Importance of Desilters in Run-Of-River Hydros

International Journal of Emerging Technology and Advanced Engineering

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Tamizhan College of Engineering and Technology (ISO 9001:2008 Certified Institution), Tamilnadu, INDIA Page 407

Importance of Desilting Basins in

Run-of-River Hydro Projects in Himalayan Region M Z Qamar

1, M K Verma

2, A P Meshram

3

1 & 2Research Officer,

3Assistant Research Officer,

1, 2 & 3 Central Water & Power Research Station, Pune (India) - 411 024

[email protected]

[email protected]

[email protected]

Abstract— The natural resources of Himalayas in terms of hydro

power generation are not only crucial for the Himalayan states of

India, but very important for the whole country. These states

seem to be keen in exploiting their vast hydro electricity potential

for net revenue earning. However, due to weak geological

conditions and steep slopes these rivers carry a huge quantity of

sediment with them. The suspended part of this sediment causes

problems after getting entry through trash racks of power

intakes in hydropower projects constructed in Himalayan region.

This paper describes the elimination of suspended sediment in

run-of-river (ROR) hydro power projects by means of providing

desilting basins.

Keywords— Desilting chamber, settling efficiency, silt flushing

tunnel, suspended sediment concentration, inlet transition, outlet

transition.

I. INTRODUCTION

Many ROR hydropower projects have been commissioned in

Himalayan region and many more are being constructed /

planned in India, Bhutan and Nepal. The suspended part of

sediment load carried by Himalayan Rivers mainly consisting

of quartz particles (hardness 7 on Mohs scale) enters into the

water conductor system through power intake. These projects

generally utilize a very high water head sometimes ranging

from 700 to 800 m. If this water along with huge quantity of

suspended sediment load is allowed in the power house with

such a great velocity, it will cause lot of damage to the

turbines and other under water parts due to abrasion and

wearing effect. One such example of damage to the turbines

due to sediment is shown in photo 1.

To tackle this suspended sediment problem, some approach

has to be planned during the design stage of the project.

Provision of desilting basins in these projects is one of the

widely used methods to deal with this problem.

Photo 1: Damage to the runner due to sediment

II. TYPES OF HYDRO POWER PROJECTS

Broadly speaking, there are three types of hydro power

projects namely, storage or impoundment type, run-of-river or

diversion type and pumped storage type.

A. Storage / impoundment type hydropower project

The most common type of hydropower plant is with an

impoundment facility. In this system a large reservoir is

created by constructing a dam to store river water for major

hydro power project. Water released from the reservoir flows

through a turbine, spinning it, which in turn activates a

generator to produce electricity. The water is released through

power intake either to meet peak electricity demands or base

load needs or through spillways to maintain a constant

reservoir level.

B. Run-of-river or diversion type hydropower project

In run-of-river or diversion type hydro power projects, a small

reservoir is created by constructing a dam / diversion weir to

divert river flow through head race tunnel to an adjacent

valley utilizing the available head for power generation. In

run-of-river hydroelectric stations diurnal storage is used to

meet peak electricity demands. These projects have small or

no reservoir capacity.

Damaged runner New runner

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SFT 1.5 m deep

Bottom slab

C. Pumped storage type hydropower project

When the demand for electricity is low, pumped storage

facility stores energy by pumping water from a lower

reservoir to an upper reservoir. During periods of peak

electrical demand, the water is released through turbines and

stored in the lower reservoir. This method produces electricity

to supply high peak demands by moving water between

reservoirs at different elevations. At times of low electrical

demand, excess generation capacity is used to pump water

back into the higher reservoir.

III. DESILTING BASIN

Desilting basins have become an integral part of the water

conductor system of ROR hydropower projects to minimize

the impact of damage due to suspended sediment. Desilting

basins are provided just after power intake and discharge is

passed through them before entry into the head race tunnel.

Desilting basins are huge and costly underground structures,

generally, constructed inside the hills and sometimes size is as

large as 525 m long 15 m wide and 27.5 m deep (Nathpa

Jhakri). A typical layout of water conductor system for a run-

of river hydropower project is shown in figure 1.

Figure 1: Typical layout of water conductor system for

ROR hydropower project

In case of desilting basins the cross sectional area of flow is

increased so as to achieve a reduction in forward velocity

which ultimately induces the settlement of the suspended

sediment. The silt flushing tunnel (SFT) is provided below the

desilting basin which is connected with main basin by

provision of openings at the bottom slab of the desilting basin.

This settled sediment is flushed out through silt flushing

tunnel and discharged into the river downstream of the dam.

An excess discharge of 15 to 20 % of design discharge is

taken from the power intake into the desilting basin for

flushing of the settled sediment from SFT. The discharge

through desilting basin and SFT is controlled by provision of

control gates at the outlet. Generally, desilting basins are

designed for 90 % removal of suspended sediment particles of

size 0.2 mm and above. However, basins may be designed to

eliminate particles finer /coarser than 0.2 mm which will

increase / decrease length of the basin and in turn adds /

reduces cost of the project. This can be decided by carrying

out the comparative study considering aspects such as

replacement / maintenance of underwater parts, revenue loss

and local site conditions. A typical plan and longitudinal

section of desilting basin is shown in figure 2 and cross

section in figure 3.

Figure 2: Typical plan and L-section of desilting basin

Figure 3: Typical cross section of desilting basin

A. Types of desilting basins

http://en.wikipedia.org/wiki/Reservoir_(water)





Various types of desilting basins and their mode of

classification are indicated in table 1.

TABLE 1

TYPES OF DESILTING BASINS

Basis of Classification Type of basin

Mode of construction Natural or artificial

Method of cleaning Manual or mechanical or

hydraulic removal of deposition

Mode of operation Continuous or intermittent

Type of flow Open channel or Pressure flow

Configuration / layout Single or multiple unit

B. Design aspects of desilting basins

The performance of desilting basins depends upon the

reduction in velocity and turbulence, provision of adequate

length of the basin for achieving the desired settlement and the

skimming arrangements at the outlet [1]. However, the settled

sediment is required to be removed periodically or

continuously to maintain its settling efficiency. Thus, though

the design of the desilting basin includes two main parts viz.

i. Settling efficiency and

ii. Flushing system

Following aspects are also required to be taken into

consideration:

1) Location and orientation: Generally, desilting basin

should be located as near the power intake as possible to

achieve the desired control and to minimize the sedimentation

in the approach channel. However, the location of the basin

too near the intake would create a problem due to the

turbulence downstream of the intake. The basin is also

required to be properly oriented with respect to the

alignment of the inlet tunnel on upstream to achieve

satisfactory distribution of flow as naturally as possible.

For this purpose, the basin may be located in the reach where

at least a straight length equal to ten times the average width

of the channel or diameter of the inlet tunnel is available on

the upstream.

2) Inlet transition: The flow area in the desilting basin

is required to be increased for reducing velocity to induce the

settlement of sediment. This increase in area is achieved by

suitable horizontal and vertical divergence. For obtaining the

satisfactory distribution of flow, the flow with relatively

large velocity at the inlet has to mix satisfactorily in a

desilting basin and a proper diffusion / dispersion is to be

achieved. From the study of the mechanism of the dispersion

of the jet in the water body, it has been seen that the region

of the expansion of flow is the region of appreciable

modification of mean flow pattern and the region of

appreciable eddy motion. From the model studies of these

basins a bed slope of between 2.0 and 2.3 was found to be

satisfactory.

3) Size of the basin: The ideal horizontal settling basin

as shown in figure 4, demonstrates the basic theory of

sedimentation developed by Hazen [2]. The following

assumptions are made: - uniform distribution of flow and

suspended solids at entry to settling zone; quiescent flow;

solids entering deposition zone are not re-suspended.

Consider a sediment particle entering the basin at point x:

4)

Figure 4: Concept of ideal settling basin

Settling time, st = d/w

Retention time, Rt = basin volume / discharge = d A / Q

Where d = flow depth; A = mean plan area of basin; Q =

discharge.

For quiescent settling, all particles of settling velocity w are

removed when retention time equals settling time:

i.e. d A / Q = d/w, or Q/A = w

In general for both ideal and real basins, the ratio wA /Q can

be regarded as a dimensionless indicator of the physical

ability of a basin of plan area A to remove particles of settling

velocity w at supply discharge Q.

In case of ideal settling basins, for discrete particles:

x




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Removal is independent of basin depth and flow-

through velocity,

For a given discharge and suspended sediment load,

removal is a function of basin surface area.

In case of real desilting basins the higher velocity incoming

flow enters the main basin which causes turbulence in its

initial reaches. Therefore, effect of turbulence is also being

considered as opposed to ideal settling basins. Camp [3] based

his classic approach to settling basin design of the work of

Dobbins [4]. After making simplifying assumptions that (a)

fluid velocity, and (b) the turbulent mixing coefficient are the

same throughout the fluid, Camp derived a relation for settling

efficiency :

= f

*

,v

w

Q

wA

Where *v is the shear velocity and w / *v can be regarded as

a dimensionless indicator of distribution of suspended

sediment in vertical. Camp’s solution to equation is shown

graphically in Figure 5.

Figure 5: Camp’s solution for settling basin efficiency

The shear velocity in the relation is given by:

Shear velocity, *v = gRS

Where R = hydraulic mean depth, and S = hydraulic gradient

which is calculated from a boundary resistance equation such

as Manning’s and essentially depends on flow through

velocity. For dimensioning of desilting basins, various

sediment removal functions such as proposed by C.P. Vetter –

[5], T.R. Camp – [3], Hunter Rouse [6] Technical Conditions

and Standards of USSR i.e. TCaS [7], H.A. Einstein [8] and

Hippola [9] are in use. These functions are based on

gravitational, diffusion or probability theory of the sediment

transport.

5) Outlet transition: The centre line of outlet should

coincide with the axis of desilting basin for uniform

withdrawal / skimming of top layers of flow over the entire

width of basin. The outlet should be as high and as wide as

possible. Narrow outlets or outlets located on the side would

result in a reduction in the effective length of the basin.

6) Size and slope of the hopper: The slope of the

hoppers is required to be steeper than the angle of repose of

the suspended sediment to allow the sediment to slip into

the openings at the bottom connecting to the flushing

conduits/pipes underneath. In the case of narrow desilting

basins, instead of individual rectangular hopper, a continuous

hopper bottom side with sediment accumulation trench below

is preferable. The spacing of the openings between the settling

trench and flushing conduit is decided in such a way that the

top of the dunes formed between the successive openings

would not protrude in the settling zone above. Based on model

studies, the preferable side slope of hopper is 400.

7) Size of silt flushing tunnel: Size of the flushing tunnel

is required to be decided for efficient transport of the

sediment. From the experience of studies carried out at

CWPRS, Pune, it is seen that minimum velocity of 3.0 m/s is

required for efficient functioning of the tunnels. In flushing

system of desilting basins, the concentration is likely to be

more due to higher settling efficiency. The flow in flushing

system is a pressure flow since the sediment enters in

flushing tunnel through the openings from main basin.

The flushing discharge is controlled by a gate at downstream

end.

8) Size and spacing of openings: The first opening from

the desilting basin to flushing conduit is required to be larger

to allow removal for higher rate of deposition and larger size

of particles. Though, no definite criteria can be suggested,

from experience of model studies for desilting basins for

various projects, size of the first opening should be adequate

to pass 20 to 30 % of the flushing discharge with a velocity

of 3.0 m/s. The total area of the openings can be broadly

estimated for passing the remaining discharge with velocity

of 3.0 m/s. The size of the openings may decrease

progressively towards the downstream as concentration and

size of the sediment settling goes on decreasing towards

downstream. This reduction could be done in steps on the

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basis of the practical considerations. It has also been observed

that the smaller size of the material settling near the outlet

end forms a reverse ramp at the upstream edge of the

skimming weir. The last opening should, therefore, be a little

larger than the opening just on its upstream.

IV. HYDRAULIC MODEL STUDIES

In absence of any definite criteria, the design of desilting

basins is based on many assumptions, broad guidelines and

site specific conditions. Verification of these assumptions and

adequacy of layout as well as other design aspects is therefore

required to be tested by conducting hydraulic model studies.

The basic aim of conducting these model studies is to judge

the hydraulic performance of desilting basin in terms of

settling efficiency and flushing efficacy of the settled

sediment. Central Water & Power Research Station, Pune has

conducted about thirty five physical model studies for

desilting basin for various hydropower projects in India and

abroad. On the basis of drawings supplied by concerned

project authorities, the model of desilting basin is fabricated

partly in fibre glass with transparent perspex windows and top

dome to observe the flow conditions and sediment movement

/ deposition pattern. These models are fabricated to a

geometrically similar scale ranging from 1: 20 to 1: 35

depending upon discharge, their shape and size and

availability of water head.

The inlet transition, outlet transition and silt flushing tunnel

below the desilting basin are also fabricated in fully

transparent perspex sheets. Generally, the model of desilting

basin is tested for inlet sediment concentration of 5000 ppm or

as otherwise indicated by the project authorities. A typical

view of desilting basin in model is shown in photo 2 and inlet

and outlet transitions in photo 3 and 4.

Photo 2: View of Desilting Basin Model

Photo 3: View of Inlet Transition in Model

Photo 4: View of Outlet Transition in Model

For simulation of suspended sediment crushed and sieved

walnut shell powder is used. This is a light weight material

with specific gravity of 1.32. The material is injected into the

desilting basin as per designed inlet concentration along with

flow at the inlet. The simulation of sediment between model

and prototype is done by fall (settling) velocity criteria.

V. DISCUSSIONS

The known volumetric quantity of sediment on the basis of

inlet discharge and sediment concentration is injected in the

model. There are two sediment collection chambers

constructed at the downstream of the model one each for head

race tunnel and SFT respectively. The sediment deposited in

desilting basin and flushed through SFT is collected in SFT

collection chamber and measured volumetrically. The settling

efficiency of the desilting basin is found out as follows:

( )

The settling efficiency obtained by above equation is the

overall settling efficiency for entire range of particle sizes of

inlet gradation curve. However, the objective is to find the

settling efficiency of 0.2 mm sediment particle size. To find

out the efficiency for particle size of 0.2 mm, calculations are

Silt Flushing Tunnel





carried out using the T.R. Camp’s criteria and these analytical

results are compared with physical model study results to

obtain settling efficiency for 0.2 mm particle size.

As indicated earlier, desilting basins are designed for 90 %

removal of sediment particle size of 0.2 mm and above. If the

efficiency is considerably less or more than 90 % then length

of basin is increased or decreased and again tested on the

model.

The efficacy of flushing system is judged by visual model

observations. In case there is some sediment deposition on bed

of inlet transition or in SFT, the design is slightly modified

and again tested on the model. An example of sediment

deposition at the inlet transition is shown in photo 5.

Photo 5: Sediment deposition on bed of inlet transition

On the basis of experience of CWPRS on findings of model

studies, a Technical Memorandum titled “Guidelines for

design of desilting basins (pressure flow)” was published in

year 2005. Another Technical Memorandum titled,

“Guidelines for operation of desilting basins” has been

published in year 2008 for practicing hydro engineers and

designers for efficient and trouble free use of these devices on

prototype. These memoranda are the only guidelines available

at present for the designers and are being immensely used for

the design and operation of desilting basins by various

agencies like NHPC, NTPC, SJVNL, Central Water

Commission and WAPCOS etc.

VI. CONCLUSIONS

Desilting basins are integral part of water conductor system of

ROR hydropower projects in Himalayan region and are huge

and costly underground structures. Once put into operation, it

is very difficult to maintain and repair them. On the other

hand, each project has its own site specific design considering

various parameters viz. inlet discharge, inlet sediment

concentration & gradation, gross head, desired settling

efficiency and flushing efficacy. Moreover, design of desilting

basins is based on various assumptions and broad guidelines.

Therefore, in spite of proper site planning and designing, their

hydraulic performance is required to be tested on a hydraulic

model to obtain optimum design for desired settling efficiency

of suspended sediment for each project.

Acknowledgements

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

CWPRS for his constant encouragement, guidance and kind

permission for publishing this paper. The authors are also

thankful to the various project authorities for providing the

necessary financial support and data for conducting the model

studies in absence of which it would not have been possible to

conduct so many model studies at CWPRS, Pune.

References

[1] CWPRS: “Guidelines for Design of Desilting Basins (Pressure Flow)”, 2005.

[2] Hazen, A. on Sedimentation, Trans ASCE, Vol LIII, 1904, p 63.

[3] Camp, T.R. Sedimentation and the Design of settling tanks, Trans ASCE, Vol 111, 1946, Paper No.2285.

[4] Dobbins, W.E. Effects of turbulence on sedimentation, Trans ASCE, Vol

109, 1944, p 629.

[5] Vetter, C.P.: Technical aspects of silt problem on Colorado river Civil

Engineering Vol.10, No.11, Nov.1940, pp 698-701.

[6] Rouse Hunter: "Engineering Hydraulics", John Wiley and Sons Inc. New York-1949, pp 811-814.

[7] T. CaS: Technical Conditions and Standards for designing settling basins

of hydropower stations- Moscow 1949.

[8] Einstein, H.A.: Final report spawning ground’ University of California

Hydraulic Engineering Laboratory 16 p, 2 tables 10 figs., 1965.

[9] Hippola, U.T.B.: Influence of suspended sediment distribution on settling basin design’ International symposium of river mechanics Bangkok

Jan.1973, pp 277 to 288.

Sediment deposition

Importance of Desilters in Run-Of-River Hydros

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