Hydraulic Jump1
Hydraulic Jump1
Hydraulic Jump1
GATES
.1, Introduction
y Dissipator Arrangement
.4 Types of Energy Dissipators
11.4.1 Bucket Type Energy Dissipators
11.4.2 Ilydraulic Jump Stilling Basins
.5 Hydraulic Jump
45
this unit you will be familiar with
the various types of energy dissipators,
/ dropping shutters. .
11-2 DEFINITION
The water discharged over the spillway ot a dam usually falls with a great force through a
height equal m the difference of headwater and tail water elevation. nu?fallng water
acquires a velacity which is gnerally much higher than the natural stream veloc~tyat the
given site. This upsets the equilibrium of tbe stream by the w m n b a t i o n of excessively
high veliocitks and abmmal undergmmd pressure gradients and presenting a pixs~ble
ty f w stxiow erosion. Devices employed to protect the dam tiom damage and to
uMutrnl p m b m d sericws emsicw xe called energy diss~pators.
SAQ 1
How ~ v o u l dyou dd'ino energy diusipatorc?
SAQ 2
Why rlo you ueed an cncrgy tlissip;rtc>rarrangement tlown~lrs;r!~~
of :I \l?~llway')
3 SolklR&rBucke%s
A ~ k t e t i s u s e d w h a t & e P a i l w a t e r c l eisgeaterlhsn
~ 1.1 t i m s the required
conjugate depth Eor the f o r m a t b of the hydraulic jump and the river bed rock IS pwd. The
lip csC the bucket difec'ts upward the high~wlwitytlow and thus creates a high hod
boilm;ryhe~(eb
e m causing objectionable eddy currents which may adversely affect the river Energy Dissipaters Pod
Gates
igure 11.1(a) shows a solid bucket and Figure 11.1(b) lhe solid bucket in action.
Figure 11.1
STANDING W E
FSgure 112
RESERVOIR
WATER SURFACE
where 8 = lip angle (angle between the tangent to the curve of the bucket at the lip and the .
horizon),
E = specific energy, and
KI = a value of 0.85 to compensate for loss of energy and velocity reduction '
due to the effect of the air resistance, internal turbulence and disintegration
of the jet.
The horizontal range of the jet at the level of the lip (i.e. y = 0) is given by
~ = K1
4 E tan8cos28
= 2 K1 Esin 28 ...(11.2)
The maximum value of x will be 2K1E when 8 = 43'. The lip angle depends on the radius
of the bucket and the height of the lip above the bucket invert. It usually varies from 20' to
45O. The bucket radius should be large enough to ensure a concentric flow along the bucket.
SiiQ 3
1) What u e the di,fferenttypes of energy dissipators provided downstream of a
spillway'?
ii) %%atis a solid roller bucket?
iii) What is a slotted roller bucket?
iv) What is a ski-jump bucket?
11.5 HYDRAULIC JUMP Energy Dissipptorsurd I
Gabs
I
t rise in water surface which may occur in an open channel
ity is retarded. The flow is supercritical before the jump
jump. The formula for the hydraulic:jump is obtained by equating
ctuig to retard the mass of flow to the rate of change of the
eneral formula for this relqtionship is :
1 vi = the velocity before the jump, also called the supercritical velocity.
II v2 = the velocity after the jump and also called the subuitical velocity,
a2 = the areas before and after the jump, respectively, and
d Y2 = the corresponding depths from the water surface to the centre of gravity of
the cross section.
Thh keneral formula expressed in terms of discharge is:
= flow depths before and after the jump, respectively. They are also called
st-jump depths, and conjugate depths.
I dl
d2 = - - 2+ d [ 2 4 d l / g + & / 4 ]
aulic jump is defined as the distance from the front of the jump to a
the flow immediately downstream of the roller associated with the
important design parameter, the length of a hydraulic jump, L,,
theoretical considerations. The length of a hydraulic jump is
five times the height of the jump itseIf which is (d2- d l ) .The ratio
e upstream supercritical Froude number, and is given by
Control Structures SAQ 4
i) What is a hydraulic jump?
iii) How c:ul you determine the length of a hydraulic jump hlowinp t l ~ eFroudz's
number and the coiljugate depths?
For Froude numbers between 2.5 and 4.5, an oscillating form of jump occurs, with the
entering jet intermittently flowing near the bottom and then along the surface of the
downstream channel. This oscillating flow causes objectionable surface waves which cany
considerably beyond the end of the basin. The action represented through this range of flows
is designated as Form B (Figure 11.4).
For the range of Froude numbers for the incoming flow between 4.5 and 9, a stable and
well &fined jump occurs. Turbulence is confined to the main body of the jump, and the
water surface downstream is comparatively smooth. As the Frou& number increases above
9, the turbulence within the jump and the surface roller becomes increasingly active,
resulting in a rough water surface with strong surface waves downstream from the jump.
action for the range of Froude numbers between 4.5 and 9 is designated as FO& C Energy Dissipaters d
Gates
11.4)>and that above 9 is designated as Form D.
g basins are designed so as to be suitable to provide stilling action for the various
of l~ydraulicjump which are designated and described as b-1 c ow:
1.6.2 Stilling Basins for Froude Numbers between 1.7 and 2.5
low phenomena for basins where the iricoming factors art:in the Froude number range
en 1.7 and 2.5 will be in the Form A. Since such flows are attended by active
ence, baffles or sills are not required. The basin should be sufficiently long to contain
nd basin lengths shown
Figure 11.5, will provide acceptable basins. The figure shows the properties of hydraulic
mps in relation to the Froude number.
LENGTH OF JUMP
11.6.3 Stilling Basins for Froude Numbers between 2.5 and 4.5
Jump phenomena where the incoming flow factors are in Froude number range between 2.5
and 4.5 are designated as the transition flow stage, since a true hydraulic jump does not fully
satisfactory dissipation, since the attendant wave action ordinarily cannot be colltrolled by
&heusual basin devices. Waves generated by the flow phenomena will persist beyond the
e,ild of the hastn and must often be dampened by means apart from.the basin.
Where a stilling basin must he provided to dissipate flows for this range of Froude number,
i.e. 2.5 to 4.5, the basin shown in Figure 11.6, which is designated as Tjqx I basin. has
proved to be relatively efkctive for dissipating the bulk of the energy of flow. However, the
wave aciion propagated by the oscillatil~gflow cannot be entirely dampened. Auxiliary
wave dampeners cx wave supressors must sometimes be employed to provide smooth
surface flow downstream.
-
, C I A m emcs
'4r-WU(.TWlH WOW dl
--SPACE 2-5W
w~aaorrenw
F
F llr~:S(B~~CS~~~~~fwFradcNrmbasbctnwo~adU
Brxauseoftktentlencyofthe~tosweepout andas a n ~ i n ~ i O g w a action, v e
the wabzr depths in the basin sbouId he h t 10percent greater than t
k computed
-jum@ *@"-
Ofh the Deed for utilizing this type of basin in design can he avoided by sekdng srilliog
basinCtimensionr whicb will provide flow mdilims whicb fall clrulskk the ranp of
t l a m s i flow.
~
11.6.M Stilling Basins for Froude Numbers Higher than 4.5
re the Froude number is higher than 4.5, a true hydraulic jump will form. The
jump vary according to the Froude number as shown In Figure 11.5. The
essory devices such as blocks, baffles, and sills along the f l m of the
uce a stabilizing effect on the jump, which allows shortening the basin and
factor of safety against sweep-out due to inadequate tailwater depth.
nK basin shown in Figure 11.7, where incoming velocity does not exceed 15 m per second,
which is designated as a II basin, can be adopted where incoming velocities do not
exceed 15 mls. Tbis basin utilizes chute blocks, impact baffle Mocks, and an end sill to
shotten the jump length and to dissipate the high-velocity flow within the shortened basin
length. l b i s basin relies an dissipation of energy by the impact blocks and also on the
turbulence of t Mjump phenomena for its effectiveness. Because of the large impact forces
to which the baffles are subjec@d by the impingement of high incoming velocities and
because of the possibility of cavitation along the surfaces of the blocks and the floor,the use
of this basin must be limited to heads where the velocity does not exceed 15rnls.
The added loads placed upon the SlNCture flm by the dynamic force brought against the
upstream face of the baffle Mocks must be taken into account. This dynamic force will
C o d Structures The force (kg) may be expressed by the fennula:
F = 2wA(dl +k,l)
where,
w = unit weight of water (kg/cunl),
A = the area of the upsueam face ofthe block (rn"), and
(dl+ h,,) = the specific energy of the flow entering the hasir] (m).
Negative pressure on the back face of the blocks will further increase the total 1i)acl.
However, since the baffle bltxks are placed at a distance equal to 0.8& beyr>ndthe start of
the junq, there will be some cushioni~igeffect by the tilnc the incoming jet reaches the
bltxks and the force will be less than that indicated by the above equation. If the lull force
computed by Eq.( 11.11) is used, the negatjve pressure force can be neglected.
Where ~ncorningvelocities exceed 15 m/s, or where the Impact bafne blocks are noi
provided, the basin designated as Type I11 (Figure 11.8) can he adopted. Since the
dissipation is achieved primarily by hydraulic jump action, the basin lerlgth wlll be greater
than that indicated for the Type I1 basin. However, rhe chute blocks and dentatcd end sill
will still be effective in reducing the length from that which would be necessary if they
were not used. Because of the reduced margin of safety against sweep-out, the water depth
in the basin should Se about 5 percent greater than the computed conlugate depth
'DENTATE0 SILL.,
\
Energy Dissipatom and
Gates
What are the different types of stilling basins?
be maintained upto
spillway crest to the extent necessary to dispose off that flood.The top level of the
rlonoverflow section and the acquisition of land for the reservoir liave to be detenniued by
the maximurn rise of floc~dabove the spillway crest. It would be advantageous to have
gates, so that while storage is conserved to the top of the gates, they can be removed frorn
the path of i flood so that the flood rise is relatively small. Gates can he provided on all
types of spillways except siphon spillways. The advantages of providing gates is either a
savi~lgin the height of the dam and the value of the property to be acyuired for a given
useful storage or more useful storage for the same height of dam. The disadvantages of
gates are additional initial cost and additional recurring expenditure in the maintenance and
operation of gates. The height of gates to he provided will be determined by the most
advantageous balance between these opposing factors. As a general rule it will always be
advantageous to provide gates on conservation or multipurpose reservoirs.
.'
gates. A radial gay comirts of sIY,pLte whxh hrm
SegmentOf a "'joder? the venicdelements being circular arcs,
.~ ,. ,.._,. .
Of stiffeners or nurlin~m ic o,,+n, L-
shafts or pins which are anchored in the piers and cany the thrust of the wilter bad
(Figure 11.9).The skin plate is concentric to the pln and h c e the resultant water pressure
passes through the centre of the pm creaking no moment against the lifting of the gate. The
hoisting load consists only ofthe weight dkhe gate, €he frWon between the side seals and
the piers and Ihe frictimd resistance at (he pins. Radial gates ate hoisted by chains or cables
attached to the upstream face neat the b c x n by a shad& and e x t d i n g to the drum of a
hoist overhead. The gate is often c m n m w M~partially ~ -awe the effect af
its weight, which further reduces the nquuedcrpecity d the hoist,
The small hoisting e f f d needed to operate rabid gsta makes hand operatian practical on
small installations which otherwise might require power. The small hoisting forces involved
also make Ute radial gate more adaptable to operaha by mhtively simple autamatic control
equipment. Where a number of gates are wed on a spiHway, they mi* be arranged to open
automatically at successively increasing reservoir levets, or o d y me cx fwo might be
eqipped with automatic controls, while the remaining gates would be whbh band or power
ItOistG.
For spans, which are not too large and for moderate heigh@radial gates are &ten the
simplest and least expellsive. They requite less head room than vertical lift gates.
(H o h W O U ~d~ e
gabs haye beenused in a size as \me a 41.5 m (L) x 0.5
\wp r*ui\u tmimd ia tire crest, th~s
type d gate i s not suitedm low
d by a framework of steel members
ded in piers at their two ends. The
secrionoftherequked
minimist? frictim and provide Ihe necessary be* mmgdt. If the gate
eavy frictionnl resistance wwld have to be counteracted
pressm-e.S!idbg gates me & e M m se- et&sed o~
crests. A t r a h r o f r o l f e r s a w h e e f s i s r m n r t r d i y ~ i d e e b d n e e n t h c ~ d *
so that the sliding friction is sdM&kd by r ow
~~are~sibleforrpwisimofthesenheels:
swCto~goovesba~
. lhis tl~mgemen(ek&ates(heaxfe
--astlle*Is~rhewtlerlosd
, t h e r e i s a o ~ a ~ a & e a x f c . A 6
els rotate on their radirs whereas (he gates mvel on (he nbeel diameter, the tram9 of
e almost tbe entire &ad wemt of (he gate. Ibe lifw frnoe is, therefore,reqsh.ed to
alame the frictiaaal resistamz only. Figtue 11.11 shows a vecticd lift gate in sedan.
Control Structu~w Vertical lift gates have k e n used for spans as wide as 15 m or more and tor heights of upto
15 m. At larger heights flie problem of head room becomes important as the operating
platform has to he located above the raised position of the gate. The bottom of the pate 111 its
lifted position must provide sufficient clearance from the high flood level to enable the
floating debrls to pass. To reduce the height of the operating platform, high gates must be
broken up into two or sorrletimes three sections, one above the other. Either they may he
placed in different grooves (Figure 1 1.12 (a)), each section overlapping the lower one to
some extent, or they [nay be placed m the same groove (Figure 11.12 (b)), the upper
sections k i n g taken out of the groove altogelher when the gate is fully raised.
B AND C SECTIONS
TAKEN OUT OF THE
SAQ 7
i) What are the various types of gates?
ii) What are radial gates and how are they operated?
iii) Where are drum gates provided and how are they operated'!
iv) Wliat is a vertical lift gate? How does it function'?
V) What are fixed wheel gates and Stoney wheel gates and where are they
provided'? How are they operated?
119 SUMMARY
Energy dissipators are required downstream of hydraulic structures to reduce the damage to
the foundation -
. . . .and the structure itself due.. to. the heavy ,...-
" .. . 4
energy content
---- 2
in the water
A -..,
falling
A---*%.-n
Energy Dissipaters and
Gates