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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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    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)

    429

    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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    430

    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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    431

    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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    434

    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.