Dam Outlet Works with Design of Spillways And Energy Dissipator designs .
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Transcript of Dam Outlet Works with Design of Spillways And Energy Dissipator designs .
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Design of ogee spillway
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Offsets and Risers on Upstream Face :
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Offsets and Risers on Upstream Face
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professor, DIET 3
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Design of spillway
Design an ogee spillway for concrete gravity dam, for the following data :
(1) Average river bed level = 100.0 m
(2) R.L. of spillway crest =1:04.0 m
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(3) Slope of d/s face of gravity dam = 0.7 H : 1 V
(4) Design discharge = 8000 cumecs
(5) Length of spillway = 6 spans with a clear width of 10 m each.
(6) Thickness of each pier = 2.5 m
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Step-1 : Computation of design head
Hd = He + Ha
Where Ha = Va2/2g
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If h/Hd is greater than 1.7 than high spillway so effect of velocity is neglected
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Its high spillway Ha = 0
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d/s profile
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The co-ordinates from x = 0 to x = 27.4 m are worked out in the table below :
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u/s profile :
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This curve will extend up to,
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Design of d/s bucket :
The radius of the bucket is generally kept equal to,
The bucket will subtend an angle of 60 at the centre.
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ENERGY DISSIPATORS
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Water flowing over a spillway acquires a lot of kinetic energy because of the conversio of the potential energy into kinetic energy.
If the water flowing with such a high velocity is discharged into the river it will scour the river bed.
If the scour is not properly controlled it may extend backward and may endanger the spillway and the dam.
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In order to protect the channel bed against scour, the kinetic energy of the water should be dissipated before it is discharged into the d/s channel.
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Commonly measures adopted
1. By developing a hydraulic jump
2. By using different types of buckets
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Hydraulic Jump : Hydraulic Jump is the sudden rise of water that takes place 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 (i.e. supercritical flow) strikes another stream of water moving with low velocity and high depth (i.e sub-critical flow), a sudden rise in the surface of water takes place. This phenomenon is called Hydraulic jump.
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High velocity water with low depth strikes low velocity water with high depth
Rise in water
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y1
y2
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For different discharges q, we can obtain different y2. if we plot graph of these q
and y2 it is known as jump height curve
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For different discharges q, we can obtain different y2 from actual observation of tail water depth. if we plot graph of these q and y2 it
is known as tail water rating curve
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There is five condition governs type of energy
dissipator
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When JHC =TWRC or both coinside
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This is the stable condition
Jump will ocurrs at toe of dam
Simple horizontal filter is provided with small rise
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Provide ski jump
Provide sloping apron below bed
Provide subsidary dam with baffle wall
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Provide sloping apron such that jump will ocurrs on apron
Roller bucket
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Sloping apron with stilling basin
Sloping apron partially above partially below river bed
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Arrangements
Height Width Length Location
spacings
Chutes (1)
2Y1 Y1
2Y1 At toe 2.5 Y1
Baffel (3) NA NA NA
NA NA
End sills (4)
1.2 to 3 y1 2 in 1 slope
Throughout spillway section
At the end of basin
NA
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Arrangements
Height Width Length Location
spacings
Chutes (1)
Y1 Y1
2Y1 At toe y1
Baffel (3) 1.5 to 3.3 Y1
0.75 h3 2.3 TO 2.8 y1
0.8 y2 from toe
0.75 h3
End sills (4)
1.2 to 3 y1 2 in 1 slope
Throughout spillway section
At the end of basin
NA
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Arrangements
Height Width Length Location
spacings
Chutes (1)
Y1 Y1
2Y1 At toe y1
Baffel (3) NA NA NA NA NA
End sills DENTATED END SILLS (4)
O.2 Y2 0.02 Y2 0.15 Y2 At the end of basin
NA0.15 Y2
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USBR TYPES
FR NO VELOCITY
PROVISIONS
I 2.5 TO 4.5 NA CHUTES END SILL
II MORE THAN 4.5
LESS THAN 15
CHUTES BAFFLE END SILL
III MORE THAN 4.5
MORE THAN 15
CHUTES DENTATED SILL
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Arrangements
Height Width Length Location
spacings
Chutes (1)
2Y1 Y1
2Y1 At toe 2.5 y1
Baffel/basin blocks (3)
0.8 to 3.3 Y1
0.02 y1 Height of blocks
0.8 y2 from toe
Height of blocks
Dentated sills (4)
0.20 y2 0.02 y2 Height of sills
At the end of basin
Height of sills
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Spillway is a structure constructed at or near the dam site to dispose of surplus water from the reservoir to the channel downstream.
Spillways are provided for all dams as a safety measure against overtopping and the consequent damages and failure
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A spillway act as a safety valve for the dam, because as soon as the water level in the reservoir rises above predetermined level, excess water is discharged safely to the downstream channel and the dam is not damaged.
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Essential requirements of a spillway :
1. The spillway must have sufficient capacity.
2. It must be hydraulically and structurally safe.
3. The surface of the spillway must be erosion resistant.
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4. It should be provided with some device for the dissipation of excess energy
5. The spillway must be so located that it provides safe disposal of water i.e. discharge must not erode d/s toe of the dam,
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Factors affecting spillway capacity :
1. inflow flood
2. available storage capacity
3. discharge capacity of other outlet works
4. whether the spillway is gated or ungated.
5. Possible damage if the capacity is exceeded.
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Location of a Spillway
A spillway may be located either within the body of the dam or at the end of the dam near abutment.
In some cases, the spillway is located away from the dam as an independent structure in a saddle or flank. the spillway can be best built independently of the dam.
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If a deep narrow gorge with steep banks, separated from a flank by a hillock with its level above the top of the dam, is available,
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Under such circumstances, a concrete dam or an earth dam can be constructed across the main valley and a spillway can be constructed independently into the saddle.
Sometimes, a concrete or masonry dam along with its spillway can be constructed in the main valley, while the flank or flanks are closed by earthen embankments.
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The top level of such an embankment is kept at maximum reservoir level (MRL). The material and design of these embankments are such that they fail as soon as water overtops their.
Hence, if by chance, either due to excessive flood above design flood or due to failure of gates of main spillway, etc. the water rises above the maximum reservoir level, it overtop such embankment, which at once fails, providing sufficient outlet for the dispose of excessive water.
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This type of secondary safety arrangement is generally provided for large dams especially on earth and rockfill dams, and is known as subsidiary spillway or emergency spillway or breaching section.
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For earthen dams, a separate independent spillway is generally preferred, although there is non-availability of spillway site, a concrete spillway is sometimes constructed with the dam or at one of the ends of an earth dam. If the main spillway is situated in a flank,
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Overflow Spillway
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Chute Spillway
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Side-Channel Spillway
Burrinjuck Dam on the Murrumbidgee River near Yass. 13/2/14 16 Prepared by v.h,khokhani, assistant
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Shaft Spillway(s)
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SPILLWAYS
Major Damage Caused By:
Cavitation
when water breaks contact with the spillway surface at
high velocities, reduced pressures cause the formation
of cavities filled with vapor, air, and other gases in the
water; when this cavity reaches a point where the absolute
pressure is much higher, an implosion occurs. Extremely
high pressure result from the collapse of this cavity, which
result in damage to spillway structure (Pitting: the sponge-
like appearance of spillway surface)
*Smoother, properly designed ramps prevent water leaving
spillway surface, which reduces cavitation.
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When actual head is greater than design head
or increase in flow / velocity of flow
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Extensive experiments were conducted by U.S. Bureau of Reclamation (U.S.B.R.) for obtain the nappe-shaped profiles for the crests of the overflow spillways with their u/s face either vertical or inclined.
On the basis of the U.S.B.R. data, the U.S. Army Corps of Engineers has developed several standard shapes of the crests of overflow spillways
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at its Waterways Experiment Station (WES) at Vicksberg. Such shapes are known
as "WES standard spillway shapes".
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1. Down stream profile : [when u/s face vertical]
x, y = co-ordinates of the points on the crest profile with the origin at the highest point C of the crest, called the apex.
Hd = design head excluding the velocity head
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THUS FOR VERTICAL FACE
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Different upstream curves were given by
WES for different slopes, as
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UPSTREAM profile of the crest :
When u/s face vertical :
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