MINISTRY OF SCIENCE AND TECHNOLOGY

DEPARTMENT OF TECHNICAL AND VOCATIONAL EDUCATION

CE-05016 DESIGN OF HYDRAULIC STRUCTURE

B.E.

Civil Engineering

CONTENT

CHAPTER 1 SPILLWAYS

CHAPTER 2 ENERGY DISSIPATOR

CHAPTER 3 RESERVOIR

CHAPTER 4 GRAVITY DAMS

CHAPTER 5 EARTH DAMS

CE 05016 DESIGN OF HYDRAULIC STRUCTURES

REFERENCES:

1. Irrigation Water Power and Water Resources Engineering (in SI Units)

by Dr. K.R. Arora

2. Theory and Design of Irrigation Structures (Volume II)

by R.S. Varshney, S.C. Gupta and R. Gupta

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DESIGN OF HYDRAULIC STRUCTURES I

CHAPTER I

SPILLWAYS

1.1 Introduction

A spillway is a structure constructed at or near the dam site to dispose of surplus water from the reservoir to the channel downstream. Spillways are provided for all darns as a safety measure against Overtopping and the consequent damages and failure. A spillway acts as a safety valve for the dam. because as soon as the water level in the reservoir rises above a predetermined level, excess water is discharged safely to the downstream channel, and the dam is not damaged.

The spillway must have adequate discharge capacity to pass the maximum flood d/s without causing any damage to the dam or its appurtenant structures. At the same time, the reservoir level should not rise above the maximum water level (M.W.L.). The maximum water level is estimated from the inflow flood hydrograph, storage capacity of the reservoir and the spillway capacity by flood routing. A spillway of inadequate capacity may lead to the overtopping of the dam, which may cause serious damages and even the failure of the dam. On the other hand, a spillway of much larger capacity than that required would be an uneconomical design

In addition to providing adequate discharge capacity, the spillway must be hydrodynamically and structurally safe. The spillway surface should be erosion-resistant to withstand the high velocities created by the fall of water from the reservoir surface to the tail water. Moreover, the spillway should be located so that the spillway discharge will not undermine the downstream toe of the dam. Generally, some energy-dissipating device, such as hydraulic jump or a bucket, is provided at the toe for the dissipation of excess energy.

A spillway may be located either in the middle of the dam [Fig 1.1(a)] or at the end of the dam near abutment. In some cases, the spillway is located away from the dam as an independent structure if there is a suitable saddle [Fig. 1.1 (b)]. (A saddle is a depression of the shape of saddle used for riding a horse). Such a spillway is called a saddle spillway. Generally, a saddle spillway is designed as an auxiliary or an emergency spillway. which is in addition to the main spillway at the dam site.

The design of a spillway requires utmost attention. Many failures of dams occurred in the past because of improperly designed spillways or by spillways of inadequate capacity. For earth and rock fill dams, a liberal spillway capacity should be provided because they fail as soon as they are overtopped. However, concrete dams can withstand some moderate overtopping and may have less liberal spillway capacity.

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Fig. 1.1

1.2 Essential Requirements of a Spillway

The essential requirements of a spillway, as discussed above, may he summarized as follows:

1. It must have adequate discharge capacity

2. It must he hydraulically and structurally sat&

3. The surface of the spillway must be erosion resistant.

4. The spillway must be so located that the spillway discharge does not erode or undermine the downstream toe of the dam.

5. It should be provided with some device for the dissipation of excess energy.

6. The spillway discharge should not exceed the safe discharge capacity of the downstream channel to avoid its flooding.

1.3 Required Spillway Capacity

The required spillway capacity is usually determined by flood routing. The spillway capacity should be equal to the maximum outflow rate determined by flood routing. The following data are required for the flood routing:

(i) Inflow flood hydrograph, indicating the rate of inflow with respect to time. It is the same as the design flood hydrograph of the spillway.

(ii) Reservoir-capacity curve, indicating the reservoir storage at different reservoir elevations.

(iii) Outflow discharge curve, indicating the rate of outflow through spillways at different reservoir elevations.

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By flood routing, the maximum outflow rate and the maximum rise in water surface can be determined. Factors affecting the required spillway capacity. The following factors affect the spillway capacity.

1. Inflow flood hydrograph

2. Available storage capacity.

3. Capacity of outlets -

4. Gates of spillway

5. Possible damage, if the capacity is exceeded.

1. Inflow flood hydrograph The inflow flood hydrograph should be selected according to the degree of protection that ought to be provided to the dam. It will depend upon the type and height of the dam. its location with respect to inhabited and developed area, and consequences of its failure.

Obviously, a high dam storing a large volume of water and located upstream of a town should have a much degree of protection as compared to that in the case of a small dam storing a small volume of water and on whose downstream the area is uninhabited. In the former case, the inflow flood is usually taken as the maximum probable flood (MPF); whereas in the latter case, a smaller flood such as standard project flood (SPF) may be taken.

2. Available storage capacity If the available: storage capacity of the reservoir is quite large as compared to the inflow; a spillway of smaller capacity will normally be required.

3. Capacity of outlets If the dam outlets can be used to discharge a portion of the flood, the spillway capacity can be correspondingly reduced.

4. Gates in Spillway If the spillway is gated; its discharge capacity can be modified. For a gate controlled spillway. the water can be stored upto the top of the gates, whereas in the case of an ungated spillway, the water can be stored only upto the crest level. By operation of gates, higher heads may be created above the crest so that greater outflow rate through the spillway is achieved

5. Possible damage If there is a possibility of extensive damage on the downstream, large spillway capacity should he provided.

Best combination of the Storage capacity, and the spillway capacity

For determining the best combination of the storage capacity and the spillway capacity to accommodate the selected inflow design flood, it is necessary to consider all pertinent factors of hydrology, hydraulics of spillway, design cost and possible damages.

After a spillway of a particular type and dimensions has been selected, the maximum water level and the maximum spillway discharge can be determined by flood routing. Various combinations of the spillway capacity and the dam height (or storage capacity), for the

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assumed spillway types are selected and flood routing is done. The process is repeated with alternative types of spillways. Cost estimates are made for different combinations. The combination which gives the most economical spillway type and the optimum relation of the spillway capacity to the height of the dam, is selected. However, the process is quite tedious. It would require a number of flood routings, spillway layouts, spillway estimates and dam estimates. Even then, the study is never complete because many other spillway arrangements could have been considered. An experienced designer, however, will be able to select only those combinations for study which show definite advantages either in cost or in adaptability.

1.4 Component Parts of a Spillway

A spillway generally has the following component parts

1. Entrance channel

2. Control structure 3. Discharge channel (or waterway)

4. Terminal structure (energy dissipator)

5. Exit channel

However, entrance and exit channels may not be required for some spillways.

1. Entrance channel Entrance channels are required in those types of spillways in which the control structure is away from the reservoir. The entrance channel draws water from the reservoir and carries it to the control structure. Entrance channels are not required for spillways which draw water directly from the reservoir.

2. Control structure The control structure (also called control) is the most important component of the spillway. It controls the outflow from the reservoir. The control structure is designed such that it does not permit the outflow from the reservoir when the water level is lower than a predetermined level but permits the outflow as soon as the water level rises above that level,

Generally, the control structure is located at the upstream end of the spillway structure. However in some cases, the control structure may be at the downstream end of the spillway structure. For example, in a shaft (or morning glory) spillway, the downstream tunnel controls the outflow at higher heads.

The control structure usually consists of either an orifice or a weir. In most of the spillways, the control structure is an overflow crest of a weir. The weir may be sharp-crested, board-crested or ogee-shaped. In plan, the overflow crest may be straight, curved, U-shaped, semi-circular or circular. The straight, ogee-shaped crests are mostly commonly used in spillways.

Likewise, orifices used as control may have different shapes. They may be placed in a horizontal, vertical or an inclined position and may be sharp-edged, round-edged or bell-mounted.

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In order to regulate the flow of water from the reservoir, gates are usually provided on the crest of the control structure.

3. Discharge channel (or waterway) The outflow released through the control structure is usually conveyed to the terminal structure through a discharge channel or waterway. Thus the discharge channel conveys the water safely from the control structure to the river downstream. It is also called a conveyance structure. The conveyance structure may have different forms. It is usually the downstream face of an overflow darn for the spillway constructed as an overflow spillway in the body of the dam. It may be in the form of an open channel, a closed conduit placed through or under a dam, or a tunnel excavated through an abutment, depending upon the type of spillway. The discharge channel may have a variety of cross-sections, depending upon the geologic and topographic characteristics of the site and the hydraulic requirements.

4. Terminal structure (energy dissipator) When the water flows from the reservoir over the spillway, the static energy is converted into the kinetic energy. This results in very high velocity of flow at the downstream end of the spillway. It may cause serious scour at the downstream end. It may also damage the dam, the spillway and other appurtenant structures. It is, therefore, necessary that the high energy of flow must be dissipated before the flow is returned to the river downstream. Terminal structures (or energy dissipators) are provided at the downstream end of the discharge channel to dissipate the excess energy. Generally, a hydraulic jump basin, a roller bucket, a ski-jump bucket, or some other suitable energy dissipating device is provided for the dissipation of excess energy. However, if the stream bed consists of an erosion-resistant strong rock, the overflowing water from the spillway may be delivered directly to the river downstream without a terminal structure.

5. Exit channel In some types of spillways, the exit channels are provided to convey the spillway discharge from the terminal structure to the river downstream. However, an exit channel is not required for the spillways which discharge water directly into the river downstream. On the other hand, in the case of spillways placed through abutments, saddles or ridges, the exit channel is usually required.

1.5 Classification of Spillways

The spillways can be classified into different types based on the various criteria, as explained below.

A. Classification based on purpose 1. Main (or service) spillway

2. Auxiliary spillway

3. Emergency spillway

B. Classification based on control 1. Controlled (or gated) spillway

2. Uncontrolled (or ungated) spillway

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C. Classification based on prominent feature

1. Free overfall (or straight drop) spillway

2. Overflow or Ogee spillway

3. Chute (or open channel or trough) spillway

4. Side-channel spillway

5. Shaft (or morning glory) spillway

6. Siphon spillway

7. Conduit (or tunnel) spillway

8. Cascade spillway

1.5.1 Classification based on purpose

1. Main (or service) spillway A main (or service) spillway is the spillway designed to pass a prefixed or the design flood. This spillway is necessary for all dams and in most of the dams, it is the only spillway. Therefore, in general terms, the spillway means the main spillway.

2. Auxiliary spillway In some dams, where the site conditions are favourable, an auxiliary spillway is usually constructed in conjunction with a main spillway. In such a case, the main spillway is usually designed to pass floods which are likely to occur more frequently. When the floods exceed the designed capacity of the main spillway, the auxiliary spillway comes into operation and the total flood is passed by both the spillways. In that case, the capacity of the main spillway is kept less than that required for the design flood.

An auxiliary spillway cannot be provided alone without the main spillway. The crest of the auxiliary spillway is kept higher than that of the main spillway. The auxiliary spillway therefore, comes into operation only after the flood for which the main spillway is designed. is exceeded. As already mentioned, the capacity of the main spil1way when an auxiliary spillway is also provided, is kept less than that required for the design flood Thus the total spillway capacity is equal to the sum of the capacities of the main and auxiliary spillways, Therefore,

am QQQ += (1.1)

where Q is the design flood, Qm is the capacity of the main spillway and Qa is the capacity of the auxiliary spillway.

If no auxiliary spillway is provided,

Q = Qm (1.2)

The site conditions favourable for the adoption of an auxiliary spillway are as follows:

(i) When there is a saddle or depression along the rim of the reservoir which leads to a natural drainage

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(ii) When there is a gently-sloping abutment where an excavated channel can be carried sufficiently beyond the dam so that there is no possibility of the erosion of the dam or its appurtenant works.

An auxiliary spillway is designed like a main spillway, but control gates are seldom provided on the crest of an auxiliary spillway. Sometimes, only a fuse plug, which is a simple earth embankment, also called fuse plug dike, is provided The fuse plug allows the water surface to rise above the crest of the auxiliary spillway, but as soon as it is overtopped, it gives way and the flood water passes over it. Instead of a fuse plug, a flashboard or any other such device is also sometimes used.

3. Emergency spillway An emergency spillway is sometimes provided in addition to the main spillway. It comes into operation only during an emergency which may arise at any time during the life of the dam. Thus an emergency spillway is an additional safety valve of the dam.

The emergency may arise when such conditions occur that have not been anticipated and considered in the design of the main spillway. Some of the conditions which may lead to emergency are as follows:

(i) When the actual flood exceeds the design flood.

(ii) When there is an enforced shutdown of the outlets.

(iii) When there is a malfunctioning of spillway gates.

(iv) When there is damage or failure of some part of the main spillway.

(v) When a high flood occurs before the previous flood has been evacuated by the main spillway.

An emergency spillway is usually provided in a saddle or a depression along the reservoir rim or by excavating a channel through an abutment or a ridge. Because an emergency spillway is not required to function under normal reservoir operations, its crest is placed at or slightly above the design maximum water level in the reservoir. Thus an encroachment on the minimum free board is usually permitted for the design of an emergency spillway. The emergency spillway is generally in the form of a fuse plug or a breaching section which is washed out as soon as the water level in the reservoir reaches a predetermined elevation. The breaching section is sometimes called fuse plug spillway.

Because a fuse plug is also sometimes provided as an auxiliary spillway, the following differences between the auxiliary and emergency spillways should be noted.

(i) An auxiliary spillway is designed to discharge a portion of design flood. An auxiliary spillway operates when the flood is less than the design flood but it is more than the capacity of the main spillway; whereas an emergency spillway operates only when the design flood is exceeded.

(ii) An auxiliary spillway may be of any type, but the emergency spillway is usually a fuse plug.

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(iii) An auxiliary spillway may also be designed to work as an emergency spillway when the design flood is exceeded. It works as an auxiliary spillway when the flood exceeds the capacity of the main spillway but it is less than the design flood. ‘As soon as the design flood is exceeded, it works as an emergency spillway.

In actual practice, the main spillway is always provided. In addition, either an auxiliary spillway or an emergency spillway may also be provided, depending upon the purpose served, It is always safe to provide an emergency spillway.

1.5.2 Classification based on Control

1. Controlled (or gated) spillway A controlled spillway is one which is provided with the gates over the crest to control the outflow from the reservoir. In the controlled spillway, the full reservoir level (F.R.L.) of the reservoir is usually kept at the top level of the gates. Thus the water can be stored up to the top level of the gates. The outflow from the reservoir can be varied by lifting the gates to different elevations. It may be noted that in a controlled spillway the water can be released from the reservoir even when the water level is below the full reservoir level.

2. Uncontrolled (or ungated) spillway In an uncontrolled spillway, the gates are not provided over the crest to control the outflow from the reservoir. The full reservoir Level (F.R.L.) is at the crest level of the spillway. The water escapes automatically when the water level rises above the crest level. Thus the main advantage of an uncontrolled spillway is that it does not require the gates and the operator and lifting power to operate the gates. Besides there is no problem related to the maintenance and repair of gates.

However, to pass a certain design discharge, a much longer spillway crest is required for an uncontrolled spillway as compared to that in a controlled spillway because the head over the crest is smaller in the former. Moreover, the useful storage in the case of uncontrolled spillway is less. Further, the discharge in the river downstream cannot be controlled to prevent flooding. Therefore, the spillways for most of the dams are controlled spillways.

1.5.3 Classification based on the pertinent feature

There are 8 different types of spillways based on the pertinent feature, as already mentioned above.

1.5.3.1 Free Overfall Spillway

A free overfall spillway (or a straight drop spillway) is a type of spillway in which the control structure consists of a low-height, narrow-crested weir and the downstream face is vertical or nearly vertical so that the water falls freely more or less vertical [Fig. 1.2 (a)]. The overflowing water may discharge as a free nappe, as in the case of a sharp-crested weir, or it may be supported along the narrow section of the crest. However, in both cases, the water flowing over the crest drops as a free jet clear of the downstream face of the spillway.

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Sometimes, the crest of the spillway is extended in the form of an overhanging lip for directing the discharge away from the downstream face [Fig 1.2(b)]. In all cases, the nappe is properly ventilated to prevent pulsating and fluctuating jets.

If the stream bed does not consist of strong sound rock, the falling jet will scour the stream bed and form a deep plunge pool. It may cause damage to the structure. In order to protect the stream bed from scouring, an artificial pool is usually constructed by excavating a basin in the bed and then covering it with a concrete apron. Alternatively, an auxiliary low dam is constructed downstream of the spillway to form a pool above the river bed [Fig.1.2 (c)].

If the tail water depth is adequate, a hydraulic jump may form after the jet falls from the crest, which can be used for the dissipation of energy. However, a long flat apron would be required to contain the hydraulic jump. Moreover, the floor blocks and an end sill may also be required for the establishment of the jump.

A free overfall spillway is commonly used for a low arch dam whose downstream face is almost vertical. This type of spillway is also used as a separate structure for low earth dams. The design of a free overfall spillway is similar to that of a vertical drop weir

A free overfall spillway is not suitable when the foundation is weak and yielding, because the apron at the stream bed is subjected to large impact forces at the point of impingement of the falling jet. The impact forces also cause vibrations, which may cause cracking or displacement of the apron and even its failure by piping or undermining. The free overfall spillways are not suitable when the drops are high. These are usually limited to a maximum drop of 6 m, measured from the head pool (reservoir) to the tail water.

Fig. 1.2

1.5.3.2 Ogee - Shaped (or Overflow) Spillway

An ogee-shaped (or overflow) spillway is the most commonly used spillway. It is widely used with gravity dams, arch dams and buttress dams. Several earth and rockfill dams are also provided with this type of spillway as a separate structure. An ogee-shaped spillway is an improvement upon the free overfall spillway, discussed in the preceding section. The

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essential difference between the free overfall spillway and the ogee-shaped spillway is that in the case of a free overfall spillway, the water flowing over the crest of the spillway drops vertically as a free jet clear from the downstream face whereas in the case of an ogee-shaped spillway, the water flowing over the crest is guided smoothly over the crest and is made to glide over the downstream face of the spillway.

An ogee-shaped spillway has a control weir of ogee-shaped, which is like the elongated English letter S [Fig. 1.3(b)]. The shape of the crest of the ogee spillway is generally made to conform closely to the profile of the lower surface of nappe (sheet of water) of a ventilated jet issuing from a sharp-crested weir when the head over the highest point of nappe is equal to the design head (Fig. l.3(a)]. The upper surface of the spillway is properly shaped to form the crest. The nappe-shaped profile is an ideal profile because at the design head, the water flowing over the crest of the spillway always remains in contact with the surface of the spillway as it glides over it Moreover, for this shape. no negative pressure will develop on the spillway surface at the design head. However, when the head is greater than the design head, the overflowing water tends to break contact with the spillway surface and a zone of separation is formed, in which a negative or suction pressure occurs.

This may cause vibration, pitting of the spillway surface and a number of other problems. However, the coefficient of discharge of the spillway is increased. On the other hand, if the head is less than the design head. the water overflowing over the crest of the spillway remains in contact with the surface of the spillway and a positive hydrostatic pressure is exerted by the flowing water because the nappe tends to be depressed. As the spillway surface supports the sheet of flowing water which creates a backwater effect, the coefficient of discharge is reduced.

Fig. 1.3

Thus ideal conditions for an ogee-shaped spillway occur when the head is equal to the design head for which the spillway has been shaped. At the design head, it attains nearly the maximum efficiency without any determental effect.

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Shape of the crest of the overflow spillway The shape of the ogee-shaped spillway depends upon a number of factors such as (1) head over the crest, (2) height of the spillway above the stream bed or the bed of the entrance channel and (3) the inclination of the upstream face of the spillway. The U.S.B.R. conducted extensive experiments to obtain the profile of the overflow spillways with the upstream face either vertical or inclined at various angles. The U.S. Army Crops of Engineers developed several standard shapes of the crests of overflow spillways on the basis of U.S.B.R. data. Because the shapes were developed at U.S. Waterways Experiment Station at Vicksberg (U.S.W E.S.), the shapes are known as the W.E.S, standard spillway shapes.

1. Downstream profile The d/s profile of the spillway can be represented by the following general equation:

yKHx nd

n 1−= (1.3)

where x and y are the coordinates of the point on the spillway surface, with the origin at the highest point 0 of the crest. Hd is the design head, excluding the head due to the velocity of approach, and K and n are constants, which depend upon the inclination of the upstream face of the spillway. Fig.1.4 shows the profile when the upstream face is vertical.

Fig. 1.4

The values of K and n for the vertical upstream face and three different inclinations are given in Table 1.1.

Table 1.1

S. No. Slope of U/s face K n

1. Vertical 2.000 1.850

2. 1:3 (H:V) 1.936 1.836

3. 2:3 (H:V) 1.939 1.810

4. 3:3 (H:V) 1.873 1.776

[Note The slopes are also designated on 3 on 1,3 on 2 and 3 on 3 instead of 1:3, 2:3, 3:3 respectively.]

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Fig. 1.5

For intermediate slopes, the values of K and n may be obtained from the plot given in Fig. 1.5 for different values of the inclination β, where β is the angle which the upstream face makes with the vertical.

It may be noted that x is taken as positive towards the downstream and y is taken as positive in the downwards direction. Eq.1.3 is applicable only for the positive values of x and y, and can be used to obtain the crest shape downstream from the origin of the coordinates. The curved profile of the crest section is continued till it meets tangentially the straight sloping surface of the downstream face of the overflow dam (Fig. 1.6). The location of this point of tangency depends upon the slope of the downstream face of the overflow dam, which is determined from the stability requirements of the overflow section.

The slope of the d/s face of the overflow dam usually varies in the range of 07:l to 0.8:1 At the end of the sloping surface of the spillway, a curved circular surface, called bucket, is provided to create a smooth transition of flow from the spillway surface to the river downstream of the outlet channel. The bucket is also useful for the dissipation of energy and prevention of scour, as discussed later.

Fig. 1.6

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The radius R of the bucket can be approximately obtained from the relation,

aR )10(= (1.4)

where )52.196.3/()88.44.6( +++= dd HHVa (1.4.a)

in which V is the velocity of flow at the toe of spillway (m/s), and Hd is the design head (m). The velocity of flow V may be approximately determined from the relation.

)(2 yHZgV a −+= (1.5)

where Z is the total fall from the upstream water level to the floor level at the d/s toe, Ha is the head due to velocity of approach, y is the depth of flow at toe and g is the acceleration due to gravity. Eq.1.5 neglects the losses over the spillway.

Generally, a radius of about one-fourth of the spillway height is found to be satisfactory. Thus

4/PR = (1.6)

where P is the height of spillway crest above the bed.

2. Upstream profile of the crest

(a) Vertical upstream face The upstream profile of the crest should be tangential to the vertical face and should have zero slope at the crest axis to ensure that there is no discontinuity along the surface of flow. The upstream profile should conform to the following equation. with usual notations.

625.0375.085.0

85.1

)270.0()(4315.0126.0)(

)270.0(724.0ddd

d

d HxHHH

Hxy +−+

+= (1.7)

The details of the upstream profile are shown in Fig.1.7. It may be noted that the values of x are negative according to the chosen axes of coordinates. The maximum absolute value of x is 0.270 Hd, for which the value of y is equal to 0126 Hd when the u/s face is vertical.

Fig. 1.7

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The values of (y/Hd) for different values of (x/ Hd ) can be obtained from Table 1.2

(b) Sloping upstream face The coordinates of the upstream profile in the case of sloping upstream face can be determined from Table l.2 for slopes of 1:3, 2:3 and 3:3. For intermediate slopes. the values may be interpolated.

Table 1.2 Values of y/Hd for the u/s profile

x/Hd Slope 1:3 Slope 2:3 Slope 3:3 Vertical

0.000

-0.020

-0.040

-0.060

-0.080

-0.100

-0.120

-0.160

-0.170

-0.180

-0.190

-0.200

-0.210

-0.220

-0.230

-0.240

-0.250

-0.260

-0.270

0.0000

0.0004

0.0016

0.0037

0.0067

0.0106

0.0156

0.0291

0.0330

0.0376

0.0425

0.0480

0.0550

0.0650

0.0800

-

-

-

-

0.0000

0.0004

0.0016

0.0036

0.0066

0.0104

0.0153

0.0283

0.0365

-

0.0412

0.0554

-

-

-

-

-

-

-

0.0000

0.0004

0.0016

0.0036

0.0065

0.0103

0.0150

0.0275

0.0313

0.0354

0.0399

0.0450

-

-

-

-

-

-

-

0.0000

0.0004

0.0016

0.0038

0.0068

0.0108

0.0158

0.0296

0.0339

0.0386

0.0437

0.0494

0.0556

0.0624

0.0701

0.0787

0.0889

0.1016

0.1260

3. Offsets and risers on upstream face If structural requirements permit, offset and risers can be provided on the upstream face by removing some portion of concrete, and thus economy can be effected The maximum permitted projection from the crest line is 0.315 Hd

and the vertical depth of the maximum bulging is 0.25 Hd [Fig. 1.8 (a)].

Fig. 1.8

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In the case of a vertical-faced overhang, the vertical depth M of the projection (called riser) should be equal to 0.5 Hd [Fig. 1.7(b)]. The ratio M/N should not be less than 0.50. However, it can have a zero value. For M/N ratio between 0.0 and 0.50, the flow conditions are extremely unstable. Moreover, the ratio of the vertical depth M to design head Hd should not be in the range 0.0 to 0.50 to avoid extremely unstable conditions.

4. Pressure over spillway surface The profile shapes discussed above are for one value of the design head (Hd). The design head is generally chosen to give the maximum practical hydraulic efficiency, in keeping with the operational requirements, stability and economy.

If the actual head is less than the design head, the pressure on the crest will be positive (i.e. above atmospheric). However, for heads greater than the design head, the pressure on the crest will be negative (i.e. less than the atmospheric pressure) and it may lead to cavitation. Model tests have shown that the design head may, however , be exceeded by 25 percent, without any harmful cavitation (IS: 6934-1973).

5. Orifice Flow In a gated spillway, orifice flow occurs at part gate openings. Sub-atmospheric pressures develop on the crest immediately below the gate if the crest profile is steeper than the one conforming to the trajectory of the orifice flow. When the crest is shaped to the nappe profile of the weir flow, the sub-atmospheric pressure at part gate openings can be reduced if the gate sill is placed slightly downstream of the crest. In that case, the trajectory becomes steeper and conforms more nearly to the free-overflow lower nappe profile of the weir flow. However, model tests have shown that even when the gate is located at the crest axis, the negative pressure is less than 0.15 Hd when the actual head exceeds the design head by 33.3 percent. A small negative pressure is sometimes permitted considering the rareness of its occurrence and very short duration during which it occurs. U.S.B.R. permits a negative pressure of 4.3m of water (about 42.0 kN / m2). As far as possible, the negative pressure should be avoided, because it has the following ill effects.

(i) It increases the overturning moment on the crest.

(ii) It increases the force required for lifting the gates.

(iii) It causes a decreases in capability for automatic control.

(iv) It causes vibrations which eventually extend. all over the structure. The vibrations also cause cracks in the mortar of stone lining of the masonry crest, for which special anchoring bolts have to be provided.

6. Corbel When the profile of the crest of an ogee spillway overflow section is plotted along with the profile of the non-overflow section of the gravity dam, with their upstream faces coinciding, it is found that it extends beyond the downstream face of the non-overflow section [Fig. 1.9 (a)]. In other words, the spil1way section is thicker than the non-overflow section of the gravity dam.

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The concrete required for the overflow section can be saved to some extent by shifting the spillway profile in the upstream direction until the downstream curve becomes tangential to the downstream face of the non-overflow and removing the concrete in the portion shown hatched in Fig.1.9(b). The projection so formed is called corbel. Thus a saving can be effected by providing a corbel on the upstream face of the spillway section. The construction of the spillway is carried out as in the case of a non-overflow section upto the height of corbel. However, after reaching that height, a smooth curve forming the corbel is provided. It may be noted that a corbel cannot be provided in a dam in which the gates are installed on the upstream face to control the flow to the outlets, because that will interfere with their operation.

Fig. 1.9

1.5.3.3 Discharge Computation for an Ogee Spillway

The discharge over an ogee spillway is computed from the basic equation of flow over weirs, given below:

2/3eed HLCQ = (1.8)

where Q is discharge (cumecs), Cd is the coefficient of discharge, Le is the effective length and He is the actual effective head including the head due to the velocity of approach. i.e.

ade HHH +=

(Note Sometimes, the coefficient Cd is also, written as C]

1.Coefficient of discharge (Cd) An ogee spillway has a relatively high value of the coefficient of discharge (Cd) because of its shape. The maximum value of Cd is about 2.20, if no negative pressure occurs on the crest. However, the value of Cd is not constant. It depends upon the shape of the ogee profile, and also upon the following factors.

(i) Height of spillway crest above the stream bed

(ii) Ratio of actual total head to the design total head.

17

(iii) Slope of the upstream face of spillway

(iv) Extent of the downstream submergence of crest

(v) Downstream apron

All these factors .are briefly discussed below:

(i) Height of spillway above stream bed The height P of spillway above the stream bed affects the discharge coefficient because the velocity of approach depends upon this height. With an increase in the height P, the velocity of approach deceases but the coefficient of discharge Cd increases. Fig. 1.10 shows the variation of Cd with the ratio (P/HD) where HD is the design total head, including the head due to the velocity of approach. Thus

adD HHH +=

where Ha is the head due to the velocity of approach.

It may be noted that there is a marked increase in the value of Cd when the height of spillway increases upto about twice of the design total head HD. With further increase in the height of spillway, there is not much increase in the value of Cd, and it remains almost constant as 2.20.

Model tests of spillways have also shown that the effect of velocity of approach on the coefficient of discharge is negligible when the height (P) is equal to or greater than 1.33 Hd, where Hd is the design head excluding the head due the velocity of approach. Such spillways are known as high overflow spillways, In high overflow spillways, the velocity of approach is sometimes neglected. Thus

HD ≈ Hd

Fig. 1.10

(ii) Ratio of actual total head (He) to the design total head (HD) Fig 1.11 shows the variation of actual discharge coefficient Cd’. It is plotted between (Cd’/ Cd), as ordinate, and (He /HD), as abscissa. The plot is applicable to high overflow spillways; with P≥ 1.33 Hd. Similar curves are available for low overflow spillways.

18

Fig. 1.11

It may be observed that with an increase in the value of (He /Hd), the value of (Cd’/ Cd) increases. The ratio (Cd’/ Cd) is less than unity for (He /HD) less than unity but greater than unity for higher values of (He /HD). Thus if the spillway is designed for a lower design head, a high value of coefficient of discharge Cd will be obtained for most of the range of heads actually occurring in operation. However, the design head should not be less than about 80 percent of the maximum head (i.e. the maximum head should not be more than 1 25 times the design head) to avoid the possibility of cavitation.

(iii) Slope of the upstream face of spillway Fig. 1.12 shows the variation of (Cd’/ Cd) with the ratio (P/HD) for three different slopes of the upstream face. For small ratios of (P/HD). the actual coefficient Cd’ is slightly more than the coefficient Cd for the vertical face. However, as the ratio (P/HD) increases, the ratio (Cd’ / Cd) decreases.

Fig. 1.12

19

(iv) Extent of downstream submergence The actual coefficient of discharge Cd’ is decreased due to downstream submergence, Fig. 1.13 shows the variation of Cd’/ Cd, with the degree of submergence h/Hd, where h is the depth of water over the crest on the downstream and Hd that on the upstream. It may be noted that the effect of submergence is negligible for smaller degree of submergence. It is about 5% for the degree of submergence of 60%.

Fig. 1.13

(v) Downstream apron Fig. 1.14 shows the effect of downstream apron on the coefficient of discharge. When the value of (hd + d)/ HD exceeds about 1.70, the d/s floor apron has little effect on the coefficient of discharge, but for lower value, the coefficient of discharge Cd is lower. In this expression d is the tail water depth, and hd is the depth of d/s water level below u/s TEL. Thus

hd + d = P + HD

Fig. 1.14

20

2. Effective length of crest The effective length of crest of an overflow spillway is given by

eape HKNKLL )(2' +−= (1.9)

where Le is the effective length of crest; L’ is the net (clear) length of crest, which is equal to the sum of the clear spans of the gate bays between piers; He is the actual total head of flow on crest, including the head due to velocity of approach; N is the number of piers, K is the pier contraction coefficient and Kp is the abutment contraction coefficient.

(a) Pier contraction Coefficient The value of the pier contraction coefficient Kp depends upon several factors, such as (i) shape and location of the pier nose, (ii) thickness of pier, (iii) the velocity of approach, (iv) the ratio of actual total head on crest He to the design head HD.

For the flow at the design head, the average values of Kp are given in Table 1.3

TABLE 1.3 S.No Pier shape Coeffience (Kp)

1.

2.

3.

Square-nosed piers, with corners rounded on a radius equal to about 0.10 of pier thickness Rounded-nose piers Pointed -nose pier

0.02 0.01 0.00

(b) Abutment contraction coefficient The value of the abutment contraction coefficient Ka depends upon a number of factors, such as (i) shape of abutment, (ii) the angle between the upstream approach wall and the axis of flow, (iii)approach velocity and (iv) ratio of the actual head to design head. For the flow at design head, the average values of the coefficient Ka are given in table 1.4. Higher value of Ka should be used for spillways involving extreme angularity of approach flow.

S.No Abutment shape Coefficient (Ka) 1.

2.

3.

Square abutment, with head wall at 90º to the direction of flow Rounded abutment, with head wall at 90ºto the direction of flow, when 0.5 HD = radius = 0.15 HD

Rounded abutment where radius = 0.5 HD and head wall is placed at not more than 45ºto the direction of flow.

0.20 0.10 0.00

21

Discharge formula at partial gate opening Eq 1.8 is the discharge formula for an ungated overflow spillway or for a gated overflow spillway at full gate opening. The discharge for a gated spillway at partial gate opening is given by the low-head orifice formula (or large orifice formula),

( )Q C gL H HD e

= −23

21

3/2

2

3/2 (1.10)

Fig. 1.15

Where H1 and H2 are the total heads, including the head, due to velocity of approach,

above the bottom and top of the opening, respectively fig. (1.15), CD is the coefficient of discharge of the orifice, and Le is the effective length of crest.

The coefficient of discharge CD will have different values for different gate and crest arrangements. It is also influenced by the approach conditions and the downstream conditions. Fig 1.15 gives the value of CD for different values of (d/H1), where d is the height of opening.

22

Fig 1.16 ACTUAL VELOCITY OF FLOW IN M / SEC VELOCITY AND AIR ENTRAINED WATER DEPTH ON SPILLWAY FACE FOR DIFFERENT DISCHARGES

Fig 1.17 ACTUAL VELOCITY AND SOLID WATER DEPTH ON SPILLWAY FACE FOR DIFFERENT DISCHARGES

23

Design of Side Walls

The profile of flow on spillway surface determines the height of sidewalls required to retain the flow on the spillway. The profile determined by rigid calculation is not the true profile of flow, since air entrainment occurs in the flow giving the phenomenon of white water. To the solid stream profile could thus be added the effect of air entrainment which would increase the water depth. The pressure on the training wall is taken as the component of weight of water normal to the surface of flow. The flow profiles with and without air entrainment are given in fig 1.16 and 1.17, which can be used in the design of height of side training walls.

The U.S Army W.E.S coordinates of upper nappe are given in the table 1.5 for different values of H / Hd (0.5, 1.0 & 1.33) of upper water surface profile Fig 1.18

Fig. 1.18

24

Table 1.5

Co-ordinate of Water Surface Profile Without piers Centre line of span

H/Hd 0.50 1.00 1.33 H./Hd 0.50 1.00 1.33 x/Hd y/Hd x/Hd y/Hd -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

-0.490 -0.480 -0.475 -0.460 -0.425 -0.371 -0.300 -0.200 -0.075 0.075 0.258 0.470 0.705 0.972 1.269

-0.933 -0.915 -0.893 -0.865 -0.821 -0.755 -0.681 -0.586 0.465 -0.320 -0.145 -0.055 0.294 0.563 0.857

-1.210 -1.185 -1.151 -1.110 -1.060 -1.000 -0.912 -0.821 -0.700 -0.569 -0.411 -0.220 0.002 0.243 0.531

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

-0.482 -0.480 -0.472 -0.457 -0.431 -0.384 -0.313 -0.220 -0.088 0.075 0.257 0.462 0.705 0.977 1.278

-0.941 -0.932 -0.913 -0.890 -0.855 -0.805 -0.735 -0.647 -0.539 -0.389 -0.202 0.015 0.266 0.521 0.860

-1.230 -1.215 -1.194 -1.165 -1.122 -1.071 -1.015 -0.944 -0.840 -0.725 -0.564 -0.356 -0.102 0.172 0.465

Along Piers H / Hd 0.50 1.00 1.33

x / Hd -1.0 -0.8 -0.6 -0.4 0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

-0.495 -0.492 -0.490 -0.482 -0.440 -0.383 -0.265 -0.185 -0.076 -0.060 -0.240 0.445 0.675 0.925 1.177

y / Hd

-0.950 -0.940 -0.929 -0.930 -0.925 -0.779 -0.651 -0.545 -0.425 -0.285 -0.121 0.067 0.286 0.521 0.729

-1.235 -1.221 -1.209 -1.218 -1.244 -1.103 -0.950 -0.821 -0.689 -0.549 -0.389 -0.215 -0.011 -0.208 0.438

25

Example 1.1 Design an ogee spillway with the following data: (i) Height of spillway crest above river bed = 100 m (ii) Design discharge = 12,000 cumecs (iii) Number of spans = 6 (iv) Clear distance between piers = 15 m

(v) Thickness of pier = 3 m Slope of d/s face of the overflow section = 0.8:1

Assume any other data if required.

Solution L' = clear waterway = 15 x 6 = 90 m

Negllecting the end contractions and assuming the value of Cd as 2.20 in Eq. 1.8, we have

Q = Cd le He3/2

or 1200 = 2.20 x 90 x He3/2

or He = 15.43 m

The maximum value of Cd is 2.20. However, it is affected by the various factors, as discussed above.

Effect of height of spillway In this case, (P/HD) = (100/15.43) = 6.48. As (P/HD) is greater than 2.0, the effect on Cd is negligible. Thus the velocity of approach is small, and HD * Hd = 15.43 m.

Effect of actual head If the design head is taken equal to the actual head, (He/HD) ratio is 1.0, and, therefore, there is no effect on Cd.

Effect of upstream slope The upstream face of the spillway is assumed to be vertical. Hence, there is no effect on Cd.

Effect of downstream apron In this case, d + hd = 100.00 + 15.43 = 115.43 m. Therefore, (hd + d) /HD = 115.43/15.43 = 7.48. As this greater than 1.70, there is no effect on Cd.

Thus there is no effect of different factors on the assumed value of Cd = 2.20.

Effect of end construction Let us assume Kp = 0.02 and Ka = 0.20.

Therefore, effective length, Le = L' – 2(N Kp + Ka) HD (a)

Le = 90.0 – 2 (5 x 0.02 + 0.2) x 15.43 = 80.74 m

26

Now from Eq. 1.4, Q = Cd Le He3/2

or 12000 = 2.20 x 80.74 x (He)3/2 or He = 16.59 m

Substituting this value of He in Eq. (a),

Le = 90 – 2(5 x 0.02 + 0.20) x 16.59 = 80.05 m

Therefore, 12000 = 2.20 x 80.05 x He3/2 or He = 16.68 m

Let us take design head (HD) of 16.70 m.

Downstream profile From Eq. 1.8 xn = K Hdn-1 y

Velocity of approach = 98.016.70)(100x3)590(

12000=

+×+=

AQ

m/s

Head due to velocity of approach = (0.98)2/19.62 = 0.05 m

This is very small and therefore neglected. Thus Hd ≅ HD.

Now for the vertical upstream face, K = 2.00 and n = 1.850.

Therefore x1.85 = 2.00 x (16.70)0.85 y = 21.895 y

y = 0.0457 (x)1.85 … (b)

For the d/s face of the overflow section, 1.25 0.801.00

==dxdy

… (c)

From Eq. (b), 0.850.85 )( 0.0845 )( 0.0457 85.1 xxdxdy

=×= … (d)

From Eq. (c) and (d), 0.0845 x0.85 = 1.25

or x = 23.80 m

Therefore, the tangent point of the profile is at a distance of 23.80 m from the origin. The coordinates y for different values of x are obtained from Eq. (b) as follows:

x 0 1 2 3 4 5 6 8 10 12 14 x 16 18 20 20 23.8

y 0 0.046 0.165 0.349 0.594 0.897 1.257 2.141 3.235 4.533 6.483 y 7.719 9.600 11.663 13.912 16.091

Upstream profile The upstream profile is determined from Eq.

27

0.625d

0.375dd0.85

d

1.85d )0.270H (x )0.4315(HH 0.126

)(H)H 0.270 (x 0.724

y +−++

=

For Hd = 16.70 m, Eq. 14.7 can be simplified as

y = 0.06614 (x + 4.509)1.85 + 2.1042 – 1.2402 (x + 4.509)0.625

The values of the coordinates y are determined below for different values of x up to the maximum value of x/Hd = -0.27 or x = - 4.509 m and y/Hd = 0.126 or y = 2.1042 m

x -0.50 -1.00 -1.50 -2.00 -2.50 -3.00 -3.50 -4.00 -4.509

y 0.0133 0.0609 0.1429 0.2631 0.4266 0.6419 0.9242 1.3100 2.1042

Alternatively, the values of y/Hd for different values of x/Hd can be obtained from Table 1.2 for Hd = 16.70 m.

1.5.3.3 Chute Spillway

A chute spillway (or trough spillway or open channel spillway) consists of a steep-sloped open channel called a chute or trough, which carries the water passing over the crest of spillway to the river downstream [Fig. 1.19 (a)].

For earth dams and rockfill dams, a separate spillway is generally constructed in a flank or a saddle away from the dam if a suitable site exists. Sometimes, even for a gravity dam, a separate spillway is required when the valley is narrow and an overflow spillway cannot be provided at the dam site. The chute spillway is generally most suitable for such conditions. It can be conveniently provided independently in a saddle at a low cost. Sometimes, it is also provided along one abutment when a separate site does not exist. A chute spillway may be constructed on any type of foundation provided it is strong enough to bear the load.

A control structure may or may not be provided for this type of spillway, depending upon the natural level of the saddle. If the natural ground level of the saddle is higher than the full reservoir level, excavation is done in the saddle upto the full reservoir level to form a flat-crested weir. However, if the natural ground level of the saddle is lower than the full reservoir level, an ogee-shaped weir is usually built to achieve a high discharge coefficient. The weir is generally of low height and with upstream face inclined [Fig. 1.19 (b)].

28

Fig 1.19

1.5.3.4 Side Channel Spillway

In the side channel spillway, the crest of the control weir is placed along the side of the discharge channel. The crest is approximately parallel to the side channel at the entrance. Thus the flow after passing over the crest is carried in a discharge channel running parallel to the crest. Water flows over the crest into the narrow trough of the discharge channel opposite the weir, it turns approximately at right angle and then continues in the discharge channel [Fig. 1.20(a)]

The side channel spillway is usually constructed in a narrow canyon where sufficient space is not available for an overflow spillway. A side channel spillway is also usually required in a narrow valley where there is neither a suitable saddle, nor wide side-flanks to accommodate a chute spillway. In such cases, if a crest of length required for flow to occur perpendicular to the crest is provided, heavy cutting would be required. Therefore, the cost of an overflow spillway or a chute spillway would be prohibitive.

The crest of a side channel spillway is usually an ogee-shaped section made of concrete [Fig. 1.20(b)]. Sometimes it consists of a flat concrete pavement laid on an earthen embankment or the natural ground surface.

29

Fig. 1.20

1.5.2.5 Shaft Spillway

A shaft (or morning glory) spillway consists of a large vertical funnel, with its top surface at the crest level of the spillway and its lower end connected to a vertical (or nearly vertical) shaft. The other end of the vertical shaft is connected to a horizontal (or nearly horizontal) conduit or tunnel, which extends through or round the dam and carries the water to the river downstream (Fig. 1.21). When the water level rises above the crest level, it starts overflowing the crest and drops from the rim of the funnel into the vertical shaft and then flows in the horizontal conduit, which conveys it past the dam. The transition between the shaft and the horizontal conduit should be smooth to avoid cavitation.

A shaft spillway is used at the sites where the conditions are not favorable for an overflow spillway or a chute spillway. It is generally considered undesirable to construct a spillway just adjacent to an earth dam. Therefore, an overflow spillway is ruled out if there is not an adequate space. If the topography of the site is also such that a chute spillway cannot be constructed, a shaft spillway may be considered as an alternative to a side channel spillway.

For low dams, where the shaft height is small, no special inlet design is usually necessary. However, for high dams, a flared inlet, called morning glory, is generally used. Small shaft spillways may be constructed entirely of metal pipe, concrete or even clay tile. However, the vertical shafts of large projects are invariably of reinforced concrete and the horizontal conduit is usually a tunnel in rock. Frequently, the diversion tunnel used during the construction of the dam is planned so that it can be used as a horizontal conduit for the shaft spillway after the construction. The shaft is sometimes driven into rock instead of constructing it as a reinforced concrete shaft.

30

On the crest of the shaft spillway, radial piers are provided to guide the water radially. These piers also prevent spiral flow and are used a supports for a bridge to go around the spillway crest. Because a shaft spillway is surrounded by water on all sides, a bridge is also provided to connect it to the dam or a hill.

Ideal site A shaft spillway is ideally suited for a site where a rock spur projects into the reservoir a little distance upstream of the dam. If the top of the spur is lower than the full reservoir level, a standard-crested spillway has to be constructed in concrete, above the spur is higher than the full reservoir level, it has to be excavated down to the required crest level and a flat-crested spillway is constructed. A shaft spillway is generally more economical than a side channel spillway where a diversion tunnel, which is used for diversion of river water during construction, is already available for the shaft spillway.

Fig. 1.21

1.5.3.6 Siphon Spillways

A siphon spillways operates on the principle of siphonic action. There are basically two types of siphon spillways.

1. Saddle siphon spillway

2. Volute siphon spillway

1. Saddle siphon spillway A saddle siphon spillway (also called saddle siphon) is a closed conduit of the shape of an inverted U-tube with unequal legs. Saddle siphon spillway is commonly used in practice. Saddle siphon spillways are usually of two types: (a) Hood type and (b) Tilted outlet type, as discussed below.

31

Fig. 1.22

(a) Hood siphon spillway The various component parts of the hood saddle siphon spillway are shown in Fig. 1.22 (a). This type of spillway is also called the hood spillway. The siphon duct is formed by an air tight reinforced concrete cover, called hood, over an ogee-shaped body wall made of concrete. The top of the body wall forms the crest of the spillway and is kept at the full reservoir level (F.R.L) of the reservoir. The top of the hood is called crown. The space between the crown and the crest is known as throat. Fig. 1.22 (b) shows a hood siphon with its outlet submerged.

(b) Titled outlet type siphon spillway Fig. 1.23 shows another type of saddle siphon spillway, called tilted-outlet type. In this type of spillway, the siphon duct is formed within the body of the dam. The lower limb of the siphon is vertical with a tilted outlet. In this case, the draught of the water falling over the crest is sufficient to cause priming, and, therefore, no separate priming device is required. The outlet is tilted upwards so as to develop water seal at the bend. It is required for sealing the air entry from the exit end without which priming is not possible. For depriming of the spillway, a deprimer is provided as shown.

Fig. 1.23

32

2. Volute Siphon Spillway

The volute siphon spillway (or volute siphon) is a special type of siphon spillway which makes use of volutes (curved vanes) for priming. This type of spillway was designed by Ganesh Iyer in India and hence it is also called Ganesh Iyer siphon. The volute siphon spillway consists of a vertical shaft (or barrel), which has a funnel shape at its top. At the bottom end, it is connected to a horizontal or nearly horizontal outlet conduit through a right-angled bend, which leads the water to the downstream channel (Fig. 1.24). The top or lip of the funnel is kept at the full reservoir level (F.R.L). The inner sloping surface of the funnel is provided with a number of volutes. The volutes are the curved vanes like the blades of a centrifugal pump or a turbine [Fig. 1.24 (b)]. A cylindrical drum is constructed around the upper portion of the vertical shaft. The drum is supported on a number of pillars. The drum is open at the sides near the bottom so that water can enter into it. A dome is constructed over the drum. On the top of the dome, a small air-vent pipe (deprimer) of reinforced concrete is formed. One end of the air-vent pipe is connected to the interior of the dome at its crown and the other end is kept slightly higher than the full reservoir level. These air-vent pipes serve as deprimers. Sometimes, a deprimer dome is constructed over the main dome for this purpose.

Fig 1.24

1.5.3.7 Conduit (or Tunnel) Spillway

A conduit (or tunnel) spillway consists of a closed conduit to carry the flood discharge to the downstream channel. Fig 1.26 It is constructed in the abutment or under the dam. The closed conduit may take the form of a vertical or inclined shaft, a horizontal tunnel, or a conduit constructed in an open cut and then covered. Such a spillway is suitable for dam sites in narrow canyons with steep abutments.

The conduit should be designed to flow partly full, because if it runs full, the negative pressure may develop due to siphonic action. The area of the flow is usually limited to 75% of the total cross-sectional area of the conduit.

33

Fig 1.26

1.5.3.8 Cascade Spillway

A cascade spillway consists of a cascade of falls, with a stilling basin at each fall (Fig. 1.27). It is ideally suited for very high dams in which the energy cannot be dissipated by a hydraulic jump or a bucket. In the case of a high rockfill dams, already excavated quarry benches on d/s may be utilized for the formation of cascades.

Fig. 1.27