Category Archives: dams

Cofferdams-Types of Cofferdam and construction

Cofferdams are enclosures built inside bodies of water such as lakes and rivers to provide a dry working environment throughout the construction period. Cofferdams are temporary dykes that are built across a body of water. They allow the water to be pumped outside, ensuring a clean and dry construction site.

This article is about the significance and definition of Cofferdam and about the different types of cofferdams preferred in construction works.

  1. Significance of cofferdams
  2. What is a Cofferdam?
  3. Types of cofferdams
    1. Earthen cofferdam
    2. Rock-fill cofferdams
    3. Single walled cofferdams
    4. Double walled cofferdams

Significance of cofferdams

Construction in water is the most challenging task in civil engineering. A safe and dry working environment is necessary to preserve the project’s safety and construction quality. However, various strategies are used to construct structures in the water and maintain the area’s dryness. One of the most popular and widely utilised ways is the use of cofferdam.

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What is a Cofferdam?

Making cofferdam involves building watertight barriers all around the construction site, pumping the water out to expose the water, and then erecting the cofferdams. For bridge piers, marine jetties, ports, etc. cofferdams are preferred. Size, water depth, water flow velocity, and other factors affect the design and types of cofferdam. Let us have a look into the types of cofferdams popularly used in construction.

Types of cofferdams

Depending on the design requirements, water depth, soil conditions, type of material used, etc., coffer dams are classified into many types.

  • Earth cofferdam
  • Rock fill cofferdam
  • Single sheet pile cofferdam
  • Double-wall sheet piling cofferdam
  • Braced cofferdam
  • Cellular cofferdam

Earthen cofferdam

Earthen cofferdam is the most common and simplest type of cofferdam. They are appropriate for locations with minimal water depth and water current. Sand, soil, clay, and boulders that are readily available locally are used to construct earthen cofferdam. The earthen cofferdam must be at least one metre above the maximum water level.

When an area of excavation is quite extensive, earthen cofferdam is used and require a sizable base area. To withstand water pressure and seepage, impervious clay core or sheet piles are driven in the centre. In order to prevent scouring and possible dam failures, the upstream side is stone-pitched. These technologies do not, however, completely provide waterproof zones. Generally, to remove the water, pumps and waterproofing systems must be installed.

Earthen Cofferdams
Earthen Cofferdams

Rock-fill cofferdams

When compared to earthen cofferdams, rockfill cofferdams are superior. The choice of rockfill dams is influenced by the cost and availability of rocks in the area. Generally, the rockfill dam’s maximum height should be limited to under 10 feet. The rockfill area is pervious and will be lined with an impervious clay layer to prevent seepage and dam failure.

Rock filled Cofferdams
Rock filled cofferdam

Single walled cofferdams

When the depth of the water is less than 6 metres and the area of work is localised, such as on a bridge pier, single-walled cofferdams are preferred. Basically, single-walled coffer dams are primarily built by driving steel sheets into the inside as a support layer after driving timber sheets into the exterior as guide piles. In situations where the water is deep, guide piles may be steel sections.

After the guide piles have been driven, wales or runners made of wood logs are bolted to the guide piles at appropriate vertical intervals. Wales are used to position the inside sheets’ distance from the wooden planks at a specific distance as shown in the figure. Mostly, these wales are fastened to the sheets using bolts from both sides.

Single walled Cofferdams
Single walled cofferdam

The inside sheet piles have strong bracing. Sandbags are positioned on both sides of the walls to increase stability even more. For clay, the penetration depth should be approximately 1 metre, 0.5-0.75 metres for sand, and 0.25-0.5 metres for gravel, etc. Construction can begin when the interior water has been pumped out.

Also Read : Reservoirs vs Dams – Reservoir – types and functions

Double walled cofferdams

Double-walled cofferdams are preferred when the construction area is large, the water depth is higher than 6 metres, and single-walled cofferdams appear to be uneconomical. Double-walled cofferdams Consist of two straight, parallel vertical walls of sheet piling coupled together, with the space between them filled with soil. If the height is greater than 3 mtr, double wall sheet piles must be strutted as illustrated in the figure.

Double walled cofferdams
Double walled cofferdams

In order to give stability to the cofferdam, the filling materials must be carefully chosen while taking the coefficient of friction into account. The sheet piles are driven into the bed in the upstream area to a good depth to avoid leaking from the ground below.

Components of dam – 12 dam components explained

Components of dams have specific functions in managing their primary responsibility of water management. Dams are structures built across water bodies to control water flow and levels. Furthermore, dams are also required for a wide range of projects, including small irrigation and water supply schemes as well as larger hydropower and disaster management schemes. Dams’ primary responsibility of managing water is managed by each component in a specific way. The components of the dam and their functions are discussed in this article.

The upstream side of a dam is the area where the water is collected. The water level is high on the upstream side. The downstream is the section of the barriers with low water levels.

Components of Dams - Audio
Components of Dams – Audio
  1. Advantages of Dams
  2. Components of Dams
    1. Water-retaining structure – Components of Dam
      1. Heel 
      2. Toe
      3. Abutment
      4. Crest/Roadway of Dams
      5. Cut off 
      6. Parapet wall
    2. Water-releasing structure: Components of dams
      1. Galleries
      2. Spillway
      3. Diversion tunnel
      4. Sluice way
      5. Freeboard
    3. Water conveying structure – Components of dams
      1. Conduit
  3. Examples of some major dams
    1. Bhakra Dam ( Gravity dam )
    2. Idukki dam ( Arch dam )
    3. Nagarjuna Sagar Dam (Masonry Dam)
    4. The Hirakud Dam (Earthern dam)
    5. KARIBA DAM (Double curvature arch dam)

Advantages of Dams

The dams provide a range of environmental, economic and social benefits. One of the most important benefits of dams is water storage. The stored water is used for drinking, cleaning, bathing, gardening, irrigation purposes, and industrial purposes.

Dams may be constructed to meet the following functions

  • Water storage: Dams are one of the major sources of water for domestic uses like cooking, cleaning, bathing, washing, drinking water, gardening, agricultural uses, industrial purposes, etc. The water is supplied through the canal or with the help of a pipe system from the dam.
  • Irrigation – The water from the dams is diverted through canals to the field where the water level is low for irrigation purposes.
  • Hydroelectric power – The water stored in the dam is passed through turbines for hydroelectric power generation.
  • Flood prevention – The water level of rivers, streams, etc is maintained by constructing dams across it. This prevents the loss and damage from unexpected floods. 
  • Recreation – The water stored in the dam is used for fishing, boating, and other recreational activities.
  • Debris control – The dam also provides the retention of hazardous material and protects the environment.

Components of Dams

The components of dams play an important role in maintaining the primary responsibility of water management. The parts of the dams are broadly classified as follows.

  • Water-retaining structure
  • Water-releasing structure
  • Water conveying structure
Components of dams
Components of dam

Components of Dam – Youtube video

Components of a dam

Water-retaining structure – Components of Dam

The water-retaining structure is the dam’s walled structure that resists water while allowing a controlled amount to flow downstream. Accordingly, the side of the barrier where water is collected is known as the upstream side, and where the water flows is known as the downstream side. Generally, the following component of dams makes up the dam’s water-retention section

  • Heel
  • Toe
  • Abutment
  • Crest
  • Cutoff
  • Parapet wall


The part of the dams meeting with the groundwater or upstream side is called the heel. (Ref fig.)


The portion of the dams meeting with the groundwater or downstream side is called the Toe.(Ref fig)


Abutments support the lateral pressure. These are the sides of the valley. These are concrete or masonry structures. 

Crest/Roadway of Dams

The section of the dams used as a roadway or walkway is the crest. It is the upper area of the dam.

Cut off 

The cut-off is an impervious barrier constructed beneath the earthen dams. The main function is to reduce the loss of stored water in the reservoir by preventing seepage.

Cut off of earthen dams
cut off – Earthen dams

Parapet wall

The parapet wall is seen below the crest near the roadway. This assists in the dam investigation and safety barriers.

Water-releasing structure: Components of dams

Mainly, the components of dams that allow water to flow downstream are known as the water-releasing structure. Generally, these dam components are technically known as the dam’s spillways. Generally, the spillway’s mechanism allows for controlled water volume. A spillway contains the following components.

  • Galleries
  • Spillways
  • Diversion tunnel
  • Sluiceway
  • Freeboard


These are hollow openings passing through the dam as shown in fig. The main purpose of providing a drainage gallery is to collect seepage water from the foundation and body of the dam and drain it out. The seepage water received by foundation galleries is drained away under gravity. The galleries are broadly divided into …

  • Grouting gallery
  • Inspection Gallery
  • Drainage gallery
  • Valve gallery
  • Transformer Gallery
Components of dam - Drainage gallery
Components of dam – Drainage gallery
Drainage gallery of dams
Components of dam – Drainage gallery


The role of the spillway is to convey excess water and prevent damage. The water passes from upstream to downstream. The spillway helps in the emergency discharge of water. 

They are two varieties

  • Controlled spillway 
  • Uncontrolled spillway

In a controlled spillway the flood flow is regulated by the gate. 

Also Read : Spillway types and features – A comprehensive guide

Diversion tunnel

The purpose of the diversion canal is to redirect the water. Diversion tunnels are constructed during the construction stage of dams.

A diversion tunnel may also be constructed to divert floodwater to divert water from mountainous regions to low-lying areas experiencing a water shortage supply.

Sluice way

The role of the sluiceway is to remove the silt accumulated. 

Sluice way
Sluice way


The interval between the dam heads to the maximum water level on the upstream side.

Water conveying structure – Components of dams

Water-conveying structure mainly conduit and conveys the water from reservoirs through, around, or under an embankment dam


These are closed pipe structures. Conduits act as a passage for the water supply. Bottom discharge conduits are pipes that cross the body of the dam from the upstream to the downstream sides enabling water flow.

Examples of some major dams

Bhakra Dam ( Gravity dam )

The Bhakra Dam is an Indian gravity dam built across the river Sutlej in Himachal Pradesh. This dam is constructed in 1963. The height of the dam is 226 meters. The length of the dam is 518 meters.
Gobind Sagar is a reservoir of this dam. The Bhakra Dam is composed of alternating layers of light red clays and sandstone.
This dam has four spillways. It helps in irrigation, hydroelectric power generation and recreation. The major source of irrigation water supply in Haryana, Punjab and Rajasthan is this dam.

Idukki dam ( Arch dam )

Idukki dam is an arch dam constructed across the Periyar river in Kerala. It is 554 feet high. One of the biggest arch dams in Asia. The dam provides hydroelectricity, irrigation and tourist destinations. It is built between Kuravan and Kurathi hills.

Idukki Dam - Arch dam
Idukki dam

Nagarjuna Sagar Dam (Masonry Dam)

Nagarjuna Sagar Dam is a stone masonry dam completed in 1967. The dam is a symbol of modern architecture. The purpose of this project was to generate hydroelectricity. It has a 26-crest gate.

The Hirakud Dam (Earthern dam)

The Hirakud dam is located in Orissa state over the river Mahanadi near Sambalpur. The length of the dam is 4800 meters and 59 meters high. It is the oldest multipurpose dam completed in 1957.
The Hirakud Dam is the 4800-meter long and 59 meters high. The gross storage capacity of the dam is 1841 million cum.

KARIBA DAM (Double curvature arch dam)

Kariba Dam is a double curvature arch dam constructed in 1960. It has been built over the Zambezi river. The crest length is 620m and 128m high. The dam provides an example of improving the quality of rocks.

Types of Dams – 3 Classification Criteria Explained

Types of Dams : Dams are large impounding structure or barriers which are outlined and constructed on a river or stream. Dams provide a wide variety of environmental, economic, and social benefits. The configuration and type of dam built should fulfil one or more purposes. This article is about the criteria for the selection of dam types.

Also Read : Dams- Components of dam – 12 dam components explained

Types of dams – Selection criteria

While selecting the type of dams the following criteria is taken into consideration. Geology, Topography, Hydrology, etc are the essential requirements for planning and selecting dam types.

  • Geological features like slopes, valleys, mountains, channels, etc. to be taken into account while selecting the types of dams.
  • The topography of the dam site has to ensure sufficient slope on the upstream side of the dam. For small dams narrow sites to be chosen. For low-lying areas, arch dams are more suitable. Earthen dams are preferred in steep areas.
  • The hydrological data like groundwater table, precipitation, stream flow, etc. serves as a perfect guideline.
  • In rivers, streams, etc. dams serve as flood prevention mechanisms. They collect, store, and manages floods with the help of reservoirs.
  • Since dams are massive structures, the materials required for construction should be available nearby. The soil wrenched from the reservoir can be used as earth fill and reduces transportation costs.
  • The foundation of the dam must have a uniform bearing capacity. Earthen dams are good for gravel-type soils. For clay-type soils, there are chances of unequal settlement. Therefore foundations have to be designed with strict precautions and factors of safety.
  • Depending on the height of the location, dams are selected. Concrete gravity dams have high altitudes. Earthen dams have less elevation.
  • Earthquake zone is another criterion for dam type selection. An earthquake can cause structural failure and damages in dams. Therefore Seismic analysis and intensity to be investigated before choosing the type.

Types of dams

Dams are broadly classified into three categories based on the following criteria.

  1. Classification based on function
  2. Classification based on the structure
  3. Classification based on the material used

Also read : Dams – Importance – Selection Criteria, Advantages, Disadvantages.

Types of dams – Based on Functions

Based on function, Dams are grouped into four types. 

  • Storage dam
  • Diversion dam
  • Detention dam
  • Cofferdam

Storage Dam

Storage dams are the most commonly used dam type constructed across high water discharge rivers and streams for water storage purposes. This surplus water stored in the upstream area is utilised for hydroelectricity, human needs, water for local wildlife, recreation, domestic water supply schemes, irrigation purpose, etc.

Diversion Dam

The diversion dam is used to divert water and is also known as weirs or barriers. These dam types can divert water from a wide to a fine-scale which provides the requisite pressure to push water through channels and conduits. Diversion dams are low-height dams having a small upstream storage area.

Detention Dam

The detention dams impede the flow of water in the downstream area. These dams serve as flood controllers by retarding the downstream flows. Flash flood chances to some extent by providing detention dams. They are also known as debris dams named debris dams as it retards sediments like debris.


Cofferdams are temporary enclosures used for evacuating water from construction sites in marine or wet areas involving bridge rehabilitation, piling works, oil exploration works, bridge foundations, etc. The main purpose of cofferdams is to provide a dry working area by retaining soil and water. They are portable and reusable structures.

Types of Dams – Based on structure

Based on the type of structure used for construction of dams the dam typse are classified as follows.

  • Gravity dams
  • Arch dams
  • Buttress dam
  • Embankment dam

Gravity Dam

Gravity dams are massive solid masonry or concrete structures designed to hold a huge volume of water. The horizontal trust of water and gravity holds the dam structure to the ground. It has a triangular profile. The strength of foundation rock is contemplated in the design of this dam.

Arch Dam

An arch dam is a concrete dam that has a curved upstream in the plan.
An arch dam works on the principle of hydrostatic pressure exerted into the curved structure straightens the structure and strengthens by pushing the structure into the foundations or abutments. Thus the hydrostatic pressure is equalized by arch actions.

Compared to gravity dams, the Arch dam bears less weight and got thin walls. An arch dam is built-in narrow gorges and is arch-shaped.

Arch dams are economical and considerable.

Types of Dam - Arch dam
arch dam

Buttress Dams

Buttress dams assemble in bowed or straight shape. They got a sloping deck supported by buttresses at regular intervals. Mostly they are preferred for soil with poor bearing capacity. The main components of the dam include a face slab, standing slab, and base slab.

This dam provides stability and endurance. The pressure in the deck is held by the buttress. 

Types of Dams - Buttress dams

Embankment dam

Embankment dams are antiquated types of dams built with natural materials like rock, soil, sand, clay, etc, and industrial waste materials like compacted plastics, etc. They are built-in wide valleys. They are of two types; Earth fills and rock fill. The core consists of impermeable materials.

Reinforced concrete and asphalt concrete are used in rock fill dams.

Types of Dams – Based on Materials Used.

Based on material used, Dams are classified as

  • Masonry dams
  • Steel dams
  • Timber dams

Masonry dams

Types of Dams built by using earth, stone/bricks and joined with mortar are known as masonry dams. In a masonry dam, seepage is less.

Steel dams

steel dam are types of dams that is made of steel, rather than the more common masonry, earthworks, concrete or timber construction materials, They possess good strength when compared to other dams. On the downstream side, the footing is provided at frequent intervals. For construction, it uses steel trusses and anchors.

Timber dams

These are temporary dams made of wood. Diversion of water is the main function of this dam. They have a short life span and limited height.

Concrete Dams

Concrete dams are large structures. Concrete dams are types of dams constructed using cementitious materials like concrete etc. They possess good strength. The construction cost of this dam is high.

Also read : Components of a bridge – Bridge Components – Types and functions

Spillway – Types,components and features

A spillway is an inevitable component of any dam that helps in the emergency discharge of water. Spillways is an important topic in hydraulics. Design of dam and related information comes under the topic. In this blog, I will show you the importance of spillway and their types. We will go deeper into each of these in separate sections.

Let’s start from the definition.

What is a Spillway?

A spillway is a structure that allows water from a dam or levee to be released in a regulated manner downstream, usually into the riverbed of the dammed river. Before going into their classification, let’s first understand why dams require a spillway.

Spillways are important because,

  • Any reservoir has a certain amount of water storage space. 
  • If the reservoir is full and flood water enters, the reservoir level will rise, potentially causing the dam to overflow. 
  • To stop this, the flood must be carried on downstream. 
  • This is accomplished by providing a spillway that draws water from the reservoir’s roof. 
  • A spillway may be built into or separate from the dam.

Also read: Dams – Importance, Site selection, Advantages and Disadvantages

Let’s go on a trip to see the different types of spillway.

Also read: Types of Dams – 3 Classification Criteria Explained

Spillway Types

Depending on the suitability of the site and other conditions, various types of spillways may be given. A spillway is made up of three parts:

  • Control structure – It is the spillway’s most critical feature. The control system is designed to prevent outflow from the reservoir when the water level falls below a predetermined level. But also allows outflow if the water level rises above that level.
  • Conveyance channel –  Dam spillways that use open-channel flow rules to transport impounded water are known as open channel spillways. They can serve as principal spillways, emergency spillways, or both. They can be found on the dam itself or on a natural incline in the dam’s vicinity.
  • Terminal structure – Static energy is converted to kinetic energy as water flows from the reservoir over the spillway. As a result, the flow rate at the downstream end of the spillway is extremely high. It causes damage to the dam, spillway, and other nearby structures. As a result, before the flow is returned to the river downstream, the high energy of the flow must be dissipated. To dissipate the excess energy, terminal structures are given at the discharge channel’s downstream end.

The first two can be combined in some cases. Now we have seen the main parts of a spillway. Time to learn about the different categories. The following are some of the most popular types:

  • Drop Spillway
  • Ogee Spillway
  • Chute Spillway
  • Side-Channel Spillway
  • Shaft Spillway
  • Siphon Spillway
  • Conduit Spillway
  • Culvert Spillway

Let’s discuss each of its features in detail.

Drop Spillway – Spillway types

Drop spillway
Drop Spillway

This is a weir structure and the flow passes through the weir opening, then drops to a level apron or stilling basin before entering the downstream channel.

  • The overflowing water falls freely and almost vertically on the downstream side of the hydraulic system. 
  • Ideal for low dams or weirs. 
  • The crest of the spillway has a nose to prevent the water jet from striking the structure’s downstream foundation. 
  • On the downstream side, a horizontal impervious apron should be provided to protect the structure from the impact of scouring.
  •  A basin is often built on the downstream side to create a small artificial pool known as a water cushion. 
  • The aim of this cushion is to dissipate the energy of falling water.

Ogee type Spillway

Ogee Spillway
Ogee Spillway
  • This is an overflow spillway with a controlled weir and a profile that is ogee-shaped (S-shaped). 
  • It’s designed to track the lower surface of a horizontal jet that emerges from a sharp-crested weir. 
  • At the design head, the pressure at the ogee crest remains atmospheric. 
  • At a lower head, the pressure on the ogee crest becomes positive. 
  • This results in the backwater effect which decreases the discharge. 
  • At a higher head, the pressure on the crest becomes negative, resulting in the backwater effect, which increases the discharge.

Chute Spillway – Spillway types

The chute spillway is an open channel-like structure with a fitting inlet and outlet built on the steep slope of the gully face.

Chute spillway
Chute spillway
  • Also called Trough Spillway. 
  • The aim of the Chute Spillway is to protect the valley walls from damage that could put the dams in danger. 
  • It is made up of a reinforced concrete slab that is a steeply sloping open channel.
  •  The spillway is not always of the same width.
  •  It is normally narrowed for the economy and then widened at the end to reduce the discharging velocity. 

Side-channel spillway – Spillway types

A side channel spillway has a control weir that runs alongside and roughly parallel to the spillway discharge channel’s upper half.

Side channel spillway
Side channel spillway
  • Unlike a chute spillway, the water spilling from the crest is turned 90 degrees and flows parallel to the crest of a side-channel spillway.
  • When flanks of adequate width are not accessible, side-channel spillways are preferred over chute spillways, typically to prevent heavy cutting.
  • The angle of turn of the water flow after passing through the weir crest can be held between 0 and 90 degrees. The spillway discharge falls into a shallow trough after flowing over the weir crest.

Shaft Spillway

  • The water from the reservoir enters a vertical shaft in the shaft spillway, which transports the water into a horizontal tube. 
  • A Shaft Spillway is made up of a horizontal crest and a vertical shaft, with the top surface at the spillway’s crest level and the lower end attached to a vertical shaft. 
  • The vertical shaft’s other end is attached to a horizontal conduit or tunnel that runs through or around the dam and transports the water to the river below. 
  • When the conditions aren’t conducive to an overflow or a chute spillway, a shaft spillway is used.
Shaft spillway
Shaft spillway

Siphon Spillway

Siphon spillway
Siphon spillway
  • The theory of siphonic action governs the operation of a Siphon spillway. 
  • A syphon spillway is made up of two syphon pipes. 
  • One of them is held upstream and in contact with the reservoir, and the other spills water on the downstream side. 
  • An airtight RCC cover, known as the hood, is placed over an oggy-shaped concrete body wall to form the syphon duct. 
  • The crest of the spillway is formed by the top of the body wall, which is held at the reservoir’s F.R.L.

Conduit Spillway

  • Made up of a closed conduit that transports floodwaters to a downstream channel. 
  • A vertical or inclined shaft, a horizontal tube, or a conduit built in an open cut and then covered are all examples of closed conduits. 
  • It is built in the abutment or underneath the dam. 
  • Dam sites in narrow canyons with steep abutments would benefit from such a spillway.

Labyrinth Spillway

  • A labyrinth spillway is one in which the weir wall is built in a zigzag pattern to maximise the effective length of the weir crest in comparison to the channel width. 
  • This improvement in effective length increases the weir’s discharge capacity, allowing for more water flow at small heads to be easily transported downstream.

That’s it about spillway and their types. Hope you found it insightful. Next time you visit a dam site, make sure that you are able to identify the type of spillway present there. If you get confused don’t forget to check out our detailed articles on dams.

Reservoir vs Dams || Reservoir- Types and functions

Reservoirs are artificial or natural structures used for collecting and storing for further usages. The reservoir is formed by constructing dams across rivers, valleys, and streams. The Reservoir is located on the upstream side of dams and prevents floods by managing the water flow apart from serving as a storage basin.

Also Read : Types of Dams – 3 Classification Criteria Explained.

Also Read : Components of a bridge – Types and Functions Explained

Functions of Reservoirs

Reservoirs are man made lake or a fresh water body that plays an important role a lot of ways as listed below.

  • Water supply
  • Flood control
  • Environmental management
  • Hydroelectric power management
  • Navigation
  • Irrigation
  • Development of fish and wildlife
  • Soil conservation
Reservoir behind dam
Reservoir behind dam

Also Read : Components of a dams-12 Dam Components explained.

Types of reservoirs

Reservoirs are broadly classified into three types.

  • Valley dammed reservoirs
  • Back-side reservoirs
  • Service reservoirs

Valley dammed reservoirs

Valley dammed reservoirs are located between mountain valleys where there is an existing water body. A dam is built in the narrowest portion for holding the water.

Bank-side reservoirs

Bank side reservoirs are made by diverting water from rivers and streams to an existing reservoir. These reservoirs can be located in different geographical locations.

Service reservoirs

Service reservoirs are man made reservoirs located above the ground or below the ground. The huge water towers/ water tanks and sumps are service reservoirs.

Service Reservoir
Service reservoir

Reservoir components and terms

There are some specific terms for defining the water level and storage capacity of a reservoir

Pool level

The water level at a particular point of time is called the pool level. No water can be collected above the maximum pool level. No water can be drained below the minimum pool level.

Useful Storage

The volume of water within the normal pool level and minimum pool level is known as useful storage.

Dead storage

The volume of water underneath the minimum pool level is known as dead storage.

Surcharge Storage

The volume of water within the normal pool level and maximum pool level is known as surcharge storage.

Reservoir yield

The volume of water discharged within a certain interval of time is reservoir yield.

Reservoir Planning

Accurate planning is necessary for locating a reservoir. For planning a reservoir the following points are taken into consideration.


A Broad natural valley opts as the site of a reservoirs. The valley can be U- shaped or V-shaped. For reservoirs sites, flatland and plains are not considered.

Ground water condition

The groundwater investigation is done by analysing the water table level and reservoir level.Hence it is an ideal situation for the reservoirs site if the water table level and the top reservoir level is the same. No loss or gain is ensured.


Permeability is a default nature as the rocks are porous. It is considered when the water table is below the reservoirs level. It is a significant characteristic for investigation.

Hydrological Investigation

This involves the study of runoff pattern. Leakage quantity should be minimum. Storage capacity calculation and maximum discharge rate are also estimated.

Difference between dams and reservoirs

Structural barriers built across,rivers
valleys,streams etc for the purpose of
storing and managing water flow.
Reservoir is a large water body formed behind the constructed dam structure across a river,stream,etc
Dams are reinforced concrete structures or structures made of bricks rubble etc erected across water bodies to control the water flow.The reservoir is the water that accumulates behind the constructed dam.
Create site for hydro electric power generation. This can improve the industrial development and living standards of people living in that region.Stores water for later usage, water for human consumption and excess water for agricultural and industrial uses.
Dams fluctuates oxygen levels and restricts migration of fishes in the river.Reservoirs leads to the displacement of people.
Dams are tourist attraction sitesHelps in water transportation

Reservoir Sedimentation

Rivers transport sediments. During heavy rainfall, sediments are carried by the water to the reservoir. Silts are accumulated due to soil erosion. These get collected in the dead storage.
Sedimentation depends on Rainfall intensity, Soil nature, Soil type, topography and vegetation. The sediment deposit replaces the water storage. Thus water supply and flood control are affected.
The sediments should be removed eventually. Hydraulic flushing, dredging, Explosive mobilization are methods adopted for sedimentation removal.


Environmental impacts of Reservoirs.

  • The construction of the reservoir prompts severe effects on the environment.
  • Since the water is stored, the rate of transpiration is high. This changes the moisture concentration.The climate of the surrounding area is affected. It causes natural temperature fluctuation. This causes heavy rainfall and deprives the traditional rainfall patterns.
  • Reservoirs disturb the flow and composition of the rivers. The water from the downstream has high energy, it enhances the water flow by eroding the river bed.
  • The change in the river composition affects the upstream and downstream habitats. The reservoirs disturb the fish and birds migration.
  • Studies show that reservoirs emit greenhouse gases. Anaerobic bacterias are seen in the river bed releases carbon dioxide and methane. This occurs in tropical regions.
  • They also cause troubles for people living near the reservoir site. Old and failing dams lead to disasters.
  • Reservoir induced seismicity is a phenomenon of provoking earthquake by reservoirs. This happens when water sweeps into the minor cracks under the bed.

Dams – Site selection, Advantages and Disadvantages

Dams are obstructions constructed across the water bodies to control the water flow and water levels. Dams serve as a water source for human consumption, irrigation purpose, power generation, prevention of floods when there is a sudden discharge of floodwater. The water that impounds in the dams is also used for recreation purposes and industrial uses. They are mostly made up of rock fill or earthen.

What are dams?

The dams are barriers built to restrict and manage the natural flow of rivers, waterways, or streams for human needs. A dam may be a natural one or artificial. They help to confine water and reduces the water rise and flood problems. Dams retain water by forming a reservoir behind the structure. They got an Upstream side and Down stream side. The place where water is collected is called the Upstream side. The water impounded is used as storage and is called Reservoir.


Also read : Type of Dams – 3 main classification criteria explained

Benefits of dam

The dams provide a range of environmental, economic and social benefits

One of the most important benefits of dams is water storage. The stored water is used for drinking, cleaning, bathing, gardening, irrigation purposes, industrial purposes.

The large dams and reservoirs serve as recreational spots for fishing and boating purposes. Dams play a major role in preventing damages due to floods by collecting and storing excess water during rains and are released as and when required.

The dam is the main source of providing water for irrigation purposes in areas where rain and water are scarce.

Also read : Methods of irrigation – 3 methods full guide

Site selection of dams.

The dam is a massive and strategic structure and needs a perfect site selection before proceeding with construction.

  • Topography -Making maximum use of the available volume of water is the main objective of a dam. The slope of the area, groundwater table, contour, weather condition, etc is other factors for dam site selection.
  • The bed level at the dam site should preferably be higher than that of the river basin. This is for reducing the dam height and facilitating the drainage of water.
  • Foundation – The foundation should bear the whole weight and pressure of the dam. It should be permeable, thick, and should have adequate strength.
  • Economic criteria – This includes the construction and maintenance cost of the dam. The land value, cost of construction materials and construction time, etc. to be taken into account while selecting a dam site. Quality construction materials required for dam construction have to be available near the proposed location. Always opt for a small length dam with maximum water holding capacity for economising the project. 
  • Height of Dam – The height of the dam should be economical, and should store the maximum amount of water.
  • Earthquake zone – The dam site should not be an earthquake zone. It may cause a collapse of the dam and losses. 
  • Material availability – Materials like rock, aggregate, soil, filler, etc. should be available near the site.
  • Climate condition – The climate of the site should be bearable, possibilities of landslides should not occur.
  • Spillway site: A suitable location for the spillway should be available in the nearby vicinity of the dam. 
  • The dam site should be easily accessible and economically connected to nearby areas. Area required for establishments and labour camps of construction group shall be available nearby in a safe zone.
  • Environmental issues should be taken into consideration while selecting the dam site.
Dam with sluice gate
Dam with sluice gate

Advantages of dams

  • Water storage : Dams are one of the major sources of water for domestic uses like cooking, cleaning, bathing, washing, drinking water, gardening, agricultural uses, industrial purposes, etc. The water is supplied through the canal or with the help of a pipe system from the dam.
  • Irrigation – The water from the dams is diverted through canals to the field where the water level is low for irrigation purposes.
  • Hydroelectric power – The water stored in the dam is passed through turbines for hydroelectric power generation.
  • Flood prevention – The water level of river, streams, etc are maintained by constructing dams across it. This prevents the loss and damages from unexpected floods. 
  • Recreation – The water stored in the dam is used for fishing, boating, and other recreational activities.
  • Debris control – Dam also provides the retention of hazardous material and protects the environment.

Disadvantages of Dams

  • Groundwater table – Stagnation of water in the dams alters the groundwater table.
  • Ecosystem – The dams prevent the fish migration and help in the breeding of mosquito which affects the ecosystem.
  • Water temperature – The aquatic temperature is varied by the dams, which causes a serious problem for aquatic lives.
  • Greenhouse effect – The reservoirs emit greenhouse gases which leads to global warming.
  • Rehabilitation – The discharge of water from the river affects the peoples living in the downstream area.