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Chapter 9 Gravity flow water supply system
Nepal consists of several rural villages. The rural villages of hilly area of Nepal are suffering from
hardship in drinking water. Several agencies are trying to make water available for drinking in such areas.
Gravity system of water supply is that system in which water flows under gravity without use of any
external energy to deliver water from source to tap stand. The gravity flow water supply system is most
appropriate for rural sloppy terrain of Nepal where the source is usually situated at the uphill and the
community is as far down.
In this system, water san be supplied to the community with or without using reservoirs. Technically this
is called closed or open systems. Open system is used when the design demand can meet by the safe yield
of the source. If safe yield cannot meet the demand we use closed system. The closed system can also be
continuous or intermittent type.
The component of gravity flow water supply systems are:
1. Intake: (Spring/ Stream/River/Infiltration Galleries)
2. Collection Chamber
3. Interruption chamber (IC) and Break pressure tank (BPT):
4. Distribution chamber (DC)
5. Reservoirs (RVT)
6. Air valve
8. Sectional valve chambers
9. Valve chamber
10. Sedimentation tanks
11. Pipeline: (a) Transmission main (b) Distribution main
12. Pipe crossings: (a) Simple crossing (b) Suspended crossing
Feasibility and Detailed Surveying
Village Development Committee (VDC) carries out the need assessment of water supply for the
communities and it is submitted to District Development Committee (DDC). Screening of the project is
carried out by the DDC and they are proceed for feasibility study. It consists of the study of the technical,
economical and social aspect of the project. Viewing and analyzing the project objective, technical aspect,
tentative cost, funding source, resources, social and economical aspect the feasibility report is prepared
and submitted to DDC for approval. After approval by DDC, water users committee (WUC) is formed
and detail survey is carried out for technical, social and economical aspect for detailed design, preparation
of drawings and cost estimate.
All hydraulic theories are applicable for the system. The design is based on the hydraulic theories of open
channel flow as well as pipe or pressure flow. The following theories are applicable in gravity flow water
1. Continuity Equation
2. Bernoulli Equation
3. Principle of Headloss (major and minor)
4. Pressure theories
The design of pipes is based on head loss principle. The elevation and length of the pipe are very
important in the design. So, profile leveling is the main data used in the design of the system.
In the route of pipeline for gravity flow water supply, there may be several U-profiles in hilly terrains. In
this case, the valley side is liable for silting problem and hillside is for air clogging. In that case, washouts
in the valley side and air relief valve in the hillside should be provided. The valves are liable to be
maintained frequently, so it is preferred to avoid u-profiles as far as possible.
It is constructed to break the static pressure if pipeline exceeds its maximum pressure rating to protect
pipe from burst. This tank allows the flow to discharge in the open atmosphere so reduces the hydrostatic
pressure to zero. It may be rectangular, circular or square in plan and made of masonry or RCC or Ferro
cement. BPT has provision of a closed chamber with inlet, outlet, overflow and a float valve to stop the
wastage of water and allow reserving at upstream tanks.
A reservoir may also act as a BPT. BPT are provided in the distribution system. If it is used in
transmission line it is called interruption chamber (IC). BPT usually consists consists of float valve but
not in IC. For economical reason, Ferro-cement BPT is preferred.
Ferro-cement water tank
Tanks made with chicken wire mesh, cement and sand is called ferro-cement tanks. The ferro-cement
RVT are also designed for balancing demand and supply. The BPT and IC can also be made of ferro-
cement. It reduces the cement, sand, brick, stone and workers but requires skilled person. For a good
ferro-cement tank cement sand ratio must be 1:1.5. Sand should be free from soil otherwise cracks will be
formed. After plastering inside and outside, it must be covered with plastic sheets otherwise water is
evaporated leaving cracks to the tank. It should be cured with water for at least 7 days.
Public tap stand
Public stand post is the point from where people collect water normally in rural area. Tap stand location
in Nepalese practice must be chosen by the villagers themselves and located at the place of non-dispute.
One tap stand for every 8 – 10 households and fetching distance not more than 200 m horizontal and 50 m
vertical is considered in design.
Pipes are subjected to internal forces due to flowing water so suitable anchor blocks are designed to resist
it. In case of stream or river crossing, simple pipe crossing can be provided but if the crossing length is
very high, suspended crossings may be appropriate.
Plantation, barbed wire fencing, catch drains, waste water drain, regular filling of exposed pipelines etc. is
the common practice used for the protection of source, tanks and valve, pipes and tap. Barbed wire
fencing protects from animals and children, human interferences. Regular maintenance for pipe and valve
leakage, clog, tap break, pipe exposure, infrastructure damage etc make structure sustainable. Emergency
maintenance during floods, landslide or natural disasters is needed for resupplying of water. Preventive
measure reduces the maintenance.