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Civil - Environmental Engineering

Explain about Estimation of Water Demand

   Posted On :  09.07.2016 04:03 pm
Explain about Estimation of Water Demand

Estimation of water demand is necessary to: 1. Calculate design flow 2. Determine the pumping power of machines to be used 3. Reservoir capacity 4. Pipe capacity

Explain about Estimation of Water Demand ?

While estimating the water demand, the above factors should be considered e.g. the size of the city; its population does matter when estimating the water demand. The more the size of population, more will be the demand. Estimation of water demand is necessary to:


  Calculate design flow

  Determine the pumping power of machines to be used

  Reservoir capacity

  Pipe capacity


To estimate water demand, following parameters must be determined or calculated.


1.     Average daily water consumption: It is based on complete one year supply of water. It is the total consumption during one year, divided by the population. q = (Q / P x 365) lpcd (liters per capita per day)


2.     Maximum daily consumption: It is the maximum amount of water used during one day in the year. This amount is 180% of the average daily consumption


MDC = 1.8 x Avg. daily consumption. It is usually a working day (Monday) of summer season.

3.     Maximum weekly demand: The amount of water used by a population during awhole single weekin a study span of 1 year.

Maximum weekly demand =   1.48   x        Avg.  D.      C

Maximum   monthly      demand       =       1.28   x        Avg.  D.      C

Maximum   hourly         demand       =       1.5     x        Avg.  D.      C

Maximum daily demand = 1.8 x Avg. D. C

4.     Fire water demand | Fire Demand: Theamount of water usedfor fire fighting is termed as fire demand. Although, the amount of water used in fire fighting is a negligible part of the combine uses of water but the rate of flow and the volume required may be so high during fire that it is a deciding factor for pumps, reservoirs and      distribution mains. Minimum  fireflow should be 500gpm (1890L/m) Minimum fire flow should be 8000 gpm  (32,       400 L/m) Additional flow may be required to protect adjacent buildings.     

Sources   of  Fresh water Documentary Video on Concrete

Flowchart of the sources of clean drinking water


Waste Water Treatment Disposal & Management


The quantity of water required for a community depends upon:


1.     Forecasted population

2.     Types and variation in demand (e.g. seasonal variation)

3.     Maximum demand (Per day/Per month)

4.     Fire demand

5.     Rural demand and supplies

6.     Appropriate / Available technology


Main sources of water are


  Surface water sources: Lakes impounding reservoirs, streams, seas, irrigation canals

  Ground water sources: Springs, wells, infiltration wells


Above are the common sources of clean drinking water, other different sources of drinking water are


Merits of surface sources


Merits of ground water sources


1.     Being underground, the ground water supply has less chance of being contaminated by atmospheric pollution.


2.     The water quality is good and better than surface source.

3.     Prevention of water through evaporation is ensured and thus loss of water is reduced.

4.     Ground water supply is available and can even be maintained in deserted areas.


5.     The land above ground water source can be used for other purposes and has less environmental impacts.


Demerits of ground water source


1.     The water obtained from ground water source is always pressure less. A mump is required to take the water out and is then again pumped for daily use.


2.     The transport / transmission of ground water is a problem and an expensive work. The water has to be surfaced or underground conduits are required.


3.     Boring and excavation for finding and using ground water is expensive work.


4.     The modeling, analysis and calculation of ground water is less reliable and based on the past experience, thus posing high risk of uncertainty.


Demerits of surface source


Chemical Characteristics of water





  Turbidity Acidity:


Acidity or alkalinity is measured by pH. PH measures the concentration

of Hydrogen ions in water. Ionization of water is



HOH H+ + OH-


In neutral solutions [OH] = [H] hence pH = 7


If acidity is increased, [H] increases and pH reduces from 7 (because H is log of [H]). The value of pH of water is important in the operations of many water and waste water treatment processes and in the control of corrosion.




The values of pH higher than 7, shows alkalinity. The alkaline species in water can neutralize acids. The major constituents of alkalinity (or causticity) are OH-, CO32- and bicarbonates HCO3 ions. Alkalinity in water is usually caused by bicarbonate ions.


Hardness of water: Definition of hard water


Hardness is the property that makes water to require more soap to produce a foam or lather. Hardness of water is not harmful for human health but can be precipitated by heating so can produce damaging effects in boilers, hot pipes etc by depositing the material and reducing the water storage and carriage capacity.


Absolute soft water on the other hand is not acceptable for humans because it may cause ailments, especially to heart patients. Hardness in water is commonly classified in terms of the amount of CaCO3 (Calcium Carbonate) in it.


Concentration of CaCO3             Degree of hardness


0 - 75 mg / L                       Soft

75 - 150 mg / L                  Moderately hard

150 - 300 mg / L                Hard

300 up mg / L                      Very Hard


Low level of hardness can be removed just by boiling but high degree of hardness can be This method has also the benefit that iron and manganese .removed by addition of lime

.organisms are reduced-including micro contents are removed and suspended particles




A reservoir (etymology from French r�servoir a "storehouse ) or an artificial lake is used to store water. Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.


The term reservoir may also be used to describe underground reservoirs such as an oil or water well.


A dam is any barrier that holds back water; dams are primarily used to save, manage, and/or prevent the flow of excess water into specific regions. In addition, some dams are used to generate hydropower. This article examines man-made dams but dams can also be created by natural causes like mass wasting events or even animals like the beaver.


Another term often used when discussing dams is reservoir. A reservoir is a man-made lake that is primarily used for storing water. They can also be defined as the specific bodies of

water formed by the construction of a dam. For example, the Hetch Hetchy Reservoir in

California's Yosemite National Park is the body of water created and held back by the O'Shaughnessy Dam.


Types of Dams

Today, there are several different types of dams and the man-made ones are classified by their size and structure. Typically a large dam is classified as being higher than 50-65 feet (15-20 meters) while major dams are those over 492-820 feet (150-250 meters).


One of the most common types of major dams is the arch dam. These masonry or concrete dams are ideal for narrow and/or rocky locations because their curved shape easily holds back water via gravity without the need for a lot of construction materials. Arch dams can have one large single arch or they can have multiple small arches separated by concrete buttresses. The Hoover Dam which is on the border of the U.S. stateAnother type of dam is the buttress dam. These can have multiple arches, but unlike a traditional arch dam, they can be flat as well. Normally buttress dams are made of concrete and feature a series braces called buttresses along the downstream side of the dam to prevent the natural flow of water. The Daniel-Johnson Dam in Quebec, Canada is a multiple arch buttress dam.


In the U.S., the most common type of dam is the embankment dam. These are large dams made out of soil and rock which use their weight to hold back water. To prevent water from


moving through them, embankment dams also have a thick waterproof core. The Tarbela Dam in Pakistan is the world's largest embankment dam.


Finally, gravity dams are huge dams that are constructed to hold back water using only their own weight. To do this, they are constructed using extensive amounts of concrete, making them difficult and expensive to build. The Grand Coulee Dam in the U.S. state of Washington is a gravity dam.


Types of Reservoirs and Construction

Like dams, there are different types of reservoirs as well but they are classified based on their use. The three types are called: a valley dammed reservoir, a bank-side reservoir, and a service reservoir. Bank-side reservoirs are those formed when water is taken from an existing stream or river and stored in a nearby reservoir. Service reservoirs are mainly constructed to store water for later use. They often appear as water towers and other elevated structures.


The first and usually largest type of reservoir is called a valley dammed reservoir. These are


reservoirs that are located in narrow valley areas where tremendous amounts of water can be held in by the valley's sides and a dam. The best location for a dam in these types of


reservoirs is where it can be built into the valley wall most effectively to form a water tight seal.


To construct a valley dammed reservoir, the river must be diverted, usually through a tunnel, at the start of work. The first step in creating this type of reservoir is the pouring of a strong foundation for the dam, after which construction on the dam itself can begin. These steps can take months to years to complete, depending on the size and complexity of the project. Once finished, the diversion is removed and the river is able to flow freely toward the dam until it gradually fills the reservoir.

Dam Controversy


In addition to the high cost of construction and river diversion, dams and reservoirs are often controversial projects because of their social and environmental impacts. Dams themselves affect many different ecological components of rivers such as fish migrations, erosion, changes in water temperature and therefore changes in oxygen levels, creating inhospitable environments for many species.


In addition, the creation of a reservoir requires the flooding of large areas of land, at the


expense of the natural environment and sometimes villages, towns and small cities. The construction of China's Three Gorges Dam, for example, required the relocation of over one

million people and flooded many different archaeological and cultural sites.


Main Uses of Dams and Reservoirs


Despite their controversy, dams and reservoirs serve a number of different functions but one of the largest is to maintain an area's water supply. Many of the world's largest urban areas


are supplied with water from rivers that are blocked via dams. San Francisco, California for example, gets the majority of its water supply from the Hetch Hetchy Reservoir via the Hetch Hetchy Aqueduct running from Yosemite to the San Francisco Bay Area.


Another major use of dams is power generation as hydroelectric power is one of the world's major sources of electricity. Hydropower is generated when the potential energy of the water


on the dam drives a water turbine which in then turns a generator and creates electricity. To best make use of the water's power, a common type of hydroelectric dam uses reservoirs with


different levels to adjust the amount of energy generated as it is needed. When demand is low for instance, water is held in an upper reservoir and as demand increases, the water is released into a lower reservoir where it spins a turbine.


Some other important uses of dams and reservoirs include a stabilization of water flow and irrigation, flood prevention, water diversion and recreation.

Tags : Civil - Environmental Engineering
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