This document discusses the forces acting on gravity dams and their environmental impacts. It outlines various forces like water pressure, weight of the dam, uplift pressure, earthquake pressure, and wave pressure. It also explains how these forces are calculated. Regarding failure, it notes dams can fail through overturning, sliding, compression, or tension. The document concludes by covering environmental impacts of dam construction like pollution, and impacts of reservoirs like habitat destruction and sedimentation.

1. Dams Stability & Environmental
Impact
CE-402: Irrigation Engineering
(CH-03)
By
Dr. Muhammad Ajmal
Lecturer, Agri. Engg. UET, Peshawar

2.  A number of forces acting on a gravity dam. Some of them are:
(a) Water pressure
(b) Weight of Dam
(c) Uplift pressure
(d) Pressure due to Earthquake
(e) Ice pressure
(f) Wave pressure
(g) Silt pressure
(h) Wind
Forces acting on a Gravity Dams

3. (a) Water Pressure
 Water pressure is the major force acting on a dam.
 When the u/s face of dam is vertical, water pressure act horizontally.
 When u/s face is partly vertical and partly horizontal the resultant
force is resolved into Horizontal component PωH and vertical
component Pωv due to weight of water supported by inclined face.
 The horizontal force is PωH =
𝝎𝑯𝟐
𝟐
 This acts at a height of H/3 from the base of the dam.
 𝝎 is the unit weight of water.
Pwh
h/3
h
Forces Acting on a Gravity Dams

4.  The vertical component is the weight of water contained in dam
and is acting at the centre of gravity of the area.
 Similarly if there is the tail water of certain height, then the
horizontal pressure is:
P2 =
𝜔𝐻′2
2
 The vertical pressure is weight of water contained in d/s level
Forces Acting on a Gravity Dams

5. (b) Weight of the Dam
 Weight of dam is a major resisting force. For this purpose unit
length of dam is considered.
 The cross section of dam is divided into several triangular
and rectangular sections and their corresponding weights w1,
w2, w3, etc.
 The total weight of a dam act at the centre of gravity.
Forces Acting on a Gravity Dams

6. Forces Acting on a Gravity Dams

7. (c) Uplift Forces
Forces Acting on a Gravity Dams

8. (c) Uplift Forces
 Uplift forces (U) is the upward pressure of water as it flows or seeps
through the body of the dam or its foundation due to pores, cracks,
and joints within the body of a dam.
 The pressure distribution depends upon the size, location, and
spacing of internal drains.
 According to United States Bureau of Reclamation (USBR) the
uplift pressure intensities equal to the hydrostatic pressure of water
at toe and heel.
✓ Uplift pressure at the heel = ωh = ρgh
✓ Uplift pressure at the toe = ωh/ = ρgh/
✓ Uplift pressure at the gallery = ρg [h/ + 1/3(h-h/ )]
Forces Acting on a Gravity Dams

9. (d) Wave Pressure
 Waves are generated on the reservoir surface because of wind
blowing over it.
 The waves pressure depends on the height of the waves developed.
 The wave height hw can be calculated using the following relations:
hw = 0.0322 (V.F)1/2 + 0.763 – 0.271 F1/4 for F < 32 km
hw = 0.0322 (V.F)1/2 for F ≥ 32 km
where hw = height of waves in meters between the trough and the crest,
V is the wind velocity (km/h) and F is the fetch (straight length of
water expanse in km).
Forces Acting on a Gravity Dams

10. Forces Acting on a Gravity Dams

11. (d) Wave Pressure
 The pressure intensity due to waves is given as:
Pw = 2.4 ωhw (t/m2)
 The maximum wave pressure Pw occurs at (1/8)*hw meters above
the still water level, however for design purpose, the pressure
distribution may be assumed to be represented by a triangle of
height equal to (5/3)*hw and hence the total pressure is given by:
Pw =1/2*(5hw/3)*(2.4 ωhw )
= 2ωhw
2(t/m)
= 2000 ωhw
2(kg/m)
 This pressure acts at (3/8) hw above the reservoir surface
Forces Acting on a Gravity Dams

12. (e) Earthquake pressure
 The earthquake waves impart accelerations to the foundations
under the dam and cause its movement.
 In order to avoid rupture, the dam must also move along with it.
 This acceleration produces an inertia force in the body of dam and
sets up stresses initially in lower layers and gradually in the whole
body of the dam.
 Earthquake wave may travel in any direction.
 The general direction of acceleration is taken as horizontal and
vertical.
Forces Acting on a Gravity Dams

13. (e) Earthquake pressure
 Effect of Horizontal Acceleration: horizontal acceleration cause two forces
(i) Inertia force in the body of dam
(ii) hydrodynamic pressure of water
(i) Inertia force
 The inertia force acts in a direction opposite to the acceleration by
earthquake forces and is equal to the product of the mass of dam and
acceleration.
✓ If W is the weight of dam;
✓ g is the acceleration due to gravity,
✓ αh is acceleration coefficient = earthquake acceleration/ acceleration due
to gravity.
Forces Acting on a Gravity Dams

14. (e) Earthquake pressure
(i) Inertia force
o The horizontal earthquake acceleration = αh*g
o Inertia force= F= Mass* Acceleration
= (W/g) * αh*g = W* αh
o This force should be at the center of gravity of the mass.
Forces Acting on a Gravity Dams

15. Forces Acting on a Gravity Dams

16. (ii) Hydrodynamic Pressure
 The horizontal acceleration of the dam and foundation towards
the reservoir causes an increase in the water pressure since the
water resists the movement owing to inertia.
 If the hydrodynamic pressure variation is assumed to be
parabolic, the increase in water pressure Pe is given by:
Pe = 0.555 αhωh2
It acts at a height of
𝟒𝒉
𝟑𝝅
= 0.424h above the base
Forces Acting on a Gravity Dams

17. (ii) Hydrodynamic Pressure
❖ Von-Karman Method
 For elliptical-cum parabolic, the hydrodynamic pressure intensity at a depth y
below the maximum water level is given by:
Pey = Cyαh ω h
where Cy= dimensionless pressure coefficient at depth y below the water level
Cm = max. value of the pressure coefficient for a given constant slope
θ = angle in degree the u/s face of dam makes with horizontal;
h= depth of water in reservoir;
y = the vertical distance from reservoir surface to the elevation in question.
Forces Acting on a Gravity Dams
2 2
2
m
y
C y y y y
C
h h h h
 
   
   
 
   
   
 
 
0.735 1
90
m
C

 
 
 
 

18. ❖ Zanger’s Method
 For elliptical-cum-parabolic, the total pressure at depth y will be equal
to the average of the areas of the quarter ellips and semi-parabola .
Hence
Similarly the moment of pressure is
Forces Acting on a Gravity Dams
   
1 2 1
0.7854 0.6667 0.276
2 4 3 2
ey ey ey ey ey
T
P P y P y P y P y

 
    
 
 
   
2 2
1 1 4
. 0.299 .
2 3 15
ey ey ey
T T
M P y y P y y
 
  
 
 

19.  Effect of Vertical acceleration
 Due to vertical acceleration, a vertical inertia force F = W αv is
exerted on the dam in the direction opposite to that of acceleration.
 The inertia act vertically downward. When acceleration is
vertically downward the inertia force F = W αv acts upward and
decreases.
 Hence the altered unit weights of the dam and water, due to vertical
acceleration are ωm (1 ± αv) for dam materials and ω ( 1 ± αv ) for
water ; where ωm is the unit weight of dam materials.
Forces Acting on a Gravity Dams
±

20.  Combination of Loading For design
 All the forces simultaneously not acting on a dam. There are various
combination of loads which act at the same time.
 Dam design must be based on worst combination of the probable load
conditions. Following are the normal and extreme load combinations
A) Normal Load Combination
(i) For normal water surface elevation; ice pressure, silt pressure, and normal
uplift pressure consideration is required (in snow fall regions).
(ii) Normal water surface elevation, earthquake force, silt pressure and normal
uplift.
(iii) Maximum water surface elevation, silt pressure and normal uplift.
Forces Acting on a Gravity Dams
±

21. B) Extreme Load Combination
 Maximum flood water elevation, silt pressure, and extreme uplift pressure
Load.
C) Reservoir Empty Condition
 The design will also be checked for the reservoir in empty condition.
 The condition is important for testing stresses inside the dam and round
opening provided in the body of the dam.
Forces Acting on a Gravity Dams

22.  The modes of failure of a gravity dam may be due to
1. Overturning 2. sliding 3. Compression or crushing 4. Tension
1. Overturning
❖ The overturning of the dam section takes place when the resultant force at
any section cuts the base of the dam downstream of the toe.
❖ In this case the resultant moment at the toe becomes clockwise (negative).
❖ If the resultant force cuts the base within the body of the dam, there will be
no overturning.
❖ For stability requirements the dam must be safe against overturning.
❖ The factor of safety against overturning is defined as the ratio of the
positive moment to the overturning moment.
Modes or Failure / Stability Requirements
R
o
Right moments M
. .
Overturning M
F S  
 
 
❖ The factor of safety against overturning must not be less than 1.5.

23. 2. Sliding
 A dam will fail in sliding at its base, or at any other point, If the horizontal
forces causing sliding are more than the resistance available to it at that level.
 The resistance against sliding may be due to friction alone, or due to friction and
shear strength of the joints.
 If the shear strength is not counted, the factor of safety is known as factor of
safety against sliding.
 Factor of safety: the ratio of actual coefficient of static friction on the
horizontal joint to the sliding friction.
 The sliding factor is the minimum coefficient of friction required to prevent
sliding.
 If Σ H = sum of horizontal forces causing the sliding and Σ (V-U) is the net
vertical forces, the sliding factor (tan θ) is
Modes or Failure / Stability Requirements
.F. tan
H
S
V

 



24.  The factor of safety against sliding is:
❖ The coefficient of friction  varies from 0.65 to 0.75.
❖ The friction of safety should be more than 1.0.
❖ A low gravity dam should be safe against sliding considering friction
alone.
❖ For large dams, shear strength of the joint should also be considered.
The factor of safety in such case is known as shear friction factor S.S.F
and is defined as
S.S.F =
𝛍∑ 𝐕−𝑼 +𝐛𝐪
𝜮𝑯
where q = shear strength of the joint (usually 14 kg/cm2)
b = width of joint or section.
Modes or Failure / Stability Requirements
F.S.S.
tan
V
H



 



25. 3. Compression or Crushing
 To calculate the normal stress distribution at the base or at any section, let H
be the total horizontal force, V be the vertical force and R be the resultant
force cutting base at eccentricity “e” from the centre of the base of width b.
 The normal stress at any point on the base will be the sum of the direct
stress and the bending stress. (See page 385 in book)
 Hence the total normal stress pn is given by:
Modes or Failure / Stability Requirements
Direct Stress
1
V
b


2
2
. V.e 6V.e
Bending Stress
1
6
M y
I b
b
     
6
1
n
V e
p
b b
 
 
 
 

26.  Positive sign will be used for calculating normal stress at the toe.
 Negative sign will be used for calculating normal stress at the heel.
 The maximum compressive stress occurs at the toe and for safety,
this should not be greater than the allowable compressive stress (f)
for the foundation materials. Hence from strength point of view:
 When the eccentricity “e” is equal to (b/6) then Pn (toe) is:
Then the stress at heel will be zero.
Modes or Failure / Stability Requirements
6
1
V e
f
b b
 
 
 
 
6
1 2
6
n
V b V
p
b b b
 
   
 
 

27. 4. Tension
 The normal stress at heel is Pn (heel)
 If e> (b/6), the normal stress at the heel will be negative or tensile.
 No tension should be permitted at any point of the moderately high dam.
 The eccentricity should be less than b/6. or the resultant should be lie
within the middle third of its length.
Modes or Failure / Stability Requirements
6
1
heel
V e
p
b b
 
 
 
 

28. ENVIRONMENTAL IMPACTS
OF DAMS

29. ENVIRONMENTAL IMPACTS of CONSTRUCTION
PHASE of DAMS
 River pollution
 Erosion
 Loss of aesthetic view
 Air pollution
 Noise pollution
 Dust

30. ENVIRONMENTAL IMPACTS of RESERVOIRS
 Loss of land
 Habitat Destruction :
 The area that is covered by the reservoir is destroyed, killing
whatever habitat existed there beforehand.
 Loss of archeological and histrorical places
 Loss of mineral deposits
 Loss of special geological formations
 Aesthetic view reduction
 Sedimentation
 Change in river flow regime and flood effects
 Reservoir induced seismicity
 Change in climate and plant species

31. EFFECTS of DAMS to WATER QUALITY
 Change in temperature
 Turbidity
 Dissolved gases in the water
 Water discharged from the spillway contains 110-120%
saturated nitrogen. This amount may be destructive for fish
life.
 Eutrophication
 It means increase in vegetation. If moss and other plants
exist in water, quality of that water gets worse.