open power channels – code of practice

8/16/2019 OPEN POWER CHANNELS – CODE OF PRACTICE

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For official use only DOC: WRD 14 (547)PJuly 2010

BUREAU OF INDIAN STANDARDS

PreliminaryDraft

Indian Standard

OPEN POWER CHANNELS – CODE OF PRACTICE(Second Revision IS 7916)

(Not to be reproduced without the permission of BIS or used as standard)-----------------------------------------------------------------------------------------------------------------------------------

Last date for receipt o f comments is 31-08-2010-----------------------------------------------------------------------------------------------------------------------------------

FOREWORD(Formal clauses will be added later)

This Indian Standard was first published in 1975. The first revision was taken up in the light ofexperience gained during the use of this standard.

The salient features of the first revision were as under:

a) Provision of 25 percent extra freeboard to be taken at the time of design of cross-section ofpower channels as due to passage of time the freeboard originally kept gets enchroached andalso due to deposit of silt in the bed during the lean discharge period, the carrying capacity ofthe power channel gets reduced;

b) Provision of some arrangements so as to facilitate grouting behind concrete lining as due towave action in the power channel, the drainage material behind the lining gets sucked andcavities is created on the back of the lining;

c) Provision of crest at the outlet to control the rate of drawdown was deleted;

d) Provision of wave suppressors in the head reach for dissipation of energy; and

e) Optimum design of power channels carrying considerable sediment.

The present revision has been taken up in the light of the experience further gained during the use ofthis standard. The salient features of this revision are as following:

a) The terminologies in 3 have been enlarged.

b) Few additional points have been added in the clause on planning and layout.

c) In 6 surge analysis has been elaborated and the freeboard due to wave runup and wind setuphas been added.

d) A new clause on the design on forebay has been added.

e) A new clause on the instrumentation has been added.

1 SCOPE

1.1 This standard covers planning, layout, design, construction and maintenance of open powerchannels meant to serve as water conductor system for a hydro-electric power project. A powerchannel may be used simultaneously as a headrace channel or a tailrace channel if the hydro powerscheme is provided with series of run-of-river plants or pumped storage plants.

1.2 Generally, the forebay is provided with the power channel, and therefore this standard also coversthe design of forebay.


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2 REFERENCES

The Indian Standards given in Annex A contain provisions which through reference in this text,constitute provisions of this standard. At the time of publication, the editions indicated were valid. Allstandards are subject to revision, and parties to agreements based on this standard are encouraged toinvestigate the possibility of applying the most recent editions of the standards.

3 TERMINOLOGY

3.0 For the purpose of this standard, the definitions given in IS 4410 (Part 5) and the following shallapply.

3.1 Surges

Surges are a class of hydraulic transients in water conductor systems in which variations of flow andpressure occur when one steady state changes into another steady state due to rejection and/oracceptance of load on hydro-power station units. In power channels surges cause sudden changes inwater level which have to be taken into account in design.

3.1.1 Positive Surge and Negative Surge

A surge producing increase in depth is called ‘positive surge’ and the surge causing decrease in depthis called ‘negative surge’. A surge can travel either in the upstream direction or in the downstreamdirection. Therefore, four basic types of surge can be formed.

3.1.2 Positive Surge Travelling Upstream

This is caused by reduction of flow in the downstream of power channel either due to decrease in thepower output of load rejection. Another example is entry of tide in a estuary (see Fig. 1).

3.1.3 Positive Surge Travelling Downstream

This is caused by sudden opening of upstream turbine guide vanes wicket gate, rapid flooding of riverinto power channel, sudden increase in the flow upstream of power channel, dam break etc (see Fig.1).

3.1.4 Negative Surge Travelling Upstream

This is caused by increase in flow in the downstream reach, sudden opening of turbine gates toincrease the power generation (see Fig. 1).

3.1.5 Negative Surge Travelling Downstream

This is caused by rapid reduction of flow in the upstream area rapid drop in the water level in the riveretc (see Fig. 1).

3.2 Drawdown

It is the extent of fluctuation of water levels in the channel, the volume of water contained therein beingused as pondage for peaking purposes.

3.3 Forebay

The forebay is a transition between reservoir or power channel and power intake. Generally, deepintakes do not require forebay.


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3.4 Celerity

The velocity of propagation of wave front is called celerity and normally expressed with respect to initialfluid velocity. If fluid is at rest, the absolute velocity of wave will be identical to celerity. If fluid itself is inmotion, the absolute velocity of wave will depend upon velocity of fluid and the celerity of wave.

3.5 Wave Runup

The rush of water up the bank or side slopes of power channel on the breaking of waves from thereservoir. The amount of run up is the vertical height above still water level to which the rush of waterreaches.

3.6 Signif icant Wave Height

The average height of the one-third highest waves of a given wave group or wave series.

3.7 Wave Steepness

The ratio of wave height to wavelength.

3.8 Wind Setup

Wind blowing over an enclosed body of water exerts a horizontal force that causes build up in the levelalong the leeward side. There will be a similar reduction in the water level in the opposite direction. Thevertical rise in the still water level caused by the wind stresses on the surface of the water is defined aswind setup.

3.9 Fetch Length

The horizontal water distance in the direction of wind over which wind blow or creates wind setup.

4 NOTATIONS

A1 = Cross-sectional area at section 1 (see Fig. 1) A2 = Cross sectional area at section 2 (see Fig. 1)

B = Width of rectangular power channel

C = Celerity of wave or celerity with respect to initial flow velocity V1.

D´ = Average water depth in feet along the fetch line

Eh = Form loss in the forebay transition

Ev = Change of velocity head in the forebay transition (V2/2g).

F = Effective fetch in miles

F´ = V/(gY)½

F1 = V12/g.Y1

g = Acceleration due to gravityh = Height of surge wave in power channel ( h = Y1 –Y2)

H0 = Height of any specific water wave due to wind in feet

Hs = Significant wave height in feet

K = Velocity head in the power channel ( K = V2/2g)

k = Form loss coefficient in the forebay transition

L = Length of power channel

Lf ́ = Wind fetch in miles (this is equal to twice the effective fetch that is Lf ́=2.F)

L0 = Length of wave in feet ( L0 = 5.12 T2)


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q1 = V1.Y1

q2 = V2.Y2

Q = q1/q2

Q1 = Power channel discharge before surge

= Power channel discharge after surge

R = Wave runup

Sw = Wind setup in feet above the reference level.

T = Time between wave crest in seconds that is wave period

V = Average flow velocity in power channel/forebay at any time t

V1 = Initial velocity in power channel/initial velocity at time t = 0

Vw = Absolute velocity of wave propagation

Vwi = Average wind velocity in miles per hour over water

V´ = Reduced velocity of power channel

V = Ratio of discharge released by spillway or waste weir of power channel to the wetted crosssectional area of power channel

Y = Flow depth at any time ( t) in power channel or forebay

Y = Y1

.Y2

Y1 = Depth of flow before surge or depth of flow at time t = 0 or depth of flow at section 1 ( see Fig.

1)

Y2 = Depth of flow at section 2 (i.e. depth after surge, see Fig. 2)

Y1' = Centroid depth for cross-sectional area A1 ( see Fig. 1)

Y2' = Centroid depth for cross-sectional area A2 ( see Fig. 2)

= Slope of power channel embankment with the horizontal

´ = Angular variation of power channel side wall with respect to power channel centre line

5 PLANNING AND LAYOUT

5.1 Planning

Provision of Power Channel versus Pressure Conduit

The power channel is provided as an alternative to the pressure conduit. In the case of pressureconduit, it need not follow surface contours and there is greater freedom for the conduit alignment,gradient and higher permissible velocity. On the contrary, the power channel can be used for irrigation,navigation and also to minimize head losses preferably in the low head plants by providing large cross-section and permitting low velocity. The choice between power channel and pressure conduit isdictated by many factors such as

(i) Site conditions,

(ii) Convenience,

(iii) Economy,

(iv) Low velocity and low head losses.

5.1.1 Classification of Power Channel Based on Operations

The design of power channel is determined by the type of operation. The following three type ofoperations are normally adopted for the low head power plants:

a) Constant discharge in channel and constant water levels upstream and downstream withbypass arrangements from upstream to downstream (see Fig. 2);


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b) Channels with balancing reservoir to take care of diurnal fluctuations. This will in many casesresult in reduced capacity requirements of the channel upstream of the reservoir (see Fig. 3);and

c) Lock operation of channel, in which the channel is used similar to a lock and the discharges inchannel fluctuate with load of generating stations (see Fig. 4).


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5.1.2 Discharge Capacity of Power Channel

A prerequisite in the planning of power channel is to fix the discharge capacity. This discharge capacityshould be fixed on the water power studies to be made for arriving at the installed capacity of the powerplant. These studies include the flow duration curve and mass curve for available discharge or storagecapacity of reservoir and extent of balancing storage/pondage requirements to be provided at suitable


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points of water conductor system to suit the load demand and type of operation of power station(namely, peaking station, base load station, etc).

In the case of stage development of the project, the capacity of the channel may have to be fixed forthe ultimate stage of power development as permissible by economic considerations.

5.1.3 Power channels shall generally be lined to avoid loss of water.

5.1.4 The slope of power channel shall be as flat as possible to avoid loss of net head on turbine.

5.2 Layout

For designing the layout, reference shall be made to IS 5968. While it is desirable to fix the take-off atthe highest possible elevation for obtaining maximum head for power generation, topographical andgeological features of the terrain shall be kept in view while planning the alignment of power channelwith sufficient care taken to avoid continuous high embankments.

5.2.1 Economic Length and Cross-Section of Power Channel

To achieve economy, various alternative layouts may be examined. For example, the alternativelength, head and discharge for power generation may be examined as mentioned below:

— A layout with a nominal height of diversion structure or without any diversion structure mayrequire longer length of power channel and it is likely that this type of arrangement may requiremany cross diversion works. But provisions of navigation may be easier in this type of layout.

— Similarly, an alternative arrangement with a high diversion structure may require smaller lengthof power channel. But this alternative may involve large scale submergence of upstream areasand provision of navigation channel may be required to be developed separately for this type oflayout.

5.2.2 If the flow approaching the power channel is more than 15° skew, flow separation at the leadingedge of power channel and circulation will be significant. The circulation generated at the leading edgeof power channel is a function of power channel length to width ratio and flow approach angle. Tominimize flow separation, the power channel length (L) to width (B) ratio that is (L/B) should be as high

as possible and the minimum L/B should be at least three. Excessive curvature in the power channelmay also cause flow separation.

5.3 Data Required

The following data are required for planning and layout of open power channels;

a) Topographical map of the area;

b) Sub-surface data on soil characteristics including mechanical properties and shear parameters;in case the sub-grade is in rock, data regarding the structure and fabric of the rock, depth anddegree of rock strata variation, effect of weathering and data regarding stability of rock andearth slopes;

c) Texture and composition of soil;

d) Permeability of sub-grade strata in relation to seepage loss and for design of filters;

e) Water availability;

f) Sub-soil water level, annual fluctuations in its levels in the area and quality of sub-soil water;

g) Drainage facilities of the areas including possibility of water logging and salination;

h) Quantity and gradation of sediment load expected in the power channel;

j) Permissible sediment grade through water turbines;


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k) If any problem due to floating or submerged debris is anticipated in the river or the reservoir,suitable type of debris control measures such as floating booms, breast wall etc (see Fig. 5).

l) Discharge requirements of irrigation system, if the channel has to cater for irrigation also;

m) Power demand and load curve of the area and future extension of power demand;

n) Seismicity of the region and value of ground acceleration;

o) Availability of suitable construction materials, including filter materials, for proposed drainagesystem lining;

p) Existing communication and transport facilities; and

q) The effect of wind, wave runup scour, erosion etc.

5.4 Adequate investigations shall be carried out to collect data indicated in 5.3 in all respects.

5.5 For general guidance regarding suitability of soil for use in power channel embankment reference

shall be made to IS 1498 and IS 4701.

6 DESIGN OF POWER CHANNEL

6.1 Design of Cross-Section of Power Channel

— The cross-section of power channel, bed slope, etc., are designed on the basis of economicstudies considering the cost of construction and cost of energy lost due to head loss in friction.

— At constant bed slope, the rugosity coefficient (n) decreases by lining of power channel andtherefore higher velocity in the power channel can be obtained which in turn reduces cross-section of channel.


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— Alternatively, for a constant cross-section, the unlined channel will require steeper bed slopethan a lined channel.

— The side slopes of the channel section should be designed to suit the drawdown conditions inthe power channel given in 6.5. For designing the channel section reference may be made to IS7112 and IS 10430.

6.2 Freeboard for Power Channel

6.2.1 General

The freeboard in a power channel is determined by the following factors:

a) Freeboard due to surges in power channel.

b) Freeboard due to wave runup and effects of wind setup.

c) Decrease in the channel depth caused by sedimentation.

d) In the case of earth embankments of unlined power channel, the scour and erosion by waveaction are also required to be considered.

Adequate freeboard should be provided to avoid overtopping of water on channel sides which may

endanger the channel section. At the time of design of cross-section about 25 percent extra freeboardmay be provided to avoid reduction in power generation in subsequent years.

In addition to above mentioned factors, IS 10635 may also be referred.

6.2.2 The freeboard due to surges in power channel requires analysis of surges.

— The phenomenon of surges due to variation of discharge on account of load demand orrejection should be analyzed fully.

— Depending upon the topography, provision of suitable spillover or balancing reservoir ofsufficient capacity to act as an open surge basin at forebay may be considered.

6.2.3 Factors involved in the analysis of surge phenomenon are as follows:

a) Hydraulic section, slope of the channel and velocity of flow in the channel;b) Magnitude of load rejection or load acceptance;

c) Rate of closure of units or acceptance of load;

d) Size of forebay or surge basin on the channel; and

e) If the power channel has bypass channel, than surge analysis should be carried out for thebypass channel also.

6.2.4 Types of Surge Analysis

The following four types of analysis should be carried out.

a) Load rejection: Positive surge travelling upstream

b) Load acceptance: Negative surge travelling upstream

c) Rapid flooding from river into power channel: Positive surge travelling downstream in the powerchannel

d) Rapid drop in water level in the river: Negative surge travelling downstrem in the powerchannel.

In the abnormal operation of turbine with rapid change in the river water level, it may be possible toinduce positive waves at both ends of power channel which would be additive.


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6.2.5 Criteria for analysis of the maximum and minimum surges in power channel should be the sameas for the surge in head race tunnels. The maximum surge height in a power channel due to loadrejection may be calculated by methods given in 6.2.7.

6.2.6 The method of analysis of a positive surge is different than a negative surge because the shapeand profile of a positive surge is more stable and preserved whereas the shape of negative surge isunstable, varies with time, gradually flattens and wave profile gets stretched.

The method of analysis for the surge given in 6.2.7 and 6.2.8, friction is not considered and thereforesuch analysis is valid for power channel having shorter length. In case of very long power channel andbypass channel, friction is important.

6.2.7 Analysis of Positive Surge

The following three methods can be used for the analysis of positive surge:

a) First method

For a rectangular channel with a horizontal bed or negligible bed slope and negligible friction,the height of surge wave is obtained from the following equation.

h = ( Q1 – Q2/C.B. …(1)

C = [ g.y1 {1 + (3 h/2y1)}]½ …(2)

where

h = height of surge wave,

C = celerity of wave,

Q1 = discharge before surge,

Q2 = Discharge after surge,

B = width of rectangular channel, and

y1 = depth of flow before surge.

The equation (2) is called Saint-Venant celerity equation. The derivation of equation (2)assumes that the power channel has a small longitudinal slope and the kinetic energy

correction factor is one.

For the non-rectangular channel, the ‘flow depth’ (y1) in the rectangular channel may be

replaced by ‘mean depth’ of non-rectangular channel which is defined as

wetted area of cross-section of channeltop width of flow surface of channel

Equations (1) and (2) are solved by trial-and-error method. As an approximation to theequations (1) and (2), the momentum theorem is applied to the flow in a rectangular channel,

ignoring higher order terms of h2, h3 and assuming total closure (final flow velocity zero), thefollowing explicit equations are obtained (E. Feifl’s equation).

hmax = K + ( K2 + 2 Ky1)

½] (for sudden total closure) ....(3a)

For gradual and complete closure, the time required for complete closure should be longer thanthe time required for the first wave to travel twice the length of power channel. In that case, thesurge height is:

hmax = [ (K/2) + V1 . ( Y1/g)1/2] …(3b)

where hmax = maximum surge wave height, K = ( V1)2/2g = Velocity head and V1 = mean

velocity of flow before surge.

The equation (3a) can be used for sudden total closure as well as for sudden partial closure.For the sudden total closure initial velocity is V 1 and final velocity is zero and change of velocity


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is (V1 – 0 = V1). In the case of sudden partial closure, initial velocity is V1 and reduced velocity

is V´ and change of velocity is (V1 – V´). Therefore, in the case of sudden partial closure, V1

will be replaced by (V1 – V´).

If the power channel is provided with waste weir or spillway to discharge the surplus flow, themean velocity ‘V1’ will be replaced by (V1 – V´ – V ) where V = (ratio of discharge released

by Spillway or weir to the wetted cross-sectional area of power channel).

(b) Second Method

Case (1) - for positive surge moving downstream (see Fig. 1).

Vw = V1 + C ….(4)

(Vw – V2) Y2 = ( Vw – V1)y1

for a non-rectangular channel ….(5)

C = [{ A2Y2' – A1Y1'}g/{ A1 – ( A1/ A2)}]0.5 ….(6)

for a rectangular channel ….(7)

C = [{ gy2(y1 + y2)}/2y1]0.5

Case (2) – for positive surge moving upstream (see Fig. 1).

Vw = C – V1 ….(8)

(V2 + Vw)Y2 = ( V1 + Vw)y1 ….(9)

for a non-rectangular channel

C = [{ A2Y2' – A1y1'}g/{ A1 –( A1/ A2)}]0.5 ….(10)

for a rectangular channel

C = [{ gy2(y1 + y2)}/2y1]0.5 ….(11)

where A1 and A2 = cross-sectional area at section 1 and 2;

y1' and y2' = centroid depths of area A1 and

A2; y1 = depth of flow at section 1 (that is depth before surge),

y2 = depth of flow at section 2 (that is depth after surge);

C = celerity with respect to initial flow velocity;

V1 and Vw = Absolute velocity of wave propagation.

The equation (4), (5) and (6) or (7) for the case (1) and equation (8), (9) and (10) or (11) for thecase (2) contain five variables V1, V2, Vw, Y1 and Y2. If any three known variables are

specified, the other two variables can be evaluated by trial and error.

By eliminating Vw

and C, from equation (4), (5) and (7) for the case (1) or from equation (8), (9)

and (11) for the case (2), the following common equation for positive surge moving upstream ordownstream is obtained and this relation is known as Johnson equation.

2 2 2

2 1 1 2 1 2

2 1

( ) ( )

2 ( )

Y Y Y Y V V

g Y Y

− −

=

−

….(12)

(c) Third Method

The solution of equation (12) requires trial and error. It can also be solved graphically by

substituting Y = Y1/Y2, V1.Y1 = q1, V2.Y2 = q2, Q = q1/q2 and F1 = V12/g.y1 in the equation

(12) which transforms in the following equation


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(Y – 1). [( Y ( y+1)0.5] = (2F1)0.5.[(Y – Q)] …..(13)

Each side of equation (13) is plotted as an ordinate against Y as abscissa. The plotting of ( Y –

1). [(Y.(Y+1)0.5] versus Y will be a curved line while plotting of (2 F1)0.5.[(Y – Q)] versus Y will

be a straight line with slope of (2F1)0.5 and ordinate intercept of ‘minus’ (2 F1)

0.5.Q and

therefore this line can be constructed for a given F1 and Q. The intersection of curved and the

straight line represents a solution of the equation (13) (see Fig. 6).


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6.2.8 Analysis of Negative Surge

The negative waves are not of practical interest for the freeboard but it may be important to determinethe minimum depth (see Fig. 1, Type 3 and Type 4).

V = [ V1 – 2 ( g)0.5.{(y1)

0.5 – ( y)0.5}] …..(14)

C = [ V1 – 2. ( gy1)0.5 + 3 ( gy)0.5] …..(15)

Height of surge = (y1 – y) …..(16)

The equation of liquid surface is X = C.t …..(17)

where

V = velocity at any time t,

y = depth at any time t,

V1 = initial flow velocity at time t = 0,

y1 = depth at time t = 0, C = celerity ( see Fig. 1).

6.2.9 When the power channel is part of a water conductor system including tunnels, hydraulictransient conditions for the whole system should be studied for load variations so the power units.

6.2.10 Determination of Freeboard Due to Wind Setup

This can be determined from the Zuider formula,

Sw = {( Vwi.Vwi). Lf ́ }/(1 400.D´) …..(18)

where Sw = wind setup in feet above the reference level, Vwi = average wind velocity in miles per hour

over water, Lf ́ = wind fetch in miles which is equal to twice the effective fetch and D´ = average depthin feet along the fetch line.

6.2.11 Determination of Freeboard Due to Wave Runup

The wave runup for deep water wave depends on structure shape, relative surface roughness,permeability, water depth at structure toe, bottom slope in front of a structure and characteristics ofwind generated waves which is measured by the ratio between wave height and wave length. Thecharacteristics of wind generated waves depends on the dimensionless parameters, gT/Vwi,

gHs/(Vwi)2 and gF/(Vwi)

2,

where g = acceleration due to gravity, T = time between wave crest in seconds that is wave period, F =effective fetch in miles, Vwi = wind velocity in miles per hour and Hs = significant wave height in feet.

Deep water is defined as depths greater than about one-third to one half the length of the wave. Thefollowing relationship for deep-water waves in reservoir relating to significant wave heights, fetch, windvelocity and the minimum duration of wind velocity (ratio of fetch to wind velocity component indirection of fetch) can be used.

gHs/(Vwi)2 = 0.002 6.{ gF/(Vwi)

2}}0.47

…..(19)

The relation between wave period (T), wind velocity and fetch is

gT/Vwi = 0.45.{ gF/(Vwi)2}0.28 …..(20)

Fig. 7a is a definition sketch for wave runup and Fig. 7b provides the curve for the wave runup (R). In

Fig. 7b, H0 = height of specific wave in feet, L0 = length of wave in feet ( L0 = 5.12 T2) and T iscomputed from the equation (20) and = slope of embankment with the horizontal.


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The Shore Protection Manual [U.S. Department of the Army, Coastal Engineering Research Centre(CERC)] provides several curves for the wave runup of regular waves (monochromatic waves) onsmooth, impermeable slopes, rip rap slopes etc. The curves pertain to the relative runup (R/Ho) as a

function of structure slope and Ho/gT2.

6.3 Lin ing for Power Channel

Power channel should preferably be lined since;

a) It is hydraulically more efficient thus ensuring smaller cross-section, relatively flatter slope forthe same discharge capacity resulting in economy;

b) Loss of water due to seepage or leakage is minimized;

c) Closure of power channel for repairs, if any, are remote and also they would be of shortduration only thus interrupting power generation for brief periods only;

d) Cost of operation and maintenance is lower;

e) Weed growth is minimized; and

f) Some suitable arrangements be provided originally on cement concrete panel to enablefacilitate sand grouting behind concrete linings. Sand grouting is essential to avoid settlement ofcement concrete.

6.3.1 Construction of Lining


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The type of lining to be provided in the power channel depends on the field conditions. For selection ofthe type of lining reference may be made to IS 10430. For construction of lining reference may bemade to IS 3872, IS 3873, IS 4515, IS 7113, IS 7873, IS 9698 and IS 11809.

6.3.2 Under Drainage

Adequate precaution should be taken for safety of lining of the channel where the alignment is throughterrain with very high ground water level and is subjected to high seasonal variations or where the soilis sufficiently impervious to prevent free draining of seepage or leakage from the channel. For this,suitable under drainage should be provided to protect the lining. Reference may be made to IS 4558 fordesigning the under drainage system for the power channels.

6.4 Drainage

In the case of fluctuation of water levels due to surges and diurnal fluctuations in channel operations asmentioned in 5.1.1(b) and 5.1.1(c), the effect on lining should be provided by suitable drainage system.It is preferable to utilize natural drainage to drain the system rather than a flap value arrangement.


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6.5 Drawdown

Depending upon the pattern of load and extent of storage provided, fluctuations of water level occur inthe power channel due to utilization of balancing storage. The effect of drawdown and the rate at whichit occurs govern stability of the side slopes and lining of the channel. When such cases of excessiveand rapid drawdown occur, suitable automatic control structures may have to be provided to regulatethe rate of drawdown. For instance automatic gates at the outlet may be provided to control the rate ofdrawdown.

6.6 Sediment Contro l

6.6.1 Necessary desilting arrangement should be provided to remove sediment content to safe degreefor operation of generating units. The quantity of sediment that can be permitted depends on the typeof turbine, its head, the size and mineralogical content of the sediment. The exact requirement shouldbe based on the design of turbine. The channel upstream of desilting arrangement, should be providedwith extra capacity to allow for discharge required to flush out the sediment.

6.6.2 The power channel should be so designed as to take care of power requirements along withbalancing reservoir capacity including peaking periods and control of entity of sediment beyondpermissible limits.

6.6.3 For design of sediment control devices reference may be made to IS 6004, IS 6522, IS 7495, IS7871 and IS 7880.

6.7 Head Regulator

For design of head regulator reference may be made to IS 6531.

6.8 Trash Rack

Suitable trash racks should be provided at the exit end of power channel (that is at the forebay portion),at the location of the penstock off-take, to avoid trash entering the penstock which would otherwisedamage parts of the generating unit. To design the trashrack, reference may be made to IS 11388.

6.9 Bypassing Arrangements

Provisions should be made for bypassing arrangements in the balancing reservoir or in power channelsas near to the balancing reservoir as possible, in the event of sudden load rejection (see Fig. 5). Also inthe case of very long power channels provisions may be made for escape regulators (see IS 6936) inpower channel for emptying it in the case of emergency/periodical closure, if any.

6.10 Wave Suppressors

Provision may be made for wave suppressor in the head reach in the event of incomplete dissipation ofenergy upstream of power channel.

7 DESIGN OF FOREBAY

The hydraulic design of forebay should consider the following:

(i) The upstream transition between the power channel and the forebay and the downstreamtransition between the forebay and the intake should be uniform.

(ii) The introduction of transition profile should lead to minimum energy loss and velocity distributionat the end of forebay should be uniform.

The following guideline should be adopted for the design of transitions.


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First method: The transition between the power channel and forebay is designed on the basis offollowing criteria.

Tan ( ´) = [1/3F´)], where ’ = Angular variation of side wall with respect to channel centre line,

F’ = V/(gy)½, V and Y are average velocity and the depth at transition entry and exitrespectively.

Second method: The length of tradition varies from 2(B2 – B1) to 2.5 (B2 – B1), where B2 is the width

at the end of transition and B1 is the width at the beginning of the transition. The width Bx at anydistance x from the beginning of the transition:

Bx = ( B1.B2.L)/[(LB2 – ( B2 – B1)x] (Hyperbolic transition)

Third method: This method is commonly employed in the design of flume and syphon transitions and itis called Hinds’ transition.

a) The form loss (Eh) along the transition is calculated from Eh = k.Ev’ ́where k = 0.2 to 0.3

and Ev = the change of velocity head ( v2/2g) between the two sections under

consideration. The loss due to friction is neglected.

b) The depth of flow downstream of transition is assumed to be normal depth and this acts asthe control section for the upstream flow. The normal depth at the downstream of transition

is calculated. Based on this normal depth, the total energy line at the downstream oftransition is calculated.

c) Then the position of total energy line at the upstream end of transition is determined. The

bed of channel at the upstream section should be (y + V2/2g) below this energy line, wherey and V are the depth and velocity at the beginning of the transition.

d) A straight line variation or two reverse parabola may be assumed between the bed level atthe two transitions.

(iii) The flow separation and verticity should be minimum and velocity should be uniform. Excessivecurvature should be avoided.

(iv) Excessive curvature may cause super elevation in the water levels. The difference in theenergy grade line (hydraulic grade + velocity head) in the lateral direction induces unevendistribution of flow and flow separation.

(v) In some case, hydraulic model studies are required to determine the optimum geometry andstreamlined boundary of forebay. Generally, the uniform velocity patterns in the forebay areachieved by provision of splitters, turning vanes etc.

8 INSTRUMENTATION

Water level sensors are used to measure the water level in the power channel and in the tailracechannel to determine the gross head. The gross head is adjusted for the loss of head and net head isfed electronically to the turbine governor which automatically modifies the angle of the runner forefficient operation of the turbine.

9 MAINTENANCE

9.1 For useful and economic operation of power plants, proper maintenance of power channels isessential. Reference in this regard may be made to IS 4839 (Part 1), IS 4839 (Part 2) and IS 4839 (Part3).


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18

ANNEX A

(Clause 2.1)

LIST OF REFERRED INDIAN STANDARDS

IS No. Title

1498 : 1970 Classification and identification of soils for general engineering purposes(first revision)

3872 : 2002 Lining of Canals with Burnt Clay Tiles - Code of Practice (first revision)

3873 : 1992 Code of practice for laying cement concrete and store slab lining on canals(second revision)

4410 (Part 5):1982

Glossary of terms relating to river valley projects: Part 5 Canals (firstrevision)

4515 : 2002 Store pitched lining for canals—Code of Practice (second revision)

4558 : 1995 Code of practice for underdrainage of lined canals (second revision)

4701 : 1982 Code of practice for earthwork on canals

4839 : Code of practice for maintenance of canals

(Part 1): 1992 Unlined canals (second revision)

(Part 2) : 1992 Lined canals (second revision)

(Part 3) : 1992 Canal structures, drains, outlets, jungle clearance, plantation and regulation(second revision)

5968: 1987 Guide for planning and layout of canal system for irrigation (first revision).

6004 : 1980 Criteria for hydraulic design of sediment ejector for irrigation and powerchannels (first revision)

6522 : 1972 Criteria for design of silt vanes for sediment control in offtaking canals.

6531 : 1994 Criteria for design of canal head regulators (first Revision)

6936 : 1992 Guide for location, selection and hydraulic design of canal escapes (firstRevision)

7112 : 2002 Criteria for design of cross-section for unlined canals in alluvial soil (firstRevision)

7113 : 2003 Soil-Cement Lining for Canals - Code of Practice (first Revision)

7495 : 1974 Criteria for hydraulic design of silt selective head regulator for sedimentcontrol in offtaking canals

7871 : 1975 Criteria for hydraulic design of groyne walls (curved wing) for sedimentdistribution at offtake points in a canal

7873 : 1975 Code of practice for lime concrete lining for canals

7880 : 1975 Criteria for hydraulic design of skimming platform for sediment control inoffttaking canal

9698 : 1995 Lining of canals with polythene film - Code of practice (first revision)

10430 : 1982 Criteria for design of lined canals and guidelines for selection of type oflining

10635 : 1993 Freeboard requirements in embankment dams—Guidelines (first revision)

11388 : 1995 Recommendations for design of trashrack (first revision)

11809: 1986 Lining for canals by stone masonry - Code of practice (first revision)


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19

REMARKS BY MEMBER SECRETARY

Comments/ suggestions received during previous circulation is given below and may be lookedby members along with the P-Draft for specific comments/ inputs:

Cl No. Commentator Comment/ Proposed change

Cl. 4 CWPRS Dimensions in S.I. units should be mentioned.Cl. 6.2.7,6.2.8, 6.2.10,6.2.11, 7.0

CWPRS Formulae should be expressed in equivalent S.I. units.

Title CES Title be changed to ‘Guidelines For Design of Open Power Channels’

ForewordPara 2, (b)

(Grouting behind concrete lining)b) Proper inverted filter material/porous concrete could be better choicethan grouting as the later will defeat the purpose of drainage.Therefore filter arrangement with suitable outlet may be adopted.

3.0 CDO Terminology : Definition given in IS 4410(Part V) are made applicable.It appears that definitions given in IS 4410(Pt X) may also be applicablefor this clause. This may please be confirmed.

4.0 CDO For cross sectional area section 1-2 (i.e. A1, A2), reference of figure 1is cited, whereas in respective Fig. 1 location of section 1 & 2 are notshown. This aspect needs attention.

Units under description of Notation are not given/mentioned. It shouldplease be mentioned wherever relevant for more clarify.

Values of D, F, Ho, Hs etc. are shown in FPS units. But as per currentpractice it should be appropriate to use S.I. units and formulae for thesame can be corrected to account for this conversions.

In notation Ev, change of velocity head is shown as V2/2g, instead itshould be V

2/2g (Sq.V/twice if),

Nomenclature for F, F’ needs to be mentioned in Notations clause q1,q2, Q1, Q2 locations needs to be shown on figure of surges for moreclarify. Also location of Y1’, Y2’, needs to be shown on referred figure.

Page 4 : In notation Y2’, reference is given of Fig. 2 which needs to becorrected as fig. 1.

4.0 BODHI Notations – In figure (1) on page 19, Y1, and Y2 have notbeen shown as reference under clause notations.

On cl. 4 for Y2’ and Y, of figure (2) is given, while it should be figure (1).Definition of V2 is not given on cl. 4.

6.2.1, 6.2.10,

6.2.11

CES Freeboard due to wind generated waves described in IS 10635 is

applicable to inland reservoirs. This is now proposed to be applied inPower channel. These waves are fetch/effective fetch dependent. Iffetch is considered along the length of channel, there needs to be someobstruction in the channel for wind waves to break and tide over theslope of such obstruction to effect the wave run. However, if cross-section of channel is considered, the fetch will be confined to width ofchannel at water surface. All these factors need clarification.

In case IS 10635 is considered applicable in channel also, thenreference of the code is adequate. Repetition of its contents at 6.2 to6.2.11 in this code may not be necessary.


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6.2.7 CDO To calculate the height of surge wave, the formula is described. But indescriptions of notations used in this formula units are not mentioned,which needed to be described please.

6.2.7 & 6.2.8

BODHI In the analysis of surge, it is mentioned that analysis is valid for powerchannel having shorter lengths. And, in case of very long powerchannels and byepass channel, friction is important.

In the above para, it is pointed out, that the definition for long and shortpower channel should be given alongwith friction analysis in the BIScode of practice to make it more explanatory and illustrative.

6.3(f) CES No settlement is expected in the well compacted layer of filter material.Moreover, grouting of sand will defeat the basic purpose of filter.Properly designed filter layer will avoid its getting sucked.

6.4 CES Following be added at the end:“Suitable cross drains leading the water from filter layer to the naturaldrains will be necessary in such cases.”

6.9 BODHI Reference of fig(5) on page (21) is not applicable to byepassingarrangement as

BODHI - Bureau of Design, BhopalCWPRS - Central Water & Power Research Station, PuneCDO - Central Designs Organization, NashikCES - Consulting Engineering Services (India) Ltd, New Delhi

open power channels – code of practice

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