CHAPTER 3
Dams and Spillways
Ercan Kahya
Department of Civil Engineering, I.T.U.
Figure uses by courtesy of Prof. Recep YURTAL
I.T.U., Department of Civil Eng.
I.T.U., Department of Civil Eng.
Embankment (Fill) Dams
I.T.U., Department of Civil Eng.
Environmental Effects of Dams
 Social
and economic effects
 Ecologic effects
 Regional climate effects
 Vegetation effects
 Fishery
 Navigation effects
 Upstream and downstream navigation effects
 Tourism effects
3.1 Classification of Dams
 According

to dams height
If crest elevation and foundation level greater than 15 m
 Large Dam




If dam height less than 15 m  Small Dam

If dam height greater than 50 m  High Dam

More specifically
The height of the dam > 15 m
The crest width of the dam > 500 m
The storage volume of the dam > 106 m3
“LARGE DAM”
Classification of Dams
 According

to construction purpose
Single purpose
■ Storage Dams
■ Diversion Dams
■ Detention Dams
■ Hydropower Dams

Multiple purpose

(Serves for all or most of the above purposes)
Drinking water
Irrigation
Flood control
Energy
Navigation
Recreational purposes
I.T.U., Department of Civil Eng.
Classification of Dams

According to Hydraulic Design
■ Overflow Dams
(i.e., diversion dams)
■ Non-overflow Dams
(i.e., earth fill & rock fill dams)
Classification of Dams

According to Materials of Construction
■ Embankment Dams
• Earth-Fill Dams
• Rock-Fill Dams
■ Masonry and Rubble Dams
■ Concrete Dams
■ Steel and Timber Dams
Classification of Dams

According to Structural Design
 Gravity Dams
 Arch Dams
 Buttress Dams
 Earth-Fill
 Rock-Fill
 Pre-stressed Concrete Dams
According to Structural Design
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Embankment (Fill) Dams
3.2 Parts of Dams
Structural components:
- Body
- Spillway
- Outlet Facilities
(i.e., sluiceways & water intake tower)
- Others (i.e., hydropower stations, roads,
fish ladder, etc.)
3.3 Planning of Dams
Three steps:
- Reconnaissance surveys
(Infeasible alternatives eliminated)
- Feasibility study
- Planning study
Planning of Dams
3.3.1 FEASIBILITY STUDY
A) Determination of water demand
B) Determination of water potential
C) Optimal plans
◘ Check out the relation D (demand) versus S (supply).
Planning of Dams
FEASIBILITY STUDY
D) Determination of dam site
◘ Factors should be taken into consideration:













Topography
Geology and dam foundation
Available of construction materials
Flood hazard
Seismic hazard
Spillway location and possibilities
Construction time
Climate
Diversion facilities
Sediment problem
Water quality
Transportation facilities
Right of way cost
Planning of Dams
E) Determination of type of dam
◘ Comparative characteristics of dams should be considered
F) Project design
◘ involves the computation of dimensions of the dam.
- Hydrologic design (max. lake elevation + spillway cap. + crest elevation)
- Hydraulic design (static & dynamic loads + spillway profile + outlet
dimensions)
- Structural design (stress distribution + required reinforcement)
◘ Failure of the dam

“
”
It is rapid for a concrete dam. See the textbook for the examples.
Planning of Dams
3.3.2 PLANNING STUDY
◘ Followings need to be done in planning certain type of dam,
since dimensions are already determined:
a)Topographic surveys (1:5000 scaled map)
b)Foundation study (seepage permeability etc. tests)
c)Materials study (quantity of materials)
d)Hydrologic study (measurements of hydrologic parameters)
e)Reservoir operation study (is to be performed periodically)
3.4 Construction of Dams
Four principal steps are followed during the construction:
1) Evaluation of Time Schedule and Equipment
◘ a work schedule is prepared using CPM.
(characteristics of dam site; approx. quantities of
works; diversion facilities; urgency of work)
2) Diversion
◘ before the construction, river flow must be diverted
from the site
◘ see the below figure for two possible ways to
divert water:
Cofferdam
Cofferdam
Construction
zone
Cofferdam
(b)
Flow in stream bed
(a)
Construction zone
First
stage
Diversion tunnel
Downstream
Flow through sluiceway
Diversion by tunnel
Second
stage
Completed portion of dam
River Diversion facilities
Construction zone
Diversion tunnel
Upstream
(c)
3) Foundation Treatment
◘ Concrete & Rock-fill dams  hard formations
Earth-fill dams  most of soil conditions
◘ Highly porous foundation  excessive seepage, uplift,
settlement
“Grouting Operation” is applied to solidify the foundation
& to reduce seepage
Formation of the Dam Body
For Concrete Gravity dams:
• Low-heat cements  to reduce shrinkage problem
•
•
Concrete is placed in “blocks”
“Keyways” are built between sections to make the dam act
as a monolith
Upstream face
Upstream face
Keyways
Downstream face
Downstream face
•
“Waterstops” are placed near upstream face to prevent leakage
Copper strip
Copper strip
Waterstops
“Inspection galleries” permit access to the interior of concrete
Dams and are needed for seepage determination, grouting
operations and etc.
For Earth-fill dams
•
•
•
•
•
Constructed in multi-layer formation
(Layers: impervious, filter and outer)
First place the materials in layers of 50 cm and then
compact these materials.
For high dams, horizontal berms are constructed to
enhance slope stability
Protect the upstream face of dam against wave action
(i.e., concrete or riprap)
Protect the downstream face against rainfall erosion
(i.e., planting grass or riprap)
Cross section of typical earth dams
Silt
Silt clay
Sandy
gravel
(a) Simple zoned embankment
Silt
Pervious strata
Clay
core
Silt
Rock-fill toe
Transition zone
Pervious foundation
(b) Earth dam with core extending to impervious foundation
Cross section of typical earth dams
Clay blanket
Silt clay
Silt
Sandy
gravel
Pervious material
Concrete cutoff wall
(c) Earth dam on pervious material
For Rock-fill dams:
• Core and filter zones
are similarly constructed as the
earth dam
• Due to heavy rocks on the sides, these dams
• have steeper slopes
• have less materials
• are economic
• Construction period is shorter and easy to increase the
crest elevation
 Width of dam crest: There are two traffic lanes
 Elevation of dam crest: There is no overtopping during
design flood
 Freeboard: See the table for recommendations
Select Compacted Rock
Rolled
Medium
Size
Rock
1.3
1
1.3
1
Coarse
Dumped Rock
Reinforced Concrete
Membrane
Cutoff wall
(a) Impermeable face
Cross-section of typical Rock-fill dams
1.4
Graded transition
sections
1.4
1
1
Dumped or
Rolled rock
(b) Impermeable earth-core
Rolled rock
(0.2m)
Grout curtain
(1.5m)
GRAVITY DAMS
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Concrete Gravity Dams
Resist the forces by their own weight
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Concrete Gravity Dams
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Concrete Gravity Dams
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Concrete Gravity Dams
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Concrete Gravity Dams
Why





& Where we prefered?
Sağlam ve geçirimsizliği sağlanabilecek yeterli kalınlıkta kaya
temellerin uygun bir derinlikte bulunduğu orta genişlikteki vadilerde
Yeterli miktarda ve istenen özellikte agrega malzemesinin
bulunduğu, çimento naklinin ekonomik olduğu yerlerde
Büyük taşkın debilerinin baraj gövdesi üzerinden mansaba
aktarılması gereken durumlarda
Baraj üzerinden bir ulaşım yolu geçirilmesi gereken durumlarda
tercih edilir
Savaş ve sabotaja karşı daha dayanıklı olması da ayrıca bir tercih
nedeni olabilir.
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Concrete Gravity Dams
 Types:
• Straight Gravity Dams
• Arch Gravity Dams


Baraj ekseni, iki yamaç arasındaki en kısa
bağlantıyı sağlayacak şekilde bir doğru ile
birleştirilir.
Temel kayasının yapısına, derzlere veya
emniyet ihtiyacına bağlı olarak kavisli de
yapılabilir.
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Concrete Gravity Dams

Design Criteria:




En uygun kesit, etki eden en önemli dış kuvvet olan haznedeki
hidrostatik su basıncı dağılımına uyum sağlayan, tabana doğru
genişleyen üçgen kesit seçilir. Üçgenin tepesi genellikle
haznedeki en yüksek su seviyesidir.
Memba yüzeyi düşey veya %10 ‘u geçmeyecek şekilde eğimli
yapılır.
Baraj boş haldeyken çekme gerilmelerini önlemek, dolu
haldeyken kayma ve devrilme emniyetini artırmak için yüksek
barajlarda memba yüzeyi genellikle eğimli planlanır.
Üçgenin tepe kısmında, duvar kalınlığını artırmak, yamaçlar
arası ulaşımı sağlamak gibi nedenlerle dikdörtgen kesitli bir
başlık bulunur.
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Concrete Gravity Dams
Design Criteria:
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Concrete Gravity Dams
Design Principles:



Ağırlık barajı hesaplarında
üçgen profil gözönüne alınır.
H
Üçgen kesitin minimum
boyutları, barajın kendi ağırlığı,
hidrostatik su basıncı ve taban
su basıncının etki ettiği normal
yükleme durumunda çekme
gerilmeleri meydana
gelmeyecek şekilde belirlenir.
Bunun için:
tg  
b

H
b
b

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1
m
Concrete Gravity Dams
 For the dam dimensions:
 Check out the safety for
• Overturning
• Shear & sliding
• Bearing capacity of foundation
• No tensile stresses are allowed in the dam body
Overturning Check
1/md
H
B
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Overturning Check
H
B
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Overturning Check
H
B
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Overturning Check
H
B
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Overturning Check
H
B
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Overturning Check
H
B
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Sliding Check
1/md
H
B
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Sliding Check
H
B
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Sliding Check
H
B
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Sliding Check
H
B
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Sliding Check
1/md
H
B
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Bearing Capacity Check
1/md
H
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3.5.1 FORCES ON GRAVITY DAMS
Free body diagram showing forces acting on a gravity dam
The following loads should be considered:
A) WEIGHT (WC): Dead load and acts at the centroid of the
section
B) HYDROSTATIC FORCES:
Water in the reservoir + tailwater causes Horizontal Hu Hd &
Vertical Fh1v Fh2v
C) UPLIFT FORCE (Fu): acts under the base as:
D) FORCE OF SEDIMENT ACCUMULATION (Fs):
Determined by the lateral earth pressure expression
where
• Fs : the lateral earth force per unit width,
• γs : the submerged specific weight of soil,
• hs : the depth of sediment accumulation relative to reservoir
bottom elevation,
• θ : the angle of repose.
 This force acts at hs /3 above the reservoir bottom.
E) ICE LOADS (Fi): considered in cold climate
Ice force per unit width of dam (kN/m) can be determined
from the following table:
Thickness of ice
sheet (cm)
Change in temperature (oC/hr)
2.5
5
7.5
25
30
60
95
50
58
90
150
75
75
115
160
100
100
140
180
F) EARTHQUAKE FORCE (Fd):
Acting horizontally and vertically at the center of gravity
k (earthquake coefficient): Ratio of earthquake acceleration to
gravitational acceleration.
G) DYNAMIC FORCE (Fw) :
In the reservoir, induced by earthquake as below

Acts at a distance 0.412 h1 from the bottom
• Fw : the force per unit width of dam
• C : constant given by
'
• θ’ : angle of upstream face of the dam from vertical (oC)
• For vertical upstream face 
C = 0.7
H) FORCES ON SPILLWAYS (∑F):
Determined by using momentum equation btw two successive
sections:
• ρ : the density of water
• Q : the outflow rate over the spillway crest
• ΔV: the change in velocity between sections 1 and 2 (v2-v1)
 Momentum correction coefficients can be assumed as unity.
I) WAVE FORCES :
Considered when a long fetch exists
LOADING CONDITIONS:
 Usual loading
B &Temperature Stresses at normal conditions + C + A + E + D
 Unusual loading
B & Temperature Stresses at min. at full upstream level + C + A +D
 Severe loading
Forces in usual loading + earthquake forces
3.5.2 STABILITY CRITERIA

Dam must be safe against

(1) Overturning for all loading conditions
FS O 
Mr
Mo
Safety factor:


F.SO  2,0 (usual loading)
F.SO  1,5 (unusual loading)


resisting moments
overturning moments
STABILITY CRITERIA

(2) Sliding over any horizontal plane
FSs =

f åV
åH
f = friction coef. btw any two planes
Safety factor:
 FSS  1,5 (usual loading )
 FSS  1,0 (unusual or severe loading)
STABILITY CRITERIA

(3) Shear and sliding together
FSss =
f åV + 0.5A t s
åH
A : Area of shear plane (m²)
τs : Allowable shear stress in concrete in contact with foundation
Safety factor:
 FSss  5,0 (usual loading)
 FSss  3,0 (unusual or severe loading)
STABILITY CRITERIA

(4) Between foundation and dam contact stresses (σ) > 0
at all points
There are two cases for the base pressure:
Mr - Mo
x=
åV
B
e= -x
2
Base Pressure Check
 CASE
1: e  B/6
æ åV ö æ 6e ö
÷ ´ ç1+ ÷ Pt  s
Pt = ç
ç B ÷ è Bø
è
ø
æ åV ö æ 6e ö
÷ ´ ç1- ÷ Ph  s
Ph = ç
ç B ÷ è Bø
è
ø
B
DAM BASE
Ph
Pt
e
x
ΣV
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Base Pressure Check
B
CASE 2: e >
B/6
DAM BASE
æ
ö
÷
æ åV ö ç
1
÷´ç
÷
Pt = ç
ç B ÷ ç æ 3 ö æ 1 e ö ÷ Pt  s
è
ø çç ÷´ç - ÷÷
èè 2 ø è 2 B øø
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Pt
x
e
ΣV