Some Important Aspects in Civil Works Design of a Mini Hydro Power Project

8/12/2019 Some Important Aspects in Civil Works Design of a Mini Hydro Power Project

1/16

SOME IMPORTANT ASPECTS IN CIVIL WORKS DESIGN OF A

MINI HYDRO POWER PROJECT

By Eng Upali Mallaa A!a"#"#i

A$%&!a"&

Civil Works of a mini hydro power project have some distinct characteristics. One of thesignificant features is that the true contribution of the quality of Civil Works to theprojects performance is not adequately reflected in the capital cost component. Unlike inthe case of lectro!mechanical Works where products of standard designs which can bedirectly incorporated in the power plant are available" location specific factors restrict theuse of standard designs in Civil Works. #herefore" quality of civil works largely dependson a thorough understanding of site conditions" accuracy of surveys" involvement ofe$pertise in different fields of civil engineering and more particularly" a very systematicdesign procedure which ensures an economical and workable design. $amination of the

site to assess the relative merits of alternative solutions to location of components of theproject would be the first step in the design process. %ecision on the best alternative istaken based on economy and technical feasibility. &n the detailed design too" a similarprocedure should be adopted in selecting the most suitable type and si'e of the structure.Use of the most appropriate design parameters in both hydraulic and structural design isalso vital in ensuring a good quality design. (anagement of the design process mustensure obtaining appropriate e$pert opinion and reference to literature to decide suchparameters and recording them for reference. #he )uthor highlights in this paper" somespecial considerations in designs and the methodology to achieve a good quality designof civil works of a mini hydro power project. &t is e$pected that the paper would providesome useful guidelines to civil engineers and hydro power developers regarding the

procedure to be followed in the design.

'( In&!)*+"&i)n

*ydro power generation means the conversion of potential energy available in waterflowing in a stream to electrical energy with the use of a turbine. #his process essentiallyinvolves diversion of water at a point of higher elevation and conveying it to the turbineinstalled at a lower elevation. #he energy generated is directly proportional to thedifference in elevation +available head, and hence the process of diversion of water

should be done with great care to save the potential energy i.e. head loss should be keptto a minimum. ) series of Civil ngineering structures are built for this purpose" namely"diversion dam" intake" head race channel" sedimentation tank" forebay tank" penstock line"power house and tail race.

#he above set of structures is common for any hydro power project. #here are somedistinguished features in a mini hydro power generating facility +Capacity within therange -. / - (W,. 0ome of those features of interest are1


2/16

(ini hydro power projects are generally 2un Off the 2iver +2O2, type and hence

storage of water is not required. #herefore a short weir is used across the streamfor diversion. nvironmental impact is generally less compared to reservoir typeprojects.

Cost of Civil Works is generally in the range of - / 34 5 of the total capital cost.

#his cost ranges between -- / 34- U06 per kW capacity +2s 7---! 894--,. #hiscan also be represented as -.-9 / -.-7 U06 per kWh of average annual energygeneration +2s 3.-- / 9.4-,. &f the cost of Civil Works is close to the upper limitof the range" financial viability of the project is in doubt.

#hese facilities are operated at relatively low flows and hence any deficiency in

performance of Civil Works +canal structures etc, would significantly affect thegeneration efficiency.

:or the above reasons" strict controls during design phase of civil works is a very criticalfactor for the success of a mini hydro power project.

#here is a well known phenomenon that the cost control be best achieved at the earlystages of a project. :rom project feasibility to the construction stage" the freedom for costcontrol measures gradually diminishes. #his phenomenon is applicable to mini hydropower projects as well. ;e$t section of the paper deals with the special considerationsand the procedure in design of Civil Works in view of achieving an economical solutionappropriate to the prevailing conditions. Only the part of project feasibility aspects whichhas direct relevance to Civil Works design has been included in the paper as feasibilitystudies as a whole is a broad subject which should be dealt with separately.


3/16

#his is basically site visits with minimum amount of measurements. &t consists of thefollowing simple steps 1

a. Identification of a prospective site

#his is doneusing contour maps or any other means of information. )vailabilityof a significant head within a short distance is the main criterion +eg. 9- m headwithin km distance,. %istance to the nearest transmission line is also a factor tobe considered.

b. Inspection of possible diversion sites

&t is generally economical to have the weir close to the starting location of the fall.Other considerations are1

! &f the river is narrow" weir length is less" hence less cost.

! &f the sides are high" less erosion problems

! &t is better to have strong rock outcrops either side

! :oundation condition should be good. &f different rock layers +foliations, arefound" there is a possibility of water leakage underneath the weir.

c. Measurement of the head and deciding on approximate location of power house

)ppro$imate difference in elevation between the diversion point and the toe ofthe fall should be measured. #his can be done either using a level instrument or awater tube. =ariation of the level of stream downstream of the fall should bee$amined. &f the variation of the level is insignificant at a certain location" powerhouse be best located at a relatively flat site +close to the stream, at that particularlocation.

d. Inspection of possible routes for head race channel and penstock

&n most cases" head race channel is constructed along a contour starting from the

intake location. #here may be several contours with the elevation of weir startingfrom intake location running in downstream direction. 8 can be assumed as the ratio of per metre cost of channel > penstockunder general circumstances. *owever" if any geotechnical problem e$ists" e$pertadvice should be sought before selection of the best route.


4/16

e. Flood Characteristics

&t is possible to assess the general flood heights by local enquiries. *owever"e$pert advice should be obtained on flood with different return periods +84" 4- ?-- year,.

f. Land Availability

%uring e$amination of alternative locations" it is essential that" land ownership"purchase price and associated legal problems are studied. @ocation of a particularstructure may have to be changed due to problems of land availability.

/(0 P!,li.ina!y C)%& E%&i.a&, )1 Ci-il W)!2%


5/16

d. Approximate si"e of #ower ouse

#his depends on the equipment layout and specific water level requirements forthe turbines.

e. Location of discharge and si"e of tail race canal


6/16

&f alternatives are available with respect to materials and technology" those factors can becomputed for the most e$pensive alternative with respective probabilities" so that therewill be greater fle$ibility during the detailed design stage.

0( Sp,"ial C)n%i*,!a&i)n% in D,&ail,* D,%ign

'( W,i!

Weir of a mini hydro power project is generally a gravity structure. *ydrostatic force forthe highest flood condition" uplift forces due to seepage and impact of floating objects arethe loading conditions to be considered.

#he shape of the cross section of the weir is something similar to that of a large dam. )typical shape is shown in :igure . 0cour protection measures should be adopted if theweir is founded on soil.

)nalysis involves normal stability checks such as safety against sliding" overturning andbearing. %owelling or keying into rock is normally done under the foundations as ane$tra precaution.

Other special considerations are the provision of minimum flow and silt removal pipes atappropriate points of the weir.

0(/ In&a2,

#his is the structure which draws water from the river. #his can be a channel" pipe or atunnel depending on the site conditions.


7/16

c. Controls during flood conditions

) flood condition results in a rise of water level at intake" which would give rise

to a high flow in the head race channel. #his is undesirable because spilling mayoccur along the channel path which can cause severe erosion on the sides of thechannel. *ence flood water should be diverted from the channel path within theintake channel itself. ) series of breast walls +walls above normal flow level, andspillway arrangements are suitable measures for this. *ydraulic jumps may occurin breast wall arrangement causing rise of water level in downstream of thosewalls. 0pillways at each of these locations and also surface protection measures+by applying a rip rap, are some important elements to be designed. 0pillwaycapacity can be computed taking the broad crested weir formula as a guide. Ao$sections can be used as an alternative to the open channel" which should beanalysed using pressure flow principles.

d. Control &ate Arrangements

Control gate is a very important component in intake structure. #he following factors tobe considered.

Date should be located at a position where diverting flood water before the gate is

possible. @evels should be decided by taking the factors such as opening required" easy

access for control specially during floods" maintenance and installationrequirements into consideration.

#ype and si'e of gate +manualmotorised" radialrectangular" steeltimber, aredecided based on the forces +thrust, and convenience in operation.

a. #revention of $ntry of 'rash

) trash rack should be provided near the entrance to avoid floating objects entering thechannel. #his generally consists of steel flat sections welded to a frame. 0pacing betweenflat sections should be decided based on the si'e of trash e$pected in the stream+generally adopted value E 4- mm,. @arger objects such as logs" bushes etc can beprevented from reaching the trash rack by having a screen mechanism upstream. #ominimise the head loss" velocity of water through trash rack should be very small"

preferably less than -.4 ms. *ence the total area of trash rack should be about 9!4 timesthe si'e of intake" and it should be ideally located upstream of the intake entrance. :ordesign calculations" a condition with one half of the trash rack blocked with trash isgenerally considered. Cleaning mechanism +a rake operated manuallymechanically and aplatform to collect and remove materials, is a must for a trash rack arrangement.


8/16

'( H,a* !a", C#ann,l

(ain criteria of head race channel design should be minimising the head loss betweenintake and the forebay tank. *ence" the bed slope is kept at a fairly low value.

0ome important considerations are1

a. !election of !i"e and shape of the channel

#his mainly depends on the terrain conditions. ) trape'oidal section would be the mosteconomical one in most cases" but this section needs a relatively flat terrain condition.2ectangular sections are common. )queducts have to be provided at places where bedlevel is considerably above the ground level. (annings equation is widely used indeciding on dimensions" bed slope etc. Ao$ sections can be used where deep e$cavationsmay cause severe stability problems. )lternative materials available for channelconstruction are discussed elsewhere in this paper.

(b) $ffect of *ain +ater and $xtraneous Items

(ain problems that the rain water may cause to smooth functioning of head race channelare1

#ransport of soil and debris into the channel


9/16

'( S,*i.,n&a&i)n an* F)!,$ay Tan2%

#he purpose of sedimentation tank is the separation of fine material +silt, which would beharmful to the turbine from the water flow. :orebay tank is meant for providing somestorage for operation of penstocks in conveying water to the turbines. Common practice

is to combine both the functions in one tank. #wo separate tanks would be required if thetopography of the location of entrance to the penstocks is not favourable for a tanksufficiently long for sedimentation function.

&mportant consideration of sedimentation tank design are1

a. !i"e and !hape

#his is governed by the criteria that the tank should be capable of settling allmaterials with a particle si'e bigger than which is allowed to enter the turbine+#his si'e should be obtained from the turbine manufacturer,. *ori'ontal velocity

of water through the tank should be maintained at a low value +around -.8 ms, tofacilitate settling" hence the cross section of the tank is governed by this aspect.@ength is determined by applying 0tokes @aw. ) special shape += or W, shouldbe provided to the bottom part of the tank for accumulation of fine material. &f thetank is combined with forebay" two sections should be separated with an internalweir to avoid passage of silt to penstocks.

b. *emoval of !ediments

Washout pipes with valves should be provided at the bottom to flush the tankperiodically to remove sediments. Aottom of the tank should be adequately sloped

+appr$ >9-, to facilitate removal. *owever" since these arrangements are not veryeffective" provision for manual cleaning should also be introduced. #he outlet ofthe pipes should be directed to a water course with adequate protection fromerosion.

c. 'rash *emoval

#ank is a good location to have a trash rack" since the head loss will be very small due tolow velocity of flow.

0ome considerations for a forebay tank are1

a. !i"e and ,imensions

0i'e of the forebay is generally governed by the minimum storage required.Fuantity required for operation during time needed for would be a useful guide.%epth of the tank at the outlet area should be such that a depth of water of at least


10/16

.4 times the penstock diameter should be available above the penstock pipes" toavoid vorte$ formation.

b. #rotection

Covering of the tank to avoid foreign materials entering the penstocks would bevery important.

c. !tability of $xcavations

&t is always important to consider the stability of slopes around the tank. )ppropriateslopes and proper drainage systems are essential.

'( P,n%&)"2 An"#)!% an* S+pp)!&%


11/16

:orces due to thermal e$pansion contraction of the pipe +%irection / along the

pipes,.

#his can be eliminated by introducing e$pansion joints at appropriate places+#hese are normally provided just after an anchor location,.

*ydrostatic forces +%irection / along the pipes,

! :orce due to %ead Weight +%irection / vertical,

! Centrifugal force +%irection / perpendicular to the pipe,

! :orce due to friction at supports +%irection / along the pipe,

! :riction between anchor blockground interface

:riction coefficient takes a value between -.3 and -.G depending on the material +soil /rock,

Certain components of these forces act as disturbing forces and other components alongwith the weight of the anchor block act as stabilising forces. Weight of the anchor blockis determined using an appropriate safety factor +preferably around .4,.

a. &round Conditions

Aearing capacity of the ground should be correctly assessed for deciding the hori'ontaldimensions of the anchor block. 2estrictions due to physical barriers such as rock

outcrops" steep slopes" large trees or permanent structures should also be taken intoaccount. )nchors on sloping ground would require benching of ground and stepping ofthe foundation. %owels are provided for foundations on rock" as an additional precautionagainst sliding.

0(3(/ P,n%&)"2 S+pp)!&%

a. !pacing

0pacing between the supports is determined by two main considerations" i.e. pipe stressesand convenience in handling.


12/16

0upport spacing should be such that the combined effect of these stresses should note$ceed the allowable values.

a. Forces

(ain forces acting on a support are reaction of the pipe and the friction force. #hetwo forces are perpendicular to each other. :riction coefficient between pipe andthe support depends on the material used at the interface. &f properly lubricatedrollers are used the coefficient can be reduced to -.. :or normal steel concreteinterface the value will be around -.3" whereas rusting of steel may increase thecoefficient to even -.G. %irection of the friction force during e$pansion of pipewould reverse during contraction.

b. !tructural form

0upports can be designed as either gravity structures or framed structures. Ay having thesides of the structure parallel to the direction of the resultant forces" moment at the centrala$is of the base can be eliminated. %ifferent types of structural forms are shown in :igure3. #he most economical type should be selected for a given situation.

0(4 P),! H)+%,5 Tail !a",

%esign considerations of power house and tail race mainly depend on the followingaspects1

a. 'ype of turbine

2equirements for impulse type and :rancis type differ considerably. &n :rancistype turbines" turbine runner is fi$ed inside a casing the water head will be thedifference in water surface elevation at forebay tank and at just outside the draft

tube. quipment need not be installed above high flood level as the outside wallof the power house can be sealed to avoid inflow of flood water. &n case of animpulse type turbine" water passing the turbine flows under gravity. 0ince theinflow of water under high flood conditions cannot be prevented" generatorsshould be installed above the high flood level" to avoid damage under floods.

b. ,ischarge point on the stream.


13/16

#he head required for ma$imum efficiency of the equipment should have beenfinalised before commencement of detailed designs. %ischarge point on thestream should be selected considering the possibility of maintaining the tail waterlevel to obtain the head required.

c. $-uipment layout and the elevation details provided by the e-uipmentmanufacturer.

d. !ub surface conditions at site


14/16

Weight of travelling overhead crane" sway" impact and braking loads are to beconsidered.

Wind forces

%epending on the terrain conditions and height of the building

a. ther Considerations

0ome other aspects to be considered are1

Overall appearance and landscaping

Cable trenches" transformer and metering rooms

Office and accommodation space

0(4(/ Tail!a",

#his can be an open channel or a closed conduit. 2equired level at the draft tube can bemaintained by constructing a regulating structure +like a weir, at the location ofdischarge.

3(6 Ma&,!ial S,l,"&i)n an* D,&ailing

3(' W,i!

0ince the weir is a gravity structure" mass concrete with plums can be used. *owever" asforces from flowing water and impact from floating objects occur" its advisable toconstruct outer layers with concrete of strength of about 84 (


15/16

(a) $arth and *ubble Masonry

:rom the above" earth canals are not preferred in mini hydro power projects for reasonssuch as difficult terrain conditions" maintenance difficulties and high possibility ofsediment accumulation.

2ubble masonry can be used for a smaller canals of less than .4 m depth" but the mainproblem arises in protection from decay and erosion of mortar in the masonry wall. &t isrecommended that interior rubble masonry walls should be covered with a ferrocementcoating to ensure durability of the structure.

(b) /nreinforced Concrete Linings

Canal sections with a thin +4- / -- mm, unreinforced concrete lining is also aneconomical solution. *owever" this type requires a trape'oidal section +preferably with> side slopes, which occupies a larger area" and also reasonably good soil condition.(inimum strength of concrete to be used is 84 (


16/16

&t is necessary to provide e$pansion contraction joints +with water stops, at appropriateintervals. Cover to reinforcement at the inner face should be high +about G- mm, towithstand erosion effect of water.

a. &eneral Considerations

Documents

Some Important Aspects in Civil Works Design of a Mini Hydro Power Project