Distribution system is a network of pipelines that distribute water to the consumers (Picture 6.1).
They are designed to adequately satisfy the water requirement for a combination of:
A good distribution system should satisfy the followings:
Adequate water pressure at the consumer's taps for a specific rate of flow (i.e., pressures should be great enough to adequately meet consumer needs).
Pressures should be great enough to adequately meet firefighting needs.
At the same time, pressures should not be excessive because development of the pressure head brings important cost consideration and as pressure increases leakages increases too.
Purity of distributed water should be maintained. This requires distribution system to be completely water-tight.
Maintenance of the distribution system should be easy and economical.
Water should remain available during breakdown periods of pipeline. System of distribution should not such that if one pipe bursts, it puts a large area without water. If a particular pipe length is under repair and has been shut down, the water to the population living in the down-stream side of this pipeline should be available from other pipeline.
During repairs, it should not cause any obstruction to traffic. In other words, the pipelines should not be laid under highways, carriage ways but below foot paths.
A. Branching Pattern with Dead End
Similar to the branching of a tree- it consists of:
Main (trunk) line
Main line is the main source of water supply. There is no water distribution to consumers from trunk line.
Sub-mains are connected to the main line and they are along the main roads.
Branches are connected to the sub-mains and they are along the streets.
Lastly service connections are given to the consumers from branches.
Picture 6.1. An example for a water network
It is a very simple method of water distribution. Calculations are easy and simple to do.
The required dimensions of the pipes are economical.
This method requires comparatively less number of cut-off valves.
The area receiving water from a pipe under repair is without water until the work is completed.
In this system, there are large number of dead ends where water does not circulate but remains static. Sediments accumulate due to stagnation of the dead end and bacterial growth may occur at these points. To overcome this problem drain valves are provided at dead ends and stagnant water is drained out by periodically opening these valves but a large amount of water is wasted.
It is difficult to maintain chlorine residual at the dead ends of the pipe.
Water available for fire-fighting will be limited since it is being supplied by only one water main.
The pressure at the end of the line may become undesirably low as additional areas are connected to the water supply system. This problem is common in many less-developed countries.
B. Grid Pattern In grid pattern, all the pipes are interconnected with no dead-ends. In such a system, water can reach any point from more than one direction (Picture 6.2).
Since water in the supply system is free to flow in more than one direction, stagnation does not occur as readily as in the branching pattern.
In case of repair or a break down in a pipe, the area connected to that pipe will continue to receive water, as water will flow to that area from the other side.
Water reaches all points with minimum head loss.
During fires, by manipulating the cut-off valves, much of the water supply may be diverted and concentrated for fire-fighting.
Cost of pipe laying is more because relatively more length of pipes is required.
More number of valves are required.
The calculation of pipe sizes are more complicated.
C. Grid Pattern with Loops
Loops are provided in a grid pattern (similar to the above diagram) to improve water pressure in parts of a city (industrial, business and commercial areas).
Loops should be strategically placed so as a city continues to develop the water pressure can be continuous.
The advantages and disadvantages of this pattern are the same as those listed under the grid pattern section.
Hydraulic Analysis of Distribution Systems
Most commonly methods used are:
Determine the locations of "dead-ends" providing that water will be distributed in the shortest way. At the dead-end points there will be no flow distribution.
To apply the dead-end method for loop systems, convert it to branch system. To do this, a dead-end point is identified for each loop. The location of a dead-end point is based on the distance travelled to reach dead-end point from 2 different directions will almost equal to each other.
This method is applicable to closed-loop pipe networks.
The outflows from the system are assumed to occur at the nodes (NODE: end of each pipe section). This assumption results in uniform flow in the pipelines.
The Hardy-Cross analysis is based on the principles that:
At each junction, the total inflow must be equal to total outflow. (flow continuity criterion)
Head balance criterion: algebraic sum of the head losses around any closed- loop is zero.
For a given pipe system, with known junction outflows, the Hardy-Cross method is an iterative procedure based on initially estimated flows in pipes. Estimated pipe flows are corrected with iteration until head losses in the clockwise direction and in the counter clockwise direction are equal within each loop.
Equivalent Pipe Method
Equivalent pipe is a method of reducing a combination of pipes into a simple pipe system for easier analysis of a pipe network, such as a water distribution system. An equivalent pipe is an imaginary pipe in which the head loss and discharge are equivalent to the head loss and discharge for the real pipe system. There are three main properties of a pipe: diameter, length, and roughness. As the coefficient of roughness, C, decreases the roughness of the pipe decreases. For example, a new smooth pipe has a roughness factor of C = 140, while a rough pipe is usually at C = 100. To determine an equivalent pipe, you must assume any of the above two properties. Therefore, for a system of pipes with different diameters, lengths and roughness factors, you could assume a specific roughness factor (most commonly C =100) and diameter (most commonly D = 8"). The most common formula for computing equivalent pipe is the Hazen-Williams formula.
A water pipe is any pipe or tube designed to transport treated drinking water to consumers. The varieties include large diameter main pipes, which supply entire towns, smaller branch lines that supply a street or group of buildings, or small diameter pipes located within individual buildings. Materials commonly used to construct water pipes include cast iron, polyvinyl chloride (PVC), copper, steel or concrete.
Photograph 6.1. Construction of pipes
Photograph 6.1. Construction of pipes
For many centuries, lead was the favored material for water pipes, because its malleability made it practical to work into shape (this use was so common that the word "plumbing" derives from the Latin word for lead). This was a source of lead-related health problems in the years before the health hazards of ingesting lead were fully understood; among these were stillbirths and high rates of infant mortality. Lead water pipes were still in common use in the early 20th century and remain in many households. Lead-tin alloy solder was commonly used to join copper pipes, but modern practice uses tin-antimony alloy solder to join copper in order to eliminate lead hazards. If the water is treated before distribution or at the point of use (POU) depends on the context. In well planned and designed water distribution networks, water is generally treated before distribution and sometimes chlorinated, in order to prevent recontamination on the way to the end user. The varieties of water pipes include large diameter main pipes, which supply entire towns, smaller branch lines that supply a street or group of buildings, or small diameter pipes located within individual buildings. Water pipes can range in size from giant mains of up to 3.65 m in diameter to small 12.7 mm pipes used to feed individual outlets within a building. Materials commonly used to construct water pipes include polyvinyl chloride (PVC), cast iron, copper, steel and in older systems concrete or fired clay. Joining individual water pipe lengths to make up extended runs is possible with flange, nipple, compression or soldered joints.
Types of Pipes
Pipes come in several types and sizes. They can be divided into three main categories: metallic pipes, cement pipes and plastic pipes. Metallic pipes include steel pipes, galvanized iron pipes and cast iron pipes. Cement pipes include concrete cement pipes and asbestos cement pipes. Plastic pipes include plasticized polyvinyl chloride (PVC) pipes.
Steel pipes are comparatively expensive, but they are the strongest and most durable of all water supply pipes. They can withstand high water pressure, come in convenient (longer) lengths than most other pipes and thus incur lower installation/transportation costs. They can also be easily welded.
Galvanized Steel or Iron Pipes
Galvanized steel or iron is the traditional piping material in the plumbing industry for the conveyance of water and wastewater. Although still used throughout the world, its popularity is declining. The use of galvanized steel or iron as a conveyer for drinking water is problematic where water flow is slow or static for periods because it causes rust from internal corrosion. Galvanized steel or iron piping may also give an unpalatable taste and smell to the water conveyed under corrosive conditions.
Cast Iron Pipes
Cast iron pipes are quite stable and well suited for high water pressure. However, cast iron pipes are heavy, which makes them unsuitable for inaccessible places due to transportation problems. In addition, due to their weight, they generally come in short lengths increasing costs for layout and jointing.
Concrete Cement and Asbestos Cement Pipes
Concrete cement pipes are expensive but non-corrosive by nature. Their advantage is that they are extremely strong and durable. However, being bulky and heavy, they are harder and more costly to handle, install and transport.
Plasticized Polyvinyl Chloride (PVC) Pipes
PVC pipes are non-corrosive, extremely light and thus easy to handle and transport. Still, they are strong and come in long lengths that lower installation/transportation costs. However, they are prone to physical damage if exposed above ground and become brittle when exposed to ultraviolet light. In addition to the problems associated with the expansion and contraction of PVC, the material will soften and deform if exposed to temperatures over 65°C.
Cost Considerations Installation costs make up a major part of the total cost of a project. Differences in the cost of the actual pipe do not change the total cost of the project much. However, the following factors should be considered concerning installation costs and the choice of pipe:
Weight of the pipe: A lightweight pipe can be handled easier and faster.
Ease of assembling: Push-on joints can be assembled much faster than bolted joints.
Pipe strength: If one type of pipe requires special bedding to withstand external pressures while another pipe does not, the choice can impact installation costs significantly.
Health Aspects A leaking distribution system increases the likelihood of safe water leaving the source or treatment facility becoming contaminated before reaching the consumer. Moreover, leaking can result in considerable water loss on the way to the end-user. The distribution system must be designed, managed and maintained to guarantee a minimal level of leakage. The internal pipe pressure constantly must be greater than the external hydrostatic pressure. This will ensure the delivery of the water reducing loss from leaks and minimizing excess growth of pathogenic microorganisms. A certain level of free residual chlorine or chloramine disinfectant will reduce the risks of recontamination within the distribution system. Inflows of contaminated water during distribution are major sources of waterborne pathogens and thus cause of waterborne diseases. Water pipes are often made of copper and bath fixtures may be made from alloys containing copper (brass, bronze). (The U.S. Environmental Protection Agency (EPA) has established a Maximum Contaminant Level Goal (MCLG) for copper in public drinking water systems at 1,300 parts per billion (ppb). MCLGs are non-enforceable health standards for drinking water.) The principal source of copper in drinking water results from the leaching of copper from pipes and bath fixtures due to corrosive (acidic) water. The blue-green stain left in some bath fixtures is a sign of the presence of copper in water. Usually, excess copper in drinking water comes from the leaching of the plumbing system into the water that has been sitting in the pipes for several hours. Therefore, letting the water run for 30 to 60 seconds before using it for drinking or cooking will often significantly reduce copper levels.
Applicability Water pipes are required almost everywhere, especially for drinking water distribution. The most robust and durable type of water pipes is probably made from cement. Due to their heavy weight they are however difficult and expensive to install. PVC pipes are easier to install and much lighter, and thus particularly suited for remote areas that are difficult to access.
6.3. Engineering Structures
A catchment is an area of land, usually surrounded by mountains or hills, where all water that falls, flows to a common point. When rain falls on the ground in a catchment, it flows by gravity to the ocean or a lake. If the water flows on top of the ground it is called run-off. If it soaks into the ground it is called groundwater. Runoff and groundwater flow into waterways like creeks, rivers, lakes, lagoons and wetlands. This water eventually flows into the ocean. Within catchments, dams are built on rivers to store water and give us a more permanent water supply. Ten millimeters of rain falling on 100 square meters of catchment surface equals 1,000 liters of water; and ten millimeters of rain falling on 1 hectare of catchment surface equals 100,000 liters of water. A water harvesting scheme will only be sustainable if it fits into the socio-economic context of the area and fulfills a number of basic technical criteria.
Slope:The ground slope is a key limiting factor to water harvesting. Water harvesting is not recommended for areas where slopes are greater than 5% due to uneven distribution of run-off and large quantities of earthwork required, which is not economical.
Soils:Should have the main attributes of soils which are suitable for irrigation: they should be deep,notbe saline or sodic and ideally possess inherent fertility. A serious limitation for the application of water harvesting are soils with a sandy texture. If the infiltration rate is higher than the rainfall intensity, no runoff will occur.
Costs:The quantities of earth/stonework involved in construction directly affects the cost of a scheme or, if it is implemented on a self-help basis, indicates how labor intensive its construction will be.
What Is a Drinking Water Catchment? A drinking water catchment is an area of land where rainfall collects in rivers and streams that flow into reservoirs, or seeps into the soil to become groundwater where it is stored in underground aquifers. The captured water later becomes drinking water for the community.
Rivers as a Drinking Water Source The land use around a river and its catchment affects the quality and amount of water in the river. It is important that human land use is managed carefully to make sure rivers remainhealthy. Managing the impacts of all the activities around a river and its catchment is called total catchment management. Rivers are the main source of water that is captured in dams. Once water is collected in dams it can be stored, treated and then be evenly distributed between areas where and when it is needed.
Why Is It Important to Protect Drinking Water Catchments? Protected drinking water catchments provide a significant ‘natural’ barrier to contamination and yield high quality water. By protecting drinking water at the source, the risk is minimized of contamination and reduce the level of treatment required before it is supplied to the community. Source water protection is a crucial step to ensuring safe, good quality drinking water. Land uses and activities within drinking water catchments may adversely impact water quality. There are three main types of contamination:
Microbiological (protozoa, bacteria, viruses) — often associated with fecal material from humans (from septic tanks or direct water body contact) or domestic animals (such as cows).
Chemical — often associated with fuel spills, rubbish dumping, pesticides or fertilizers.
Physical — such as turbidity (cloudiness). This may be caused by erosion and runoff associated with fires, pigs wallowing, and vehicles or horses on unsealed roads or reservoir banks.
How Can We Protect Drinking Water Catchments? When a new drinking water source is developed, existing approved land uses on private land within the catchment may continue. The expansion or development of high risk land uses may not be supported, and some activities may be restricted to protect the source water from contamination and minimize the risk to public health.
A reservoir (etymology: from French reservoir a "storehouse") is an enlarged natural or artificial lake, storage pond or impoundment created using a dam or lock to store water. Reservoirs can be created by controlling a stream that drains an existing body of water. They can also be constructed in river valleys using a dam. Alternately, a reservoir can be built by excavating flat ground and/or constructing retaining walls and levees. Tank reservoirs store liquids or gases in storage tanks that may be elevated, at grade level, or buried. Tank reservoirs for water are also called cisterns. Underground reservoirs store almost exclusively water and petroleum below ground.
Types of Reservoirs
Reservoirs Dammed in Valleys A dam constructed in a valley relies on the natural topography to provide most of the basin of the reservoir. Dams are typically located at a narrow part of a valley downstream of a naturalbasin. The valley sides act as natural walls, with the dam located at the narrowest practical point to provide strength and the lowest cost of construction.
Photograph 6.2. Reservoirs Dammed in Valleys
In many reservoir construction projects, people have to be moved and re-housed, historical artifacts moved or rare environments relocated. Construction of a reservoir in a valley will usually need a river to be diverted during part of the build, often through a temporarytunnelor by-pass channel. In hilly regions, reservoirs are often constructed by enlarging existing lakes. Sometimes in such reservoirs the new top water level exceeds the watershedheight on one or more of the feeder streams.In such cases, additional side dams are required to contain the reservoir. Where the topography is poorly suited to a single large reservoir, a number of smaller reservoirs may be constructed in a chain.
Bank-side Reservoir Where water is pumped orsiphonedfrom a river of variable quality or quantity, bank-side reservoirs may be built to store the water.
Photograph 6.3. Bank-side Reservoir Such reservoirs are usually formed partly by excavation and partly by building a complete encircling bund orembankment, which may exceed 6 km (4 miles) in circumference (https://en.wikipedia.org/wiki/Reservoir - cite_note-ICEQueenMary-7).
Both the floor of the reservoir and the bund must have an impermeable lining or core: initially these were often made ofpuddled clay, but this has generally been superseded by the modern useofrolledclay. The water stored in such reservoirs may stay there for several months, during which time normal biological processes may substantially reduce many contaminants and almost eliminate anyturbidity. The use of bank-side reservoirs also allows water abstraction to be stopped for some time, when the river is unacceptably polluted or when flow conditions are very low due to drought.
Service Reservoir Service reservoirs store fully treated potable water close to the point of distribution. Many service reservoirs are constructed aswater towers, often as elevated structures on concrete pillars where the landscape is relatively flat. Other service reservoirs are entirely underground, especially in more hilly or mountainous country. Service reservoirs perform several functions, including ensuring sufficient head of water in thewater distribution systemand providing water capacity to even out peak demand from consumers, enabling the treatment plant to run at optimum efficiency. Large service reservoirs can also be managed to reduce the cost of pumping, by refilling the reservoir at times of day when energy costs are low.
Photograph 6.4. Service Reservoir
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. 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.
Photograph 6.5. A view from a dam
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.
Photograph 6.6. 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. 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.
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.
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