Technical brief

Gravity-fed schemes
Part of a series ofWaterAid technology
briefs.
Available online atwww.wateraid.org/technologies
January 2013

Introduction
A gravity-fed supply from a small upland river, stream or spring, impoundedwithin a protected catchment, is an example of a sustainable water supply
technology requiring no treatment. An additional benefit is that, using the forceof gravity, water can be transported by pipework to tapstands placed near tohomes, reducing the work involved in carrying water.
The capital costs of gravity-fed schemes are, on average, higher than the costs ofschemes that obtain water from underground sources. This is due mainly to thecost of long pipelines from the upland sources down to the villages and partly
to the cost of providing storage tanks. However, running costs are usually low;maintenance includes simple tasks such as replacing tap washers and cleaning
screens. Reliability is usually high and consequently the level of service is good.
The usual components of a gravity-fed scheme are the source (stream, spring,catchment, dam or intake), main pipeline, storage and break-pressure tanks,
distribution pipelines and tapstands.

Fig 1: A gravity-fed scheme

Technical brief

Gravity-fed schemes

Advantages of gravity-fed schemes✓ Use of gravity negates the need for expensive pumps✓ Low maintenance and running costs✓ Consistent level of service due to low maintenance needs✓ Tap stands within reasonable distance (250 metres) of all households

Disadvantages of gravity-fed schemes✗ More expensive construction costs than underground sources✗ Difficult terrain can restrict pipe-laying✗ Yield can diminish or dry up during extreme drought periods✗ Furring of pipes in hard water areas

Feasibility of the scheme
The first phase in the creation ofa gravity‑fed scheme is a survey
to determine the feasibility of theproject. Influencing factors include
village population, locally available
materials, supply of skilled labour,and the quantity and quality of flow.
The communities that benefit from thescheme usually commit large amountsof time and effort to the construction
work.
SourceSprings and small streams are themost common sources for theseschemes. Whenever possible, a springshould be considered first as they
are generally of better water quality
and easier to protect against furthercontamination.
If a spring or stream is to be thesource, it must be one that flows
throughout the year, and the flow
must be measured in the dry season
in order to know the yield that cansafely be relied upon. The maximum
flow should also be determined byquestioning the villagers for anecdotal
evidence. While the safe/minimum
yield is important for pipeline andreservoir design, the maximum flowis also necessary for estimating
structural protection for the intake

and overflow requirements. When aspring is used, the spring head must
be protected. The water must be piped
directly from the eye of the spring toprevent pollution of the supply.
Catchment
The catchment area of a spring or
stream must be free of animals andcultivation to prevent contaminationof the source from chemical or
biological matter. If only a small
area is involved, it may be fenced
off completely. Communities often
enforce bylaws to exclude people andanimals from the area.
Source development
When investigating a possible source,
the surveyor must develop ideas
about how the intake work will bebuilt, for example, how to protect thestructure against erosion, floods orcontamination by surface water runoff. What amount of excavation will becalled for and will a dam or drainage
channel be required?
Dams in streams are generally small;their purpose is to provide a small
pond so that a controllable draw-off
pipe can be built into the wall of thedam at a level higher than the bed of
the stream. Unlike larger dams, which
impound water to provide storage
2Technical brief

Gravity-fed schemes
over a dry season, these small dams
overflow for most of the time. The
crest of the dam acts as an overflow
weir, except at the sides where it
is raised to prevent scouring of thebanks.
A dam is usually constructed outof concrete, blockwork or masonry,
preferably founded on solid rock.
Rock, or some other impermeable
material, should also form the basinof the impoundment. Twin intake
pipes (one in use, one in reserve)
are built into the wall of the dam; onthe upstream side of the dam they
have strainers or screens, on thedownstream side they are fitted with
control valves. A scour pipe is also
built into the dam, at a low level, witha stop valve on the downstream side,and is used periodically to drain thepond and to clear accumulated silt.

DesignIf the project is considered feasible,
the surveyor must then conduct a
topographical survey. The route of thepipeline from the intake to the storagetank must be surveyed and a drawing
made of the optimum hydraulicgradient line, in order to determine
the pipe size needed to deliver thedesign flow.
Main pipeline
In rocky areas, the main pipeline
will probably be laid above groundand will be made of galvanised mild
steel tubing, anchored on saddles.
Elsewhere, the pipeline will be laid intrenches, to protect it from damage,
and will usually be plastic pipe (MDPE– medium density polyethylene).

Fig 2: Components of a typical gravity-fed system

Stream source

Catchment
IntakeSedimentation tank
Break pressure tank
Inlet pipe

Outlet pipe

Branch line

Distribution chamber
Reservoir

Reservoir tank

Tap
Valve box

Tap standTap standSub-system
Distribution line
Sub-system3Technical brief

Gravity-fed schemes
Break-pressure tanks
To reduce operating pressures it issometimes necessary to introduce
break-pressure tanks, which
are usually made of concrete orferrocement. If break-pressure tanks
are used, the hydraulic gradientstarts again at the tank water level.
If suitably sized, these tanks can beused within the system as storage
tanks to meet peak demand.
Reservoir storage tanks
Although a village’s water needs are
based upon a minimum requirement(eg 25 litres per person per 24 hours),
in reality, nearly all of this water will
be required during the day. Reservoir
storage tanks serve to store waterprovided by the source during low
demand periods (overnight) foruse during high demand periods(early morning). They are usuallyconstructed within the system toprovide a total volume of storage
equivalent to one day’s consumption.
If there are no break-pressure tanks,
they may also be sited so as either
to limit the maximum pressure in
distribution pipelines or to sustaina pressure of at least three metres
head at each tapstand, while meetingthe peak demand in the morning andevening.
Capacities of tanks range from 10 to
100 cubic metres, depending upon
the size of the population to beserved. Various materials have beenused to construct them: masonry,reinforced concrete, concrete
blockwork, ferrocement, galvanised
mild steel, and fibreglass panels.
In flat areas, tanks may have tobe elevated on blockwork support
structures. Tanks are roofed and,typically, are provided with a float
controlled inlet valve, twin outlet
pipes with stop valves, a scour pipe
at low level for emptying and cleaning
out, and an overflow pipe led wellaway from the tank. The roof of thetank should have a sealed accessmanhole and ventilators, covered inmesh fly screen, to allow air to be
exhausted or admitted when raising orlowering the water level in the tank.
When it comes to designing thereservoir tank, it is a common
assumption to think the bigger thebetter. While this is desirable, there
is little use in building any tanks solarge that the source will never beable to fill it up during the overnightrefilling period. The storage capacityof the reservoir is actually determinedby the projected village water needs
and the safe yield of the source.
The actual dimensions of the tankare determined by its capacity, theconditions at the selected site, andthe rules of economical design.
Distribution pipelines and tapstands• A distribution system of small
diameter MDPE pipes, laid in
trenches, feeds tapstands aroundthe village. Each tapstand should
serve about 150 people and should
be positioned so as to uniformly
reduce the maximum distancepeople have to carry water.
• Tapstands have several
components: a concrete post
supporting a 15 milimetre mild
steel riser pipe from the pipeline
up to a bibcock which shoulddischarge at least 0.1 litres persecond; a concrete stand on whichto place a bucket; a concrete apron
to collect spillage; and a gutter anddrainage to a soakaway in order toprevent the breeding of mosquitoes
and to keep the area clean.
• Tapstands should have a fence
around them to keep animals awayand each should have a nominatedperson, or caretaker, to keep thearea clean and tidy.

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Technical brief

Gravity-fed schemes

Fig 3: Various designs of tapstands

Adapted fromA handbook of gravityflow water systems byJordan T D Jnr

References
Jordan T D Jnr (1984) A handbook of gravity-flow water systems. IT Publications
Watt S B (1978) Ferrocement water tanks and their construction. IT Publications
5Technical brief

Case study

Gravity-fed schemes

Construction of a gravity-fed scheme in Hitosa woreda
of central Ethiopia

Work underway
on the gravity-fed
scheme in Hitosaworeda, Ethiopia

WaterAid/Caroline Penn

Since 1995, some 75,000 people
in more than 40 communities inHitosa woreda (district) in centralEthiopia have been provided withwater through a spring-fed gravity
water supply project that is one of thelargest in the country.
The water for the main Hitosa scheme
is sourced from two springs which arelocated at the head of a watercourse
known as Burkitu, in the Hitosaworeda, some 164 kilometres southeast of Addis Ababa. The combined
yield of the springs when surveyed in
the 1990s was approximately 19.4
litres per second.
A retaining wall and collection chamber
impound water from the main spring,
prevent contamination and convey
water into the reticulation (piped)
system. The second spring, which ismuch smaller, is impounded in asimilar fashion and piped to thesame chamber.
The main pipeline is 203 milimetre
diameter ductile iron and 152
milimetre galvanised iron. Other
sections are in PVC in diametersfrom 76-152 milimetres. The branchlines from the reservoirs are in
MDPE with diameters from 25-63
milimetres. There is one breakpressure tank between the sourceand escarpment to the lowlands,
where the villages supplied by thissystem lie.
Flow controls are installed on thereservoir inlets, ensuring a steadybut even high pressure in thesystem and avoiding considerable
maintenance.

6Watersource
options:
howwells
gravity-fed
compareto other water source options
Watersource
options:
See howSee
hand-dug
compare toschemes
other water source
options

Technical brief

Gravity-fed schemes

Bacteriologicalwater quality

Situation in which technology ismost applicable

Water source

Capital cost

Running cost

Yield

Springprotection

Low or medium if
piped to community

Low

High

Good if springcatchment isadequately protected

Reliable spring flow required
throughout the year

Sand dams

Low – local labourand materials usedLow

Medium/high –depending on method
used to abstract water.
Water can be abstracted
from the sand andgravel upstream of thesand dam via a well or
tubewellGood if area upstreamof dam is protected

Can be constructed acrossseasonal river beds onimpermeable bedrock

Sub surface
dams

Low – local labourand materials used

Low

Medium/high –Good if area upstream
depending on methodof dam is protected
used to abstract water.
Water can be abstracted
from the sand, gravel orsoil upstream of the subsurface dam via a well or
tubewellCan be constructed in sediments
across seasonal river beds onimpermeable bedrock

Infiltration
galleries

Low – a basic
infiltration gallery
can be constructedusing local labourand materials

Low

Medium/high –depending on method
used to abstract waterGood if filtration
medium is wellmaintained

Should be constructed next tolake or river

Rainwater
harvesting

Low – low costLowmaterials can beused to build storage
tanks and catchment
surfaces

Medium – dependent on
size of collection surface
and frequency of rainfall

Good if collection
In areas where there are one orsurfaces are kept clean two wet seasons per year
and storage containers
are well maintained

Hand-dug wellcapped with arope pump

Low

Medium – spare parts
required for pump

MediumGood if rope andpump mechanisms aresealed and protectedfrom dust. Area around
well must be protected

Where the water table is not
lower than six metres – althoughcertain rope pumps can lift
water from depths of up to 40
metresHand-dug wellcapped with ahand pump

MediumMedium – spare parts
required for pump

MediumGood if area around
well is protected

Where the water table is not
lower than six metres

Tube wellor borehole
capped with ahand pump

Medium – welldrilling equipment
needed. Boreholemust be lined

Medium – hand pumps
need spare parts

MediumGood if area aroundborehole/tubewell isprotected

Where a deep aquifer must beaccessed

Gravity supply

High – pipelines and Low
storage/flow balancetanks required

High

Good if protectedspring used as source

Stream or spring at higher
elevation – communities served
via tap stands close to the homeBorehole
capped withelectrical/diesel/solarpump

High – pump andstorage expensive

High – fuel or power
Highrequired to run pump.
Fragile solar cells need tobe replaced if damaged

Good if source isprotected

In a small town with a large
enough population to pay forrunning costs

Directriver/lakeabstraction
with treatment

High – intake must
be designed andconstructedHigh – treatment andpumping often required.
Power required for
operation

Good following
treatment

Where large urban populationmust be served

ReverseosmosisHigh – sophisticated High – power required for Highplant andoperation. Replacement
membranes required membranes requiredGood

Where large urban populationmust be servedHouseholdfilters

High – certain filters
can be expensive topurchase/produce

Filters can be fragile.
Replacement filters canbe expensive or difficult
to source

Low

Good as long as
regular maintainanceis assured

In situations where inorganic
contaminants are present
in groundwater sources orprotected sources are notavailable

SODIS (solardisinfection)

Low – although
clear bottles can bedifficult to source inremote areas

Low

Low

Good

In areas where there is adequate
sunlight – water needs to be
filtered to remove particulatematter that may harbour
pathogens before SODIS can becarried out effectively. SODIS
is not appropriate for use withturbid water= most preferable= preferable

High= least preferable

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Technical brief

Gravity-fed schemes

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