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10 FLOWMANAGEMENT

10.1 The Intake10.2 Canals10.3 Forebay Tanks10.4 Reservoirs

10.1 The IntakeThe intake of pico hydro schemes can be a simpleand inexpensive arrangement. Non-permanentsolutions are favoured over elaborate weirs dueto their lower cost and greater flexibility. Theeffect of floods must always be considered whendesigning the intake.Pipe intake – for ample flowsBoulders used to divert part of a river flow intoa simple canal or submerged length of pipe asshown in Figure 10-1 are often sufficient. Withsuch simple and inexpensive solutions, stormdamage can be repaired with local materials.Though careful construction is required toprevent frequent repair being necessary. Thepipe must sometimes be quite long to ensure thatthe entrance is higher up than the exit to thecanal. Flexible pipe is easier to use than rigidpipe and it should be anchored in the flow withlarge stones. The entrance should be raisedslightly off the bed of the river or stream. Thiswill prevent silt or debris from blocking theentrance.

Figure 10-1 The simplest intake design is a pipe anchoredin the flow

Weir intake – for low flow sitesA small weir can be constructed out of concreteto ensure that all the available water is divertedduring the dry season. This may be a practicalsolution at some sites. The flexible pipe, whichremoves the water, is set into the weir. Thefoundations and the sides of the weir should bejoined to solid rock to prevent water fromleaking round and eventually undermining thestructure.

Figure 10-2 An intake using a small concrete weir isuseful if the flow is very low during the dry season

Construction of Small Concrete WeirsThe proportions for small concrete or masonryweirs should follow approximately those given inFigure 10-3. Often large stones cementedtogether can be used for the construction of theweir. The strength of the structure issignificantly improved by using a gabion. This is awire cage that holds the structure together. Itis particularly useful where strong flows areexpected. The wire mesh is usually made from 2or 3mm diameter wire with a mesh size of 50mmto 100mm.

Figure 10-3 The construction of a weir should follow theabove proportions.

Figure 10-4 A concrete weir can be used to form a smallreservoir (Nepal).

0.7 x h

h (m

)

0.4 x hrock concrete

flushingpipe

intake pipe

0.50

m

intake pipe river bed

water level

flushing pipe

concrete weir

wire filter

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Filters

Figure 10-5 Filter designs for a) intake and b) penstock

Filters are required to prevent pipes that areused for pico hydro installations from becomingblocked with materials such as silt, wood andleaves. There are two situations where a filter isrequired:1. the intake pipe leading to a canal, forebay or

reservoir as illustrated above.2. the entrance to the penstock pipe (see

Section 11).For the first case, the size of the holes in thefilter is not important. The only thing toconsider is how to prevent the pipe frombecoming blocked. One method of building asuitable filter is to use a tube of wire mesh. Thetube can be fixed to the pipe with a metal clampsuch as a jubilee clip.

In the second situation, when the filter ispositioned at the entrance to the penstock, it isimportant that the holes in the filter are smallerthan the nozzle at the other end. Actually, theyshould be about half the diameter of the nozzleto be sure that any particles that enter the pipecan not cause a blockage at the other end. (Ifthe nozzle becomes blocked then there is adanger that the penstock will burst due to thesudden pressure.) A filter with small holes canbe made out of a piece of plastic or metal pipethat is capped at one end. Lots of holes are madeusing a correctly sized drill. It is important thatthe total area of the number of holes drilled ismore than the area of the pipe to make sure thatenough water can be drawn in. The filter isthreaded or clamped over the pipe to ensure atight seal.

10.2 Canals

Figure 10-6 Canal installed in a difficult locationsupplying a small hydro scheme for local electrification(Peru)

A canal can be a low cost method of conveyingwater from a source some distance to a morefavourable intake position. For some sites, thiscan really improve the economic viability byreducing the length of penstock and cablerequired. At other sites a canal is an expensiveaddition and can often require regularmaintenance due to leaks, soil erosion andlandslides.

Using a canal and deciding on the route it shouldtake, are decisions that must be consideredcarefully. The following factors are important:• Local experience and history with building and

managing similar water systems such asirrigation canals.

• The availability of cheap / free labour to digand maintain the canal

• Soil type, lining requirements and cost oftransporting materials such as cement.

• The cost of other options such as a longerpenstock / longer distribution cable comparedto the cost of a canal. The use of low-pressure pipe to convey water to the intakemay also be a lower cost alternative.

• Could a new canal serve a dual purpose,irrigating land during the dry season inaddition to supplying the turbine?

• Are there any existing (or abandoned)irrigation canals that could be directly usedor improved or extended if necessary?

If the canal is lined with concrete or stones andcement, then its strength and reliability will beimproved. However, such a lining will increase thecost considerably. In many remote areas this willnot be practical because of the difficulty of

wire mesh coil

holes drilled in pipe(half nozzle diameter)

b) to turbine

a) to forebay

metal clip

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transporting building materials. Lining a canalalso increases its efficiency. Water can travel athigher speed without causing the sides to erodeso for a particular volume flow rate, thedimensions can be reduced (see Table 10-1).Canal designThe flow of water in a canal depends on:• The speed or velocity of the water• The cross-sectional area of waterThe equation for flow rate is

vAQ =where:Q = flow rate (m3/s)(multiply by 1000 for flow in litres per second)v = velocity (m/s)A = Cross-sectional area of water (m2)

1. The velocity of water flowing in a canal (v)depends on the slope of the canal and the‘roughness’ of the material which has beenused to line it.

There is an upper limit to the velocity fordifferent building materials. Above this value thesides can quickly become eroded.Canal material Max. velocity to avoid

erosion in shallow canalsSandy soil 0.4 m/sClay soil 0.6 m/sConcrete / masonry 1.5 m/s

Table 10-1 Velocity limits for shallow canals (<0.3mdeep)

From Table 10-1 it is apparent that for earthlined canals, the maximum velocity of the wateris much less than for those with a durable lining.If the water contains silt then there is also alower limit to the velocity of 0.3 m/s. This is toprevent the silt being deposited and blocking thecanal. If the water is clear then the lower limitis not important. A low velocity means that thedrop required over the length of the canal issmall. For this reason, the canal dimensions givenin Table 10-2 have been calculated using a designvelocity of 0.3 m/s. This allows the head loss inthe canal to be reduced and maximises the headavailable for the penstock.

In the following examples of canal dimensions aroughness of 0.07 has been assumed. This issuitable for a shallow canal with some vegetation.The roots of vegetation growing along the canalcause some friction but they are useful becausethey support the sides.

2. Design of canal cross-sectional area (A)

Naturally, the canal cross-sectional area must bebigger than the cross-sectional area of waterneeded to give the flow required. The extraheight (called the freeboard allowance) is usuallyabout 30%. This reduces the risk of the canalwalls being damaged by overfilling.The sides of an earth canal should slopeoutwards. This reduces the risk of the wallscollapsing due to the erosion caused by the flowof water. It is possible to construct a canal withvertical walls if it is strengthened by lining thesides with stones and cement or with concrete.In this case the top and bottom dimensions arethe same. However, masonry and concrete liningsare expensive and are rarely cost-effective forpico hydro schemes.

In order to simplify the process of canal design,suitable dimensions for the cross sectional areaare given for different flow rates in thefollowing table. The head loss is the drop for100m of canal. If the required length of canal is200m then multiply the head loss by 2 and buildwith this drop over its length.

Canal lining and minimum dimensions10 l/s Sandy soil Clay soil Concrete/

masonryHeight H 13 cm 15 cm 15 cmTop Width 59 cm 44 cm 29 cmBottom Width 6 cm 13 cm 29 cmHead Loss(100m length)

1.6 m 1.3 m 1.4 m

20 l/sHeight H 19 cm 22 cm 21 cmTop 84 cm 62 cm 42 cmBottom 9 cm 18 cm 42 cmHead Loss(100m length)

1.0 m 0.8 m 0.9 m

30 l/sHeight H 23 cm 27 cm 25 cmTop 103 cm 75 cm 51 cmBottom 11cm 22 cm 51 cmHead Loss(100m length)

0.8 m 0.6 m 0.7m

Table 10-2 Suitable minimum canal dimensions fordifferent flow rates and lining materials

H

Bottom

Top

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SeepageEarth lined canals loose a significant proportionof the water due to seepage. In sandy soil areasexpect at least 5% to be lost through seepageper 100m (e.g. 0.5 l/s for a flow rate of 10 l/s)Providing additional water is available, it is worthsizing the canal for a larger flow rate than thatrequired by the turbine. Assume that anadditional 10%-20% will be required. This willallow for some loss through small leaks as well asseepage. Sometimes water in the canal will alsobe used for a number of purposes such asirrigation or domestic supply. The requirementsshould be taken into consideration at the designstage.Canal Construction.The route taken by the canal must be carefullyselected. If possible the following areas shouldbe avoided:• Excessively porous ground• Rocky areas which prevent excavation• Steep and unstable sections

Sealing the canal with clay or concrete in porousareas may be an option but large rocky outcropsshould be avoided. Steep sections and stormgullies are also difficult but in many rural areas,ingenious solutions have been found. Canals have,been successfully constructed in some verydifficult locations (see Figure 10-6 and Figure10-9). These require careful planning, localmotivation and persistence but not necessarilygreat expense.

Figure 10-7 A length of pipe can be used to bridge adifficult gulley with a small canal. The pipe is securedwith rocks on either side.

When crossing a storm gulley, for example, it isparticularly important to allow adequate drainagefor rainwater that could otherwise destroy thesides of an earth canal. Short lengths of pipe

(Figure 10-7) or small wooden aqueducts (Figure10-8) can sometimes be used to bridge difficultsections of the route.

Figure 10-8 A wooden aqueduct can be used to transportwater in the canal across uneven ground

Figure 10-9 and Figure 10-10 show a raised canal thatruns on a narrow ledge underneath a vertical rock face.The canal has a masonry lining and is supported by a highstone wall underneath.

Low Pressure PipeAn alternative method to convey water to theintake is to use low-pressure, plastic pipe. This isavailable in some countries as drainage pipe. Low-pressure pipe is cheaper than penstock pipebecause the walls are thinner. It will often be acheaper alternative than a concrete lined canal.

canal

stone wall

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Figure 10-11 Land drainage pipe

Some consideration needs to be given to thebest diameter and the slope of the pipe sincethis will affect the flow rate and head loss. SeeSection 11.

10.3 Forebay Tanks

Figure 10-12 A small forebay is suitable where flow issufficient throughout the year

The forebay tank provides a sufficient depth ofwater to ensure that the top of the penstock isalways covered. In some cases the penstock isextended to the intake and then no forebay isrequired. A forebay is required where a canal isused or if water from more than one source iscollected.The design of the forebay may vary depending onfactors such as:• accessibility of location• availability of building materials

• type of ground and soil• cost of labour and local skillsHowever, it is highly recommended that alldesigns include an overflow facility and somemeans of draining to allow the tank to becleaned.

Figure 10-13 Suggested design of forebay tank fed from

an irrigation canal.

Depth of WaterThe depth of water in the forebay tank shouldbe sufficient to cover the penstock by 4 timesits diameter. The penstock should also beapproximately one diameter clear of the bottom.

Example What depth should the overflow in the forebaybe set at if the penstock is 75mm in diameter?AnswerDepth above penstock = 4x75 = 300mmDepth below penstock = 75mmPenstock diameter = 75mmTherefore approximate depth of overflow

= 300+75+75 = 450mmSiltSince the water in the forebay tank is slowmoving, silt falls to the bottom where it forms athick layer of mud. This can block the penstockif it is allowed to become too deep. Larger hydroschemes often have desilting basins to remove

Height difference (100mm)

0.5mPenstock Pipe

Flushing Pipe

1.0m

Direction of flowin irrigation canal

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the silt. This is rarely necessary for pico hydroproviding that a sluice gate or a flushing pipe isincluded (see Figure 10-13). This makes the jobof cleaning out the silt much easier.

OverflowIf the forebay becomes full, the water mustescape without causing damage. The overflow canbe a notch or channel cut into the lowest wall ofthe forebay (see Figure 10-15). An alternativemethod is to use a pipe that combines theoverflow and the flushing facility. This isillustrated in Figure 10-13. The vertical pipe canbe removed from the elbow to flush out thetank. This example shows a forebay that is fedby an irrigation canal although the water couldequally well be provided by a low-pressure pipe.The flow can be diverted into the forebay whenrequired. Otherwise the water always continuesto flow down the canal.Whichever method is used, the overflowingwater should be directed away from the forebay,preferably into another stream or a ditch.

Forebay ConstructionProviding that labour is available a small forebay,using stones and clay as a seal, is not expensiveto construct. The intake to the penstock iseasier to secure if stones are used in theconstruction of the walls. If the forebayfrequently overflows then the clay will be quicklyeroded and will require regular repair.Cement and stones or concrete are used toconstruct forebays with a longer life and greatererosion resistance. A suitable method forcementing stones or bricks to line a forebay,canal or reservoir is shown in Figure 10-14

Figure 10-14 Masonry techniques for pico hydro civilstructures

Figure 10-15 Construction of stone forebay (Kushadevi,Nepal)

Figure 10-16 The flow of water into the forebay iscontrolled by a sluice gate. An overflow is cut into thewall on the downhill side so that excess water is returnedto the stream without undermining the masonry. Aflushing pipe, sealed with a wooden bung has beenincorporated at the lowest part of the tank.

10.4 Reservoirs

Figure 10-17 A small reservoir provides low-cost energystorage which can be useful during the dry season.(Sankhuwa Sava, Eastern Nepal)

The forebay can be enlarged to form a smallreservoir if the flow during the dry season isinsufficient to operate the turbine continuously.

Mortar for cementing stonetogether during constructionof civil structures should bemixed to the ratio of 1:4(cement to sand).

A more water resistant cement, ratio 1:2, can then be used for pointing between the stones and preventing leaks.

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For example, water is stored up during the dayand night when the turbine is not in constant useand then released to provide power for eveninglighting.

Water storage can add a useful degree offlexibility and security to the scheme if eitherof the following are true:1) The water will be collected from more thanone source, some of which is likely to beintermittent.2) The water is or will soon be used for otherpurposes apart from hydro-power, such asirrigation.

The storage requirements are easily calculated ifthe flow required by the turbine and the lowestexpected flow into the reservoir are known. Themanufacturer should provide information aboutthe flow requirements of the turbine. Thereservoir should be sized according to theminimum expected flow. This must be measuredduring the driest part of the year using one ofthe techniques described in Section 0.Alternatively it must be estimated by relying onlocal knowledge.

Example Storage Capacity of a Reservoir

A 1.5kW turbine and generator have beenselected for the site in Western Nepaldescribed in the example on page 7-1. The headhas been measured at the site and found to be70 meters after friction losses in the penstockhave been taken into account.

In order to generate this power with this headthe turbine manufacturer recommends a flow of5 litres per second. Measurements of the flow inthe nearest stream to the village have beentaken during the driest part of the year, afterseveral weeks of no rain and found to be only 2litres per second. This is also the lowest flowthat has been observed in the stream accordingto local knowledge (thought to be reliable as farback as 10 years). During the rest of the year,however, a flow exceeding 5 l/s is available.Storage will be required for the dry periods toenable the scheme to supply electricity to thevillage for 5 hours of lighting in the evenings and

for two hours of mechanical power to drive a sawin the workshop during the mornings.

How big does the reservoir need to be?Flow in during driest period = 2 l/sFlow into penstock = 5 l/sMaximum period of operation = 5 hrsShortfall in supply during this period

= 5 - 2 = 3 litres per second

The reservoir must have enough capacity tosupply an extra 3 litres per second during theperiod of operation.Volume of water required as storage:

5hrs = 5 x 60 x 60= 18000 seconds

Volume = 18000 s x 3 litres per second= 54000 litres or = 54 m3

(1 cubic metre = 1000 litres)

What dimensions should the reservoir be inorder to store 54 m3?

The reservoir will be allowed to fill up overnightso that the saw can be driven in the morning.

The ground is rocky at the proposed site of thereservoir and it is unfeasible to dig a reservoirthat is more than 1.5 m deep.Suitable dimensions would therefore be 1.5metres depth x 6 meters width x 6 metreslength.1.5 x 6 x 6 = 54 m3

An alternative solution for this village is tosupply the reservoir with water from a springthat is some distance away. They can transportapproximately 1 litre per second though a narrowbut inexpensive pipe from the second sourceduring the driest period. What is the newstorage capacity required?

[Answer = 36 m3 so suitable dimensions would be1.5m x 6m x 4m]