Inversin R. Allen Micro-hydropower Sourcebook

where the bottom and the banks both have the same

slope, as is usually the case. Assuming an obstruction

along a canal as shown, the water will start backing up

and rising, until it reaches point B. Water will then

start overspilling the banks of the canal unless a spill-

way has been located somewhere between points A and

B. Over each distance “L” which can be found from the

formula shown, there must therefore be at least one

spillway to avert uncontrolled overspilling. If the canal

is short or has a very small slope, “L” will be greater

than the canal length and spillways may not be neces-

sary-

The value of “L” derived above is only approximate and

serves as a maximum limit. All the excess water in a

canal will pass over the spillway anly if the water level

in the canal is high enough above the level of the spill-

way (Fig. 5.87). By lengthening the spillway suffi-

ciently, it is possible to reduce this extra head needed

to permit all the excess water to leave. If the spillway

is not wide enough, it may not prevent the canal banks

from overflowing. (See Spillways, p. 159, for desuip-

tions and sizing of various types of spillways.)

r

head above

spillway which

is necessary

-

Fig. 5.87. Water

level may

have to rise significantly above

the spillway crest to permit all excess flow to leave.

Unless a canal is very level

or

-short, the magnitude of

the waterflow ateringthe canal will not be affected by

obstructions to the flow downstream. In this case, the

dimensions of a spillway must be adequate to permit the

entire flow which might enter the canal to overspill.

If a canal is short or its slope is vsry small, its banks

can be constructed horizontally from intake to forebay

(Fig. 5.88). The banks will not overspill unless a major

flood submerges the intake. As that figure shows, this

approach cannot be adopted

for

long can& or for those

with large slopes, because the canal would become too

deep at its lower reaches. III this case, the banks should

be built parallel to the canal bottom, and spillways

should be incorporated at appropriate intervals.

For some canals, the drop in elevation between the pro-

posed intake on a stream and the proposed forebay loca-

tion is more than that necessary to provide the

required

flow. One solution is to use a steeper canal with

snaller crossectional area; however, this would

increase velocities, and the canal may then have to be

lined. Another alternative is to use segments of canal

with the original slope and to connect these by one or

more drops (Fig. 5.89). Appropriate design velocities

are thereby maintained along the entire length of the

canal except at the drops. At these points, properly

designed drop structures should be included to ensure

that the energy gained by the falling water dissipates

without undermining the canal.- If the drop is not over

natural rock, it is usually necessary to construct a drop

structure of concrete, brick, stone masom-y, or timher.

Timber is usually not chosen because of its relatively

short life. Low-cost drop structures can also be built of

discarded oil drums or the equivalent.

(a)

nstock

me-

maximum level that the water will

attain if the flow is obstructed

anywhere up to the turbine

\ intake

area

(b)

Fig 5.88. Comparison

of canal depth for a short (a) and

tong lb) canal where the benks

are

constructed horizontally

to

prevent 0wwspiZZing if flow is obstructed.

110 Civil works

slope required to

carry design flow

f9. 5.89.

Canal drops perntlt small slopes and velocities to be mainburea even wfmre a large avenge slope fs r%qufreQ

Strictly speaking, a drop is used only when the differ-

ence in elevation from one section of the canal to the

next is small and can be effected over a relatively short

distance, sften by a vertical fall. Fig. 5.90 illustrates

the basic configuration of one design for a drop. In this

case, the drop is approximately 1 m. Drops usually

include a section along which the drop in elevation takes

place and a stilling basin in which the excess energy

gained by the falling water is dissipated. They can also

include an inlet structure with a weir or gate designed

to control the level of water in the upstream stretch of

the canal. The water leaving a drop is often turbulent,

and this can cause erosion in the section of canal imme-

apron toewall’-

‘end sill

F-+-l---I

0 100 cm

Fig. 5.90. Isometric view of a typical

brick-masonry drop

stmcture used along irrigation canals in India. (83)

diately downstream of the structure. If the canal is not

lined, it is necessary to place riprap over a length of

several meters. Also, sufficient freebnard must be pro-

vided around the stiUiig basin because of the splashing

which occurs there.

Another design for a drop is a pipe drop structure

(Fig. 5.91). A stilling basin, suitably lined, is still

needed to dissipate the energy of the incoming water.

To prevent seepage along the outside of the pipe, a con-

crete apron around the inlet or coilars along its length

are often used. A lid on the inlet to the pipe can

func-

tion as a check valve. The required pipe diameter for a

specific flow is found in the same manner as that for an

inverted siphon (see Inverted niphone, p. 115).

upstream canal

stilling basin

pipe drop

Fig. 5.91. A sectional view of

a

pipe drop.

With conventional projects in some areas, there has

been a tendency to abandon vertical drops in favor

of short, inclined, concrete drop structures. ?be

one shown in Fig. 5.92 is designed for a flow of

0.15-2.0 m3/s and a maximum fall of about 5 m. These

often have a rectangular cross-section, but trapezoidal

nr semicircular cross-sections have also been used.

.,e ---

Engineering Field Manual for Conservation Practices (9)

includes a detailed design for an inclined concrete drop

which c

T

be made without formwork and can

convey

up

to 1.0 m /s through a maximum drop of 1.5 m.

Civil works 111

platform

/

Plan View

this wall section warped

from vertical (at left) to

slope of bank (at right)

stilling basin

Lon~i tuclinal Section

riprap

prbtcction

L

Fig. 5.92. A view of

a

rectangular ir.clined drop (40).

Where larger

drops are required and a series of drop

structures might be used, an inclined lined channel

called a chute may be more economical (Fig. 5.93). It

has esseImy the same design as an inclined drop

(Fig. 5.92! a n generally contains the Sam? features. d

Detailed descriptions of larger drop structures can be

found

in

Dt>sip Standards No. 3, Canals and Related

StructuresiS).

Circumventing obstacles

Obstacles may be encountered along the alignment of a

canal. It may be necessary for a canal to be supported

above or to go arrjund or below these obstacles. A

flume, inverted siphon, or culvert can be used for this

purpose.

Flumes. A flume is often used when it ia necessary to

traverse a stream, ravine, gulley, or ether depression, to

detour around an obstacle such as a large boulder or

part of a cliff face (see Fig. 5.111, or when the ground is

too rough or too steep to permit excavation for a canal.

The flume is simply an extension of the canal, often

continuing with the canal’s slope but generally supported

on concrete, steel, or wooden piles, piers, or trusses

above the ground. A flume may be an open channel

made of timber with a rectangular or sometimes a

triangular cross-section or of wood staves, sheet metal,

or reinforced concrete, frequently with a semicircular

cross-section.

112 Civil works

Fig. 5.93. A chute permits controlled dissipation of

enargy

along a

power canal.

A flume may also be a pipe of reinforced concrete,

steel, or other material. Although a pipe seems the

optimum choice to minimize labor requirements, pipes

are expensive in rural areas of developing countries and

often must be imported. Also, it can be

very

difficult

to remove sediment and debris deposited within the pipe

during operation of the scheme, especially

If

the pipe is

more than several meters long. Removing settled silt or

clay is not simply a matter of re-establishing the design

velocity through the pipe by putting the plant back in

operation, because the velocity ht which the deposited

clay or silt is eroded, picked up, and carried back in sus-

pension is considerably greater than the velocity at

which it was deposited.

In sizing a flume constructed of any material, the same

principles apply as those for canals. Even if the slope of

the flume and canal are equal, the difference in the

roughness coefficients “n”

of

the two sections may

imply that different cross-sectional dimensions can be

used.

The joint between the canal excavated in the ground and

the inlet and outlet of the flume should be built care-

fully to prevent leakage that might undermine the canal

at that point. To minimize losses as well as turbulence,

any transition between the canal and flume cross-sec-

tions should be gradual. Depending on the velocities

encountered and the nature of the canal bed, riprap or

other protection against scour may be needed for a

short distance beyond the outlet.

Ample freeboard also must be provided, especially if

overtopping of the flume can erode the ground below

and undermine the supports. If the flume is long, it may

be necessary to include spillways at appropriate points

to safely convey any unexpected excess water away

from the flume. Freeboard for a flume is usually less

than that for a canal. A common rule is a freeboard of

about one-tenth the width of the flume, with a minimum

of about 5-7 cm (40). Curved flumes require greater

freeboard, especially along the outside edge.

In areas with a ready supply of sawn timber, wood is a

cheap and convenient material with which to construct

a flume. Small wooden flumes are made of plain boards

nailed together with a triangular or rectangular cross-

section, reinforced with timber collars at l-2 m inter-

vals, and supported on the ground or on timber posts

(Fig. 5.94). The walls of large flumes are built of multi-

ple boards with a rectangular (Fig. 5.95) or semicircular

cross-section, and collars become mare important.

Semicircular wood-stave flumes are similar to wood-

stave pipe and generally require staves that have been

specially milled. Timbers for rectangular flumes are

easier to prepare and, because wider planks can be used,

there are fewer joints through which water might leak.

To make the joints more watertight, various types of

joints can be used (Fig. 5.96). Good-quality timber, free

of warps and knots and at least 3 cm thick, is often

recommended even for small flumes, because thinner

caulking

\

butt

end

ship

lap

tongue

and

groove

butt

sphne

double

lining

Fig. 5.94. A

commcjn design for wooden flumes used to

convey water to mills.

Fig. 5.96. Various joints that can

be

used

between

the

wailhoar&

or floorboards of a wooden flume.

100 x 250

x

5 m

Cross-set t ion

Elevaticn

--.

I

Fig. 5.95. A wooden flume

on

a supporting trestle.

Nominal timber sizes

are given ip millimeters wlless otherwisenoted (40).

Civil works 113

boards tend to warp or crack. Boards properly seasoned

before use will swell and tighten when wet.

The life of a timber flume depends on the type of tim-

ber and the treatment it was given. A.n untreated tim-

ber flume may last 10-50 years, and creosoting may

extend its life 50%-100% (40). Intermittrnt service

shortens its life.

Collars for the rectangular flumes usually consist of a

rectangular frame around the three sides of the flume

tied across the top. If floating debris

may

pass along a

flume, there must be sufficient freeboard to permit this

debris to continue unhindered. Steel bands with wooden

crossties are used as collars for semicircular timber

flumes.

Support trestles can be pairs of posts for smaller

flumes; multi-column trestles can be used for larger

flumes. The posts should be inclined slightly and

anchored to supports-concrete

or

masonry piers-to

increase stability. The structure should be secure

enough to resist overturning by strong winds and earth-

quake loads when either full or empty. All frame joints

should be connected by bolts, except for very light con-

struction; side and floor boards can be nailed in place.

If the flume is placed on a bench cut into a hillside, the

sills can rest on the ground

or

on blocking. Broken

stones

or

gravel under the sill facilitates draining and

prolongs its life. If the flume is on trestles, the sills xt3

supported by longitudinal stringers; with small flumes,

the flume itself acts as a girder between supports.

A flume can also be built of sheet metal. Generally,

such flumes consist of thin steel sheets curved to semi-

circular form and suspended from wooden or steel

stringers or crossties (Fig. 5.97). The design of trestles

is similar to that for wooden flumes described above.

Overlapping edges of the sheets forming the flume bar-

rel are generally pressed together between an outside

rod or hanger and an inside compression member. The

rod is oupported by a crosstie and the inside compression

member reacts against the underside of the crosstie as

shown in Fig. 5.98. Various types of joints have been

used. The Newcomb and American types are simple to

manufacture and install, particularly on curves. These

upper stringer

n/

crosstie

I

\

galvanized steel sheet

bolted to stringers

Fig. 5.97. Election and sectional views of the flume

shown

in Fig. 5.11.

stringer) ’

tension

member 1

compression

member

Section AA

(cl

compression

member

tension

/

members ’

\

elastic tj’, 9,

compound

Fig. 5.98. Some examples of

metal flume

joints incknffng

(a) the

Nevcomb type, 0 the Amerfcan type, and ICI the

Lenncm type (40).

joints can accommodate moderate curvature without

specially mitered sheets. Tightening the outside (ten-

sion) member of an American-type joint

forces

the out-

side sheet against the inner sheet which

serves

as the

compression member.

More

sophisticated designs such

as the Lennon type have also been used. These provide

greater structural security and watertightness while

minimizing resistance to flow, but they require specially

formed beads or grooves at the ends of the metal

sheets, and mitered joints have to be custom-made for a

specific curvature (40).

The metal sheets and all metal parts that come in con-

tact with them should be galvanized. When the galva-

nixing wears away, the interior surface should be

treated periodically with coal-tar paint or enamel.

Considerable other details concerning sheet metal

flumes can be found in “Design, Construction and Use of

Metal Flumes” (57).

At the Christian Radio Missionary Fellowship station at

Rugli in Papua New Guinea, sheet-metal flumes are

used both to cross depressions in the terrain and to tra-

verse steep areas. In this case, stringers are reinforced

by welding steel rods as shown in Fig.

5.9?

and crossties

are constructed of angle iron. Bolts along the edges of

114 Civil works

the galvanized metal sheets secure them to the angle-

iron stringers. Consecutive sheets of metal simply

overlap, with no attempt to seai these areas, but leaking

has not been a major problem. During several decades

of operation, these sheets have had to be replaced about

every 10 years. Rusting through the galvanized surface

seemed to be most severe at areas of overlap. The

number of overlapping joints can be reduced by using

long rolls of galvanized steel sheet rather than small

rectangular sheets.

As part of one shorter sheet-metal flume at Rugli which

traverses a stream, a simple gate was fabricated by not

bolting one edge of a short section of sheet metal to-&

stringers.

When the canal has to be emptied or the flow

:\as to be prevented from flowing beyond a specific

point, this edge is lifted, permitting the water i? the

canal to drain into the stream beneath (Fig. 5.99).

Fig. 5.99. A simple but effective gate

in

a sheet-metal

flume.

Reinforced concrete flumes can also be built but are

rarely used for micro-hydropower schemes. They are

the most permanent type, but also the heaviest and

most expensive. It is also difficult to make concrete

products of reliable quality in thin sections, and heavy

sections pose handling problems if they are precast. If

they are cast at the site, appropriate skills are required

and formwork costs may be excessive. Although they

can be supported above ground level, concrete flumes

are generally built as bench flumes, resting directly on

the ground.

Flumes made of other materials or designs are possible.

One example proposed in “Micro-Hydro: Civil Engineer-

ing Aspects” (72) has R basic appearance that resembles

a sheet-metal flume; however, rather than sheet metal,

a steel mesh covered with a sheet of rubber or other

suitable material is hung from the stringers (Fig. 5.100).

concrete or

Fig. 5.100. A flume

design

proposed

as appropriate for

remote areas where the

quantity of materials to be tmns-

ported

should be

minimized (72).

Inverted siphons. At times, an obstacle such as a road

or streambed may lie just below the elevation of the

canal. A flume might not permit sufficient clearance

underneath or it might be difficult to support ade-

quately. At other times, the canal may have to traverse

a long and deep depression, and a flume might be too

expensive or objectionable for some other reason. In

these cases, the canal discharge could be carried in an

inverted siphon (Fig. 5.101).

An inverted siphon consists of an inlet and outlet struc-

ture connected by a pipe. If the canal is unlined, includ-

ing inlet and outlet tanks may be advisable to facilitate

removal of debris or sediment which accumulates in the

siphon (Fig 5.101a). These should be adequately sized to

permit access. If possible, a valve should be included at

the low point along a longer siphon (Fig. 5.lOlb) so that

the siphon can be drained.

The size of a pipe used in an inverted siphon is governed

by the same principle that governs flow in a penstock

pipe. (See Selecting pipe diaroeter, p. 125, for more

information). Assuming that the velocity of water in

the canal is the same at the inlet and outlet, Bernoulli’s

equation gives the following relationship between the

head loss through the siphon “ht”--the difference bet-

ween elevation of the water at the inlet and at the out-

let--and the velocity through it:

h _ 120n’ Lv2

VZ

I

--

D4/3

2g

+ (Ke + Kb + 1) --

2i3

(5.5)

Civil works 115

road or

streambed

section of

power canal

------------

(a) inverted siphon w-

(b) inverted siphon

Fig. 5.101. Two examples

illustrating how an

inverted siphon

may

be

used along

a canal alignment

to

circumvent obstacles.

where

hf = head loss through inverted siphon (m)

n = roughness coefficient (see Fig. 5.126)

D = diameter of siphon (m)

L = length of siphon (m)

V

= velocity in siphon (m/s)

is

= gravitational constant (9.8 m/s2)

K, = coefficient of losses at entrance (see

Fig. 5.128)

Kb = coefficient of losses at bends (see Fig. 5.128)

The first term represents friction losses along the

siphon [see Eq. (5.13)]; the other terms represent

entrance and exit losses caused by turbulence and losses

at any bends included along the siphon.

If a coefficient of friction losses is defined as

120 n2

Kp=-QXjT

Eq. (5.5) can be solved for the velocity in the siphon:

v =JT (5.7)

Using Eq. (5.7) and the relationship

it is possible to derive the equation

Q=+/X

or

,

Q = 3.5 D2

J

hf

.itpL+Ke+Kb+l

(5.9)

which must be solved for the required diameter of the

siphon. However, because K depends on pipe diameter

as is apparent in Eq. (5.61, it % not possible to solve

Eq. (5.9) to obtain a simple expression for the required

diameter for a given flow and head loss. Rather, a trial

diameter should be selected and the flow derived

from

Eq. (5.9) should be compared with the desired flow.

Repeating this process several times will quickly lead to

the required diameter.

If the water contains eilt or other sediment, velocities

in an inverted, siphon should be maintained at levels high

enough to prevent it from settling. Minimum velocity

should

range

from 0.3 to 0.5 m/s. To determine the

maximum siphon diameter required to maintain this

velocity, the design flow and minimum velocity can be

introduced into Eq. (5.8). This value of diameter can

then be substituted into Eq. (5.9) to determine the head

loss across the siphon required to obtain this velocity.

A smaller diameter pipe would lead to increased velo-

city but also to an increased head loss.

Fig. 5.102 shows the typical design and dimensions of an

.‘lnverted siphon used along irrigation canals in India.

The inlet and outlet tanks act as settling basins because

of the increased area of flow and lower velocities

through them. Any silt carried by the water is likely to

be deposited in these tanks; therefore, their interior

dimensions are commonly 0.6 m square to permit access

for cleaning out the accumulated sediment. The bottom

of the tank is at least 0.15 m below the invert (bottom)

of the pipe to prevent this sediment from interfering

with the operation of the siphon. The pipeline should be

buried deep enough to protect it from any traffic over

it.

Culverts. When a canal has to avoid an obstacle such as

a road 01 streambed which is just slightly higher in ele-

vation, a common culvert can be used (see Fig. 5.12).

This is simply a pipe which flows partially full. To pre-

vent piping-the flow

of

water along the outside of the

pipe--which may lead to collapse of the ground above,

masonry headwalls should be constructed at the ends of

the culvert

or

buried pipe; alternatively, collars could

be included along the pipe.

On steep slopes, where providing for dr4inage around a

canal can prove difficult because it would require too

116 Civil works

road, track, or

riverbed to be

circumvented

canal

Fig. 5.102.

Typical

dimensions

(ml

of

an

inverted

siphon used in

India to

carry

irrigation

water below obstacles (83).

Fig 5.103. The

steep,

rocky soil slope at this site makes

it

difficult to provide dminage.

In

this

case,

flat concrete

covers

will

eventually be placed along the length

of the

canal

to prevent

debris

from entering.

much excavation (Fig. 5.103), one approach is to cover

the canal and construct a culvert. Flat covers made of

timber or reinforced concrete can be placed over the

canal (Fig. 5.104). These

covers

can be removed to

permit access to the canal for repair

or

cleaning. Any

debris coming off the hillside would either settle on

these covers or be carried over. Because some rain-

water and sediment may still enter the canal, some set-

tling at the forebay may be necessary.

At a site at Bundi, Papua New Guinea, where it seemed

better to backfill over the canal, a rectangular canal

with a width slightly larger than the diameter of a 200 P

‘ig. 5.104. A canal section showing flat covers

to

prevent

ntry of debris.

Civil works 117

oil drum was built. When it was completed, a half-drum

was used as formwork and placed over the canal on tim-

ber supports, and concrete was laid over it (Fig. 5.105).

When the concrete had set, the timber supports were

removed. The oil-drum forms dropped down and were

removed for use farther down the canal. The outside

surface of the drum was oiled before use to facilitate

its removal. Soil was backfilled over this “culvert.”

Occasional access holes along the canal (Fig. 5.106)

permitted a workman to enter and crawl along the canal

to remove sediment carried from the intake area.

At Kagua, Papua New Guinea, a culvert was cast in

place (Fig. 5.107). Automobile tires were placed side by

side along their common axis, and planks were then laid

around this core of tires, also parallel to this axis. This

served as cylindrical formwork around which concrete

was poured. When the concrete had set, the tires were

pulled out and the planks were removed, leaving a con-

crete culvert.

For large canals, brick arches over the canal have been

built (Fig. 5.108). However, this is generally costlier

and requires brick-masonry skills.

Sizing low-pressure conduits

Although canals are the most common lype of power

conduit used with micro-hydropower schemes, low-pres-

sure pipes are also used for the reasons mentioned in

Power conduit (p. 68). The appropriate pipe diameter

can be determined from Figs. 5.125 and 5.129, the same

figures used to determine losses in a penstock pipe.

Forebays

A forebay is’lbasically a basin with dimensions that need

be no larger than those required for it to perform either

of the functions mentioned earlier. Any of several

components--settling basins, spillway, gates, and trash-

rack--may be incorporated within this basin as described

in Forebay (p. 72). This section describes various fore-

bay designs to illustrate how these components have

Fig. 5.106.

An access hole permits access to

the covered

canal.

been incorporated. (For information on design and siz-

ing

of

these components, see Other components, p. 154).

It also discusses design considerations for the shape and

location of the inlet to the penstock to minimize losses

and the chance for air entrainment into the penstock.

One version of a forebay design frequently recom-

mended by the Ossberger Turbinenfabrik of Germany at

installations using its turbines illustrates how all the

basic components are incorporated into the forebay

(Fig. 5.109). The initial portion of the forebay can be

sized as a secondary settling basin if necessary. A stop-

excavated area

Fig. 5.105.

Steps

in

the construction

of a covered stretch of

cmal at Bun&, Papua

New Guinea (Source: B. Vijstli).

118 Civil works

Fig. 5.107. This culvert was cast in

place

using tires and

sawn timber

as

formwork

Fig. 5.108. An irrigation canal is covered with a brick arch

to prevent

slides

from obstructing the flow

of

water.

-

stoplog

groove

(not to scale)

stoplog

groove

nstock

Civil works 119

Inversin R. Allen Micro-hydropower Sourcebook

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