DAMS

What is a Dam?

• Dam – Hydraulic structure – across river – stores water – upstream side.

• Reservoir – U/S side.• Water level – U/S side – rises• Water used – generate HP, irrigation, domestic,

industrial water supply etc

Classification of DamsClassification

According to use

Storage

dam

Diversion dam

Detention Dam

According to hydraulic design

Non overflow

Overflow dam

By materials

Rigid dam

Gravity dam

Arch dam

Buttress dam

Steel dam

Earth dam

Non rigid dam

Earth dam

Rock fill dam

ACCORDING TO USE

1. STORAGE DAM

• Stores water – excess supply

• Behind dam – reservoir• Water – irrigation,

power etc• Stone type, concrete,

earth fill, rock fill types

2. DIVERSION DAMS• Rises the water level• Water to – canals, ditches• Small height• No reservoir • Eg : 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

• Two types – one type – stored water – released – outlet

• Second – no outlet – water seeps – lift irrigation is possible – This type – also called – water spreading dam

• Also used to trap and release sediments – 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 discharge – carried – crest

• Crest level – lower – non over flow dams

• Water glides down- strong material usage

• Generally concrete or masonry• Overflow dam – called –

spillway• At times non overflow and

overflow types are combined

ACCORDING TO MATERIAL1. RIGID DAMS

a. GRAVITY DAM• External forces – resisted – weight of dam• Constructed either of masonry or concrete• Masonry Gravity dams – small height• Major dams – concrete • May be straight or curved in plan

ADVANTAGES• Strong and stable than earth dams• Can be used as overflow spillway • Can be constructed to any height • Built – areas of heavy rainfall

• Least maintenance• Failure not sudden• Deep set sluices can be used• Cheaper in long run

Disadvantages• 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 by arch action• Water load – amount of curvature• Balance water load – to foundation

Advantages• Adopted in gorges of length < height• Requires less material• Problems of uplift pressure is less

Disadvantages• Requires skilled labour• Speed of construction is slow• All sites are not possible for this dam

ARCH DAMS

c. BUTTRESS DAM• A number of piers – divide – space – number of spans• To hold water – panels – placed between piers• Panels – flat or arched.

Advantages• Less massive than gravity dam• Extension of height is possible• Concrete volume used is less.

Disadvantages• Skilled labour is needed• More susceptible to damage

BUTTRESS DAM

d. STEEL DAMS• Consists of framework of steel• No Dams in India• Three dams in USA. One failed – due to underpinning• Two types of steel dams 1. Direct strutted type• 2. Cantilever type• Direct strut type – load directly to foundation – through struts• Cantilever type – Struts anchored into foundation

Advantages• Speedy construction possible• Cheaper then rigid dams• Greater resistance to settlement• Not affected by frost action• Repairing can be done easily

Disadvantages• Lighter – can absorb shoocks• Life shorter than concreted dams• Requires constant maintenance

STEEL DAM

e.TIMBER DAM• Made of timber struts and beams• Temporary dam… proper construction – 30 to 40 years• Adopted – timber is plenty

Types

i) A – frame type

ii) Rock filled crib

iii) Beaver type

Advantages

Low initial cost

Suitable for any foundation

Speedy construction

Disadvantages• High maintenance cost• Short life• Small height is only

possible• Seepage loss is more

2. NON RIGID DAMS

EARTH DAMS• locally available materials• Moderate height• Materials used in natural state• With modern machinery – height greater

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– any foundation• Easy construction • No skilled labour needed• Cheaper than other types of dams• Height can be raised without difficulty

Disadvantages• Easily damaged by floods• Cant be used as over flow dams• Not suitable in areas of heavy rainfall• Maintenance cost is high

EARTH DAMS

ROCK FILL DAM

EARTH AND ROCK FILL DAM

FACTORS FOR SELECTION OF TYPE OF A DAM

1. Topography• 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

5. Roadway

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

SELECTION OF SITE FOR A DAM1. Foundations• 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

2. Topography• River cross section – narrow gorge – reduces the length

of the dam• Gorge – open up at the upstream – for reservoir• Major portion of dam – high ground – reduce cost, easy

drainage

3. Site for spillway• Good site for spillway – for earth and rock fill dams• Gravity dams – space for spillway at the centre

4. Materials

Nearby – reduce cost of construction

5. Reservoir and catchment area

a. Adequate storage capacity of reservoir

b. Cost of land – minimum

c. Leakage through bed rock – minimum

d. Percolation losses - minimum

e. Site - formation of deep reservoir

f. Deep reservoir – better than shallow one

a. Lower cost of submerged land

b. Less evaporation loss

c. Less weed growth

6. Communication• Site – connected – rail, road• Convenience for transportation of materials

7. Locality• Surrounding area- healthy