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The Importance Of Dams To Water Quality Environmental Sciences Essay

5431 words (22 pages) Essay in Environmental Sciences

5/12/16 Environmental Sciences Reference this

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Water plays an exceptional significant role in the economy and in the life of all countries. It is of vital importance for the existence of people, animals and vegetation. The settling of people in different regions of earth has always been closely dependant on the possibilities for water supply parallel to those providing with food, shelter and heat. The increase in the population, development and enrichment of mankind in number of places has reached a level at which the water supply needed for the population, industry, irrigation and production of electric power has been brought to a critical point. Only one-fifth of the fresh water which is available is suitable for human consumption.

There is uneven distribution of water regarding space, time and quality. That is why artificial redistribution of water is needed by undertaking expensive engineering in order to obtain water for given place in a defined quantity and quality.

Definition of Dam

Dam is a manmade barrier constructed to contain flow of water built across a river, lake, sea forming a reservoir behind it. They are mostly constructed from earth, natural materials, concrete & rocks.

Types of dams:-

Arch dams – Arch dams are made from concrete. They are curved in the shape of an arch, with the top of the arch pointing back into the water. Arch dams are usually constructed in narrow, steep valleys. They like all dams need good rock for their foundations, to resist the force of water on the dam. An arch shape is used for resisting the pushing force of the flowing water behind the dam.

Buttress dams – These dams are made from concrete & masonry or both. The watertight upstream side supported is by triangular shaped walls, called buttresses. The buttresses are spaced at regular intervals on the downstream side. They resist the force of the reservoir water trying to push the dam over.

The buttress dam was developed from the idea of the gravity dam, except that buttress dam uses a lot less because there are a lot of empty spaces between the buttress dams. Like gravity dams, they are made to suit to both narrow and wide valleys and they must be constructed on stable rock.

Since 1945, 14 buttress dams have been constructed in the UK, mainly for hydroelectric schemes in Scotland.

Cross-section through gravity dam (http://www.britishdams.org/about_dams/gravity.htm)

Gravity dams -A gravity dam is made from concrete or masonry, or sometimes both. It is called a gravity dam because gravity is the force that holds it down to the ground stopping the water in the reservoir pushing it over.

Cross-section through gravity dam

A cross-section (or slice) through a gravity looks roughly like a triangular shape.

Gravity dams are mostly used at sites which have narrow valleys, but they do need to be built on a stable rock. They can be constructed fairly on any terrain.

There are more than 260 gravity dams in Britain. Masonry was used in many early dams, as far back as the 17th Century. However, concrete became more common from about 1900.

Embankment dams & their main features- There are several types of embankment dams; the designs have varying degree of inbuilt conservation. For e.g.

Relating to the degree of seepage within the dam which is controlled by provision of filters and drains

The use of free draining rock fill in embankment ,

The control of foundation seepage by grouting, drainage and cut-off constructions.

The shape of dam looks like a small hill or pile. The hill or the pile is made up of impermeable [1] material, so that the water cannot pipe through the dam. The core of the dam is made up of clay or asphalt concrete; these types of dams are mostly used on wide valleys. The wide base helps water to exert less pressure on the foundation of the dams this is the main reason these dams could be built on soft soil. There are two types of embankment dams earth filled dam & rock filled dam

For rock filled dams the rock pieces are cut precisely to form a dam. Earth filled dams & rock filled dams can further be divided into different categories of dams:-

Concrete earth fill- Concrete is used in the core of dam as impermeable material.

Asphaltic core rock fill-This type of dam is build with asphalt core (i.e. rock & gravel) as the main fill material in the dam.

Bituminous concrete face earth and rock fill-Bituminous concrete membrane is used in the water barrier for a dam as it is easy to maintain and economical viable.

Types of spillways

Spillways are structures which are found on or near dams, these structures are used when reservoir is full to pass floodwater safely, and in a controlled way, across a dam.

There are 3 types of spillways:-

Overflow spillway- These types of spillways are mostly created on gravity dam or a buttress dam. The spillway section in these dams are lower than the other sections of the dam allowing water to flow over its top and down from the front face of the dam.

Side channel spillway- These types of spillways are used mainly with embankment dams. They are just located upstream and towards the side of the embankment dam. The water flows over the spillway, into a side channel of the dam and flows out safely through a tunnel

Shaft spillways- This type of spillways is also known as Morning Glory, this spillway resembles like inverted funnel over a tower which looks like a shaft in the reservoir as soon as the water level rises in the reservoir it falls into the shaft and it is removed through the bottom of shaft via a tunnel.

Shaft Spillway (http://www.britishdams.org/about_dams/shaft.htm)

How are dams built?

To build a dam some sequence of events are required for building the foundations of the dam and creating a reservoir. Dam is usually constructed across a river to create a reservoir in the valley which helps in storing the water that flows into it naturally. Sometimes, they are built across dry valleys, or valleys with small streams, to create a storage area for water that is transported from elsewhere.

(I)Diverting the river flow from the site – Streams and rivers are diverted to create a dry land for the construction of the dam. Small rivers, streams or canals are usually diverted through a tunnel, or a channel that is constructed around the side of the dam. Soft soils and rocks are removed to make a route, while harder rocks have to be blasted with explosives, to loosen up the soil and form soft debris.

If dams are built across wide rivers with large volumes of flowing water. It would be impracticable and too expensive to construct a separate channel or a tunnel to divert the flow of water. Instead, “a dry construction pit is formed on one side of the river, leaving the other side open for the water to flow through. The first portion of the dam is constructed in the dry pit. When it is finished, another dry area is formed on the other side of the river, and the remaining part of the dam is built. Meanwhile, the river flows through openings in the completed portion of the dam, and the reservoir can start to fill behind it” (http://www.britishdams.org/about_dams/divert.htm)

(ii)Constructing a foundation for the dam-A dam is usually built on a soil or on a hard rock foundation, different foundations are used depending on the type of dam and type of topography [2] around it. The foundation needs to be strong enough to hold and maintain the weight, structure of the dam, and the force of water pressure acting on the walls of the dam. The dam foundation generally needs to be below the ground level of the dam. To construct foundation soft soil and rocks need to be removed by excavation, drilling or by blasting rocks with explosives.

Rarely, areas of weaker soils or rocks are found below the planned foundation level of the dam and this need to be removed and replaced with stronger materials for a stronger foundation.

Some rock foundations have crevices [3] and fissures [4] in them. These have to be filled with grout or sealing material to stop water seeping out from the reservoir through the cracks, once the dam is made. This is usually done by drilling holes down into the weak rock with cracks and by pumping grout in them, which spreads outwards to fill up the cracks and making the rock hard.

(iii)Building a dam (structure) -There are two major types of dam constructions (i) Concrete dams

(ii) Embankment dams

Concrete dams-As limited amount of concrete is required in a concrete dam, the concrete is mixed together in a special building called a concrete batching plant, which is built near the dam site. Sometimes for very large dams, there can be a number of batching plants. The concrete in the dam is placed by two different methods.

(a) Pouring concrete -The traditional method of constructing a dam is to pour a wet mixture of concrete into a mould made in the required shape of the dam. The mould is usually made from sheets of timber it is also known as formwork. The mould is not made to the full length and breadth of the dam, as the dam is built upwards in small stages of about 1 to 2 metres at a time. The concrete is left to dry and harden before they can work on the next section on the top.

The concrete needs to be transferred from the batching plant to the dam this can be done by number of methods. Like a system of conveyor belts is used from the batching plant to the dam site or sometimes the concrete is taken by the trucks to the dam site from where it is poured into skips which are then lifted to the top of the dam by cranes and poured.

(b) Compacting concrete with rollers- Roller Compacted Concrete (RCC) dams are formed by spreading a dry layer of mix of concrete and compacting and mashing it down using a heavy steel rollers. The dam is raised slowly step by step to about 600 mm. The edges of the dams are formed first, by making low concrete walls on the upstream and downstream sides of the dam. Concrete is then moved to the area between the walls and spread in a thin layer using bulldozers. Rollers are then driven over the concrete to compact it down and harden it up.

(iv)Filling up the reservoir with water

(v)Testing that does the floodgates and valves work

(vi) Monitoring and recording the behaviour of newly built dam

USES OF DAMS

Water supply -This water is used to supply water to homes and factories. Reservoirs are designed in such a way to store the rain that falls during the rainier parts of the year, so that there is a continuous supply of water for the drier period

Hydroelectricity- Waters in the reservoirs could be used to store water to feed hydroelectric power stations. Hydroelectric power is produced by using the potential energy from falling water to drive water turbines that in turn, turn electric generators. The reservoir water is stored at a height higher than the turbines, which are in the power station. Sometimes, the power station is directly in front of a dam, and pipes through the dam feed water directly to the turbines. In some other cases, the power station is downhill from the reservoir, and the water is fed to it through long pipes or tunnels called penstocks.

Flood control-A reservoir can be used to control the amount of water flowing in a river after heavy rain or snow fall. The water level in the reservoir is kept low during the wetter periods of the year. When heavy rain occurs, it is gated by the dam and held in the reservoir like a collecting tank. When the reservoir gets too full, the floodwater can be passed downstream over a spillway.

Sometimes, floodgates are used on top of spillways, and they can be fully or partly opened to control the amount of water let out into the river downstream. If dam weren’t there the water level in rivers can rise dramatically. Sometimes, the water flows over riverbanks or walls causing flooding of farmland, property, and in the worst cases, loss of life.

Irrigation-Plants grow naturally on fertile soil that is watered by rain. However, in different regions of the world where the climate is very dry for some parts of the year, the soil becomes so dry that it restricts the growth of vegetation. This problem can be overcome by irrigation, a man-made system for watering the land. As water is released through spillway into downstream and to the drier parts

Navigation -The construction of a dam across a river forms a reservoir that holds water raises the water level upstream, , and it slows down its rate of flow. This improves the navigation conditions upstream of the dam for ships and boats. Dangerous areas of rocks and sandbanks, previously in shallow water, become well covered, and rapids in the river disappear. Also, water from the reservoir can be released into the river downstream during the drier seasons of the year to make sure that it is deep enough for navigation all the year round.

How do dams affect people and environment around it?

Reservoirs are generally created to act as a resource or to give benefit to people. Rarely , inundation, or flooding of land and the management of the water in the reservoir can have an adverse effects on people, the wildlife ,the environment and the habitats, not only inside and around the valley, but also downstream of the dam or the spillway. The advantages and disadvantages of creating a new reservoir and a dam should be considered during the planning stages. Suitable methods should be decided and worked upon to eliminate or to reduce the disadvantages, so that the reservoir provides an overall resourceful benefit to people.

These are some of the issues that are considered:-

People- Peoples livelihood and their quality of living are also affected when the land is inundated by the reservoirs, on which they used to live and earn living .For some large reservoirs, or dams, hundreds of thousands of people have had to leave their homes and well established businesses had to migrate elsewhere and set up everything from scratch. In the past, there have been cases that many of these people that were forced to migrate for the betterment of society had not been given adequate compensation for their losses, and some had not even been given new places to live let alone the compensation. During relocation some of the communities who had been living together for a long time were broken up and moved to different areas due to construction of the dam, which also led to emotional and mental traumas. Some people made their living from farming and used the water from river for irrigation and some used the river for fishing. Majority of them suffered when they were relocated, as they were not given new land to work, if land was given it was far more less than they actually used to have or the land was infertile and the land was too far from a river so river could not be used for irrigation. They needed different skills to get another job and adequate training was not always provided. These days, authorities responsible for the dams are becoming more aware of these issues. Resettlement plans have been worked on, developed and applied to minimise the disruption and suffering caused to people during relocation, which used to live in the reservoir areas. Good plans make sure that fair compensation and employment opportunities would be provided. In some cases, efforts have been made to resettle migrants in their own chosen communities.

Plants and animals Dams are generally constructed across rivers or lakes to store water that would naturally find its path to the lower reaches of the river and finally into the sea. The building of a dam upsets the natural flow of water and the balance of the river which affects the animal and plant life in and around it. some of the reasons how it may change the natural balance are :-

At the upstream of the dam, the dam is made, river is flooded and it becomes a reservoir.

The nature of the flow of the water downstream is changed.

The dams generally accumulate sediments that normally flow downstream in a natural flow

When the river valley is flooded with water, animals are forced to leave there natural habitat and plants and trees are cut. In some cases rare species have been extinct due to the negligence of the authorities. Now for some large projects, plans for nature reserves have been made .In some natural reserves same species of plants and trees have been replanted in them and some of the native affected animals have been moved there. However, the natural reserves can only really work when authorities carefully give a thought that how the plants and animals depend on each other in an environment.

A dam across a river generally forms a barrier to fish that migrate, such as salmon, pomfret, sailfish. Fish passes should be included in the design of a dam. This would allow adult fish to swim upstream to spawn, and then come back downstream later with their young ones. Fish passes usually take the form of a fish lock or fish ladder. These fish passes need to be designed very carefully and precisely to make sure that the conditions are just about right for the fish to use them.

Sedimentation- Rivers always carry sediments. When river is flooding a reservoir, the speed of the flowing water comes to almost a halt and the sediments fall down and get deposited on the reservoir bed. Over a number of years, the sediment in the reservoir can pile up, and form a hard rock which may reduce the space available for storing water in the reservoir. Most of the sediments that are held back in the reservoir would normally be moved downstream by the flowing water. If too much sediment is accumulated in the reservoir, the natural balance of the sediments in the river downstream can be changed, affecting people, wildlife and plants as far away as the river estuary, meets the sea. Farming land, which uses water from reservoirs for growing crops, can be deprived of its nutrients and silt which are normally accumulated when the river floods the reservoir. Nutrients are essential for fertilising the soil for agriculture. When designing a reservoir, the quantity and the quality of sediment that will flow into the reservoir have to be monitored and controlled. The reservoir needs to be designed to reduce the amount of sediments that are deposited upstream, and to maximise the sediment flow downstream. Positioning spillways and outlet pipes and tunnels can control and regulate the flow of sediments downstream. Sometimes sediment is knowingly accumulated in the reservoir. Then at regular intervals, it is removed. This can be achieved by letting water flow out of the reservoir through the outlet pipes at the bottom of the dam, so that the sediments easily get flushed out.

Water quality – The quality of water can slowly deteriorate when it is stored in a reservoir as the water is stagnant .Flowing river water maintains a constant supply of dissolved oxygen. Dissolved oxygen is vitally needed to support aquatic flora and fauna, and to prevent some chemical reactions to pollute the water. There are many numbers of factors that can reduce oxygen levels in a reservoir, for e.g. organic material in the water can readily use up oxygen as it decomposes. The depth of the water in the reservoir, temperature difference between the water at the top of the reservoir and the bottom and its flow can also affect the oxygen levels. The type and the chemical nature of the land that is covered by a reservoir may affect the water quality. Pesticides and other chemicals from farms and toxic materials from industries can pollute the water. The streams and rivers flowing into the reservoir act as a major source of pollution as they may be carrying pollutants. The designers of a reservoir and the dam have to consider all these factors and whether if any of these factors will have an adverse effect on the quality of the water. If the answer is yes, the designers need to include special safety measures to maintain good quality of water.

Historical site- river valleys have high heritage values as throughout history people have settled in them. Many of the world’s archaeological sites, historical buildings and monuments can be found on the beds of these river valleys. Often these sites include sacred buildings such as churches, temples and their burial sites, which are areas of high sentimental values to the local inhabitants. Such heritage sites could be lost forever when a reservoir is made and the site is inundated with water. In the past, no such steps were taken in these dam projects to explore, preserve or save any of the local heritages. Recently, special measures have been taken in account for building of some projects. They have included following guidelines:

Intensive archaeological investigations at the sites believed to have traces by ancient civilisations, before dam or a reservoir constructions proceeds.

Relocation of burial sites with proper respect and dignity.

Redesigning the structure of the dam, to minimise the loss of heritage site.

The dismantling, relocation and re-erection of ancient monuments to preserve the interests of locals

Safety and the maintenance of the dam

In 1925, the two dams failed causing a flood that swamped a village in North Wales called Dolgarrog, this flood killed 16 people. The disaster was initially started when the Eigiau Dam, a small gravity dam failed. The released water from the reservoir of the broken dam flooded downstream and overtopped the reservoir of Coedty Dam which was an embankment dam; the high pressure of water inside the reservoir forced this dam to fail. Thus, releasing the huge volume of water that that swapped Dolgarrog and killed people.

This disaster at Dolgarrog led the British government to pass the” Reservoirs (Safety Provisions) Act in 1930 that introduced laws on the safety of the reservoirs. This act has been updated, and the current one is known as the Reservoirs Act, 1975″. According to this acts all the dams in the United Kingdom that can hold at least 25,000 cubic metres of water would be maintained and looked after to ensure the safety of the general public. Under the Act, there is a usually “Panel” (or group) of civil engineers who are responsible for checking the safety and maintenance of these reservoirs and their dams. These are also known as “Panel Engineers”, and to qualify, these panel engineers need to be well qualified and vastly experienced in dam and reservoirs safety maintenance. After 10 years every existing reservoir is thoroughly checked that if they meet all their safety requirements. Panel Engineers are also required to actively participate with the new construction of dams and reservoirs, or if the existing reservoirs needs to be repaired and changed entirely. According to this act reservoir owners are legally responsible for the safety and the maintenance of their reservoirs, dams. These dam owners are forced to employ suitably qualified civil engineers to make periodic checks on safety of the dam during the intervals between the Panel Engineer’s inspections.

Safety check by Engineers

Some of the safety checks are listed below

Visual inspections – An important part of a reservoir safety check is visual inspections. Unusual features like cracks on the surface of a dam may reveal important clues about the condition of the internal body of the dam which may not be visible to the naked eye of the inspector.

On embankment dams, the civil engineers always look for hollows, bulges cracks and leaks on the outer face and on the upstream face of the dam, when the water is low. These features could possible indicate number of factors like the dam becoming more unstable, and that if there is a possibility that, with time, the portion of the slope could collapse, just like a landslide or the dam could crumble down on its own weight. Hollows on the dam can also indicate that floodwater has been overflowing the dam and slowly eroding the downstream slope. Engineers also look through or underneath the dam for any signs of water leaking. Water leaking through the dam can erode away the material inside the dam gradually creating a large cavity. This may lead to the collapse of the cavity itself which may reduce the stability of the dam. If the water is leaking under the dam or near the foot it can be a problem in its foundations in this case the entire dam needs to be reconstructed as the foundations get mushy and dam can collapse any second. Concrete dams have inspection galleries theses are tunnels running through the entire dams. These galleries just between 5-6 feet high just about big enough for a person to walk through. This helps engineers as the can inspect the conditions of the dam both inside and outside. The concrete surface is checked for signs of any erosion, deterioration, cracking or moulding. The engineers also look for the stability of the dam and look for any signs that could suggest that the dam or sections of it are moving. Checks are also made for any leaks on every part of the dam. Inspector checks the dam for all types of conditions, water pipes, valves and floodgates and even the quality of the water. The valves and gates are operated and greased regularly to check that they work and would work when required.

Monitoring behaviour- Although dams look like a solid structure, they generally do move under the forces of nature, and which may cause water to seep through tiny pores in the structure. The height of an embankment dam actually reduced due to its weight over the years. Reduction of the height of dam is called settlement. A reservoir filled with water pushes a dam downstream a little and tries to make it slant and slide on its foundation. The structure of the dams is mostly made out of metal and concrete which may contract and expand the dam as the temperature changes throughout the year. Civil engineers have to often ensure the stability of dams by designing dams that can minimise water seepage and could withstand the effects of natural forces. Changes in the behaviour of a dam does not occur overnight, they occur slowly, and gradually so these changes cannot be detected by naked eye so the dam needs to be monitored regularly using scientific instruments that can measure changes at very small values . This way any unwanted behaviour can be detected in early stages before it causes a major structural problem. Some instruments are permanently fitted in dams, while others are generally taken to the site to make measurements.

These are some the things that are monitored:-

Seepage or leakage- A crack in a dam can indicate there would be increased leakage or seepage. In embankment dams, it could indicate that there is internal erosion of the fill material is taking place.

Settlement- Optical instruments are used to measures levels on the crest of an embankment dam, to check if the height of the dam is reduced. If a crest appears to settle it can be a warning of problem a occurring within the body of the dam. The crest levels are generally designed to be of a certain height or above the normal full reservoir level. This ensures that the dam is not overtopped if in case it is flooded by excess floodwater or by waves on windy days.

Tilting -Pendulums are installed in the vertically shafts of the dam to monitor any tilting in the dam

Considering floods – Dams are usually designed to hold back and collect water that flows into the valley behind it in the reservoirs; dams are also used to control the amount of water that passes out through it. Hydrologists are used to estimate the quantity and the quality of water that flows into a reservoir. Civil engineers use this information to find a location and plan the appropriate height of the dam. The dam needs to be of certain height, high enough to ensure that it will not be overflow excess flood water, unless it is designed to do so via tunnels or spillways. Accidental over toppling of water could damage the property downstream and in worse can be fatal or it could wash away part of the downstream side of an embankment dam which may affect the stability of the dam and it could collapse. These events can form catastrophic floods, as the water is released from the reservoir, could cause death and destruction to the people who live downstream. Floodwater is usually passed through a reservoir using a spillway, tunnels or safety canals safely. If people and property are at risk, spillways are opened safely so that they could pass the huge amounts of flood water controllably. Hydrologists often estimate the size and the danger of the flood by studying past rainfall records for the area and considering recent climate changes or the sudden change in the topography e.g. a large slope formed due to a landslide. They use this information to design new, better, efficient, spillways and also check the existing ones. Often, small-scale models are created to check the design.

Checking for earthquakes- Every year there are more than 200 earthquakes are detected just alone in Britain. The majority of them are extremely small and have very small value on the Richter scale, but after regular intervals of every eight years at least a Magnitude 5 earthquake on Richter scale occurs. Luckily Magnitude 6 earthquake is about the largest and biggest that could ever occur in Britain, but the odds of that happening is very small or even negligible. A Magnitude 6 earthquake can easily damage structures that are poorly constructed, have weak foundations or that have not been designed to absorb earthquake shocks.

If a dam fails during an earthquake it can be very serious, because the water released from the reservoir could cause havoc and severe flooding downstream. Due to this reason, new and existing dams in Britain are checked that if they could absorb the shocks of an earthquakes.

There are a few handful dams in Britain which are believed to have been damaged or have been severely affected by earthquakes. The most important case was found in 1957 when the Black brook Dam, in Leicestershire, was severely affected by a Magnitude 5.3 earthquake. Tithe tremors of this earthquake caused heavy coping stones to shift and cracks and crevices appeared on the faces of this gravity dam which was constructed from concrete and masonry.

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