DAMS and Its Types
What is a Dam?
Dams are massive barriers (hydraulic structures) built across river and streams to confine and store water on upstream side reservoir and utilize the flow of water for multipurpose.
Water uses for Irrigation, Hydropower, Water Supply (domestic and industrial), Flood control, Navigation, Fishing and Recreation.
Different Parts and Terminologies of a Dam
Classification of Dams
ACCORDING TO USE
1) STORAGE DAM
- Stores water – excess supply
- Behind dam – reservoir
- Water – irrigation, power etc
- Examples Stone type, concrete, earth fill, rock fill types
2. DIVERSION DAMS
- Rises the water level
- Water to – canals, ditches
- Small height
- No reservoir
- Examples : Weir, Barrages
- Flood water diverted to Diversion structures
- Normal flow diverted to canal
- USES: Irrigation, Industrial uses
3) DETENTION DAM
- Store water – floods – releases after flood
- One type: stored water & releasing through outlet
- Second type: no outlet, water seeps and lift irrigation is possible.
- The second type also called water spreading dam.
- Also used to trap and release sediments and called debris dam.
ACCORDING TO HYDRAULIC DESIGN
1) NON OVER FLOW DAMS
- Top of dam – higher level than high flood level
- Water not permitted to overtop the dam
- Materials – rock fill, earth fill, masonry, concrete
2) OVER FLOW DAMS
- Surplus water is discharged above the crest.
- The crest level is lower than the non over flow dams
- Water slides down and hence need strong construction material.
- Generally concrete or masonry
- Overflow dam expels surplus water through spillway
ACCORDING TO MATERIAL USED
1) Rigid Dams
A rigid dam is constructed of stiff (rigid) materials such as concrete, masonry, steel and timber. Further classification is as under:
a). Gravity Dam
- These dams resists the water pressure and other forces due to its weight (or gravitational forces).
- Stability depends upon its weight
- Usually made of masonry or concrete.
- The masonry dams are constructed for very small heights.
- Height upto 100 ft (30.48 m) generally considered as low dams.
- Height between 100 ft (30.48 m) and 300 ft (91.44 m) are designated as medium-height dams.
- Dams higher than 300 ft (91.44 m) are considered as high dams.
- Strong and stable than earth dams
- Can be used as overflow spillway
- Constructed to very great heights
- Built – areas of heavy rainfall
- Least maintenance
- Failure not sudden
- Deep set sluices can be used
- Cheaper in long run
- Can be constructed only on strong rocks
- Initial cost is high
- Construction time is longer
- Require skilled labour
- Height cant be increased unless provisions are made
b). ARCH DAMS
- Curved in plan
- Carries its part load to the abutments (braces) by arch action
- Water load – amount of curvature
- Balance water load – to foundation
- Adopted in gorges of length < height
- Requires less material
- Problems of uplift pressure is less
- Requires skilled labour
- Speed of construction is slow
- All sites are not possible for this dam
c). BUTTRESS DAM
- A number of piers – divide – space – number of spans
- To hold water – panels – placed between piers
- Panels – flat or arched.
- Less massive than gravity dam
- Extension of height is possible
- Concrete volume used is less.
- Skilled labour is needed
- More susceptible to damage
d). STEEL DAMS
- Consists of framework of steel
No Dams in India and Pakistan
Three dams in USA.
- One failed – due to underpinning
Two types of steel dams
- Direct strutted type
- Cantilever type
1. Direct strut type: load directly to foundation – through struts
2. Cantilever type: Struts anchored into foundation
- Speedy construction possible
- Cheaper than rigid dams
- Greater resistance to settlement
- Not affected by frost action
- Easy maintenance and repair
- Lighter – can absorb shocks
- Life shorter than concrete dams
- Requires constant maintenance
e). TIMBER DAM
- Made of timber struts and beams
- Temporary dam,
- 30 to 40 year life span
- Adopted – timber is plenty
- Frame type
- Rock filled crib
- Beaver type
- Low initial cost
- Suitable for any foundation
- Speedy construction
- High maintenance cost
- Short life
- Small height is only possible
- Seepage loss is more
2. NON RIGID DAMS
a). EARTH DAMS
- locally available materials
- Moderate height
- Materials used in natural state
- With modern machinery – height greater
b). ROCK FILL DAM
- Rocks of various sizes
- Rock fill dam has rock fill at the downstream, impervious membrane, upstream cut off to check seepage
Advantages of Earth and Rock fill dam
- Earth dams possible almost on any foundation
- Easy construction
- No skilled labour needed
- Cheaper than other types of dams
- Height can be raised without difficulty
- Easily damaged by floods
- Cant be used as over flow dams
- Not suitable in areas of heavy rainfall
- Maintenance cost is high
ANOTHER CLASSIFICATION OF DAMS
ACCORDING to the SIZE of the DAM
- Large (Big) dam
- Small dam
- International Commision on Large Dams, (ICOLD) assumes a dam as big when its height is more than 15 m.
- If the height of the dam is between 10 m and 15 m and matches the following criteria, then ICOLD accepts the dam as big:
- If the crest length is more than 500 m
- If the reservoir capacity is larger than 1 million m3
- If the flood discharge is more than 2000 m3/s
- If there are some difficulties in the construction of foundation
ACCORDING to only HEIGHT of DAM
High Dam or Large Dam
If the height of the dam is more than 100 m
If the height of the dam is between 50 m and 100 m
Low Dam or Small Dam
If the height of the dam is lower than 50 m
FACTORS FOR SELECTION OF TYPE OF A DAM
- Low rolling plains land – Earth dam with separate spillway
- A low narrow V shaped valley – Arch dam
- A narrow stream between high rocky walls – Concrete overflow dam
2) Geology and Foundation Conditions
- Foundation of Sound Rock – Any dam type
Granite, Gneiss, Schist – good for gravity dam
- Poor rock – Earth dam, rock fill, low concrete gravity dam
- Silt, fine sand foundations – problems of settlement – used as foundations for earth dams, low gravity dams but not rock fill dams
- Clay soils – problems of settlement – earth dams – No gravity or rock fill dams
3). Materials of construction
- Cost – availability of materials nearby – transportation charges reduced
- Sand, Gravel, Crushed stone – Concrete Gravity dam is suitable
- Coarse, fine grained soils – Earth dam
4). Spillway size and location
- If large spillway area needed – Overflow concrete gravity dam
- Small spillway capacity – earth dam
- Large discharges during construction – Concrete gravity dam
- No site for spillway – Concrete gravity dam having overflow section
- If roadway needed – Earth dam or Gravity dam
6). Length and Height of Dam
- Length long, height low – earth dam
- Small length, height more – gravity dam
7). Life of the dam
- Concrete and masonry dams – very long life
- Earth, Rock fill dams – Intermediate life
- Timber Dams – Temporary life
- Gravity dams – Sound rock is needed
- Earth dams – Any foundation with proper treatment
- Foundation – free from holes, faults
- Formations of hard and soft layer – not suitable
- Alternations of sand, shale, stone – leads to slipping
- Beds with dip along the upstream and strike along valley – best foundation conditions
- No dam – on faulty zones
- Best condition – dam – uniform formation
9). Reservoir and catchment area
- Adequate storage capacity of reservoir
- Cost of land – minimum
- Leakage through bed rock – minimum
- Percolation losses – minimum
- Site – formation of deep reservoir
- Deep reservoir – better than shallow one
- Lower cost of submerged land
- Less evaporation loss
- Less weed growth
- Site – connected – rail, road
- Convenience for transportation of materials
- Surrounding area- healthy
- Advantages and Disadvantages of Dams
Advantages and Disadvantages of Dams
- Clean, efficient, and reliable source of energy.
- Does not emit any direct pollutants or greenhouse gases.
- While the initial cost is high, they are very inexpensive to operate.
- Electricity generated by hydro-electric power plants is the cheapest electricity generated.
- Dams store water for irrigation in summer seasons and dry months. Many desert areas can now be cultivated due to dams and canals that supply water.
- Dams supply water for local drinking needs.
- Allows for fish farming.
- Flood prevention and control by storing water.
- Supply of water to less rainfall areas.
- Picnic spot (Boating) etc.
- In flat basins large dams cause flooding of large tracts of land, destroying local ecosystem.
- Displacement of community from the dam construction nearby area resulting change in life style, customs, even casing emotional scarring.
- Approx. 40 to 80 million people have been displaced physically by dams worldwide.
- Causes submergence of large amount of plants and decay anaerobically generating greenhouse gases like methane.
- It is estimated that a hydroelectric power plant produces 3.5 times the amount of greenhouse gases as a thermal power plant burning fossil fuels.
- The migratory pattern of river animals like fished are affected.
- Dams restricts sediments that are responsible for the fertile lands downstream. Farmers use chemical fertilizers and pesticides to compensate for the loss in productivity.
- Salt water intrusion into the deltas means that the saline water cannot be used for irrigation.
- Large dams are breeding grounds for mosquitoes and cause the spread of disease.
- Farmers downstream who used to wait for flooding of the fields to plant their seeds are affected.
- Dams serve as heat sink, and the water is hotter than the normal river water. This warm water when released into the rive downstream can affect animal life.
- Peak power operations can change the water level thirty to forty feet in one day and can kill the animal staying at the shorelines.
- Approx. 400,000 km2 of land worldwide has been submerged due to the construction of dams.