design of spillway

Bhongle Vishal 241023Bhuse Anil 241025Dighewar Rajesh 241044Jagadale Nilesh 241059

INTRODUCTIONA structure constructed to discharge the

surplus water from a reservoir into the river, on the downstream side without endangering the safety of dam is known as SPILLWAY.

It is also known as Waste Weir or Surplusing Structures

Thus spillway essentially acts as safety valve of dam.

Component parts of spillwayApproach Channel

Control Structure

Energy Dissipater

Tail Channel

APPROACH CHANNELCarry water from the

reservoir to the control structure.

It may be shallow or deep, depending upon the topography.

The width of the approach channel should be adequate to have uniform distribution of water.

The approach should be at right angle to the spillway weir for a sufficient length

CONTROL STRUCTUREthe major component of a spillway as it

regulates and controls the discharge.It generally consists of an ogee weir with or

without gates. the gates can also be operated manually or

electricalyA road bridge is also designed over the weir

supported by piers and abutments.

ENERGY DISSIPATOREnergy dissipation arrangement at the toe

of the dam weir is indirectly a must.The type of the energy dissipation

arrangement depends upon1. The discharge,2. The geological conditions at the site,3. The magnitude of the energy to be

dissipated,4. Tail water rating curve

THE TAIL CHANNELThe tail channel conveys the flood discharge from

the energy dissipater to the main river channel.

The tail channel design should be such that there is neither silting nor scouring.

The tail channel is required to be kept straight for some distance so that the performance of the energy dissipater will be satisfactory.

The junction of the tail channel with the main river should not be at right angles and should be at as small angle as possible.

Special types of spillway

Side Channel SpillwayChute or Trough SpillwaySiphon SpillwayConduit SpillwayShaft SpillwayBreaching Section

SIDE CHANNEL SPILLWAY

In a narrow valley power house ungated spillway

weirparallel to the river

discharg. trapezoidal in cross-

sectionopen channel

Chute or Trough SpillwayThe spillway at flank.

Heavy rock excavation at higher level.

Excavation not economical or feasible.

Steeply sloping channel.

SIPHON SPILLWAY

closed conduit in a shape of inverted U tube.

Advantages – It can discharge with full

capacity.The operation is

automatic. It has no moving parts or

mechanical devices,

SIPHON SPILLWAY

Disadvantages – It cannot pass ice or

debris.possibility of clogging the

siphon conduit.water freezes, siphon

action will not start.Energy dissipation

arrangement is difficult to design.

CONDUIT SPILLWAY

water through reservoir by closed channel.

no symphonic action.

river gorge is very narrow

SHAFT SPILLWAY

Four elements 1. A circular overflow

control weir. 2. A vertical tunnel. 3. horizontal conduit 4. An energy

dissipation arrangement at the end of the horizontal tunnel.

Hydraulic Design of SpillwayCase Study Upper Wardha Project

salient features of the project

1. Name of the river – Wardha2. Location – Near Simbora Village, Taluka – Morshi, District –

Amravati.3. Catchment Area –4302 Sq.Km

Hydrology of SpillwayMaximum Flood DischargeThe maximum flood discharge may be

estimated by 1. Using available data2. Unit hydrograph method3. Empirical formula4. Envelope curveIn our case we calculated by,Inglis Formula,

Where,M=catchment area in sq.km

)24.10(

125

M

MQ

Location of SpillwayThe spillway may be located –In the gorge portionOn either of the flanks In a saddle.

In our case,Good rock in the river portion was available at

61.00 m i.e. 6.00 m from the river bed levelIn view of this the spillway was proposed to be

located in the gorge portion.

Control StructureUpstream Profile:The control structure will be an ogee type weir. The profile of ogee weir will be according to

USWES profile, Xn = K Hd

n-1 y

Where, x = Co-ordinates in x direction. K = constant. Hd = Design head.

n = constantDesign Head Hd =HFL-Crest Level

=(108-100) =8 m

The values of n and K can be calculated by following table

As the spillway located in gorge portion Upstream face is Vertical therefore,

n=1.85 & K=2.00

Upstream Profile n K

Vertical 1.85 2.000

1:3 1.836 1.936

2:3 1.850 1.939

3:3 1.776 1.873

Upstream ProfileThe Upstream Profile is designed as per

recommendations of USWES a = 0.175 Hd = 1.40 m

b = 0.282 Hd = 2.256 m

r1 = 0.5 Hd = 4.00 m.

r2 = 0.2 Hd = 1.60 m .

Upstream Curve

Downstream ProfileFor the structural stability the maximum

downstream slope of the weir is restricted to 0.8 horizontal to 1.0 vertical.

The tangent point is worked out follows  x1.85 = 2 Hd

0.85 y

y =

x=11.40m y=7.70m

85.0

85.1

2 dH

x

85.0

85.0

2

85.1

8.0

0.1

dx

y

H

x

d

d

Downstream Curve

The co-ordinates of the ogee weir downstream of the crest are as follows –

Sr.No X Y Remark

1 1.00 0.0854

2 3.00 0.65

3 5.00 1.67

4 7.00 3.125

5 9.00 4.97

6 11.00 7.21

7 11.40 7.70 Tangent Point

Radial GatesThe number and size of the gates is finalized to

suit theWidth of river.The flood discharge.The storage between the gates. The U.S.B.R. recommendations regarding the

radial gates as followsThe ratio of width to height of the radial gate be

as far as possible 2:3The radial should rest on the weir at a point 0.5

m to 1m lower than the crest of the weir.

The radius of the skin plate be equal to 1.25(Hd – 1)

R=1.25(Hd -1)

=1.25(8-1) =8.75mHeight of trunion axis above the sill beam level be

3/4 to 1/2 (Hd -1)

=.75 Hd to 0.5(Hd -1)

=6 to 3.5mTrunion axis level=5.00m above crest level

Sill Beam Level= 1.00m below crest level

Radial Gates

ENERGY DISSIPATING ARRANGEMENTMethods adopted are as follows –

1. Hydraulic jump type stilling basin.2. Bucket type energy dissipation. 3. Flip or ski-jump bucket

4. Interacting jet, splitter etcAs the rock at the down-stream side of the dam

was available at about 6 meters below the ground, the last two alternatives, viz.

Flip or ski-jump bucket.Interacting jet, splitter etc.Have been discarded

Hence we will design the tail channel with ‘Hydraulic jump type stilling basin’

TAIL WATER RATING CURVE

Developed on one cross section at the dam site.

Coefficient of rugosity was assumed equal to 0.03 and the slope of the river was assumed to be in equal 1 in 1000.Sr.No Discharge Tail Water

levelDepth

1 750 75.5 74.0

2 1500 78.0 77.0

3 2250 79.5 79.0

4 3100 81.85 81.85

TAIL WATER RATING CURVE

HYDRAULIC JUMP TYPE STILLING BASINThe pre jump depth (d1) and post jump depth (d2)

are calculated as follows.

E = F.R.L. level – foundation level of spillway.

E = 108 – 67  E = 41mNow,E=d1+

d1=1.55m

gd

E

21

2

V1 =

q = discharge/total length of spillway (Discharge per unit length passing through a single gate)q =

q = 43.05 m3/sec/m . V1=

V1=27.77 m/sec

Fr1=

1d

q

126

3100

X

55.1

05.43

1

1

gd

V

Fr1=7.12Now,d2=

d2=14.85m

11812

1 2 Frd

ConclusionFor the Upper Wardha Project the design of spillway withslight modifying the data was carried out as perrecommendations of USWES and USBR for stilling basinAn ogee shaped gated spillway was designed to pass flooddischarge of 3100 cumecs. Also 6 radial gates were designed control the flood discharge. For energy dissipation purposethe hydraulic jump curve and tail water rating curve arestudied.Due to tail water deficiency, to match the hydraulic jumpcurve and tail water rating curve, the stilling basin wasdepressed.