Tye ye �o f
SPILLWAYS
�s p i l l wa �c a �b �c l a s s i fi e �i n t �di ff e r e n � t ye � b a s e �o �t h �v a r i o u �c r i t e r i �
1
Cl a s s i fi c a t i o n �b as e �o �PURPOSE
2
Cl a s s i fi c a t i o n �b as e �o �CONTROL
3
Cl a s s i fi c a t i o n �b as e �o �PROMINENT FEATURE
1
Cl a s s i fi c a t i o n �b as e �o �PURPOSE
( o rs e r v i c � ) s p i l l wa � Main s p i l l wa � Auxiliary Emergency s p i l l wa �
S e r v i c �
SPILLWAY
i �a r �d e s i g n e �f o rfrequen u!e i �c o nv e i n � b o t �n o r ma � a n �fl o o �r e l e a s e �f r o ��r e s e r v o i rt �t h � wa t e r c o u r s �d o wn s t r e a � f r o � �d a � "i#$u �t h �d a �o rd i k � . !ignifican %amage t
GATED SPI SPILLW LLWA AY
Morning Glory
STEPPED STEP PED SPILLW SPILLWA AY
Service spillways from top to bottom, Pineview Dam, Utah; Monticello Dam, California; and Upper Stillwater Dam,
Ai l l a r �
SPILLWAY
i �a r �d e s i g n e �f o rinfrequen u!e a n � ma �!u!ain limie% %amage wh e �u s e �. i �a r �u s e �i �c o mb i n a t i o �wi t �s e r v i c �s p i l l wa � a n �s o me t i me �a l s �wi t �fl o o �o u t l e t � . I �i �d e s i g n e �t �funci$n au$maically wh e �r e q u i r e � wi t h o u �a g g r a v a t i n � do wn s t r e a � fl o o d � .
GATED SERVICE SPILLWAY GATED AUXILLIARY SPILLWAY
OGEE SERVICE SPILLW SPILLWA AY
FUSE PLUG AUXILLIAR AUXIL LIARY Y SPILLWAY spillways
Auxiliary from top to bottom, Stewart Mountain Dam, Ariona; !ew "addell Dam, Ariona
E me r g e n c �
SPILLWAY
i �a r �d e s i g n e �t �p r o v i d � � re!er&e o f�da � 'r$eci$n again! $&er$''ing a n �a r �i n t e n de �f o ru!e un%er exreme s u c �a �mi s o p e r a t i o �o rma l n c t i o �o f c$n%ii$n! , �s e r v i c �s p i l l wa �o ro t h e re me r g e n c �c o n d i t i o n � .
GATED SERVICE SPILLWAY
#mer$ency spillway , %olsom Dam, California
GATED EMERGENCY SPILLWAY
2
Cl a s s i fi c a t i o n �b as e �o �CONTROL
o rg a t e �) s p i l l wa � C$nr$lle% ( o ru n g a t e � ) s p i l l wa � Unc$nr$lle% (
Co n t r o l l e �
SPILLWAY
e s �s p i l l wa � ena(le !$rage $ (e �c o n t r o l l i n �wa t e rl e v e l � . maximi)e% b Ge n e r a l l �m$re c$m'lex a n �m$re c$!ly t �b u i l �a n �ma i n t a i �t h a �u n c o n t r o l l e �s p i l l wa � . I � s h o u l �b �b a c k e �u pb �a i l i a r �s p i l l wa �a �t h � g a t e � ma �b �s u b j e c �t �a u t o ma t i �o p e r a t i o � ma l n c t i o � , h u ma �e r r o ra n �d e b r i �l o c k a g �
Un c o n t r o l l e �
SPILLWAY
M$! c$mm$nly u!e% a !mall %am!
b e c a u s �o ft h e i rr e l i a b i l i t � ,s i mp l i c i t �a n �a b i l i t �t � p as �d e b r i �a n �t �r e d u c �t h �ma g n i t u d �o f i n c o mi n � fl o o �p e a k � , a �we l � a �b e i n �c#ea'er t �b u i l �a n � ma i n t a i � .
3
Ogee s p i l l wa � o ro p e �c h a n n e � o rt r o u g � ) s p i l l wa � C#ue ( p i l l wa � Si%e*c#annel s o rmo r n i n �g l o r � ) s p i l l wa � S#af ( p i l l wa � Si'#$n s C$n%ui ( o rt u n n e � ) s p i l l wa � p i l l wa � Ca!ca%e s
Cl a s s i fi c a t i o n �b as e �o �PROMINENT FEATURE
Tye �o f
SPILLWAYS O,EE
C+UTE
SI-E*C+ANNEL
S+AFT
SIP+ON
TUNNEL
Og e �
SPILLWAY
Ogee spillways are also called Overflow spillways This type of spillway allows the passage of the flood wave over its S-shaped crest. Can be classified under controlled or uncontrolled. Widely used on Gravity dams, Arch dams, and Buttress Dams.
EM.AN/MENT -AM
Ogee spillways from top to bottom, Fier Dam, Pankshin; and Takato Dam, Nagano
UN,ATE0 ,ATE-
,RA1ITY -AM
Og e �
SPILLWAY
C h u t �
SPILLWAY
Chute spillways are common and basic in design as they transfer excess water from behind the dam down a smooth decline into the river below. The spillway’s slope and it’s side are lined with concrete. In case of having sufficient stiff foundation conditions at the spillway location, a chute spillway may be used instead of overflow spillway due to economic consideration
ROC/*FILL -AM
UN,ATEC+UTE SPILLWAY
0 ,ATE- C+UTE 2 UN,ATE-
ROC/*FILL -AM
Chute spillway from top to bottom, Mohale Dam, fri!a; and Pantabangan Dam, Philippines
S i d �Ch a n n e �
SPILLWAY
If a sufficient crest length is not available for an overflow or chute spillway in narrow valleys, excess water is removed from the reservoir through a side channel spillway. The side channel through which water is discharged can also be lined with concrete to prevent erosion and subsequent sedimentation in dams on the course of the river.
,RA1ITY -AM
SI-E C+ANNEL SPILLWAY
S h a
SPILLWAY
It discharges excess water from a reservoir through a shaft that is constructed near the crest of the am with height less than that of the crest. The shaft spillway is constructed when the other types of spillways cannot be constructed due to a lac! of space. When the shaft is completely submerged, further increased in head will not result in appreciable increase in discharge. It is not suitable for large capacity and deep reservoirs because of stability problems.
MORNIN, ,LORY
ARC+ -AM
Monti!ello Dam, California
S i p h o �
SPILLWAY
" siphon spillway is similar to a shaft spillway but instead is incorporated into the dam The presence of a siphon spillway wea!ens a dam at certain points, so the dam has to be reinforced at these wea! points incurring extra cost. #aintenance of this spillway is very difficult $iphon spillways comprise usually of five components which include an inlet, an upper leg, a throat or control section, a lower leg and an outlet.
Co n d u i �
SPILLWAY
Conduit spillway or tunnel spillway is the one in which a closed channel is used to convey the discharge around or under a dam. The closed channel may be in the form of a vertical or inclined shaft, a hori%ontal tunnel through earth dam or a conduit constructed with open cut and bac!filled with earth materials. These spillway are designed to flow partly full. To ensure free flow in the tunnel, the ratio of flow area to the total tunnel area is often limited to &'( and air vents are provided at critical points along the tunnel or conduit to ensure an adequate air supply which will avoid unsteady flow through the spillway
Ov e r fl o w
STRUCTURE
epending on the site conditions and hydraulic particularities an overflow structure can be of various designs) " Frontal overflow, " Side-channel overflow, and " Shaft overflow. *ther types of structures such s labyrinth spillway use a frontal overflow but with a crest consisting of successive triangles or trape%oids in plan view. $till another type is the orifice spillway in the arch dam.
ain !ypes of Overflow Structures
Frontal Overflow
Side Overflow
Shaft Overflow
Ov e r fl o w
STRUCTURE
fr$nal $&erfl$"
" The frontal type of overflow is a standard overflow structure, both due to simplicity and direct connection of reservoir to tailwater. It can normally be used in both arch and gravity dams. " The frontal overflow can easily "e e#tended with gates and piers to regulate the reservoir level, and to improve the approach flow to spillway. " +ated overflows of - m gate height and more have been constructed, with a capacity of -- m /s per unit width. $uch overflows are thus suited for medium and large dams, with large floods to be conveyed to the tailwater. " 0articular attention has to be paid to cavitation due to immense heads that may generate pressure below the vapor pressure in the crest domain.
F r o n t a �
O1ERFLOW
$rest Shapes *verflow structures of different shapes are) 1. $traight 2standard3 .
Curved 0lan view
.
0olygonal
4.
5abyrinth
The labyrinth structure has an increased overflow capacity with respect to the width of the structure.
%a"yrinth spillway
S t a n d a r �
CREST S+APES
" When the flow over a structure involves curved streamlines with the origin of curvature below the flow, the gravity component of a fluid element is reduced by the centrifugal force. If the curvature is sufficiently large, the internal pressure may drop below the atmospheric pressure and even attain values below the vapor pressure for large structures. Then cavitation may occur with a potential cavitation damage. "s discussed, the overflow structure is very important for the dam safety. Therefore, such conditions are unacceptable. 6or medium and large overflow structures, the crest is shaped so as to conform the lower surface of the nappe from a sharp7crested weir. weir.
" "
"
The transverse section of an overflow structure may be rectangular, rectangular, trape%oidal, or triangular. triangular. In order to have a symmetric downstream flow, and to accommodate gates, the rectangular cross section is used almost throughout.
1.
The longitudinal section of the overflow can be made either8 9road7crested.
. .
Circular crested, or $tandar dard cr crest shape ape 2oge ogee7t e7type3 pe3
Broad $rested
$ircular $rested
Ogee $rested
" 6or heads larger than m, the standard overflow shape should be used. " "lthough its cost is higher than the other crest shapes, advantages result both in capacity and safety against cavitation damage.
Fig. 1
$rest Shape of Overflow Spillways The lower surface of a nappe from a sharp7crested weir is a function of 1. the head on the weir, . the slope or inclination of the weir surface, . the height of the crest, which influences approach velocity. *n the crest shape based on a design head :, when the actual
head is less than &D, the tra;ectory of the nappe falls below the crest profile, creating positive pressures on the crest, thereby reducing the discharge. *n the other hand, with a higher than design head, the nappe7 tra;ectory is higher than crest, which creates negative pressure poc!ets and results in increased discharge. "ccordingly, it is considered desirable to underdesign the crest shape of a high overflow spillway for a design head :, less than the head on the crest corresponding to the
:owever, with too much negative pressure, cavitation may occur. The <.$. 9ureau of =eclamation 21>??3 recommendation has been that :e/: should not exceed 1.. The Corps of @ngineers 2C*@3 has accordingly recommended that a spillway crest be designed so that the maximum expected head will result in an average pressure on the crest no lower than 27 4.'-m3 of water head 2<.$. epartment of "rmy, 1>?A3. 0ressures of 274.'-m3 can be approximated by the following equations 2=eese and #aynord, 1>?&3. 6or :e, : B 1- m, &D ' (.)*&e+. 2without piers3 &D ' (.*&e+. 2with piers3 6or :e, : 1- m, &D '(.(&e 2without piers3 &D ' (.)&e 2with piers3 "nother empirical equation given for the maximum head on the crest
The <.$. 9ureau of =eclamation described the complete shape of the lower nappe by separating it into two quadrants, one upstream and one downstream from the crest 2apex3, as shown in previous figure. The equation for the downstream
1uadrant is expressed as,
Eq. 1 Where : D esign head excluding the velocity approach head. x, y D Coordinates of the crest profile, with the origin at the highest point 2*3 E D Constant that depends on upstream inclination and approach velocity. Constant E can be varied from .-- for a deep approach to .- for a very shallow approach
In a high7overflow section, the crest profile merges with the straight downstream section of slope F, as shown in 6ig. 213 2i.e., dy/dx D F3. ifferentiation of @q. 213 and expressing that in terms of x yield the distance to the position of downstream tangent as follows)
Eq. where xD! D :ori%ontal distance from the apex to the downstream tangent point F D $lope of the downstream face.
Fig. $oordinate coefficients for spillway crest 23.S. Department of the Army, +4/5
The discharge efficiency of a spillway is highly dependent on the curvature of the crest immediately upstream of the apex. To fit a single equation to the upstream quadrant had proven more difficult. "n ellipse, of which both the ma;or and minor axes vary systematically with the depth of approach, can closely approximate the lower nappe surfaces. With respect to origin at the apex, the equation of the elliptical shape for upstream
1uadrant is expressed as, Eq. !
where x D :ori%ontal coordinate, positive to the right y D Gertical coordinate, positive downward ", 9 D *ne7half of the ellipse axes, as given in 6ig. 2.b and c3 for various values of approach depth and design head.
6or a inclined upstream face of slope 6$, the point of tangency with elliptical shape can be determined by the following equation.
Eq. "
De s i g �Di s c h a r g �o f
SPILLWAY
" "
The lifespan of a dam is of the order of 1-- years. The design discharge may be related to the maximum flood discharge that may occur within this period. " 0robability of occurrence of a discharge that can seriously damage the system should be minimum. "s an example 2depending on pro;ect si%e and country regulations3 H1-- 6or optimum flow conditions observed. H1--- 6or some adverse flow conditions may be tolerated, but there should be no damage. H1---- 6or minor damage may be tolerated but system should not fail. " "nother approach is based on the concept of the possible maximum flood 20#63. "ccordingly, a rainfall7runoff model with the most extreme combination of basic parameters is chosen, and no return
Discharge $haracteristics
"
The discharge over an ungated ogee crest is given by the formula)
Eq. # Where) " HDdischarge, " CDdischarge coefficient, " 5Deffective length of crest, " :eDtotal head on the crest, including the velocity of approach head, h a. The discharge coefficient, C, is influenced by a number of factors) 1. The depth of approach, . =elation of actual crest shape to the ideal nappe shape, .
Fig. !
Coefficient of discharge for ogee crests with vertical faces 2=oberson, Cassidy, Chaudhry, 1>>?3
Fig. "
Coefficient of discharge for ogee crests with vertical faces 2=oberson, Cassidy, Chaudhry, 1>>?3
Overflow Gates6
" The overflow structure has a hydraulic behavior that the discharge increases significantly with the head on the overflow crest.. " The height of the overflow is usually a small portion of the dam height. " 6urther, gates may be positioned on the crest for overflow regulationJ. " uring the floods, if the reservoir is full, the gates are completely open to promote the overflow. " " large number of reservoirs with a relatively small design discharges are ungated.
" Currently most large dams are equipped with gates to allow for a flexible operation. " The cost of the gates increases mainly the magnitude of the flood, i.e.) with the overflow area. " Improper operation and malfunction of the gates is the ma;or concern which may lead to serious overtopping of the dam. " In order to inhibit floods in the tailwater, gates are to moved according to gate regulation. " +ates should be chec!ed against vibrations.
!he Advantages and Disadvantages of Gates !he advantages of gates at overflow structure are6 # Gariation of reservoir level, # 6lood control, # 9enefit from higher storage level.
!he disadvantages are6 # 0otential danger of malfunction, # "dditional cost, and maintenance.
epending on the si%e of the dam and its location, one would prefer the gates for) # 5arge dams, # 5arge floods, and # @asy access for gate operation.
Three types of gates are currently favored) " &inged flap gates, " 7ertical lift gates, " 8adial gates.
Flap Gate
7ertical Gate
8adial Gate
VERTICAL LIFT GATE
RADIAL GATE
FLAP GATES
The flaps are used for a small head of some meters, and may span over a considerable length. The vertical gate can be very high but requires substantial slots, a heavy lifting device, and unappealing superstructure. The radial gates are most frequently used for medium or large overflow structures because of " their simple construction, " the modest force required for operation and " absence of gate slots. They may be up to -m K -m, or also 1 m high and 4- m wide. The radial gate is limited by the strength of the trunnion bearings.
" The ris! of gate ;amming in seismic sites is relatively small, if setting the gate inside a stiff one7piece frame. " 6or safety reasons, there should be a number of moderately si%ed gates rather than a few large gates. " For the overflow design, it is customary to assume that the largest gate is out of operation. " The regulation is ensured by hoist or by hydraulic ;ac!s driven by electric motors. " $tand7by diesel7electric generators should be provided if power failures are li!ely.
FLAS$%OARD
NEEDLES
ROLLING GATES
RU%%ER DAM
*verflow spillways frequently use undershot radial gates for releases over the dam. The governing equation for gated
flows, Eq. &
Where C is a coefficient of discharge, and :1 and : are total heads to the bottom and top of the gate opening. The coefficient C is a function of geometry and the ratio d/:1, where d is the gate aperture. 6ig. 2&.3.
Fig. #
Crest piers and abutments cause contraction of the flow, reduction in the effective length of the crest, and cause reduction in discharge.
where 5 D @ffective length of the crest for calculating discharge 5 ’ D Let length of the crest L D number of piers Ep D 0ier contraction coefficient Ea D "butment contraction coefficient :e D Total head on the crest
Eq. &.1
9ier and A"utment effects The nose of piers and abutments should be rounded sufficiently to minimi%e the hydraulic disturbance. 9iers may extend downstream on the chute as a dividing wall in order to suppress shoc! waves. A"utments are extended towards the reservoir to facilitate gentle flow conditions at the entrance of spillway. 0iers on overflow structures are provided) " to improve approach flow conditions " to mount overflow gates " to divide the spillway into subchannels " to aerate the chute flow at the pier ends
P
Pier
D Divide "all
A
Abutment
I' #&ective %low width 'etween Piers
:FF:$!;7: $rest %ength
Fig.
Eq. 1
(
Eq. "
Eq. !
Fig. "
Su"merged Discharge on Overflow Spillways The coefficient of discharge decreases under the condition of submergence. $ubmergence can result from either excessive tailwater depth or changed crest profile. The effect of tailwater submergence on the coefficient of discharge depends upon the degree of submergence defined by hd<&e and the downstream apron position, 2hd=d5<&e shown in 6ig. 2A3. 6or a value of 2hd=d5<&e up to approximately , the reduction in the coefficient depends on the factor 2hdMd3/:e and is independent of hd<&e as shown in 6ig. 2A.a3, i.e., it is sub;ect to apron effects only.
Su"merged Discharge on Overflow Spillways
Fig. / 8eduction of discharge coefficient for su"merged spillway6 a5 apron effects, when 2hd=d5<&e> 0
Su"merged Discharge on Overflow Spillways When 2hdMd3/:e is above ', the reduction depends only on hd/:e as shown in 6ig. 2A.b3, i.e., tailwater effects control. 6or 2hdMd3/:e between and ', the reduction of the coefficient depends on both factors, given in 6ig. 2A.c3. The effect on the discharge due to crest geometry is not well defined. #odel studies are the best way to determine the coefficient.
Figure/. 2c5 8eduction of discharge coefficient for su"merged
(
$%&'
(%($&) * (%+
%i$)
(
0 (%(-.) * (%+
(%+
23456 m
�Sp i l l wa � -YNAMIC FORCE o When water flows over the curved surface of ogee spillway there is continuous change of velocity, and hence, there is change in momentum from section to section. "ccording to LewtonNs second law of motion, this change in momentum causes a force on the spillway structure. this force is !nown as the dynamic force. Consider an element of water between two sections " and 9 on a curved surface. The resultant of the forces on the element of water is given by where O D mass density of water D w/g H D discharge
�Sp i l l wa � -YNAMIC FORCE o =ewriting eq. in hori%ontal and vertical directions separately, we get
#*) +a #*) +bfree The force 6: and 6G are those acting on a significant body of fluid and include gravity forces, hydrostatic pressure and the reaction of any ob;ect in contact with water.
Fig. * +(, -o/- (n og'' -0ill/(y /i 2i-(rg'/(3'r /i3 ( '(2 o4 1. 5 o6'r 3' r'-3. T(7ing 3' o'8i'n3 o4 2i-(rg' (- .9 o50:3' 3' 2yn(5i 4or' on 3' :r6'2 -'3ion A% /i (- ( on-3(n3 r(2i:- o4 ! 5.
$olution. The discharge over the spillway is given by, H D C5:/ or qDH/5 D C:/ D . 21.3/ D.> cumecs/m 5et d1 and d be the depth of sheet of water at " and 9 and v 1 and v be the velocities. "ssuming that there is no loss of energy and neglecting approach velocity, we may apply 9ernoulliNs theorem at u/s water surface, and at sections " and 9. Thus we get 11. D 1.' M d1 cos A- M v1/g D d M v/g PP213 9ut G1d1 D q D .> D G d
:ence G1 D .>/d1 and G D .>/d $ubstituting these in 213 and solving these by trial and error, we get, d1 D -.1 m and d D -.1>& m :ence G1 D 1.& m/sec and G D 14.& m/sec 6ig. A 2b3 shows the free diagram of the curved element sections " and 9 in which 6x and 6y represents components of force on the water by the curved section "9. If 61 and 6 are the resultant hydrostatic forces at section " and 9, we have
The weight W of the water body in the curved portion between sections " and 9 is
"pplying @q. & 2a3 for 1 m length of spillway, we get
6rom which 6x D 4 !g/m
/imilarly, from 01% 22%2+ 3b) we ha4e
From whi!h Fy * +2&& kg5m 6esultant for!e
* +7( kg per meter length
Side channels " $ide channels are often considered at sites where) " a narrow gorge does not allow sufficient width for the frontal overflow, " impact forces and scour are a problem in case of arch dams, " a dam spillway is not feasible, such as in the case of an earth dam, " when a different location at the dam site yields a simpler connection to the stilling basin. " $ide channels consist of a frontal type of overflow structure and a spillway with axis parallel to the overflow crest. " The specific discharge of overflow structure is normally limited to 1- m /s/m, but for lengths of over 1-- m. " The overflow head is limited to say m. Lot equipped with gates.
orning Glory " T' -(43 3y0' -0ill/(y (- 0ro6'2 3o ;' 'ono5i(l9 0ro6i2'2 3' 2i6'r-ion 3:nn'l (n ;' :-'2 (- ( 3(ilr('. T' 5(in 'l'5'n3- (r'< " T' in3(7'9 " T' 6'r3i(l -(43 /i3 ( ;'n29 " T' (l5o-3 ori=on3(l -0ill/(y 3:nn'l9 (n29 " En'rgy 2i--i0(3or. " Air ;y ('r(3ion on2:i3- i- 0ro6i2'2 in or2'r 3o 0r'6'n3 (6i3(3ion. " Al-o9 3o (o:n3 4or >oo2 -(4'3y9 only non? -:;5'rg'2 >o/ i- (llo/'2 -: 3(3 4r'' -:r4(' >o/ o:r- (long 3' 'n3ir' -3r:3:r'9 4ro5 3' in3(7' 3o 3' 2i--i0(3or. " U-'2 4or 2(5- /i3 -5(ll 3o 5'2i:5 2'-ign
orning Glory *verfall is advantageous when) " seismic action is small, " the hori%ontal spillway may be connected to the existing diversion channel, " floating debris is insignificant, " space for the overflow structure is limited, " geologic conditions are excellent against settlement, and Q 5ocation of the #orning +lory " The inta!e is prone to rotational approach flow, which should be inhibited with a selected location of the shaft relative to the reservoir topography and the dam axis. " The radial flow may be improved with piers positioned on overfall crest.
$rest shape " The shape of the #orning +lory overfall is a logical extension of the standard overfall crest. @xperiments were performed on circular sharp crested weir.
" "ll quantities referring to the weir are over barred. " The overflow head relative to the sharp crest is R and the (coordinate system 2R , R 3 is located at the weir crest.
Discharge Q The discharge over a #orning +lory overfall structure is in analogy with the straight7crested overfall
for the range of
Q "n initial value of : or = may be assumed for a fixed :/= ratio to start the computations. Q $haft radius =s can be determined from =s D 1 M -.1= 2in meters3
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