Inversin R. Allen Micro-hydropower Sourcebook

to regulate water level in the intake and canal. Even if

there is a gate between the intake structure and canal,

an operator will not be stationed there continuously.

Then, if the water level in the intake structure

increases suddenly, significantly more water will flow

through the same gate opening, possibly causing the

banks of the canal to overflow, erode the ground down-

hill, and undermine the canal.

If the canal depth may increase unr.xpectedly, the

placement of spillways at suitable points along a canal

must also be considered. This increase may result from

excess water entering the caual from the intake, from

runoff which enters the canal, or because of obstruc-

tions (such as landslides) which prevent normal flow

along the canal (see Freeboard and spillways, p. 109).

A spillway should also be incorporated in the design of a

forebay to prevent unexpected overflowing. This will

happen when water enters the forebay faster than it is

taken up by the penstock, either because the canal is

conveying excess flow or because the flow into the pen-

stock has been reduced or stopped.

In all cases, the length of the spillway and the elevation

of its crest must be selected to permit all excess water

to overflow through it before it occurs elsewhere

(Fig. 5.185). The dimensions

of

the spillway required to

pass the expected flows can be calculated as described

in the following paragraphs. And wherever a spillway is

used, the water which overflows must be led away to

avoid erosion.

Overflow spillways. Of the several typ*s of basic spill-

way designs, the most common is the overflow spillway

where water overflows the structure itself--the crest of

a dam, the wall of the intake or forebay, or the side of a

canal. If the spillway is not submerged, that is, if the

downstream water level is below the crest of the spi!i-

way, the discharge over such a spillway can be deter-

mined by the following equation:

Q = C L h3i2

spillway

trashrack

conduit

where

Q = discharge (m3/s)

L = length of spillway (m)

h = head above crest

(m)

C = coefficient of discharge

= 1.4 - 2.3 (see Fig. 5.186)

Figure 5.186 illustrates the variation

of

the discharge

coefficient with spillway profile. For broad-crested

spillways, the coefficient of discharge increases as head

is increased or crest width is decreased. Rounding the

upstream

corner

of a broad-crested spillway also

slightly increases the discharge for a given head.

The spillway profile which passes the largest flow per

unit length is a rounded crest shaped like the underside

of the nappe of water passing over a sharp-crested weir.

This is often designated as a “standard crest.” Although

equations for the shape of this profile are available, an

extra level of effort would be required in both the

design and construction of such a spillway; therefore,

this profile is not generally used for micro-hydropower

schemes. Most frequently, a portion of ordinary

masonry wall is used as a spillway section. Because it

has a lower coefficient of discharge, the length of such

a spillway section compared to that of a rounded crest

would have to be correspondingly greater.

Shaft spillways. Another type of spillway is the shaft

or

“glory-hole” spillway more commonly found behind dams

associated with large hydropower plants. This spillway

usually incorporates a funnel-shaped inlet to increase

the length of the crest and therefore the flow which it

can accommodate. Some micro-hydropower schemes

using unlined earth canals have applied a variation

of this type of spillway to prevent any excess water

from overflowing the bank of the canal and causing a

major breach (Fig. 5.187). The spillway illustrated in

Fig. 5.188 has been made by using an oil drum as the

inlet section. A smaller-diameter pipe can lead the

overflowing water away from the canal. The flow into a

pipe used as a shaft spillway can be approximated using

Fig. 5.185. The intake at this site has several features

which were

poorly

desfgned-the area of

the tmshmck Is

160 Civil works

too small and

qutckly fiils

with debris

and the spillway

does

not overflow until the tmshmck

itself

overflows.

broad-crested spillways

c = 1.4-1.9

sharp-

crested

spillway

C = 1.8

“standard crest”

C = 1.7-2.3

(C = 2.2 if head

equals design head

associated with

crest profile used)

Fig. 5.186.

Discharge coefficients

for spillways of different

profiles.

For broad-crested profiles,

a

head and crest width

each less than 300

mm is assumed.

the same e:quation as for flow over an overflow spillway

[Eq. (5.35)j, but “L” now represents the circumference

of the spillway. If the pipe flows full, then it acts as a

bends sre incorporated in the downstream leg. Water

flowing down the pipe is deflected across at the second

bend and seals off this leg. Water which continues to

siphon and passes increased flow. This flow can be

approximated by using Eq. (5.9). In this case, “hf” repre-

sents the head across the portion of pipe flowing full.

Siphon spillways A siphon spillway represents a third

type, At large powerplants, this spillway is of major

dimensions and is often incorporated in the dam itself.

Although it has rarely been used with micro-hydropower

plants, its principle of operation is described here

because it might find some application. Because it uses

the potentially large head across a siphon, it produces a

significantly larger flow velocity than would be attained

with an overflow weir. It is therefore useful where

space available for another form of spillway is limited

<and where the water level must be kept within narrow

limits. This is frequently the case at a forebay, for

example.

The basic operation of a siphon spillway is illustrated in

Fig. 5.189. When the water level rises above the elbow

of the siphon, water begins to flow down the pipe. The

siphon will only operate properly when all the air within

has been removed, but this might not happen until the

water level is considerably above the maximum level

indicated in the figure. To prime the siphon sooner,

flow down the pipe entrains the entrapped air, gradually

reducing :he pressure until all the air is removed and

the siphon is primed. The larger the gross head across

the siphon-the vertical distance between the water

level in the basin and the outlet of the siphon-the

greater will be the flow which is then conveyed. With

the rapid outflow of water, the water level in the basin

will lower until the air vent hole is uncovered. Air

which is then drawn into the siphon breaks the prime

and the siphon stops operating. The air vent is simply a

small hole drilled into the wall of the pipe. Alterna-

tively, the air vent can be eliminated, in which case the

water level would descend until the inlet to the siphon is

exposed.

Fig.

5.188. An shaft spillway improvised from

an oil

drum.

Civil works 161

I33

“‘I’““: max

bend used to initiate

priming of the siphon

Fig. 5.189. Proposed

variation of

a siphon spillway for use

at a micro-hydropower installation

It is not possible to accurately predict the water level

required to initiate the siphon, because that depends on

the siphon’s size and configuration. However, when the

siphon is primed, the flow through a siphon spillway is

governed by the same equation that governs the flow

through an inverted siphon [Eq. (5.9)]. In this case, “ht”

is the gross head across the siphon.

Tra8hracks and skimmers

Any debris carried in the incoming water can have

adverse impacts on a hydropower scheme:

e It can obstruct flow along the conveyance struc-

tures, interrupting power generation or causing the

water to overflow and possibly undermine the struc-

tures.

I It can cause rapid deterioration of the penstock or

turbine

or

cause a catastrophic failure, such as rup-

ture of the penstock through a sudden blockage of

flow through the nozzle or fracture of the runner

blades.

It is therefore essential that the quantity of debris

which enters a hydropower scheme be minimized.

A trashrack intercepts the entire flow and removes any

large debris, whether it is floating, suspended, or swept

along the bottom. Frequently, one is located in the

intake structure to prevent debris from entering, and

another is placed just before the inlet to the penstock to

remove smaller debris as well as other debris which may

have entered the water downstream of the intake.

Sometimes a trashrack is also located at the entrance to

a power conduit or along a lengthy canal. A skimmer is

used only to hold back debris floating on the water sur-

face.

162 Civil works

In certain geographic areas

or

during specific seasons,

the large quantity of floating debris can wreak havoc

with the operation of a hydropower scheme. Trashracks

might seem to compound the problem, because debris

accumulates at these points and rapidly obstructs the

flow. During these times, operators at some sites have

been stationed around the clock to remove this debris.

The proper placement of skimmers and the proper loca-

tion and design of the intake and weir or dam can signl-

ficantly reduce the problem of debris. This is where

initial efforts should be focused.

lbshracks. A trashrack is made up of one or more

panels, each generally fabricated of a series of evenly

spaced parallel metal bars. The bars are parallel and

evenly spaced because a rake is commonly used to clear

the debris off the rack (Fig. 5.190). In this case, it is

essential that the teeth of the rake mesh into the paral-

lel bars without binding so that the rake can be pulled

along the bars easily to scrape off accumulated debris.

If a rake is used, trashracks made of expanded metal or

screen mesh should be avoided because they would be

difficult

to

clean. With small schemes where the trash-

rack is small and easy to manipulate, the rack can be

withdrawn and cleared simply by inverting and striking

it on the ground. A mesh can be used although removing

grasses and debris intertwined with the mesh can be dif-

ficult, In all cases, trashracks should be removable and

not permanently set in concrete.

Ftg. 5.190. A tmshmck bar spacing equal to

the

teeth

spacing

of

a rake facilitates removal

of

debris.

Trashracks can be installed by sliding them into grooves

in the concrete walls of the intake, canal, or forebay

structure. When closure for maintenance or repair is

desired, the trashracks can be removed and replaced by

stoplogs. A platform on which the plant operator can

stand while removing the debris is usua!ly erected just

downstream of the rack.

Trashracks fabricated of steel can be unexpectedly

heavy. Therefore, they should be assembled in panels

small enough to je handled easily for repair or replace-

ment.

Bars with a rectangular cross-section set on edge are

used at large hydropower schemes and are structurally

superior in resisting water pressure to those with a cir-

cular cross-section. However, shape may be critical for

very low-head sites. At such sites; the head loss

(Fig. 5.191) through the trashrack may represent a

larger portion of the gross head. If the trashrack is

located in front of tl;e inlet to the penstock, head loss

through the trashrack will reduce the submergence of

the inlet that is necessary to prevent vortex formation.

If the trashrack is located at the beginning of a canal,

the losr, in bead will reduce the depth of water in the

canal and therefore the flow it l:an convey. Fortu-

nately, liead loss across a trashrack rarely is mere than

a couple of centimeters, and including a trashrack in a

water conveyance structure generally has no major

impact on the plant’s operation even if this head loss is

not accounted for.

trashrack

cleaning

platform

Fig. 5.191. The

head loss “h” across

a

tmshruck

located

at

the inlet

to

a

penstock.

The gross wetted area of a trashrack-the area of the

rack below minimum water surface-is selected to limit

the velocity through the rack. This facilitates the

removal of debris and minimizes head losses. A design

approach velocity of 0.5 m/s permits some trash to

cover the rack between cleanings. If a trashrack is

located immediately in front of the inlet to a penstock

and the penstock velocities are significantly higher than

0.5 m/s, the trashrack can be built in a circular arc or

some other configuration (Fig. 5.192). This approach

increases the area of the trashrack and correspondingly

decreases velocity through it.

Bars on a trashrack before the inlet to the penstock

should be spaced no closer than is necessary to remove

debris which might be detrimental to the turbine’s oper-

ation. Otherwise, head losses may be high and the rack

Flg. 5.192. lf a small area would

result in too hfgh a wlo-

city through a tmahmck (aI, other

configumtbns can

reduce thIa veloctty (b).

may fill up quickly with debris. Letting small floating

debris through a turbine will have no adverse impact.

With a Pelton turbine, the space between bars usually is

not more than half the nozzle diameter (or a quarter, if

a spear value is used) to prevent the nozzle from chok-

ing. A similar criterion can be set for crossflow tur-

bines. For Francis turbines, the space between bars

should not exceed the distance between nmner vanes.

Numerous expressions for predicting head loss across

trashracks are available. One such expression is (39)

h=I$(i)

4/3,2

2g sin6

(5.36)

where

= head loss across trashrack (m) (see Fig. 5.191)

it = trashrack loss coefficient (Fig. 5.193)

t/b

= ratio of maximum bar thickness to space het-

ween bars

V

= approach velocity

= (design flow)/(cross-sectional area of water

just upstream of rack)

s

= gravitational constant (9.8 m/s2)

= angle of bars with the horizontal

The trashrack’s angle of inclination is largely one of

personal preference. An inclined rack facilitates the

Civil

works

163

I

flow direction

2.4 1.8

1.8 1.7 1.0 0.8

%

-I

Fig. 5.193. Trashrack loss coefficients for vurious

bar sec-

tions 141).

manual removal of debris when a rake is used and gives

less resistance to flow.

A well screen also can be used as a trashrack. This is

usually a plastic

or

metal pipe perforated with a series

of slots which is used in tube wells to restrain the

granular material whose pores contain the water. An

improvised trashrack of this type is shown in Fig. 5.194.

In using a well screen as a trashrack, an adequate length

is extended into the stream, settling basin, or forebay,

with a cap over the end. The length ‘Ii.,” (m) of well

screen required is

L’&

0.

where

Q = m&,um expected flow through the screen

8

A, = effective open area per meter length of screen

(m2/m or m)

V

= entrance velocity at screen

Fig. 5.194.

A segment

of

drilled PVC

pipe,

installed in

the

forebay as an extension of

the penstock, serves the

func-

tion

of

a tmshrack

To minimize head losses, the entrance velocity “v”

should be in the range of 0.05-0.10 m/s (83). The effec-

tive open area per unit pipe length, which can be

increased by increasing the density of the slots or using

larger-diameter well screen pipe, is specified by the

manufacturer.

As mentioned, a rake can facilitate the removal of

debris from the trashrack. LI this case, its teeth should

fit easily between the bars

of

the r ick. To facilitate

cleaning, the design illustrated in Fig. 5.109 places an

inclined trashrack below water level. Floe ting debris is

carried away by excess water. Any debris which does

accumulate on the trashrack can be raked up and into

the dowrztream trough, where it is carried away.

With a continuously operating hydropower plant, the

task which requires the most frequent attention is

removing debris from the trashrack. Large hydropower

schemes have automatic rack-raking machines to per-

form this chore. These are rarely used with micro-

hydropower schemes. But rather than performing this

task manually, other approaches have been attempted

or

are being used.

One

design

for

a self-cleaning trashrack is incorporated

in the forebay described in Settling basins (p. 166) and

shown in Figs. 5.204 and 5.205. This design incorporates

a horizontal trashrack which is cleared periodically by

briefly reversing the flow through it. This design is

probably more effective where most of the floating

debris is leaves rather than

grass

which can get

intertwined in the trashrack.

The design shown in Fig. 5.117 also uses reverse flow to

clean the trashrack. In this case, a cylindrical trashrack

is used. Flow is introduced from inside and moves out-

ward. As debris is caught in the trashrack, the trash-

rack is placed under tension and will not collapse as it

would if the water flowed inward. The trashrack

is

cleaned by raising the cylinder gate to open the drain in

the center. Water which has already passed through the

trashrack will flow back in and, in so doing, wash off

and carry away through the drain most of the debris

restrained by the trashrack.

Some creative powerplant operators have replaced the

electric motor incorporated in automatic rack-raking

machines with other sources of motive power. Most of

these devices, such as the one illustrated in Fig. 5.195,

use a drum filled with water to provide the motive

power (110). Water, under pressure from the higher

level of water in the forebay, is fed continuously into an

oil drum which is normally held in a

raised

position by a

counterweight. When the weight of the water in the

drum is sufficient, the drum lowers, pulling up the

scraper to rake debris off the trashrack. Then when

sufficient additional water has entered the drum and the

water level reaches the bend in the siphon, the siphon is

automatica!ly primed, empties the drum, and permits

the counterweight to raise the drum and lower the

scraper to their orIgina positions to repeat the cycle.

Several problems were encountered initially in the oper-

ation of this device:

64 Civil works

x -fl counterweight

Fig.

5.195. A

diagrammatic

representation of

a mechani-

cal rake driven by a waterfilled oil drum (ill)). This mke

was designed by John

Wood

and is in opemtion in EMi&)+

mon,

Ireland.

l

Initially, the scraper carriage carried the counter-

weight, but the sizable

force

against the trashrack

and the increased drag on the carriage as it moved

up led to unreliable operation. The drum was then

counterbalanced as shown in Fig. 5.195.

l

The cycling period depends on the size of the fecd-

Fipe and siphon, and the resulting 14 minute cycle

sas too long for the increased volume of leaves the

device had to handle in the autumn. Consequently,

the feedpipe diameter was increased from 20 mm to

50 mm, and a valve was placed along the feedpipe to

control the filling time and therefore the cycling

period. In addition, the siphon diameter was

increased to 80 mm.

l

Feedwater was originally drawn from the forebay.

Consequently, just when sufficient pressure was

needed to clear a clogged trashrack, water level

behind the trhshrack was lowering, reducing the

pressure head and flow in the feedpipe and increas-

ing the cycling period. The simple solution to this

problem was to locate the inlet to the feedpipe

ahead of the trashrack. The intake to the feedpipe

then had to be designed to ensure that it could not

itself become clogged easily.

Skimmers. A skimmer is an obstruction placed at the

water’s surface, usually at an angle to streamflow,

which skims floating debris from the passing water. If

the water level changes markedly as, for example, at

the intake off a stream, the skimmer can be a floating

piece of timber secured at both ends (see Fig. 5.17).

Because some debris usually passes under the skimmer,

a trashrnck is still necessary. However, a skimmer

reduces the frequency with which the trashrack has to

be cleaned.

If changes in water level are small, a fixed skimmer can

be used. Figure. 5.196 illustrates such a skimmer

installed in a forebay. A trashrack is still included over

the inlet to the turbine. Another approach is simply to

place the trashrack under the minimum water level, as

shown in Fig. 5.59. In this case, floating debris is

deflected by the wall of the intake and carried down-

stream by the excess streamflow.

Fig. 5.196. Water is being drained out through the sluice

gate

at the rfght to

facilitate

cleaning out

the forebay.

A

skimmer in

the form of a screen can be seen extending

acrass the forebay. A wall behind the skimmer prevents

sediment

from entering the

penstock

through the tmsh-

rack. The powerhouse

at

this very-low-head

site is at the

left.

Civil works 165

A settling basin can serve two functions:

l

When located at tbs intake, it removes any sediment

from the incoming water which might otherwise set-

tle in the intake or power conduit. A large accumu-

lation of sediment there can reduce the flow avail-

able for

power

generation, cause a intake or canal to

overflow, and require a sizable effort to clear.

l

When located before or at the inlet to the penstock,

it removes sediment which might cause rapid wear

and reduce the operating life of the turbine.

Proper placement and design of the intake structure and

the weir or dam are important factora in reducing the

sediment and bed load admitted in the first place and in

facilitating the maintenance of settling basins. If a

scheme includes a power canal, proper design of the

canal can minimize the additional debris which can

enter along its length and simplify the job of a settling

basin at the inlet to the penstock.

A settling basin

works

on the theory that if flow is not

turbulent, particles denser than water will gradually

settle out. When such a particle moves without inter-

ference through a fluid because of the difference bet-

ween its density and that of the fluid, its settling velo-

city with respect to the fluid quickly becomes constant,

the drag on the particle equalling its weight in the fluid.

Usually, settling basins are roughly rectangular in shape.

Of the particles entering such a basin (Fig. 5.197), the

last particle ta settle out before the water leaves the

basin would be one located approximately at point (a),

To settle out, the slowest settling velocity this particle

may possess is “v

settling velocity ess than this value would not settle to

P

“.

Any particle at point (a) with a

the bottom. The physical characteristics of the settling

basin would therefore imply that

where

V

= average water velocity through the basin (m/s)

Q

=DW

v. = particle settling velocity (m/min)

It is assumed that the velocity is uniform across the

basin cross-section.

If this value is substituted into Eq. (5.38), then

where

%

= LW = surface area of basin

166 Civil works

rectangular prism of

\

Y

J

settling basin

Fig. 5.197.

A rectangular setting basin sired to settle out

all particles with settling velocities greater than “vo”.

Therefore, with the assumptions made above, the small-

est particle size which can settle out from a given flow

is solely a function

of the

surface area of the basin and

not its depth.

Rather than constructing a settling basin approximately

the same depth as the inlet canal (Fig. 5.198a), it could

be significantly deeper than the inlet or outlet conduit

(Fig. 5,198b). This increase in cross-sectional are&

would appear to decrease the velocity of water “v”

through the basin and permit a

basin

of smaller surface

area to remove particles above a given size. However,

(4 _

settling basin

A

Fig. 5.198. Although the velocity through a deeper basin

(bl is generally lower, flow largely short-ctrcdts the basin

In this case

if

special baffles are not used.

The velocity,

therefore, may closely resemble that of the velocity of a

shallow basin (a).

the velocity would not be uniform

over

its cross-section,

and most of the flow would short-circuit the basin-it

would flow directly from inlet to outlet by the shortest

route.

The judicious use of baffles could increase the

effectiveness of a deeper basin but would result in a

more complicated and expensive design.

Moreover,

it

would require additional excavation, and if the settling

area were close to the intake, flushing of a deeper basin

back into the main stream might not be possible because

of the relative elevation of the stream level and basin

floor.

Equation (5.39) implies that a long, narrow basin would

be as effective as a short, narrow one. However, the

flow velocity in the former would be greater and

because of turbulence, sediment may get stirred back

into suspension. To prevent this, the velocity of the

flow throueh the basin nenerallv should not exceed 0.3-

0.5 m/s (se; Velocity - * in a canal, p. 96). If the approx-

imate depth of the basin is set equal to the depth of

water in a canal, the width of the basin “W” (m) is found

to be

(5.40)

where

Q = flow (m3/s)

V

= desired velocity through the basin (m/s)

D = basin depth (m) = canal depth in this case

When the basin width has been determined, Eq. (5.39)

can be solved for the basin’s length:

L = 60+w

0

where:

L = basin length (m)

PO

= settling velocity of smallest particle to be

removed (m/min)

For sand (quartz) particles, the settling velocity “vO”

ranges from 1 m/min for 0.1 mm particles to 5 m/min

for 0.8 mm particles. A basin for micro-hydropower

schemes is often designed to remove particles with dia-

meters greater than 0.3 mm and a corresponding set-

tling velocity of about 2 m/min.

Although a rectangular basin was considered when

deriving the previous expressions for its dimensions, a

more widely used configuration includes a base sloping

toward the gate through which the basin is draiued and

cleaned out. A base with a slope ranging from 3% to

10% is often used. In addition, the side walls sometimes

have a significant lateral slope to facilitate removing

the sediment (see Fig. 5.56a). Because the dimensions

derived above are only approximate, incorporating a

sloping base does not change the previous conclusions.

If a canal opens up into a wider settling basin, the

accompanying decrease in flow velocity will cause

sediment and bed load to start settling out as soon as

they enter. The upper portion of the basin will contain

the coarser sediment; the finer sediment will be depos-

ited at the downstream end of the basin. Therefore,

although settling basins conventionally slope down-

stream toward a sluicing gate so that water entering the

basin can scour its entire length, most of the sediment

which will have to be removed may be at the other end.

If it is essential that water for power generation flow

through the canal at all times, parallel settling basins

may be incorporated (Figs. 5.199 and 5.200). One basin

Fig. 5.199. This

settltig

basin just after the intake

Incor-

pomtes two sections-one can be cleaned out whfle the

other

remains

in use.

No

scouring

outlet has been incor-

pomted,

and

sediment

has to be shoveled out.

Fig. 5.200. Parallel settling basins permit use of one while

the other is

being cleaned. Cations were placed downhill

of a portion of the canal to protect this

area at a

bend

in

the stream from being undermined by the stream.

Civil works 167

can then be cleaned while the other is in use. Because

each basin has to be adequately sized, this approach

requires doubling the size of this structure, which dou-

bles its cost and requires that a location with double the

area be found along the canal route. Another approach

would be to bypass the flow around the settling basin

temporarily, because an insignificant amount of sedi-

ment would pass through the turbine during the brief

time that the settling basin is being cleared (see

Fig. 5.58). Because a well-designed settling basin should

not require much time to clean, it would be simpler to

incorporate only a single, well-designed basin and to

shut down the turbine briefly during a low-demand per-

iod while the basin is cleared of sediment.

During site layout, in the initial stages of planning for a

hydropower scheme, the settling basin should be located

so that the suction head--the difference in eletation

between the settling basin and the outlet of the scouring

or drain pipe (Fig. 5.201)-permits adequate scouring

velocities in the vicinity of the inlet to the drain pipe

when the gatd is opened. Insufficient scouring veloci-

ties make it more difficult for the water to draw out

the sediment and more shoveling would be required.

Even though a high velocity may exist through the

scouring pipe, the flow velocity quickly diminishes as

the area of flow increases, i.e., as one moves father

back from the inlet of the pipe. Therefore, opening the

scouring gate will not necessarily result in velocities

throughout the basin that are sufficient to pick up and

carry away all the sediment. Some shoveling will still

be necessary. The objective is to design the settling

basin to minimize manual inputs, even if they cannot be

eliminated completely.

-power conduit d+-- settling basin-1

intake

Fig. 5.201. If a settifng

basin

Ls lncorpomted In

the

intake

structwe, sefectfng a

site with

adequate auct[on head will

facWate

the scourfng of sediment from the b&n.

To facilitate scouring, it may be advantageous to open

the drain pipe as quickly as possible; otherwise, the set-

tling basin may be empty of water before the pipe is

fully open, and optimum use therefore cannot be made

of the scouring effect of rapidly flowing water. This is

especially a problem with sliding gates raised by screw

hoists. A cylinder gate (see end of Sliding gates, p. 155)

overcomes this problem since it can be raised quickly

with little effort (Fig. 5.202).

To reduce the effort required to clean out the settling

basin, the floor of each basin shown in Fig. 5.203 is unu-

sually steep, closely resembling a funnel, with a sliding

gate located at the bottom. It is envisioned that the

cylinder

gate

Fig. 5.202.

A

long,

narrow

forebay serves as a serrung

basin which is

conveniently flushed out using a cylinder

gate.

Excess water overflows the tmshmck, taking with it

some of the

floating debris.

sediment will be directed toward the bottom as it

accumulates. With sn excess of water in the stream,

the gates wiI1 always be slightly open to remove the

sediment on a continuous basis. (There are two settling

basins in series here because the instructions for design

168 Civil works

Fig. 5.203. A view of the settling area taken

from !he dnm

crest at

a site

under

construction.

Two

sluice gates permit

this area to be scoured out. Tha sediment returns to the

stream at the right.

were interpreted incorrectly; two parallel basins had

been envisioned.) Such deep settling basins are rarely

considered for several reasons:

e additional excavation usually would be necessary,

m flushing out the sediment would be difficult unless

the streambed elevation dropped signific‘antly just

after the intake, cand

a additional construction materials would be required.

Fig. 5.204 shows a self-cleaning settling basin in opera-

tion. At this site, the forebay serves as a primary set-

tling area. Its design is shown schematically in

Fig. 5.205. Water is taken from below the stream’s sur-

face, permitting debris floating on the surface to con-

tinue unaffected. After entering the structure, water

passes upward through a trashrack before proceeding

into the penstock. The trashrack restrains debris in sus-

pension and any sediment drops to the floor.

A scouring outlet is controlled by a radial gate which is

an integral part of a pivoted container. When in opera-.

tion, water fed to the container from the penstock

through a length of tubing gradually fills it. When this

container/gate unit is nearly full, the weight of the

Fig.

5.204. A self-cleaning foreboy designed by

Rupert

Armstrong Evans: (a)

sluicing gate in the normally closed

position

and

(b) gate

open

to

permit scouring.

water contained inside forces it to tip. Several events

then occur:

o The scouring outlet is opened, permitting a large

velocity within the settling area to wash out any

sediment.

Civil works 169

Inversin R. Allen Micro-hydropower Sourcebook

Подождите немного. Документ загружается.