D A M S

DAMS & 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 ( ‫ )عائق‬built on a stream or a river to collect water behind it

A Reservoir:

An artificial (‫) ص ناعي‬, seasonal or permanent lake, that is created at the US of a dam and used of Irrigation, Drinking, Land reclamation, Electricity generation, Fishing, Recreation and (or) Pr 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 ar 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 fi 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

•

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 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 stru

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

•

Bhakra Dam is across river Sutlej in Himachal Pradesh

•

The construction of this project was started in the year 1948 and was completed in 1

• It is 740 ft. high above the deepest foundation as straight concrete dam being more tha 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

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

Gravity Dam from the Down straw side

Arch Dam

Buttress Dam:

• Buttress Dam – Is a gravit by structural supports

• Buttress - a support that from a roof or wall to another suppo

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 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]} Unknown = { K n o w n }

main eq. used in hydrologic analysis for flood routing
Assumed Given Estimated Given for t1 O Determined Determined from equation from table above 2*S/∆t – O1 2*S/∆t + O2 h Estimated

t

I

It + It+∆ t

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
function of time.

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

Benefits of routing:

1. Determination of water level at peak at different locations to open gates and make precautio 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 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992

Inflow 100 100 110 40 120 180 150 30 100 50 20 210

Storage 100-80=20 120-80=40 150-80=70 110-80=30 150-80=70 250-80=170 150+120-80=190 30+120-80=70 100+70-80=90 50+90-80=60 20+60-80=0 210+0-80=130

Spillway

50 70

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. 2. 3. 4. 5. 6.

Keep pleasant appearance of surrounding areas Construction of required satisfying structures Minimum disturbance of area ecology Excavation depths and tools available Esthetic considerations Economy

Required Data

1. Hydrologic data: max storage; normal storage; dead storage; reservoir water surface elevati hydrograph 2. 3. 4. 5. 6. 7. Required capacity of power plant Topography Geology Climate Soil testing (geo-technical data) Material testing

A deep reservoir is better than a shallow reservoir because:
1. 2. 3. 4. It has a lower cost per unit capacity It has less evaporation rates It has less possibility of weed growth 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) b) Cost of dam/ storage capacity = minimum; Benefit / cost ratio > 1.0

Structure of Dam

Definitions
• • • • • •
side

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 operatio

Diversion tunnel: Tunnels are constructed for diverting water before the constructio

helps in keeping the river bed dry.

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

•

Sluice way: An opening in the dam near the ground level, which is used to clear the sil

in the reservoir side.

Earth Dams: are the most simple and economic (oldest dams) Types of earth dams:
1. 2. 3. Homogeneous embankment type Zoned embankment type Diaphragm type

Stability of earth dams covers the following points:
Hydraulic  Determination of seepage pattern and magnitude  Determination of hydrostatic forces from US and DS heads and from seepage, on dam and foundation  Phreatic line must lie inside dam body Structural

 Study of US and DS slope stab saturation conditions

 Pre-determination of settlemen

 Precautions against piping thro

 Precautions against sliding of D 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 filte

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 2. Seepage failure causes 3. Structural failure causes 4. External causes

40% 30% 25% 5%

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

Hydraulic Failure is due to:

1. 2. 3. 4.

Overtopping (design level is underestimated) Erosion of US face (wave action – height) Cracking in upper portion of dam due to frost action (additional freeboard allowance up to Erosion of DS face due to rain action (maintenance – berms - grass)

Seepage failure is due to:
Uncontrolled seepage (causes scour through DS wet zone – needs adequate filters)

1. 2.

Piping (through dam foundation – either prevention or control of percolation…may cause d

Structural failure is due to:
Foundation slide by soft soil (fine silt – soft clay, …all dam body slides on foundation) Slide of slopes (US or DS slopes)

1. 2.

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=kiA

i = dy/dx {y = saturated m for q: q = k dy/dx y

= k d (S2+2SX) 0.5 / d = k * 0.5 * (S2+2SX) (S2+2SX) 0.5

= k S m2/sec almost f filters Q = k.S.L

Precautions of earth dams:

1. 2. 3. 4. 5. 6. 7. 8.

Filling earth is of sufficiently low permeable soil Provision of spillway + outlets to avoid overtopping Provision of sufficient freeboard Seepage line remains inside DS face of dam No possibility of free flow from US to DS US face should be protected against wave action DS face should be protected against rains 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 whe 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 tha 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

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

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) i) Sliding: 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:

123-

Remove all loose soil up to hard bed rock By excavation, avoid damage of underlying soil 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 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 s produce local effects as per St. Vennant’s principal.

ANALYTICAL METHOD
The vertical stresses at the toe and heel

GRAPHICAL METHOD



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



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

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

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

but shall reduce it on d/s faces. The most economical top width, without considering earthqua been found by greater or equal to 14% of the dam height. Its usual value varies between 6 to 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 comp membrane settlement. (C)Best quality, higher strength rock, compacted to provide section stability

Details of concrete membrane at cutoff wall in rock fill dam

Galleries


Galleries are openings or passages through dams



They are either horizontal or slightly sloping openings left inside the dam body They are parallel to the dam axis in the longitudinal direction





Sometimes, additional galleries are found normal to the dam axis, i.e., in the tr direction They are provided at various elevations They are fitted with stairs or mechanical lifts





Galleries through dam body serve in:
Drainage: Inspection: Grouting: drain water seeping from dam body provide windows to control dam behavior 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) b) Horizontal section (arch); Vertical sections – profiles of dams: 1. 2. 3. 4. 5. 6. The Tin Dam (H = 180 m, eο = 44.5 m, L/H = 1.63); The Mori Dam (H = 65 m, eο = 18 m, L/H = 2.86); The Anshane Dam (H = 75 m, eο = 11 m, L/H = 3.07); The Val-Galina Dam (H = 92 m, eο = 11.2 m, L/H = 2.48); The Oziletta Dam (H = 77 m, eο = 10.8 m, L/H = 2.91); 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 stru

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