dam outlet works with design of spillways and energy dissipator designs

Design of ogee spillway

Offsets and Risers on Upstream Face :

13/2/14 Prepared by v.h,khokhani, assistant

professor, DIET 2

Offsets and Risers on Upstream Face


professor, DIET 3

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


professor, DIET 4

(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


professor, DIET 5

Step-1 : Computation of design head

Hd = He + Ha

Where Ha = Va2/2g


professor, DIET 6

If h/Hd is greater than 1.7 than high spillway so effect of velocity is neglected

Its high spillway Ha = 0


professor, DIET 8


professor, DIET 9

d/s profile


professor, DIET 10


professor, DIET 11

The co-ordinates from x = 0 to x = 27.4 m are worked out in the table below :

u/s profile :

This curve will extend up to,

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.

ENERGY DISSIPATORS

17/2/2014 1 PREPARED BY V.H.

KHOKHANI,ASSISTANT PROFESSOR, DIET.

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.

17/2/2014 PREPARED BY V.H.


2

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.



3



4

Commonly measures adopted

1. By developing a hydraulic jump

2. By using different types of buckets



5

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.



6



7

High velocity water with low depth strikes low velocity water with high depth

Rise in water



8

y1

y2

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



9

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



10

There is five condition governs type of energy

dissipator



11

When JHC =TWRC or both coinside



12



13

This is the stable condition

Jump will ocurrs at toe of dam

Simple horizontal filter is provided with small rise



14



15



16

Provide ski jump

Provide sloping apron below bed

Provide subsidary dam with baffle wall



17



18



19



20



21

Provide sloping apron such that jump will ocurrs on apron

Roller bucket



22



23

Sloping apron with stilling basin

Sloping apron partially above partially below river bed



24

17/2/2014 1 PREPARED BY V.H.


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

Arrangements


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

Arrangements


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

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

Arrangements


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

13/2/14 1 Prepared by v.h,khokhani, assistant

professor, DIET

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


professor, DIET

2

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.


professor, DIET

3

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.


professor, DIET

4

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,


professor, DIET

5

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.


professor, DIET

6

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.


professor, DIET

7

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,


professor, DIET

8

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.


professor, DIET

9


professor, DIET

10

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.


professor, DIET

11

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.


professor, DIET

12

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,


professor, DIET

13

Overflow Spillway


professor, DIET

Chute Spillway


professor, DIET

Side-Channel Spillway

Burrinjuck Dam on the Murrumbidgee River near Yass. 13/2/14 16 Prepared by v.h,khokhani, assistant

professor, DIET

Shaft Spillway(s)


professor, DIET

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.


professor, DIET

When actual head is greater than design head

or increase in flow / velocity of flow


professor, DIET

19


professor, DIET

20

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


professor, DIET

21

at its Waterways Experiment Station (WES) at Vicksberg. Such shapes are known

as "WES standard spillway shapes".


professor, DIET

22

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


professor, DIET

23


professor, DIET

24

THUS FOR VERTICAL FACE


professor, DIET

25

Different upstream curves were given by

WES for different slopes, as


professor, DIET

26

UPSTREAM profile of the crest :

When u/s face vertical :


professor, DIET

27


professor, DIET

28

dam outlet works with design of spillways and energy dissipator designs

Documents