Introduction to Dams.pptx

Dam: Dam is a man made barrier built
across a river to hold back river water for safe
retention and storage of water or control the
water flow .
Natural processes as landslide and rock
falling into the river may obstruct the river
flows for some time and create a dam like
condition .

Max. Length=21 km
Max. Depth = 109 m
Stored Vol. = 0.33 MAF
Lake was created in 2010 as result of Attaabad disaster

Reservoir: Reservoir is defined the as a man-made
lake or fresh water body created or enlarged by the
building of barriers on which man exerts major
control over the storage and use of the water (Golze
1977, P-619).
Need:
1. River supply usually does not match with the demand at all
times/months. Dam’s storage reservoir is created to match
releases with the water demand.
2.Dams are created to substantially raise water level and thus
provide working head for hydropower production or to direct
water into off taking canals (e.g. irrigation canal).

PURPOSES
1. Irrigation (e.g. Tarbela and Mangla dams)
2. Hydropower development (e.g. Bunji dam)
3. Domestic, municipal, industrial water supply (e.g. Hub dam,
Simly dam)
4. Flood control
5. Recreation (picnic, camping, fishing, swimming, kayaking,
white water rafting)
6. Fish and wildlife protection and development, and
improvement of river ecology
7. Stream flow regulation for various purposes
8. Navigation
9. Mine tailings dam (to store mine processing waste product)

Multipurpose dams:
Most dams are multi-purpose, serving more
than one purpose. Mostly these additional
purposes are achieved as byproduct
outcome, e.g., hydropower, recreation, etc.
For multipurpose dams, the storage is
allocated and prioritized for different
purposes.

Classification of Dams
Dams can be classified according to many different
features as location, release pattern, hydraulic design,
size, filling and emptying mode, service region, type
of materials etc.
1. According To Location
On-Channel: Dam is constructed across the main water
feeding river. Examples Tarbela, Mangla, Simly, Hub
dam. Water from other rivers may be diverted to the dam
through feeder channels to increase the water
availability, e.g. Kurram Tangi dam.

Off-Channel: Dam is constructed on a channel having
much smaller flow. Major storage water is transferred from
a different nearby river. This is done due to non-availability
of suitable/economic dam site on the major flow river.
Example Akhori dam.
2.According to Release Pattern
Storage dam: Water is stored and later released through an
outlet for consumptive or non-consumptive purposes as per
requirements. The outflow is controlled as per need.
Recharging dam. There is no outlet provided to release
water and all incoming water is retained. The main purpose
of the dam is to induce recharge to ground water system in
the area. Small release in d/s channel may be made to allow
seepage in the channel bed.

Delay action dam : These dams are used to retard the peak
flow of flash floods. There may or may not be any control over
the outflow. For no control over the outflow the outflow rate
varies as function of storage volume / water depth in the dam.
These dams are usually meant to reduce flood damages as
well as to induce maximum recharge in the area.
Diversion dam These are hydraulic structures with a main
purpose to raise water level to divert flow into the off taking
channels / canals/ hydropower pressure tunnels and
penstock of run-of-river hydropower projects. The storage
created by these is minimal.

3.According to Hydraulic Design
Non Overflow Dam: Flow is not allowed over the
embankment crest for reasons of dam safety. (earth, rock)
dams.
Overflow Dam: The dam body is made of strong material as
concrete and flow is allowed over the dam crest Concrete
dams
4.According to Size
Small Dam: USBR defined small dam as one having
maximum height < 15 m (50 ft)
Medium Dam: Intermediate size 40 ft to 70 ft

Dam height > 15 m (50 ft) measured from lowest portion of
the general foundation area to the crest .
Large Dam: ICOLD defined large dam as: a dam that
follows one or more of following conditions. (Thomas
1976)
A dam height 10-15 m but it compiles with at least
one of the following condition:
a. Crest of dam longer than 500 m
b. Capacity of the resulting reservoir more than one
million cubic meter
c. Maximum flood discharge more than 2000 cumics
d. Dam has specially difficult foundation problems
e. Dam is of unusual design

Unique: Dams exceeding 100 m are considered as unique.
Every aspect of its design and construction must be treated as a
problem specifically related to that particular site.
5.According to Filling and Emptying Mode
The storage of a dam may be filled and emptied in short time
(one season) or long time (several seasons).
Seasonal: Seasonal dams are filled and then emptied within the
same water year (September to August). Example Tarbela dam.
Thus water level in the dam varies from maximum (normal
conservation level) to minimum (dead storage level) in most
years. Such dams have annual releases usually equal or little
more than the minimum annual flow. The seasonal dams spread
the water stored in wet months over to dry months in the same
year thus provide service for a single season only.

Carry over: Filling and emptying of a carry-over dam
reservoir continues over more than one year (e.g. 2 to 5
years). Example. Hub Dam, Kurram Tangi Dam. Thus water
stored in wet years may be released during subsequent dry
years .
The annual releases are usually more than minimum annual
flow but equal to long term average annual flow. Carry over
dams are applicable where wide variations occur in annual
flows.
Carry over dams spread storage during wet years/months over
to dry years and months and thus provide service for multiple
seasons.

According to type of material
A dam can be made of earth, rock, concrete or wood. Dams
are classified according to the materials used .(Novak et. al.
2001 P: 11-18, 33)
Embankment Dams
The embankment dams are made by use of natural
materials of earth and rock only and no cementing
materials are used. Same or varying materials are used to
construct the dam embankment. There are two main
types:

Earth fill Dam: These are constructed of selected soils
(0.001 ≤ d ≤ 100 mm) compacted uniformly and intensively in
relatively thin layers (20 to 60 cm) and at controlled
optimum moisture content. Compacted natural soils form
more than 50% of the fill Material. Dams may be designed
as: Homogeneous, Zoned or with impermeable core. core
part is made of relatively finer material that reduces seepage
flow, e.g. clay.

2. Rock fill dam: Over 50% of fill material be of class ‘rock’
usually a graded rock fill (0.1 ≤ d ≤ 1000 mm) is filled in bulk
or compacted in thin layers by heavy plant. Some impervious
membranes/materials are placed in the interior or on u/s face
of the embankment to stop/reduce seepage through the dam
embankment .
Dam section may be homogeneous, zoned, with
impermeable core, or with asphalt or cement concrete face.
Zoned part is made of relatively finer material that reduces
seepage flow, e.g. clay. Core is made of clay, concrete, asphalt
concrete etc.

B. Concrete Dams
Concrete dams are formed of cement-concrete placed in the
dam body. Dam section is narrow with steeper side slope.
Concrete dam section designed such that the loading produces
compression stress only and no tension are induced any where.
The reinforcement is minimum mainly as temperature control.
Concrete is placed in two ways: as conventional plain/reinforced
concrete (RC dam) or as roller compacted concrete (RCC dams).

1.Gravity dam: Stability due to its mass. Dam straight or slightly
curved u/s in plan (no arch action). The u/s face is vertical or nearly
vertical, d/s sloping.
2. Arch dam: Arch dam has considerable u/s plan curvature. U/s and
d/s faces are nearly straight / vertical. Water loads are transferred
onto the abutments or valley sides by arch action. Arch dam is
structurally more efficient than concrete gravity dams (requires
only 10-20% concrete). However abutment strength and geologic
stability is critical to the structural integrity and safety of the dam.
3. Buttress dam: It consists of continuous u/s face (i.e. deck)
supported at regular intervals by d/s buttress or crib. Types include
massive buttress, diamond head, round head with each section
separate.

DAM SITE SELECTION
A dam can be built anywhere if you can spend enough money. However
preferred site have following characteristics which lead to lower project
costs.
1. A wide and flat sloping valley upstream of the dam site and narrow and
steeply sloping valley at site of dam.
2. Deep valleys - Deep reservoir possible – require less area and lesser land costs,
less surface evaporation .
3. Enough water flow/yield available to meet requirements/demand
4. High sediment load tributaries are excluded

7. Geology favorable for foundation (foundation can be designed at any
site, but it increases costs), competent hard rock is most suitable.
8. Abutments are water tight, and reservoir rim allow minimum
percolation and seepage losses.
9. Reservoir area not very sensitive to environment (wild life parks,
endangered species, historical places, monuments etc).
10. No seismic and tectonic activities or active faults in and near the site.

11. Socio-political stability (no unstable gestures) (Gomal-Zam, Mirani,
Kalabagh dams).
12. Reservoir and dam area less populated.
13. Site is easily accessible; approach road is present or can be developed
easily.
14. Construction material available nearby easily.
15. Site near load center (demand area) for water+ power.
16. No mineral resources in reservoir area (present or future)

AREA –STORAGE-ELEVATION CURVE
To develop the curves following procedure is adopted
Topographic survey of the area is carried out.
 The contours map of the area is developed with contour intervals of say
5m
Area enclosed by each contours is calculated.
The area of intervening contours at small interval say 0.5 m is calculated by
using following relation.
o Suppose area of reservoir at 200m contour is “A1” hectare and at 205m contour
is “A2” hectare. Then area at 200.5m contour is

Incremental volume (ΔS), enclosed between two successive contours is
computed by using simple average method.
Where
a1 & a2 = Plan area of two consecutive contours with Δh contour interval.

The reservoir level corresponding to normal reservoir storage is
called as normal conservation level NCL and is determined from
the elevation-volume relationship of the dam. Referring to Figure
below, the normal conservation level is determined as 2076.2 for
gross storage capacity of 0.716 MAF.
In Fig. blow, the level corresponding to the point where both
curves are intersecting will NCL .It is thumb rule and used for
preliminary information.
Generally NCL is fixed after computing demands and dead storage.

El = 2.5821 x (Area)0.5226 + 1805

Area = 0.163 (Elevation ft - Datum)1.9132

El = 2.6905 x (Vol)0.3432 + 1805

Vol.= 0.05595 (Elevation - Datum)2.913

DAM HEIGHT
Height of dam determined from
(i)The gross storage (live storage + dead storage)
capacity of the dam
(ii)The space required to accommodate the
maximum flood (called flood surcharge)
(iii) The wave height generated from extreme winds
(iv) The wave run-up over the upstream sloping face
due to wind gusts
(v) The free board.
 Free board of 5 to 10 ft is generally provided

DAM HEIGHT
For Gross storage = 0.716 MAF (Live storage = 0.55 as
determined from mass curve / reservoir operation studies,
and dead storage = 0.166 MAF as determined from
sedimentation analysis), workout the required dam height.

DAM LAYOUT FOR EARTH & ROCKFILL DAM
Data: Dam crest level = 2100 ft, u/s face slope = 3.5:1 (H:V), d/s face
slope = 3.0:1; contour interval = 50 ft, river bed level = 1805 ft
Crest:
1. Locate the centerline of dam crest by connecting two points on
2100 ft contour line along right and left abutments such that the
dam has smallest crest length. The geologic makeup of the
foundations and abutments is also considered. Measure the crest
length.
2. Mark the crest width (e.g. 30 ft) parallel to the selected centerline.
3. Mark chainage along the dam crest with 0+00 mark at one of
abutments, e.g. right abutment. Determine the dam crest length.

U/s face:
4. Determine the horizontal distance corresponding to 50 ft vertical height
for u/s face ( = 50 x 3.5 = 175 ft). [3.5 :1 is slope of u/s face]
5. Mark a line A-A’ on u/s face parallel to crest edge spaced 175 ft apart
between 2nd contour line of 2050 ft.
6. Mark lines B-B’, C-C’, D-D’, E-E’ 175 ft apart between other contour lines
of 2000, 1950, 1900, 1850 ft, respectively.
7. Mark location of point F of lowest elevation in the river channel.
8. Connect points A-B-C-D-E-F-E’-D’-C’-B’-A’ with a smooth line and
connect the outline with crest edge on u/s face. This defines the dam
outline or footprint along u/s sloping face.

D/s face:
9. Determine the horizontal distance corresponding to 50 ft vertical height
for d/s face (= 50 x 3.0 = 150 ft). [3:1 is slope of d/s face]
10. Mark a line G-G’ on d/s face parallel to crest edge spaced 150 ft apart
between 2nd contour line of 2050 ft.
11. Mark lines H-H’, I-I’, J-J’, K-K’ 150 ft apart between other contour lines of
2000, 1950, 1900, 1850 ft, respectively.
12. Locate point L of lowest elevation in river channel on d/s side.
13. Connect points G-H-I-J-K-L-K’-J’-I’-H’-G’ with smooth line and connect
this with crest edge on d/s side. This defines the dam outline or footprint
along d/s sloping face.

Crest length, Longitudinal Section and Cross section
14. Draw longitudinal section (L-section) along centerline of dam crest. This
will provide valley profile between the river’s left and right abutments.
15. Draw dam cross section at maximum depth (section F-L at Ch 7+45),
and also at other chainage, e.g. at every 200 ft apart.
NOTE: The layout of concrete gravity dam is similar to earth fill dams
with the exception that u/s and d/s face slopes are very small (u/s ~ 1
H:10 V, d/s ~ 0.7 H:1 V)

Dam appurtenants
The layout of dam appurtenants (spillway, outlet, diversion tunnel,
power house, etc) is determined such that space requirement of all dam
components is adequately met. Few trials may be needed to finalize the
layout of dam embankment and dam appurtenants.

Construction of dams significantly alters the flow
regime which may affects;
 Ecology and Echo system in d/s reaches
 Sailaba area
 River Bed-aggradation of river bed
River at the entrance of ocean

Construction of dam will deprive the current
occupants of the area from productive benefits.
The affected persons will not only loose their
residential houses but most often their means of
livelihood (agriculture, small to medium business
etc.)
Dams and reservoirs may inundate some places of
regional nature
Transportation Corridors may get submerged

TERMINOLOGY
Catchment Yield: Annual runoff that is collected from a catchment area
measured at a point. It is expressed in Mm or M.ha and it is represented by
mass-inflow curve.
Reservoir Yield: Whatever flow is obtained from the reservoir (monthly, bi-
monthly or annually). It is represented by “Demand Curve” or “Mass Outflow
Curve.”
Safe / Firm Yield: The maximum of water that can be guaranteed during a
critical period.
Secondary Yield: Water available in excess of the firm yield during the year
of higher inflows is designated as the secondary yield.
Dependable/Yield: Yield that can be guaranteed with certain probability P
(e.g. Irrigation 75 % Hydropower 95% , Water Supply 100% )

COMPUTATION OF DEPENDABLE YIELD
i) Arrange annual flow volume data of N years in descending order
ii) Assign serial number n (n = 1 to N)
iii) Dependability (p%) of ‘n’ the discharge event = n/(N+1) * 100
iv) For pre-selected dependability P% ,read out the flow value from graph
or find m where m=(N+1) * P/ 100.
v) This procedure is valid for a seasonal storage only where volume stored
in one season is released in next irrigation season within one water cycle
of one year.
vi) For a large size carry over dam dependable flow equals the average flow
over a couple of years since storage reservoir will considerably alter the
outflow volumes.

COMPUTATION OF DEPENDABLE YIELD
The annual synthesized inflow of Kurram and Kaitu Rivers
at Kurram Tangi dam site is given in Table Determine the
dependable yield.
The dependability of different flows is determined in Table
and shown in Figure below. From the Table and Figure it is
seen that 50, 60, 70, 80 90 and 95% dependable yield of the
river at dam site is as 808, 775, 745, 630, 460, and 400 Th AF
per annum, respectively.

0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
300 400 500 600 700 800 900 1,000 1,100 1,200 1,300 1,400
Dependability
/
exceedance
(%)
Annual Deprndable Flow (ThAF)
Inflows to Kurram Tangi Dam from Kurram and Kaitu Rivers

 Assessment of water availability at dam site
a) Flow Data
The length of flow record preferably 100+ years should
available.
Minimum flow data of 20 to 30 years is needed to undertake
meaningful hydrological analysis.
Following methods may be used to determine the river
flows.
1. Historic stream flow is data available at the dam site
for sufficient long period……By using direct
observation method river flows at dam site are
computed

2.Flow data at dam site (Qd) is available for short period
but flow data of same river at a u/s or d/s distant location
(QL) is available for long period.
3. Short flow data at dam site and long rainfall data for the
catchment area…..Develop rainfall runoff correlation. This
method was used by NESPAK for generating long term flow
synthesis for Mirani Dam.
4. Short flow record at dam site but a long flow record at a
nearby river having similar hydrologic conditions (rainfall,
catchment hydrologic characteristics, etc)…… Develop
correlation b/w two sites.

5. No flow data for the dam site river but satisfactory flow
record for a nearby basin of similar or different hydrologic
characteristics in the region. Precipitation data is available
for the two sites/basins….Use derived P-Q relationship
using the P data of dam site. In case hydrological
conditions are not similar, modify the underlying factor of
P-Q relationship .
6. No flow record at dam site or nearby location. Rainfall
data available at dam site or a nearby location……Use
regional P-Q model
.

b) Stochastic Data Generation from Short Data
Various models used to extend data include Auto-correlation
(AR) models, Moving Average (MA) models, ARMA model,
ARIMA models, Seasonal/non-seasonal flow models (e.g.
Thomas-Fierring).
The generated data have the same statistical properties as the
original short term data.
Seasonal models will provide monthly flows, and Non-
seasonal models will provide annual flows

c) Data Processing
 Flow data is processed to find out average annual flows,
average monthly flows ,10-daily flows ,standard deviations etc.
Flow data is processed to determine flow duration curve
(FDC) for run-of –river hydropower projects.
FDC describes the exceedence probability for selected flow
discharge.

Kurram Tangi Dam: Average Annual Synthesised Inflow (Th.AF)
441
748
979
638
781
765
559
786
955
897
1017
993
1350
808
541
715
781
749
654
927
1219
1295
1056
813
874
865
910
1294
621
370
414
833
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400 1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
Annual
inflow
(Th.AF)
Average
Flows

Kurram Tangi Dam: Average Monthly SynthesisedInflow(Th.AF)
43
39
84
116
107
66
99
102
56
45
33
41
0
10
20
30
40
50
60
70
80
90
100
110
120
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Average
Monthly
inflow
(Th.AF)

Kurram Tangi Dam: 1971-2001 10-daily Synthesised Inflow (Th. AF)
0
20
40
60
80
100
120
140 Jan
01-10,
71
Jan
01-10,
73
Jan
01-10,
75
Jan
01-10,
77
Jan
01-10,
79
Jan
01-10,
81
Jan
01-10,
83
Jan
01-10,
85
Jan
01-10,
87
Jan
01-10,
89
Jan
01-10,
91
Jan
01-10,
93
Jan
01-10,
95
Jan
01-10,
97
Jan
01-10,
99
Jan
01-10,
01
Month,10-Day period and Year
10-day
KTD
inflow
(Th.AF)

Golen Gol Hydro Power Project
Flow Duration Curve (1993-2006)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
Exceedence Time (%)
Discharge
(m
3/
sec)
Av 93-06
1993
1995
2004

 Reservoir Live Storage Capacity
a) Ripple mass curve analysis (Mass Curve)
RMC is a plot between accumulated inflow and time

Procedure
 Determine accumulated flow ΣQ and demand ΣD. Plot
accumulated flow discharge against time ( as shown in Figure ).
Mark the apex point on mass inflow curve. Draw the tangents
on apex points parallel to demand curve.
 For small demand the ΣD curve will meet the ΣQ curve before
next apex point P. This ensures that reservoir will become full at
this time of the year.
For large demand the cumulative demand curve may meet the
cumulative flow curve after more than 1 year

Procedure
 Determine supply deficit for each year as the maximum
difference between supply ΣQ and demand ΣD curves. This
gives required storage for each year corresponding to the
demand.
For large demand, the reservoir may not become full at end of
each water year (example demand D3 in Fig.). This represents
condition for a carry over dam.
 In case demand varies during the year, use appropriate data to
determine accumulative demand and deficit for each flow
period

Procedure
 Analysis is started from a time when reservoir is most likely to
be full (e.g. by 1st Sept.) each year depending upon average flow
pattern of the particular river.
 Determine maximum deficit and the required storage S for
each year of analysis.
 The calculated storage requirements represent live storage
for the particular purpose e.g. irrigation.
Determine the required reservoir capacity that will ensure
supplies for selected probability level by probability procedure.

KT Dam: Commulative Inflow
0
200
400
600
800
1000
1200
1400
Jan
01-10,71
Feb
01-10
Mar
01-10
Apr
01-10
May
01-10
Jun
01-10
Jul
01-10
Aug
01-10
Sep
01-10
Oct
01-10
Nov
01-10
Dec
01-10
Jan
1-10,72
Feb
01-10
Mar
01-10
Apr
01-10
May
01-10
Jun
01-10
Jul
01-10
Aug
01-10
Sep
01-10
Oct
01-10
Nov
01-10
Dec
01-10
Jan
1-10,73
Inflow
(ThAF)
P
P

KT Dam: Commulative Inflow
0
200
400
600
800
1000
1200
Jul
01-10
Aug
01-10
Sep
01-10
Oct
01-10
Nov
01-10
Dec
01-10
Jan
1-10,72
Feb
01-10
Mar
01-10
Apr
01-10
May
01-10
Jun
01-10
Jul
01-10
Aug
01-10
Sep
01-10
Oct
01-10
Nov
01-10
Dec
01-10
Inflow
(ThAF)
P
P
D1
D2
D3
S1
S2
S3

Ripple Mass Curve Analysis
 Arrange data in columns (time, Q, D) for all years. The flow and
demand may be available on 10-daily basis or on monthly basis.
 Start the analysis at latest apex point P (e.g. 1st Sept.) when
dam may be considered full every year.
 Determine storage deficit SD for subsequent periods as:
 SDt = MAX [{(Dt-Qt)+SDt-1}, 0]
 Determine largest value of the storage deficit SD for each water
year of the analysis period. This is the required live storage for
that year
 Above steps explained in table 2.5 (DRE, by Dr.Tariq )

Ripple Mass Curve Analysis
The deficit for Kurram Tangi Dam is shown in Fig. for annual
demand of 785 Th.AF.
KTD: Annual Storage Deficit
277
274
221
335
315
434
580
565
405
310
132
132
137
208
372
427
439
502
613
467
105
88
107
93
146
169
108
166
465
0
100
200
300
400
500
600
700
Jan
1-10,71
Jan
1-10,72
Jan
1-10,73
Jan
1-10,74
Jan
1-10,75
Jan
1-10,76
Jan
1-10,77
Jan
1-10,78
Jan
1-10,79
Jan
1-10,80
Jan
1-10,81
Jan
1-10,82
Jan
1-10,83
Jan
1-10,84
Jan
1-10,85
Jan
1-10,86
Jan
1-10,87
Jan
1-10,88
Jan
1-10,89
Jan
1-10,90
Jan
1-10,91
Jan
1-10,92
Jan
1-10,93
Jan
1-10,94
Jan
1-10,95
Jan
1-10,96
Jan
1-10,97
Jan
1-10,98
Jan
1-10,99
Jan
1-10,00
Annual
Storage
Deficit
(ThAF)

Required Storage Capacity
 Storage may be provided to meet the maximum deficit
determined during the period of analysis. This is true when 100%
dependable supplies are required for the purposes, e.g. domestic
water supply .
 For other cases, storage is provided for selected probability level
in concordance with the scope of water delivery, e.g. 75% for
irrigation, 90% for hydropower, etc.
 Following procedure is followed to determine the storage
required to avert deficits for selected probability levels
 Determine the yearly maximum deficit for N years from Ripple
curve analysis for known inflows and selected annual demand.

Required Storage Capacity
 The storage required to meet all deficit in any year equals the
maximum deficit of that year.
 Arrange yearly required storage (i.e. live storage) data of N years
in ascending order.

Reservoir Total Capacity
 Determine required live storage capacity from Ripple mass
curve analysis described in previous slides.
 Dead storage volume is selected in view of annual sediment
inflow volumes such that dead storage space is filled up in not
less than 50 to100 years
Flood storage space (for a dam with part objective of flood
control) is determined by knowing flood volume which has to be
temporarily stored in the dam and then released.
 Total gross storage = live storage + dead storage + flood storage.

International Commission on Large Dams (ICOLD) defined embankment
dam as “any dam constructed of excavated materials placed without
addition of binding material “

An Earthfill Dam is an embankment dam, constructed
primarily of compacted earth materials, either homogeneous
or zoned, and containing more than 50% of earth granular
materials.
Rockfill Dam is an embankment dam constructed of natural
rock materials, usually broken down to smaller fragments.
An embankment dam where large quantities of both
granular materials (earth) and rock fragments are used is
called as Earthfill-Rockfill dam

TYPE OF EARTHFILL DAMS
Homogeneous Dams
The dam embankment is made of a single type of material .These include fine-
grained particles with minor amounts of coarse-grained materials.
The fill material is required to possess following properties.
The fill material must be sufficiently impervious to provide an adequate
barrier and prevent excessive loss of water through the dam
The fill material should develop maximum practical shear strength under
compaction and maintain most of it after the filling of the reservoir.
The fill material must not consolidate, soften or liquefy upon saturation.
Due to relatively finer materials, the slopes must be able to avoid sloughing.
The u/s slope is relatively flat to ensure safety against sloughing under rapid
drawdown conditions after prolonged high-level storage. The d/s slope must
be protected to resist sloughing when saturated to high level rainfall.

A homogeneous embankment should not be used for storage dam. A
homogeneous type of dam is applicable in localities where readily available
soils show little or no variation is permeability
A homogeneous dam provided with the measures to intercept the seepage .
Such a dam is called as modified homogeneous dam

Zoned Embankment Dam
A zoned embankment dam is constructed of materials of more than two types.
When rock is used in shell then it is then as earthfill-rockfill dam (Tarbela,
Mangla dams).
The dam is considered as zoned dam only if the horizontal width of the
impervious zone at any elevation equals or exceeds the height of the dam
above that elevation, and is not less than 10 feet [w ≥ h and w > 10 ft]

The maximum width of the core is controlled by stability and seepage
criteria and the availability of the material.
When a variety of soil materials are available, the choice of an earthfill
dam should always be a zoned embankment type because of its
inherent advantage in reduced cost of construction.

RAISING OF MANGLA DAM
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Diaphragm or Thin Core Dam
This dam is similar to a zoned embankment dam with the exception that a
thin diaphragm of impervious material is provided to form a water barrier
A thin core dam becomes more economical for reasons as:
1. Unit cost of placing impervious materials (acquiring, processing, haulage,
and compaction) may be less than the unit cost of placing pervious
materials.
2. The amount of embankment volume can be reduced in a thin core dam
more effectively.
3. The construction time available and weather conditions may not permit
the use of an impervious core of large thickness.

The core may be vertical oriented or inclined.
If it is strong to resist cracking under load, a location near u/s is often
the most appropriate. However, if core material is weak, a central
location is better.

The core is preferably located in the center of the dam embankment due to
following advantages.
1.The core is equally supported and is more stable during a sudden drawdown
(if constructed from earth).
2.Settlement of dam induces compressive stresses in the core, tending to
make it more compact. In inclined bending and sagging can cause cracking in
the core.
3. There is less core volume.

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