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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459,ISO 9001:2008 Certified Journal,Volume 3, Issue 12, December 2013)

428

Design Aspects and Model studies for Silt Flushing Tunnels inHydro Power Projects

M K Verma1, M Z Qamar2, A P Meshram31,2Research Officer, 3Assistant Research Officer, Central Water & Power Research Station, Pune (India)-411 024

Abstract The Himalayan Rivers due to their steep bedand side slopes, narrow width and fragile geology of the

region carry huge amount of sediment with them. Many run-

of-river hydro power projects are coming up on these rivers

and suspended sediment enters into the power intake of these

projects. Desilting chambers are provided to remove /

minimize the suspended sediment from water conductor

system. The settled sediment in the desilting chamber is

flushed out continuously through the flushing tunnels

provided just below the chamber. These flushing tunnels

carry the settled sediment laden water up to their outfall in

river downstream of the dam. Various design parameters of

silt flushing tunnels which can only be finalized, after

conducting physical model studies are discussed in this paper.

Keywords Desilting chamber, sediment transportcapacity silt flushing tunnel, suspended sediment

concentration.

I.

INTRODUCTION

The rivers in the Himalayan region possess very high

power potential and many cascade type run-of-river hydro

power plants have already been commissioned and many

more are coming up in the near future. When a hydro

power project is constructed on such a river, the suspended

part of the sediment load finds its way into the power

intake and causes a lot of damage to the turbine blades and

other costly under water equipments in the power house.

Desilting chambers form an integral part of water

conductor system of these projects to remove / minimize

the suspended sediment from water diverted for power

generation. In these desilting chambers, cross sectional area

of flow is increased to a designed length and settlement of

suspended sediment occurs in this length. The settled

sediment in the desilting chamber is flushed out through the

silt flushing tunnels (SFT) provided just below thechamber. An excess flushing discharge of 15 to 20% of the

head race tunnel (HRT) discharge is taken through the

power intake for flushing of sediment. These flushing

tunnels carry the settled sediment laden water upto the

outfall in the river downstream of the dam. Silt flushing

tunnel connect with the desilting chamber through the

openings of designed size and spacing which are provided

at bottom of the chamber.

The flushing discharge along with the settled sediment is

sucked into the flushing tunnels through these openings.

The size and spacing of these openings are so designed to

ensure that the velocity in the silt flushing tunnel is more

than 3.0 m/s throughout its length for efficient transport of

settled sediment.

II.

DESIGN ASPECTS OF SILT FLUSHING TUNNELS

Control gates are provided in silt flushing tunnels at the

end of desilting chamber and beyond the control gates the

tunnels are designed as open channel flow. The flow in the

flushing tunnels upto the control gate is pressure flow. If

two or more units of desilting chambers are provided than

the individual flushing tunnels are combined into a single

tunnel and is discharged to the outfall into the river

downstream of dam. The layout and slope of individual as

well as the combined flushing tunnel are so designed that

the desired sediment transport capacity is achieved and the

sediment is discharged to the outfall without any deposition

or chocking. The length of the combined flushing tunnel is

kept as minimum as possible from economic point of viewwithout compromising in its sediment transport capacity.

Various design aspects are summarized below:

A.

Layout of the individual silt flushing tunnel

Many-a-times the desilting chamber is required to be

divided into multiple units for achieving flexibility in the

operation or for limiting the size of the excavation in the

hills [1]. In the case of Shanan Hydel Project, Himachal

Pradesh, the chamber has been divided into six

compartments for obtaining continuous flow to the power

house with intermittent flushing arrangement for each unit

separately. In case of Chukha, Dul-Hasti and Nathpa Jhakri

Projects, the chambers have been divided into 2, 2 and 4units respectively, depending upon the discharge and

permissible size of excavation. Similar arrangements has

been done in many other hydro power projects situated in

Himalyan region such as Chamera Stage II and III, Parbati

Stage II & III, Teesta Stage IV & V, Dhauliganga,

Punatsangchhu stage-I & II and Mangdechhu etc.


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Layout of flushing tunnels is easier in hydro power

projects where only one or two units of desilting chambersare provided. When three or more units of desilting

chambers are provided, same number of flushing tunnels

comes out of the desilting chambers and their layout

becomes complicated. The proper site specific layout of

these tunnels is very important in terms of their sediment

transport capacity. The proper alignment of the individual

tunnels is absolute necessity so that these are able to carry

the sediment laden water towards the combined tunnel. The

initial straight reach of the individual flushing tunnels is

kept sufficient so that the hydraulic jump is safely

accommodated inside this straight reach without causing

damage to the walls and top of the tunnels.

B.

Cross section of silt flushing tunnel

The cross section of the flushing tunnel is designed such

that the required minimum flushing velocity is maintained

throughout the length. Adequate free board inside the

tunnels is also necessary so that enough space is available

between the water surface and the top surface of the tunnel

to prevent the abrasion/cavitation damage of the tunnels

top surface and locking of air inside.

C. Slope

To economize the construction, instead of constructing

all the individual flushing tunnels up to the outfall, these

are joined together in a combined tunnel which carries the

sediment laden water up to its outfall. The slope of each ofthe individual tunnel as well as the combined tunnel is kept

such that the required flushing velocity is maintained

throughout for sediment disposal and flow should be

supercritical.

D.

Junctions of silt flushing tunnel

Sometimes due to maintenance of one unit of desilting

chamber, the discharge through it is stopped by closing the

respective intake gate while other units remain in

continuous operation. Junctions of flushing tunnels should

be designed in such a way that in case of one tunnel not

running, back water effect to this tunnel from the other

tunnel which is in operation is minimum. The velocities at

the junction points and some reaches upstream of junctions

remain very low due to back water effect thus causing

settlement of sediment in this region which is not desirable.

Hence, in order to streamline the flow in forward direction

into combined flushing tunnel, the length of the nose at the

junction may be increased. One example of Tapovan

Vishnugad H.E. Project, Uttarakhand is shown below in

Photo 1 and 2 where the junction was modified based on

the model studies conducted at CWPRS, Pune.

Photo 1: Original Proposal

Photo 2: Modified layout of Junctions

E. Location and invert level of outlet

The invert level and location of outfall of combined silt-

flushing tunnel, carrying sediment laden water from all theindividual tunnels may be modified / finalized based on

model studies to generate adequate velocities for sediment

transport. This can be achieved by lowering or raising the

outfall level for maintaining optimum bed slope of the

whole flushing system.

1 2 3 4

Elongated noseat junctions

1 2 3 4


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III. NECESSITY OF MODEL STUDIES FOR SFT

The silt flushing tunnels carry settled sediment from

desilting chambers and the concentration cannot be known

without conducting model studies for settling efficiency of

the silt particles for desilting chamber. This serves as input

parameter for silt flushing tunnel model studies. The

preliminary design of silt flushing tunnel is based on

empirical formulae and certain assumptions. The manifold

arrangement, junctions of the individual flushing tunnels,

the slope of individual as well as the combined tunnels,

discharge and sediment transport capacity is verified on a

physical model to achieve the best hydraulic design. Many

physical model studies have been conducted at Central

Water & Power Research Station, Pune for various projects

in India and abroad which helped in improving the designparameters for flushing tunnels and one such case study is

described below.

IV.

CASE STUDY FOR TAPOVAN VISHNUGAD

The Tapovan Vishnugad Hydro Electric Project (520

MW) is located on river Dhauliganga in Chamoli district of

Uttarakhand. This is a run-of-river scheme with diurnal

storage and would utilize a net head of water of 481 m. It

will feature a 113 m long and 22 m high barrage across

Dauliganga River with 4 gates and the pondage will have a

maximum depth of 22 m and a live storage depth of 13 m

with a capacity of 0.57 MCM. The intake structure has

been designed for drawing the discharge of 147.18 m3/sthrough the head regulator that is connected to four

numbers of desilting basins. The size of the each desilting

basin was proposed as 140 m (L), 16 m (W) and 18.5 (D)

and design discharge for the system is 122.18 m3/s and

flushing discharge is 25 m3/s for four basins. Hydraulic

model studies were conducted for desilting basin [2] which

modified the inlet tunnels and inlet transition and also

reduced the length of the basin. On the basis of these

studies it was found that a 113 m long, 18.5 m deep and 16

m wide desilting basin is adequate for 90% settlement of

particles of 0.2 mm diameter and above. The reported

overall settling efficiency of the desilting basin arrived at

by Camp's criteria worked out to be 63.24%. Hydraulicmodel studies were carried out to study the appropriateness

of geometrical configuration of individual as well as

combined silt flushing tunnel (SFT) beyond desilting

chamber for estimating sediment transport capacity [3].

A.

Studies for original proposal

The studies were conducted on a 1:15 geometrically

similar scale model as per the drawings received from

project authorities.

Figure 1: Layout of SFT Original Proposal

The drawings were studied from hydraulics point of

view and it was found that the Froude Number at various

locations along the SFT varies between 1.4 to 1.6. As such,

lowering of the exit portal (at point S7) of the SFT by atleast 1.0 m from EL 1760.5 m to El 1759.5 m was

recommended. This lowering of the exit portal of SFT was

incorporated in the model to obtain favorable hydraulic

conditions particularly at junctions. The layout plan and

cross section are reproduced in figure -1 and 2.


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Figure 2: Original Cross Sections of SFT

Four units of the flushing tunnels coming out from end

of the desilting basins and up to the control gates (S1),

manifold arrangement for branch tunnels upto S5 and

combined silt flushing tunnel beyond point S6 were

reproduced in the model as shown in Photo3 (a & b). The

actual length of combined flushing tunnel beyond point S6

to its outfall into the river is 1546 m. However 200 m

length reproduced in the model was adequate to establish

the flow conditions.

The tunnels were reproduced in polished Kadappa stone

slabs downstream of the control gates and with open top for

visualizing the flow conditions as seen in Photo 3 (a & b).

All the tunnels from end of desilting basin up to controlgates were fabricated in clear transparent Perspex sheets.

B. Studies for estimation of roughness

The Mannings roughness coefficient n equal to 0.014

was assumed for the design of silt flushing tunnel. Initial

studies were therefore taken up for estimation of

Mannings roughness coefficient n of the combined silt-

flushing tunnel reproduced in the model.

Based on these model studies values of Mannings

roughness coefficient n in the model were calculated to be

0.00936 and 0.00890, which is equivalent to 0.01433 say

0.14 in the prototype. As such it is seen that the model

roughness is correctly simulated in accordance with the

prototype roughness. The supercritical flow prevails in thecombined silt-flushing tunnel under all operating

conditions.

Photo 3 (a): Upstream view of original Proposal

Photo 3 (b): Downstream view of original Proposal

C.

Velocity observations

The discharge equivalent to 25 m3/s equally distributed

in four tunnels was simulated and reservoir water level was

maintained above MDDL on the upstream. It was observed

on the model that 2.5 m depth of individual silt flushing

tunnel as proposed is not adequate to accommodate the

discharge of 6.25 m3/s. Also, to streamline the flow in line

with the flow in combined flushing tunnel, the proposed

layout of the flushing tunnels needs modifications.

Considering these aspects, the depth of individual SFT was

increased from 2.5 m to 3.0 m in the reach 40.0 m to 81.70

m downstream of control gate for SFT No 3 & 4. Similarlyfor SFT No 1 & 2 the depth was increased from 2.5 m to

3.0 m in the reach 40.0 m to 69.25 m. Further downstream

of this reach the depth of flushing tunnel was maintained as

3.50 m.

1 2 3 4

1

234

Combined SFT


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433

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10

%F

iner

Particle Dia (mm)

SFT d = 0.3

Figure 4: Gradation of suspended sediment (in model)

Figure 5: Gradation of suspended sediment (in model)

F.

Sediment transport capacity by Analytical Estimation

The overall settling efficiency of the desilting basin

arrived at by Camp's criteria worked out to be 63.24% as

mentioned in Para IV. On the basis of these results, the

gradation curve of suspended sediment likely to flow

through the silt flushing tunnel was estimated and is shown

in figure 5.With this overall settling efficiency of 63.24%

and the concentration of 5000 ppm at the inlet, the

concentration of suspended sediment in the flushing tunnel

would be of the order of 17000 ppm as shown in table I.

The slope of the combined flushing tunnel is 1:127.24

and super critical flow prevails in the flushing tunnel under

all operating conditions. As such direct estimation ofsediment transport capacity was not possible. In view of

this, as an initial approximation, sub critical flow with

Froude No. 0.88 was assumed to prevail in the flushing

tunnel with the present conditions. With this assumption,

the depth of flow worked out to be 2.09 m in the combined

silt flushing tunnel for discharge of 25 m3/s.

TABLE I

ESTIMATIONOFCONCENTRATIONINSFT

Average discharge in one desilting

basin (m3/s)33.67

Inlet concentration (ppm) 5000

Qty. of sediment in the basin w.r.t.

Inlet concentration (kg/s)168.35

Analytical overall settling

efficiency of desilting basin (%)63.24

Material that will settle in the

desilting basin (kg/s)

168.36 x 0.6324 =

106.46

Discharge in one silt flushing

tunnel (m3/s)6.25

Concentration in the silt flushingtunnel (ppm)

17034 Say 17000

The suspended sediment transporting capacity of

combined flushing tunnel would be of the order of 27600

ppm for the design discharge of 25 m3/s and d50of 0.2 mm,

using Engelund and Hansen formula [4] as given below:

qs= 0.05 V2

1

50

sg

d

2/3

50)(

ds

Where;

s = Unit weight of sediment;

= Unit weight of water;

V = Forward velocity of flow;

g = Acceleration due to gravity;

= Shear stress.

This equation is dimensionally homogeneous and can be

solved by using any set of homogeneous units. It may be

mentioned here that Engelund-Hansen formula is not

applicable for super critical flows. However, as the flow in

the combined SFT is super critical the sediment transport

capacity would be much more than that with sub critical

flow. Similar calculations were made for the model

parameters and the material used in the model and it would

be seen that sediment-transporting capacity of the

combined silt flushing tunnel in the model would be of the

order of 16250 ppm.

0

10

20

30

40

50

60

70

8090

100

0.01 0.1 1 10

%F

iner

Particle dia (mm)

Gradation of suspended sediment

SFT d50 = 0.2 mm

Inlet d50 = 0.15


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V. DISCUSSIONS

The analytical results for the prototype and that for the

model are as follows:TABLE II

ESTIMATIONOFSEDIMENTTRANSPORTCAPACITY

Parameter Prototype Model

Flushing discharge for

four units (m3/s)25.0 0.0287

Width of tunnel (m) 3.0 0.2

Slope (assumed for

calculations)1 in 267.88 1 in 267.88

Froude number 0.88 0.88Mannings roughness

coefficient n0.014 0.0089

Velocity (m/s) 3.987 1.030

Shear stress (kg/m2) 3.2598 0.2173

d50of material (mm) 0.2 0.3

Sediment Transport

capacity (ppm)27587 Say

27600

16250 Say

16250

From table 2, it would be seen that if the slope of the

combined silt flushing tunnel would have been 1 in 267.88against the provided slope of 1 in 127.24, the flow in the

silt flushing tunnel would be sub-critical with Froude No.

0.88 and the velocity would be of the order of 3.987 m/s

where the flow attains the normal depth. With these

conditions, the sediment transport capacity of the combined

flushing tunnel works out to be of 27,600 ppm for

prototype parameters and 16250 ppm for model

parameters.

Whereas, the slope provided in combined silt flushing

tunnel is 1 in 127.24 and supercritical flow prevails in the

flushing tunnel.

As mentioned earlier the sediment transporting capacity

of the flushing tunnel observed in the model is of the orderof 22,150 ppm, which is much more than estimated

capacity of 16250 ppm. Hence, it is expected that the

sediment transporting capacity of the flushing tunnel in

prototype would be more than 27,600 ppm against the

estimated concentration of 17000 ppm with the proposed

bed slope of 1 in 127.24 and n = 0.014.

VI.

CONCLUSIONS

Based on the case study mentioned in this paper, it was

concluded that the hydraulic design was improved by

testing various alternatives and combinations such as

increase in depth of individual flushing tunnel, provision of

transition in combined SFT, modifications in junctions andlowering of exit portal. The designed layout of the silt

flushing tunnels, if tested on a physical model helps in

evolving a better hydraulic design thus improving the

sediment carrying capacity. The deposition of sediment at

critical points is observed in the model and the layout, cross

section and the slope of the flushing tunnels can be

modified accordingly.

Acknowledgements

The authors sincerely thank Mr. S. Govindan, Director,

CWPRS for constant encouragement, guidance and kind

permission for publishing this paper. The authors are also

thankful to the various project authorities for providing thenecessary financial support and data for conducting the

model studies.

REFERENCES

[1]

CWPRS: Guidelines for Design of Desilting Basins (Pressure

Flow), 2005.

[2]

CWPRS Technical Report No. 4659 of October 2009 Hydraulic

Model Studies for desilting basin for Tapovan Vishnugad H.E.Project, Uttarakhand.

[3]

CWPRS Technical Report No. 4687 of January 2010 HydraulicModel Studies for Silt Flushing Tunnel beyond desilting chamber for

Tapovan Vishnugad H.E. Project, Uttarakhand.

[4]

Morris, Gregory and Fan, Jiahua. 1997. Handbook of ReservoirSedimentation. McGrawHill Book Co. New York.

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