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close this bookWater and Sanitation Technologies: A Trainer's Manual (Peace Corps, 1985)
close this folderSessions
close this folderSession 34 - Gravity water systems: Part II
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View the documentAttachment 34A: Design guidelines and layouts for simple gravity water systems

(introduction...)

TOTAL TIME

Four Hours

OBJECTIVES

* Discuss the following design considerations for a gravity water system: project life, growth rate, consumption figures, and source identification


* Describe some common design layouts for a simple rural system with these basic components: source intake, storage, distribution, and operations/maintenance plan


* Design a sample gravity water system

RESOURCES

Rural Water and Sanitation Projects; USAID, pp. 35-42, 49-57


Attachment 34-A: "Design Guidelines and Layouts for Simple Gravity Water Systems"


Small Community Water Supplies; IRC, pp. 317-360

PREPARED MATERIALS

Newsprint and felt-tip pens


Copies of Attachment for all trainees, reproductions of Figures 1-4


Pieces of graph paper, straight - edge rulers, and pencils for all trainees

FACILITATORS

One or more trainers

Trainer Introduction

This is the second part of the two part session on gravity water systems. It builds upon the information presented in Part I to describe the basic design steps for a simple small-scale gravity system. The design standards used are general guidelines, based on field experience, and when used have given acceptable results. The type of systems discussed are simple branch rural systems, serving from 500 to a few thousand people, with usually under ten kilometers of pipeline, and using standpipes for distribution. At all times the emphasis is placed on general standards and practical methods of design. It is believed that these practical methods can be used successfully by the Peace Corps water/sanitation technician and engineer.

The reading assignment from Water for the World is a carryover from Part I. The reading in Small Community Water Supplies is additional, more technical, resource information which trainees may choose to study on their own after the session.

PROCEDURES

Step 1

10 minutes


Present the objectives and format for the session.

Trainer Note

Point out that this session builds on the information and principles presented in Part I. Mention that the water systems which will be discussed are simple branched rural systems, serving from 500 to a few thousand people, with under ten kilometers of pipeline, and using standpipes for distribution. Emphasize that such systems are relatively easy to design and construct, and can be operated and maintained by trained local community members.

Step 2

20 minutes


Lecturette on design considerations for a gravity water system; Project life and Growth rate.

Trainer Note

Begin by discussing project life. Point out that it is important to estimate how long the project will serve the community. Each community, or local government agency, should have set guidelines for project life. General standards are either 10 years, 15 years, or 20 years.

Next, explain growth rate. This is also a estimation based on the present population and the percentage of population growth in the area. Many governments have figures on the growth rate of their populations, if so, the designer should use the official percentage. If not, a general figure of 2% per year can be used.

When tied together, project life and growth rate play an important role in the design process. Together they determine the future population figure used for the design process. Here is the formula for finding the future population. Write on newsprint

*present population x (1 + growth rate) project life = future population

The simplest way to use the formula is to refer to a population growth rate table where the number for (1 + growth rate) project life has been worked out. Here is such a table:

Growth Rate

Project Life

% per Year

10 Yrs.

15 Yrs.

20 Yrs.

1%

1.10

1.16

1.23

2%

1.22

1.35

1.49

3%

1.34

1.56

1.81

4%

1.48

1.80

2.19

5%

1.63

2.08

2.65

Therefore, to use the table the formula would be:

*present x table figure = future

Write the following example problem on newsprint:

Example:

Growth Rate = 2% Project Life = 15%
Present Population = 1,000
1000 x 1.35 = 1,350

This computation can also be worked out mathematically by multiplying the population figure for each year times the growth rate. Such as:

Year One = 1000 x.02 = 1020
Year Two = 1020 x.02 = 1040.4
Year Three = 1040.4 x.02 = 1061.2
Year Four = 1061.2 x.02 = 1082.4 and so on

Step 3

40 minutes


Lecturette on design considerations: Consumption Figures, Source Identification.

Trainer Note

Begin by discussing consumption figures. These figures result from the relationship between the user population of the system and the per capita demand. You receive the population figure from the project life/growth rate computations. The per capita demand is determined by the designer to fit the type of distribution system. During Session 23, Water Supply Improvements, some guidelines were given. The range fell between 20 and 80 liters/per person/per day depending upon the degree of distribution convenience, such as the difference between using communal standpipes or individual single tap connections.

Next, explain Average Daily Use (ADU). Mention that once the per capita consumption has been agreed upon, this figure can be used to compute ADU. The formula is simply (write on newsprint):

*number of users x liters/per person/per day = ADU

Example:

Number of Users = 1000
per capita consumption = 30 liters/per person/per day
1000 x 30 = 30,000 liters/day (30m3) ADU

NOTE: At this point the approximate number of users per distribution point, or water tap, may also be decided upon. This number will too vary depending upon the degree of distribution convenience. The general guideline is between 30 and 100 users per tap. By determining the desired number of users per tap, you can determine the approximate number of water taps needed in the system. Write on newsprint:

Example: Population = 1000
Number of users per tap = 50
1000 : 50 = 20 taps

Emphasize that the ADU (and number of users per tap) are figures controlled by the designer. These figures must match local acceptable standards and the guidelines presented here may be adjusted to fit individual systems.

Mention that the degree of distribution convenience, in other words where the water taps are located in the system, will affect the actual ADU and number of users per tap. The designer must match the distribution system with the design figures she/he uses for ADU and number of users per tap, or actual consumption figures will be different from those used to design the system.

Now, mention that identification of the source is a very important step in the design process. In general, the water source must be free of contamination, provide a continuous minimum amount of water to the community, and be able to be supplied to the community in a convenient manner. There are three basic questions to ask:

1. Is the source accessible for development?
2. What is the water quality?
3. What is the flow? (quantity)

Here are important points to emphasize for each question:

1. The source must be accessible for development, if not, the water quantity and quality mean little. Most intakes require substantial construction work and materials. If there is no available road or path access, the cost of the project will be considerably increased to provide one.

2. Water quality standards were also discussed in Session 23, Water Supply Improvements. In general, to establish water quality, measurements must be taken over a period of time during high and low flows. The main considerations in assessing quality of the water source are bacterial quality, chemical quality, and consumer acceptability. The first two are derived from lab testing. The third, consumer acceptability, varies according to local standards.

It is difficult to set rigid standards for water quality however, no source should be used if it will not significantly improve the quality of water used by the local community. This is especially true when the source will be used for drinking purposes, good quality is the number one priority.

3. Measuring the source flow should be done on numerous occasions, especially during the driest part of the year. There is a simple rule with regard to the quantity of source flow: cumulative inflow must be greater than outflow. Simply stated, this means that the minimum source flow should at least meet, and if at all possible exceed, the ADU.

Source flow is stated in units of volume per unit of time, such as liters per second or gallons per minute. You can compare the source flow to the ADU by mathematically reducing the ADU, which would be stated as liters per day for example, to liters per minute, or liters per second. This is called the Average Daily Flow (ADF). It is the continuous flow of water necessary to supply the ADU. You can find the ADF by this formula (write on newsprint):

* ADU in meters3 - 86.4 = ADF in liters per second (Note: there are 86,400 seconds in one day)

Example:

ADU = 30,000 liters/day
30,000 liters = 30 m3
30 - 86.4 =.35 liters/second ADF (21 liters/minute)

Emphasize that the designer would compare the source flow to the ADF to decide if water quantity was sufficient for development. Mention that the ratio between source flow and ADF will be used later to calculate storage requirements for a system.

Step 4

20 minutes


Lecturette on common design layouts for a simple rural system.


Handout Attachment 34-A

Trainer Note

Refer to the four drawings (Figures 1-4) on the attachment. Review each design layout, discussing their relative strengths and weaknesses. Comments on each system are included in the attachment. Point out, in all four layouts, the basic components of the system: source intake, storage, distribution, and operations/maintenance.

Step 5

30 minutes


Lecturette on basic components of a gravity system; source intake and storage.

Trainer Note

Begin by discussing the source intake. Point out that there are a variety of intake designs and methods to construct them. It is not the purpose of this session to discuss them. However, all intakes must perform two basic functions; to collect the source flow for distribution, and to improve the water quality by protecting it from contamination.

The construction method and procedures for the intake must satisfy these two criteria to at least a minimum level of sufficiency.

NOTE: A more detailed discussion of intake design for springs will take place during Session 35.

Next, discuss storage requirements. Emphasize that in most cases, storage of water is necessary. The size and placement of the tank(s) are dependent upon the following:

1. The size of the tank, or tanks, is usually determined by comparing the supply curve with the demand curve. This is done by looking at the ratio of the minimum source flow over the ADF. Write on newsprint:

*


To begin with, if the ratio is less than one, the source does not have enough water to supply the system as discussed during Step 3. If the ratio falls between one and two, then storage should be equal to one half the ADU of the system. If it falls between two and three then storage should be one quarter ADU. If it is between three and four then, one-eighth ADU. And, if the ratio is greater than four, then no storage is necessary. Illustrate on newsprint:


Example:

MSF = 2 liters/second
ADF = 1.2 liters/second


Ratio

Storage

< 1

= not adequate

1-2

= 1/2 ADU

2-3

= 1/4 ADU

3-4

= 1/8 ADU

> 4

= no storage required

1.6 = 1-2 ratio

= 1/2 ADU for storage

Mention that in many field cases, the ratio falls between 1 and 2. Therefore, as a rule of thumb, storage capacity can often be fixed at one half the ADU. However, point out that all such figures are approximations based on field experience and may be adjusted to fit specific circumstances.

2. In general, the placement of storage tanks depends on two factors: the design layout for the system and the system hydraulics or available head.
Refer to the design layouts shown in Figures 1 and 2 of the attachment.

In Figure 1, the design layout called for storage at point of use and there are five distribution points. Illustrate the following on newsprint:

Example:

ADU = 105 m3
ADF = 1.2 liters/second
MSF = 2.0 liters/second


Storage = ½ ADU = 50m3
Storage capacity at each point = 10m3

Point out that this example assumes that the population is evenly distributed. If it were not, the storage tanks at each point would have to vary in size to match the population distribution. Emphasize that in all cases, however, the total storage capacity would be fixed at ½ ADU; only their placement would be altered to fit field conditions.

In Figure 2, the design layout calls for central storage at some point along the pipeline. Determining total storage capacity is done the same way as in figure one and because only one tank is built, the capacity would be the total storage capacity of the system.

In this case, the placement of the tank would be based on the hydraulic conditions of the system. It would be placed to regulate the available head, breaking pressure from the source, and determining the head available at the distribution points. Also, mention that it should be built on an easily accessible site.

Step 6

40 minutes


Lecturette on basic components of a system; distribution and operations/maintenance.

Trainer Note
.
Begin by discussing the distribution system. Laying out of the distribution system requires two basic procedures: sizing the pipeline and determining the number and placement of the distribution points. Both are dependent upon the data received from the design survey and discussions with the village users.

To size the pipeline, the designer must select the design flows she/he will use in the system. Whatever flow is chosen, is then used in the calculation of head losses to determine pipe diameter. As was stated in Part I, the designer will select the smallest diameter pipe with a calculated head loss less than the available head.

Explain that the design flow used to size the pipe going from the source to storage is the same as the ADF. Write on newsprint:

*design flow for source to storage = ADF

Refer to the design layout shown in Figure 1 of the attachment. In this case, a design flow equal to the ADF would be used to size the entire pipeline.

Now refer to Figure 2. Here a design flow equal to the ADF would be used to size only the pipe going from the source to the storage tank.

Next, explain peak demand. The demand of water in any system is not a constant, it fluctuates up and down throughout a 24-hour period. Peak demand is the highest demand of water expected to occur over that 24-hour period. This is the time when the most water is used and often times, all taps are fully open. In general, peak demand accounts for 20-25 percent of the operating time (4-6 hours per day) usually in the daylight hours around meal times.

Emphasize that the design flow used to size the pipe going from storage to the distribution points must take into account peak demand.

From field experience it has been found that the design flow used for this section of pipe can be taken as four times the ADF. Write on newsprint:

*design flow for storage to distribution points = 4 x ADF

Refer to the design layout shown in Figure 2. In this case, a design flow equal to four times the ADF would be used to size the pipe from the storage tank to the distribution points.

Explain to the trainees that this design flow would be taken as the flow rate for the entire distribution system after the storage tank. Because this pipeline has two branches, the flow would be divided after the branch. Flows used in calculated head losses for subsequent pipeline sections, would be reduced by the amount used at each distribution point. Emphasize that the designer would make his/her decision about what percentage of the flow went into each branch and subsequent distribution point, by looking at the population density at each point and what the water demands are.

Now, explain the next step in the design process, calculating the number of water taps necessary for the system. The number of taps needed for the system is determined by the ratio of peak demand over the maximum flow per tap. For a standard 3/4" tap, that maximum flow is .225 liters/sec. Write this formula on newsprint:

*


Example:

Peak Demand = 4.6 liters/sec.
Maximum flow per tap =.225 liters/sec.


NOTE: There is an alternative way to calculate peak demand and you may explain that method now if desired. During Step 3 a method to calculate the desired number of water taps was discussed. It was done by dividing the population figure by the desired number of users per tap. To find peak demand, multiply that number of taps times .225 liters/sec.

Peak Demand = number of taps x.225 liters/sec.

You may use this method to calculate the design flows in each branch line, just multiply the number of taps in each pipeline section by .225 liters/sec.

Next, mention that the placement of each specific distribution point is important. The village users should be consulted to ensure that the placement is acceptable. In general, no more than 20% of the users should have to walk more than 100 meters to obtain water. However, this figure is a guideline and can be adjusted to fit local standards.

Also, emphasize that at each distribution point, a certain amount of head is needed to supply pressure at the taps. This is called residual head. A general guideline for the amount of residual head is 10-15 meters at any standpipe along the pipeline. This guideline may vary according to field conditions. However, the residual head of a standpipe should never fall below seven meters or there may not be sufficient pressure to supply water. If the residual head is more than 25-30 meters, the tap may not be able to withstand the pressure and consequently, blowout. In this case, valves or a short length of small diameter pipe can be used to reduce pressure.

Point out that when discharging into a storage tank, a greater amount of residual head is allowable (up to 50 meters) because a straight pipe is used at the discharge point, rather than a water tap.

One other general guideline should be mentioned. All points along the pipeline should have a minimum of 7 meters residual head to ensure a smooth continuous flow of water regardless of ground profile.

Lastly, discuss the operations/maintenance component of a system. Point out that any system is only as good as its operating and maintenance procedures allow it to be. Countless systems all over the world have been built with skill and the best intentions only to become useless because of failures in operating procedures or lack of proper maintenance.

Operation/maintenance plans may vary to fit local standards and practices. In some places, the government may assume responsibility, in others the local users may have communal responsibility. At times, an individual is trained and paid to operate and maintain the system. However, in all cases, the plans should be included in the design phase of the project, specifically delegate responsibility for various tasks, and be agreed upon by all participants.

Step 7

15 minutes


Lecturette on other design features of a gravity system

Trainer Note

The guidelines discussed during this session provide the general information and format for the design of a simple rural system. However, there are numerous other design considerations that may or may not be applicable to individual systems.

Take this time to discuss such design considerations. Here are some factors to consider:

1. Excessive Head. For most systems a maximum static head of 50 meters is desirable at any one point. GI pipe can withstand much greater pressure but most types of plastic pipe can not, especially when values and fittings are used. It is advised not to push acceptable limits of pressure head. Break pressure tanks or storage reservoirs can be used to bring pressure back to zero in the pipeline.

2. Number of Fittings and Valves. Head losses are increased along the pipeline by the use of fittings and values. A few fittings and values used to control standpipes or storage tanks do not significantly increase head losses. However, if a large number are used throughout the system, to regulate flow for example, or there are a significant amount of bends and joints in the pipeline, allowances should be made for the increase in head losses. In general, 5% may be added to the calculated losses to account for this. A further 5% may be added to account for errors in measuring head or distances. There are tables which give specific head losses for individual fittings and values but in most cases, these general guidelines may be used.

3. Air Blocks. Air bubbles can accumulate at high points in a pipeline and interfere with the flow of water. Normally, air blocks are not a problem if a tank is located at a point lower than the air block and the block is at least 10 m below static level. However, if necessary, air values can be placed at such points to release the block and allow water to flow.

4. Washouts. Over time, suspended particles in the water will tend to settle out at low points in the pipeline, especially when flows are low. Washouts can be placed at these low points to allow for periodic cleaning of the pipeline.

Step 8

60 minutes


Work through sample problems, Attachment 34-A.

Trainer Note

Two sample problems have been provided in the attachment. Have the trainees work through each problem individually, or in groups of two to three. Each trainee should draw the pipeline profile and plot the HGL on graph paper. Encourage them to follow the design steps discussed during the session.

When plotting the HGL encourage them to use the following steps. Write on newsprint:

(1) Starting at the end of the pipeline, indicate points of desired head. At least 10m at each distribution point.

(2) Rough in an approximate HGL by plotting these points.

(3) Refer to the charts and select specific pipe diameters for each reach of pipe.

(4) Plot the true HGL according to exact head losses at each point. Remember, when determining the available head at each new point along the pipeline, be sure and subtract the head losses from preceding points.

(5) Recheck head losses (and residual heads) at each point, add them together to determine total head loss for the system.

Trainers should walk around the room and check the progress of the trainees as they work, and answer questions they may have.

When the trainees have completed the problems, review each one as a large group. Ask trainees to assist in the process. Use newsprint to illustrate the design steps, pipeline profile, and HGL.

Step 9

5 minutes


Review the objectives and conclude the session by pointing out the Peace Corps Water Technicians and engineers around the world have designed and constructed simple gravity water systems. Emphasize that by learning the design process presented during Parts I and II of this session, and through hard work, the trainees themselves will be able to participate in the same kind of work as Volunteers.

REFERENCE: Practical Design Notes for Simple Rural Water Systems; A. Scott Faiia, CARE, Indonesia, 1982.
Handbook of Gravity Flow Water System for Small Communities; Thomas Jordan, UNICEF, Nepal, 1980.

Attachment 34A: Design guidelines and layouts for simple gravity water systems

Simple Design System Layouts: The following are four common layouts for a simple rural system

Figure 1 depicts the general schematic for placing storage at the point of use.


Figure 1

General Comments for Figure 1:

- The inflow to each reservoir can be regulated so that each area receives a set allotment of water. If the people at that reservoir tend to waste water then they can only waste their allotment and not that of others. In a standpipe system with storage at the source, wastage would be much greater if taps were left open.

- The small reservoir acts to break pressure in the system. This means that faucets at the point of use have only the head of the reservoir itself, and will last for a longer period of time because of the reduced pressure. Reduced head at the point of use also reduces wastage.

- Storage at the point of use means that the main distribution pipe is in use at all times. It can therefore be of smaller diameter and thus reduce costs. (Influence on total cost will depend on the flow of the source compared to average daily usage as this will influence storage costs.)

- It is sometimes easier to obtain community support and cultivate feelings of ownership and consequently, improve maintenance through the construction of small scattered reservoirs as compared to one large distant reservoir and standpipes. Additionally, the construction of small reservoirs allows each segment of the community to work at its own pace during construction and a lack of community organization will be less likely to impede the project.

Figure 2 depicts the general schematic for placing storage between source and distribution. This system also has many beneficial features:


Figure 2

General Comments for Figure 2:

- The pipeline to the point of storage is small as in Figure 1.

- The tank serves as a break pressure point in the system and can be placed to regulate pressure at the distribution points.

- Only one storage tank (larger in size) need be constructed and maintained.

- Water may be treated to improve quality at one center point.

- Standpipes can be easily placed at the desired distribution points.

In Figure 3, no storage is proved, and distribution is direct to public standpipes. This type of system requires a larger diameter pipeline, which in many cases, increases the cost of the overall system substantially. This system also requires a strong source flow to provide peak demand without storage.


Figure 3

In Figure 4, storage is provided at or near the water source, then distribution is direct to public standpipes. The pipeline diameter is the same as in Figure 3 and this is the most expensive option. In certain cases, however, storage may be included with construction of the intake and prove cost-effective for small systems.


Figure 4

SUMMARY OF SUGGESTED GUIDELINES

1. Quality: The quality of the water should always meet local standards and be acceptable to the users. Optimally, there should be no fecal coliforms in any sample from the proposed source. If this is not possible, then there should be an average of less than 50 fecals/100 ml for all samples with no single sample exceeding 100 fecals/100 ml. For levels above this, simple treatment is advisable such as slow sand filtration or chlorination. Other characteristic quality problems such a turbidity or odor may be treated by aeration or settling chambers.

2. Quantity: Optimally, the source should be able easily to supply the ADF. When calculating storage capacity for a system, the rule of thumb is, use 1/2 the ADU. More specifically, storage capacity is dependent upon the ratio of source flow over ADF. For a ratio between one and two, then storage is 1/2 ADU, between two and three, then storage is 1/4 ADU, between three and four, 1/8 ADU, and if greater than four, no storage is required. In regards to per capita consumption, the figure should be between 25-80 liters/per person/per day.

3. Convenience: This criteria is dependent on the standards of each local community; however, it is desired that no more than 20% of intended users have to walk more than 100 meters to obtain water. Also, no more than 100 users per standpipe is desirable.

4. Design Flows: For lengths of pipe from source to storage, use ADF. For lengths from storage to distribution, use ADF multiplied by four.

5. Pipe Size: The smallest diameter of pipe with a calculated head loss less than the measured available head is desirable. Five percent should be added to losses to account for fittings and bends, 5% more to account for errors. Ten to fifteen meters of head should be available at any distribution point to ensure adequate pressure at the tap. The HGL should always lie above the ground profile of any system, and if possible 7 meters of residual head should be available at all points.

6. Number of Taps: This is determined by dividing peak demand by the maximum flow per tap. For a standard 3/4" tap, the maximum flow is .225 liters/sec.

APPENDIX A

GLOSSARY

AVAILABLE HEAD

The actual difference in elevation between the two points in question.

AVERAGE DAILY USE

The average volume of water which flows through the water system during a 24-hour period. It is based on the total population served and the projected per capita usage and is usually expressed in cubic meters (m3). The Average Daily Use is a hypothetical quantity. In reality, on some days water usage is greater and on some days it is less; it is only rarely the same. Moreover, this figure is the basis used for determining other design parameters such as storage volume and design flows. The Average Daily Use for a system supplying 1,000 persons with 100 liters per day is:
1,000 x 100 liters or 100 m3.

AVERAGE DAILY FLOW

The flow of water necessary to supply the Average Daily Use if the water were flowing continuously. It is used as the basis for selecting design flows for pipes. The Average Daily Use in m3 divided by 86.4 gives the Average Daily Flow in 1/s that is necessary to supply that amount.

DESIGN FLOWS

The flow used in the calculation of head losses to determine the pipe diameter. It is chosen by the designer based on the number of users, level of service, and type of storage to be provided and is thus related to the Average Daily Flow.

ESTIMATED MINIMUM FLOW

The best estimate of the low flow from the water source that can be made with the available data.
If long-term estimates are not able to be made, flow measurements should be taken during the dry season in the area. It is best to be conservative in estimating the minimum flow to be used in designing the system.

HEAD

The pressure or force per unit area that is avail able or must be overcome in order to transport water. Head may be supplied by gravity or by mechanical means such as a pump. Although it is a pressure, it is generally referred to in meters or feet of elevation, which is the equivalent pressure that would be exerted by a standing column of water of that height.

HEAD LOSS

A loss of pressure (or head) in a closed pipeline due to friction between the pipe and the flowing water. The head loss is affected by flow of water, the distance it is carried, the diameter of the pipe, all fittings and valves in the system, and the inside surface of the pipe. Calculated head losses are compared with the available head to determine if the desired flow of water will be obtained using selected pipe diameters.

PEAK DEMAND

The highest flow of water expected to occur on any given day. This flow usually lasts for only a very short time period and does not necessarily occur every day. For designing standpipe systems, it is taken as four times the Average Daily Flow.

STATIC HEAD

The various pressures that would be obtained in the water system if it were full of water and the water was not flowing. It is different for each point in the system and depends on the elevation relative to the highest point in the system.

DYNAMIC HEAD LEVEL

The pressure head levels of a flowing system caused by the loss of head through friction. These points vary throughout the system and plot the HGL.

HYDRAULIC GRADIENT LINE (HGL)

An imaginary line that plots the head loss at any given point in the pipeline. It is determined by friction loss factors and always slopes downward along the direction of flow.

RESIDUAL HEAD

The difference in elevation between any point on the pipeline and that point's dynamic head level.

STATIC HEAD LEVEL

Highest point in the system.

STEPS IN SURVEY AND DESIGN

I. The Field Survey

1. Become familiar with the water source and the village; consult with the community with regard to the acceptability of the water source and proposed system and their commitment to participation.

2. From community survey results, decide on the feasibility of constructing the water system.

3. For systems considered feasible, a detailed survey is made including: source flow measurement, pipeline distances, and ground level profile.
Someone from the village should assist in this survey. At this time, tentative locations for distribution points are decided upon in consultation with the villagers, taking into account their own wishes, population distribution, etc.

II. The Design Process

The survey data is used to design the system generally as follows:

1. Decide on general system design, type of distribution facilities, etc.

2. Calculate the project life and growth rate to find the total population to be served. Decide on per capita usage and calculate the average daily use (ADU).

3. From the ADU, calculate the average daily flow (ADF) and by comparison to the minimum source flow, determine the desired storage and its placement. The placement of the distribution points should be set at this time as well.

4. From the average daily flow and the type of system, the design flows for the pipe are selected, taking into account peak demand.
The number of taps required is calculated.

5. The pipeline profile is drawn and an approximate hydraulic gradient line (HGL) plotted by making points of desired head.

6. From the design flows and elevations, the exact head losses for various diameter pipes are calculated for each unbroken section of the pipeline.

7. The most appropriate pipe is chosen for all sections of the pipeline based on the calculated head losses. The true hydraulic gradients are then plotted on the profile.

8. From the elevation profile and general scheme of the system, the need and placement of break pressure tanks, air valves, wash-outs, etc. are determined and recorded on the general sketch.

9. Detailed drawings, specifications, materials lists and budgets can now be prepared.

REFERENCE: Practical Design Notes for Simple Rural Water Systems; A. Scott Faiia. CARE, Indonesia, 1982.

RIGID PVC FRlCTIONAL HEADLOSS FACTORS

These are the approximate headless factors, in m/100m (%), for new rigid PVC pipe. Flows are in liters/second.

FLOW

1/2"

3/4"

1

1¼"

1½"

2"

2½"

3"

4"

0.1

4.2

1.0

0.25

0.08






0.15

8.8

2.2

0.53

0.17

0.07





0.2

15.0

3.7

0.9

0.28

0.12





0.25

22.0

5.5

1.35

0.44

0.18





0.3

31.0

7.8

1.9

0.6

0.25





0.35

41.0

10.0

2.45

0.8

0.34





0.4

53.0

13.0

3.1

1.0

0.43





0.45

66.0

16.3

4.0

1.25

0.54

0.13




0.5


19.0

4.8

1.5

0.65

0.16




0.55


23.5

5.6

1.8

0.78

0.19




0.6


27.5

6.6

2.1

0.9

0.22




0.65


32.0

7.8

2.4

1.04

0.25




0.7


36.0

8.7

2.7

1.19

0.28




0.75


41.0

9.9

3.1

1.32

0.33

0.1



0.8


45.0

11.0

3.5

1.5

0.37

0.12



0.85


52.0

12.5

4.0

1.7

0.41

0.14



0.9


57.0

14.0

4.5

1.9

0.45

0.15



0.95


63.0

15.0

4.9

2.1

0.5

0.17



1.0



16.5

5.4

2.25

0.55

0.18

0.08


1.05



18.0

5.8

2.5

0.6

0.20

0.09


1.1



19.5

6.3

2.7

0.67

0.22

0.1


1.15



21.5

6.9

2.95

0.71

0.24

0.11


1.2



23.0

7.3

3.2

0.78

0.26

0.12


1.3



26.5

8.6

3.75

0.9

0.29

0.13


1.4



30.0

10.0

4.25

1.0

0.34

0.15


1.5



35.0

11.2

4.9

1.15

0.39

0.17


1.6



39.0

12.5

5.5

1.3

0.43

0.19


1.7



44.0

14.2

6.05

1.45

0.49

0.21


1.8



49.0

15.9

6.9

1.6

0.54

0.24


1.9



55.0

17.4

7.5

1.8

0.6

0.26


2.0



60.0

19.0

8.0

2.0

0.66

0.28


2.2




22.5

9.7

2.35

0.79

0.34


2.4




26.8

11.5

2.75

0.9

0.4


2.6




31.0

13.3

3.2

1.05

0.45


2.8




35.1

15.2

3.7

1.2

0.52


3.0




40.0

17.0

4.2

1.36

0.6


3.2




45.0

19.3

4.7

1.52

0.68


3.4




50.0

21.9

5.25

1.7

0.75


3.6




56.0

24.0

5.8

1.9

0.84

0.2

3.8




62.0

26.0

6.3

2.1

0.9

0.22

4.0




69.0

29.0

7.0

2.3

1.0

0.24

4.5





36.0

8.8

2.8

1.2

0.3

5.0





44.0

10.5

3.5

1.5

0.37

5.5





62.0

12.5

4.2

1.75

0.44

6.0






14.7

4.9

2.1

0.52

6.5






17.0

5.6

2.4

0.6

7.0






19.5

6.5

2.8

0.7

GI FRICTIONAL HEADLOSS FACTORS

These are approximate headloss factors, in m/100m (%), for new GI pipe. Flows are in liters/second.

FLOW

1/2"

3/4"

1"

1¼"

1½"

2"

2½"

3"

4"

0.1

5.9

1.58

0.38

0.12






0.15

12.25

3.4

0.82

0.26






0.2

21.45

5.65

1.4

0.44

0.19





0.25

31.65

8.5

2.1

0.68

0.28





0.3

44.91

11.9

2.9

0.92

0.4





0.35

58.2

15.8

3.8

1.2

0.52





0.4

75.5

19.9

4.8

1.55

0.67





0.45

91.9

25.0

6.0

1.93

0.84





0.5


30.0

7.3

2.35

1.0

0.25




0.55


36.0

8.7

2.75

1.2

0.3




0.6


42.0

10.2

3.25

1.4

0.35




0.65


48.0

11.9

3.8

1.63

0.4




0.7


55.0

13.6

4.35

1.82

0.46




0.75


63.0

15.4

4.9

2.15

0.52

0.17



0.8



17.4

5.55

2.4

0.59

0.19



0.85



19.4

6.15

2.65

0.68

0.21



0.9



21.8

6.9

2.9

0.74

0.23



0.95



24.0

7.5

3.25

0.82

0.26



1.0



26.2

8.2

3.6

0.88

0.28

0.12


1.05



28.5

9.0

3.9

0.97

0.31

0.13


1.1



31.0

9.8

4.2

1.05

0.34

0.15


1.15



34.6

10.6

4.6

1.15

0.37

0.16


1.2



36.0

11.5

5.0

1.25

0.39

0.17


1.3



42.5

13.3

5.7

1.45

0.45

0.2


1.4



48.0

15.3

6.6

1.65

0.52

0.23


1.5



55.0

17.5

7.65

1.9

0.59

0.26


1.6



62.0

19.5

8.45

2.1

0.67

0.29


1.7



69.0

22.0

9.5

2.35

0.75

0.33


1.8




24.2

10.5

2.6

0.82

0.36


1.9




26.5

11.7

2.85

0.9

0.4


2.0




29.5

12.8

3.2

1.0

0.44


2.2




35.0

15.3

3.8

1.2

0.52


2.4




42.0

17.9

4.45

1.4

0.61


2.6




48.5

20.5

5.15

1.6

0.71

0.17

2.8




55.0

24.0

5.95

1.85

0.82

0.2

3.0




62.5

26.7

6.7

2.1

0.92

0.22

3.2





30.0

7.6

2.35

1.02

0.25

3.4





34.0

8.4

2.65

1.15

0.28

3.6





38.0

9.4

2.95

1.28

0.32

3.8





41.0

10.3

3.25

1.42

0.35

4.0





45.0

11.2

3.55

1.55

0.38

4.5





56.0

14.0

4.45

1.95

0.46

5.0






17.0

5.45

2.25

0.56

5.5






20.0

6.5

2.8

0.68

6.0






24.0

7.5

3.35

0.8

6.5






28.0

8.65

3.9

0.92

7.0






32.0

10.0

4.45

1.05

SAMPLE PROBLEMS

Problem I:

Design for system with central storage at some point along the pipeline, serving one village


Central storage

Given:

Population = 1000
Growth Rate = 1.0%
Project Life = 10 yes.
Per capita consumption = 50 liters
Minimum Source Flow = 1.0 1/sec.
Pipe = PVC
Tap size = 3/4"


Pipeline Profile - Problem I

Step 1. Determine design population figure_________________________
Step 2. Determine average daily use_________________________
Step 3. Determine average daily flow_________________________
Step 4. Determine storage size_________________________
Step 5. Determine peak demand_________________________
Step 6. Determine number of required taps_________________________
Step 7. Determine storage placement_________________________
Step 8. Determine pipe size for all reaches along the pipeline_________________________
Step 9. Determine exact head losses and residual head at points B. C, D and E. _____________
Step 10. Plot HGL.

Problem II:

Design for system with storage at point of use, serving two villages


Storage at point of use

Given:

Population = 1500 divided evenly between the two villages
Growth Rate = 2.0%
Project Life = 15 yes.
Per capita consumption = 40 liters
Minimum Source Flow = 1.5 1/sec.
Pipe = PVC
Tap Size = 3/4"


Pipeline Profile - Problem II

Step 1. Determine design population figure_________________________
Step 2. Determine average daily use_________________________
Step 3. Determine average daily flow_________________________
Step 4. Determine storage size_________________________
Step 5. Determine peak demand_________________________
Step 6. Determine number of required taps_________________________
Step 7. Determine storage placement_________________________
Step 8. Determine pipe size for all reaches along the pipeline_________________________
Step 9. Determine exact head losses and residual head at points B. C, D and E._____________
Step 10. Plot HGL.

Problem I: Answer Sheet


Figure I

Step 1. 1100
Step 2. 55,000 liters = 50m3
Step 3. .6 liters/second
Step 4. 25m3
Step 5. 2.4 liters/sec.
Step 6. 10
Step 7. Point C
Step 8. See Figure 1

Step 9.


pt. B

pt. C

pt. D

pt. E

Head Loss

4.2

20

8

23

Res. Head

10.8

31

12

14

Step 10. See Figure I

Problem II: Answer Sheet


Figure II

Step 1. 2025
Step 2. 81,000 liters = 80m3
Step 3. .9 liters/second
Step 4. 40m3
Step 5. 3.6 liters/sec.
Step 6. 16
Step 7. Point D 20m3
Point E 20m3
Step 8. See Figure II
Step 9.


pt. B

pt. C

pt. D

pt. E

Head Loss

7.6

13.5

42

20

Res. Head

12.4

29

12

17

Step 10. See Figure II