilization
to require
	protection
to navigate
	supply
to construct

verb	noun
to improve
	conservation
to manage
	dependence
to destruct
	design
	development

 

UNIT II HYDROLOGY

Warming-up

Choose the correct variant.

1) The sun’s heat (evaporates / takes / precipitates) water from the oceans.

2) The water rises as invisible (liquid / moisture / vapour), and falls back to the earth as rain, snow, or some other form of (moisture / droplets / fog) which is called precipitation.

3) Most (drop / evaporation / precipitation) drops back directly into the oceans.

4) The unending circulation of the earth’s waters is called (hydrology / the water cycle / evapotranspiration).

5) At one time or another, all the water on earth enters the air, or (ocean / atmosphere / cloud), as water vapour.

6) As the air cools, the vapour (discharges / dew points / condenses) into droplets of liquid water, forming (rain / clouds / hydrologic cycle).

 

Reading Task: A

 

 

Text A Hydrology

 

Hydrology is a scientific discipline concerned with the waters of the Earth, including their occurrence, distribution, circulation via the hydrologic cycle, and interactions with living things. It also deals with the chemical and physical properties of water in all its phases.

The world's supply of fresh water is obtained almost entirely as precipitation resulting from evaporation of sea water. The processes of moisture transfer from the sea to the land and back to the sea again are known to be called the hydrologic cycle. An understanding of these processes is considered very important to the water-resources engineer.

The first stage in the hydrologic cycle is the evaporation of water from the ocean. The heating of the ocean water by the sun is the key process that starts the hydrologic cycle (water cycle) in motion. The vapour is carried over the continents by moving air masses. If the vapour is cooled to its dew point, it condenses into visible water droplets which form cloud or fog. Under favourable conditions the tiny droplets grow large enough to fall to earth as precipitation. About two-thirds of the precipitation which reaches the land surface is returned to the atmosphere by evaporation from water surfaces, soil and vegetation and through transpiration by plants. The remaining part of precipitation returns to the ocean through surface or underground channels.

Precipitation includes all water which falls from the atmosphere to the earth's surface. Precipitation occurs in various forms. The hydrologist must be interested only in liquid precipitation / rainfall / and frozen precipitation / snow, hail, sleet, etc./. The amount of precipitation, however, can vary greatly from season to season in any location. Some regions experience very heavy precipitation in some seasons, and relatively little in others. The results of these variations mean that pipelines and large-scale dams must be used to supply some cities with water throughout the year.

Evaporation is the transfer of water from the liquid to the vapour state. Transpiration is the process by which plants remove moisture from the soil and release it to the air as vapour. More than half of the precipitation which reaches the surface of the earth is returned to the atmosphere by the combined process, evapotranspiration.

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 transporting wastes through sewers; for irrigation of farms and parklands; and for production of electric power. After use, water is returned to another part of 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 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. Water resources problems are also the concern of meteorologists, oceanographers, geologists, chemists, physicists, biologists, economists, specialists in applied mathematics and computer science, and engineers in several fields.

Hydrologists apply scientific knowledge to solve water-related problems in society: problems of quantity, quality and availability. They may be concerned with finding water supplies for cities or irrigated farms, or controlling river flooding, soil erosion, or in environmental protection.

Scientists and engineers in hydrology may be involved in both field investigations and office work. In the field, they may collect basic data, oversee testing of water quality, direct field crews and work with equipment. In the office, hydrologists do many things such as interpreting hydrologic data and performing analyses for determining possible water supplies.

The work of hydrologists is as varied as the uses of water and may range from planning multimillion dollar interstate water projects to advising homeowners about backyard drainage problems.

Comprehension Check

UNIT III RESERVOIRS

Warming-up

 

Text A

Dams are built in the river channels to create reservoirs for storing water. Rivers in natural conditions never carry a constant amount of water. In dry summer and in winter it is very small but in spring the water in the rivers is so high that it flows out of the rivers and floods their banks, whereas water consumers require a constant amount of water invariable during the whole year. To meet these requirements the run-off of the rivers must be controlled. The control of the river run-off can be effected by creating a reservoir. There are some types of the reservoirs: storage (conservation) reservoirs and distribution reservoirs. Stock tanks or small ponds may also conserve water. Whatever the size of the reservoir, its main functions are to regulate the flow of water and to provide storage of water, their most important physical characteristic being storage capacity.

Capacity of reservoirs constructed in natural conditions must usually be determined from topographic surveys.

Picture 2 (Gelmersee reservoir in Switzerland)

The storage volume between the minimum and normal pool levels is called the useful storage. Water held below minimum pool level is dead storage. In multipurpose reservoirs the useful storage may be subdivided into conservation storage and flood-control storage. Reservoir banks are usually permeable and water enters the soil when the reservoir fills and drains out as the water level is lowered. This bank storage effectively increases the capacity of the reservoir. But if the walls of the reservoir are of badly fractured rock, permeable volcanic material or limestone, serious leakage may occur. This leakage may result in a loss of water. If leakage occurs through a few channels or within a small area of fractured rock, then it is quite possible to seal this area by pressure grouting. If the area of leakage is large the cost of grouting may be too great.

The computation of the water-surface profile is an important part of reservoir design since it provides information on the water level at various points along the length of the reservoir.

Docks, houses, roads and bridges along the banks of the reservoir must be located above the water level.

Probably the most important aspect of storage-reservoir design is an analysis of the relation between yield and capacity. Yield is dependent upon inflow and it may vary from year to year. Most water-storage reservoir must have safe and firm yield, i.e. the maximum quantity of water which can be guaranteed during a critical dry period.

Comprehension Check

 

Standard reservoir

a) retention time	c) released	e) treatment	g) hold
b) water level	d) live	f) well

 

A raw water reservoir doesn't simply 1) _______ water until it is needed. It is the first part of the 2) _______ process. The time the water is held for before it is released is known as the 3) _______ _____, and is a design feature that allows larger particles and silts to settle out as 4) _______ as time for the biological treatment of algae and bacteria by plankton-like creatures that naturally 5) _______ within the water.

Water is then 6) _______ from the reservoir, generally by gravity, to be cleaned (for drinking water). In the event that major rain occurs, water can be released, decreasing the reservoir's 7) _______ _______.

 

Hydroelectric reservoir

a) electricity	c) built	e) released
b) reservoir	d) rely	f) drinking

A hydroelectric power station consists of large turbines that 1) _______ on a gravity flow of water through pipes from the dam to turn a turbine to generate 2) _______. The water can then be either 3) _______ to the surrounding water course or pumped back into the 4) _______ and reused. Generally, hydroelectric dams are 5) _______ specifically for electricity generation and are not used for 6) _______ or irrigation water.

 

Irrigation reservoir

a) drinking water

b) water

c) full

d) canals

 

1) _______ in an irrigation reservoir is released into networks of 2) _______ mainly for use in farmlands or secondary water systems. Water in an irrigation reservoir is generally not used for 3) _______, but in some cases is. As with all reservoirs, water can be released if the reservoir is too 4) _______.

 

Flood control reservoir

a) flooding

b) rainfall

c) collect

d) "Attenuation"

Commonly known as an 1) _______ reservoir, these are used to prevent 2) _______ to lower lying lands. Flood control reservoirs 3) _______ water at times of unseasonally high 4) _______, then release it slowly over the course of the following weeks or months.

Recreational reservoir

a) safety

b) purpose

c) are built

d) fishing

Very rarely is a reservoir built solely for a recreational 1) _______. Most reservoirs 2) _______ to a civic purpose, but still allow 3) _______, boating, and other activities. At most reservoirs, special rules apply for the 4) _______ of the public.

 

UNIT IV DAMS

 

Warming-up

 

PART 1

Reading Task: A

 

Text A

 

A dam is a barrier across flowing water that obstructs, directs or retards the flow, often creating a reservoir, lake or impoundment. Most dams have a section called a spillway, over which or through which it is intended that water will flow.

Dams may be classified according to structure and used material, intended purpose or height.

Based on structure and material dams may be subdivided into concrete dams (gravity, buttress, arch and multi-arch), timber dams, embankment dams (earth fill, rock fill) and masonry dams (such as gravity, buttress and multi-arch). The selection of the best type of a dam for a given site is a problem in both engineering feasibility and cost. Feasibility is governed by topography, geology and climate. The relative cost of the various types of dams depends mainly on the availability of construction materials near the site.

According to height, there are low dams (less then 30 m high), medium-height dams (between 30 and 100 m high) and high dams (over 100 m high). The height of a dam is defined as the difference in elevation between spillway crest and the lowest part of the excavated foundation.

Dams are built for the following reasons:

· to meet demands for human consumption, irrigation and industrial uses;

· to provide head for generating hydroelectric power;

· to protect against flooding from rivers or the sea;

· to improve navigation by increasing the depth of water in a river;

· to provide lakes for recreation and fisheries.

A dam must be relatively impervious to water and capable of resisting the forces acting on it. The most important of these forces are known to be gravity (weight of a dam which is the product of its volume and the specific weight of the material), hydrostatic pressure (it may act on both the upstream and downstream faces of the dam), uplift pressure (created when water under pressure finds its way between the dam and its foundation), ice pressure (exerted on the upstream face of a dam) and earthquake forces. The material underlying a dam also must be capable of withstanding the foundation pressures. Most failures of dams have been caused by failure of the underlying material.

Construction of dams and reservoirs requires the closest cooperation of engineers, soil-mechanics experts and geologists in the planning, design and construction so as to assure a maximum degree of safety of these structures.

 

Comprehension Check

 

 

Make a plan of the text.

Make a summary of the text.

Build up the correct words.

Model: amd → dam

1) owlf → …	4) alek → …	7) lyswpali → …
2) upeprso → …	5) tgiheh → …	8) udinfaoon → …
3) uetctrusr → …	6) asnero → …	9) fyaste → …

Text B What does a dam do?

1) to conserve - …   2) to elevate - …   3) neighboring - …   4) to generate - …   5) invariable - …     6) discharge - … 7) amount - …   8) shelter - … 9) to protect - …   10) quick - … 11) take away - …

As a barrier across a river or stream, a dam stops the flow of water. A dam stores the water, creating a lake or reservoir above it. The stored water is then made available for irrigation, town and city water supplies, and many other uses. The dam also raises the water surface from the level of the original riverbed to a higher level. This permits water to be diverted by the natural flow of gravity to adjacent lands. The stored water also flows through hydraulic turbines, producing electric power that is used in homes and industries. Water released from the dam in uniform quantities assures water for fish and other wildlife in the stream below the dam. Otherwise, the stream would go dry there. Water released in larger quantities permits river navigation throughout the year. Where dams create large reservoirs, floodwaters can be held back and released gradually over longer periods of time without overflowing riverbanks. Reservoirs or lakes created by dams provide recreational areas for boating and swimming. They give refuge to fish and wildlife. They help preserve farmlands by reducing soil erosion. Much soil erosion occurs when rivers flood their valleys, and swift floodwaters carry off the rich topsoils.

Comprehension Check

 

 

Text C Types of dams

 

Dams are classified by the material used to construct them. Dams built of concrete, stone, or other masonry are called masonry dams. Dams built of earth or rocks are called embankment dams. Engineers generally choose to build embankment dams in areas where large amounts of earth or rocks are available.

Today nearly all masonry dams are built of large blocks of concrete. There are three main kinds of masonry dams: gravity, arch, and buttress.

Gravity dams depend entirely on their own weight to resist the tremendous force of the oncoming water. They are the strongest and most massive dams built today. A gravity dam is built on a solid rock foundation. The dam transfers the force of the water downward to the foundation below. Gravity dams can hold back enormous amounts of water. However, they are costly to build because they require so much concrete.

Arch dams curve outward toward the flow of water. They are usually built in narrow canyons. As the water pushes against the dam, the arch transfers the water's force outward to the canyon wall. An arch dam requires much less concrete than a gravity dam of the same length.

Buttress dams depend for support on a series of vertical supports called buttresses. The buttresses run along the dam's upstream face—that is, the side facing away from the water's flow. The upstream face of a buttress dam usually slopes outward at about a 45-degree angle. The sloping face and the buttresses serve to transfer the force of the water downward to the dam's foundation. Buttress dams, like gravity dams, are usually built in wide valleys where long dams are needed.

Embankment dams are constructed of materials dug out of the ground, including rocks, gravel, sand, silt, and clay. They are also known as fill dams. An earth-fill dam is an embankment dam in which compacted earth materials make up more than half the dam. Earth-fill dams are constructed by hauling the earth materials into place and compacting them layer upon layer with heavy rollers. The materials are graded by density, and the finest, such as clay, are placed in the center to form a waterproof core. In some cases, concrete cores are used. The coarser materials are placed outside the core and covered with a layer of rock called riprap. The riprap serves as an outer protection against water action, wind, rain, and ice. In addition, thinned-out cement, called grout, is pumped into the foundation to fill cracks. This process makes the foundation watertight.

Where rocks are available, it may prove most economical to build a rock-fill dam. Most dams of this type are constructed of coarse, heavy rock and boulders. Many of them have a covering of concrete, steel, clay, or asphalt on the upstream side. This covering makes the dam watertight. Combinations of rock and earth result in a type of dam called an earth-and-rock-fill dam.

There are other types of dams. Timber dams are built where lumber is available and the dam is relatively small. The timber is weighted down with rock. Planking or other watertight material forms the facing. Metal dams have watertight facings and supports of steel.

Dams with movable gates are built where it is necessary to let large quantities of water, ice, or driftwood pass by the dam. A roller dam has a large roller located horizontally between piers. It can be raised and lowered to allow ice and other materials to pass through the dam without much loss of reservoir water level.

Comprehension Check

Choose the best variant.

1) The finest material in earth-fill dam is placed…

a) in the center

b) outside the core

c) on the upstream face

2) Embankment dams are known as…

a) rock-fill

b) fill dam

c) riprap

3) Rip rap serves as an outer protection against…

a) water action, wind, rain, ice

b) fill cracks

c) leakage

4) An earth-fill dam is an embankment dam in which compacted earth materials make up…

a) the whole dam

b) 1/3 the dam

c) more than ½ the dam

Match the following words.

 

1) будущий, предстоящий	a) riprap
2) пиломатериал	b) planking
3) транспортировка, перевозка	c) oncoming
4) обшивка досками	d) a roller dam
5) каменная наброска	e) hauling
6) сплавной лесоматериал	f) lumber
7) разреженный цемент	g) thinned-out cement
8) плотина с вальцовыми затворами	h) driftwood

 

PART 2

 

Reading Task: A

 

 

Skim the text.

Text A Gravity Dams

 

The stability of gravity dams depends on their own weight. They use its own dead weight to resist the horizontal force of the water. Gravity dams are usually straight in plan although sometimes they may be slightly curved. The shape of the gravity dam resembles a triangle. This is because of the triangular distribution of the water pressure. The deeper the water, the more horizontal pressure it exerts on the dam. So at the surface of the reservoir, the water is exerting no pressure and at the bottom of the reservoir, the water is exerting maximum pressure.

Generally the base of a concrete gravity dam is equal to approximately 0.7 times the height of the dam. For this type of dams, good impervious foundations are essential.

The Gilboa Dam in the Catskill Mountains of New York State is an example of a "solid" gravity dam.

Gravity dams are classified as “solid” or “hollow”. The solid form is more widely used, though the hollow dam is more economical to construct. Gravity dams can also be classified as “overflow” (spillway) and “non-overflow”.

Before the construction work in a river channel can be started, the stream flow must be diverted. In two stage construction the flow is diverted to one side of the channel by a cofferdam while work proceeds on the other side. After the work on the lower side of the dam is completed, flow is diverted through outlets in this portion while work proceeds in the other half of the channel.

The foundation must be excavated to solid rock before any concrete will be poured. After excavation cavities or faults in the underlying strata are sealed with concrete or grout. Frequently a grout curtain is placed near the heel of the dam to reduce seepage and uplift. Grout cement and water sometimes mixed with a small amount of fine sand is forced under pressure into holes drilled into the rock.

The Gilboa Dam in the Catskill Mountains of New York State is an example of a "solid" gravity dam.

Concrete for the dam is usually placed in blocks depending on the dimensions of the dam. On large dams the maximum width of a block is usually about 50 ft. Maximum height of a single block is usually about 5 ft. Sections are poured alternately so that each block can stand several days before another one is poured next to it or on its top. After individual sections are poured, they are sprinkled with water and so protected from the drying. After the forms are removed, the lateral surfaces of each section are painted with an asphaltic emulsion to prevent adherence of adjoining sections. Keyways are provided between sections. They carry the shear from one section to the adjacent one and make the dam act as a monolith. Metal water stops are also placed in the vertical construction joints near the upstream face to prevent leakage. Inspection galleries to permit access to the interior of the dam are formed as the concrete is placed. These galleries may be necessary for grouting operations, for inspection and maintenance of gates and valves and as intercepting drains for water which seeps into the dam.

When concrete sets, a great deal of heat is liberated and the temperature of the mass is raised. As the concrete cools, it shrinks, and cracks may develop. To avoid cracks, special low-heat cement may be used. For obtaining best results the temperature of the concrete mix should be between 50 and 80 ⁰ F.

Comprehension Check

 

 

Choose the odd word.

1) to liberate / to restrict / to discharge / to release / to free

2) to preserve / to protect / to attack / to defend

3) to permit / to allow / to let / to admit / to prohibit

4) access / exit / path / entrance / entry / admission

5) surface / covering / exterior / top / inside

6) to form / to build / to destroy / to create / to make / to construct

7) to accumulate / to leak / to seep / to escape / to pass / to discharge

Text B Arch Dams

 

An arch dam is built in a convex arch facing the reservoir, and owes its strength essentially to its shape, which is particularly efficient in transferring hydraulic forces to supports. In the arch dams, 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 directions. When the upstream face is sloped the distribution is more complicated.

The Hoover Dam, a concrete gravity-arch dam in the Black Canyon of the Colorado River

An arch dam is curved in plan and carries most of the water load horizontally to the abutments by arch action. The thrust requires that sidewalls of the canyon be capable of resisting the arch forces. So arch dams can be used only in narrow canyons. Practically all arch dams constructed in recent years are of concrete, and only few arch dams have failed, in comparison with the more numerous failures of other types of dams.

The same forces which act on gravity dams also act on arch dams, but their relative importance is different. Because of the narrow base width of arch dams, uplift forces are less important than for gravity dams. However, internal stresses caused by ice pressure and temperature changes may become quite important in arch-dam design.

The simplest approach to arch analysis is to assume that the horizontal water load is carried by arch action alone. Most early arch dams were designed on this basis.

Arch dams must be built on solid rock foundation. Seams and pockets in the foundation and abutments are grouted in the usual manner. Since the cross-section of an arch dam is relatively thin care must be taken in the mixing, pouring and curing of the concrete in order to secure its resistance to seepage and weathering. Concrete is placed by techniques similar to those utilized for gravity dams, usually 5-ft lifts, although I0-ft lifts may be also placed at the upper part of an arch where the section is very thin. A layer of mortar is usually placed between lifts to ensure better bond. Small arch dams are provided with only radial construction joints, while large arch dams have circumferential joints as well. All joints must have keyways and in order to prevent leakage water-stops must also be provided. To minimize temperature stresses, the closing section of the dam is poured only after the heat in the other sections is largely dissipated.

 

Comprehension Check

 

 

Choose the odd word.

1) shape / profile / form / silhouette / outline / surface

2) to obtain / to achieve / to gain / to create / to acquire

3) to act / to carry out / to execute / to fail / to function / to work

4) approach / attitude / manner / technique / achieve

5) to stand / to remove / to withdraw / to displace / to eliminate / to delete

6) to resist / to move / to confront / to counteract / to oppose / to withstand

Text C Buttress Dams

 

A buttress dam consists of a sloping membrane which transmits the water load to a series of buttresses at right angles to the axis of the dam. Concrete-buttress dams reduce material in the wall itself by using support buttresses around the outside base. There are several types of buttress dams, the most important ones being the flat-slab and the multiple-arch.

In locations where aggregate for concrete is not available in a sufficient quantity and the foundation rock is acceptable, flat-slab buttress dams may be constructed as well. In cross-section buttress dams resemble gravity dams, but with flatter upstream slopes. In a buttress dam a slab of reinforced concrete rests on a succession of upright buttresses. Their thickness and spacing between them must be sufficient to support the concrete slab and the load exerted by the water in the reservoir. Buttress spacing varies with height of dam. Closely spaced buttresses can be less massive and the slabs can be thinner but then more formwork is required. Concrete beams of diaphragms used as stiffeners between adjacent buttresses or concrete braces may be utilized to resist buckling of the buttresses. Sometimes hollow buttresses are used to increase the effective buttress width. One of the most interesting features of the flat-slab dam is its articulation, i.e. the slab is not rigidly attached to the buttresses. The joint between the slab and buttress is filled with asphaltic putty or some other compound. This permits each slab to act independently, and even minor settlement of the foundation may seriously harm the structure. Flat-slab and buttress dams are particularly adapted to wide valleys where a long dam is required and where foundation materials are of low strength. Placing the buttresses on spread footings one can reduce the foundation pressures. Flat-slab dams may be built on materials ranging from fine sand to solid rock. But the maximum practical height must be necessarily less on poor foundations.

The multiple-arch dams require a better foundation than the flat-slab dams do. Arches for a multiple-arch dam are designed in the same manner as for a single-arch dam, but cantilever action is commonly not taken into account. The design of a multiple-arch dam is more economical for high dams, where the savings in concrete and reinforcing steel are considerable.

Buttress dams are subjected the same forces as gravity and arch dams. Because of the slope of the upstream face, ice pressures are not usually important as the ice tends to slide up the dam. Uplift pressures are relieved by the gaps between the buttresses. The total uplift forces are usually small and can generally be neglected except when a mat foundation is used.

Buttress dams usually require only one-third to one-half as much concrete as gravity dams of similar height but they are not necessarily less expensive because a great deal of formwork and reinforcing steel is required. Since a buttress dam is less massive than a gravity dam, the foundation pressures are less and a buttress dam may be used on foundations which are too weak to support a gravity dam. If the foundation material is permeable, a cut-off wall extending to rock should be provided. The height of a buttress dam can be increased by extending the buttresses and slabs. Usually buttress dams are used where a future increase in reservoir capacity is expected. Powerhouses and water-treatment plants are usually placed between the buttresses of dams in order to obtain some savings in cost of construction.

The removal of overburden down to a suitable foundation and excavation of a trench for the cutoff wall are the first steps in the construction of buttress dams. Great care must be taken in the construction of formwork, handling of concrete and placing of reinforcing steel in order to obtain the strength and water tightness of the thin sections inherent in buttress dams. Deck and buttresses are placed in lifts of 12 ft or more, the buttress construction being kept well in advance of the deck. Keyways are required in all construction joints. Since buttress dams require much less concrete than gravity dams, the time for construction is usually less and the problem of water diversion somewhat simplified.

Comprehension Check

 

Text D Earth-fill Dams

Earth-fill (or embankment) dams are usually used across broad rivers to retain water. The profile of an earth-fill dam is a broad-based triangle.

For the construction of earth dams natural materials with a minimum degree of processing are utilized. These dams may be built with primitive equipment under conditions, where any other construction material would be impracticable. It is not surprising that the earliest known dams were built of earth.

Earth dams are readily adapted to earth foundations. It should not be assumed that the construction of earth dams is a simple operation. Numerous failures of poorly designed earth embankments make it evident that earth dams require as much engineering skill in construction as any other type of dam.

The simple embankment is essentially built from a homogeneous material, although a blanket of relatively impervious material may be placed on the upstream face. There are several types of embankments: zoned embankments, diaphragm-type dams, rock-fill dams.

Hydraulic-fill dams are constructed by using water for transporting the material to its final position in the dam. The material is discharged from pipes along the outside edges of the fill and the coarse material is deposited soon after discharge while the fines are carried into the central pool. The result is a zoned embankment with a relatively impermeable core. Hydraulic fill is best suited for placing well-graded materials containing a considerable amount of coarse sand and gravel. An adequate water supply is necessary as well. Because of the slow drainage of the water from the core, considerable settlement is to be expected over a long period.

Semihydraulic-fill dams are constructed by dumping the material from trucks into its approximate position in the dam. Like the hydraulic-fill method this procedure requires careful control to assure a satisfactory construction of the embankment.

Rolled-fill dams are constructed by placing selected materials in thin layers and compacting them with a heavy roller. Some compaction may also be obtained by proper routing of trucks and other construction equipment. Usually, however, special equipment is used for compacting the fill. Both sheepsfoot rollers and heavy pneumatic-tired rollers are used singly or in combination.

The required height of an earth dam is the distance from the foundation to the water surface in the reservoir when the spillway is discharging at design capacity plus a freeboard allowance for wind-tide as well as for wave and frost action. Recent studies of earth-dam failures indicate that 40 % resulted from overtopping of the dam because of insufficient freeboard or inadequate spillway capacity. Frost in the upper portion of a dam may cause heaving and cracking of the soil that, in turn, may cause dangerous seepage. An additional freeboard allowance up to a maximum of about 5 ft should be provided for dams in areas subject to low temperatures. Parapet walls 2 or 3 ft high as an additional safety factor and as an element of freeboard are sometimes provided on the upstream side of the crest of an earth dam exceeding 30 ft in height.

No earth dam can be considered fully impervious. Some seepage through the dam and its foundation must be expected. Seepage through earth dams may be reduced by the use of a very broad base, by the placing of an impervious blanket on the upstream face, by use of a clay core, or by a diaphragm of timber, steel or concrete. A grout curtain formed by forcing cement grout down through closely spaced drill holes is also effective means of checking leakage through fractured rock. In any case drains are normally provided near the toe of the dam to permit the free escape of seepage water.

The upstream slope of an earth dam should be protected against wave action by a cover of rip-rap or concrete. The rock should be sound and not subjected to rapid weathering and should be placed over a filter layer of graded gravel at least 12 in. thick. Concrete slabs are often employed for facing the upstream slope of earth dams but must be very carefully constructed. The usual failure results from the washing of embankment material through the joints between the slabs. It is therefore desirable that a filter layer be provided under the slab. Upstream slope protection should extend from above the upper limit of wave action to a berm.

The downstream slope of an earth dam may be subjected to erosion resulted from rainfall. On dams having a rock shell this is no problem, but earth slopes should be planted to grass as soon as possible after completion of the dam. Since the erosive action of water increases as the slope length increases, berms should be placed at about 50 ft intervals to intercept rainwater and discharge it safety.

Comprehension Check

 

 

Text E Rock-fill Dams

Rock-fill dams have characteristics both gravity dams and earth dams.

The rock-fill dam has two basic structural components - an impervious membrane and an embankment which supports the membrane.

The embankment usually consists of an upstream section of dry rubble or masonry and a downstream section of loose rock fill. The rock used should be capable of resisting erosion and strong enough to withstand loads of high intensity even when wet. Rocks may vary from small stones to boulders 10 or more ft in diameter. Large stones of regular shape are used to obtain a flat surface on which membrane can be placed.

The impervious membrane is most commonly constructed of concrete. The membrane is sometimes poured as a monolith without expansion joints but with ample steel reinforcement in both horizontal and vertical directions. Most membranes have expansion joints at intervals of about 50 ft with asphaltic joint filler to minimize leakage. Slab thickness is usually between 6 and 18 in with greater thickness often used near the base of the dam.

Rock-fill dams are subject to considerable settlement, which may result in cracking of the membrane. This is perhaps the greatest disadvantage of rock-fill dams, although in many instances leakage can be controlled by periodic repair of the membrane. A rock-fill dam of good design and careful construction has high resistance to earthquakes. Moreover, much less material is required for a rock-fill dam than for an earth dam. Because of the narrow base width and the possibility of high seepage, foundation requirements for rock-fill dams are more rigid than for earth dams. Rock-fill dams are generally cheaper than concrete dams and can be constructed more rapidly if the proper material is available.

Comprehension Check

 

Build up the correct words.

Model: amd → dam

1) ibremt → …	6) ahetr → …	11) ionditcon → …
2) ircb → …	7) erlifl → …	12) ledubar → …
3) elip → …	8) veridt → …	13) poryramet → …
4) twidh → …	9) tihegh → …	14) meaperble → …
5) stoc → …	10) urextim → …	15) damferofc → …

Text A Spillways

In the design of almost every dam there must be a spillway to permit the discharge of water. A spillway is necessary to discharge floods and prevent the dam from being damaged. Gates on the spillway crest, together with sluiceways, permit one to control the release of water downstream for various purposes.

A spillway is the safety valve for a dam. It must have the capacity to discharge major floods without damage to the dam or any appurtenant structures at the same time keeping the water level in the reservoir below some predetermined maximum level.

An overflow spillway is a section of dam designed to permit water to pass over its crest. Overflow spillways are widely used on gravity, arch and buttress dams.

Water flows over the crest of a chute spillway into a steep-sloped open channel which is called a chute, or trough. The channel is usually constructed of reinforced- concrete slabs 10 - 20 in thick. Such a structure is relatively light and is well adapted to earth or rock-fill dams when topographic conditions make it necessary to place the spillway on the dam. A chute spillway may be constructed around the end of any type of dam when topographic conditions permit. Such a location is preferred for earth dams to prevent possible damage to the embankment. The chute is sometimes of constant width. But it is usually narrowed for economy and then widened near the end to reduce discharge velocity. Expansion joints are usually required in chute spillways at intervals of about 30 ft. They should be as watertight as possible. Drains under the spillway are highly desirable. The drains may be rock-filled trenches, clay tile or perforated steel pipes.

A side-channel spillway is one in which the flow after passing over the crest is carried away in a channel running parallel to the crest. This type of spillway is used in narrow canyons where sufficient crest length is not available for construction of overflow or chute spillways.

In a shaft spillway the water drops through a vertical shaft to a horizontal conduit which conveys the water past the dam. A shaft spillway can often be used where there is no adequate space for other types of spillways. Small shaft spillways may be constructed entirely of metal or concrete pipe or clay tile. The large vertical shaft is usually of reinforced concrete.

If space is limited, the siphon spillway may be used. Siphon spillways have the advantage that they can automatically maintain water-surface elevation within very close limits.

 

Comprehension Check

 

 

UNIT VI INTAKES

 

Warming-up

 

Text Intakes

An intake structure is required at the entrance to a conduit through which water is withdrawn from a river or reservoir. Intake structures vary from a simple concrete block supporting the end of a pipe to elaborate concrete intake towers, depending upon reservoir characteristics, climatic conditions and other factors. The primary function of the intake structure is to permit withdrawal of water from the reservoir and to protect the conduit from damage or clogging.

Intake towers are often used where there is a wide fluctuation of water level. Ordinarily they are provided with ports located at various levels. They may aid flow regulation and permit some selection of the quality of water to be withdrawn. It is necessary to place the lowest ports far enough above the bottom of the reservoir so that sediment will not be drawn into them. A wet intake tower consists of a concrete shell filled with water to the level of the reservoir and has a vertical shaft inside connected to the withdrawal conduit. Gates are normally provided on the inside shaft to regulate flow.

A dry intake tower has no water inside of it since the entry ports are connected directly to the withdrawal conduit. Each entry port is provided with a gate or valve. An advantage of the dry tower is that water can be withdrawn from any selected level of' the reservoir. Intake towers should be located so as not to interfere with navigation and must be designed to withstand hydrostatic pressures and forces from earthquakes, wind, waves and ice.

A submerged intake consists of a rock-filled crib or a concrete block which supports the end of the withdrawal conduit. Because of their low cost, submerged intakes are widely used on small projects.

The entrance to intakes and sluiceways should be provided with trash racks. These racks are usually made of steel bars spaced on 2 to 6 inches depending on maximum size of debris which can be present in the conduit. Sometimes the rack is constructed in the form of a half cylinder. Debris which accumulates on the rack is sometimes removed by hand when necessary, but where much debris is expected automatic power-driven rakes are preferable.

Most intakes and sluiceways are provided with some type of gate or valve at their entrance, for example, high-head tractor gates, cylinder gates which can be used to close the flow into large intake towers of the wet types; interior gate valves which are located downstream from the conduit entrance. For smaller heads interior gate valves are often used to regulate flow but for greater heads they are ordinarily used only in the fully open or fully closed position. A butterfly valve is best suited for moderate heads, although they have been used under high heads; high head regulating valves: slide gates are not suitable for flow regulation under high heads. On high-heads installations needle valves and tube valves are widely used for flow regulation. They are usually placed at the downstream end of sluiceways and discharge directly into the atmosphere.

Water flowing over a spillway or through a sluiceway is capable of causing severe erosion of the stream bed and its banks below the dam. Whenever flow changes from supercritical to subcritical, a hydraulic jump will occur. The problem facing the designer is to find the location of the hydraulic jump under various conditions of flow. In order that a hydraulic jump may occur, the flow must be below critical depth. The location and character of the jump depend on tailwater elevation. For the jump to form at the toe of the dam, the tailwater elevation must coincide with the upper conjugate depth. If the tailwater depth is greater than conjugate depth, the overflowing water will underrun the tailwater without a jump.

If the tailwater depth is less than conjugate depth, flow will continue below critical depth for some distance downstream from the toe of the dam. Because of the energy loss to friction, the depth of flow will gradually increase until a new depth is reached which is conjugated with the tailwater depth.

The measures necessary to control erosion and dissipate the energy of the spillway flow are dependent on the relation between the upper conjugate depth and tailwater depth.

 

Comprehension Check

Translate into English.

1) Водозабор - в широком смысле - забор воды из водоема, водотока или подземного водоисточника для орошения, водоснабжения, использования водной энергии. В узком смысле, это - инженерное сооружение по захвату подземных вод или воды из реки, водохранилища в водоотводные, оросительные, гидроэнергетические и другие системы.

2) Глубинное водозаборное сооружение - водозаборное сооружение, при помощи которого забирают воду под уровнем свободной поверхности. Донное водозаборное сооружение - водозаборное сооружение, при помощи которого забирают воду с самой низкой части русла водотока. Поверхностное водозаборное сооружение - водозаборное сооружение, при помощи которого забирают воду на уровне свободной поверхности.

3) По характеристикам источника водозаборы бывают: подземных вод; поверхностных вод (речные, озерные