Inversin R. Allen Micro-hydropower Sourcebook

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Flg. 4.22. With the canal walls largely completed, stone

masonry is being laid for

the

base of the canal. Before the

wall and canal bottom in the foreground

can be

completed,

a small drainage ditch will be placed

wrderneath.

imizing available head with the minimum length of COR-

veyance structures. Because considerable flow is

required to generate the approximately 90 kW which

may eventually might be needed and because stone-

masonry skills were available locally, it was decided

that a canal rather than imported penstock pige would

be used to convey the water to t!le powerhouse area

(Fig. 4.23).

The location of the intake was straightforward to

determine, but the construction of a stone-masonry weir

from the actual intake across the river to a sandy

oxbow, rather than orienting it perpendicular to this

alignment, has caused problems. Plans had been to

begin construction of the weir on the right bank, at the

intake, while directing the flow toward the left bank.

Then, as the left bank was approached, the dry-season

flow would be bypassed through both the intake and

openings in the base of the completed portion of the

weir. However, the sandy left bank gradually eroded

and it became apparent that this bank would not be

reached. Work then proceeded on extending the weir at

right angles, toward a rocky bank. When the photograph

in Fig. 4.24 was taken, cofferdams had breached due to

the early start of the rainy season, and work on the

remaining several meters of weir had to await the next

dry season. (Also see Dams and weirs, p. 78).

Fig. 4.24.

Problems due initially

inadequate investiga-

tion into the nature of the foundation material

at the

oxbow

left bank was later compounded

the early

arriwl of the rainy season.

At this installation, an initial 30 kW unit comprising a

manually-adjustable propeller turbine directly driving a

submerged generator will be placed in a short length of

penstock at the end of the canal. There is no need to

construct a powerhouse, because the generator will be

under*ater and sealed. The civil works have been

designed to accommodate two more similar units as the

electrical load increases in the future.

Fig. 4.23. This low-head installation at GihLta, Burundi,

designed

to accommodate three 30 kW turbines, illustrates

60 Site selection and basic layout

the

use of a power canal to reduce

penstock length and

maximize

local inputs.

Fig. 4.25. From the head of a waterfall near Bishalalo

power canal and

through a penstock to the powerhouse

(center of photograph), water is conveyed along a short

llower left of center).

_Bishalalo (Zaire)

The layout at this site was straightforward to develop.

A steep series of cascades falling lo-15 m is located by

thr roaclsid(J (Fig. 4.25). A power canal is used to con-

vey water from the head of the fall a short distance

away from the steep section adjacent to the Call to

facilitate> construction of the penstosk and powerhouse.

A penstock then convcays water to the turbine, after

which the water is divertecl back to the stream below

the fall.

Ruyigi (Burundi)

Gently rolling terrain is found in the vicinity of the

Ruyigi micro-hydropower scheme. Along the Sanzu

River, a small stone-masonry dam is used to create

several meters of head and an additional 5-6 m is gained

by means of a 800 m canal which carries water to the

forebay. But from this point, there is a considerable

distance back to the stream. With a low head, a signifi-

cant flow is required to generate 60 kW, and it would

have been necessary to import a long, costly length of

steel penstock pipe. By excavating for the powerhouse

(Fig. 4.26), it could be located near the forebay, reduc-

ing significantly the length of penstock pipe required

(Fig. 4.27). This also required the excavation for a tail-

race in rock to return the water emerging from the tur-

bine back to the stream.

Though a.11 the excavation was

done manually, this approach proved more appropriate

in this situation.

Mardan (Pakistan)

One case in which laying out a hydropower scheme

becomes trivial is along irrigation canals. The designers

of these canals have already addressed the problem of

,.- forebay

(a)

ib)

Fig. 4.26. To minimize penstock

length, the

powerhouse

was built in rock below ground level. The

foreby is

located

10-20 m behind

the

powerhouse but is not visible.

The structure

the right houses an 11 kV tmnsformer.

Fig.

4.27. Two possible options for developing the sanle

site at Ruyigi: (a) using a

long penstock

and (b) using

short penstock but

with

considerable extra excavation

required

for the

powerhouse

and tailmce.

Site selection and basic layout 61

tapping a stream or river, and the quality and quantity

of the flow in a canal should therefore already be con-

trolled.

Where the terrain drops off more rapidly than is neces-

sary for the water in the canal to attain the required

velocity, drops frequently several meters high are

included (see Drop structures, p. 110). These are

obvious sites for hydropower schemes, although the low

heads available restrict the power potential of these

sites.

Fig. 4.28 illustrates a micro-hydropower plant

which straddles a drop along a small feeder canal. The

water that would otherwise tumble over the drop and

dissipate its energy through turbulence is conveyed to

the turbine by means of a very short flume.

Fig. 4.28. At

this

mill, a traditional wooden vertical-axis

turbine

harnesses the

energy available at a drop along an

irrigation

canal

to mill

gmin. The

Intake to the

drop is

shown at

Zhe tight, with mill behfnd.

b2 Site selectioa and basic layout

vi CMLWORKS

INTRODUCTION

The civil works for a hydropower scheme can be assem-

bled from the principal components shown in Fig. 5.1.

In addition, other components, such as spillways, gates,

trashracks and skimmers, and settling basins, may be

incorporated at one or more locations. Each of these

components serves specific purposes. If commonly

heard arguments against micro-hydropower schemes--

their high cost and the relatively large efforts required

to implement them--are to be countered so that micro-

hydropower will be more widely accepted as an appro-

priate technology, it is important to incorporate only

those components which are necessary at a specific site.

If unnecessary components are included, overall costs

are unnecessarily increased and new problems in imple-

menting and operating the scheme might be incurred.

On the other hand, if necessary components are not

included, the problems which these components were

meant to address will remain, and these may also lead

to increased cost of, and frustration with, implement-

ing, operating, and maintaining a micro-hydropower

scheme.

The first section of this chapter, QUAIJTATlVJ3 OVER-

VIEW, provides the information a designer of a micro-

hydropower scheme needs to decide which components

are necessary for the most efficient design of the speci-

fic project under consideration. To attain this objec-

tive, this section will:

describe briefly each component and subcomponent;

dam or 1

penstock

reg. 3.1.

An lflustmtion

of all

principal

components which

might be included

a micro-hydropower

site. This

scheme has

been laid out at

the site shown

in Fig. 1.9.

Civil works 63

* explain the function(s) each serves, how it serves

?se, and its relationship to other components; and

identify the factors which determine when each

component should be included as part of the overall

scheme.

In addition to incorporating only components necessary

for cost-effective implementation of a scheme, it is

also necessary to be careful in the design of each com-

ponent used. Although designs of individual components

might be standardized, there is still no single standard

design to suit all sites. The second major section of this

chapter, DESI- AND CONSTRUCTION DETAILS,

covers the design of individual components. Its objec-

tives are:

to provide a more detailed look at each component;

to raise awareness of the

range

of potential prob-

lems which might be encountered in the installation,

operation, and maintenance of that component;

a to identify factors which must be considered in

designing and sizing each component and explain how

these actually affect designs and dimensions;

to present alternative materials that might be used

to reduce cost of construction; and

to document some design options for each compo-

nent, which might be adapted as presented or serve

as a catalyst for new designs.

This chapter deals primarily with projects in the micro-

hydropower range (less than 100 kW). As projects

increase in size, however, the same factors must be

considered, although their impact of the final designs

may differ as project size increases. In addition, only

components relevant to micro-hydropower schemes are

described; surge tanks, arch dams, and power tunnels,

which are rarely

used

with micro-hydropower schemes,

are not covered.

QUALITAnvE OVERVIEW

Dam or weir

A dam is generally thought to be an intrinsic part of any

hydropower scheme, and this is frequently the case with

large hydroelectric projects. However, a dam, like any

component of a hydropower scheme, serves specific

functions, and it is necessary to become familiar with

these functions to determine whether a dam is actually

required. Because of the number of disciplines-such as

soil mechanics, geology, and structures--which must be

considered, an experienced civil engineer is frequently

required to design and supervise construction of a dam.

Including a dam when it is not essential will:

require that technical expertise be found;

incur the expense of building the structure;

lengthen the time before the scheme is operational;

and

possibly increase problems with maintenance.

On the other hand, excluding a dam when it should be

included may result in inadequate potential for meeting

present and future power generation needs. Rather than

a dam, a weir is sometimes used. Although the terms

“dam” and “weir” are commonly used interchangeably,

they are not regarded as synonymous terms in this pub-

lication; they are each defined by the functions they

serve. As used in this text, a dam is a structure that

can fulfill either or both of two basic functions:

A dam can be used to increase available head.

Where the terrain is relatively flat, there may be a

need to create head so that the required power can

be generated from the available streamflow. A dam

serves this function by raising the level of water

behind it, thereby increasing the difference between

the elevation of the water upstream and downstream

of the dam.

v A dam can also be used to create a reservoir to store

water. At micro-hydropower sites, the streamflow

may be insufficient to meet the peak power demand

with the available head, and a dam cannot add signi-

ficantly to that head. In these cases, a dam might

be included to store excess water in times of high

flow or low power demand to make it available at

times of low flow

increased demand. Except at

very-low-head sites, most dams are used only to

store water; they do not significantly increase the

available head.

When adequate head and flow are both available, no dam

is required. Water is essentially used at a rate no

grester than that with which it “runs” down the river.

Such a scheme is called a “run-of-river” scheme. A por-

tion of the flow in the steam is simply directed toward

the powerhouse and then rejoins the original stream.

The vast majority of micro-hydropower schemes are

run-of-river.

A run-of-river installation is sometimes misunderstood

to be one in which the turbine is placed directly in the

river. However, this is more properly referred to as an

installation using an in-stream or water-current turbine

(p. 187). As can be seen from the preceding definition,

although an in-stream turbine installation is also run-of-

river, the term “run-of-river” is much broader.

As noted in LOCATING THE INTAKE (p. 52), the intake

to a hydropower scheme should be situated to prevent

riverborne debris and unusually large flows from being

funneled into the intake, especially during heavy rains.

At the same time, there is a need to ensure that an ade-

quate flow of water is diverted toward the intake to

generate power during times of low flows. To accom-

plish this, a low diversion structure of temporary or

permanent construction, commonly called a weir, is

often constructed across a part or all of the stream. As

used in this publication, a weir is a structure specifi-

cally designed to divert the required flow into the

intake, not to store water.

It might either channel the

flow toward the intake or simply provide the required

depth of water at the intake for flow to enter of its own

64 Civil works

accord. Where the streambed is susceptible to erorjon,

a weir also maintains the level of the streambed con-

stant near the intake; otherwise, the streambed might

erode so badly that the stream eventually will be too

low for water to enter the intake.

The following discussion describes the circumstances

under which a dam or weir would be incorporated in a

hydropower scheme to fulfill each of these three func-

tions.

Inaeasing head

If the terrain in the vicinity of a site is relatively flat,

the head available for power generation can be

increased by conveying the water over a considerable

distance by means of a pressure pipe or penstock

(Fig. 5.2a). However, in areas with little slope, the

increased length and diameter of a penstock necessary

to obtain the required head and power would made this

option costly. This may be true even in fairly hilly ter-

rain if the gradient or slope of the stream itself is

small. For a stream with a very small gradient or slope,

incorporating a dam (Fig. 5.2b) may be the only viable

option for increasing head. All the head would then be

concentrated near the dam and powerhouse, markedly

reducing the dimensions and cost of the penstock. If a

dam is to be constructed, the topography of the area

must be suitable, most probably a section of a stream

with relatively steep banks. However, even at micro-

hydropower sites with very low head, a dam is rarely

built to increase head because of significant cost and

diameter pipe

. ...-.,,&

power canal

Fig.

5.2. Three approaches to

obtaining

the

same head an

power

potential.

effort required for its construction. It should be noted

here that, regardless of the head, a power conduit-fre-

quently an open canal-is a most common means of

increasing head (Fig. 5.2~). If the terrain is suitable,

this is often more cost-effective than either a dam or a

long penstock.

A dam incorporated at a low-head site can significantly

increase the head and therefore the power potential at

that site. At higher-head sites, dams are generally not

incorporated to increase head because the incre-3se due

to the added height of water behind the dam does not

justify the additional construction effort required

(Fig. 5.3).

/dam

(a) Using a dam at this

lower-head site

increases available head

and power by 200%.

20 m

(b) Using a dam at this

higher-head site

increases head and

power by only 10%.

Fig. 5.3. Comparison of

the

relatlw! increases In

power

obtained by

incorporating a 2 m

dum

at a very-low-head

and at a

higher-head site.

Providing storage

Incorporating a dam to store water and make it avail-

able when water demand for power generation outstrips

available streamflow requires careful forethought.

There are at least two occasions when such an approach

would be considered:

Storage would be considered when streamflow is

inadequate to meet the peak demand which occurs

during part of each day-for example, for lighting

and cooking during early evening hours. In this case,

the streamflow should be adequate to provide excess

water for storage during off-peak times, and the

Civil works 65

impoundment behind the dam should have the capa-

city to store this excess to meet peak power

demands on a daily basis.

e Storage would also be considered when there are dis-

tinct dry seasons. In this case, the volume of the

impoundment must be adequate to generate power

not only for several hours a day, but for weeks or

possibly months. Because this would require a dam

and storage reservoir of significant dimensions, stor-

age for more than several days is rarely considered

with micro-hydropower schemes.

In both cases, the required storage capacity and the

associated dam height can be derived simply from the

projected load patterns and streamflow data. Whether a

dam of reasonable size can provide sufficient storage to

meet present and future needs can then be determined.

In so doing, it must be remembered that only the volume

of water located above the outlet to the power scheme

is available for power generation. This, i-s called the

active storage. The dead storage, or the volume of

water located below the outlet, is clearly of no use for

power generation.

Even if a dam can provide adequate storage, its cost

might Frovide an incentive t.o look for a higher-bead site

so that the same energy and power could be generated

from a smaller volume of water. If a dam of re,asonable

size and cast cannot meet the expected needs and no

higher-head site can be found, twc options remain:

o to consider a load pattern which makes more effi-

cient use of the power that can be genera.ted without

an inordinately large dam, or

e to conclude that hydropower alone is not an option

at this site.

It should be noted that when a dam is used to create

pondage, the dam obstructs the normal flow of the

stream and the water is stored i.n the ri.verbed itself. In

countries with a pronounced difference between wet and

dry season flows, flood flows can

bt?

significant and have

a devastating effect on any structure in the stream. In

addition, sediment and bed load would be deposited

behind tbr dam, reducing its elfecti,veness in storing

water. Under tbece circumstances, it might be advisa-

ble to store watr:r off the stream. A forebay can then

serve this function fF~g. 5.4).

In considering the construction of a dam for storage,

several factors must be kept in mind:

Because the power which can be generated is propor-

tional to the product of net head and flow, the

higher the available head, the lower the flow

required to generate a given power. Lower required

flows reduce the required storage volume and there-

fore the dam’s size and cost. In effect, the higher

the head, the more effective a given storage volume.

In addition, the smaller storage volume required for

a higher-head scheme implies potentially fewer

adverse environmental impacts.

Fig. 5.4. An

enlarged

forebay stores

water

fw peak

eserring loads wher streamflows alone ure inadequate.

A dam

for

strrage fulfills its function only insofar as

its reservoir capacity is filled with qwater. However,

in many areas-particularly in the :ropics--rivers and

streams COT tain large silt and bed ioads which settle

as the streitm slows down upon entering the reser-

voir. In these areas, the reserbvoir may fill rapidly

with sedinlent, stones, and other debris, rendering it

largely ineffective. Even when a gate is included to

scour out this area, it will remove only the sediment

in the inmediate vicinity of the gate, where suffi-

ciently high velocities exist. Therefore, where

streams carry large sediment or bed loads, con-

structing a dam for storzge should be carefully con-

sidered.

The ad.verse social, economic, and environmental

impacts of a dam for micro-hydropower schemes are

small but must be considered. Of all the components

in a hydropower scheme, a dam has the largest

potential for beneficial, as well as adverse, impacts

on the surrounding area. Although it is generally

small, a dam associated with a micro-hydropower

scheme can increase the incidence of certain dis-

eases by providing breeding grounds for disease-

carrying organisms, particularly in the tropics.

Malaria and schistosomiasis are two such debilitating

diseases. By decreasing streamflow, reducing levels

of dissolved oxygen, or changing water temperature,

a dam might also adversely affect the fish in a

stream, which might be a source of both food and

income for a riverine population.

If a dam is to be a major component of a micro-

hydropower scheme, its inclusion must be deter-

mined by calculations based on realistic streamflow

information. If streamflow information is not avail-

able but flow seems to be sufficient for most of the

year, a scheme might initially incorporate a simple

weir across the stream to bring the powerplant on

line with minimum delay. Then, after more data are

obtained, a decision can be made on the advisability

of replacing the weir with a dam.

66 Civil works

A micro-hydropower project with a capacity of 30 kVZ

under a head of 60

is being implemented near the

village of Yandohun in the fairly remote northern

corner of Liberia. Based on occasional observations

of the stream to be used and comments by the villag-

ers? the streamflow was thought to be above the

80 t/s required to generate full power for all but two

months

of the year. The only existing streamflow

data were gathered during a week at the end of the

dry season when the low flow was measured as 7 f/s.

Initial plans were to build a sizable stone-masonry

gravity dam about 40 m long and 2 m high. This was

intended to retain sufficient water to rur~ the plant at

half power for a 12-hour period for several days at

the minimum observed flow or, equivalently, for

shorter periods over correspondingly more days.

Because of its design, the turbine could not operate

at less than half power without modification.

When the plant was visited, work on the dam had

been underway for some time but with little progress

(Fig. 5.5). Construction proved time-consuming. The

volume of the dam was large, and

the stone was

obtained by building fires on bedrock to crack it. The

stone was then broken with hammers to obtain aggre-

gate.

It was recommended that the dam not be completed

but built only high enough to serve as a weir. The

weir could be built up at some later date

desirable.

This recommendation had the following advantages:

e The plant could be operational with minimum

delay. The project was ambitious even without

the dam, and the villagers needed to begin realis-

ing some of the benefits of their efforts as soon

as possible.

e Streamflow observations showed that without a

dam, flow was adequate 85% of the time (10

months) to generate full power and that, for

another 5%‘0-10% of the time, the flow would

probably be sufficient to operate at half power all

day. Construction of the dam would only permit

Ffg. 5.5. A comptxite view

the dam

site

for

a 30 kW

hydrrrpower scheme in Yandohun, Lfberla.

half power to be generated part of each day for

the remaining 5%10% of the year. Because the

impoundmentwas too small to permit storage of

water from the earlier rainy season, it would still

be impossible to guarantee water

for

power gea-

eration at all

times.

Therefore, not much would

be gained by constructing a dam. In this case, an

effort to modify one of the two fixed nozzles by

reducing its size would be more appropriate than

to construct a dam because it would permit at

least some power to be generated at all times

with no storage.

o A delay in building the dam to full height would

permit the operators of the plant to determine

whether the silt and bed loads carried downstream

and caught behind the weir would present prob-

lems should a dam be built later. After the plant

begins operation, more flow data during the dry

season-information needed for proper dam siz-

ing-could be gathered. At the same time, the

villagers could decide whether having electricity

for part of the day during the remainder of the

dry season would be worth the additional time and

expense required to complete the dam.

Diverting flow

With micro-hydropower schemes, a dam is rarely

required to increase head or store water; however, a

weir might be required to divert adequate flow into the

intake for power generation. At some locations, the lay

of the !md and natural formations within the stream

may direct sufficient water into the intake without a

weir. Simply placing a few stones across the streambed

also could achieve this purpose (Fig. 5.6).

Where the streambed is likely to erode, a weir can be

included to maintain its level near the intake. If the

streambed is composed of bedrock or streamflows are

not large enough to erode the bed, a weir for this pur-

pose may be unnecessary.

A weir is also useful where streamflows normally fill

only a portion of a wide streambed, at least during the

drier seasons, and the low flows may never reach the

intake (see Fig. 5.48). In this case, a weir should be

designed to deflect low flows toward the intake yet

permit high flows and waterborne debris to proceed

downstream unhindered. Such a design will prevent sill

bed load, or other debris from having the adverse

impact on the life of a weir that they do on the life of

storage reservoir formed by a dam.

Intake

An intake permits a controlled flow of water from a

river or stream into a conduit which eventually convey

Civil works

Fig. 5.6. No permanent welr has been included at this site.

A few boulders placed across the stream will permit addi-

tional flow

enter

the intake when required for power

genemtion.

it to the turbine to generate power. An intake is a

component of virtually every hydropower scheme. The

intake serves as a transition area between a stream

which can become a raging torrent and a flow of water

which must be controlled in both quality and quantity;

therefore, its design requires careful consideration. An

improperly designed intake may become a source of

continuing and frustrating maintenance and repair prob-

lems, not only with the intake but with all other major

components of a scheme. For this reason, the long-term

success of a hydropower scheme depends largely on the

design of the intake.

The selection of a design is complicated by the variety

of possible intake configurations ranging from simple,

excavated canals opening onto a stream to large, rein-

forced-concrete structures. They can be either sepa-

rated from, or an integral part of, a weir or dam. The

most appropriate design for a specific site is determined

by a number of factors, including:

o the quality and quantity of water required;

o the extremes in the stage of the stream;

e the topography and soil conditions of the site;

o the nature of the catchment area;

e whether a dam or weir is to be constructed at that

site;

o the financial capital available for its construction;

and

e the relative capital vs. labor resources available for

construction, operation, and maintenance of the

structure.

3ne of the major functions of the intake is to minimize

the amount of debris and sediment carried by the

incoming water. Trashracks (p. 162) are often placed at

the entrance to the intake to prevent the entrance of

floating debris and larger stones. Skimmers (p. 165) are

sometimes used to prevent floating debris from entering

the intake. Depending on the water quality, a settling

basin (p. 166) might also be incorporated to remove

some of the more troublesome waterborne sediment.

Without these features, the sediment and debris could

settle along the power canal, eventually obstructing the

flow, or they could pass tbzough the turbine, damaging

it or reducing its effective life. If a spring or other

source of clean water is used, trashracks, skimmers, or

settling basins may not be needed.

The intake also controls the flow of water which is

admitted under all streamflow conditions. Several types

of gates (p. 154) can be used to control this flow. Spill-

- (p. 159) also serve as a backup to the gates by

permitting excess water which has entered the intake

gate to overflow back into the stream. If the waterflow

is not controlled at the intake, excess water may cause

the power canal or forebay to overflow unexpectedly,

leading to serious erosion which can undermine the

canal, forebay, penstock, or powerhouse. Although the

canal and forebay should be designed to prevent damage

from flows larger than those expected, it is still best to

minimize this possibility by incorporating spillways in

the intake structure.

Proper design of the intake is essential for troub!e-free

operation of the civil works; however, proper location of

the intake along the stream reduces the burden placed

on the intake itself (see

LOCATING THE INTAKE,

p. 521. Adequate initial planning is necessary to place

the intake where it will be protected from excess water

and debris which might otherwise be funneled into it

during flood stage.

Power conduit

As used in this book, the term “power conduit” signifies

the component of a hydropower scheme used to convey

water a relatively large distance from the stream to the

inlet of the penstock, with minimum loss of head and at

minimum cost.

A power conduit is most frequently a canal excavated in

soil, because this approach reduces cost (Fig. 5.7). Such

canals are sometimes lined or constructed with concrete

or other impervious material to reduce seepage of water

which might eventually undermine the canal or to per-

mit increased velocity and flow along 1’ e canal. They

may a!so be built of concrete or stone masonry (Fig. 5.81

or of half-round wood, sheet-metal, or concrete sections

supported above the ground. Because an open canal

needs to have only enough slope to produce the required

Civil works

Fig. 5.7. An unlined power

canal cut into a

steep

hillside.

flow, its alignment shoulcl near!y coincide with a con-

tour line.

Depending on the topography of the area,

portions of this conduit may be supported above the

ground (such as when ir crosses ravines) or placed

underground in a pipe (such as when it crosses a road).

A pipe can serve as a low-pressure power conduit to

convey water from the intake to the beginning of the

pcnstock. The pipe has an advantage over the c‘anal in

that it eliminates components such as spillways, drop

structur(?s,

and drainage which are usually incorporated

in the tIcsign of power canals. It also does not nred to

remain level but can follow the rise and fall of the ter-

rain it traverses.

On the other hand, a pipe often must

be imported and generally costs more than a canal,

especially a simple canal excavated in earth. However,

lengths of pipe are sometimes included along sections of

the power conduit where a canal is not possible

(Fig.

5.9).

With larger schemes, power conduits may take the form

of tunnels; over rocky terrain, few other alternatives

may exist. Among .he several advantages they possess,

tunnels can:

e make a more direct route possible;

o protect the conduit from landslides, heavy runoff,

and debris; and

m avoid interference with surface features such as

roads, gardens, or houses.

Fig. 5.8. A stone-masonry

power

canal.

However, tunnels must usually be large enough to

accommodate men and equipment necessary for the tun-

neling operations and therefore are too large for the

flows used by most micro-hydropower schemes. With

tunnel boring machines, it is impractical to drill a tun-

nel much smaller than 2.5 m in diameter. In Nepal, a

local company uses compressors and jackhammers to

construct tunnels as small as 1.50 x 1.75 m.

Although tunnels 6re rarely included within micro-

hydropower schemes to convey water a significant dis-

tance, short tunnels may prove useful along power

canals where a rock outcropping must be crossed. These

can be seen alung traditional irrigation canals in coun-

tries such as Nepal.

Fig. 5.9.

Pipes are

used to connect two

sections

canal

where a steep, unstable section

had

to be traversed. Three

lengths

were used because a single pipe

the necessary

diameter was not available.

Civil works