Contents preface (VII) introduction 1—37

EXERCISES

1. 
   Describe the common forces acting on a gravity dam.
   

2. 
   What are the main causes of failure of a gravity dam ?
   

3. 
   Write notes on
   

(i) Galleries in gravity dams,

(ii) Foundation treatment for gravity dams, (iii) Structural joints in gravity dams, and (iv) Keys and water seals in gravity dams.

4. 
   Check the stability of the gravity dam shown in Fig. 16.11 and calculate the stresses at the toe and heel for both empty and full-reservoir conditions. For full-reservoir conditions, assume an earthquake acceleration equal to 0.1 g. Assume coefficient of shear friction = 0.70, specific grav-ity of concrete = 2.40, and shear strength at concrete-rock contact surface = 140 × 10⁴ N/m². Other data, if required, may be suitably assumed.
   

Max. res.

10 m

level

0.75

70

m

1

Drainage

TWL

50 m

gallery

10 m

15 m
Fig. 16.11 Sketch for Exercise 16.4

5. 
   Figure 16.12 shows cross-section of a gravity dam which has the following data:
   

Angle of internal friction of silt	= 30°
Submerged unit weight of silt	= 15 kN/m^3
Horizontal earthquake acceleration	= 0.15 g
Vertical earthquake acceleration	= 0.075 g
Shear strength between dam and foundation	= 1500 kN/m^2
Coefficient of friction	= 0.75

Determine, for extreme load combination (without ice pressure), the following: (a) various relevant stresses at the toe and heel, and

(b) factors of safety against overturning and sliding, and the shear friction factor.

GRAVITY DAMS 561

FRL 797.00

793.00

681.00
Silt

16 m
802.00

792.00

Drainage gallery
640.00

5
630.00

10 m
172.70 m

Fig. 16.12 Sketch for Exercise 16.5
REFERENCES
1. ...... Design of Gravity Dams, USBR Design Manual for Concrete Gravity Dams, 1976. 2. ...... IS:6512-1984, IS Code for Criteria for Design of Solid Gravity Dams.

3. 
   Golze, AR (Ed.), Handbook of Dam Engineering, Van Nostrand Reinhold Company, U.S.A., 1977.
   

3. 
   Sharma, HD, Concrete Dams, Metropolitan Book Co. Pvt. Ltd., New Delhi, 1981.
   

3. 
   Zanger, CN, Hydrodynamic Pressures on Dams due to Horizontal Earthquake Effects, ‘USBR’s Special Assignment Section Report No. 21, 1950.
   

6. ...... ACI Manual of Concrete Practice, Pt. 1-1983, American Concrete Institute, Michigan, U.S.A.

17
SPILLWAYS
17.1. GENERAL
The occurrence of a flood in an unobstructed natural stream is considered to be a natural event for which no individual or group is held responsible. However, if a flood occurs on account of the failure of an artificial obstruction (such as a dam) constructed across a natural stream, the agency responsible for the construction of the obstruction is held responsible. Embankment dams constructed of earth or rockfill material are very likely to be destroyed, if overtopped. Concrete dams may, however, tolerate moderate overtopping. The damage to life and property on account of the failure of a dam would be catastrophic. As such, there must always be a provision to release excess water safely when the reservoir has been filled to its capacity so that the dam itself is not overtopped. This is achieved by constructing a spillway. Spillways release safely the surplus water which cannot be contained in the reservoir created by the dam. The surplus water is usually drawn from the top of the reservoir and conveyed through an artificial waterway back to the river downstream of the dam or to some other natural drainage channel. Spillway can be constructed either as part of the main dam, such as in overflow section of a concrete dam or as a separate structure altogether. Besides being capable of releasing surplus water, a spillway must be able to meet hydraulic and structural requirements and must be located such that spillway discharges do not damage the toe of the dam. Insufficient spillway capacity and/or the failure of a spillway will cause widerspread damage and loss of life. As such, the design criteria for a spillway are usually conservative. The inflow design flood, used to determine the spillway capacity, is also estimated conservatively.

The frequency of flow over a spillway would mainly depend on the runoff characteristics of the drainage area, reservoir storage, and the available outlet and/or diversion capacity. For example, at a dam with storage and outlet capacities relatively small (compared to normal river flows), spillway will be used almost continuously. Under favourable site conditions, one should examine the possibility of providing an auxiliary spillway in conjunction with a smaller service spillway. In such a situation, the service spillway is designed to pass frequent floods of smaller magnitude. The auxiliary spillway operates only when flood of magnitude larger than the discharging capacity of the service spillway is passing. Sometimes, the capacity of outlet structures may be increased so that these may also serve as service spillways. The auxiliary spillway is used infrequently and, hence, is not designed for the same degree of safety as required for other spillways.

At some projects, emergency spillways are also provided for additional safety to meet emergencies not anticipated in the normal design. Such emergencies could be on account of malfunctioning of regular spillway gates, damage to the regular spillway, shutdown of outlet works, or the occurrence of two floods in quick succession.
A dam always has some storage capacity above its normal storage level. This storage capacity is termed surcharge storage. If a dam could be made so high as to provide ample

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SPILLWAYS

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surcharge storage to contain the entire volume of the incoming flood, theoretically no spillway, except an emergency type, would be needed provided that the outlet capacity of the dam can evacuate the surcharge storage well before the arrival of the next flood. Such an ideal situation permitting retention of entire incoming flood by surcharge storage, however, would never exist. In the absence of surcharge storage, spillway must be sufficiently large to pass the peak flood discharge. In such a situation, the peak rate of inflow is more important than the total volume of the incoming flood. On the other hand, if relatively large surcharge storage can be made economically at a dam, a portion of the incoming flood is retained temporarily in the surcharge storage of the reservoir and the discharging capacity of spillway can be reduced significantly. Economic considerations will usually require that a reservoir be designed to have reasonable surcharge storage as well.

Using the overflow characteristics of an assumed spillway type and known inflow design flood, the maximum spillway discharge and the maximum reservoir water level can be determined by flood routing. For known maximum spillway discharge, the components of the trial spillway can be designed and a complete layout of the spillway prepared. Cost estimates of the trial spillway and dam can now be made.
All relevant factors of topography, hydrology, geology, hydraulics, design requirements, costs, and benefits must be considered for determining the best combination of storage and spillway capacity for the chosen inflow design flood. Some of these important factors are as follows (1):
(i) The characteristics of the inflow flood hydrograph,
(ii) The damages which would result if the inflow flood occurred (a) without the dam, (b) with the dam in place, and (c) after the failure of the dam or spillway.
(iii) The effects of various dam and spillway combinations on the upstream and down-stream of the dam on account of the resulting backwater and tail-water effects.
(iv) Relative costs of various combinations of storage and spillway capacity including the outlet facilities which can be utilised for the duration of the flood.
The cost estimates of various combinations of spillway capacity and dam height for the trial spillways will form the basis for the selection of an economical spillway and the optimum combination of surcharge storage (or the height of the dam) and the spillway capacity.