D A M S

&

RESERVOIRS

 

 

Classification according to type

  • Earth Dams

§        Homogeneous Earth Dams

§        Zoned Earth Dams

§        Diaphragm Dams

  •    Rockfill Dams

  •   Gravity Dams

  •   Arch Dams

  •   Buttress Dams

 

Classification according to reservoir behind

  •    Flood Control Reservoir

  •   Storage Reservoir

 

Economic Dam Height

 

Fish Ways

 

A Dam:

An obstruction ( ΪΗΖή) built on a stream or a river to collect water behind it

 

A Reservoir:

An artificial (ΥδΗΪν), seasonal or permanent lake, that is created at the US of a dam and used for the purpose of Irrigation, Drinking, Land reclamation, Electricity generation, Fishing, Recreation and (or) Protection of towns from flood danger 

 

 

 

Layout of the Bonneville Dam Site

 

 

Layout of the Almendra Arch Dam (photograph)

 

 

A View of a Gravity Overflow Dam ( from the Down Stream side)

 

Earth Dams:

•             They are trapezoidal in shape

•             Earth dams are constructed where the foundation or the underlying material or rocks are weak to support the masonry dam or where the suitable competent rocks are at greater depth.

•             Earthen dams are relatively smaller in height and broad at the base

•             They are mainly built with clay, sand and gravel, hence they are also known as Earth fill dam or Rock fill dam

A view of a gravity over flow Dam

Arch  Dam (photograph)

 

Embankment Dam:

 

 

 

 

Arch Dams

 

•             These type of dams are concrete or masonry dams which are curved or convex upstream in plan

 

•             This shape helps to transmit the major part of the water load to the abutments

 

•             Arch dams are built across narrow, deep river gorges, but now in recent years they have been considered even for little wider valleys.

 

Arch Dam(photograph)

Buttress Dam:

 

•             Buttress Dam – Is a gravity dam reinforced by structural supports

•             Buttress - a support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

Buttress Dam

 

 

 

 

 

                                                     

 

 

           

 

 

 

 

 

 

Type and arrangement of incomplete transverse  baffles for fish pools

 

For Fish Elevation from Downstream to Upstream Reservoir Level

 

 

Fishway at a dam site

 

 

 

A photograph for Fish way at a adam site

 

Problems, which appear with a Dam Construction are:

 

1.Submergence problems

2.Fish problems

3.Failure problems

4.Bomb problems

 

 

 

 

Example                                                                         

 

•              Bhakra Dam is the highest Concrete Gravity dam in Asia and Second Highest in the world.

 

•              Bhakra Dam is across river Sutlej in Himachal Pradesh

 

•              The construction of this project was started in the year 1948 and was completed in 1963 .

 

 

 

 

•               It is 740 ft. high above the deepest foundation as straight concrete dam being more than three times the height of Qutab Minar.

•               Length at top 518.16 m (1700 feet); Width at base 190.5 m (625 feet), and at the top is 9.14 m (30 feet)

•               Bhakra Dam is the highest Concrete Gravity dam in Asia and Second Highest in the world.

 

 

Gravity Dam from the  Down straw side

 

Arch Dam

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Buttress Dam:

 

 

•             Buttress Dam – Is a gravity dam reinforced by structural supports

•             Buttress - a support that transmits a force from a roof or wall to another supporting structure

 

 

 

 

 

This type of structure can be considered even if the foundation rocks are little weaker

 

• DamReservoirs are created from the storage of water which is utilized for following objectives:

 

•            Hydropower

•            Irrigation

       •           Water for domestic consumption

•            Drought and flood control

       •             Avigational facilities

•            Other additional utilization is to develop fisheries

Storage & Reservoir Levels

 

 

Determination of Reservoir Capacity                                    

 

Hydrograph: Is the relation between discharge & time at a certain location on a stream

 

Flood Routing

 

Given:

·        Inflow Hydrograph (I versus time)

·        Storage S & Outflow O versus Elevation h  (from topography)

·        H at time= 0, for section b-b

 

Elevation h
(m)


Storage S
(m3)


Outflow
O (m3/s)

 

(2S/∆t)+ O

(m3/s)

 

 

 

 

 

Over–Year Storage

Required:

Elevation h with time

I(t) – O(t) =  ∆ S

I(t) – O(t)= dS/dt  (from continuity)

 

I= I1 + I2 /2 ; O= O1 + O2 /2 ; S = S2 – S1

At t1: I1; O1; S1

At t2: (t1 +t): I2; O2; S2

(I1 + I2) /2 – (O1 + O2) /2 =(S2 – S1)/ ∆t

O2+ (2* S2)/ ∆t  = {(I1 + I2) +  [(2* S1) /∆t – O1]}

U n k n o w n      =          {  K   n   o   w   n   }

main eq. used in hydrologic analysis for flood routing

Assumed

Given

Estimated

Given

for t1

Estimated

Determined from equation

Determined from table above

t

I

It + It+ t

O

2*S/∆t – O1

2*S/∆t + O2

h

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Mass Flow Curve:   Is a Curve for the Accumulation of                          

                                              Discharge Versus Time

 

 

How to determine Demand from Reservoir of Known Capacity?

Reservoir Routing: a process to know reservoir level as a function of time and outlet from reservoir as a function of time.

 

Routing: a technique used in hydrology to estimate the effect of channel storage on shape and movement of flood wave.

 

Benefits of routing:

1.    Determination of water level at peak at different locations to open gates and make precautions

2.    Design of protection structures

    3.    Design of suitable escapes

 

Example for reservoir storage

 

Demand from an over-year storage reservoir = 80*103 cu.m/sec/ y

Maximum reservoir storage                            = 120

 

 

 

Year

Inflow

Storage

Spillway

1981

100

100-80=20

 

1982

100

120-80=40

 

1983

110

150-80=70

 

1984

40

110-80=30

 

1985

120

150-80=70

 

1986

180

250-80=170

50

1987

150

150+120-80=190

70

1988

30

30+120-80=70

 

1989

100

100+70-80=90

 

1990

50

50+90-80=60

 

1991

20

20+60-80=0

 

1992

210

210+0-80=130

10

 

 

Selection of Dam type:                                                        

-         Preliminary studies + cost estimates for several types = right choice

-         Right choice = right type + right location

 

Design considerations of Dams must cover:

 

1.     Keep pleasant appearance of surrounding areas

2.     Construction of required satisfying structures

3.     Minimum disturbance of area ecology

4.     Excavation depths and tools available

5.     Esthetic considerations

6.     Economy

 

Required Data

 

1.     Hydrologic data: max storage; normal storage; dead storage; reservoir water surface elevation; flood hydrograph

2.     Required capacity of power plant

3.     Topography

4.     Geology

5.     Climate

6.     Soil testing (geo-technical data)

7.     Material testing

A deep reservoir is better than a shallow reservoir because:

                  1.     It has a lower cost per unit capacity

                  2.     It has less evaporation rates

                  3.     It has less possibility of weed growth

                  4.     It gives minimum depth of the dam for the same storage

 

 

 

Economic Dam Height                                                           

At the location which corresponds to: Cost of Dam / Storage Capacity = minimum

Economic height of a dam is the height corresponding to:

a)     Cost of dam/ storage capacity = minimum;

    b)     Benefit / cost ratio  > 1.0

 

Structure of Dam

 

 

Definitions

•             Heel: contact with the ground on the upstream side 

•             Toe: contact on the downstream side

•             Abutment: Sides of the valley on which the structure of the dam rest

•             Galleries: small rooms like structure left within the dam for checking operations.

•             Diversion tunnel: Tunnels are constructed for diverting water before the construction of dam. This helps in keeping the river bed dry.

•             Spillways: It is the arrangement near the top to release the excess water of the reservoir to downstream side

•             Sluice way: An opening in the dam near the ground level, which is used to clear the silt accumulation in the reservoir side.

 

 

Earth Dams: are the most simple and economic (oldest dams)

 Types of earth dams:                                                                       

1.     Homogeneous embankment type

2.     Zoned embankment type

3.     Diaphragm type

Stability of earth dams covers the following points:

Hydraulic

Structural

Ø     Determination of seepage pattern and magnitude

Ø     Determination of hydrostatic forces from US and DS heads and from seepage, on dam and foundation

Ø     Phreatic line must lie inside dam body

 

Ø     Study of US and DS slope stability at existing saturation conditions

Ø     Pre-determination of settlement values (= 3%)

Ø     Precautions against piping through foundation

Ø     Precautions against sliding of DS slope due to rain forces

 

Methods of construction of Earth Dams:

 

1 – Hydraulic fill method

-         Pumping of earth + water through pipes

-         Liable to considerable settlement due to drying and consolidation

-         During rest, grains of earth are graded, thus  ,this should be considered by filter design.

 

2 – Rolled fill method:

-         Soil is prepared at certain moisture content

-         Put into layers (15 – 30) cm

-         Pressed by rollers having adequate weights

Causes of failure of earth dams:

1. Hydraulic failure causes      40%

2. Seepage failure causes        30%

3. Structural failure causes      25%

4. External causes                   5%

 

Slip Failure of Earth Dam at Down Stream Side (Structural Failure)

 

Hydraulic Failure is due to:                                                      

 

1.     Overtopping (design level is underestimated)

2.     Erosion of US face (wave action – height)

3.     Cracking in upper portion of dam due to frost action (additional freeboard allowance up to 1.5 m)

4.     Erosion of DS face due to rain action (maintenance – berms - grass)

 

Seepage failure is due to:

 

1.     Uncontrolled seepage (causes scour through DS wet zone – needs adequate filters)

2.     Piping (through dam foundation – either prevention or control of percolation…may cause dam subsidence)

 

Structural failure is due to:

 

1.     Foundation slide by soft soil (fine silt – soft clay, …all dam body slides on foundation)

2.     Slide of slopes (US or DS slopes)

 

Zoned Earth - fill Dam

Treatment of Seepage Flow through Permeable Strata

Treatment of Seepage through Earth Dam Body

Treatment of Stratified foundation below earth dam

Seepage Treatment through Stratified Soil Foundation

 

Diaphram Earth Dam

 

Diaphram Earth Dam

Typical cross section for diaphragm earth dams

 

 

A typical cross section of a zoned earth fill dam

 

 

Seepage flow through Earth Dam

 

 

q = k i A 

 i = dy/dx {y = saturated depth}, A = y 1 m          for q:

q = k dy/dx y

   = k d (S2+2SX) 0.5 / dx

   = k * 0.5 * (S2+2SX) 0.5 * 2S * (S2+2SX) 0.5

   = k S m2/sec almost for horizontal filters

                                                                                                           Q = k.S.L

 

Precautions of earth dams:

 

1.     Filling earth is of sufficiently low permeable soil

2.     Provision of spillway + outlets to avoid overtopping

3.     Provision of sufficient freeboard

4.     Seepage line remains inside DS face of dam

5.     No possibility of free flow from US to DS

6.     US face should be protected against wave action

7.     DS face should be protected against rains

8.     Provision of filter drains to drain parts DS of impervious core

9.     Stable US and DS slopes under worst loading conditions

10. Counteraction due to consolidation (about 3% of dam height)

 

 

•           Gravity Dams:

•             These dams are heavy

             and massive wall-like

            structures of concrete in

           which the whole weight

          acts vertically downwards

 

As the entire load is transmitted on the small area of foundation, such dams are constructed where rocks are competent and stable.

 

Example                                                                         

     Bhakra Dam is the highest Concrete Gravity dam in Asia and Second Highest in the world.

   Bhakra Dam is across river Sutlej in Himachal Pradesh

   The construction of this project was started in the year 1948 and was completed in 1963 .

 

 

 

•               It is 740 ft. high above the deepest foundation as straight concrete dam being more than three times the height of Qutab Minar.

•               Length at top 518.16 m (1700 feet); Width at base 190.5 m (625 feet), and at the top is 9.14 m (30 feet)

•               Bhakra Dam is the highest Concrete Gravity dam in Asia and Second Highest in the world.

 

 

Gravity Dam from the  Down straw side

Gravity Dam

Gravity Dam

 

 Forces acting on a Dam

 

 

 

 

All Forces acting on a Dam

Counteracting Force : OWN WEIGHT

 

 

Failure of a Gravity Dam is due to:

Sliding; overturning; overstressing (crushing)

 

a)     Overturning: factor of safety = 2 – 3 about Toe

b)    Overstressing: compression – crushing (fails by failure of its material)

Fall < 30 kg / cm2

No tension (if tension is developed < 5 kg / cm2)

c)     Sliding:

i)                   F.S.S (Factor of safety against sliding)

ii)                 S.F.F (Shear friction factor)

q = shear strength of joint

    = 14 – 40 kg / cm2

To resist sliding:                      

1-   Stepped bed

2-   Key wall at heel

Preparing surface of foundation:

1-              Remove all loose soil up to hard bed rock

2-              By excavation, avoid damage of underlying soil

3-              By faults, entirely excavated, washed, then filled with concrete or grouted.

 

Dimensioning of a gravity dam cross section

Assumptions for the design of a gravity dam                 

The various assumptions made in two-dimensional designs of gravity dams are:

 

(i)       The dam is considered to be composed of a number of cantilevers, each of which is 1 m thick and each of which acts independent of the other;

(ii)     No loads are transferred to the abutments by beam action;

(iii)   The foundation and the dam behave as a single unit, the joint being perfect;

(iv)  The materials in the foundation and body of the dam are isotropic and homogeneous;

(v)    The stresses developed in the foundation and body of the dam are within elastic limits;

(vi)  No movements of the foundations are caused due to transference of loads;

(vii)Small openings made in the body of the dam do not affects the general distribution of stresses and they only produce local effects as per St. Vennant’s principal.

 

ANALYTICAL METHOD

The vertical stresses at the toe and heel

 

GRAPHICAL METHOD

 

q       For each section, the sum of the vertical forces () and the sum of all the horizontal forces ( ) acting above that particular section, are worked out and the resultant force should lie within the middle third of the base.

q       Hence, a low gravity dam is the one whose height is less than that given by the following equation  

Then, if the height of the dam is bigger than this height, it is classified as a high gravity dam

Low And High Gravity Dam

For the normal values of stresses, the limiting height of a low concrete gravity dam is

                

where          w = 1 t/m3

                   Ss = 2.4

                   f = 300 t/m2           or                (30 Kg/cm2)

 

Thus, an increase in top width will increase the masonry in the added element and increase it on the u/s face, but shall reduce it on d/s faces. The most economical top width, without considering earthquake forces has been found by greater or equal to 14% of the dam height. Its usual value varies between 6 to 10 m and it is generally taken approximately equal to , where H is the height of max. water level above bed.

Empirical Dimensions of Gravity Dam                                                      

Rock fill dam

Rock fill Dam with RC facing

Rock fill Dam

 

 

 

((A)Well graded, selected, compacted rock used to provide bearing support for membrane

(B)Smaller sized rock from quarry and rock of lesser quality from foundation excavations compacted to   reduce membrane settlement.

(C)Best quality, higher strength rock, compacted to provide section stability

 

 

 

Details of concrete membrane at cutoff wall in rock fill dams

 

Galleries

q       Galleries are openings or passages through dams

q       They are either horizontal or slightly sloping openings left inside the dam body

q       They are parallel to the dam axis in the longitudinal direction

q       Sometimes, additional galleries are found normal to the dam axis, i.e., in the transverse direction

q       They are provided at various elevations

q       They are fitted with stairs or mechanical lifts

Galleries through dam body serve in:

Drainage:     drain water seeping from dam body

Inspection:    provide windows to control dam behavior

Grouting:      provide a space for movement and for grouting

                     contraction joints

Cooling:        provide enough space for carrying pipes during

                     artificial cooling 

Concrete cracks through dam bodies are caused either by:

     1.     temperature stresses

2.     shrinkage stresses

 

Arch Dam

 

Arch Dam with an Overflow Spillway

 

Sketches for typical sections of arch dams

 

Items of Arch Dams

Arch Dams

1.constant radius arch dams

  for U-shaped valleys  

  have vertical US face

  constant extrados radii for U-shaped valley

  suitable to install gates at the US face

 

2.constant angle arch dams

  for V-shaped valleys

     have curved US face

      no possibility for gate installment

 

 

  Constructed of masonry; pl. concrete; RC

     Suitable for narrow valley sections with rock abutments

    Suitable for sites having weak underlying soil foundation

      Carried loads in arch dams increase with curvature

     Carry the biggest portion of water pressure to the abutments by arch action

     Subject to same forces as gravity dams but resisted by horizontal arch action

         Arch stresses are able to adjust themselves to support any loading conditions

     Can be overtopped

      Thiner in dimensions than all other types

      Mass volume of arch dam material ≈  1/6  of gravity dam

      Concrete volume in 1m rib (for economy);

V = (r – t/2) . θ . t         where t = γ.h.r / f

    = γ.h / f [ L / 2 sin θ/2 ]2 . θ ;      

Vmin is for δV/δ θ = 0,  where θ ≈ 133o  34`

≈ (100o – 140o)

Stability Analysis of Arch Dams                                   

 

Total horizontal loads are determined and compared with max allowable abutment stresses

No danger of sliding or overturning

Foundation soil stresses are never critical

For design: a thin cylinder analysis is sufficient

                   gravity and cantilever actions are neglected

 

Sketches for typical sections of Arch Dams

 

Sections of Arch Dams

 

a)      Horizontal section (arch);

b)      Vertical sections – profiles of dams:

                          1.      The Tin Dam (H = 180 m, eο = 44.5 m, L/H = 1.63);

                          2.     The Mori Dam (H = 65 m, eο = 18 m, L/H = 2.86);

                          3.      The Anshane Dam (H = 75 m, eο = 11 m, L/H = 3.07);

                          4.      The Val-Galina Dam (H = 92 m, eο = 11.2 m, L/H = 2.48);

                         5.      The Oziletta Dam (H = 77 m, eο = 10.8 m, L/H = 2.91);

                          6.      The Abu-Sheneina Dam (H = 335 m draft).

REACTION FORCE ON ARCH  DAM

Determination of Dam Thickness

If dam thickness = t at any level

Assuming uniform stress along t;

Examples for profiles of existing Arch Dams

 

 

 

Buttress Dam:

 

•             Buttress Dam – Is a gravity dam reinforced by structural supports

•             Buttress - a support that transmits a force from a roof or wall to another supporting structure

This type of structure can be considered even if the foundation rocks are little weaker

 

 

Shapes of Butters Dam

 

To the work analysis of buttress dams                                     

a)     Solid gravity dam;

b)    Hollow dam (with wide joints);

c)     Roundhead buttress dam;

d)    Flat slab buttress dam;

e)     Multiple-arch buttress dam

 

 

 

Schematic sketch for electric power station

 

1 metric HP = 0.736 KW

Text Box:     Surge Tank:
reduces pressure due to
WL fluctuations prevents
water hammer

Energy = QH m3 m / s

HP generated = QH / 75

Electric Energy = 9.8 ζ QH  kw

                         ≈ 8 Q H

for efficiency 80 % of  T & G

              H net < 30 m  Low pressure station

 200 m >H net < 30 m  med. pressure station

              H net > 200 m  High pressure station

power station at the high Aswan Dam

 

 

 

power station at the Esna Barrages

 

 

   

              

Numerical simulation of Dam Body

New Esna Barrages site during construction of Dam

Cross sec. of High Aswan Dam