A

Project Report

On

Design of Spillways to the specification for rectangular

flume

Submitted in Partial Fulfilment of the Requirements for the award of Bachelor of Technology

In Civil Engineering

Submitted by

Pavitra Kumar 1213300107

Pawan singh 1213300108

Pankaj verma 1213300106

Aayushi Goel 1213300004

Neha 1213300102

B.TECH Final Year

DEPARTMENT OF CIVIL ENGINEERING

NOIDA INSTITUTE OF ENGINEERING AND TECHNOLOGY

19 Knowledge Park –II INSTITUTIONAL AREA

GREATER NOIDA, UTTAR PRADESH-201306

Affiliated to Uttar Pradesh Technical University, LUCKNOW

2

DECLARATION

WE, PAVITRA KUMAR, PANKAJ VERMA, PAWAN SINGH, NEHA, AAYUSHI,

STUDENTS OF BACHELOR OF ENGINEERING, CIVIL DEPT., NIET GREATER NOIDA,

HEREBY DECLARE THAT THE WORK PRESENTED IN THIS THESIS IS OUTCOME OF

OUR OWN WORK, IS BONAFIDE, CORRECT TO THE BEST OF OUR KNOWLEDGE. THIS

WORK HAS BEEN CARRIED OUT TAKING CARE OF ENGINEERING ETHICS AND

KEEPING INDIAN IP LAWS INTO CONSIDERATION.

DATE: ___-___-_____

3

ACKNOWLEDGEMENT

Successfully accomplished project work and the completion of this report would have not been

made possible without those people who believed us and supported us.

I wish to express my deep sense of gratitude to Dr. Narayan R. Chandak (HOD, Civil) who always

encourages us to learn more without his support this work would have been not possible.

I also want to thank my esteem guide Mr. Nitesh Kumar Verma (Asst. Professor, Civil) to guide us

at all stages from the beginning till the end.

Pavitra kumar

Pawan singh

Pankaj verma

Neha

Aayushi

B.Tech (CE)

Noida Institute of engineering & Technology

Greater Noida

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Table of content

CONTENT PAGE NO.

Declaration....………………………………………………………………..……………………….2

Acknowledgment…………………………….……………………………..………………………...3

Table of content……………………………………………………………..………….…………….4

List of figures………………...………………………………………..………………..……………6

List of Tables………………………………………………………………….…………..………….7

Abstract…………………………………………………………………………….………..………..8

Chapter 1………………………………………………………………..……….….…….............9-18

1.0 Introduction………………….………………………………………..……….…………9

1.1 Capacity of dam……………………….………………………….….…….….………...10

1.2 Types of spillway………………………………………………….………...………….10

1.2.1 Free Overfall Spillway……………………………………………...…….......10

1.2.2 Overflow Spillway………………………………………………...……….….11

1.2.3 Side Channel Spillway………………...…………………………...……....…12

1.2.4 Chute (Trough) Spillway…………………………………………...………....13

1.2.5 Drop Inlet (Shaft or Morning Glory) Spillway………………….……..……...14

1.2.6 Conduit (Tunnel) Spillway……………………….…….………………......…15

1.2.7 Siphon Spillway……………………………..……………………….……......16

1.2.8 Stepped Spillway……………………..………………………….................…17

Chapter 2……………………………..………………………………………………………….19-28

2.0 Literature Review………………………..………………………………..…………….19

2.0.1 Hydrological Consideration………………...…………………………………20

2.0.2 Hydraulic Consideration…………………...………………………………….20

2.0.3 History of Stepped Spillway………………...……………………………...…21

2.0.4 Spread of the spillway technology…………………...…………….…………21

2.0.5 Design techniques of Stepped Spillways……………………………………...22

2.0.6 Classification of flow…………………...........……………………………….22

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2.0.6.1 Nappe flow…………………………………………...………….….23

2.0.6.2 Partial Nappe Flow…………………...………………………….….23

2.0.6.3 Skimming flow………………………...……………………….…...23

2.0.7 Ogee Spillway………………….......……………………………………....…24

2.0.8 Background…………...……………………………………………………....24

2.0.9 Design details of downward profile of ogee spillway………………………...26

2.0.10 Discharge equation……………………...…………………………………...27

2.0.11 Factors affecting coefficient of discharge (𝐶𝑑)………………………...……27

2.0.12 Effective length (𝐿𝑒)………………………...……………………………….27

2.0.13 Dams in India and their Spillways………………………...…………………28

Reference………………………………………………………………………………..29

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LIST OF FIGURES

Figure Page no.

Fig 01 Free over fall Spillway…..…………………………………………………………………..11

Fig 02 Ogee Spillway……………………………………………………………………………….12

Fig 03 Side channel Spillway…………………...…………………………………………………..13

Fig 04 Chute Spillway…………...………………………………………………………………….14

Fig 05 Shaft Spillway……………………………………………………………………………….15

Fig 06 Conduit/ Tunnel Spillway…………………………………………………………………...16

Fig 07 Siphon Spillway……………………………………………………………………………..17

Fig 08 Stepped Spillway…………………………………………………………………………….18

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List of Tables

Table Page No .

Table 01. Major Dams in India ………………………………………………………………………9

Table 02. Major Dams in India and their Spillways……………………………………………...…28

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ABSTRACT

During our project duration at NIET Gr. Noida, we were assigned to the project “Design of

spillways to the specification for rectangular flume” which is under Mr Nitesh Kr. Verma.

There we were assigned to study about the various spillways and check the various

construction of the spillways working going on. We were guided by Mr Nitesh Kr Verma about the

spillways prject.

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CHAPTER – 1

1.0 Introduction

Before knowing about Spillway ,we will know about where the spillway is built. So Spillway is

built over the dam and the dam is constructed on a reservoir of the river to control the flow and

prevent the area from flood. Dam is constructed up to a permissible height and this depend upon

few factors such as the topography of the area , location , objective of the dam and the Investment

for the construction. Spillway provide a safe passage for the excess water or flood water in the

reservoir or in the river to flow over the dam through spillway before it overtops the dam.

Generally the excess water is drawn from the top of the reservoir created by the dam and conveyed

back to the river through the artificially created waterways ,these waterways are of sufficient

capacity to convey the excess water. the spillway is constructed in such a way that it should be

structurally safe and hydraulically adequate , also it must be located in such a way that the outfacing

water discharge back into the river don’t erode or undermine the downstream toe of the dam. The

spillway surface is also able to resist the erosion or scouring due to the very high velocity water

discharge through spillway .

In a spillway flood water flows from high elevation to low elevation through a passage ,the higher

elevation is the reservoir surface and the lover elevation is the river level , this passage is also

considered as a spillway. At the bottom of the channel where excess water meets the natural river is

generally provided with an energy dissipation device which is used for killing most of energy of

flowing water and are called as Energy Dissipators. these are provided to prevent the river surface

from getting dangerously scoured by the impact of out-falling water .

Some time we uses two set of spillways in the project such as on Indira Sagar Dam on Narmada

river, one is called Main spillway or Service spillway and other is Auxiliary spillway. Main

spillway is most of time in operation and passes most of water discharge through it , Auxiliary one

is at higher level of the dam and so passes less water through it. Some time there provided

Emergency Plug in periphery of reservoir for high flood. Generally spillway is provided with gates

for better control on the flow passing through.

Table 1. Major Dams in India

Sl. No. Name of Dam Copletion Year

1 Tehri Dam 2005

2 Lakhwar Dam U/C

3 Idukki Arch Dam 1974

4 Bhakra Dam 1963

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1.1 Capacity of Dam

The capacity of spillway depend on the following factors:

The inflow flood

The volume of storage provided by the reservoir

Crest height of the spillway

Gated or undated

According to the Bureau of Indian Std. guideline IS: 11223-1985 “Guideline for fixing Spillway

capacity”, The following values of inflow design floods (IDF) should be taken for the design of

spillway:

For large Dams (Gross storage capacity greater then 60 million m3 or hydraulic head between

(2m and 30 m))

For intermediate dams those with Gross storage between 10 and 60 million m^3 or hydraulic

head between 2m and 30 m.

For small dam Gross storage between 0.5 to 10 million m^3 or hydraulic head between 7.5 m to 12 m.

1.2 Types Of Spillway

Spillway are generally classified according to there most prominent features. The common types of

Spillway is used are the following:

Free Overfall (Straight Drop) Spillway

Overflow (Ogee) Spillway

Chute (Open Channel/Trough) Spillway

Side Channel Spillway

Shaft (Drop Inlet/Morning Glory) Spillway

Tunnel (Conduit) Spillway

Siphon Spillway

Stepped Spillway

1.2.1 Free Overfall Spillway:

This is the simplest type of spillway which is constructed in the form of low height weir having

downstream face either vertical or nearly vertical. Water drops freely from the crest and the

underside of the falling nappe is ventilated sufficiently to prevent a pulsating, fluctuating, jet.

Occasionally, the crest is extended in the form of an overhanging lip (similar to that provided in

notch falls) to direct the small discharge away from the face of the overfall section. However, after

sometime, the falling jet will form a deep plunge pool. Where erosion is not permissible, a low

secondary dam may be constructed to create an artificial pool, or a concrete apron may be provided.

If tailwater depths are sufficient, a hydraulic jump will form when the free jet falls on the flat apron.

This type of spillway is not recommended for high head since the vibrations caused by piping or

undermining.

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Fig. 01 (Free over fall Spillway) Source: http://1.bp.blogspot.com/-

_6QkrIjgS6o/UxtBlYoc2RI/AAAAAAAAAus/33Fplag6QWY/s1600/d.jpg

1.2.2 Overflow Spillway:

This is the most common type of spillway provided on gravity dams. The profile of the spillway is

Ogee or ‘S’ saved. The overflowing water is guided smoothly over the crest and profile of the

spillway so that the over flow water doesn’t break contact with the spillway surface. If this is not

assured, a vacuum may form at the point of separation and cavitation may occur . In addition to

cavitation, vibration from the alternate making and breaking of contact between the water and face

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of the dam may result in serious structural damage . Hence the upper profile of the ogee is made to

confirm with the lower nappe of the freely falling jet of water over a sharp crested weir, when the

flow rate corresponds to the maximum designed capacity of the spillway. Here,the essential

difference between straight drops spillway and the ogee or over flow spillway should be clearly

noted. In the former type, jet falls clearly away from face of the spillway and the gap between the

jet and face is kept ventilated. In the ogee or over flow spillway, the falling water is made to guide

over the curved profile of the spillway. A smooth gradual reverse curvature under the downstream

face of the spillway provided. This reverse curve turns flow on the apron of a stilling basin or into

the spillway discharge channel.

Fig. 02 (Ogee Spillway)

Source: http://smg.photobucket.com/user/willard3/media/Photography/hdr_3043.jpg.html

1.2.3 Side Channel Spillway

A side channel spillway is the one in which the flow , after passing over a wier or ogee crest, is

carried away by channel running essentially parallel to the crest. Discharging characteristics of a

side channel spillway are similar to those of an ordinary overflow spillway and are dependent on

selected profile of the weir crest. However, this flow may differ from that of overflow spillway in

that the flow in the trough may partly submerge the flow over the crest. Side channel spillway is

suitable for earth or rockfill dams in narrow canyons and for other situations where a direct

overflow is not permissible. Side channel spillway is also the best choice where a long overflow

crest is desired in order to limit the surcharge head and the abutments are steep. This type of

spillway is also desirable where the spillway discharge is to be connected to narrow discharge

channel or a tunnel.

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Fig. 03 (Side channel Spillway)

Source:http://1.bp.blogspot.com/-w7TxmbPd-

mg/UxtCb9Gg57I/AAAAAAAAAv0/EMWtWguKzkA/s1600/h.jpg

1.2.4 Chute (Trough) Spillway

A chute spillway is the one which passes the surplus discharge through a steep sloped open channel,

called a chute or trough, placed either along a dam abutment or through a saddle. Generally this

type of spillway is provided on earth or rockfill dam, and isolated from the main dam. Its crest is

kept normal to its centre line. It consist of a discharge channel to the river in an excavated trench

which is usually paved with concrete in whole or in part. The crest or the spillway proper is usually

of insignificant height or actually flat. The chute is sometime of constent width, but usually

narrowed for economy and then widened near the end of reduce discharge velocity. Factors

influencing the selection of chute spillway are the simplicity of their design and construction, their

adaptability to almost any foundation condition and the overall economy often by the use of large

amount of spillway excavation in the dam embankment.

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Fig. 04 (Chute Spillway)

Source:http://3.bp.blogspot.com/-

dxmhmcRUxmM/VRQkx1lciPI/AAAAAAAABUA/nP50xEK3ONY/s320/parts2-300x280.jpg

1.2.5 Drop Inlet (Shaft or Morning Glory) Spillways

A drop inlet or shaft spillway, as the name implies, is one in which the water enters over a

horizontally positioned lip, drops through a vertical or sloping shaft, and then flows to the

downstream river channel through a horizontal or near horizontal conduit or tunnel. The structure

may be considered as being made up of three elements; namely, an overflow control weir, a vertical

transition, and a closed discharge channel. Where the inlet is funnel-shaped, this type of structure is

often called a “morning glory” or “glory hole” spillway. Fig.5 illustrates a typical drop inlet

spillway. Discharge characteristics of the drop inlet spillway may vary with the range of head. The

control will shift according to the relative discharge capacities of the weir, the transition, and the

conduit or tunnel. For example, as the heads increase on a glory hole spillway, the control will shift

from weir flow over the crest to orifice flow in the transition and then to full pipe flow in the

downstream portion. A drop inlet spillway can be used advantageously at dam sites in narrow

canyons where the abutments rise steeply or where a diversion tunnel or conduit is available for use

as the downstream leg. Another advantage of this type of spillway is that near maximum capacity is

attained at relatively low heads; This characteristic makes the spillway ideal for use where the

maximum spillway outflow is to be limited. This characteristic also may be considered

disadvantageous, in that there is little increase in capacity beyond the designed heads, should a

flood occur. This would not be a disadvantage if this type of spillway were used as a service

spillway in conjunction with an auxiliary or emergency spillway.

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Fig. 05 (Shaft Spillway)

Source:http://2.bp.blogspot.com/-

4IdbduIQlEI/UxtCckGePWI/AAAAAAAAAwA/KPkaf107vSg/s1600/jj.jpg

1.2.6 Conduit (Tunnel) Spillway

Where a closed channel is used to convey the discharge around or under a dam, the spillway is often

called a tunnel or conduit spillway, as appropriate. The closed channel may take the form of a

vertical or inclined shaft, a horizontal tunnel through earth or rock, or a conduit constructed in open

cut and backfilled with earth materials. Most forms of control structures, including overflow crests,

vertical or inclined orifice entrances, drop inlet entrances, and side channel crests, can be used with

conduit and tunnel spillways. Tunnel spillways may present advantages for dam sited in narrow

canyons with steep abutments or at sites where there is danger to open channels from snow or

rockslides. Conduit spillways may be appropriate at dam sites in wide valleys, where the abutments

rise gradually and are at a considerable distance from the stream channel. Use of a conduit will

permit the spillway to be located under the dam near the streambed.

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Fig. 06 (Conduit/Tunnel Spillway)

Source: http://1.bp.blogspot.com/-

saS0yXrAnbk/UxtCaTeYV0I/AAAAAAAAAvw/1Q_6K0GIPqA/s1600/fvfv.jpg

1.2.7 Siphon Spillway

A siphon spillway is a closed conduit system formed in the shape of an inverted U, positioned so

that the inside of the bend of the upper passageway is at normal reservoir storage level. This type of siphon is also called a Saddle siphon spillway. The initial discharges of the spillway, as the

reservoir level rises above normal, are similar to flow over a weir. Siphonic action takes place after the air in the bend over the crest has been exhausted. Continuous flow is maintained by the suction effect due to the gravity pull of the water in the lower leg of the siphon.

Siphon spillways comprise usually of five components, which include an inlet, an upper leg, a throat or control section, a lower leg and an outlet. A siphon breaker air vent is also provided to

control the siphonic action of the spillway so that it will cease operation when the reservoir water surface is drawn down to normal level. Otherwise the siphon would continue to operate until air entered the inlet. The inlet is generally placed well below the Full Reservoir Level to prevent

entrance of drifting materials and to avoid the formation of vortices and draw downs which might break siphonic action.

Another type of siphon spillway designed by Ganesh Iyer has been named after him. It consists of a vertical pipe or shaft which opens out in the form of a funnel at the top and at the bottom it is connected by a right angle bend to a horizontal outlet conduit. The top or lip of the funnel is kept at

the Full Reservoir Level. On the surface of the funnel are attached curved vanes or projections called the volutes.

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Fig. 07 (Siphon Spillway)

Source: http://nptel.ac.in/courses/105105110/pdf/m4l08.pdf

1.2.8 Stepped Spillway

In recent years, the design flows of many dams were re-evaluated, often resulting in discharges

larger than the original design. In many cases, the occurrence of the revised discharges would result in dam overtopping because of insufficient storage and spillway capacity. The embankment dams are more prone to overtopping failure than other types of dams because of breaching or erosion of

the downstream face of the embankment. Despite the catastrophic effects of failure, dam overtopping constitutes the majority of identified dam failures. Before the 1980s, overtopping

counter-measures consisted mainly of increasing the reservoir storage or spillway capacity. Lately overtopping protection systems have gained acceptance because they safely allow controlled flows over the dam wall during large flood events.

There are several techniques to armour embankment slopes, including paving, rip-rap gabions,

reinforced earth, pre-cast concrete slabs and roller compacted concrete (RCC). RCC protection and gabion placement techniques yield embankment protections shaped in a stepped fashion. While

most modern stepped spillways are designed as prismatic rectangular chutes with horizontal steps, recent studies suggested different step configurations that might enhance the rate of energy dissipation (Andre et al. 2004, Chanson and Gonzalez 2004). Some older structures were equipped

with devices to enhance energy dissipation: some had pooled steps with vertical walls (Sorpe dam, 1932) or rounded end sills (Le Pont dam, 1882) (Fig. 2). Macro- roughness systems consisting of

concrete blocks were studied also (Manso and Schleiss 2002). All the above-mentioned techniques may effectively enhance the flow resistance, but their attractiveness is counterbalanced by the increased structural loads to the chute and the needs of extraordinary placement methods that might

increase the construction period and total costs. Hence, more effective methods to increase the energy dissipation of embankment overflows are needed. This study review a series of experimental

investigation of the hydraulic performance of moderate-slope stepped chutes with flat smooth steps,

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rough steps and of chutes equipped with different configurations of longitudinal ribs acting as turbulence manipulators (Fig. 2). The results aim to understand the turbulent energy dissipation

processes occurring down the stepped chutes. They also provide new, original insights into air-water stepped spillway flows not foreseen in prior studies and they yield new design criteria for

stepped chutes with moderate slopes typical of embankment dams (15° < θ < 25°).

Fig. 08 (Stepped Spillway)

Source: http://staff.civil.uq.edu.au/h.chanson/pictures/melton1b.jpg

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Chapter 2

2.0 Literature Review

Spillways are provided for storage and detention dams to release surplus floodwater, which

cannot be contained in the allotted storage space. In diversion works, like weirs and barrages,

spillways bypass the flow exceeding that which is released in to the system like irrigation canals,

power canals, feeder canals, link canals etc. Ordinarily, the excess flow is drawn from the top of the

pool created by the dam and conveyed through an artificial waterway i.e. spillway, back into the

same river or to some other drainage Channel. Spillways are safety devices in a dam and it is like

safety valve in a boiler. The primary function of spillway is to release surplus waters from the

reservoir in order to prevent overtopping and possible failure of the dam.

The water discharged over the spillway of a dam attains a very high velocity due to its static head,

which is generally much higher than the safe non-eroding velocity in the downstream. This high

velocity flow may cause serious scour and erosion of river bed downstream. To dissipate this

excessive energy and to establish safe flow conditions in the downstream of a dam spillway, energy

dissipaters are used as remedial devices. Many failures of dams have been reported due to

inadequate capacity or improper design of spillway, especially for earthen and rock fill type dams

which are likely to be destroyed, if overtopped, unlike concrete dams which may not fail with slight

overtopping for a small period of time. In addition to providing sufficient capacity, the spillways

must be hydraulically and structurally sound and located such that the spillway discharges will not

cause objectionable erosion downstream near the toe of the dam causing the failure of the dam and

other appurtenant structures. Uncontrolled erosion of bed and bank materials due to faulty design of

spillways and energy dissipation devices have caused not only serious safety problems, heavy

maintenance cost are also to be incurred annually after the monsoon when the spillway is in

operation. The spillway’s bounding surface must be erosion resistant to withstand the high velocity

flow created due to the drop in water surface from the reservoir level upstream to the tail water level

downstream of the dam. Spillways are to be designed as transition structures for smooth passage of

flow from upstream to downstream of a storage reservoir without causing any damage to the

structure or endangering the river system.

In determining the best combination of storage and spillway capacity to accommodate the

selected inflow design flood, all pertinent factors of hydrology, hydraulics, design, cost, and

damage should be considered. In this connection and when applicable, consideration should be

given to such factors as (a) the characteristics of the flood hydrographic; (b) the damages which

would result if such a flood occurred without the dam; (c) the damages which would result if such a

flood occurred with the dam in place; (d) the damages which would occur if the dam or spillway

were breached; (e) effects of various dam and spillway combinations on the probable increase or

decrease of damages above or below the dam (as indicated by reservoir back-water curves and tail

water curves); (f) relative costs of increasing the capacity of spillway ; and (g) use of combined

outlet facilities to serve more than one function, such as control of releases and control or passage

of floods. Service outlet releases may be permitted in passing part of the inflow design flood unless

such outlets are considered to be unavailable in time of flood. With due regard to the above

mentioned considerations, the type and size of a spillway will also be governed by the hydrological

and hydraulic considerations as discussed below:

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2.0.1 Hydrological Considerations

The peak flood for which the spillway is to be designed will govern the size and capacity

(waterway) of a spillway. The flood series for the particular river should be carefully studied and a

frequency analysis is to be made to determine the design peak flood of a given frequency which will

be governed by the importance of the structure and its safety requirement. Usually, Gumbel’s

probability equation or similar probability equations are used to find the design flood of a given

frequency, which may vary from 1 in 50 years to 1 in 500 years-return period. If the failure of a

dam leads to unprecedented loss of life and properties, maximum probable flood of high return

period (say 1 in 500 years or more) should be considered for the design of the spillway capacity so

that there is hardly any risk of failure and consequent damages. Obviously, the capacity of the

spillway and its costs will be too high in such case. On the other hand, if the dam failure does not

cause any loss of life or property, the spillway capacity can be substantially reduced considering

peak flood of low return period (say 1 in 50 years or so). The incoming peak flood should be routed

as it passes through the reservoir upstream of the dam. Higher the storage capacity (due to greater

height of dam), lower will be the routed flood peak for which the spillway capacity has to be

provided. An economic analysis should be done to arrive at the best combination of storage capacity

and the spillway size for optimum cost of spillway and appurtenant structures such as energy

dissipater etc.

2.0.2 Hydraulic Considerations

Hydraulic design of various types of spillways narrated under item 2 is to be carefully done

so as to avoid poor performance and failure of the structure and also to avoid high maintenance

cost. For the detailed hydraulic design of different types of spillways, the relevant codes (given

under references) should be followed. Hydraulic design involves consideration of the following

aspects:

(a) Fixing the crest Level.

(b) Design of Waterway

(c) Design of Spillway Profile

(d) Design of Energy Dissipation Device

(e) Design of aeration device

(f) Design of Anti vortex Device

(g) Design of Control Gates and their operation

(h) Design of Outlet Works

(i) Reservoir Operation Schedule

(j) Desalting of Reservoirs

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2.0.3 History of Stepped Spillways

Recently, spillways with a stepped profile have regained interest and favour among design engineers to pass flood waters over the dams. The stepped geometry enhances the energy

dissipation above the spillways and reduce the size of a downstream stilling basin. Recent advances in technology have led to the construction of large dams, reservoirs and channels. This progress has necessitated the provision of adequate flood disposal facilities and safe dissipation of the energy of

the flow, which may be achieved by providing steps on the spillway face. Stepped channels and Spillways are used since more than 3000 years. Stepped spillway is generally a modification on the

downstream face of a standard profile for an uncontrolled ogee spillway. At some distance in the downstream of the spillway crest, steps are fitted into the spillway profile such that the envelope of their tips follows the standard profile down to the toe of the spillway. A stepped chute design

increases higher energy dissipation and thus reduces greatly the need for a large energy dissipater at the toe of the spillway or chute. Stepped spillway was quite common in the 19th century and present

practice is confined to simple geometries. Generally, a stepped channel geometry is used in channels with small - slope: for river training, in sewers and storm waterways and channels downstream of bottom outlets, launder of chemical processing plants, waste waterways of treatment

plants and step –pool streams. Detailed investigation into its various elements started only about 1978 with the comprehensive laboratory tests by Essery and Horner (1978). During the 19th century

and early 20th century, Stepped waste - waterways (also called ' byewash' ) were commonly used to assist with energy dissipation of the flow. Now a days stepped spillways are often associated with roller compacted concrete dams.

The world`s oldest stepped spillways are probably those of the Khosr River dams, in Iraq.

Much later, the Romans built stepped overflow dams in their empire example of which is Kasserine

dam. Following the reconquest of Spain, Spanish engineers benefited from the Roman and Moslem

precedents and designed dams with overflow stepped spillways (eg. Almansa dam). In 1791, they

built the largest dam with stepped spillway, the Puentes dam, but the dam was washed out in 1802

after a foundation failure. Before 1850, the dam expertise of Spanish engineers was most

exceptional. Not surprisingly, after the conquest of America, the Spanish dam-building was

exported to the New Indies. In central Mexico several stepped overflow dams were built by the

Spanish during the 18th and 19th centuries. The Spanish expertise was known to French engineers by

the middle of the 17th century.

In United Kingdom, several dams were built near furnaces and water mills. Some included

stepped weirs and spillways. It is believed that English engineers gained experience from the

Romans who built aqueducts and dams during their occupation.

2.0.4 Spread of the spillway design technology

From Antiquity to the beginning of the 20th century, the Romans, Moslems, and Spanish

contributed successively to the dissemination of the arts of dam-building. Dams and stepped

overflow spillways were found early in the Middle East, then the practice spread through the

Mediterranean in Roman times. The Muslim conquerors of the Hispanic peninsula brought their

water traditions with them from the eastern and southern Mediterranean. Seven hundred years of

Moorish control of Iberia left a strong influence of irrigation structures. Later the Spanish

conquerors of the New World transferred deliberately their technology in turn.

22

In most early dams, the waters were discharged over the dam crests. The stepped spillways

geometry was selected initially to contribute to the stability of the dam, for the simplicity of shape,

or for a combination of the two. Later, design engineers realized the advantages of stepped channels

for reducing the flow velocities and to prevent scouring. By the fall of the 19 th century, overflow

stepped spillways were selected frequently to contribute to the dam stability and to enhance energy

dissipations. Most structures were masonry and concrete dams with a downstream stepped face

reinforced by granite blocks. The spillways of the New Croton dam is probably the first stepped

chute designed specifically to maximize energy dissipation.

In the first part of the 20th century, new progress in the energy dissipation characteristics of

hydraulic jumps favoured the design of stilling basins downstream of chute and spillways. Stilling

basins allowed better energy dissipation and smaller structure, and they contribute to cheaper

constructions. Recently (in 1970s), design engineers have regained interest for stepped spillways.

This trend was initiated by the introduction of new construction materials. Over the past decade,

several dams have been built with overflow stepped spillway around the world.

2.0.5 Design techniques of Stepped Spillways

Since Antiquity, the design of stepped spillways and channels was recognised to reduce flow

velocities and to prevent scouring. Some ancient engineers might have known the concepts of nappe

and skimming flows. But there is evidence that, even at the beginning of 20 th century, hydraulic

engineers had no quantitative information on the main flow properties. It is only recently that new

progress on the hydraulics of stepped channels has been achieved example Essery and Horner

(1978), Sorensen (1985), and Peyras et al (1991). It is certain that small weirs and drop structures

were designed for a nappe flow regime. But the author wishes to highlight that some ancient

stepped spillways were designed for a skimming flow regime.

2.0.6 Classification of flow

The concept of stepped spillway was used as early as 1892 - 1906 in New Croton dam.

Lombardi and Marquenent were first to consider stepped spillway consisting of concrete drop spillway and intermediate erodible river reaches. The slopes of these reaches were such that a hydraulic jump occurred at the base of each drop. However, the experimental studies revealed three

types of flows over a stepped spillway, namely:

(a) Nappe flow (b) Partial nappe flow (c) Skimming flow

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2.0.6.1 Nappe flow This type of flow occurs for small discharges. The flow cascades over the steps, falls in a series of plunges from one step to another in a thin layer that clings to the face of each step, with the

energy dissipation occurring by breaking of the jet in the air, impact of jet on the step, mixing on the step, with or without the formation of a partial hydraulic jump on the step. The step height should must be relatively large for nappe flow. This situation may apply to relatively flat stepped

channels or at low flow rates. The depths can be determined from the expression:

𝑦𝑐

𝑠ℎ1

= (𝑞2

𝑔𝑠ℎ13 )

0.22

(1)

Where: 𝑦𝑐= initial depth of water above weir

𝑠ℎ1 = depth of steps

q = discharge per unit width

g = acceleration due to gravity

2.0.6.2 Partial Nappe flow In this type of flow, the nappe does not fully impinge on the step surface and it disperses with considerable turbulence. Flow is super - critical down the length of the spillway. For a given step geometry, an increase in flow rate may lead to intermediate flow pattern between nappe and

skimming flow - the transition flow regime also called a partial nappe flow. The transition flow is characterised by a pool of circulating water and often accompanied by a very small air bubble

(cavity), and significant water spray and the deflection of water jet immediately downstream of the stagnation point. Downstream of the spray region, the supercritical flow decelerates up to the downstream step edge. The transition flow pattern exhibits significant longitudinal variations of the

flow properties on each step. It does not present the coherent appearance of skimming flows.

2.0.6.3 Skimming flow In skimming flow regimes, the water flows down the stepped face as a coherent stream, skimming over the steps and cushioned by the recirculating fluid trapped between them. The

external edges of the steps form a pseudo bottom over which the flow skims. Beneath this, recirculating vortices form and are sustained through the transmission of shear stress from the water flowing past the edge of the steps. At the upstream end, the flow is transparent and has glossy

appearance and no air entrainment takes place. After a few steps the flow is characterised by air entrainment similar to a self -aerated flow down a smooth invert spillway. In case of the skimming

flow, at each step, whether air entrainment occurs or otherwise, a stable vortex develops and the overlying flow moves down the spillway supported by these vortices, which behave as solid boundary for the skimming flow, and the tips of the steps. There is a continuous exchange of flow

between top layer and vortices formed on steps.

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The flow rotates in the vortex for a brief period and then returns to the main flow to proceed on down the spillway face. Similarly, air bubbles penetrate and rotate with the vortex flow, when

aeration takes place. Transition from one type of flow to another is gradual and continuous, as a result both the nappe flow and the skimming flow, appear simultaneously in a certain range, one of

them on some steps and other on the remaining, both changing spatially and temporarily.

2.0.7 Ogee Spillway The ogee-crested spillway, because of its superb hydraulic characteristics, has been one of the most studied hydraulic structures. Its ability to pass flows efficiently and safely, when properly

designed, with relatively good flow measuring capabilities, has enabled engineers to use it in a wide variety of situations. This is most common type of spillway provided on gravity dams. The profile of the spillway is Ogee or ‘S’ shaped. The overflowing water is guided smoothly over the crest and

profile of the spillway so that the overflow water does not break contact with the spillway surface. If this is not assured, a vacuum may form at the point of separation and cavitation may occur. In

addition to the cavitation, vibration from the alternate making and breaking of contact between the water and the face of the dam may result in serious structural damage. Hence the upper profile of the ogee is made to conform with the lower nappe of a freely falling jet of water over a sharp

crested weir, when the flow rate corresponds to the maximum designed capacity of the spillway. The essential difference between the straight drop spillway and the ogee spillway should be noted

that in straight drop spillway the jet falls clearly away from the face of the spillway and the gap between the jet and the face is kept ventilated. In the ogee spillway, falling water is made to glide over the curved profile of the spillway. A smooth gradual reverse curvature on the downstream face

of the spillway is provided. This reverse curve turns the flow on the apron of a stilling basin or into the spillway discharge channel.

2.0.8 Background In theory, the ogee-crested spillway’s performance attributes are due to its shape being

derived from the lower surface of an aerated nappe flowing over a sharp-crested weir. The ogee shape results in near-atmospheric pressure over the crest section for a single given upstream head. Because the flow rate is not limited to a single head, the flow rate over an ogee crested spillway

varies from that of a sharp-crested weir. At heads lower than the design head, the discharge is less because of crest resistance. At higher heads, the discharge is greater than an aerated sharp-crested

weir because the negative crest pressure suctions more flow. Designing a crest that allows small negative pressures at the design flow is a practice called under designing and it increases the efficiency of the spillway. However, the crest pressures must not be allowed to go too negative. A

large negative pressure on the crest can cause cavitation damage, destabilization of the structure, and possible failure. Large negative pressures can also be caused by discontinuities in the crest

shape. Considerable research has been done to determine the shape of the crest of an overflow spillway, and different methods are available that depend on the relative height and upstream face slope of the spillway (Maynord 1985). Bazin (Chow 1959), in 1888, completed a comprehensive

laboratory investigation and was the first to study the ogee shape. After Bazin, the majority of the existing information is derived from extensive data taken from physical models completed by the

USBR and the USACE.

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In the past few years, several researchers have attempted to solve this and similar problems

with a variety of mathematical models and computational methods. The main difficulty of the problem is that the flow transitions from subcritical to supercritical flow. In addition, the discharge

is unknown and must be solved as part of the solution. This is especially criticalwhen the velocity head upstream from the spillway is a significant part of the total upstream head. An early attempt at modeling spillway flows was completed by Cassidy (1965). By using potential flow theory and

mapping into the complex potential plane, he was able to solve for the free surface and crest pressures and found good agreement with experimental data for a limited number of solutions. The

close agreement led Cassidy to conclude that viscosity has a negligible influence on the location of the free surface. He also concluded that the point of minimum pressure for a given head is dependent on the boundary configuration. Better convergence of Cassidy’s solution was obtained

by Ikegawa and Washizu (1973) and Betts (1979) using linear finite elements and the variation principle. Li et al. (1989) completed additional improvements on the 2D irrotational gravity flow by

using higher-order elements to model the curved water surface and spillway surface. More recently, Guo et al. (1998) expanded on the potential flow theory by applying the analytical functional boundaryvalue theory with the substitution of variables to derive nonsingular boundary integral

equations. This method was applied successfully to spillways with a free drop. Bu¨rgisser and Rutschmann (1999) used finite elements and an eddy viscosity to iteratively solve the

incompressible 2D vertical steady Reynolds-averaged Navier-Stokes (RANS) equations. Given a flow rate, they successfully computed the free surface and velocity and pressure fields using a finite-element grid that adapts locally for a changing water surface. Olsen and Kjellesvig (1998)

also included viscous effects by numerically solving the RANS equations in two and three dimensions, using the standard k-e equations (Rodi 1980) to model turbulence. Olsen and Kjellesvig

(1998) showed excellent agreement for water surfaces and discharge coefficients for a limited number of flows. However, pressure data were only recorded at five locations downstream from a nonstandard crest at one flow and showed some variability.

In the history of the ogee-crest dam, it is interesting to note the different designs and shapes that have been used. It appears that the majority of these changes have been focused on the crest section upstream from the crest axis. This is logical in that the forward crest is where the flow

makes a transition from subcritical to supercritical. Maynord’s report (1985) shows four different shapes for vertical face dams. Counter to the upstream section, the downstream section from the

crest axis to the tangent section has been ‘‘standardized’’ to the equation 2. However, the coefficients and exponents may change, depending on the dam geometry and design flow rate.

The general non dimensional equation for discharge is given by

𝑄 =2

3𝐶0√2𝑔𝐿𝐻𝑒

3

2 (2)

26

Where: Q = total discharge

L = lateral crest length or width 𝐻𝑒 = total head upstream from the crest

g = gravitational constant 𝐶0 = discharge coefficient

Note that He, the total head, includes the velocity head. Usually, this requires that an iterative solution be completed because the velocity head is not known until the flow rate is calculated. However, because the velocity head is generally a small part of the total head, the

equation converges to a solution after several iterations. The discharge coefficient C0 is not constant. It is influenced by a variety of factors including the depth of approach, relation of the

actual crest shape to the ideal nappe shape, upstream face slope, downstream apron interference, and downstream submergence (Design 1977). Each of the conditions have ranges and design curves that describe the effect of the parameters mentioned.

2.0.9 Design details of downward profile of ogee spillway The curved profile of the crest is continued till it meets tangentially the straight sloping surface of the downstream face of the overflow dam. The location of the point of tangency depends

upon the slope of the downstream face, which usually varies from 0.7:1 to 0.8:1, based on the stability requirements. At the end of the straight sloping face of the spillway, a curved circular surface, called bucket is provided to create a smooth transition of flow from the spillway surface to

the outlet channel or the river on the downstream side. The bucket also serves the purpose of dissipating the energy as well as prevention of scour at the downstream side.

The radius R (in m) of the bucket is obtained from the following appropriate expression:

R=10𝑎 (3.1)

a=𝑉+6.4𝐻𝑑 +4.88

3.6𝐻𝑑 +19.52 (3.2)

Where:

V = velocity of flow at the toe of the spillway (m/s), given by

V=√2𝑔(𝑍 + 𝐻𝑎 − 𝑦𝑡)

Z = fall or vertical distance from upstream reservoir level to the floor at the downstream toe

𝑦𝑡= depth of water at the toe.

𝐻𝑑= design head

27

Generally, R=P/4 is found to be satisfactory, where P is the height of spillway crest above approach

channel of river bed

2.0.10 Discharge equation

The discharge over an ogee shaped spillway is given by:

Q=𝐶𝑑𝐿𝑒𝐻𝑒

3

2 (4)

Where:

Q = discharge

𝐶𝑑= a variable coefficient of discharge which is influenced by several factors.

𝐿𝑒= effective length of crest

𝐻𝑒= total head on the crest, including velocity of approach head=H+𝐻𝑎.

2.0.11 Factors affecting coefficient of discharge (𝑪𝒅)

(i) Ratio of actual total head to the design head

(ii) Depth of approach channel below spillway crest

(iii) Downstream submergence

(iv) Slope of upstream face of spillway

2.0.12 Effective length (𝑳𝒆) of the crest

The effective length of the crest is given by

𝐿𝑒 = 𝐿 − 2(𝑁𝐾𝑝 + 𝐾𝑎)𝐻𝑒 (5)

28

Where:

𝐿𝑒= effective length of the crest

L = net length of the crest

N = number of piers

𝐾𝑝 = pier contraction coefficient

𝐾𝑎 = abutment contraction coefficient

The pier contraction coefficient depends upon many factors such as:

(i) Shape and location of pier nose

(ii) Thickness of pier

(iii) Approach velocity

(iv) Actual head of flow in relation of design head

2.0.13 Dams in India and their spillways

A dam is a barrier that impounds water or underground streams. Reservoirs created by dams

not only suppress floods but also provide water for such activities as irrigation, human

consumption, industrial use, aquaculture, and navigability. Hydropower is often used in conjunction

with dams to generate electricity. A dam can also be used to collect water or for storage of water

which can be evenly distributed between locations. Dams generally serve the primary purpose of

retaining water, while other structures such as floodgates or levees are used to manage or prevent

water flow into specific land regions.

Table 2. Few dams in India and their spillways are as:

Dams Spillways Tehri dam Uttrakhand 1 chute spillway and 4 shaft spillways

Bhagra Nangal dam Panjab and HP 4 gated spillway

Hirakund dam Andhra Pradesh Gated spillway

Sardar sarovar dam Gujrat Chute spillway

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REFERENCES

Bhahra Nangal Dam Bridging faraway information news Retrieved from

http://bhakranangaldam.com/

NPTL. Retrieved from http://nptel.ac.in/

Punmia, B. C., B.B. Lal, Pande, Jain, A.K. & Jain, Arun Kumar. (2009), Irrigation and Water

power engineering. Laxmi Productions

Sardar Sarovar Narmada Nigam Ltd. A Wholly owned Govt. of Gujrat undertaking. Retrieved form

http://www.sardarsarovardam.org/components-of-project.aspx

Subramanya, K. (2010). Flow in open channels. Tata McGraw Hill Education Private Limited

THDC India LTD. A Joint venture of Govt. of India & Govt. of UP Retrieved from

http://thdc.gov.in/Projects/english/Scripts/Prj_Introduction.aspx?Vid=132