A dam is defined as a hydraulic structure of fairly impervious material built across a river to make a reservoir on its upstream side for impounding water for various purposes.
There are many importance dams. Among them, some major importance is Dams are used for irrigation, hydroelectricity, drinking water purposes, etc.
Classification of dam
The dam is classified in different ways. Thus, we can have different classification based on their;
- Material of construction
- Based on the function
- Based on the head
a) Classification based on the material of construction
- Concrete dams:- These dams can be defined as the dams constructed on the mass of concrete.
- Embankment dam:- these dams can be defined as dams constructed of natural materials (rock fill or earth fill) excavated or obtained from close areas surrounding the dam.
- Steel dam:- A steel dam consists of a steel formwork, with a steel skin plate on its upstream face. Steel dam used as temporary coffer dams needed for the construction of permanent dams.
- Timber dam:- These dams are constructed formwork of timber struts and beams with timber planks facing to resist water pressure. These are used very small scale of works.
b) Classification based on the function
Based on function, dams are classified as follows:
- Storage dam:- It’s a large dam constructed to store extra of water during the period of excess supply in the river. The store water can also be used in various seasons and for various purposes when it is needed.
- Diversion dam:- It’s a small dam used to rise the water level slightly within the river & thus provides a head for carrying or diverting water off taking canal or other conveyance systems to the place of use. Generally, a diversion dam is known as a weir or barrage.
- Detention dam:- It’s a dam constructed to store water during floods & release it gradually at a safe rate when the fluids recede. These dams are able to control or lower large peak floods.
- Cofferdam:- It is a temporary dam constructed on the upstream of main dam to divert water into a diversion tunnel during the construction (or development) of the main dam. Sometimes a coffer dam on downstream is also required.
c) Classification of dam based on head
Based on hydraulic head available, dams are expressed as:
- Low head:- If the head of dam(H) is less than 15m, then the dam is said to be low head dam.
- Medium head:- 15m < H < 50m
- High head :- 50 m < H < 250 m
- Very high head:- H > 250 m
Site selection of dam
A dam is a big structure & it requires a lot of funds. Extreme care shall be taken while selecting the site of the dam. A wrong decision may lead to excessive cost &may occur difficulties in construction &maintenance. The following factors is taken when selecting the site of a dam.
- Topography:- As far as possible dam should be located where the river has narrow gorge which opens out the upstream to make a big reservoir.
- Suitable foundation:- The foundation should exist at the site for a particular type of dam.
- Good site for reservoir:- AS the dam is constructed to store water in reservoir, so the site should have following characteristics to make a good site for reservoir:
- Large storage capacity:- The topography of the site in such that the reservoir has a large capacity to store water.
- The shape of reservoir basin:- The reservoir basin in upstream of the dam should preferably be cup-shaped with flat bottom but steep slopes.
- Water tightness of reservoir:- The geological condition of the reservoir site should be such that the reservoir basin is watertight.
- Good hydrological condition:- The hydrological condition of the reservoir at the reservoir site should be such that adequate runoff is available.
- Deep reservoir:- The site should be such that a deep reservoir is formed after the construction of the dam.
- Small submerged area:- The site should be such that the submerged area is minimum. It should be submerge costly land and property.
- Low silt inflow:- The dam site should be such that reservoir would not silt up quickly.
- Spillway site:- The dam site should be such that a good site for a spillway should exist at or near the dam site.
- Availability of materials:- The dam requires a large number of materials for its construction. Suitable types of material in sufficient quantity should be available at or near the dam site to reduce the cost.
- Accessibility:- The site should be easily accessible.
- Minimum overall cost:- The site should be such that it requires the minimum total cost of the project, including subsequent maintenance.
- Healthy surrounding:- The surrounding of the site should be healthy and free from mosquitoes so that labourers can comfortably live in colonies constructed near the dam site.
Importance of dams
- It provides water for farmers and thus helps them grow fruits & vegetables.
- Stored water is used in the summer season when water is not available in sufficient quantity.
- The river may experience a shortage of water due to drought or any other reason which makes it difficult to travel ships. Dams helps in reducing this shortage of water.
- The dam helps to reduce the risk of flood & saves homes, cattle & lives of people.
- Dams help to produce electricity through hydropower.
- It also preserves the environment from toxic materials by storing them.
- It helps to reduce water wastage and efficient utilization of water.
Advantage of dam
- It helps to control floods.
- Generation of electricity
- To provide water for the irrigation purpose
- A dam is used in hydropower for the generation of electricity.
Disadvantages of dam
The disadvantages of dams are as follow;
- The dam itself block fish migrations.
- The dam also traps sediment which is critical for maintaining physical processes.
- For various disease-carrying pests, reservoirs are identified as breeding grounds, including mosquitoes that transmit malaria disease.
- During the development of dams in the past, the geology of the dam was rarely or never put into consideration. The sudden filling of reservoirs with lots of gallons of water has caused seismic instability & causes minor earthquake.
- Dam affect the social and cultural structure of the region.
- Submerged large part of land.