HYDRO 2013_Pradyumna

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  • Lahmeyer International (India) Pvt. Ltd. Lahmeyer International (India) Pvt. Ltd.

    One-dimensional sediment modelling for Chuzachen and Devsari hydroelectric power projects to check the feasibility of reservoirs usage as pseudo-desanders Author and Presenter : Pradyumna Machhkhand Senior Manager (Hydropower & Water Resources)| Lahmeyer International (India) Pvt. Ltd. | Gurgaon-122002

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    Outline of the presentation

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    Introduction

    Background Information

    Reservoir Sedimentation

    Sediment Modelling

    Theory

    Case Studies

    Modelling Scheme

    Model Set Up

    Data Organization

    Results

    Comparison

    Conclusion

    Model Description

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    Reservoir sedimentation in hydroelectric projects (HEP): a major problem that interrupts the smooth functioning of hydropower plants.

    A schematic and classical illustration of reservoir sedimentation

    Introduction

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    In general, the problem is seen as a big hindrance and therefore, as a remedial measure n number of desanders, based on studies, are usually proposed to manage the sediment removal operation.

    The presented case studies foray into the primary investigation of reservoir sedimentation prior to making decisions on sediment management.

    The investigation of sediments lies on two basic searches:

    Where & How Much

    Introduction

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    To examine the location and capacity of the reservoir in discrete form

    To understand the relation between sediment concentration and discharge in the reservoir

  • Lahmeyer International (India) Pvt. Ltd.

    Two different hydroelectric projects have been studied: a. Devsari HEP (Devsari Reservoir) b. Chuzachen HEP (Rangpo Reservoir & Rongli Reservoir) Project features:

    contd...

    Background information

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    Project features Devsari HEP Chuzachen HEP

    (Rangpo)

    Location Uttarakhand, India Sikkim, India

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    Background information

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    Project features Devsari HEP Chuzachen HEP

    (Rangpo)

    Type of structure Dam Dam

    Height of structure 35m 48m

    Extent of reservoir 4.8Kms from dam axis 508m from dam axis

    Storage 9.026 Mm3 0.360 Mm

    3

    contd

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    Empirical methods

    a. Advantage : explicit methods (such as, Hazen, Vetter, and Camp), computational ease while computing the sediment trap efficiency.

    b. Limitations : i. cover only the settling phase of reservoir operation, ii. and other factors which are NOT included in the empirical methods are:

    the sediment transporting capacity of flow in a settling reach, the change in conditions as reservoir fills with sediment, the effect of variation in flow depth down a basin or reservoir, and the additional turbulence caused by inlet condition to a basin or

    reservoir. contd...

    Reservoir sedimentation

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    1-dimensional numerical model

    a. Advantages :

    models the effect of turbulence in sediment movement and mass deposition,

    not only simulates deposition, but also sluicing,

    does not require grids to approximate cross-sections,

    and requires less field data to set up.

    Reservoir sedimentation

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    contd

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    1-dimensional numerical model

    b. Limitations:

    does not offer a detailed view of hydrodynamics of reservoir

    system like 2-D or 3-D models.

    Although 2-D or 3-D models have certain advantages, but at the cost of a

    longer computational time and substantial amount of field data to

    capture the complexities of 2-D or 3-D flow.

    contd...

    Reservoir sedimentation

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    SHARC , a software developed by HR Wallingford, UK, is a suite of integrated

    programs designed to assist in the identification and solution of sediment problems at intakes in rivers and canal systems.

    DOSSBAS stands for Design of Sluiced Settling Basins.

    DOSSBAS is tool built within SHARC to model sediment depositions in basins/reservoirs and can model both regular and irregular basins.

    Two suites of DOSSBAS: 1) Deposition Model. 2) Sluicing Model.

    Model description : DOSSBAS tool of SHARC

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    contd

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    Deposition model

    Assumptions: o the flow is steady,

    o the velocities and concentrations are constant across the width of the

    channel,

    o and the concentrations in one size fraction do not affect other size fraction.

    Model description : DOSSBAS tool of SHARC

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    contd

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    Deposition model [Basic equation of turbulence (Dobbins ,1994)]

    Equation (1)

    = sediment diffusion coefficient in y-direction (m2/sec) , = sediment diffusion coefficient in x-direction (m2/sec), = and settling velocity (formula by Gibbs et al, 1971) of sediment for the sediment size fraction (j) (m/sec).

    Model description : DOSSBAS tool of SHARC

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    contd

    2

    2

    2

    2

    x

    C

    y

    C

    yV

    y

    C

    x

    Cu

    j

    x

    jy

    sj

    j

    y

    j

    where, u = flow velocity at height y above the bed (m/sec), y = height above bed (m), Cj = sediment concentration at height (y) above bed for size fraction( j), x = distance co-ordinate along channel (m),

    y

    x

    sjV

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    Deposition model

    Model description : DOSSBAS tool of SHARC

    13

    contd

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    Deposition model : Turbulent diffussion within a sub-reach

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    Deposition model : Boundary conditions

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    Overall structure of deposition model

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    Sluicing model

    Sluicing model simulates only sand movements. It is assumed that any silt in the exposed bed material is sluiced instantly and therefore, only sand transport controls the sluicing rates.

    Diffusion is not a dominant process in sluicing. Sluicing is modelled using equation,

    Note: The threshold value to differentiate between silt and sand in DOSSBAS is 63 micron (default).

    Model description : DOSSBAS tool of SHARC

    17

    contd

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    Overall structure of sluicing model

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    Modelling scheme of reservoir sediment management plan

    Modelling scheme

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    contd

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    Modelling scheme

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    contd

    Set of parameters

    X1 is the set of input parameters, viz., reservoir geometry, and sediment data

    X2 is the set of parameters that includes the temperature, and the sand concentrations to be applied in both deposition and sluicing model.

    The input parameters for deposition model can be expressed as, (X1 X2).

    The set X3 is composed of additional parameters such as, flushing/sluicing discharge, the water level at the time of sluicing, and the duration of sluicing.

    Y1 is the set of longitudinal bed profiles, which are results of deposition models.

    The input parameters for sluicing model can be expressed as, (Y2 X2 X3).

    The set Y2 comprises results of sluicing models in terms of elapsed time for flushing out the sediments and change in bed profiles at different time.

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    Hydrological year mean flow duration curves

    Data organization

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    contd

    0

    20

    40

    60

    80

    100

    120

    140

    160

    180

    200

    220

    240

    0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

    Dis

    char

    ge (m

    3/s

    ec)

    Exceedance probability (%)

    Flow duration curve divisions: Devsari HEP

    Flow duration curve

    Qdesign (Intake) = 120.76 m3/sec

    Division3 Qm = 44.8 m3/sec

    Division2 Qm = 160.1 m3/sec

    Division1 Qm = 205.3 m3/sec

    0

    20

    40

    60

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    0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

    Dis

    ch

    arg

    e (cum

    ec)

    Exceedance Probability (%)

    Flow duration curve divisions : Rangpo

    Flow duration curve

    Qm = 33.2 m3/sec (Division 3)

    Qm= 95.8 m3/sec (Division 1)

    Qm= 58.1 m3/sec (Division 2)