4 Spillway, Sluices and crest gates and how to construct it

CHAPTER
42
44
46
46
43 LoalKD of a Sp:llwa
47
4 8
49
4
IntroducKon Spltwav
Typcs of Spillways hascd on Most Prominent Features
Ogec ypllway or Overflow Spillway
3
Spillways, Sluices and Crest Gates
Designing the Crest of the Ogee Spillway
Practacal Profle of Spillway
4
Offsets and Risers on Upstrcam Face
4 10 Chuie Spilways or Trough Spiilway
4|| Spilkway Crest Gales J
D1share Computation for an Ogee Spillway
EIfectve tength of Ogre Spillway
412 ACratiom Gallerics
413 Siuces n Dam
Reiw Qucstions
4.1 Introduction - Spillways :
CONTENTS
(Oct. 2012, June 2014, Nov. 2014, Dec. 2015)
ncar the dam site to disposc of surplus watcr from the rcscrvoir
to the channel downstream. Spillways are provided for all dams as a safety mcasure against overtopping and the
conscquent damages and failure. A spillway acts as a safety valve for thec dam. bccausc as soon as the water
level in the revervoH ISes above a predeternincd level. excess water is discharged safely to the downstream channcl,
and the dam is not damaecd
A spullway i a structure oNstructcd at
Essential requirenents of a spillway :
The spillway must thave sutfcicnt capacity
I mut be hydraulu ally and structurally safe.
Ihe sufl ace of the spillway must be erosion resistant.
(November 2014, May 2015)
Il should hc providcd with some device for the dissipation of cxceSs energy.
The spullway mus bc so located that it provides safe disposal of water, ic. discharge
d/s toe ot the dam
195
Inust not crode
1.
2.
196
3.
Factors affecting spillway capacity :
1. inflow Mood
2.
3. discharge capacity of other outlet works
4
available storage capacity
whcther the spillway is gated or ungated.
5. Possible damage if the capacity is exceeded.
2
2
3.
Types of spillways based on utility :
Main Spillway
Amain spillway or service spillway is the one
which is the first to come into operation and is
designcd to pass the entire spillway design flood.
This spillway is necessary for all dams and in most
of the dams, il is the only spillway.
Subsidiary spillway or auxiliary spillway
3 Emergency spillway
Main Spillway :
Subsidiary spillway or auxiliary spillway :
Emergency spillway :
Cases of emergency :
(1) an enforced shutdown of the outlet works
(2) a malfunctioning of spillway gates
Natural
Drain
Design of Hydraulic Structures (Degree)
Componcnts of Spillway :
Free overfall or straight drop spillway
" Control structure
Ogee or overflow spillway
Dischargc channel
Side channel spillway
" Energy dissipators
" Entrance and outlct channels
4.2 Types of spillway bascd on most prominent features :
In some cases, where site condiions are
favourable it may be economical to provide an
auxillary spillway in conjunction with a smaller main spillway. In such cases the main spillway is designcd lo
pass only small floods which are likely to occur frequently and when these floods are exceeded thec main spillway
is assisted by the auxillary spillway in passing the excess flood water. The total capacity then equal o the
sum of the capacities of the main and the auxillary spillways.
Emergency
Splway
(3) the necessity for bypassing the regular spillway because of damage.
(May 2015, Nov. 2017, May 2018)
Main Dam
Reservor
An emergency spillway is one which is provided in addition to the main spillway but it comes into operation
only during emergency which may arise at any time and the same might not have been considered in the normal
design of the main spillway. Thus it is provided for additional safety during emergency
Rim of the
Reservok
Rver
-Main Spilway
Power House
Fig 4.1: Main and Emergency Spillway
(May 2012, June 2014, December-2015, May 2017)
According to the proininent features pertaining to the various componcnts of the spillway such as conrol
structures, discharge channel, etc. the spillways may be classified in the following types
Spilway
Channe

Splliways, Sluices and Crest Gates
4
Chute or open channel or trough spillway
2
6
siructure.
Tunnel spillway
Shaft or moming giory spillway
7. Siphon spillway
Free overftl or straight drop spillway :
splBway is a type of spill way in which the
control structure consists of a low-height,
narrow crested weir and the ds face is
vertical or nearly vertical so that the water
falls freely more or less vertical.
HAD H
WER
Ne overfall or straight drop
The ovcrflowing watcr may discharge
as a free jet (nappc) clcarly away from the
d's face of the spillway, as in the case of
a sharp crested weir. As such occasionally
the crest of this spillway is extendcd in the
form of an overhanging lip. Fig. 4.2(b), to
direct small discharge away from the d/s
tacc of the overfiow section. The underside
of the nappc is ventilated sufficiently to
prevent pulsating fluctuating jet.
Oge or overlow spillway
(a)
If no artificial protection is providcd on the down stream side
SGally cause the scouring the strcam bed and will form a plunge pool. As such in order to protect the stream
bod from scouriag an artificial pool may be created by constructing a low auxiliary dam downstream of the main
Hd=DESIGN HEAD OF SPILWAY
r VACUUM
IhHd
(a) SPILuWAY WITHOUT D/S PROTECTION
EXCESSM
TEPBULENT
FLOW
Fig. 4.3: Ogee Spillway
(b) SPILLWAY WITH OVCRHANGING UP
(c) SPILLWAY WITH D/S PROTECTION
(b)
197
Fig. 4.2:Straight Drop Spillway
the overflow section, the falling jct will
An ogee or ovcrflow spillway is the most
commonly uscd spillway. It is widcly uscd with
gravity dams, arch dams and buttress dams.
Several earth dams are also provided with this
Lype of spillway as a separate structure.
An ogee spillway is an improvement upon
the free overfall spillway. The Cssential
difference between the frcc overfall spillway
and ogee spillway is that in the case of frec
overfall spillway, watcr flowing over the crest
of the spillway drops vertically as a free jet
away from the downstream face, whereas in the
case of an ogee spillway, the water flowing
over the crest is guided snoothly over the cnd
is made to glide over the Ws face of thc
spillway.
198
4
Thc profile of the sp1llway is Ugce or 'S Thc overflowing water is guided sothiy (rvCr thc crest And
profile of the spillway so that overflow water does not break contact with the spilway surface If thix s o
assured, avacuum may form at the point of scparation nd cavitations may cccur.In addton to cavitatons,vibration
from the alternate making and breaking of contact betwcen the water andthc face of the dam may rusuit in ri
structural damage
3.. Site channel spillway :
A side channel spillway is the
one in which the flow, after passing
over a weir or ogee crest, is carried
away by the channel running
cssentially parallel to the crest.
The side channel spillway is
usually constructed in a narow
canyon whcrc sufficient space is not
available for an overflow spillway. A
side channelspillway isalso usually
required in a narrow valley wherc
there is neither a suitable saddle nor
wide side flanks to accommodate a
chute spillway. Side channcl spillway
is also the best clioice where a long
overflow crest is desired in order to
limit the surcharge head and the
abutment are steep.
Side
Chute or open channel or Trough spillway :
Channe
(2) When valley is narrow
(3) When the streambed is erodible
Wate
spillng
(|) When it is not possible to provide overflow spillway
Crest of ide channgt
sodway
(4) When separate site lor Np1llway is nol ava1lable.
Chule or open channel spillway ts provided in thu following situations
Dam
Rive
Fig, 4.4: Side Channcl Spillway
A chute spillway is the one which passes the surplus discharge through a steep slopd opcn chan called
a chute or trough, placed either along a dan abutment or through a saddie Generally this y ol p1liway ts
provided on carh or rock fill dam, and is isoluted from the main dan Its crest is Acpt nornmal io ts centr
l1ne. I consists of a discharge channel o the river n an excavated trench whch is uUally pavcd with concret
n whole or in part. The chute is sometimes of constant width. but usually naroWcd lor coHkoiY

Spilways. Siuices and Crest Gates
Tunnel Spillway :
US Pue
orgr
1)
400
30
2) vencal control
Certre line of
splley
20
DIS Profie
(3) closcd discharge channel
line o sphway
(b)
Pla:
Secion X-X song contre
Overfiow control weir of morning glory
la
(A shaft spillway consists of three mnain clements
R-2Hd
15
Waler level
Fig. 4.5 :Chute or Trough Spillway
Convex curve
Rvr
Acondut or tunnei spillway is the one in which a closed channel is uscd to convey the discharge arournd
or near a dam The closed channcl may be in the form of a verical or inclined shaft and a horizontal tunnel
or condust The courol structure for this spillway may be in the form of an overflow crest, vertical or inclincd
300
250
or1fice cntrance and side channel crest.The conduit or tunnel is designed to flow partly full and it is not allowed
to flow full bccausc siphon1c action may develop due o ncgative pressure being devcloped in the conduit.
6 Shaft or Moruing glory spillway :
Concave cunve
Shaft sp1lway sthe onc which has horzontally positioned lip through which water enters and then drops
through a verucal or slop1ng shaft and then to a horizontal conduit which convey water past the dam. A shaft
spillway cn ofcn be uscd where there is inadequate space for other types of spillways. Thus, on the carth dan
location., if thet Is no enough space or if the topography prevent the use of a chute or side channcl spillway.
shc best alternative wold he to use shaft spillway
400
199
7
200
Small shaft spillway may be constructed entirely of metal or concrete pipe or clay tile. The vertical shaft
of large structures is usually made of reinforced concrete and the horizontal conduit is tunnelled in rock.
There are two general types of shaft spillways
() Standard crest type
(2) Flat crest type
In the standard crest type spillway there is no weir section and water begins its free fall immediately upon
leaving the crest. The standard crest type has the advantages of having smaller crest diameter since its coeffcIent
of discharge is greater than that of flat.crest. In flat crest type, the weir section prececds frre fal! section.
Free
Falling
Section
Vertical
Shaf
Siphon spillways
River
Inlet
Diversion
Tunnel
!Reservoir Wbter Level
Standard Crest Type
(a) Secion
Plug
(b) Volue siphon spillway
Tunnel
(a) Saddle siphon spillway :
(c) Plan
Funnel Weir Setion
Verticat
Shat
Fig, 4.6 : Shaft Spillways
Outlet
Reserror Water Lsva
Flat Crest Type
(b) Section
Funsal
Tunnel
A siphon spillway operates on the principle of siphonic action. There are two types of siphon spillways
(a) Saddle siphon spillway
(December 2015)
Asaddle siphon spillway is a closed conduit of the shape of an inverted u-tube with uncquat legs Saddle
siphon spillway is commonly used in practice.

Spillways, Sluloes and Crest Gates
Saddie siphon spillways are usually of two types
() Hooded type
(ii) Tilted outiet type
Hooded type :
I he various component parts of the hooded tvpe siphon spillway are shown in Fig. 4.7 (a). Ine
syphon duct is formed by an air tight R.CC cover called hood. over an ogce shaped body wall nade
of concrete. Thc top of the body wall forms the crest of the spillway and is kept at F.RL. of the resero.
The top of the hood S called crown, The space between she crown and the crest is known as throat
The cntrance lip of the hood is called inlet. It is shaned to form a bcll-mouthed entry. The inlct
is kept below the FRL so that it remains submerged and the floating dcbris cannot cnter the siphon
duct.
DHS. (Degree) / 2021 / 26
The exit lip of he hood is called outlet. The outlct of the siphon duct is usually kept subncrged
in a cupHike basin which forms a water seal to prevent the cntry of air.
When the water level in the reservoir-rises, the water starts flowing over the crest. As the CxIt
cnd is submerged. the air cannot enter from that end. The air already entrapped in the top portion o!
the s1phon duct above the sheet of water flowing over the crest is gradually sucked by the flowing waler.
SiphonK action is established after the air in the bend over the crest has been exhausted. This action
is known as priming. The pressure in the siphon duct drops and a suction pressure less than atmospheric
pressure develops. Ii creales a suction pull due to which more water is drawn over the crest.
During receding flood, when water level has gonc down just to the reservoir level, air entcrs through
the mouth of deprimer dome and the siphonic action is broken. This action is called depriming of the
siphon
Deprimer
iniet mouth
Air vent
Throat
Crest
Crown
(a) Hooded Type
A
Limb or
-Lower
Leg.
Air
Inlet
Tail water
Level
LWater
Cistern
201
Fig. 4.7 : Saddled Siphons
Dan
Siphon
.Outlet
(b) Tilted outlet type
202
4.3 Location of aSpillway :
Aspillway may be lcatcd cither with1n the bxty of thg dam or a thg end of the dom ncat shutnent in
sorme cases. the sp1llway is lox atedaway from the damas an nckepcndent strcturc n a akikc or flank
deep narrow gorge w
ith stecp banks, separated from a flank by a hiliock with its levci sbovc th p of the dan
Is avalable, the spillway can be best built ndepTenaly of the dam (tg 4 *)
Too o Dam
Man Gorge
a spillway can be constructcd independently into the saddle
Top of ernergercy
embankment
Fig 4.8:Spillway Location
Frea bcas
Under such circumstances, a coneTeLe dam or an carth dam can be constructed aros th man valky ar
-Top of Dam
-Normal Pool Lav
Fig. 4.9
Oxtotxer 2012. emher 20j4
Sometimes, a concrete or masonry dam along with its spillway can be constnwtcd n the mn valley. whik
the flank or tlanks are closed by earthen embankments. The top lcvel of such an embankMcnt S kpn at maumur
reservoir levcl(MRL). The matcr1als and design of these cmbankments are sucth that thv fail s soon às walcr
overtops them. lHence, if by chance, either due to excessive lood above design flood or der to fasure of gate
of main spillway, etc. the water rises above the maximum rescrvoir level, it shal! overtop such cntuna ent. whwh
atonce fails, providing sufticien outlet for the disposal of excessive water.This type ot seondary fcty arrangenw
is generally provided on large dams especially on carth and rockilldams, and is knuwn ay sabsidiary spllsay
or emergency spillway or breching scction.
-Myrm Nar Lae
Scitav Cot
For earthen dams, aseparate independent spillway is generally preferred. althouLm de to non-avakublty
spillway site,: concrete spillway is sometimes constructed within or at one of the ends of an euth dam It he man
spillway is situatedin aflank, a seconduryemergency spillway may be located in anoher ark s shown in Fg 9
Varous ty pes ol spillways have heen described n chapter 1
-Max Resero Lve
Nommal pexot el
Yop ot main spilaay

Spiltways, Sluices and Crest Gates
4.4Ogee Spillwav or Overflow Spillwa
An ogoc overtow spullay is the mot commonly used spllway It iN wIdely uscd with gravity dams.
atch dam and hutureNA dam Several canh dams are also rovidcd wth this (vpe of spillway as a scparate structure.
An ogte pay an omprovemenl uon the free overtall spillway The enental difference betwecn the
froe ovcrfall spilwa1 and opee spullway is that in thc casc of froc overfall spillwav. water flowing over the crest
of the spiliwav drop vertall as a froe et awav from the downstream face, whercas in the case of an ogee
splhway. thc waly flowing UNCr the crest is guided smoothl over the end Is made to glide over the /s facc
ot the spillwav
(b
Cavitation
Fig. 4.10 Ogve Spillway
203
(November 2014, 2017)
Head > H
Seoaration
Exccsstvc
urbuenc8
Thc prof tic of the spillw ay Is ogee' or S The overflowng water is guidcd smoothly over the crest and
prutile ol the spilluay so that overflow waler docs not break contact with the spillway surfacc. If this is not
assurcd avacuum tnay form at the point of separation and cavitations may ocCur. In addition to cavitations, vibration
Irom th atlernatc mak.ing and break1ng of contact betwccn the water and the face of the dam may result in serious
srutural dama
(May 2017, Novemnber 2017, May 2018)
The creN ol the ugee spillway can be made to confirm only lo one particular nappe that would be obaincd
al one particular hcad. This hcad is called designed head., and represented by H, But in practicle, the actual
hcad of watc1 on the spliway crest. called operating heud may be less or more than the designed head.
If the optaling head is less than the designed head(H< H,).the pressure on the spillway crest will be
above atmospherk prussure on the other had. if the operating head on the spillway is more than the designed
hcad (H > H). the pressurc on the spillway crest will be less than the atmospheric pressure (negative pressure)
and the lowet nape of the fallung jet may leave the ogec profile. Such acondition is callcd cavitation.
The gencratiom of vacuTn Or negat1vC pressure may lead to formalion of bubhles or cavities in thec water.
These cavitics u buhtbi lilled with alt. vapour and other gases are formed in aliquid. whenever the absolute
pressure (ic atmytcpressuc-vacuum pressure) of the liquid is close to its vapour pressure, so asto commence
vaporalion Suh acomdion may arie wlen the hcad of water s Inore than the designcd head andtheconscquenu
hugh velocty jt Cls tcdu.cd pressutt or legauve pressure in the lower regon of the waler jet.
The cavites so formed nove downstream and enter a region where the absolute pressure is much higl1 L.e.
ore vacuum). This causs the vapour in the cavity to condernse and returm to liquid with aresuling impossible
collape of the cavity Wlen ic cavty coflapses, extremely high pressures are generaled. The continuous
onbardmcnt of these implos1ons will thus take place near the surface of the spillway, causing fatigue failure
f its material The snall partcles of conurete or masonry are thus hroken away. causing formation of pits
ts urtacc and grving the utace a spongy appearance. This daiutaging action of cavitation is called 'pitting
204
The cavitation plus the vibrations from the alternate making and breaking of contact between the water and the
face of the spillway, may thus result in serious structural damages to the spillway crest
Aeration arrangement : Aeration pipe of 25 mmo at 3 mc/c are provided along spillway facc bclow zatc lip
Hence. itcan be concluded that if the head of water over the spillway (H) is more than the designed heud
(H), cavitation may occur. On the other hand, if the head of water over the spillway is less than the designed
head, the falling jet would adheve to the crest of the ogee spillway, creating positive hydrostatic pressure ard
there by reducing the discharge coefficient of weir.
Fig. 4.11 shows the section of an ogee spillway with vertical upstream face
-Max Reservoir Level
Designod Head
-Sharp crested weir
US Face of ogee spillway
4.5 Designing the Crest of the Ogee Spillway :
-Upper nacpe
-Lower nacoe
-Crest of the ogee spliway
Fig, 4.11 : Section an Ogee Spillway with Vertical ws Face
(May 2012, 2017, 2018)
The shape of the crest or the upper curve of the ogee profile of this spillway is ordinar1ly made to confirm
closely to the profile Ùf the lower surface the nappe or sheet of water flowing over a ventilated sharp cresicd
weir when discharging at ahead cqual to the design head of the spillway. The nappe shaped profile for the crest
of an overflow spillway is an ideal profile because for discharge at the design head of water îlowing over the
crest of the spillway will remain in contact with the surface of the spillway as it glides over it and optimun
discharge will occur. Moreover, in this case no pressure will be exerted on the spillway by the lowing
water, because there will be atmospheric pressure along the contact surface between the flowing water and th
The thcorelical profile obtained for the lower nappe of a free falling jet is known as Buzin 's profile Th
adoption of such a profile, should cause no negative pressure on the crest under designed head
But, in actual practice, the operating head may exceed the designed head and there exists a lot of frction
due to roughness on the surface of the spillway. Hence, negutive pressure on such a profle scems inevitable.
The presence of negative pressure causes the danger of cavitation and sometimes fluctuations and pulstations of
the nappe. Hence, while adopting a profile for the spillway crest, the avoidance of negative pressure must be
an objective along with consideration of other factors such as practicability, hydraulic efficiency, stability and
economy
spillway crest.

1.
The shape of the ogce shaped spillway denends unon a number of factors such as
2
head over the crest
height of the spillway above the stream bed.
inclination of the ws face of the spillway
Etensive experiments were conducted by Us Bureau of Reclamaion (US.B.R.) for oblain the nappC-Shaped
proftles for the crests of the overflow spillwavs with their ws face cither vertical or inclincd. On (he bsIS o
the U SBR. data, the U.S. Army Corps of Engincers has developed several standard shapes of the crests of overtiow
Spillways at ks W'aterways Experiment Station (WES) at Vicksberg. Such shapes are known as "WES standard
spllway shapes
Down stream profile
The d's profile of the spillway can be represented by the equation
wherc.
=KH;y
[when ws face vertical)])
H, = design head excluding the velocity head
A, y = co-ord1nates of the points on the crest profile with the origin at the highest point C of the crest,
called the apex.
K and n are constants depcnding upon the slope of the uws face.
The values ofK and n are given as follows
8-2-H.y
|Slopeof ws face of the spill way
Verical
Table-4.1 : Values of Constants K and n
Vertical
1:3(H:V)
J:1.5(H:V)
1:1(H:V)
Thus, for a spillway having a vertical ws face the s crest is given
C(Origin or apex of rest)
rCrestaxis
4=0.5 H
(=0.2 Hd
2.000
a =0.175 H¡
b=0.282 Ha
1.936
CDIS Curve in
accordance with
80, 4.2
1.939
1.852
D1fferent upstream curves were given by WES for different slopes, as shown in Fig. 4.12.
1.85
1.836
.810
1.780
the cquation
(ü)
r, 0.68 Ha
(=0.21 H
a=0.139 H4
b=0.237 H
205
clnaccordance
with og. 4.1
... (4.1)
... (4.2)
206
The slope of the ds face of the
overflow dam usuallyvar1es in the rangc of
0.7:1to 0.8: 1. At the end of thc sloping
surface of the spillway, a smooth curved
circular surfacc, called bucket, is provided
to create a smooth transition of low from.
the spillway surface to apron of a stilling
basin or into the spillway discharge channel.
The bucket is also useful for the diss1pation
of energy and prevention of scour. The
radius of the bucket of about l/4 of the
spillway height is found to be satisfactory
h
Thus r =
t, )49 H.
t,=0.22 H
a -0.115H.
b0.214 H4
where, h = height of spillway
crest above the bed.
(i
Fig. 4.12 : WES Profiles for Opee Spillwars of Different ' Sopes
or table 4.2 may be used for making the ogee profile.
rin acoordance
wth eg 4.
H. ah
Fig. 4.14
-Crest aES
The profile for an ogee spillway having a vertical ws face, can be deteImiedonthe basis ot Hs WTS prnfile,
Fig. 4.13
(x, v) are the c0-ordinates of a point on thc profile as shown in Fig. 4.14
Uoper Iappe
Lowr nacp

H
0
10
025
0.50
075
L00
1=-027 H
|50
2.00
)= 0126 H,
3.00
2 Upstream profile of the crest :
400
S 00
When us face vertical :
u/s face. shoud have the following cquation
0724 (1+0.27 H, *S
(H.S
The us profile Cxtends up to.
US Profie
0.126 He
Table-4.2
Lower nappe
(ii) When u's facr is sloping :
H,
0126
-027 He
- 0.033
0
000
- 0.034
- 0.129
0.283
0.738
- i393
- 3.303
- 6.013.
The detals of the ws profile are shown in Fig. 4.I5.
- 9.523
Upper nappe
0831
Fiy. 4.15 : Details of w/s Prolile
0
807
Accord1ng to the latest studies of US. Army Corps, the ws curve of thc ogce spillway having a vertical
0 763
0.668
1312225) 0.539
0.373
0.088
- 0.743
- 2.653
+0.126 H, - 0.4315 (H, O375 .(x +0.27Ho625
- S.363
- 8.873
207
(4.3)
Tlc cordinates of the ws profile in the case of slopng upstrean tace can be deenincd from Table-4.3
for slopes I 3.2 and 3 3. For intermediale slopes, the values nay he nterpolated(S : 6934 - 1973).
208
H,
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
Table-4.3A)
-Max Reservoir
Level
Slope 1 : 3
Normal PoolLevel
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
4.6 Practical Profile of Spillway :
0.0800
Values of
rTop of Dam
Ttangular profle
H,
rOgee profle
Thickening
Slope 2:3
0 0000
0.0004
0.0016
0.0036
0.0066
0.0104
0.0153
0.0283
0.0365
0.0412
0.0554
for the uws face profile
CMax Reseroir
Lave
Nomal Pool Levei
Projecting
corbeil
Cancrete
Slope 3 : 3
0.0000
0.0004
0.0016
0.0036
0.0065
0.0103
00150
0.0275
0.0313
.0354
0.0399
0.0450
Fig. 4.16 : Practical Profile of Spillway
-Top of Dam
Vertical
-Trienguler
0.0000
0.0004
0 0016
0
0038
0 0068
00108
0.0153
0.0296
When the profile for the crest the ogee spillway is plotted over the trianguiar profile of the section of
agravity dam (non-overflow section), it is found that it goes beyond the downstream face of the dam, thus requiring
thickening of the section for the spillway. (Fig. 4.16(a)|. However, this extra concree can be saved by shifting
the curve of the nappe in abackward direction until this curve becomes tangential to thc downstream face of
the dam. (Fig. 4.16(6)).
0.0339
0.0386
0.0437
0.0494
0 0556
0.0624
0.0701
0.0787
0.0889
0.1016
0 1260

Splways, Sluices and Crest Gates
Ihe proCtion so formed is called corebel. Thus. a aving can he effected by providing a corcbel on ne
upstream fsce of the spilway section The construction of the soillwaAy is thus carried out as if It Was a o
overflow dam, up to the height of corbcl. Only the sligtht modifications are made after reaching the required eg
(up to O)at which corcbel is provided, and a smooth required curve 0CA is given shown in g "100
t may be noted that a corcbel cannot be provided in adam in which the gates are installcd on the upstrcam
face to control the flow to the outlets, because that will interfere with their operalion.
4.7 Offsets and Risers on Upstream Face :
If structural requirements permit, offset and risers can be provided on ws face by removing somc porion
of concrete, and thus economy can be effected. The maximum permitted projcction from the crest linc is 0.3151
and the vertical depth of the maximum bulging is 0.25 H,. (Fig. 4.17(a)].
Ms025 He
iS given by the equation
where.
Q
= CL, H:
4.8 Discharge Computation for an Ogee Spillway:
The discharge passing over the ogce spillway
Q= discharge
-Ss0315
In case of a vertical faced overhang, the vertical depth M of the projection (called riser) should bc cqual
to or greater than 0.5 H.The ratio M/N should not be less than ,50. However, it can have a zero valuc. l'or
M/N ratio between 0 to 0.5, the flow conditions are extremely unstable. Moreover, the ratio of the vertical depth
M to design head H, should not in the range 0 to 0.5 to avoid extremely unstable conditions.
-Crest
C= coefficient of discharge
(a) Offsets
= H, + H
L, = effective length of the spillway crest
is very small. and H, = H,
Fig. 4.17 : Provision of Offsets and Risers
D.HS. (Degree) I2021 / 27
H, = total head over the crest including the
velocty head
... (4.4)
For ogee spillways, the velocity head (H,)
M205H
UISTEL
UrS WL
209
-Crest
(b) Risers
Fig. 4.18
2
210
Factors allecting cocfficient of discharge (C):
The co-cfficient of discharge. depends upon the following factors
I. Height of spillway crest above the stream bed.
2
3. Slope of uw/s face of spillway
4
Ratio of actual total head of flow over the spillway crest to the design head tl /1)
Downstream submergence
5 Downstream apron interference
1. Height of spillway crest above the stream bed :
The height of spillway crest ahove stream bed or bed of approach channel affets thc vckety of approach
which in turn affects the coefficicnt of discharge with increase in the height of sp1llway. the veiocity of approaxh
decreases and the coefficient of d1scharge increascs. If the height of weir (h) is more than IR|,. the vclty
approach have a negligibel effect upon d1scharge.
Fig. 4.19 shows a plot of cocfficient of discharge C versus h/H., where Il, =l1, H may he obscrcd
from this plot that there is marked increase in the valuc of C ull the height of spillwav becoIC cqual to 211
Coeficient
of
Discharge
2.
0
Fig. 4.19
1.5
10
Value of (h/H.)
Plot of Coefficient of Discharge Vesus (h/t,
QCLH
2.0 2
Ratio of actual total head of flow over the spillwayerest to the design head (H, /IL,):
3.0
Fig. 4.20) shows a plot of (C/C) versus (H /L,) for a spillway of heght above strcanbcd grcater ta
1.33H,,where Cis coefficient of discharge corespond1ng to 1:, and C cactlicwnt oft dicharge corespond1ng
lo H,.
With further increase in the height of spill way there is not much increase in the valoe of C

Spillways Sluices and Crest Gates
ma h dfnm th nx ihat wath
in her wodwat irca in the had R
lowvc tx H Ha ( and for
When
Whn li
H th t
H h valoe
02
Slope of ws face of spillway :
1 04
1.03
Ratio of Head on Crest to Design Head (H d
1.02
101
C=:
1 00
04
0 99
N
tea In the value o HL,) the value of (CIC) inereases
thc coeffkent of diNharge inCrcavcS
H,> Hn. C>C
O8
The coeffwen of discharge is also affectcd by the slope of the ws facc of thc ogce wcir. The values of
Cand C lound up to now were for a vertical upstream fce. If the ws face is sloping, acorrection factor by
wtt the above valucs of Cshould he mul:iplicd can he obta1ncd trom the curves gven in Fg. 4.21
1:1
Fig. 4.20 : Plot of C Versus (L
1.
1.0
hH.
Fig. 4.2I
1:1
14
2.0
211
16
4.
212
Downstream submergence :
S.
When the tail water level is such that the top of the weir is covered by it, such that thc weir cannot discharge
frecly, the weir is then said to be a submerged weit.
Corecion
factor
by
wich
C
should
be
mutiphed
0.8
0.
0.4
of ratio
0.2
When the value of
) 02
Degree ofsubmergernce, hJd
Downstream apron interference :
hy +d= h +Hp
where,
0.3
d = tail water depth
H,
cocfficient of discharge. But, there may be adecrease in the coefficient due to tailwater submergence The correction
factor by which the value of C should be multiplied in order to get the modified value of coefficient of discharge,
can be oblained from the curve of Fig. 4.22.
0.5
Fig. 4.22:Effect of Submergence on C
excceds 1.7, the downstream apron is found
h = height of spillway
Fig. 4.23 shows the effect of d/s apron on the coefficient of discharge. It s observed that when the vatuc
(4y +d)
Hp
for lower values of this ratio the actual coefficient of discharge , is lower
From the geometry.
exceeds about I.70 the d/s floor apron has title effect on the coefficent of discharge However,
h, = depth of d/s water below w's TEL
Decth, d
have negligible effect on the
vckity of approah is negltg1bl

Ratin
(G
0.75
10
where,
H,
1.1
C, = 22x
q0.12
12
Effect of actual prevailing head on the discharge capacity of a spillway :
= 2.02
13
Ration, (hg+dHo
H, = designed head including velocity head.
H,
= 2.2 x 0.92
H
Fig. 4.23 Downstream Apron Interference
As discussed carlier, when once a spillway has been designed and constructed for adesign hcad (I1,).and
fa a corresponding coefficient of discharge (C) it will not always find the same hcad over its crest in ils actual
operations. The actual operating head (H) including velocity head, may be less or more than the designcd head.
Since the design is done for maximum head, the possibility of a head more than the designcd hcad is very small.
When the actual operating hcad passing over the spillway is less than the designed head, the prevailing cocfficient
of discharge (C) tends to reduce, and is given by the equation
TEL
16
he
47 18
Since an overflow spillway is sufficient in height (ie. h> L.33 H,), the cocfficient of discharge Cat designcd
head can be taken as 2.2. The prevailing coefficient of discharge at 50% head (includ1ng velocity head) will then be
012
213
... (4.5)
214
4.9 Effective Length of Ogee Spillway :
2
The effective length of crest of an ogee spllway
is given by the following equat1on
L, = L-2[N-K, +K,I H,
where.
L, = effective length of crest
L=net clear length of the sprltway crest
H, = total design head the crest includ1ng
velocity head
N= Number of piers
K, = Pier contraction cocfficient
K, =abutment contraction coufficient
The values of K, and K, depends upon the
hape of the pier and that of the abutnents. The
realer is the divergence from streamled flow.the
Teatcr is the contraction cocfficient and lesser is the
ffective length of the crest. A 90° cut water nose
ipe is the most efficicnt and has quite a low value
bf K.and is generally preferred Fig. 4.24 shows
he various shapes of picrs.
Round-nosed piers
The values of K, and K. are given in Table 4.3 and 44
Table-4.3
3. Pointed-nosed piers
Pier (Condition
r0.033 He
Square piar with cormers rou ded C02
Biurt nos9 CUars pier Cse91
0.311 Hq
0267 He
D267 4
90° Cut water Nose pier, C001
1. Square-noscd piers with corners rounded on a rad1us cqual to about 0! ot prcf hCkes
0267 M4
Pointed NoSe pier, Co 0.08
Fig. 4.24 Various Shapes af Piers
)

Spilways. Siuices and Crest Gates
Squarc abutmcntth hcadwall at 90
Ruundod ahement wh catall at to the dirccton of floa. when 0511 >r20I5I1,
than 45 c the duroc son of flo
wher
trample-1
Solution
Q= C L H;
C =24
L = 0 m
adis ot abtmcnt rounding and
H = 2m= H
Neglectun: Vckxty of appruact.
foripute the dscharge over an ogce weir with cocfticicent of discharge equal to 2.4 at a head of 2 m.
The length of spillwav is 100 m. The weir crest is 8 m above the bottom of the approach channel having
the ame width s that of the spillway. (Consider velocity approach) (May 2912, December 2015)
Q =24x 100 x(2)2
Abatment Condition
= 678 82 cumecs
Velocity of ppruah is given by
H
Hence.
Veaity hcad.
bcgha wIdth of channel
678 82
8-2x100
0679 m/eC
=2- o023
=2023 m
Table-44
(06791
2x981
to thc 1rcctIon of flo
690 56 cumecs
= 0023 m
Q
-CL : =24x100 x(2.023)
215
K
0.20
10
0.00
216
Example-2 :
Design an ogee spillway for concrete gravity dam, for the following data
(1) Average river bed level
(2) R.L. of spillway crest
(3) Slope of dis face of gravity dam = 0.7 H : 1V
(4) Design discharge
(5) Length of spillway
(6) Thickness of each pier
Solution
Step-1 : Computation of design head
Let us assume C = 2.2, for high weir.
Now,
Q = C-L, H;
where,
L, = L-2[N -K, +K,-H,
Let, L, = L =clear water width
8000 = 2.2 x 60 x H?
:: H =60.6
= 6 x 10
Since,
Also
= 60 m
H, = 1542 15.50 m
The height of the spillway crest above the river bed
h = 204 - 100 = 104 m
hy +d
H
104
H 155
Ws slope :
= 6.71 > L33
= 100.0 m
= 204.0 m
H+h -15.5 + 104
H,
= 8000 cumecs
It is a high spillway, the effect of velocity of approach can therefore, be neglected
15.5
=6 spans with a clear width of 10 m each.
= 25 m
= 7.7|> L7
(May 2013, Similar May 2015, October 2016)
Hence, the discharge coefficient is not affected by down stream apron interference and tail water conditions.
The uws face of the dam and spillway is proposed to be kept vertical. However, a batter of I: 10 will be
provided from stability considerations in the lower part. This batter is small and will not have any effect on the
coefficient of discharge.
Roundc abutmots whcre r>5H and bcadw all s placed not more

Effective length of spillway (L) :
L, = L-2(N K, + K,l- H,
Assuming that 90° cut water nose piers and abutments will be provided, we have
K, = 0.01
K, = 0.1
N= no. of piers = S
Assuming that the actual value of H. is slightly more than the approximate vlauc worked out (i.e. 15.5 m),
say. let it be 16.3 m, we have
L, =60-2 (5x 0.01 + 0.1]| ×16.3
Hence.
= $S.10 m
8000 = 22×55.10x H}
H: 66
H. = 16.40 16.30 m (assumed)
Hence, the assumed H, for calculating L, is correct. The correct profile will be designed for H, =l6.4 m
Igiecing velocity head.
However, the vclocity head (H,) can be calculated as follows :
Velocity of approach = V, =
Velocity head,
Step-2 : Determination of d/s profile :
Let us keep ws face vertical.
y
S= 2-Hy
H, =
This is very small and, therefore, neglected.
I85
21.6
Q
Area
D.H.S. (Degree) I2021/ 28
85
(60 + 5x 2.5)(104 + 164)
8000
8729
The d/s profile suggested by WES, is given by
2g
8000
= 0.917 m/sec
(0.917)'
2x 9.81
2(H,j035 2x(16.4)085 2x 10.8
217
= 0.043 m
... ()
Before we determine he various points of the d/s profile. we shall first determ1ne thc tangcnt point (I
The d/s slope of the darn s given to be 0.7I
218
dy
dx 07
differentiating equation (i) w.r.t x, we get
dy I85 x
dx 21.6
= 0.0856 0
85
0.0856 xo8> =
0.7
,0.55 = 16.68
X=27.4 m
(27.4)SS
216
o21.I5 m
y =
The co-ordinates from x = 0 to x = 274 m are worked oul in te labie below
r (metres)
7
12
|4
16
r.85
21.6
(metres)
0,046
0.166
0.353
0.600
().909
L274
I.694
2.169
2.697
3.277
4.592
6 T07
7820
9 723
I815
14 093
16 $55
21.150

Step-3 : ws prufile
For vertNal s 1ace. th w profike is given by
07241- 02H,
H
Tak1ng iH I64 m. c gct
024-027x164) NS
This curvc i Cend un to.
-t0-443 2.07-1232 (r+ 443)o25
=-027 x 164
48 m
RL =204 C
207 m
"0126H, -04315(H(a+027 H,'s
For vanous valucs of=-0.5, A =- 10. Ê = -2.0, A = - 3.0, x = - 4.0, A = - 4428 mthe valucs
of are gven belo.
4A28 m
US tace vetcal
- 0
12616 4) -0 431s (16 4)375r+027 x6.4,0623
10
r (metres)
-0.5
- L0
-2.0
-3.0
- 4.0
4 428
lc(Origin1
-Ans of Spillway
y (metres)
0.7
0.052
0.093
0.286
0.665
L.358
2.07
-Tangentpt (274, 21.15)
RI2
Fig. 4.25 : Spillway Section
219
'R 26 m
220
Step-4 : Design of ds bucket :
The profile of the spillway is shown in Fig 4.19 Arcverse curve at the toe is provided to form a buckct.
The radius of the bucket is generally kept equal to,
104
r=-= = 26 m
The bucket willsubtend an angle of 60 at the centre.
4.10 Chute Spillway or Trough Spillway :
Chute or open channel spillway is provided in the following situations :
() When it is not possible to provide overflow spillway.
(2) When valley is narrow
(3) When the streambed is erodible
(4) When separate site for spillway is not avaifable.
UrS Profle
Achute spillway is the one which passes the surplus discharge through a steep sloped open channel, callcd
achute or trough. placed either along adam abutment or through asaddle. Generally this type of spillway is providod
on earth or rock fill dam. and is isolated from the main dam. Its crest is kept normal to its centre line. It consists
of a discharge channel to the river in an excavated trench which is usually paved with concrete in whole or n
part. The chute is sometimes of constant width, but usually narrowed for economy
2
Onigln
400
350
300
250
200
Canteüne ol
soheay
150
DrS Profte
al
(b)
R=2Hd
SecdonXX elong oente
ine of epihey
Waluy vat
CoveA CUNe
Fig. 4.26:Chute or Trough Spillway
(May 2015, 2018)
Conceve a

1.
Design of low-oge weir as control structure
Since the chute spillway is providcd in a flank or a saddle, the height of spillway or ogce weir requircd
to be constructed in that flank, will be small, sometimes, almost flat low weir shall be required depcnding upOn
the natural level of the botom of the flank. If the flank bottom is at a level lower than the natural pool levl.
an ogoe welr shall have o be consAruced upo that level. If the lank botuom is at higher evel than the norma
pool level, cxcavations will have to be done up to that level.
The d's profile of a low-ogee weir may be represented by the cquation 4.1, as noicd below.
=K Hy
The valucs of K and n for low ogee weir depend on the ratios (H,/1|,) and (hIH,), whcre
H, = velocity heed
H, = dcsign head = H,+H,
H/H,
0.0
0.08
0
12
h
h/ii,
Table-4.5 : Downstream Profile
21.0
|-0.57
0.57 - 0.30
Fig. 4.27
I.852
I896
1905
Profile
n
1.780
1.750
1.747
c(ongin)
Equation for the d/s profile
-DIS
Profle
-|852 IHOy
x =|.89611y
747 - |.905IH*y
Profile of Small Ogce Weir
747
221
|r=2He
The co-ordinates of the ws profile, which should merge in a slope of 45° (i.e. I ) as shown in Fig. 4.27,
are given table 4.6.
|222
H,
0.000
- 0.020
shall
0.060
0.100
- 0.120
0.140
- 0.150
- 0.160
- 0.175
0.190
- 0.195
- 0.200
Table-4.6 : Co-ordinates of ws profile for ow ogee weir
Ha 0,00
H,
0 0000
0.0004
Steeper skope
0.0036
0.0103
0.0150
0.0207
0.0239
0.0275
0.0333
0.0399
0.0424
2. Design of vertical curves of the chute :
0.0450
H,
for different values of
H,
H
luncton
0.08
0.0000
0.0004
0.0035
0.0101
0.0150
0.0208
0.0235
0.0270
Design of Hydraulic Structures (Degree,
0.0328
00395
0 0420
H
-Concave Curve
0.0035
(a) Concave Vertcal Cuve
= 0.12
00147
6D199
At the sections where the bed slope of the trough changes, the adjacent sloping fioors should h interconnccted
through convcx and concave vertical curvcs.
9 0231
Concave curve : Whenever the slope of the chute changes from steeper to milder, a cocave vertical cunc
shall be provided. Fig. 4.28(a) In no case the radius of this curve should be less than 10 d. where d the dpth
of water in metres
00265
00325
Convex curve : Whenever the slope of the chutc changes from milder to stevpcr. a cunea vUrLKal curvc
have to be provided. Fig. 4.28(b).
0070
Mn Rus 10 d
Mider sope

Spillways. Sluices and Crest Gates
where.
4.
Siopng (1 S)
77177111IN11
Mider sione
gIven as
The convex Curc N USually parabolc, as given by the cquation
K|41d+- h,)cos 0j
d= depth of tlow
where.
h= wcocity head
K= fator of safety 2 i5
Approach channed of chute spillway :
h =S, xL
t= anglc of thc upstream floor just at beginning of curve
(b) Canvex Vertical Cuve
R
tion point
Steepe slopc
An approahchannel or cntrance channcl. lrapezoidal in shapc with side slopes I may be constructcd
so as to lcad the rewvOr water up lo the control structure (ie. low ogce wcir). If any curvature (in plan) is
required. t s gcnerally confned lo the cntrance channel, becausc the velocity of 1ow is low in this channel.
Velocty in channel
H= Manmg's coclfcent of roughness
L= lngth ot channel
Fig. 4.28
The frctuon hcad lost n the entrance channcl upto the spillway crest can be calculatcd by Manning's formula.
Side walls of the chule :
(1S)
Conved Curve
S, cm corgy slop Ixtwca (wo pomts
X
223
. (4.7)
A suffwCWnt freetuard must be prov ided above the top walT ape
(4.8)
The side walls called uanng walls of the chute should be of such a height that water docs not spill over
thcm The side walls of he chut may be kept vertical or sloping. But in the vicinity of gatcd ogee weirs, they
will have to be vertical. Gekrally, a rectangular chue channcl is designed.
|224
.
5.
The free board is usually determined by the equation.
Free board = 0.61 +0.04 V d
where,
V = mean velocity of water in he chute
d. = mean depth of water in the chute
Concrete Paving
Evoent in the case of solid rock with no cracks concrete paving of the trough is necessary in all cases. The
thickness of the concrete paving may generally range from 300 mm to 375 mm. Light reinforcement of about
0.25 to 19% of concrete area is generally provided each way in the top of the paving. To avoid cracking. the
concrete paving should be poured in square panels with contraction joints on all sides. The panels of size ranging
from 9 mto 12 mare usually adopted. the reinforcement should not be continuous through the contraction joints
and the surface of the joint should be treated to permit free îmovement due to contraction and expansion.
Gravel
Cutoffs:
Direcion of Flow
Dlrectton of Aow
Asphalt Paht
rExpansion Joint Fler
6mm
(a)Contraclon Jaint Normal to
Direclon of Flow Type (1)
rExpansion Jalnt Filler
RZUcut off
Looen TIle Drain
(c) Contreclion Jalnt Nomal to
Dreclon of Flow Type (2)
rExpansion Joint Fer
Steel plp
drain
LAsphalt Palnt
(b)Contracion JolntProlto
"Dlrecion of Aow
Dlrecdon of Flow
-GravelFter Surounding
End of Pipe
(4) Raleving Drain Pipe Through Puving
.. (4.9)
Fig. 4.29:Contraction Joints in Paving of uChute Spillway
II the paving panel cannot be made heavy enough to resist the uplift pressure, it may be provided with hold
down piles if the foundation is earth or anchorage rods if the foundation is rock. Anchorages for the paving pancls
'onsist of sleel rods grouted into holes drilled in the rock and tied to concrete panel
For achute spillway three types of cutoffs are provided. Acuoff at the upper end of the spillway is providcd
to reduce the uplifi pressure on the paving. At the ds end of the paving acutoff is provided to prevent undercutting
of the paving Further at the upstream end ofeach panel acutoff is provided to prevent creeping ofpanels resuting
from expansion and contraction due to changes in temperature as wellas toprevent flow of water from one panc!
to another along the underside of the paving. Atypical cutoff of this type is shown in Fig 4.29(b).
The chutc channel called thc dischargc channel is generally kept straught in plan.

Spillways, Sluices and Crest Gatees
Example-l
Design a suitable crest profile of a chute spillway for the following data :
Spillway crest level
Level of bottom of lank at which the
low ogee weir to be constructed
Design discharge
D's tailwater level
Solution :
1. Design of approach channel
Q=c.L, H;
Assume, coefficient of discharge C = 2.18
The spillay length consists ofSspans of 10.0 mclear width each. Thickness ofcach pier is 3m. Assume
any other data if necessary.
and L, = clcar wIdth = $ x 10 = 50 m.
also assumc H, = H
5000 = 2.18 x 50 x (H,)²
(H. )² = 45.8
H,= 12.8 m = H
ws water level = crest level + H
= 200 + 12.8
= 212.8 m
Bed level of river in flank = 192 m
D.H.S. (Degree) / 2021 / 29
water dcpth = 212.8 - 192
= 200.0 m
= 20.8 m
= 192,0 m
= S00O cumecs
= 103.0 m
62+ 220.8= 103.6m
y 20.8m
B 62m
225
Fig. 4.30: Approach Channel
(May 2017)
226
Assuming the trapczodal apprach channel with side skopes I
Bed width of the channel = B
Total length of spillway = (5 x 10) + (4 x 3) = 62 n
Arca of trapezoidal channel,
A= |62+ 103.6] x
=|722.24 m'
velocity of approach = V,
velocity hcad = H,
P= B+2/2 xy
wcttcd pcrimcter of the channel.
= 120.8 m
R=
= 62 +2/2x20.8
A
P
h, =
1722.24
120.8
= 14.20 m
4
R}
= 014 m
20.8
Assuming the length of approach channel = l60 m
head loss due to friction,
V
(0.019) x(2.9)° x160
(14.2)33
= 13.21 m
2g
.. H, = 213.21 - 200
H, = |321 - H,
= 12.78 m
= |3.21- 0.43
(2.9)
2x981
A
= 212.8 + 0.43
= 213.21 m
S000
1722.24
= 043 m
Level of ws TEL = Ws water level + velocity hcad - head lost upto spllway crrst
0.014
= 2.9 sec
P= B
+2y ym +1
For slopc I: 1
m = I
n = Manning's rugosity cocfficient
= 0.019 (assume)
h= 200 - 192 = 8m

|Spillways,Sluices and Crest Gates
Correctin caficient of dichange.
) Corrtion due to hcight of weir
-063<1 33
Ii, 1278
approach vclocay has arpreciable cffect on discharge
From Fig 413. For
C=215
H, 12.78
2) Correction duc lo ws sopc of 45 (ie. 1:1)
From Fig 415.
correcuon factor = L008
effectivc iength.
corrt alue of discharge coefticicnt
C= 215 x I008 = 217
= 1.03
S000
L = L-2|K, N+ K) H,
H
Thc corrCt value of H, is given by
|
- 50 - 2 (0.01 x 4 + 0.1]x 13.21
= 46.30 m
.:: H, = (49.70)
H
217 x 46.3 x (H, )?
.. Correcded H, H, - H,
(49.70,t = 13.6 m
2. Design of crest profile:
1363
0.63
H= 13.20 + 0.43 = 13.63 m
= 136 - 043
13 20 - 043)
043
= 13.17 m 13.20 m
75
=|.896 H
|3 45 y
1345
Irom Tabl 4 5. ds profile is given by cquation
I696 (1363y
= 0.0315 < 0.08
= 0.58 between | -0.58
Assume K =0.1, K,-0.01
N = Number of picrs = 4
227 228
Position d's apron of spillway :
The d/s apron should be at such an elevation that it does not affect the cocfficicnt of discharge.
H,
hy +d 2 1.7H,
> 1.7 x 13.63 = 23.2 m
Maximum apron elevation
Provide toe the spillway at RL 190,4 m
= TEL - (h, +d)
= 213.6l6 - 23.2
= 190.41 m
Discharge intensity downstream of spillway piers
velocity =
5000
(50 + 12)
80.64
d
specific energy = d +
d+31.44
but speciic energy = TE.L.
-m/s where d = depth
:. d +331.44 =23.206 d
We have.,
= 23.206
y=
80.64 m'ls
d'- 23.206 d +331.44 =0
Solving by calculator,
2g
= 23.206
d = 4.17 m say d= 4.20 m
13.45
= 213.616 - 190.41
= d+
x = 16.07 m
The d/'s profile is designed between RI. = 200 m
(crest level) and RL = 190.4| m (apron level)
r3 - 13.45 x 9.59
r.75 - |128.98
(80.64 Y
d
2x9.81
190.41
Maximum ordinate y = 200 - 190.41 = 9.59 m
TEL = 213.63 - h,
= 213.63 - 0.014
= d+31.44
= 213.616 m
d
The remaining ordinates of d/s profile between x = 0 and x = 16.07 are workcd out as below in Tablc I.

Spillwaye, Sluicee and Crest Gates
Refer Tabie-4.6 for
H,
H,
-0.000
-0.020
-0.060
-0.100
-0.120
-0.140
-0.150
-0.165
-0.175
-0.190
RL1920
Appronch Channad
Table-1 : Co-ordinates of ds profle
-0.200
r (metres)
and values: H,=13.61 m
0S
2.0
H,
4.0
6.0
8.0
10.0
The co-ordinates of ws profilk are calculated in table-2 below :
12,0
14.0
16.07
H,
0.000
0.0004
0.0035
0.0101
0.0150
0.0208
0.0235
0.0270
0.0328
0.0395
Table-2 : Co-ordinates of d/s profile
0.0420
1345
RL 200.0
1.74
0.022
0.074
(metres)
0.25
0.84
L.71
2.83
4.18
S.75
7.53
12
9.59
0
-0.272
-0.817
2.88
-1361
-1633
-1.905
- 2,042
- 2.178
- 2.382
-2.586
-2.722
4.03
10- 584
100
Fig 4.31:Section of Chute Spillway
0.0054
0.0476
0.1375
0.2042
0.2831
0.3198
0.3675
0,4464
0.5375
0.5716
-R=27.5 m
250:1
229 230|
4.11 SpillwayCrest Gates : (September 2013)
For an ungated spillway the useful storage in the reservoir can be maintancd onty up to tix ievel of th
crest of the spillway. By installing gates over the crest of the soillway additonal storagc can b adc avaslabk
When flood occurs these gates are removcd so that the full spillway capac1ty Is made valabie
1.
Some of the common types of gates used for spillways are
2
4
Flash boards, stop logs and nccdles
3. Drum gales
6
Radial gates
Vertical lift gates
Bear trap gates
Rolling gates
Flash boards,stop logs and needles :
Flash
Board
Flash bonrds:
Pier
Strut
Sop log
Plan
Stop log
(a) Flash Boards
Design of Mydraulic Structures (Degree)
Spillway
Crest
F l a s h
b o a r d
Up Stream
D/
-Bridge between
spillway picrs
(b) Stop Logs
tc) Needles
Fig. 4.32 Flash Boards, Stop Logs and Necdle
Keyway
These are the lemporary gates uscd only for small spillway ot minor importance They conststy ot woxdn
panels supportedby pins on the edges. The pins are supported on pie sockets along thc rest of th dam Iemporary
flash boards have been used up to a height of 17 m. The operation of the lempocAry Iash hxurds s utomatic

Spilhways, Sluices and Crest Gates
Stop log:
Stop log cons o honzontal woxden logs spanning th spacc betwcen groovcdpiers The logs are placcd
O OvCr thc oher h pushing them doun In to the erooes They may h placed or removed by hand or wIlh
a borst
Ncedles:
3
Ncodles conNINs ot woOden planks kept in inclncd position with lower ends resting in a keyway on the
spllwav creN and upnr cnd ol the top of a bridgc girder
2 Radial gat
A radal catc als Anon a a tainter gale has its
waler sunporung tsce skn plaie made of stecl plates.
in the shan o! a lr of a circlc. properly hinged at
th p Th ate ca thus b made to rotate about .
fIACC horzontal 3IS 1he Ioad of the cale and water
ck t carned on arings mounte on piers. The gates
an h ltfted b mcans o ropes and chains with the hcBp
of power drien winhes
Drum pates :
Skin
pate
Lumb
Train
Plate
Plate
Gate in closed
position
Gate un open
Position
Fg. 4.34 : U.SB.R. Drum Gale
Reservoir Water Level
Gate
Crest Seat
Cable
Winch
231
Fig. 4.33: Tainter Gate
(May 2017)
Bridge
Pier
Trunnion
Drun gates are nomally used for long span. It consists
of a scgment of a cyl1mder formed by skin platcs attached
to internal bracing. IL is hinged at the centre of curvaturc,
which may be either upstreanm or downstrcam. In the open
or lowered position the gate fits in a recesses in the top
of spillway. When waler is admitted to the recess the hollow
drum gatc is lorced upward to the closcd position. Fig. I.19
shows a drum gate developed by U.S. Burcau of
Reclamation.
6.
232
4. Vertical lift gates :
A vertical gate consists of a frame work of
skin plate at the w/s face along with beams and
girder suitably placed. Vertical gates are rectangular
in shape, and move vertically in their own plane.
These are three types :
5.
(1) Sliding gates
(2) Fixcd whccl gates
(3) Stoney gates.
The gate is hoisted by horizontal lifting beam,
the ends of which travel in the gate guides.
Bear trap gates :
Fig. 4.36 : Bear Trap Gate
Rolling gate :
Lowered
Position
4.12 Aeration Galleries :
tRoller
Train
(a) Plan
Water
Sea
It consists of a hollow steel cylinder spanning between the piers.
A heavy annular rim having gear teeth on its periphery encircles each
cnd of the cylinder. Each pier has an inclined rack which engages the
gear teeth encircling the cylinder. The gate is rolled up the inclined rack
by exerting a pull on the hoisting cable attached to the cylinder.
U/S
Gate
Side
Roler
Spühway Crest
Fig. 4.35 :Stoney Gate
Hoist
Room
Hnsting
Cabie
Crest
-Roler
Train
(b) Section A-A
A bear trap gate consists of two leaves of either timber
or steel hinged to the dam. These gates are lifted up by
admitting water to the space under the leaves. The downward
leaf is often made hollow so that its buovancy aids in the
lifting operation. These gates are often used for low navigation
dams.
Fig. 4.37 Rolling Gate
-Cuide
Gate seat
Tower
Spillways of high head dams are susceptible to cavitation damage due to surface deformities and high velocity.
Aeration is the most effective method for mitigating cavitation damage. Design of aerators is complex. Ilence,
recourse is always taken to refer the existing designs while designing an aeration system for a dam.

Spillways, Sluices end Crest Gates 233
In many dams heights are in excess of 100 m lcading to velocities exceeding 30 Ms. Thus, the spliWays
or these dams are susceptible to cavilation damage. Low pressure regions on spillway surfaces are crealed due
Oseparation of high velocity flows leading to cavitation damage. Aeration is the most cffective method to mitigate
cavitation damage. Natural aeration of the flow on the spillway may not be sufficient for this purpose. Theetorc,
terators are provided to supply the air undermeath the flow along the spillway surface. An aerator basically consisls
f a ramp/offset or a combination of the two to lift the jet from the floor and an acration duct to supply hc
it in the cavity s0 created below the jet. Agroove is used in conjunction with an upstream ramp andor a downstrcam
Htset or step, so that the bottom spray of the high velocity iet does not impact within the confines of the groove
nd thereby some of its air conductance capacity Continuity of airflow in the duct requires a continuous supply
f air to the space beneath the nappe.
Ampie aeration must be provided in a tunncl spillway in ordcr to prevent a fluctuating siphonic action which
Would result if some part of exhaustion of air caused by surging of the water jet, or wave action or backwatcr.
The acration system consists of a ramp, the circular ar ducts arranged along the spillway chute width
Jownstream of the ramp on the chute bottom, and a main air gallery supply. This gallery can be opened to the
tmosphere through the side walls. The ramp creates a sub pressure region by lifting up the high velocity water
et above the chute bottom. A small ramp is sufficient for this: The air is entrained into flow due to the cavity
sub pressurc via the air ducts and main gallery.
The Sardar Sarovar dam in Gujarat is provided with a 30 span spillway of width 18.3 m each separatcd
y 4.7 m thick piers and equipped with radial gates 18.3 m x 14.7 m. The service spillway of 23 spans has
sloping - cum-horizontal stilling basin as energy. dissipater and the 7 span auxiliary spillway has two chutes
erminating in to ski-jump buckets.
herators were provided on the service and auxiliary spillways for Sardar Sarovar spillway. Aerator for the scrvicc
spillway consists of an aeration groove of 2.45 m x 2.45 m with a ramp angle of 40 and ramp height of 0.4
The acrators in bays I, 3 and 5 are provided at EI 82 m whereas acrators of bays 2 and 4are providcd at
El 78 m. The selection of location of aerator was govermed by the flow cavitation index and the availability of
staght length along the glacis between the tangent point of the crest curvature and the tangenl point of the
lower circular curvature leading to the energy dissipater.
The acralors were staggered to accommodate the air ducts in the divide walls. Due to the staggercd positions
Tf the aerators in adjacent bays, the air intake towers are placed just adjacent to each other, one opening in cither
hays. Air intake towers are provided in the adjaccnt divide walls of the spillway. The intake of the tower is provided
with smooth bell mouth entries to minimize the head losses.
Roberts splitters are atype of energy-dissipating mcasure located ncar the top of dam spillways. These spliters
are typ1cally used on high dams where the spillway flow velocities are too fast for a stilling basin, or the unit
tischarge too high for a stepped spillway. The Roberts splitters system consists of a scries of projccting tceth
or splituers immediately upstream of a continuous lip or step.
Cavitation in general can be mitigated in twoways :firstly, by ensuring that the fluid pressure remains abovc
3m absolute, and secondly, by introducing at least 8% air concentralion into the flow. On a dam spillway with
Roberts splitters, air can be introduced to the water flow at atmospheric pressure via air vents connccted to an
atmospheric air source. This is called artificial aeration. This air flow to he air vents needs not be pumped or
pressurized, if properly designed, because sub-atmospheric pressures within the nappe created by thc separated
low over splitters will naturally suck air out of the vents. This is rue provided that the pressure of the air in
the cavity of the aforementioned nappe is lower than the air in the air vent (which should be atmospheric or
higher).
D.HS. (Deee) / 2021 / 30
234
4.13 Sluices in Dam :
Sluices are provided in the body of the concTte dam to rclcasc reguialed suppls of w.tr lor a vatiNty
of purposes which are briefly listed below :
1.
2.
3
5.
6
River diversion.
4 Water supply for municipal or industrial uses.
Irrigation.
8.
Generation of hydro-clectric power.
To pass the flood discharge in conjunction with the spillway
Flood control regulation to release water temporarily stored in flood control sioragr pwe or to cvacuate'
the storage in anticipation of flood inflows.
7. Depletion of the reservoir in order to faciliate inpection of the reservorrnand the upstrcam tace
of the dam for carrying out remedial neasures, if necessury
To lurnish necessary flows for satisfying prior right uNes downstrcam.
9. For maintenance of a live stream for abatement stream poliution. preservation ol nquaiIc lite. ciu
The flow through asluice may be either pressure flow or free flow along its cmire kength or a combinaton
of pressure (low in part length and free flow in the remainder part
Generally. sluices that traverse through concrete gravity dan1s havc retangular ero sthms and .are short
in comparison with conduits through embankment dams of comparabie height. Uw cÍ anuntei t smI sluke
al one or more clevalions provides flexibility in flow regulation and in quantuty of water releascd downstrcau
Sluices are controlled by gates at the upstream face and/or by gates or valves orated irO a gallkry n thc
interior of the dam. Sluices are usually designcd so that the outflow discharges onto the spliwa, face andor
directly into the stilling basin. When sBuices raverse through no-overllow sccions, a part Cnergy dissipaler
Sluices are also classificd based upon their aligoment as :
1. Straight Barrel Sluice
The barel of this sluice is kept nearly
horizontal betwcen the entry and enit transitions.
This sluice has the advantage of hav ing
minimum length due to which lesCT friction
losses take place. llorizontal suICeS are
generally uscd under the followng condtions
(a) When the sluices are drownod t the
Cxit; nd
(b When they have to h kaaldat or
ncar the river bed level. f0r exampie.
in construction sluices for rIver
diversion.
AIR VENT
-SERVICE GATE
-EMERGENCYGATE
Fig. 4.3S
DOWNSTRE AN FACE
OF SP:WAY MOHOVERFLO
SECTIO
straight Bartet Sluce
The width of the sluice barel IN generaliy kept unlorm throughout the length exep n th entrv tranen
If the sluice is designed for pressure ilow cond1los hen the op prottl ol lh sluke may h given a shght
coIstriction. On the oher hand, if free flow conditons preail then ho h) constrNlK N ICUUIrcd
must be provided.

7
1.
2
5.
3.
Sp1llways. Sluices and Crest Gates
Ihe harrc! ot this duwe s generallv kept
hzotal downtrcam of th cnrn transtt ion u
the sore c3te to fac1litatc rrstine of thc
iattet Beyond ihe enc gate thc bion of thc
stuKe conforms to th paraholx ath cf the
trapecto and Ks the downtream far of the
dam lio0 ingcntaliv
Irajectory T Sluke :
Ior decIding unon the numher and sizc of
siuwes. onc has to conder the dcs1gn discharge
at a predetermuned rexTvor clevauon Detatks of
th1s may he had írom the Burcau of Indian
Standards cok IS
Describ brcfl on Ogce Spilway
Write br scí note on
D:scuss bracfly the component parts and design for a chutc spillway.
(2
Cavitation in Ogce Spillway.
Enlia var1ous pllway crest gates and explain radial gate.
(1) Averagc rver bed level
I1485-1985 "Crteria for hydraulic ´design of sluices in concrete and masonry dams
Why spdhways ure cons1dered 'safety valve' for dams " Classify and write suitability of various spillways.
(6)
STRF AM FACE OF CAN
REVIEW QUESTIONS
Wrile des1gn pranKiples of four major parts of an Ogee Spil!way with governing equations. (May 2017)
(June 2014)
Definc sp1llway What is thc purpoN O provide it ? What are essential requircments ? Wherc the spillway
s locatcd
(Novemiber 2014, May 2015)
What s chute spillway ? Where is il preferred to ogee and other types of spillways ?
(4) Design dicharge
RL of spillway crcst : 350.0 m
(3) Siopc of ds face of gravity dam : 0.75:I
(5) Lengtb of spilhway
-ve
Ex. 1: Destgn an ogce spill way for concrete gravity darn. for he following data
Thckns uf cch per
EXAMPLES
-AR VÆNT
-SERVICEGATE
250.0 m
6500 cumecs
DOWN STREAM FACE O
SPILLWAYNON-OVERFLOW
SECTION
Fig. 4.39 : Trajectory tvpe Sluice
Mve)
-X> Hy
BUCKET
235
5spans with a clear length of 7 m each.
2 Im
(May 2018)
(November 2014, 2017)
(May 2017, 2018: November 2017, 2018)
(May 2917)
Ex. 2: Comptc the discharge over an ogee spilway with acoctficicnt of dischargc C=2.5 at ahead of 4m
The clfeetve kength of spillway is l00 mNegleu the velocity of appuouch (Ans. :Q 2010 cumccs
CHAPTER
5.1
5
5.2 Hydraulic Jump
Energy Dissipation Below Spillways
5.3 Stilling Basins
5.5
5.4 Bucket Type Energy Dissipators
Plunge Pools
% Review Questions
2
5.1 Energy Dissipation Below Spillways:
(October 2012, September 2013, December 2015, May 2017, 2018)
Water flowing over a spillway acquires a lot of kinetic energy because of the cogversion of the potential
energy into kinetic energy. If the water flowing with such a high velocity is discharged into the river it willscour
the river bed. If the scour is not properly controlled, it may extend backward and may endanger the spillw3y
and the dam. order to protect the channel bed against scour, the kinetic energy of the water should be diss1pated
1. By developing a hydraulic jump
CONTENTS
For the dissipation of the excessive kinetic energy of water, the following two methods are commonly adopted.
By using different types of buckets
Energy Dissipators
1 5.2 Hydraulic Jump : (May 2017)
(Hydraulic Jump is the sudden rise of water that takes placc when the flow changes from supercritical flow
state to the subcritical state. When a stream of water moving with a high velocity and low depth (ie. supercritical
low) strikes another stream of water moving with low velocity and high depth (ie sub-critical flow), a sudden
rise in thc surfacc of water takes place. This phenomenon is called Hydraulic juwnp This is generally accompanicd
by a large scale turbulence, dissipating most of the kinetic energy of supercritical flow. Such a phenomenon may
Occur in a canal below a regulating sluice, at the bottom of the spillway, or at a place where a steep channel
slope suddenly turns flat.
It may be noted that the depth before the jump is always less than the depth after the jump. The depth
before the jump is called the initial depth () and the depth after the jump is called the sequent depth (y)
y, and y, are called conjugate depths
The two depths at which specific encrgy is same are called alternate depths
In the specific energy diagram, the specific energyis minimum at point C. This depth of watcr in the channc.
corresponding to he minimum specitic energy (at point C) is known as eritical depth.
before it is discharged into the d/s channel.

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