Water from ponds, pans and dams

Across Africa’s rural areas, reliable water supply is at the root of all sustain-able development. In the hands of motivated technical advisors or well-or-ganized community groups, this manual holds the promise of increasing

the vital water resource.In concise and readable language, Water from ponds, pans and dams guides

the reader through all stages of community planning, design and construction, maintenance, and operation of these water storage structures. Every chapter is illustrated with fi gures that add to the text. At the back of the book, you will fi nd a kit of practical tools on how to plan, survey sites, analyse soil properties, determine water needs, and estimate construction costs.

This book is meant to help improve the water supply for rural communities throughout eastern Africa.

ISBN 9966-896-67-8

World Agroforestry Centre’s Eastern and Central Africa’s Regional Land Management Unit (RELMA in ICRAF) ICRAF Building, Gigiri, P. O. Box 00100, Nairobi, Kenya

Tel: (+254 20) 722 4000, Fax: (+254 20) 722 4001, E-mail: [email protected] www.relma.org

www.worldagroforestry.org

Water from

ponds, pans and dams A manual on planning, design,

construction and maintenance

Water from ponds, pans and dams

TECHNICAL HANDBOOK No. 32

Technical handbook No. 32

Technical handbook (TH) series

Agroforestry handbook for the montane zone of UgandaAlex Lwakuba, Alice A. Kaudia and John Okorio. 2003. TH No. 31. ISBN 9966-896-55-4

Soil fertility and land productivityCharles K.K. Gachene and Gathiru Kimaru. 2003. TH No. 30. ISBN 9966-896-66-X

Soil and water conservation manual for EritreaAmanuel Negassi, Estifanos Bein, Kifl e Ghebru and Bo Tengnäs. 2002. TH No. 29. ISBN 9966-896-65-1

Management of Rangelands: Use of natural grazing resources in Southern Province, ZambiaEvaristo C. Chileshe and Aichi Kitalyi. 2002. TH No. 28. ISBN 9966-896-61-9

Edible wild plants of TanzaniaChristopher K. Ruff o, Ann Birnie and Bo Tengnäs. 2002. TH No. 27. ISBN 9966-896-62-7

Tree nursery manual for EritreaChris Palzer. 2002. TH No. 26. ISBN 9966-896-60-0

ULAMP extension approach: a guide for fi eld extension agentsAnthony Nyakuni, Gedion Shone and Arne Eriksson. 2001. TH No. 25. ISBN 9966-896-57-0

Drip Irrigation: options for smallholder farmers in eastern and southern Africa Isaya V. Sĳ ali. 2001. TH No. 24. ISBN 9966-896-77-5

Water from sand rivers: a manual on site survey, design, construction, and maintenance of seven types of water structures in riverbedsErik Nissen-Petersen. 2000. TH No. 23. ISBN 9966-896-53-8

Rainwater harvesting for natural resources management: a planning guide for TanzaniaNuhu Hatibu and Henry F. Mahoo (eds.). 2000. TH No. 22. ISBN 9966-896-52-X

Agroforestry handbook for the banana-coffee zone of Uganda: farmers’ practices and experiencesI. Oluka-Akileng, J. Francis Esegu, Alice Kaudia and Alex Lwakuba. 2000. TH No. 21. ISBN 9966-896-51-1

Land resources management: a guide for extension workers in UgandaCharles Rusoke, Anthony Nyakuni, Sandra Mwebaze, John Okorio, Frank Akena and Gathiru Kimaru. 2000. TH No. 20. ISBN 9966-896-44-9

Wild food plants and mushrooms of Uganda Anthony B. Katende, Paul Ssegawa, Ann Birnie, Christine Holding and Bo Tengnäs. 1999. TH No. 19. ISBN 9966-896-40-6

Banana production in Uganda: an essential food and cash cropAloysius Karugaba and Gathiru Kimaru. 1999. TH No. 18. ISBN 9966-896-39-2

Agroforestry extension manual for eastern ZambiaSamuel Simute, C.L. Phiri and Bo Tengnäs. 1998. TH No. 17. ISBN 9966-896-36-8

Water harvesting: an illustrative manual for development of microcatchment techniques for crop production in dry areasMwangi T. Hai. 1998. TH No. 16. ISBN 9966-896-33-3

Integrated soil fertility management on small-scale farms in Eastern Province of ZambiaThomas Raussen (ed.). 1997. TH No. 15. ISBN 9966-896-32-5

Agroforestry manual for extension workers in Central and Lusaka provinces, ZambiaJoseph A. Banda, Penias Banda and Bo Tengnäs. 1997. TH No. 14. ISBN 9966-896-31-7

Facilitators’ manual for communication skills workshopsPamela Baxter. 1996. TH No. 13. ISBN 9966-896-25-2

Useful trees and shrubs in Eritrea: identifi cation, propagation and management for agricultural and pastoral communitiesEstifanos Bein, B. Habte, A. Jaber, Ann Birnie and Bo Tengnäs. 1996. TH No. 12. ISBN 9966-896-24-4

Agroforestry extension manual for northern ZambiaHenry Chilufya and Bo Tengnäs. 1996. TH No. 11. ISBN 9966-896-23-6

Useful trees and shrubs for Uganda: identifi cation, propagation and management for agricultural and pastoral communitiesA.B. Katende, Ann Birnie and Bo Tengnäs. 1995. TH No. 10. ISBN 9966-896-22-8

The soils of Ethiopia: annotated bibliographyBerhanu Debele. 1994. TH No. 9. ISBN 9966-896-21-X

Curriculum for training in soil and water conservation in KenyaStachys N. Muturi and Fabian S. Muya (eds.) 1994. TH No. 8. ISBN 9966-896-20-1

Soil conservation in Arusha Region, Tanzania: manual for extension workers with emphasis on small-scale farmersPer Assmo and Arne Eriksson. 1994. TH No. 7. ISBN 9966-896-19-8

Useful trees and shrubs for Tanzania: identifi cation, propagation and management for agricultural and pastoral communitiesL.P. Mbuya, H.P. Msanga, C.K. Ruff o, Ann Birnie and Bo Tengnäs. 1994. TH No. 6. ISBN 9966-896-16-3

Agroforestry manual for extension workers in Southern Province, ZambiaJericho Mulofwa, Samuel Simute and Bo Tengnäs. 1994. TH No. 4. ISBN 9966-896-14-7

Useful trees and shrubs for Ethiopia: identifi cation, propagation and management for agricultural and pastoral communitiesAzene Bekele-Tessema, Ann Birnie and Bo Tengnäs. 1993. TH No. 5. ISBN 9966-896-15-5

Guidelines on agroforestry extension planning in KenyaBo Tengnäs. 1993. TH No. 3. ISBN 9966-896-11-2

Agroforestry manual for extension workers with emphasis on small-scale farmers in Eastern Province, ZambiaSamuel Simute. 1992. TH No. 2. ISBN 9966-896-07-4

Curriculum for in-service training in agroforestry and related subjects in KenyaStachys N. Muturi (ed.). 1992. TH No. 1. ISBN 9966-896-03-1

Technical handbook series, continued...

...continued on inside back cover

IntroductionChapter 1

T he purpose and scope of this book are presented in this chapter, followed by a detailed description of the most common

kinds of ponds and dams. Pans used on seasonal basis by pastoral herders are also described.

Chapter 1 • Introduction

2

1.1 Purpose and scope of this manualBackground Africa is considered a water-scarce continent with most of the countries regularly experiencing extreme water shortage during periodic dry spells. Rapid population growth and ineffi cient use of resources increases the defi cit between available water supplies and the needs of people. Many regions in East and Southern Africa are drought prone and the vulnerability of the population to drought is high with more than 40 per cent of the region’s people living in dryland areas.

As resources dwindle and water demand increases, large scale water supply projects become unviable. There is a need to decentralize water supply to household and small community level. There is great potential to make be� er use of water resources by harvesting rainwater and storing it locally for household and productive purposes.

The need for more ponds and damsThe lack of water is the largest constraint to sustainable livelihoods in many parts of Africa. Rapid runoff during the rainy season frequently results in a high proportion going to waste or even becoming destructive. Soil erosion, vegetation degradation and decreases in soil fertility are severe problems throughout the region. Harvesting rainwater where and when it falls presents opportunities to address both water scarcity and soil degradation at a local level. Water can be harvested and used for many purposes but reliable water storage facilities are required.

Communities/individuals need help to identify suitable sites for water harvesting and storage structures, which types are most suit-able, how much water the catchment area gives during the rains, and the water needs for household, livestock and crops. Local knowledge is valuable in answering these questions. Local communities must be involved and feel ownership from the planning stage all through the construction phase, if the project is to endure.

Aim and scope of this bookThe purpose of this handbook is to provide a practical guide to extension workers and technicians who assist communities and farmers intending to construct a water storage structure for agricultural, livestock watering or domestic purposes. The reader will fi nd out how to involve the community in all steps, so they become the true owners of the project and ensure the sustainability of the water storage structures (Chapter 2). Guidance is provided on planning and feasibility study including environmental impact and legal aspects


3

(Chapter 3). Various options for water storage are discussed and the design and construction of ponds and small earth dams are covered in detail (Chapter 4). The handbook also deals with the operations and maintenance once the pond or dam is built (Chapter 5). The last chapter (Chapter 6) presents several useful tools to use in the planning, design and construction stages.

1.2 Types of ponds and pansPonds and pans are naturally occurring or excavated water storage structures without a constructed wall. They usually store surface run-off , even though there are examples of constructed ponds storing roof water.

The terms pans and ponds are o� en used interchangeably with slightly indistinct meaning. In Kenya both natural and dugout struc-tures are called pans, while in Zambia pans are larger natural water sources and ponds are smaller excavated structures. In general, the term pan is used to describe structures used by herders, while pond most o� en refers to structures used by farmers. There are also various local terms to add to the confusion.

Naturally occurring pansNatural pans have provided water for wildlife, livestock and humans since ancient times. They form in depressions in which rainwater accumulates during the rainy season and they do not have an outfl ow (see Figure 1). Today, most natural water sources are used for watering livestock during rains and a few months therea� er. Some people still use them for domestic water supply even though they are dirty and not suitable for drinking or washing.

Figure 1. Natural pan in a pastoral area with livestock and wildlife.


4

In some very dry areas such as the Kalahari Desert large natural salt pans are found. These usually hold water only for a few weeks a year and have been formed by wind action. Smaller natural pans include the silanka ya ndovu (elephant dam) common on the eastern African savannah, scooped out on fl at land by elephants, digging for water many years ago. The animals trample and compact the sediment when they enter to drink, making the pan’s fl oor watertight.

Many pans suff er from high evaporation losses. As they fi ll with sediment over time and the water becomes shallow, the problem of evaporation gets worse. Although it may not always be feasible to build new pans, it is sometimes worth deepening or enlarging existing natural or artifi cial ones. Herders in particular appreciate the benefi ts which natural pans can bring even though most are seasonal and can-not store water throughout the year.

Excavated pondsExcavated ponds come in sizes from the household level of 200 to 500 m3

up to community level of 10,000 m3. They can easily be started with a small capacity and expanded over the years by digging deeper and wider. In areas with impermeable soils and a suitable site the only cost of construction is the labour, so a group can dig their own pond with li� le cash expense.

Figure 2. Illustration of a charco pond, note silt traps and spillway.


5

Ponds should be situated at a low point in the catchment area so rainwater runoff fl ows by gravity towards the excavated pond. The catchment area can consist of any type of surface such as cropland, grasslands or compounds around homesteads. Hard road surfaces or rock outcrops may also make suitable catchment areas. Rainwater runoff can be diverted from a nearby gully, provided the pond is situated at a lower elevation than the gully. Soil excavated from the pond can be used to make soil bunds for diverting runoff water to ponds.

In Sudan the name hafi rs describes dugout enlargements of natural depressions on the savannah. They range in size from 500 to 10,000 m3

and provide water for both livestock and domestic purposes. In the past most hafi rs were dug by hand. Today however, heavy machines, i.e tractors and bulldozers, are commonly used to build them.

In Tanzania dugout ponds are commonly referred to as charco ponds, or malambo in Kiswahili. The charco are common in the drylands of Tanzania where they are used for watering livestock. In some cases, they are also used through necessity for domestic supply despite the poor water quality. Farmers build these ponds in stages during dry seasons until they are satisfi ed with the capacity. The farmers do not follow a standard design for their charco ponds and excavate them in many diff erent shapes and sizes (see Figure 2).

Borrow pitsBorrow pits (also known as murram pits) are excavated to supply soil and gravel for road construction, but opportunistically used for water storage, usually from road runoff . As the ownership o� en is unclear nobody takes proper responsibility for the borrow pits and they may even be sources of confl ict. If proper regulations were in place to deter-mine how these dugouts could benefi t both the road construction and the neighbouring communities, the structures would be more valuable for a longer time.

When borrow pits are dug in fi rm soil with li� le seepage and have a large catchment area from the road runoff , even small rain showers will fi ll them. If they do not normally fi ll this way, they can fairly eas-ily — and at low cost — be fi lled by digging a trench sloping from the road to divert rainwater runoff . This rainwater runoff may contain tar and other pollutants, making water from borrow pits unsuitable for human consumption.


6

1.3 Types of damsDams are water storage structures on slopes, with walls or embankments on the downhill side. They come in all shapes and sizes from the giant Kariba Dam on the Zambezi River to small check dams built across gullies. In drylands these include various types of earth dams, rock catchment dams, sub-surface dams and sand dams.

This handbook covers the design and construction of small earth dams with storage capacities up to 10,000 m3 and embankments of up to approximately 3 m in height. It will deal with two types of earth dams only:

• Small dams in valleys built with straight embankments, which is a common and economical type of dam.

• Small earth dams on hillsides built with curved embankments on sloping land, a less common but practical type for individu-al farmers.

Site investigations, design and construction of medium and large dams require experienced engineers and cannot be constructed by fi eld technicians and farmers. For this reason they are beyond the scope of this handbook.

A word of warning:A word of warning: It must be remembered that the construction of any dam introduces a small risk of failure such as collapse of the dam wall. Therefore always seek experienced technical advice to minimize such risks.

Earth dams in valleysIf a suitable site can be found, constructing a small earth dam at a valley site is a cost eff ective way to create a water storage reservoir (see Figure 3). This is because it has a high water storage capacity per cubic metre of soil moved. Nevertheless, the impact of a small earth dam being washed away in a fl ood could be very serious and endanger lives and property. This is particularly so for valley dams where a large quantity of water suddenly released would be channelled down the valley. For this reason experienced technical help should always be sought for the design and construction of any dam which might present a threat to downstream households or communities.

Small earth dams, below 1,000 m3 can be built manually, using draught animals, a farm tractor or a bulldozer. The medium sized, 10,000 to 50,000 m3, and large earth dams, above 50,000 m3, are nearly always built using heavy machinery.


7

Figure 3. Illustration of a valley dam. Note sill in spillway, rock toe.

Earth dams on hillsidesSmall earth dams on hillsides or sloping land are one of the simplest and least costly type of dam to design, construct and maintain. Suitable sites can be found on almost any sloping land that produces rainwater runoff . They may be built small the fi rst year and enlarged over time (see Figure 4).

Rock catchment dams Rock catchment dams store rainwater runoff collected from rock out-crops. In large rock catchments cement and stone gu� ers are used to extend the catchment area to gather runoff from several hectares of rock surface. Rock catchments typically have reservoirs with capacities of up to 5,000 m3.

Sub-surface dams and sand damsIn semi-arid areas where dry sandy riverbeds are common, their water storage capacity can be improved by building sub-surface or sand dams. These are a kind of weir constructed across the sandy riverbeds to block fl ood water. Water that infi ltrates into the sandy riverbed is trapped in the spaces between the sand particles. This form of water storage has the advantage of protecting the water from evaporation as well as helping to protect it from contamination.


8

Figure 4. Illustration of a hillside dam.

The major diff erence between sub-surface and sand dams is that sub-surface dams can be built cheaply of soil or stone-masonry to the level of sand in the riverbed, while a sand dam can be built to a height of several metres above the sand level. Although sand dams should produce more water than sub-surface dams, most of the hundreds constructed in recent years are not functioning well.


9

One of the oldest rock catchment dams built in Kitui, Kenya, in 1956.

Sand dam with masonry apron in Nyando District, western Kenya.

Chapter 2 Community participation

A ll development practitioners should realize by now how important community participation is. Many are still struggling with how to go about it. Chapter 2 provides clear guidelines,

in a logical sequence. It begins with project identifi cation, forming management structures, and setting SMART objectives. With structures in place, the text explains the community’s role in feasibility and planning, design and construction. Throughout, emphasis is given to the need for capacity building, with a detailed description of the kinds of training that need to be provided. The roles of government departments are discussed, as well as the need for monitoring of progress by the community groups themselves. At the end, there is a discussion on ways to manage confl icts should they arise.

12

Chapter 2 • Community participation

2.1 Introduction There are many examples of community water projects that were built and ended up being abandoned or broken down soon a� er the development agency le� . Such experience highlights the need to involve the community in all stages of a project in order to ensure that the community owns the project and willingly takes responsibility for it. Community participation is essential to ensure a genuinely sustainable project. Very small pond or dam projects may also be done at an individual rather than community level. In order to make sure that projects are sustainable, there is need to identify clear steps in the project implementation process. Turn to page 28 for a useful fl ow-chart that shows all steps in sequence.

Such steps include:

• Project identifi cation

• Community social organization and management structures

• Feasibility, design and planning

• Construction

• Capacity building

• Operation and maintenance

• Monitoring and evaluation

2.2 Project identifi cationIn the past, the needs of a community were taken for granted. “Top down approaches” meant that communities were given whatever projects the aid agencies or government had funds for. Communities were not consulted and their real “felt” needs were never identifi ed. This resulted in unwanted projects which were neglected and became “white elephants”.

Pond and dam projects can be identifi ed in many diff erent ways but in all cases the demand for the project should come from the community. Community needs or demands can be identifi ed by assessing their development priorities using techniques such as participatory rural appraisal (PRA). Alternatively, there may be a direct request to a local development agency or government department from an established community group. Establishing the nature of the group or individual requesting assistance should be the fi rst part of the project implementation.

13


2.3 Community social organizations and management structuresForming a management structureIn order to implement a successful water project, the community will need to select a suitable management structure. Possible approaches might include:

• Starting by understanding the existing structures, including traditional council of elders.

• Raising awareness about the project and the need for a man-agement structure with the local leadership.

• Helping the community to develop the roles and responsibili-ties of the proposed commi� ee.

Within most communities in Africa, there are traditional manage-ment structures for water resources that existed in the past and possi-bly current management structures for new projects. It is important to understand the responsibilities of these structures with regards to us-ing water resources. Analysing these structures can be done for each type of management structure using the following checklist to estab-lish the gaps in knowledge and skills:

• What do you see and think should be their responsibilities?• Which of the responsibilities mentioned above can they com-

fortably take on?• What reasons do you give for their inability to carry out the

rest of their responsibilities and what possible solutions do you suggest?

• Compare roles of the two management structures (traditional and current) and come up with important roles each can pro-vide that cannot be provided by the other.

The responsibilities that the management structure will have to take on behalf of the community for the sustainability of the project include:

• Coordinating construction and maintenance of the pond/pan or dam in the community.

• Operation and maintenance of the pond/pan or dam. Having the technical know how to carry out repairs with li� le or no reliance on external support.

• Charging for providing services. Establishing the best revenue collection method. This includes a cost recovery system for opera-tion and maintenance and managing the funds.

14


• Supervising staff who work on the water supply system, i.e. operators, fee collectors.

• A� ending meetings and having periodic elections in accept-able format for all community members.

• Implementing decisions discussed and taken in meetings.• Equitable distribution of water resources, formulation and en-

forcement of by-laws to ensure eff ective distribution.• Engaging community members on their own terms, minimising

and resolving confl ict, and identifying and resolving community problems.

Having analysed the existing management structures, it is possible to agree with the community on the most appropriate structure for the planned project. This is o� en a commi� ee that combines both tradi-tional and modern resource management structures as shown in the example below.

Formation of management structures for water projects in Mandera District, Kenya

There is a history of traditional water management in Mandera District through the council of elders or aba-heriga. Stakeholders at the start of a new water project debated the strengths and weaknesses of both this and a modern, elected committee style management structure. The stakeholders explored methods of combining modern and traditional systems and the measures to be taken to ensure representation and accountability, including how to reduce the infl uence of dominant personalities on the process. It was concluded that:

• The inclusion of elders brings considerable advantages to the management of community water supplies, but may increase confl ict within the committee.

• Comprehensive and enforceable by-laws can play an important role in ensuring genuine representation by committee members and, by clearly defi ning roles and responsibilities, can reduce the infl uence of dominant individuals.

Good record keeping and regular monitoring by support agencies is essential to check the effectiveness of a committee.

Ownership, land tenure and legal issuesSuitable sites for earth dams are normally found in valleys and seasonal water courses which are o� en boundaries between two or more landowners. In such cases, it is important that the landowners make a wri� en agreement on sharing the ownership. This agreement should

15


include construction cost, usage of water and maintenance of a pond or dam and be fi nalized before any survey and construction work takes place. It is also important for the landowners to agree on the location of an access road to the dam site and on any soil conservation methods to reduce soil erosion and siltation. Catchment protection can consist of digging trenches, making terraces and planting of grasses or trees in rows along the contours (lines of equal elevation). It also includes the building of check dams and silt traps in gullies. All land-users in a catchment area should be encouraged to participate in all the soil conservation activities including the maintenance of structures and vegetation cover.

Before a pond/pan or dam project can be implemented it is impor-tant to ensure that the ownership of the site is clear and that access to all users is guaranteed. The box below is an example of the importance of land ownership.

Gathingi dam, Sweetwaters, Laikipia District, Kenya

During the initial stages of a project to rehabilitate an old colonial dam for a community near Sweetwaters game reserve, the implementing agency surveyed the site and compared the boundary of the proposed dam impoundment area with land settlement maps of the area. During this process, it became apparent that the actual location of the dam spanned both the communal land set aside for the dam and private land owned by an individual in the community. The project could not proceed because the eventual ownership of the dam and the water resource was not clear and community access could not be guaranteed.

Following intensive facilitation by the development agency the committee and the land owner agreed to exchange the private land within the dam area for alternative land nearby. The community arranged for the legal process of transferring title deeds and establishing the whole dam impoundment area as public land. This process took three months and only then could the dam rehabilitation project start.

Initial discussions should be held with the community, or their rep-resentatives, using this checklist:

• Who owns the land?

• Who has access to the land?

• Who owns any existing water source?

• Who will own this project?

• Who will manage it?

16


• How will the money collected be used?

• Who will maintain the project?

Ideally a community water project must be established on commu-nity owned land. If any part of the water supply passes over private land, it is necessary to obtain a “wayleave” which is a legal document signed by the land owner that ensures access by the community mem-bers to the water supply facilities on his/her land.

Compliance with water resource regulations on dam construction Legal requirements will vary from one country to another. It is always advisable to ask the authorities before starting any construction work in order to avoid disappointment and legal cases.

Generally, it is understood that farmers may construct ponds on their land without asking for permission from anyone, provided the ponds are small and do not block water runoff to people living down-stream. If in doubt of the legality, the authorities should be asked be-fore starting on the construction work. In the case of earth dams built in valleys, however, these may interfere with people’s water supply downstream. Since dams can collapse during exceptionally heavy rainfall due to poor maintenance, incorrect design or poor construc-tion work, this could endanger people and structures downstream. For these reasons, approval for the design and permission for the construc-tion works must be obtained from the authorities.

Legal aspects of community organizationDiff erent countries have diff erent laws governing associations and community based organizations. The way in which a community group is registered usually dictates how they can operate and how eff ective they can be at managing a communal resource such as a dam. Typical legal guidelines for diff erent organizations in Kenya are shown in the box below.

Self-help groups registered with Ministry of Social Services

Can:

• Hold meetings without license

• Raise funds for the group

• Open a bank account

• Apply for small grants from local donors or NGOs

17


Cannot:

• Make legal transaction (e.g. legally binding contract)

• Seek legal redress against individuals or organizations (e.g. for misappropriation of funds)

• Own land on which to place project assets (e.g. dam, borehole, tanks etc.)

• Own equipment (e.g. generator, pipeline, vehicles etc.)

Associations registered with the Registrar of Societies

Can:

Do all the above plus:

• Legal transactions

• Own land

• Own assets and equipment

• Go to court to seek legal redress

In addition if registered as water users association with Ministry of Water can:

Have status and rights of water undertaker, i.e. right to legally sell water.

It is advisable to check with the relevant government department to make sure the community group is properly registered and can have the legal authority to handle any problems that might arise.

Setting project objectivesIt is important for everyone involved in a project to have a common understanding of what they are trying to achieve. Se� ing clear objec-tives at the start of the project and making sure they are achievable is part of the preparation for the project. Simple clear objectives should be defi ned with the community and bearing in mind the rule that ob-jectives should be SMART, i.e.:

S = Specifi cM = MeasurableA = AchievableR = RealisticT = Time bound

18


A typical example of an objective for community water projects which is not SMART is “improved community health.” This is not spe-cifi c, measurable, achievable, realistic or time bound because it is dif-fi cult to measure improvements in community health and even more diffi cult to relate them directly to the construction of a water facility such as a pond or dam. A more reasonable, SMART objective for a pond or dam project would be to “reduce the time women spend col-lecting water from 2 hours to 30 minutes by the end of the two year project”. This is very specifi c about what is being achieved and for whom, by when and the achievement can be measured.

2.4 Feasibility, design and planningCommunity involvement in feasibility and designThe important principle for community participation in feasibility, de-sign and planning is to remember who owns the project. Technicians must ensure that the community takes part in site selection, survey, en-vironmental impact assessment (EIA) and any other investigations or discussions forming part of the feasibility and design. This may mean spending time explaining the design to key members of the commu-nity and ensure that the project is not disowned at a later stage.

The design must also consider community preferences, hygiene and water use practices. Use the following list of questions to gather basic social data necessary to ensure that the design meets the needs of people.

1. How many people are likely to use the pond or dam? Where are they? How are they distributed? Are they se� led or nomadic?

2. Are there other water and sanitation needs apart from human or domestic? (e.g. irrigation, tree nurseries, livestock) If so, what are the demands?

3. What security factors may interfere with people’s access to the pond or dam?

4. What are the current or likely water and sanitation-related diseases? How can transmission of these diseases be reduced?

5. What are people’s normal household sanitation and hygiene practices, including disposal of children’s faeces?

6. What are the environmental conditions, including drainage, waste disposal, and location of defecation areas relative to water sources?

7. What are the typical water use habits in the community, including collection practices, preferences for washing selves, clothes and utensils and sources of drinking water?

19


Capacity assessmentAt this stage it may be useful to carry out a community capacity as-sessment. The aim of the assessment is to determine what capacity the community has to sustainably manage the pond or dam a� er construc-tion. Sustainability aspects that need to be looked at include:

• Technical sustainability, referring to balanced demand and supply of water from the ponds and dams.

• Institutional sustainability, referring to the capacity of the institutions within the community to plan, manage and operate the system.

• Social sustainability, referring to the willingness of the commu-nity to contribute to the project

• Economic sustainability, referring to sustainable economic de-velopment and improvement of the welfare of the community.

• Financial sustainability, referring to cost recovery.• Environmental sustainability, referring to there being no long-

term negative or irreversible eff ects to the environment owing to the establishment or use of the ponds and dams.

An analysis of the “gaps” in knowledge and skills required to man-age the water supply and maintain the structure can assist in the de-sign of a community training component (see Section 2.6). Once the de-sign is complete the detailed planning can take place. There is a need to facilitate a planning exercise with the community in which community and implementing agency roles are clearly defi ned.

Cost sharingMost development organizations have adopted a cost sharing policy towards their community projects. The reason is that if communities have to pay for at least part of the project cost, they are more likely to value the facility and feel a sense of ownership.

The willingness and ability to pay a cost contribution varies from community to community and should be assessed carefully. The actu-al community contribution should be negotiated with the community and not determined by an external agency. A fi xed contribution set at a percentage of the cost of the project tends to result in unaff ordable contributions in cases where mechanical construction is required. An “in kind” contribution in the form of labour and local materials is of-ten easier for poor communities with limited cash. It is important to monetarize “in kind” contributions to establish the actual value of the community contribution.

Negotiating the cost of contribution should be done alongside dis-cussions about the design and construction planning of the project. It

20


is important to make sure that communities understand the full scale and cost of the project so that they can appreciate the need for their contribution. Demanding a certain percentage without explaining where the fi gures come from can lead to communities feeling that they are being asked for a “bribe” to get the project started.

Where there is an option of mechanical, animal draught or manual construction, it may also be appropriate to give the community the choice of technology they prefer to contribute to. It is unrealistic to ex-pect communities to provide intensive labour for building a large dam or pond without payment.

Pan desilting in Wajir District

Several large pans were desilted following the drought of 1999—2001 in Wajir. The development agency decided to use Cash for Work to help drought affected families to recover. Providing cash to families who contributed labourers for desilting did not undermine the community sense of responsibility for the pan because the community fully understood that the cash was being paid as a drought recovery measure. In these communities, even if the labour had been provided as a community contribution someone within the community would have had to pay the workers. No-one is expected to work without being given something.

For large, complex projects where considerable revenue collection, operation and maintenance will be required it is advisable to carry out an awareness raising exercise with the whole community. This will help the community to understand their roles and responsibilities and to feel part of the planning and implementation process. Restricting community contact to the selected few in the management commi� ee o� en results in confl ict and confusion at later stages.

2.5 ConstructionRoles of who should do what should be clear. Typical community roles during construction include:

• Clearing the site, uprooting trees, removing stones.• Supervising earth works.• Providing labour for minor earth moving (if mechanically dug).• Organizing, supervising and monitoring work (if manually dug).

21


• Providing local materials for cement works where required (sand, ballast and water).

• Providing accommodation and/or food for skilled workers.• Fencing and other auxiliary works, i.e. planting grass on em-

bankments, stone pitching the spillway.

The management commi� ee and possibly other members of the community should be involved in measuring and approving the work carried out. This ensures that the community is in control of the proj-ect and the commi� ee can answer any queries that the community members may have about the construction. In addition, where it is not possible to employ manual labour, the community should decide on the possibilities of tendering or contracting the work to establishments with relevant equipment.

2.6 Capacity buildingRationaleHistorically, government water departments played a prominent role in the development and rehabilitation of dams. More recently, the capacity of the water departments to undertake this work has diminished and very li� le rehabilitation of community dams takes place. Under new government water policies, communities have a greater role and responsibility in the management and operation of their water supplies. This responsibility is only meaningful if the communities genuinely have the interest and capacity to manage their water supplies. Typically, the community has a strong interest, being the principle benefi ciary of the water supply. However, the operation and management of a water supply requires awareness, skills and experience that the communities do not necessarily have, especially if their dam is new or has not been operational for a long time. The capacity of the community may need strengthening so they can manage their water supply. A capacity building exercise for community operation and management of water supplies involves the following steps:

• Training needs assessment.• Development of an appropriate training programme.• Training of management commi� ee and other key community

people.• Follow up to monitor progress on operational and management

issues.

22


Capacity building should result in the following: that the commi� ee selected by the community gains skills on leadership, fi nancial management and technical operation and management; that the community gains knowledge on water related hygiene and sanitation; and that the community learns how the pond/pan or dam will be operated and learns to demand accountability from their commi� ee (community empowerment).

Training needs assessmentThe training needs assessment is a detailed exercise with the commu-nity in which participatory tools are used to:

• Discuss the overall project objectives with the community.• Establish existing organizational and management structure.• Establish the capacity of existing management system to han-

dle existing water supply system and rehabilitated system.• Discuss expectations and responsibilities involved in commu-

nity management of the proposed dam rehabilitation and the subsequent operation and maintenance.

• Identify who should be trained.• Identify external factors that can aff ect the training and project.• Identify the roles of other stakeholders (e.g. Water Department).

Where a comprehensive capacity assessment was carried out dur-ing the feasibility stage it may not be necessary to undertake a full training needs assessment. However, discussions should be held with community representatives to identify what training they think is nec-essary. The format of the training needs assessment involves discus-sions with the community using PRA tools that help the community to identify their priorities. This ensures that the opinions of diff erent members of the community (e.g. women, youths, agriculturists, pasto-ralists) are expressed. The community is able to identify their strengths and weaknesses, with the result that the topics covered in the training programme can clearly be identifi ed as arising from the community.

The output from the training needs assessment is a report which provides details on the key issues that need to be addressed during the community capacity building exercise.

Development of an appropriate training programmeEach community is diff erent. It is important to adapt approaches and topics to the needs of each individual community. The general approach is to develop a training programme which uses participatory tools (drama, role plays, picture games) to build the capacity of the

23


community to operate and manage their water supply. Historically most pans have a traditional, communal ownership

background. This means that capacity building for community man-agement should build on existing systems. The following features are recommended for pond or dam management training:

• Community mobilisation/awareness is a key part. • Training should be on-site and make use of participatory ap-

proaches.• Thorough training/capacity building in management at few

sites is preferable to partial training in many sites.• Long-term follow-up process is essential.• Training should involve and encourage women in decision-

making.

A typical training programme is divided into modules dealing with diff erent topics including:

• Community organization and optional management structures. Other issues covered include leadership, gender, equity and confl ict resolution.

• Community self reliance and organizational sustainability which deals with issues of dependency, organizational records, constitutions and by-laws.

• Building fi nancial sustainability, fi nancial records (budgeting, book keeping and accounting).

• Operation and maintenance which deals with technical sustainability and includes accessing technical services, spare parts, and routine maintenance activities. This module also covers catchment conservation measures and environmental impacts.

• Water, sanitation and hygiene education. Typically, health benefi ts from improved water supplies are only obtained through changes in water use habits. The management commi� ee members are generally cast in the role of community leaders and so have a responsibility to the community to raise awareness on good environmental health practices at the household, homestead and community level.

• Community action plans and community monitoring and evaluation. Indicators are discussed to help the community monitor their adherence to their action plans and to monitor any environmental or social impacts.

24


An additional module that is useful for dam projects would cover sustainable water and land use practises. This module would explore ways of making use of the water facility to enhance the environment (tree nurseries etc.), to create opportunities for co� age industries (e.g. fi sheries) and to encourage micro-irrigation.

Community training approachCommunity training takes place in the community area and takes the form of a one to two week exercise in which the diff erent training mod-ules are explored with the community members.

Special a� ention is given to the possibility that individuals within management commi� ees may be transitional. It is therefore important to ensure that the community as a whole is involved in the training. This helps to reduce the likelihood of generating an elite of trained individuals. The training has to be appropriate for the community as a whole, hence the limited use of wri� en material. Additionally, the training may be conducted in the local language.

Staffi ngStaffi ng for the community training component requires a community training specialist and one or two community mobilizers. The training specialist will be responsible for the content, coordination and reporting of the community training component. The community mobilizers, who speak the local language, will undertake the community training and follow-ups.

2.7 Monitoring and evaluationThe community should develop a suitable system for monitoring their performance. In addition, the project should involve the community in evaluation of their works. This is important for management, re-adjustment or introduction of new approaches towards improving on the existing systems or solving of problems. Tools for monitoring the performance of the dam or pond and planning operation and maintenance work are presented in Chapter 6.

2.8 Important considerations in working with communities

Gender Traditional gender roles relating to water in the household are o� en divided according to whether the water is for productive or

25


reproductive use. For example, domestic water for cooking, drinking or washing is the woman’s responsibility. Water for agriculture or livestock is usually the man’s responsibility. Women may be responsible for water collection for kitchen gardens or small livestock raised for the household rather than cash income.

Understanding gender issues within the community where the pond/pan or dam is to be constructed is an important factor in plan-ning a sustainable project. It is useful to analyse women’s and men’s work and their control over resources to establish who is likely to have the time, interest and authority to take on the management of a pond or dam (see box for questions for gender analysis).

Questions for gender analysis

• What role do women have in water issues?

• Who controls water sources?

• Who is responsible for maintaining water supplies?

• Who is responsible for water use in the household?

• If the community manages the water supply, should women be involved?

• How should they be involved? In the committee or through women’s groups?

• What resources do women control and what decision-making power do they have in the community?*

• Do women have time available for community activities?*

• What other constraints are there to women’s involvement in water management?

• What steps can be taken to reduce these constraints?

• Who should take these steps?

* these issues can be explored using PRA tools such as “Gender access and control to resources” and “Gender activity schedule”.

Women are clearly the ones who benefi t from improving availabil-ity of water in terms of:

• Reduced time spent fetching water• Improved family health • More opportunities for girls to go to school• Potential for market gardening and/or small-scale livestock

production to increase household income and/or improve nutrition.

26


Women therefore have higher interest in improving access to water and are eager to participate in construction. However, in many cases their ability to participate in decision making concerning the planning, operation and maintenance of the pond or dam is restricted by traditional practices. There are obvious advantages of including women in the management of water facilities because they are highly motivated to construct new water supplies and keep them operational. It is therefore important that communities are encouraged to include women in the decision making. However, the presence of women in a water commi� ee is not always suffi cient to ensure genuine participation in decision making so technicians and extension workers may need to carry out gender awareness exercises for the whole community to empower women and encourage them to actively participate.

Gender issues should also be taken into account when designing the pond or dam and thinking about how water for domestic use will be collected. Women’s priorities for water collection facilities may be diff erent from men’s and they should therefore be consulted. In some communities watering livestock is given priority over domestic water collection. It may be necessary to provide separate abstraction facilities for domestic water to ensure women have access to the water.

Understanding of gender roles in the community is also necessary when designing follow on projects which make use of pond water, such as market gardens, tree nurseries or brick making. The extent to which both women and men will benefi t from these activities needs to be considered.

Confl ict mitigationConfl icts over access to water can undermine or even destroy a com-munity water project. All potential confl ict situations should be thor-oughly explored with the communities involved and confl ict miti-gation measures agreed and put in place. This may take time and should involve traditional confl ict resolution bodies to ensure that an agreement can be reached where more than one tribe or group are in-volved.

Where confl icts over land have not been resolved, review of land acts with consequent reforms should be pursued by the local leadership in conjunction with government offi cials. The community should be educated on such issues. The box below depicts an example of confl ict mitigation in northern Kenya.

27


Confl ict mitigation in northern Kenya

Unreliable and uneven seasonal distribution of rainfall in pastoral regions of northeastern Kenya results in great differences in surface water availability from year to year. Dams and pans in some areas may fi ll up while others go dry even in the same district. Such disparities in rainfall and available water force livestock herders to migrate in search of water, sometimes far from traditional grazing grounds and water points. In northeastern Kenya, this often means crossing international borders into Ethiopia or Somalia. This perennial search for water is the underlying source of confl ict between pastoral communities. The level of confl ict will vary, and can be among individuals, between clans, between water management committee and the local community, or between communities across national borders.

Causes of confl ict

• Lack of defi ned ownership of water sources (pans and dams).

• Complex clan relations which are highly heterogeneous within communities, leads to competition between clans for access to water.

• Lack of clear by-laws for water users associations in agropastoral and pastoral communities

• Absence of, or corrupt water management committees.

• Disparities in water fees charged by management committees.

• Individuals fencing off access to pans/dams.

• Upstream water abstraction

Mitigation initiatives

Local communities, especially with support from the relevant authorities or external agencies, have taken initiatives to manage/mitigate confl ict over water at different levels. In Kenya (and other countries) the key approach has been facilitating dialogue aimed at creating mutual understanding among groups and fostering peaceful coexistence. The following are some options that have been used in Mandera for resolving confl ict.

• Forming water user associations and management committees.

• Ensuring such management structures have clear and enforceable by-laws.

• Capacity building programmes that target both users and management committees to strengthen their ability to deal with confl ict.

• Forming village and cross-border peace committees to facilitate dialogue and awareness raising.

28


Figure 5. Planning and design fl ow chart for ponds and dams

Problem identifi cation, identify project ob jec tives

Estimate water demand> Establish scale of project

Does site appear feasible?

YES

Planning[Manual labour, draught animal trac tion, or

mech a nized excavation?]

Final technical design > Detailed topographical survey

> Position of the dam wall and spillway> Capacity of the water reservoir, height

and length of the dam wall> Design the foundation> Design the dam wall> Design spillway

> Design water extraction

Communitysocio-economic survey

General feasibility and planning> Water quality-health considerations?> What quantity needed vs. available?> Is it economically feasible? > What environmental impacts?

Preliminary costing

NO

Is project still fea si ble?

NO

Site identifi cation and assessment> Site gradient, along and across valley site (if a dam)> Pond/pan or dam: is wall structural and how high?

YESYES

Community cost sharing

Legal issues, permits

Community mobilization

Community management structure,

Training needs assessment

Ownership,legal aspects, land tenure

continues to next page

If a valley dam with wall higher than 3 metres...

NO Get technical assistance!YESYES

29


Operation and maintenance

ConstructionConstruction

Community Community Approval

Project fi nanc ing

Project still justifi able,affordable

sustainable?sustainable?sustainable?sustainable?

Review costs, prepare detailed plan, including:> Construction plan

> Bill of quantities

> Design report & drawings

NO

Construction schedule

Community supervision, manage construction

Examine al ter na tive

tech nol o gies, e.g. ground water

Community contributionCommunity contribution(in cash and/or in kind)

Monitoring and Evaluation

YES

Community and water Community and water committee training

T his chapter tells you what to do to fi nd out if the proposed pond/pan or dam is feasible. It provides information on estimating how much water the community uses, determining

the quantity of runoff water a catchment can produce and how to identify the best site for the dam, pond or pan. Following a logical order, there are sections dealing with examining the economic costs and benefi ts of the project, and which construction methods are more or less expensive. The chapter ends with a section addressing the potential environmental and social impacts of building pans and dams. Don’t forget to use the ideas and methods in Chapter 2 to involve the community. After all, it is their dam or pond.

Chapter 3 Feasibilty and planning

32

Chapter 3 • General feasibilty and planning

3.1 IntroductionTo determine whether a pond or dam project is feasible requires look-ing closely at its technical and economic viability as well as the envi-ronmental and social impacts. It is important that these are shown to be positive. A detailed format for doing a thorough feasibility report appears in Annex 1.

The most successful projects are those identifi ed and implemented by community groups. This instils a greater sense of ownership by the community who are then more likely to engage in the active mainte-nance of the dam, pond, reservoir and catchment area.

Key questionsThese are some of the key questions that you will need to answer at the outset:

• What will the water be used for? • Will the water be clean enough for the intended purpose and if

not, can it be improved? • How much water is needed? • How much water will the new source provide? • What will the project cost? What percentage of this can the

community aff ord?• How much will the facility cost to maintain and operate? Can

the community aff ord this?

There are fi ve components to the feasibility assessment. These are: 1. Assessing the quality of the water that will be harvested.2. Estimating how much water will be needed, to compare with

the capacity of the catchment to supply water.3. Making a preliminary site assessment.4. Estimating the costs of constructing the pond or dam.5. Doing an Environmental Impact Assessment. This is required

by law in most countries.

3.2 Water quality and sanitationThe rainwater runoff which fi lls ponds and reservoirs fl ows over ground that is usually contaminated. The ground on catchment areas can have animal droppings, human excreta (especially from young children) and other rubbish on it that will pollute the water. While this

33


water is suitable for livestock, small-scale irrigation and construction work it is NOT safe for drinking. When planning, be sure to survey the catchment and identify all possible sources of contamination that could jeopardize water quality and users health.

Catchment pollution in northeastern Kenya

Korondille is a small settlement in Wajir District. The people rely on a large pan, located close to the settlement, for their water supply. There are no draw off structures so women collect water directly from the pan. The main catchment for the pan is on the other side of the pan, away from the settlement but population growth and livestock coming to drink at the pan has meant that a pathway has been formed through town and into the pan. In the rainy season this pathway carries runoff into the pan, collecting all the waste, including human faeces, from the town and depositing them into the pan. Even though care has been taken to protect the main catchment, the pan is still being polluted via an unanticipated catchment.

The community at Korondille are unaware of the health risks caused by the runoff through the town and have made no attempt to correct the problem. Possible measures to reduce the pollution would be to divert town runoff away from the pan. Alternatively the temporary settlements in the new catchment could be moved away from the runoff path and the catchment rehabilitated to provide better quality runoff.

If the purpose of a pond or a dam is to get clean water for domes-tic needs, then the water should be drawn from an enclosed hand-dug well below the dam wall. Drinking untreated water from open water sources is not recommended (unless it has fi rst been boiled) as it may cause water-borne diseases such as dysentery, diarrhoea or typhoid. If water based diseases such as schistosomiasis (bilharzia) carried by wa-ter snails are present in the area, people should be discouraged from entering the water.

3.3 Estimating water demand To estimate how much water is required and for how long a period some simple calculations are required. The demand for water for domestic purposes, livestock and irrigation can be estimated by fi lling in the relevant rows in Table 1 below. First, determine from available rainfall data for your district, or from asking knowledgeable community member, how long the typical dry season lasts or the length of period when stored water is being used.

To determine the water requirement for any particular household or community, information on the number of people, livestock and any

34


irrigation requirements needs to be gathered. By making a copy and fi lling in the table below you can calculate the approximate water use of a given village or group of households (to fi nd an average). At the district level, recent census data is usually available on human and livestock populations. In Tanzania this data is available even at village level.

In areas where be� er quality water sources are available for do-mestic consumption, such as a hand-dug well or rainwater tank, water from a dam or pond will only be required for livestock or irrigation. Be sure to ask about such sources when doing the feasibility study.

Estimating water demand A copy of Table 1 also appears in the feasibility report in Annex 1.

Table 1 Estimating water demand

Item Population Consumption rate (litres/day)

Total (litres/day)

People x 20

Camels x 15

Cattle x 15

Sheep/goats x 3.5

Donkeys x 15

Irrigation x 20 l/buckets/day

Other + 10% (seepage+evaporation loss)

Total (litres/day)

Total (m3/day*)*divide total litres by 1,000.

Livestock water demandThere are standard fi gures for the diff erent types of livestock, as shown in Table 1. It is o� en the case that once a large new water supply is built within a community, people will bring their animals to drink rather than go to other sources further away.

35


Irrigation water demandTo estimate the water requirement for irrigation, consider the main factors, such as the irrigation method used (furrow, bucket, drip), the soil type (sandy, loamy, clay), climate, type of crop and its growing period.

See Annex 2 for a table showing typical crop-water requirements of major crops. It is be� er to over estimate the quantity of water needed for irrigation than underestimate.

The capacity of a catchment to supply waterUsually the runoff from a catchment will be more than suffi cient to fi ll the pond or reservoir. Only in cases where a catchment is very small and there is li� le sign of runoff will further investigations need to be undertaken. Usually, the descriptions from local people who have watched rainwater runoff produce temporary streams or even fl oods during torrential downpours in the wet season should provide suffi cient evidence that there will be suffi cient water to fi ll the reservoir. More details are provided under site selection below.

3.4 Site investigation and selectionTopographical survey Potential sites for dams and ponds need to be measured to establish what volume of water they might be able to store. For very small reservoir areas (less than 500m3) you might be able to use a simple tool such as a “line-level” or “circular level” (see Tool 1 in Chapter 6). For larger dams or pans (above 3 metre dam wall height) it will be necessary to bring a survey team to site and map out the whole area.

Site selection for pondsThe best sites for constructing ponds are in places with deep clay, or silty soils, where surface run-off accumulates during the rainy season. The land surface should be fairly fl at, ideally with a slope of not more than 4 per cent (4cm per metre). A natural depression where water collects during rainy seasons is a suitable site.

The catchment area should be suffi ciently large to generate ade-quate runoff water to fi ll the pond. Ideally, a pond should be located near a gully or a natural waterway, which carries water during and af-ter rainfall events as this water can easily be diverted. Avoid digging ponds near or downstream from livestock enclosures or mines as these are likely to suff er from organic or chemical pollution.

A suitable site should have deep fi ne textured soils, preferably clay-ey. Coarse textured sandy soils should be avoided as these are highly

36


permeable and water will drain through them easily. Soils with a low permeability (e.g. soils with high clay content) should be used for the fl oor and sides of the pond to avoid seepage losses. If seepage is high, puddling and compacting of the fl oor may be necessary. Sites with un-derlying strata of sand, gravel, limestone or fractured rock at a shallow depth may result in high seepage losses and should be avoided (see Soil analysis on page 38). All other factors permi� ing, a pond should be located in such a way that the stored water may be used directly without the need for pumping and piping.

Site selection for damsWhen undertaking the site investigation the following points need to be considered.

• The seasonal runoff from the catchment feeding into the valley needs to be suffi cient to fi ll any reservoir constructed.

• The walls of the earth dam should be situated in a narrow part of the valley. Preferably at a place with a natural depression just up stream producing some additional storage capacity (see Figure 6).

• The dam wall needs to be built in a part of the valley which provides a water-tight valley fl oor and sides of either clay or uncracked rock.

• The valley fl oor should not be too steep and sloping as this will reduce the storage volume of the reservoir.

• The dam wall should be situated at least 100 m from any bends in the valley to prevent currents causing erosion when heavy runoff occurs.

• Suitable clay soils for building the dam wall needs to be avail-able. Preferably these should come from a borrow pit in the reservoir and from excavating the spillways.

• Reservoirs should not contain boulders or rock outcrops be-cause they might cause leakage unless covered with clayey soil.

• Natural depressions in the banks of a reservoir should, when present, be used for spillways in order to reduce construction costs (see Figure 7).

37


Resevoir innatural depression

Seasonal water course

620m

625m

630m635m

620m

625m

630m

635m

Contr

oulines on 1:50,000 map

Dam

sitein

narrow

valley

Figure 6. Site selection for a valley dam. Choose a narrow point in a valley with a natural depression upstream.

EmbarkmentA A

Spillway

Lowspot

Contour lines

Bench mark

Contourlines

Contour line s

Figure 7. Siting of a valley dam spillway. If possible choose natural depres-sion in the banks of a reservoir for the spillway to discharge surplus water.

Embankment spillway A -A profi le:

38


Soil survey and analysis The purpose of this exercise is to see whether the local soils are suitable to use when constructing the dam wall, and to estimate the permeability of the soil in the impoundment area to understand whether the site will hold water or lose it all through seepage.

Soils can be classifi ed on the basis of their texture. The fi nest soils are clayey and these are impermeable (watertight) and do not allow water to pass through them. Silty soils are not as fi ne as clays and more permeable and unstable. Sandy soils are coarser still and quite perme-able allowing water to pass through them easily. Gravel and soils with a high gravel content are very permeable. Most soils are made up of a mixture of clay, silt, sand and gravel.

The fi rst step is to dig adequate test pits along the dam wall and throughout the fl oor of the dam or pond to provide soil samples for testing (see Tool 2 in Chapter 6).

There are various tests which can be carried out to determine what type of soil is available at the site. The fi rst analysis is to establish the seepage rate of the soil (see Tool 4 in Chapter 6). This is a comparative test so it is helpful to compare the results with a soil with high clay content (and low permeability) in order to establish the relative perme-ability of the soil samples.

A� er testing the permeability the soil should be tested to determine the clay content. A simple fi eld tool for establishing the percent clay content is described in Tool 3, Chapter 6. It is important to recognize that some sorts of clay (e.g. black co� on soil) must not be used for construction because it cracks badly when it dries out.

In order to decide whether the available soil is suitable for storing water or dam wall construction it is necessary to have a minimum of 30 per cent clay content. Simple guidelines for dam construction are as follows:

• The soil with the highest clay content should be used for the key (cut-off trench), core and diaphragm of the dam wall.

• The soil with the next highest clay content should be used for the upstream side of the dam wall and for a blanket to cover the whole dam when completed.

• The soil with the lowest clay content (the most sandy soil) should be used for building the downstream side of the dam wall.

Estimating spillway sizeThe surplus water of a small earth dam reservoir must be discharged safely, otherwise the dam wall will be washed away. Surplus runoff must therefore pass over a spillway that is large enough to safely

39


discharge the overfl ow water from the highest recorded rainfall plus a li� le extra in case the next “El Niño” storm breaks the record. All spillways must have a fi rm surface or a constructed sill to prevent erosion and avoid reducing the reservoir storage capacity. The size of a spillway will depend on the volume of water running off its catchment during peak times. A common mistake is the belief that a small dam only needs a small spillway.

If the total runoff volume is large a spillway of matching capacity is essential. It is therefore not economical to build a small dam on a large catchment because the risk of under-estimating the spillway. On the other hand if the volume of rainwater runoff from a catchment area is too small and the earth dam reservoir does not fi ll on a regular basis the investment of building the dam will have been partly wasted.

For small earth dams built in valley sites, estimating the volume of runoff from a catchment is vital to ensure that spillways are large enough to cope. The amount of runoff depends on the size and con-dition of the catchment and other factors (rainfall intensity, soil type, slope, vegetation cover). If reliable local rainfall data and/or contour maps 1:50,000 cannot be obtained an alternative method to estimate the required spillway capacity can be used. This involves measuring the maximum fl ood level in the valley proposed for a dam and adjust-ing for the diff erent gradient and vegetation conditions between the natural water course and the improved spillway.

Maximum fl ood levels Determining the maximum fl ood level may involve enquiries with long-time residents regarding the highest fl ood water level they can remember. Using this information the maximum cross-sectional area of the water in the valley at the highest fl ood level ever observed can be estimated. See Tool 5a and 5b in Chapter 6 for detailed instructions.

3.5 Economic feasibility Methods and costs of excavation and constructionA family or community group can build a pond or dam using either manual labour, draught animal power using an ox scoop or with hired machinery (e.g. a tractor or bulldozer). Work should be carried out in the dry season when people have less work in the fi elds and when uncompleted excavation and construction work is less likely to get washed away.

40


Manual labourPonds and small earth dams can be constructed manually in several ways:

• A community can provide manual labour during a dry season for construction of their pond or dam. The value of their labour may be regarded as cash input by a donor organization willing to contribute a similar amount for payment of technicians to assist the community.

• A farmer can hire people to build his pond or dam for a fi xed price, usually between Ksh 100 and Ksh 150 (US$ 1.25 to $2) for every cubic metre of soil excavated and transported in wheel-barrows to the dam wall. An able bodied person can excavate and transport up to three m3 of soil in a day.

• A simple way to distribute the work is to divide up the area to be excavated. Each cubic metre of soil to be removed can be marked as a “plot” (see photo section) and given to a person on contract basis. Plots can be pegged out in diff erent shapes, all having a volume of 1 cubic metre.

Draught animal powerOx scoops have been used for construction of ponds and earth dams in Machakos and Kitui districts of Kenya since the 1950s, and more recently in Garissa. A man with two trained oxen, a plough and an ox scoop can excavate and transport up to 30 m3 of soil in a day, or ten times as much as could be moved manually.

In areas where few households are able or willing to provide man-ual labour, it might be viable to use ploughs, scoops or carts pulled by oxen, donkeys or camels. The cost of the scoops, ploughs and carts can be covered by the need to purchase fewer hand tools and by the lower cost per cubic metre of soil.

Mechanised excavation Mechanised excavation using soil moving equipment can be used in places where farm tractors are used instead of animal draught. In some cases, particularly for the construction of larger reservoirs where several hundred cubic metres of soil needs to be excavated, it might be economically the most viable option (see Table 2). Even if it may be slightly cheaper to hire a tractor, this needs to be balanced against the employment opportunities and degree of ownership that will result from hiring local manual labour. Such decisions should be made with the community and will also depend on the local economic conditions at the time. For example, compare the cost of hiring a farm tractor or community members’ willingness to provide free or subsidised la-bour. The most expensive option is to hire a bulldozer for earth mov-

41


ing. In addition to the cost of about Ksh 5,000 per hour, a mobilisation fee of several hundred thousand shillings has to be paid plus daily al-lowances for two or three drivers and their supervisor. In all, the total cost may be about Ksh 60,000 per working day.

Estimating costsThe table below gives an example of the theoretical costs of excavating three diff erent types of water storage reservoirs of volumes ranging from 500m3 to 5,000m3 using diff erent methods of excavation: manual (by hand using shovels and wheelbarrows); oxen (as draught power to pull ox scoops, ploughs and carts); tractor with plough, scoop and trailer and bulldozer. The same table can be used for estimating these costs, fi rst by fi nding out actual local rates for each option, then fi lling in the quantity of soil to be excavated.

Table 2 Worksheet for estimating cost of excavatingType of reservoir

Construction method*

Reservoir volume (m3)

Water to soilratio

Excavated soil (m3)

Cost per (m3)

Total cost(Ksh)

Cost per m3 of water storage (Ksh)

example: Tractor 500 1:1 500 x 150 = 75,000 150

Pond Manual* 1:1 x ___ =

Tractor* 1:1 x ___ =

Oxen* 1:1 x ___ =

example Tractor 500 1.5:1 333 x 150 = 49,950 100

Hillside dam

Manual 1.5:1 x ___ =

Tractor 1.5:1 x ___ =

Oxen 1.5:1 x ___ =

example Tractor 5,000 3:1 1,670 x 150 = 250,500 50

Bulldozer 3:1 x ___ =

Manual 3:1 x ___ =

Tractor 3:1 x ___ =

Oxen 3:1 x ___ =

*This relates to whether excavation is done manually with shovels and wheel barrows, using draught animals with ox scoops, ploughs and carts or by hiring a bulldozer.

42


Estimating the benefi tsThe main cost for a dam or pond is paid at the time of construction but the benefi ts can be calculated over the life of the reservoir or at least 10 years, assuming it will eventually fi ll with silt and need to be rehabilitated. Economic benefi ts will include the value of labour and time saved fetching water and watering livestock. Benefi ts may also re-sult from improvements in the condition of livestock and small stock, cash from sale of irrigated farm produce and value of food grown for the household.

It is helpful to estimate the cash value of the benefi ts - especially if a community is currently spending scarce cash on buying water. The feasibility study should consider additional income, time and labour saved resulting from any project and comparing these with the cost (see Table 3).

Table 3 Estimated annual value of benefi ts from a 1,000 m3 water reservoir

Examples of annual income and savings Value (Ksh)

Labour saved on fetching water (Ksh 5,000 x 3 months) 15,000

Labour saved on watering livestock (Ksh 5,000 x 3 months) 15,000

Income from sale of tomatoes and kale from one quarter irrigated acre

12,000

Savings from household consumption of tomatoes and kale

500

Total income from a 1,000 m3 water reservoir after a rainy season

42,500

Establish the most cost-effective optionsIf suitable sites exist the construction of valley dams is less expensive per cubic metre of stored water than the construction of excavated tanks and ponds. This is because less material needs to be moved for each cubic metre of storage capacity created. The most expensive option (as-suming the labour is being paid) is the manual excavation of tanks and ponds because only one cubic metre of water storage capacity is cre-ated for each cubic metre of soil excavated.

The cheapest construction method is to use oxen if available. The cost can be as low as Ksh 20 per cubic metre of storage capacity created in the case of valley dams. This type of dam is, however, the most diffi cult for a community, farmer and/or water technician to construct.

Where feasible another option is a small hillside dam constructed

43


with a reservoir volume of 500m3. Although not the cheapest option for each cubic metre of water storage capacity created, it is the most aff ordable. It will cost about Ksh 20,000 if oxen are used. In one good rainy season it can potentially fi ll and produce savings and cash income worth about Ksh 10,000. So it could pay for itself a� er just two years.

3.6 Environmental impact and other considerations

If the answers to the questions listed at the beginning suggest that building a pond or dam is technically and economically feasible then another set of questions need to be asked. These relate to the possible environmental and social impact of the project and the role of the com-munity in managing, operating and maintaining the pond or dam.

These questions should include the following:• Will the project have any major impact on the environment?• What will the impacts of the project be on local people and how

are they involved in its planning and management?• Does the project address gender issues, meaning those which

aff ect the roles and work of men and women in the commu-nity?

• Are there any laws, cultural or ownership issues associated with the project which need to be addressed?

New legislation in Kenya and Tanzania requires an environmental impact assessment for ponds and dams. In Kenya, the specifi c legislation is the Environment Management and Coordination Act of 1999, and Environmental Impact Assessment and Audit Regulation, 2003, Kenya Gaze� e Supplement #56. All new dams or pans must seek approval from the National Environment Management Authority (NEMA), which may require doing an environmental impact assessment.

Technicians should check with the local water authorities in your country for regulations which apply to your proposed project. Small earth dams and ponds do not individually have a major impact. Nev-ertheless, if many are constructed in the same catchment area their combined eff ect could be signifi cant. The larger the earth dam or pond is, the more impact it will have on the environment. The impacts can be negative or positive. If the negative impacts are signifi cant or outweigh the positive ones, then the dam should not be constructed. The list be-low can be used as a checklist for positive and negative impacts.

44


Any environmental impact assessment should include a risk as-sessment to consider the likely eff ects of a worst case event, such as an earth dam wall being washed away in a major fl ood. An alternative location for the dam should be found if households or downstream se� lements might be put at serious risk by a washout.

Checklist of impacts of earth dams, pans and ponds

Positive impacts Negative impacts

1. Irrigating fi elds and tree nurseries for generating income and re-planting forests.

2. Watering livestock near villages saves time and reduces erosion caused by cattle tracks.

3. Providing domestic water from a hand-dug well generates income and can lead to health improvements.

4. Raising ducks, geese and fi sh farming for food and income.

5. Making bricks and construction works for income generation.

6. Reducing water-borne diseases by providing improved water supply for domestic use.

7. Saving peoples’ time by reduced walking distances to fetch water.

8. Reduced impact of fl oods by storing initial fl oodwaters, controlling erosion.

9. Raising the water table down-stream of ponds and dams, higher water levels in hand-dug wells.

10. Increasing the value of land near an earth dam, because of all the above benefi ts.

11. Improving incomes using the water, through the money-making activities described above.

1. Loss of some land taken up by the pond or reservoir and its spillway(s).

2. Risk of increased cases of malaria (can be reduced by introducing Tilapia nilotica to eat mosquito larvae).

3. Risk of increased cases of bilharzia (schistisomiasis), cholera, dysentery and typhoid.

[Note: disease risk can be reduced by fencing reservoir and drawing water from hand-dug wells or draw-off pipes and if people do not bath and wash clothes in the reservoirs].

4. Increased soil erosion along roads due to people and animals coming for water at the dam or pond.

5. Risk of dam wall collapse if poorly designed or constructed incorrectly, releasing a violent fl ash-fl ood damaging everything in its path.

6. Siltation of dam reservoirs shortens the lifetime of dams unless proper soil conservation is implemented in the catchment areas.

7. Risk of people and animals drowning if they try to bath or swim across a dam reservoir.

8. Impact on downstream users who may be deprived of water or subject to pollution or increased sediment load due to upstream usage.

Once the feasibility study has shown which type of water storage structure is viable, this chapter explains:

• How to calculate the exact volume of the water reservoir, the height and length of the wall for dams and embankments for ponds.

• How to design the foundation, dam wall or embankment and spillway.

• How to prepare the bill of quantities, calculate the exact costs and develop the construction plan.

• How to peg to the site and what is involved in the construction.

Chapter 4 Design and construction

46

Chapter 4 • Design and construction

4.1 IntroductionFor small ponds and earth dams on sloping land of sizes not exceeding 1,000 m3, these calculations are fairly easy and are done using simple methods. For larger structures, always seek technical assistance for the calculations, as small mistakes in the design phase can make the whole project fail and not hold water.

In particular, earth dams in valleys involve advanced construction methods that require experienced technical assistance to design the structures and supervise the construction. This is because valley dams are situated in seasonal water courses which fl ood during heavy rains. Spillways must be designed to discharge surplus water safely. The dam wall must be strong enough to withstand several metres of water pressure from fl ash-fl oods.

4.2 Design and construction of small earth dams in valleys

This section will lead the reader through the steps of designing and constructing a valley dam. Earth dams in valleys should always be designed, calculated and supervised by an experienced person, and always seek advice or a second opinion from skilled engineers if there are any hesitations. This is because failure of a valley dam may have disastrous consequences.

DesignThrough the feasibility study we already have good information about the site. We have a fair idea how the dam will look and what costs are involved. Now we need to precisely position the dam wall and spillway, design the dam in detail and make exact cost calculations, to allow us to hire contractors and/or go ahead with the construction work.

Now we need to make a detailed topographical survey and on the map precisely locate the dam wall and the spillway, to enable us to exactly calculate its storage capacity and the height and length of the wall. Therea� er we will design the foundation, the wall and the spill-way. This will give the basis for preparing the bill of quantity (the vol-ume of soil to be moved) and planning for and calculating the costs of the construction phase.

47


Equipment needed for designing the dam is:

• All the data from the previous feasibility study, including rainfall data and the catchment area marked out on a 1:50,000 contour map.

• Survey equipment: dumpy level/engineers level and accessories.

• Drawing equipment: drawing paper, drawing board, drawing pens and other equipment, a calculator.

Detailed topographical surveyThe detailed topographical survey should be prepared by a surveyor (engineer or technician), to produce a topographical map over the dam area of the scale 1:1,000 and a vertical interval of 0.5 m between the contours.

The topographical survey starts off positioning one or two bench-marks as reference points on a tree, rock or some stones concreted to-gether near one end of the proposed dam wall. Mark the benchmark point with white paint to make it visible from a distance. The position of the bench mark is plo� ed onto the topographical map being cre-ated. All measurements and levels during the topographical survey, the design and the construction of the dam should be taken from this benchmark.

Position of the dam wall and spillwayOn the 1:1,000 topographical map, indicate the exact position for the dam wall and spillway. Mark the centre line of the proposed dam wall, which is an imaginary line drawn through the centre of a dam wall at the crest (top of the wall), (see Figure 8).

Use the map to prepare a profi le drawing of the dam site. The depth from the centre line to the fl oor of the valley must be indicated at several points (see Figure 9).

48


Figure 8. Indicate the position for the dam wall and the spillway on the map.

Bench Mark

Horizontal centreline

Rocks

DistancePoint

0 1.51

5.22

8.03

11.74

14.05

17.06

20.77

22.08

27.69

41.610

1.2 1.7 4.0 5.1 5.5 4.8 4.5 1.8 1.1

Figure 9. Profi le of the dam site.

Capacity of the water reservoir, height and length of the dam wallThe approximate capacity of the reservoir taken from the feasibility study, should guide the estimation of the height and length of the dam wall. This calculation will have to be repeated a couple of times until the height of the dam wall is fi nally established. First fi nd and mark out the contour line that you believe corresponds to the approximate water capacity. On the same topographical map, the shape of the water reservoir is marked, which gives the maximum width, maximum depth and the throw-back, that is the full length of the reservoir when it is full of water, (see Figure 10). See Chapter 6, Tool 6 for two methods of calculating the reservoir volume.

49


The top of the dam wall should exceed the estimated full supply level (normal water level). The distance between the full supply level and the top of the wall is called freeboard, and includes fl ood water level (maximum water level at heavy rain), waves generation and some allowance for se� lement a� er the dam wall is completed (see Figure 11).

Centre line

Max.depth

Max.width

Bench mark

Flow

Figure 10. Plan of maximum width, depth and throw-back of a dam reservoir.

MFL - Maximum fl ood levelNWL - Normal water levelNF - Net freeboardGF - Gross freeboard

Figure 11. Section of dam wall showing freeboard.

MFL - Maximum flood levelNWL - Normal water level

Reservoir

MFLNWL

NF GF

Spillway sill level

MFD

Crest level

MFD - Maximum fl ood depthGross freeboard = Crest level - Spillway levelGF = NF + MFD

50


Design the foundationTo prevent seepage passing under the dam wall, it is necessary to build a key or core trench. The key consists of a trench dug immediately below the centre line of the dam wall. It must extend along the dam wall and include all sections that lie below the maximum water level of the reservoir (see Figure 12).

A key must be excavated through all layers of sand and gravel until it is at least 0.6 m into watertight (impervious) soil, like clay and murram. The width of a key should be at least 2.5 m with its sides sloping at 45 degrees. The key is re-fi lled and compacted with the soil with high clay content, preferably sandy clay with a higher proportion clay than sand. Avoid pure unstable soils like black co� on soil.

Figure 12. Longitudinal section of the dam wall with the key underneath, along the centre line of the wall below the highest water level.

Design the dam wallWhen the height and length of the dam wall is calculated, adjust the height of the centre line on the profi le drawing of the dam site. Make sure the freeboard (diff erence between normal water level and crest) is included.

Convex crest. The crest (top) of an earth dam wall should always be highest at the middle and lowest at the ends (convex). This is to avoid a washout of the middle section of the dam wall in case the spillway is blocked or cannot cope with the peak discharge in a heavy storm. Should a washout happen, it is easier to repair the end of a dam wall instead of repairing the deep middle section. The height of a convex (upward curving) crest should be about 10 per cent of the maximum depth from the centre to the valley fl oor.

Se� lement allowance. Also allow for se� lement of the soil in the dam walls. No ma� er how much the soil is compacted, the height of a newly built dam wall will always sink when the reservoir is fi lled with water for the fi rst time. This se� lement occurs because the soil, made pliable and heavy by water, will press air out of the voids in the soil. Dam walls must therefore be built with at least 30 per cent allowance for se� lement, to therea� er remain higher (convex) at the middle.

Bench mark

Freeboard Water level

Clay key

Centre lineConvex crest

51


Gradient of slope of dam walls The gradient of the slope (also called ba� er) of the dam wall is determined by the height of the dam wall and the type of soil. The more stable the soils are, the steeper the slopes. Usually they range between 1:2 and 1:3. The slopes are used when calculating the outline of the base and determine the amount of material to be deposited (see Figure 13).

Figure 13. Cross section with gradients of the batters for an earth dam. Pegs and strings indicate the slopes (batters) and base of the dam wall.

Good soil for construction should be coarse grained material containing suffi cient clay to assure reasonable imperviousness. The clay content (minimum) should be in the range of 20 to 30 per cent. Dams built with soils with good granular distributions should have upstream and downstream slopes of 1:2.5. Those soils which are predominantly clay in nature should have upstream and downstream slopes of 1:3 and 1:2.5 respectively. Zoned dams should have an upstream and downstream slope of 1:2 respectively while the dam core should have slopes of 1/2:1 respectively.

The outline of the base for a dam wall is determined by multiplying the vertical measurements from the centre line to the ground with the gradient of the upstream and downstream ba� er. The upstream mea-surements are taken from the upstream side of the key and the down-stream measurements are taken from the downstream side of the key (see Table 4). For each height above the base, at the measuring points taken earlier, indicate the depth, gradient, length from the key of both the upstream and downstream ba� er.

The width of the crest of a dam wall should be wide enough to al-low traffi c to use the crest as a road spanning across a valley but should

52


not be too wide for that will increase the volume and cost of earth works. The minimum width of a crest should be 3 to 4 metres, for let-ting vehicles pass over the dam wall. On very small dams (less than 1,000 m3) two metres is enough.

The type of earth dam wall to construct depends on the availability of diff erent types of soils. At this stage, the soil analysis from the feasi-bility study may have to be complemented with further tests to make sure enough of the needed types are available. Three common types of earth dam wall are as follows:

Homogeneous dam wall. If the soil samples of a dam site have the same type of stable soil with 20 to 30 per cent clay (especially clayey gravel, clayey sands) or alternatively inorganic clay, a dam wall is built of the same type of soil throughout. This is called a homogeneous dam wall, i.e. all made of the same material (see Figure 14a). It is the easi-est type of dam wall to construct. Normally, homogeneous dam walls should only be built on smaller dams, at the most up to a height of 6 metres. Where higher dam walls are required the design should be changed to a zoned dam wall as described below.

Zoned dam wall. This is the most common type of dam wall. It consists of a key and a core of clayey soil whose sides are supported with graded gravels and sands or sandy soil (see Figure 14b). It is suit-able where clayey soils are available only in limited supply. It is also a more stable and economical design than a homogenous dam wall be-cause it is built with steeper slopes, thereby reducing the cost of earth

Table 4 Example: Calculating the outline of the base for a dam wall

Point Depth from centre to the ground (m)

Gradient of upstreambatter 3:1

Upstreamlength of base from key (m)

Depth from centre line to the ground(m)

Gradient of downstreambutter 2.5 :1

Downstreamlength of base from key (m)

1 1.2 X 3 = 3.6 1.2 X 2.5 = 3.0

2 1.7 X 3 = 5.1 1.7 X 2.5 = 4.25

3 4.0 X 3 = 12.0 4.0 X 2.5 = 10.00

4 5.1 X 3 = 15.3 5.1 X 2.5 = 12.75

5 5.5 X 3 = 16.5 5.5 X 2.5 = 13.75

6 4.8 X 3 = 14.4 4.8 X 2.5 = 12.00

7 4.5 X 3 = 13.5 4.5 X 2.5 = 11.25

8 1.8 X 3 = 5.94 1.8 X 2.5 = 4.50

9 1.1 X 3 = 5,4 1.1 X 2.5 = 2.75

61


works especially for higher walls. The width of the clay core at the bot-tom should not be less than the height of the dam. If more clayey soils are available, the soil with the next highest clay content should be used for the upstream side of the dam wall and for a blanket to cover the whole dam when completed, while the soil with the lowest clay con-tent (the most sandy soil) should be used for building the downstream side of the dam wall.

Diaphragm dam wall. In situations where plenty of rocks, stones or gravel are available on site but too li� le impermeable material, a diaphragm design may be used (see Figure 14c). In this case a watertight blanket (diaphragm) of clayey soils with a clay content of 12—40 per cent is placed over the rocks, stones or gravel on the upstream side of the dam wall. This clay soil layer should be 0.6 m thick for a dam wall up to 5 m in height. It should start in the key at the front toe of the dam wall to prevent seepage.

Figure 14a. Homogeneous dam wall.

Figure 14b. Zoned dam wall.

Figure 14c. A diaphragm dam wall.

Reservoir Shell Core Shell

Core Trench

Reservoir

ReservoirDiaphragm

62


Design spillwayA spillway should be sited at a distance of at least 10 m from the ends of a dam wall to avoid fl ood water eroding the dam wall. Further protection from erosion is achieved by building a low wall of large stones set in mortar along the side of the spillway next to the dam wall.

Where the spillway crosses the extension of the centre line of the dam wall the depth of the spillway should be equal to the lower line of the freeboard. The depth of the fl oor for a spillway is therefore found by measuring the depth of the gross freeboard down from the centre line.

Where the fl oor of a spillway does not consist of weathered rock, then small walls (called sills) of stone-masonry should be constructed across the width of the spillway to distribute water fl ow evenly across the spillway to prevent erosion.

To calculate the required size of a spillway, you need to know the maximum fl ood fl ow coming from the catchment in a heavy rainfall and the freeboard depth.

First calculate the actual size of the catchment area in hectares. This is done by either using the results from the topographical survey, or alternatively fi nd the size of the catchment area in hectares using the contour map on which the boundary of the catchment was traced during the site investigation. For example using a 1:50,000 map, each square kilometre (1 km2) is equal to 100 hectares. The size of a catch-ment in hectares is found by counting the number of squares in the area and multiplying them by 100.

Once the height of the freeboard and catchment area have been es-tablished it is possible to determine the width of the spillway, provided some basic information on soil type, soil cover, slope and mean annual rainfall are available.

There are diff erent ways to calculate the maximum fl ood fl ow (or peak runoff ) and therefore the required size of the spillway(s). Two methods are given in Chapter 6, Tools 5a and 5b.

The height of the freeboard can be reduced but that would require a wider spillway. When the reservoir has been fi lled with water for some months and the soil in a newly built dam wall has se� led com-pletely, it might be feasible to reduce the freeboard. This is done by raising the spillway by building a low wall of stones (known as a sill) embedded in mortar across it.

Design water abstractionAt this stage, the water abstraction should be designed. Diff erent options are presented in Section 4.5.

63


Design drawingsDrawings useful to prepare for the construction are:

• Α plan of the dam wall and spillway.

• Α cross section of the dam wall.

• Α profi le of the dam site (longitudinal drawing of the dam wall including key and crest).

An example of a plan of an earth dam with homogenous wall is shown in Figure 15. The plan compiles all data on the catchment, dam wall, core trench, spillway, reservoir and water abstraction method.

A spillway being lined with stone.

64


Figure 15. Complete plan and data for an earth dam.

Bench mark

Freeboard Water levelCentre lineConvex crest

Allowance for settlement

Spillway

Clay key

Rock

Concretecollars Downstream batter

Upstream batter

Crest Centre line

Pipe

Gradient

Gradient

Spillway

65


Name of dam_________________

Survey by ................. Date............

Drawn by ..................Date............

Checked by .............. Date............

Designed by ............. Date............

Checked by .............. Date............

Bench mark

Freeboard Water levelCentre lineConvex crest

Allowance for settlement

Spillway

Clay key

Rock

Concretecollars Downstream batter

Upstream batter

Crest Centre line

Pipe

Gradient

Gradient

Spillway

66


Bill of quantity for design and constructionIn order to work out the needs for soil for the dam construction, a bill of quantity needs to be prepared (see Annex 2). It is also used for costing the survey, design, tools, equipment, materials and labour. It is fi rst necessary to calculate the amount of material which needs to be excavated (the soil works). The example below shows how this is calculated.

To get the diff erent soil types needed for the construction, it will have to be taken from borrow pits. It is especially important to select the best clay soil for making a watertight key and foundation of the dam wall.

The excavation of the borrow pit within the reservoir has the advantage of increasing the reservoir volume and also of not leaving a scar on the landscape as the borrow pit will be submerged when the reservoir fi lls. If it is within the reservoir it is important that the depth of a borrow pit is never deeper than the bo� om of the key, otherwise water might seep under the key. It also must be at least 10 m upstream of the front of the dam wall to avoid seepage under the wall.

In the ideal situation, the dam wall should use the same amount of soil that is excavated from the spillway. In most cases this is not pos-sible, and additional labour is required for digging a borrow pit. The borrow pit site should be as close as possible to minimize transport cost.

The quantity from each source can now be worked out as shown below. There are also situations where the spillway has to be built up.

Calculating the embankment volume The length of the entire dam is divided into segments of equal lengths. The volume of each segment is determined and the sum gives the volume of the dam wall. The procedure is as follows (see also Table 5):

1. Plot the layout of the embankment using a suitable scale.2. Divide the entire length into segments of equal length, e.g. 5 or

10 metres.3. Calculate the cross section area of the embankment at equally

spaced distances.4. Calculate the volume of each segment by multiplying the

length with the average area of the end sections of each seg-ment.

5. The sum of the segments gives the total volume of the embank-ment.

67


The results will be more accurate if the wall is divided into several seg-ments as opposed to a few segments. Use the following example as a guide.

• total length of dam wall is 100 metres.

• top width is 4 metres.

The dam will be divided into segments each with a length of 10 me-tres. Total borrow material required = (embankment volume + core-trench volume).

Table 5 Calculating volume of embankment and core trenchX sectionchainage

(m)

Height (m)

Top width(m)

Bottonwidth (m)

Computedarea (m2)

Computed volume (m3)

0 0.5 4 6.5 2.6

10 1 4 9 6.5 45.6

20 2 4 14 18 122.5

30 3 4 19 34.5 262.5

40 4 4 24 56 452.5

50 6 4 34 114 850

60 4.1 4 24.5 58.4 862

70 2.9 4 18.5 32.6 455

80 2.6 4 17 27.3 299

90 2 4 14 18 226.5

100 1.5 4 11.5 11.6 148

Coretrenchtotalvolume

1000

Total vol. 4,725

Detailed cost analysesWhere manual labour is being hired the cost obviously has to be cal-culated. Even if the labour is being provided voluntarily it is necessary to calculate how many person days are required. It is worth estimating the value of this local contribution so the signifi cance of this contribu-tion is rightly recognized and shared with the community.

Permits and approval of designs Before the actual construction can start on the ground, make sure all permits and necessary approvals are received. For example in Kenya permits are obtained from the District Water Offi ce, and they need

68


complete design drawings, design reports, and must be signed by a qualifi ed engineer.

Make sure all documentation for the dam project is archived safely for any future extensions, repairs or other alterations.

ConstructionBefore construction work begins check that the following criteria have been met and relevant procedures followed:

1. A suitable site for the dam has been identifi ed and its feasibility investigated in terms of the issues highlighted in Chapter 3.

2. A wri� en agreement on the ownership of the dam site, an ac-cess road, usage of water from the dam and conservation of the catchment has been completed.

3. Design drawings and bill of quantity for the dam are ready.4. A decision is taken regarding the method of excavation of soil

works whether manual labour, draught power or machinery. Obtain quotations for purchases and hiring labour or machinery or make prior agreement with community regarding labour inputs.

5. Any legal requirements have been addressed.6. Funds for construction of the dam have been secured.7. The community is fully aware and involved.8. A construction schedule for the project is prepared.

Construction scheduleThe construction of valley dams should only be done during dry seasons when there is very li� le risk of heavy rainfall because a dam under construction can easily be swept away by a thunderstorm. If the water fl ow has been diverted, construction can be done in the wet season, unless there is heavy rain. In the wet season, the soil is easier to handle than in the dry season, and it does not need to be we� ed for compaction, but it can be too wet for good compaction and heavy machines can get stuck.

To make sure all work is done in the right order, and work is fi nalized at agreed times, all activities should be listed and given a timeline in a construction schedule (see the example in Table 6). A more detailed construction plan appears in Chapter 6, Tool 7.

69


Table 6 Example of a construction schedule

Itemdescription

Week 1 Week 2 Week 3 Week 4 Week 5 Week 6

Site clearing and pegging

******

Core trench and foundation prep-aration

********* ****

Abstraction sys-tem

*********

Spillway excava-tion

*********

Embankment/ wall construction

********* ********* ****

Finishing works incl. catchment protection

*********

Water points and cattle troughs

*********

Site clearing and peggingSite clearing involves excavation and disposal of the top vegetation soil up to a depth of 0.2 to 0.30 metres. from the embankment area, borrow pits and spillway section. It also involves removal of all stones, uprooting of tree stumps and disposing the same on the downstream side. Measure out and peg all designs on the site, always starting from the benchmark. Mark the pegs in diff erent colours. Peg as follows:

• Mark the centre line of the proposed dam wall by placing a peg at both ends of the dam wall and drawing a nylon string between the pegs.

• Peg the core trench.• Peg the base of the dam.• Mark the outline of the foundation.• Mark the position for the spillway. The outline of a spillway is

marked with pegs spaced about 10 m apart.

Core trench and foundation preparation When a key has been excavated to a depth of 60 cm below any layer of sand or sandy soil, the vertical sides of the key are cut to a slope of 45 degrees for stabilising the excavation. It is extremely important

70


to select the best soil and compact well to get a watertight key. The soil is fi lled into an excavated core trench in layers of 15 cm depth all along the length of the trench. If the soil is dry, water should be used to moisten the soil before compacting it.

Foundations of earth dams, as well as keys, should be made water-tight to prevent seepage under the dam walls. This is achieved by re-moving all vegetation including the roots and all patches of sandy soil within the base of dam walls. Water abstraction methodWhere an outlet pipe is required it should be laid a� er clearing the foundation of vegetation, roots and sandy soil.

Spillway excavationThe fl oor of a spillway is made level at the centre line. From there the fl oor should slope 3 cm for every 100 cm towards its upstream and downstream edge. The area to be excavated is divided into plots with a volume of one cubic metre (see Figure 16).

Figure 16. Determining the level of the spillway fl oor.

To minimize the risk of a thunderstorm fl ooding a reservoir and destroying an incomplete dam wall, be sure to excavate a part of the spillway to its fi nal depth before major construction work on the dam wall begins, so the water can escape if necessary.

Determine the height of the spillway using topographical survey instruments during the excavation phase to avoid digging too deep. Preferably a surveyor should be brought in to assist in supervising ex-cavation.

71


Embankment construction When a key has been fi lled with clayey soil, compacted and the foundation cleared of vegetation, roots and sandy soil, the construction of the dam wall can begin. It is built in layers of 20 cm and each layer compacted. If the core of the dam wall is to be constructed with a diff erent soil type this has to be placed fi rst along the centre line to its design width and the sides placed therea� er, for each layer.

Between each layer, the wall needs compaction. This step deter-mines the future strength of the wall and must be done correctly. If compaction of dam walls cannot be done with machinery (more o� en the case in remote rural areas) the allowance for se� lement must be in-creased to 30 per cent.

The construction materials should be spread uniformly to the specifi ed thickness (20cm). Roots, vegetation and boulders over 15 cm diameter should be removed. The materials must not be allowed to dry, i.e. to lose moisture. When areas of the fi ll are not fully compacted or the material is too dry to allow full compaction, that area will se� le upon we� ing and weaken the dam wall. In all cases, the embankment should be built in horizontal layers which should be of similar thickness. The degree of compaction is suffi cient if hand excavation using a shovel is not possible otherwise the compaction is not suffi cient.

The stage of constructing the wall mustn’t take too long, as there is always a risk of an unexpected thunderstorm that may produce a fl ood that could wash it away before it is completed

Finishing works, including catchment protection Upon completion of a dam wall with its convex crest and se� lement allowance, make its sides and crest even and smooth. Cut down and remove trees and bushes in the reservoir. Fill holes made by rodents in the fl oor of the reservoir with soil and smooth the ground. Place riprap on the dam side of the wall from the bo� om up to the maximum height of the water level. Riprap is a barrier of rocks to break the erosive force of waves washing against the wall.

Pack medium-sized stones at the base of the downstream side of the dam wall to form a rock toe (or backtoe) and grass planted between the stones. This stone apron will prevent erosion of the dam wall by any water seeping out through the downstream toe (see Figure 16).

To protect the sides and crest of a dam wall, plant deep-rooted grasses with runners such as Kikuyu grass where rainfall is good, or star grass in dry areas. Plant the grass on contour lines spaced 30 cm on both sides of the dam wall. The dam and especially the dam wall should be fenced, and no animals allowed onto the wall.

72


Completion certifi cateWhen all work is fi nalized, especially if it is contracted out, it is important to carry out a detailed assessment to be sure it was carried out according to the specifi cations. For example, the district water offi ce (Ministry of Water or Natural Resources) can appoint a technician or engineer to issue a completion certifi cate.

4.3 Design and construction of pondsDesignPonds or pans are excavated below the natural ground level or on in-clined slopes. These structures may be of any shape, although a circu-lar design is common. They are mostly built in areas with fl at slopes or inclined slopes, where construction of an earth dam will not be tech-nically feasible. The storage volume created is equal to the volume of excavated soil. For pans or ponds excavated using machinery, other shapes as opposed to circular ones are preferred since machines work more effi ciently in straight lines.

The size of the pond will depend on the following factors:• The water demand plus silting allowance. Normally 10 per cent

of the storage is le� for silting.• The size of the catchment area draining into the pond and the

expected volume of runoff water from the catchment.• The area available for constructing the pond.• The soil type.• The amount of money available for excavation.

If the size of the pond is small, e.g. below 1,000 m3, a simple design is done on site and sketched onto paper. Simple fi eld equipment such as tape measure, line level, strings and pegs, are suffi cient.

The design of larger ponds and pans, from 1,000 up to 30,000 m3, follow in general the same procedures as for valley dams. Planning for such structures should follow the same feasibility study as outlined in Chapter 3. A detailed topographical map (scale 1:1,000) of the pro-posed site is needed and a surveyor should be assigned to prepare it. On the prepared map, fi rst locate the position for the pond. Next in-dicate the inlet channels, location of overfl ow channels, silt traps, and the embankment. Calculate the amount of runoff as well as the maxi-mum fl ood fl ow to determine the size of pond or pan and the overfl ow channels.Detailed design of the various components should be done to enable

73


the bill of quantities to be prepared (see Annex 3) as well as the construction plan. Normally, shallow storage depths are discouraged due to high evaporation rates. The depth of storage should not be less than three metres.

Another way to reduce evaporation and conserve water towards the end of the dry season is to create a gentle slope at the bed of the pond or pan towards the inlet. As the water level drops, remaining water accumulates in the deeper side minimizing the surface area ex-posed to evaporation

The slope of the sides depends on the soil types and topography. The slopes usually vary between 1:4 and 1:5 at the inlet side and 1:3 to 1:2.5 for the rest of the sides. Generally, a slope of 1:2.5 is adequate for soils with good granular distribution as well as impervious soils. For sandy soils and heavy clay soils, a slope of 1:3 is adopted.

The bed and the sloping sides of the excavated pond or pan should be watertight. If the pond or pan is located in areas with porous soils such as sand, then lining with an impervious clay blanket of 20 to 30 cm to minimize seepage should be considered. Pans and ponds can also be lined with heavy polythene sheets if clay is unavailable, but this is costly.

Inlet design and catchment protectionA natural channel leading water into the pond may exist. If not, excavate one or two trenches to lead the water into the pond. Dig silt traps to collect sediment and minimize the silt entering the pan or pond. Their design capacity depends on the surface condition and the sediment yield of the catchment. They should be located some distance away from the mouth of the reservoir, between 5 and 20 metres, depending on the topography. The silt traps need to have reasonable size and depth, depending on the expected siltation rate. Their depth is usually one to two metres.

At the design stage, it is important to assess the vegetation cover/soil status of the catchment and take measures to control erosion with-in the catchment area.

Embankment designThe embankment should be highest in the middle (convex), opposite the inlet to the pond. There is no need for detailed embankment calculations as the construction of the embankment is a ma� er of heaping the soil. The embankment will not need any compaction.

The excavated soil should be placed to form an embankment around the pond or pan but any soil dug from the reservoir should be placed in a way that its weight will not endanger the stability of the sides. Also rain must not be allowed to wash soil back into the pond so

74


embankments are built up at a distance from the sides of the excava-tion. Embankments of large pans (1,000 to10,000 m3) should be made large, up to seven meters. For small pans below 1,000 m3, embank-ments can be less. If there is space around the pond for future enlarge-ment, place the embankment further away from the pond. If the space is limited and no future enlargements will take place, leave it at two metres.

Embankments also help to reduce wind speed and help vegetation to re-grow at the site to protect it from erosion. Planting trees outside the embankment is wise, especially on the side towards the prevail-ing wind. These will eventually form a windbreak which will reduce evaporation losses.

The overfl ow channelThere is need for overfl ow arrangements before the inlet to the pan to divert excess fl ow when the pan is full. If more water is permi� ed into the pan at this stage, the pan will act like a silt trap and overfl owing water will damage the embankments. The channel is designed in such a way that its inlet level is slightly below the lowest edge of the pan. The peak fl ood (during a 20-year period) should be used to determine the dimensions of the overfl ow channel.

Preparation of design drawing, bill of quantity, cost calculations For larger ponds and pans, complete design drawings showing the construction details should be prepared with plans and cross-sections (see Figure 15). Bill of quantity and costing should be produced to give the basis for hiring contractors.

These drawings and supporting documentation are needed for se-curing permits from the relevant government authorities. All details in Section 4.2 on valley dams also apply to ponds and hillside dams.

Construction

Construction plan The construction plan describing activities to be done, when, and by whom should be prepared. The plan forms the basis for procuring equipment and recruiting labour or contracting out the construction.

Site clearing and peggingThe construction site should be cleared of all vegetation, tree stumps and other material which will hinder the excavation works. The out-lines of the pond and the dumping site should be pegged out with wooden pegs. The overfl ow channel should also be cleared and pegged.

75


ExcavationA� er clearing the site, start the excavation work in strips from the low-est section. Excavation should be carried out to the required depths. The excavated soils should be transported to the designated locations for the embankment. The supervisor should guide the operator to de-posit the soil at the correct places to avoid long distance movements and loss of time. The pan excavations should be continued in steps progressively toward the inlet.

The pan should be shaped according to the designed slopes once the correct depths have been achieved. The embankment should be progressively shaped as the excavation work continues. Once the pan is completed, construct the overfl ow channel to the required depth and slope.

The fi nal phase involves the completion of the abstraction struc-tures and fi nishing works including silt traps, fencing, other catch-ment protection works and riprap protection of the ca� le ramp and silt traps.

Manual excavation of pans and pondsConsider the following issues in planning and supervising construc-tion, or when rehabilitating ponds or pans:

• Provide labourers with the necessary tools so they work effi ciently.• Carefully mark out the area to be manually excavated to make

sure the volume of soil removed can be measured and the cost of work estimated. For smaller ponds each worker may remove one m3 of soil at a time. For larger constructions have the labourers work in pairs digging 3 m3 plots.

• Check the depth of the excavation regularly to ensure the re-quired depth is not exceeded and the excavation has not gone into permeable material.

• Organize labourers into teams of two people, one to dig/loosen soil and one to carry it away. Draught animal traction teams will also need two tools, one for loosening soil and one for scooping.

• Plan construction to coincide with the dry season.• Drinking water will have to be provided on site for labourers

and/or draught animals.• Labourers who are going to be on site all day, for weeks at a

time may need lunch provided. Make arrangements for camp-ing at the site if necessary.

• Arrange accommodation for machinery drivers and assistants.

76


Charco ponds

Charco ponds, which are commonly built in Tanzania, are usually excavated manually by individuals near their homesteads for watering livestock. The water may also be used for some domestic purposes, although, as it is easily contaminated, it is not suitable for drinking. The size and shape of charco ponds varies depending on the owner’s preference. A preferred shape in some areas is that of a calabash used for scooping water. The “handle” is used for the infl ow channel and for giving access to people and livestock. Farmers dig their ponds during dry seasons and may enlarge them every year until the owner is satisfi ed with the capacity of the pond. The main problem with most charco ponds is that their storage capacities are too small to supply suffi cient water throughout the long dry season. High evaporation losses are diffi cult to address on the hot, windswept plains where most are located. Reduced storage capacity due to siltation is sometimes made worse by a lack of silt traps or where the sides of ponds are so steep that they collapse.

Run-offA A

Dam Resevoir

inflowSilt traps

Spillway

Spillway

Staircase

Dam Wall

PLAN

B

B

Catchment Reservoir Dam WallMax. WL

Profile A-A

Berm

Profile B-B

Max. WL

Figure 17. Plan of charco pond with silt traps, stone sides on both spillways and staircase/cattle ramp.

Figure 18. Cross section view showing cattle ramp design.

Figure 19. Cross section at centre of reservoir.

77


4.4 Design and construction of small earth dams on sloping land

Small earth dams on sloping land described in this section are very small dams and their construction does not in general need expert advice. As the embankments must withstand the pressure of water they still need thorough planning as well as proper compacting during construction.

One very positive feature of this design is that it is possible to start off constructing a relatively small earth dam and reservoir for storing water from the fi rst rainy season and then enlarge it during the follow-ing dry seasons. This is done a number of times until the reservoir has been signifi cantly enlarged to the desired capacity as shown in Figure 23.

DesignThe design of this dam consists of a semi-circular dam wall shaped like a new moon as shown in Figure 20. The wall is made of compacted earth and each end, which is strengthened with rocks, is designed to act as a spillway. Because of this arrangement it is not necessary to estimate the volume of runoff fl owing into the reservoir. Once full any surplus water will simply spill over the ends and continue its normal downhill course. It is important that the crest of the dam wall is always higher in the middle than at the ends, to prevent any water spilling over the middle and washing out the earth dam wall see Figure 21 a and b). Although it is not essential to know the runoff volume for this design, the runoff will need to be suffi cient to fi ll the reservoir.

Construction

Excavation workThe excavation and soil works for a small earth dam on a hillside site can be done manually, with oxen or machinery. Construction involves excavating soil from a central pit and placing it in a semi-circular line along the downstream side of the excavation. The curved heap of soil will become the dam wall with the excavated pit as the reservoir. The size of the dam wall and its reservoir depends on the capacity for removing soil from the reservoir and placing it on the dam wall. Initially, communities might typically build a reservoir with a capacity of about 200m3 in the fi rst year but continue to enlarge the reservoir over a number of years until it is large enough to store water throughout the year.

78


Constructing a hillside pond in Zambia

During a training course in Zambia organized by RELMA/ASALCON in 2000, a local farmer built a small hillside pond near his homestead using a farm tractor with a plough for the earth works.

The farmer’s family had previously fetched water from a hand-dug well about 3 kilometres away from their homestead. The farmer wanted to save time on watering his livestock so he decided to construct a small hillside earth dam to enable him to water his livestock at home and to grow vegetables for cash income.

Rainwater running off his compound started to create a gully on his farm, eroding the land from where his only income was generated. He felt the dam might also help to protect his land from erosion.

Water for domestic purposes would be drawn from a hand dug well situated in a seepage line downstream of his pond. In case insuffi cient seepage occurred, as a back up, he relied on a roof catchment tank for harvesting clean rainwater.

The farmer used his tractor to plough against the pond wall repeatedly, thus moving every line of the ploughed soil away from the excavation pit and towards the wall. This way he completed the construction of his pond within two weeks, while also enlarging it from the proposed storage volume of 150 m3 to about 600 m3 (see Figure 20).

Dam

Figure 20. Using a tractor to enlarge the reservoir of a hillside pond.

79


Profile A-A

Catchment Reservoir Dam wall

Max. WL

Reservoir

(15m)A

B

B

Dam wallWater reservoir

Spillway

Spillway

Plan

Profile B-B

Max. WL

Figures 21, 22a and 22b. Plan and profi les for a hillside dam.

80


The gradient (slope) of the sides of the dam wall should be 2:1, that is for every 2 m in width there is 1 metre of height. The width of a crest varies from 1 metre for a low dam wall and up to 2 m for a high dam wall. The longitudinal section of a dam wall should have a crest that is higher in the middle than at its ends (convex) to prevent surplus water spilling over it.

Pegging the outline of the reservoirPlace a peg at the proposed centre of the reservoir. Preferably, the centre should be in, or near, a place where run-off collects. Decide whether the pond should be situated in the compound or at a distance from the compound. The safest option is to site the pond some 100 m outside a compound to reduce the risk of small children falling in. Tie a nylon string to the peg in the centre and draw half a circle, using the string as a radius to mark the pond wall on the lower side of the centre peg. The length of the radius is determined by the required size of the pond and the available space.

The two ends of the dam wall must be at a horizontal level to function as two spillways (see Figure 21). This is measured using a line level or circular level (see Tool 1 in chapter 6).

Building the dam wallWhether using manual labour, draught equipment or a farm tractor, the soil to be excavated should be ploughed to loosen it. Ploughing should start along the inner side of a dam wall with the plough share turning the soil towards the dam wall (see Figure 20).

Aerial photo of a hillside dam in Kajiado District, Kenya.

81


Manual labour may be used to throw or transport the ploughed soil in wheelbarrows against the centre line of the dam wall. Whenever the ploughed soil has been removed, the site is ploughed again and the loose soil thrown onto the dam wall, and so on, until a dam wall is built to its fi nal height.

Hillside dams are structural and required to hold water so good com-paction is essential to achieve a strong water-proof wall. Compaction of the soil in a dam wall with minimum water, which is usually scarce, can be done using;

• A tractor while adding 10 per cent height to the dam wall for se� lement.

• Oxen while adding 20 per cent height to the dam wall for se� lement.

• No compacting but adding 30 per cent height to the wall for se� lement.

Procedures for compaction by these three methods are described in Chapter 6, Tool 8.

Maintaining the inlet gradientThe inlet to the pond should be gently sloping to avoid erosion.

Reinforcing the spillwaysThe two ends of the curved wall of hillside dams function as spillways to allow surplus water from a fi lled reservoir to overfl ow the dam reservoir safely. Heavy rain showers on large catchments produce huge volumes of runoff water that must pass over the spillways without eroding the ends of dam walls otherwise water might destroy the whole dam wall.

Spillways should therefore be reinforced by placing large stones against the ends of dam walls. Long-rooted grass with runners should be planted between the stones to prevent overfl owing water from eroding away the stones.

The fl oor of the spillways should also be covered with stones in-ter-planted with grass to prevent erosion. If the fl oor of the spillways is steep a concreted stone-masonry structure may be needed.

Enlarging the catchment Should the volume of runoff water not be suffi cient to fi ll a pond, then enlarge the catchment by diverting runoff water from another catchment into the pond by making a soil bund or diversion channel.

82


Enlarging the reservoir Dams having catchments with suffi cient runoff can be enlarged to hold water throughout the year by deepening the reservoir and using the excavated soil to heighten the dam wall (see Figure 23).

Phase 14m

2m

5m

1m

Phase 24.5m 6m2.5m 1.5m

Phase 37m5m

3m

2m

Phase 48m5.5m

3.5m 2.5m

Phase 59m6m

4m

3m

4m

Figure 23. Enlarging the capacity of a hillside pond in stages.

83


4.5 Options for water extractionThe most common options for extracting water from small earth dam reservoirs are:

• Direct extraction• A hand-dug well below the dam wall• An outlet pipe beneath the dam wall• A siphon pipe (where no outlet pipe was laid during construction)

Drawing water directly from dam reservoirs Most ponds and dams are not equipped with any means of extracting water from their reservoirs. People simply draw their water directly from the reservoirs and their livestock walk in when drinking. O� en the water in these reservoirs has a soup-like consistency and muddy colour, from animal dung and mud mixing with water. Such practice is a health hazard and shortens the lifetime of ponds and dams due to siltation of reservoirs and erosion of dam walls. It is therefore a good investment to install an extraction device.

Hand-dug well for domestic water Domestic water should never be drawn directly from a reservoir but from a hand-dug well located downstream of a dam wall where seepage is found. Dirty water from a reservoir is fi ltered as it seeps through the soil and is cleaner when drawn from a well. However, it should still be boiled before drinking. The benefi ts of having a hand-dug well downstream of an earth dam are cleaner water for domestic use, and using water that would otherwise be lost by seepage, while reducing contamination of water in the reservoir. To fi nd the best site for digging a well look for an underground line of seepage by digging test pits, dowsing or looking for greener vegetation.

Outlet pipe for watering livestock and irrigationWater for livestock and irrigation can be piped to an outlet below the dam reservoir by means of an underground pipe going through the base of a dam wall. An outlet pipe is used for draining water by gravity for watering livestock downstream of an earth dam. Two-inch diameter (50 mm) galvanized iron (GI) pipe should be adequate.

To install the draw-off pipe, lay it in a trench about 30 x 30 cm under the base of a dam wall. To avoid seepage along the pipe, concrete collars, 20 cm thick and 60 cm wide, are built around the pipe in the trench every fi ve metres. Clay soil is therea� er compacted against the pipe.

84


The outlet pipe should reach the deepest point of a dam reservoir where it should be li� ed up without bending it to avoid blockage. The end of the pipe should be covered with plastic mosquito mesh. A large heap of stones are piled around the intake pipe to support it and pre-vent damage by people and livestock.

3:1 2.5:1

AfterSettl

ementBefo

re settlem

ent

Key

Figure 24. Cross section of a dam wall with an outlet pipe.

On the downstream side of a dam wall, the pipe should extend to a point at least two metres below the intake to allow water to fl ow by gravity. Fit a lockable water tap at the end of the pipe.

Siphon pipe for watering livestock and irrigationWhere an outlet pipe was not installed during construction a siphon pipe can be laid in the spillway as an alternative to an outlet pipe (see Figure 25). A siphon pipe is more complicated to use because it must be primed to start water fl owing.

Make a siphon pipe as follows:

1. Lengths of two-inch (50 mm) GI pipes are joined together with a tee placed at the highest point of the spillway. The intake pipe is placed in the lowest part of a reservoir, while the tap stand is situated downstream of the dam wall at a point at least two metres below the intake.

2. A 1 metre length of 2 inch GI priming pipe is joined vertically onto the tee in the spillway and closed with a cap of G.I.

3. A non-return valve, which blocks water from fl owing out of the pipe, is screwed onto the end of the intake pipe.

4. Plastic mosquito net is wrapped around the non-return valve to function as a fi lter to prevent blockage of the pipe.

5. Stones are piled around and over the intake to prevent damage to it. 6. A lockable water tap is screwed tightly onto the end of the outlet

pipe.

85


The siphon is started by closing the water tap and unscrewing the cap on the primer. The primer is fi lled with water until it overfl ows and all air bubbles have emerged. The cap is then screwed on airtight. Water will now fl ow out of the water tap when opened.

If the primer does not fi ll up with water the reasons may be:• Τhe water tap is not completely closed.• The non-return valve has not closed automatically as it should.• The pipe is not watertight.

Figure 25. Cross-section through a spillway with a siphon pipe.

2 inchtee

Now that the hardest part of the job is done — building the dam or pan — the most important one begins: that of maintaining it. This chapter explains ways to control livestock, so they

don’t damage the structures or contaminate the water. It offers suggestions for collecting fees for maintaining the dam or pan. Most important, it gives details protecting the catchment area and reservoir, to reduce the risk of soil erosion and silting up of the pan or dam. As with all previous stages of the work, the more closely you involve the community, the better. A long life for the new pond, dam or pan is the concern of everyone who benefi ts from it.

Operation and maintenance

Chapter 5

88

Chapter 5 • Operation and maintenance

5.1 IntroductionDuring the operation and maintenance stage it is assumed that a commi� ee or individual has already taken on responsibility for managing the pan or dam. Their mandate is to ensure that the agreed by-laws are adhered to and that funds are handled properly. For a community project, a monitoring and evaluation system should also be in place for the commi� ee to follow, and when necessary seek advice or technical help from external sources.

Operation entails balancing water demand and supply and sched-uling withdrawal/abstraction. Maintenance entails prolonging the lifespan of dams and pans through routine maintenance, repairs and desilting.

5.2 OperationControlling accessIn most ponds/pans and dams it is advisable to control access to the water both to protect the reservoir and embankments and to reduce contamination. Where practical build a thorn fence around the reser-voir to keep people and livestock away from the water and dam walls and put up a lockable gate. Open water is dangerous because small children and animals can fall in and drown.

In pastoral areas livestock are the primary users of water. During the dry season when there may be large concentrations of thirsty ani-mals, it is not practical to keep them out of the fenced area. However, there are ways to control where the livestock enters and how they are watered.

Demand managementVery few pans and dams hold suffi cient water to meet all demands throughout the dry season, especially during droughts. There is a need for the managers to restrict the water abstraction in order to make sure that some water remains for essential uses. To do this the manager should:

• Look at the volume of water impounded.• Estimate the livestock, irrigation and domestic demands (see

Section 3)• As the dry season progresses, use a gauge or marker (e.g. a

concrete post set in the reservoir) to estimate the volume of water remaining and decide if further restrictions/rationing are needed.

89


Controlling livestock and human access to pans in Mandera, Kenya

Pans in northeastern Kenya are normally fenced with separate and adequate gates or entrances for people and livestock. Different approaches are used to water livestock inside the fence, bringing them closer to the water without contaminating the reservoir. Here are two examples.

Mergacho systemThis watering system avoids direct contact between the people/livestock and the water in the reservoir but allows for direct access. It involves creation of soil bunds inside the reservoir but close to the waters edge. As the water recedes, the bunds are also moved back by the users. This allows for systematic seasonal desilting and continuous pollution control. The animal faeces are removed by the owners as they water their animals.

Daar system (portable trough)This system entails drawing water by means of small plastic containers (often 20 litre jerrycans) cut in half. The water is then transferred into portable troughs that are made of either a drum cut in half, oval shaped wooden containers or plastic sheet placed in a raised, wooden frame. This watering method is laborious but avoids direct contact between animals and the reservoir.

• Look ahead to the dry season and prioritize what demands can be met.

Operation of abstraction devicesThe pan or dam may have abstraction devices such as pumps or pipes and taps which need to be operated effi ciently. In the case of several wells and/or pumps there may be a need to appoint an operator. In all cases the equipment should be made as durable and simple to operate as possible. During times when water is rationed the managers may need to prepare a schedule for water collection to minimize queuing and arguments.

Revenue collectionAll pans and dams require money for maintenance whether this is provided by an individual owner or raised in the community. Charging for the use of the water is the most common way of raising funds and payment can be done in several ways:

• In kind — in the form of labour for maintenance.• In cash — per animal watering or per jerrican collected.• Through a monthly/annual fee.

90


For community projects the community or dam commi� ee should es-tablish by-laws to govern the sale of water and set tariff s for diff erent water uses. Collection and use of funds should be transparent and ac-countable.

Good management skillsCommunity projects require good management to ensure sustain-able operation and maintenance. Commi� ees and the community as a whole may require capacity building to assist them to take on new roles and responsibilities for managing and maintaining pans and dams.

5.3 MaintenanceCatchment protectionCatchment protection is actually another technical term for soil and water conservation. It is important to make soil conservation struc-tures on farmland to prevent siltation of dam reservoirs, otherwise layer upon layer of silt and soil will fi ll up a reservoir. Heavy silt loads reduce the volume of water a dam reservoir can hold, and once it be-comes shallow the evaporation loss increases as well.

In the worst scenario, dam reservoirs will be fi lled to the brim with soil and cannot hold any water at all. Since desilting of such reservoirs is more expensive than building new reservoirs, silted-up dams are of-ten abandoned. However, for dam reservoirs situated on sandy soils, a thin layer of siltation is benefi cial because the silt seals the fl oor of a reservoir against seepage.

Usually there is no need for protection of a catchment having pe-rennial vegetation such as forests and evergreen grassland provided livestock are not watered at a dam in that catchment. However, as soon as a newly built earth dam is holding water, but has not been fenced properly, people bring their livestock to drink which speeds up silt-ation. Later on, if people start building houses near the dam and clear-ing land for agriculture without soil conservation, then siltation may reduce the lifetime of an earth dam to eight years, or even less. In these cases it is advisable to agree on a management plan for the catchment with all stakeholders. This is particularly necessary where the catch-ment is used by several diff erent groups (or tribes) who may not nec-essarily benefi t from the dam or pond/pan water.

Catchment protection on farmland can be implemented in several ways. Maintaining the vegetation cover within the catchment by tak-ing steps to avoid overgrazing by livestock and deforestation are key aspects in the ba� le against soil erosion. The adoption of agroforestry

91


is also helpful along with some of the physical measures and planting strategies described below.

Contour lines of fodder grassesContour lines of fodder grasses, such as Napier grass or the more drought-resistant Bana grass, can be planted at intervals depending on the gradient of the land. On land sloping about 3 cm per 100 cm (such as the fl oor in a spillway) the distance between the contour lines should be about 20 metres. On steeper land the distance should be re-duced accordingly.

Contour lines with multi-purpose treesContour lines planted with multi-purpose trees such as leucaena, Melia volkensii and Azadirachta indica (neem, or mwarubaini in Swahili) make good windbreaks for reducing wind erosion of bare cropland. Planting trees, especially in contour lines, in catchment areas will:

• Reduce soil erosion and siltation of dam reservoirs.• Improve rainwater infi ltration into the soil for growing crops.• Provide a windbreak, fi rewood, charcoal, fodder, timber and

shade.• Improve the overall micro-climate.

Fanya juu contoursFanya juu contours are made by placing excavated soil on the uphill side of a trench. Although digging contour trenches is heavy work (best done by groups), the trenches increase growth of crops by improving soil moisture retention.

Silt traps made of vegetation planted in stripsSilt traps can be made of perennial vegetation planted in several strips across the infl ow channel to ponds and earth dams. The silt traps reduce the speed of the infl owing water thereby giving soil particles time to se� le in and above the silt traps. A� er fl ooding, most of the accumulated silt should be removed and used for fertilising adjacent farmland if possible. In areas where vegetation is scarce it may be necessary to construct reinforced silt traps.

Check damsCheck dams are usually made of large stones placed across infl ow channels. Perennial grasses are planted in soil packed in between the stones for “cementing” them together.

92


5.4 Reservoir protection and maintenanceFence the reservoirWhere feasible it is o� en appropriate to fence off reservoirs to keep livestock out. This helps to maintain be� er quality water and avoids the problem of ca� le ge� ing stuck in the mud. Since fencing material is expensive the planting of live fence may make be� er sense. In sandy soils it is actually benefi cial to let livestock walk into ponds because they mix soil, silt and dung to form an almost watertight fl oor which reduces seepage loss. Water mixed with urine and dung from animals is also valuable for irrigation as a natural fertilizer. Such ponds cannot be used for domestic water supply.

Do not dig wells in reservoir fl oorIt is common practice in dry areas for people to dig wells in the reservoir fl oor to abstract water a� er the pan/pond has dried up. This should be discouraged as such wells are dangerous and can open up seepage paths through the reservoir fl oor.

Control pollution/contamination To prevent contamination of dam reservoirs by people bathing, washing clothes, and watering livestock directly in the reservoirs, build washing stands and bathing facilities downstream of a dam wall next to the draw-off point. The waste water from washing stands, bathing facilities and draw-off points can be diverted to irrigate a small vegetable garden. Where the slope is suffi cient , a watering trough for livestock can be included.

Build pit latrines During the rainy seasons rainwater runoff washes the excrement into dam reservoirs where pathogens can multiply and transmit human diseases. The construction of pit latrines is therefore encouraged.

The latrines should be situated well away from dam reservoirs, and downstream of hand-dug wells. The usual types of pit latrines with walls of burnt bricks and tin roofs may be too expensive to build. Due to poor cra� smanship, some latrines collapse during rainy sea-sons. Lack of ventilation gives latrines bad smell and fl ies. VIP latrines equipped with ventilation pipes and built by experienced artisans have been constructed at very li� le cost using local materials.

Plant windbreaksEvaporation losses from ponds can be high in dry, windy areas. Plant a stand of trees such as neem on the windward side of the reservoir to

93


reduce evaporation. To maintain the windbreak the trees have to be densely planted.

Hold a desilting harambeeRainwater transports topsoil and other light surface particles from a catchment to a dam reservoir where some of it se� les to the fl oor of the reservoir as a layer of silt. A layer of silt that is only a few centimetres thick is good because it reduces seepage, but thicker layers of silt decrease the water storage capacity, reducing the period during which water can be drawn from a dam reservoir. Catchments without soil conservation and ponds or dams without silt traps may result in dam reservoirs that cannot store any water a� er only ten years.

Desilting can be done using any of the techniques suitable for con-struction (manual, draught animal traction or mechanical). Desilting should be done regularly, preferably once a year in areas where heavy siltation occurs. The depth of silt deposited (and hence the quantity to be removed) can be measured easily if a marked post is installed in the reservoir fl oor at the time of construction. Desilting should be carefully supervised to ensure that the very bo� om layer of silt, which helps seal the reservoir, is not removed.

5.5 Dam and embankment preventative maintenance and repair

Preventing dam leakageNewly built dams do not usually hold water for as long a period as expected during the fi rst couple of years, due to leakage. There are various reasons for leaks through dam walls but the most common in-clude:

• Inadequate compaction resulting in air and water fi lled voids which become water channels over time.

• Old tree roots and tunnels of small animals which form channels for water to escape through.

• Porous materials which are not sealed properly with water tight materials in the dam wall.

• Dam walls which do not key into impervious rock or soil layers under the dam wall.

Preventing dam walls from washing out (breaches)There are several reasons why a dam wall gets washed out but the most common one is that the spillway becomes blocked, or was made

94


too small and fails to discharge fl ood water fast enough. The water level in the reservoir therefore overfl ows the dam wall at its lowest point and causes a washout of that section, or perhaps of the whole dam wall.

The most common problem with pond embankments is that blocked and silted overfl ows cause the water level to rise so that the embankments are acting as structural dam walls. These are not strong enough to hold water and will breach when the pan/pond gets too full. This usually damages the embankment and the excavation.

There are several ways to prevent washout of dam walls and ponds:• Obstructions such as trees and bushes, carried into a reservoir

by fl oods and block the spillway should be cleared immedi-ately.

• Dam walls must always be maintained with their crests at least 10 per cent higher at the middle (convex) than at the ends to prevent a breach at the centre.

• Dam walls should always be carefully compacted. The height of dam walls must be increased 30 per cent to compensate for the se� lement of soil when the reservoir is fl ooded.

• The freeboard may reduce to 1.2 m a� er a reservoir has been fl ooded several times and the soil in the dam wall has se� led completely.

• Erosion of the dam wall should be controlled. This can be done using riprap (stones placed along the upstream face) or by planting grass (but not trees or shrubs) along the dam wall.

• Routine maintenance of dam walls should include removing roots, repairing cracks and sealing tunnels of burrowing animals.

Breached dams are diffi cult to repair and the cause of the breach should be fi xed before repair work is started. Repairing a breach requires re-constructing part or all of the dam wall and compacting it thoroughly (see the photo section for an example of a serious washout in process).

Preventing spillways from washing outSpillways can be washedout to such depths that they drain all fl ood water out of their dam reservoirs either due to erosion caused by excessive fl ood water, or because the fl oor of a spillway was not made to withstand erosion. Spillways should be designed to take the maximum possible fl ood fl ow without damage. In some cases the runoff from the catchment is greater than originally estimated (due to erosion or changes in land use) and the spillway may need to be enlarged or the design changed to accommodate the increased fl ood fl ow.

95


Here are several ways to prevent washed out spillways:• Planting drought-resistant and short perennial grasses with

runners (star grass or Kikuyu grass) in contour lines spaced about 30 cm across the fl oor of spillways.

• Cover the fl oor of spillways with stones packed closely togeth-er and inter-planted with the types of grass mentioned above.

• Construct low walls of stone-masonry, called “sills”, as horiz-ontal steps across the fl oor of spillways where they will function as a staircase for the overfl owing water. The walls should be built 30 cm below ground level and about two metres apart.

• The slope of the spillway fl oor should be maintained at less than 3 per cent.

5.6 MonitoringPreventative maintenance is the key to ensuring a long life for a pan/pond or dam. Regular monitoring of the embankments, reservoir and catchment area should be carried out to make sure that maintenance needs are identifi ed early enough to take action.

Monitoring should be carried out by the individual or commi� ee responsible for management and needs to be more frequent in the fi rst year a� er construction, preferably once a month. Ideally a technician should assist in the monitoring for the fi rst year. A� er the fi rst year monitoring can be carried out once per year, preferably just a� er the rainy season.

ToolsChapter 6

T hroughout the previous sections, especially chapters 3 and 4, reference is made to several techniques and tools for assessing and surveying sites. All such tools have been gathered in this

chapter, to make them easier to fi nd and use. Before going to the fi eld, you may wish to make a photocopy of this section to use as a reference, or as a handout for training. Then you can make notes on the pages without marring the book.

For more complex formulas and site or soil survey methods, there is a list of reference materials immediately after the Tools chapter.

98

Chapter 6 • Tools

Tool 1. Equipment for surveying valley sites for small earth dams

1. A 1:50,000 contour map of the catchment area if available.2. A circular water level to measure horizontal levels (described below).3. A panga for cu� ing pegs.4. Approximately 20 marker pegs cut from branches.5. A mason’s hammer.6. A shovel, spade and ma� ock for digging test pits.7. Two tape measures, 30 m and 50 m long.8. Long nylon string.9. Notebook and pencils.10. Ten transparent plastic bo� les and plastic bags for soil samples and

marker pen.

How to make a circular level Use a 1 metre length of transparent hose pipe. First, the pipe is half fi lled with water, then bent into a circle. Fit the two ends of the pipe together by heating the ends and sealing with tape (see Figure 26).

Figure 26. Using a circular water level made of transparent pipe.

99

Chapter 6 • Tools

Two or more horizontal points can be located by sighting along the two water levels in the pipe towards another person standing at same eye level. When the two persons stand on sloping ground, the gradient is found by knowing a) the horizontal distance between the two persons and b) the vertical distance to the sighting level and the eye level of the person standing some distance from the person using the circular level.

How to use a line level to measure slope A simple line level can be used to estimate the slope at a site. The tool requires three people to use it. The equipment required is:

• Two graduated wooden boards (graduations at 5cm intervals)• A spirit level• 10-metre length of string

Procedure

• The string is held at the same graduation mark on both wooden boards

• One person moves down the slope while the other remains upslope. The third person remains at the middle of the string to read the spirit level.

• The upslope person moves the string mark from the fi rst grad-uation downwards until the bubble of the spirit level centres.

• The graduations are counted and then expressed as a percentage drop for that part of the slope. For example: 1 graduation = 5cm = 0.5 per cent slope.

100

Chapter 6 • Tools

Tool 2. How to collect soil samples1. Determine the centre line of the top of the proposed dam wall. Peg

out a nylon line to mark it. Dig test pits spaced 5 to 10 m apart under the centre line. The pits should be dug deep enough to go through all layers of sand and soil until bedrock or another impervious layer is reached.

2. Take soil samples of the various types of soil in each test pit. Note carefully the depth from the surface from which the sample is taken. The amount of soil taken in each sample should be at least suffi cient to almost fi ll the plastic bo� le being used for the soil testing.

3. Put the soil samples in plastic bags marked with the number of the test pit and the depth from where it was taken in that test pit. Test the soil texture as described in Tool 3.

4. If the test pits prove that the base for a dam wall is fi rm and water tight, further test pits should be dug in the fl oor of the reservoir in rows 5 m apart. These rows should be at a distance of about 15 m from the dam wall centre line (see Figure 27). These pits will show the best place for a borrow pit from where soil could be excavated for construction of the lower part of the dam wall. Soil for the upper part of the dam wall can be taken from excavation of the spillway(s).

5. Draw a sketch showing the test pits and their soil profi les. Show on this sketch the number and depth of each soil pit and their location relative to the proposed dam wall. For each pit, sketch the soil profi le. Show the depth of topsoil, amounts of sandy, silty or clayey soil and the depth to the subsoil or bedrock.

Figure 27. Plan of dam site with test pits and their profi les.

101

Chapter 6 • Tools

Tool 3. How to test soil textureMaterials needed:

- Transparent plastic bo� les (all of equal size) with caps - Soil samples from test pits - Clean water- Salt

1. Take the soil sample from the plastic bag and fi ll each bo� le one-third full with soil (see Figure 28).

2. Add water to the bo� les until they are two-thirds full and add a pinch of salt.

3. Replace the cap and shake the bo� le vigorously for one minute.

4. Leave the bo� les for one hour and then shake them again.

5. A� er four hours, measure the thickness of each soil layer.

6. If there are four layers the top layer will be clay, the next silt, then sand and gravel at the bo� om.

7. By dividing the thickness of each layer by the total thickness of the soil and multiplying by 100 per cent we can calculate the percentage (%) of each soil type according to it texture, as shown below. Clay is the fi nest (top) layer and gravel the coarsest (bo� om).

Figure 28. Texture test for soil samples using plastic bottles.

102

Chapter 6 • Tools

Poor soil

SoilNo.6

SoilNo.1

water

SoilNo.2

water

Bestsoil

SoilNo.3

water

SoilNo.4

water

SoilNo.5

water water

less seepage more seepage

Tool 4. How to measure soil seepageMaterials needed:

- Transparent plastic bo� les, all of equal size with bo� oms cut off - Soil samples from test pits - Clean water

1. Remove caps and cut off the base of the bo� les and place them upside-down in sand. Support the bo� les with stones if necessary (see Figure 29.

2. Take the soil sample in its plastic bag and make sure the soil is bro-ken into small particles.

3. Fill each bo� le half way with soil from each sample and pour water onto the soil until the soil is saturated, or until it can absorb no more water.

4. Fill the bo� les with more water and then compare the rate of seep-age of water through the soil. The slower the water seeps through the soil the be� er it is for constructing the dam.

Note: A slow seepage rate suggests a high clay content. This can be verifi ed in a separate test to determine the soil texture shown in Tool 3.

Figure 29. Seepage test for soil samples using plastic bottles.

103

Chapter 6 • Tools

Tool 5a. Calculating maximum fl ood levelTo determine the maximum fl ood level in a valley, fi rst stretch a long tape across the valley at the correct level. Next, using a shorter measuring tape, measure the depth of the fl oor at 1 metre intervals. The profi le of the maximum fl ood cross-sectional area is then drawn on a piece of graph paper. The area can be estimated from counting the squares (see Figure 30). If a scale of 1 metre to 1 centimetre is used on standard 1cm graph paper, each square centimetre on the paper will equal 1 square metre. Each smaller square (millimetre squares) will equal 0.01 square metres. By adding the squares together the cross-sectional area of the fl ood water in the valley at the height of the fl ood can be calculated.

Max. flood level 10m

Tool 5b. Calculating approximate spillway size

The following is a simple way of calculating the size of spillways at the feasibility stage for very small dams. First, estimate what the maximum fl ood level is on the proposed dam site (above). For example, the illustration shows the width of the fl ood water in the valley at peak fl ow is 10 metres, and the average depth is 0.5 metre (see Figure 31). The cross-sectional area is 10m x 0.5 = 5m2. To estimate the cross-sectional area of the spillway, add 20% to the cross-sectional area of the maximum fl ood level, or multiply the fi gure by 1.2. In the example shown, the spillway will have a cross-sectional area of 1.2 x 5m2 = 6m2.

Figure 30. Figure 30. Measuring the maximum fl ood area of a valley. Measuring the maximum fl ood area of a valley.

10m5sq.m.1m

Max flood area =10m width x 1m depth/2 = 5m sq

6m

min 6sq.m.1m

Minium required spillway width:Max. flood + 20% =5m sq max. flood area + 20% =6 m width

Figure 31. A spillway should be about 20% larger than the maximum fl ood area.

104

Chapter 6 • Tools

Tool 6. How to calculate required storage volume: two methods

Method AEstimating the approximate capacity of the reservoir

To estimate the approximate capacity of a reservoir use the formula: V = W x T x D /6

Where V is the reservoir capacity in cubic metres. W is the maximum width of the reservoir when full in metres.T is the throw back (length) of water of a full reservoir in metres.D is the maximum depth of water of a full reservoir in metres.

Example: To estimate the storage capacity of a curved dam reservoir with maximum width of 40 metres, throw back length 150 m and maximum depth 4.5 metres.V = W x T x D / 6Reservoir volume = 40 m x 150 m x 4.5 m / 6 = 4,500 m3

Method B

Use the contour map of the dam site to determine the storage capacity of the reservoir, as follows:

1. For each contour starting from the base of the dam, determine the surface area enclosed by the contour.

2. This can be obtained by using graph paper. The area is determined from the graph paper by counting the total number of squares enclosed by the contour.

3. The actual area enclosed by each contour is then obtained by mul-tiplying the total area of the squares with the area scale factor from the map.

4. If the scale used is 1:1,000, then 1 cm on the map represents 10m on the ground and 1cm2 represents 100m2.

5. For an enclosed area of 10cm2 from the graph paper, the actual area enclosed by this contour will be 10 x 100 m2 or 1000m2..

6. Calculate the storage volume between two contours by adding the two surface areas together and dividing them by 2 to get the average area which is then multiplied with the distance between the two areas (which should be 1 metre).

105

Chapter 6 • Tools

Table 7 shows the calculations for each contour.

Table 7 Calculations for each contour

Contour (m) Surface area enclosed by

contour

Computed volume (m3)

Cumulative volume m3)

86 0

87 39.1 19.5 19.5

88 234.8 136.9 159.2

89 778.6 506.7 665.9

90 1,580.6 1,184.6 1,850.5

91 5,871.6 3,731.0 5,581.5

7. From the tabulated results, plot a graph of dam height against cu-mulative storage.

8. From the graph determine the required dam height which will give you the required water storage capacity. Don’t forget to subtract a small percentage for siltation storage.

9. Add 1.5 m of free board to the dam height to establish the gross dam height.

106

Chapter 6 • Tools

Tool 7. Sample construction plan

Activity How it will be done Who does what

Technical survey and design Existing water reticulation and proposed extentions to be surveyed and designed by a team from the Ministry of Water Development

Unskilled labour (clearing etc.)CommunitySurvey teamMinistry of WaterCoordination: TA

Rehabilite intake TA to coordinate desilting, weir rehabilitation and bush clearing

Manual works:Community

Masonry works:Hired artisan and plumber

Construction of storage reservoir/dam embarkments

Technical team and the community will undertake the construction works

Bush clearing, digging foundation:Community

Design and technicalsupervision: TA

Pipeline A qualifi ed plumber will be hired to install the pipeline

Bush clearing, trenching, back-fi lling and transportation of pipes: Community

Pipe laying and fi xing of fi ttings: qualifi ed plumber Supervision: TA

Construction of community water points

The water point will be constructed by the community, plumber and mason

Excuvation and provision of unskilled labour: Community

Masonry works and fi xing of tap stand: plumber and mason

The construction plan details activities that will be undertaken and who will do these activities. Note TA = Technical Assistant.

107

Chapter 6 • Tools

Resources needed Resources available(Provided by community)

Resources from external sources

Ø Unskilled labourØ Survey teamØ Meals/accommodationØ Transport

Ø Unskilled labourØ Meals

Ø Survey teamØ TransportØ Accommodation

Ø Unskilled labour Ø Artisan & plumberØ CementØ Reinforcement barsØ Water proof cement Ø Gunny bags

Ø FittingsØ TransportØ SandØ BallastØ MealsØ Manual transportØ Supervisor

Ø Unskilled labourØ SandØ BallastØ Manual transportØ Meals

Ø CementØ Reinforcement barsØ Water proof cementØ Gunny bagsØ FittingsØ Artisan and plumberØ Hardcore

Ø ArtisansØ Unskilled labourØ CementØ BRC wireØ Reinforcement barsØ Pipes/fi ttingsØ Sand

Ø WaterØ BallastØ BlocksØ Fence supervisorØ StorageØ Meals

Ø Unskilled labourØ SandØ WaterØ BallastØ FenceØ StorageØ Meals

Ø CementØ Reinforcement barsØ Waterproof Ø CementØ Pipes & fi ttingsØ ShuttersØ MasonsØ Hardcore

Ø PipesØ TransportØ PlumberØ StorageØ Meals/accommodationØ SupervisorØ Unskilled labourØ Manual transport

Ø StorageØ Unskilled labourØ MealsØ Manual transport

Ø PipesØ TransportØ AccommodationØ PlumberØ Hardcore

Ø Stand pipesØ ConcreteØ SandØ ArtisanØ Unskilled labourØ Cement

Ø TapsØ DoorsØ FittingsØ Meals/ accommodationØ TransportØ Supervisor

Ø Unskilled labourØ SandØ WaterØ Meals

Ø CementØ PipesØ ArtisansØ TapsØ FittingsØ AccommodationØ TransportØ Supervisor

108

Chapter 6 • Tools

Tool 8. Options for compacting the dam embankment

Motorised machineryA scraper is a specialised machine for scooping and depositing soil on an embankment (see photo section, photo 12). A scraper can compact the soil to some extent but a “sheep foot” roller is required for full compaction of larger dams.

Draught animal tractionEquipment required for compaction using draught animal traction:

• Two 200 litre drums with a piece of pipe welded to each end.• A wooden or metal axle to join the two drums so they can rotate

freely.• Harness and rigid sha� s for oxen or donkeys to tow the

drums.Procedure

• Fill drums with water and close the bung holes.• A� er each 50cm layer of soil is applied, add suffi cient water to

moisten then roll the drums across the layer several times until fully compacted.

• Oxen hooves also assist in compaction as they move across the dam.

ManualWhere machines or draught animals are unavailable, small dams have been compacted manually but the compaction achieved is not adequate and should only be used for small, household ponds. It is preferable to increase the se� lement allowance to 30 per cent (of embankment height).

Procedure• Where water is easily available (rarely the case in drylands!),

apply just enough water to moisten the soil.• Use a heavy wooden plank or concrete mallet to compact each

layer of soil.

109

Chapter 6 • Tools

Tool 9. Leakage problems in reservoirs and recommended solutions

Problem Reason Solution

1. Water disappears through the fl oor of a dam reservoir.

The fl oor was not prepared for being water-tight.

• Holes made by rodents, rotten tree roots, old ant-hills, forgotten pits and trial pits drainwater into the underground and must therefore be closed with clayey soil and compacted. Stones and boulders must be removed from the fl oor so water does not seep along them into the underground. If some boulders are too large to remove, these should be covered with a thick layer of clayey soil to prevent seepage.

• Should a dam reservoir still leak after the fl oor has been prepared as described above, the fl oor should be compacted by either driving a tractor or a herd of cattle over the fl oor of the reservoir repeatedly until the soil has been compacted fi rmly.

• Should the fl oor of a reservoir still leak after compaction, it can be sealed (puddled) with a layer of water-resistant materials, such as clay, powdered ant-hills or lime, which is compacted onto the fl oor.

• Floors can be covered with high density polyethelene sheets to make them water tight. This is an expensive solution and the sheeting is easily damaged by livestock. Desilting without damaging the sheeting is also diffi cult.

2. Water seeps through a newly built dam wall.

The soil in the dam wall contains air and water- fi lled voids.

• The voids will be compressed and the seepage sealed by the weight of the soil in the dam wall itself, when the soil gets moist and softened by water infi ltrating from the reservoir fi lling with water.

• If leakage continues, further compaction of the dam should be considered

3. Water seeps under the key of a dam wall

The key does not seal — a sandy layer is situated deep under the key.

• The layer of sand can be sealed by placing a vertical membrane or barrier made of thick plastic and/or ferro-cement along either the upstream or the downstream toe of the dam wall.

110

Further reading

Gould, J. and Nissen-Petersen, E. 1999. Rainwater Catchment Systems for Domestic Supply. Intermediate Technology Publications, London, UK.

Hatibu, N. and Mahoo, H.F. 2000 Rainwater Harvesting for Natural Resources Management: a planning guide for Tanzania, Technical Handbook No. 22, RELMA/Sida, Nairobi.

Kenya-Belgium Water Development Programme. 1992. Guidelines for the design, construction and rehabilitation of small earth dams and pans in Kenya, Nairobi, Kenya

Mburu, C.N. 1995. Management of watershed and silt load. Kenya-Belgium Water Development Programme, Nairobi.

M. T. Hai. Water Harvesting: An illustrative manual for development of microcatchments, techniques for crop production in dry areas, Technical Handbook No. 16, RELMA/Sida, Nairobi, Kenya.

Nissen-Petersen, E. 1990. Small earth dams built by animal traction.Danida, Kenya.

Norton, J. 1997. Building with Earth. Intermediate Technology Publications, London, U.K.

Orlate, M.J. 1995. Guidelines for community participation in dams and water pans construction and rehabilitation. Kenya-Belgium Water Development Programme, Kenya.

Smout I. and Shaw R. 1996. Technical brief 48: Small earth dams, Waterlines, Volume 14, No. 4, p.15-19, Intermediate Technology Publications, London.

Bibliography

111

Annex 1. Sample feasibility report for dam/pan

INTRODUCTIONExplain why this feasibility is being undertaken.

PROJECT BACKGROUNDProject location and history

Objectives of the projectProvide SMART objectives (see p. 17) as agreed with the community.

Proposed project activitiesEither construction of dam or pan or rehabilitation of pan plus any abstraction works and follow on activities.

Current water demandDescribe the benefi ciary community and the likely water uses.

Water demand formItem Population Consumption

rate (l/day)Total (l/day)

People 20Camels 15Cattle 15Sheep and goats 3.5Donkeys 15IrrigationTotal (l/day)Total (m3/day)*

* divide total litres by 1,000

Other water sourcesDescribe alternative water sources and their distance from the users.

Health and hygiene issuesDescribe hygiene and sanitation practices and describe contamination potential from catchment. State likely water quality and propose measures to address poor quality.

Annex 1

112

PROJECT ORGANIZATION AND MANAGEMENTMembership and project committeeDescribe community organization and management structure. Include details about by-laws and/or any proposed measures to ensure sustainability of the project.

Legal status of the landEstablish ownership of land and/or take measures to ensure access for all users.

Operation and maintenance issuesDescribe plans to ensure proper operation and maintenance.

Confl ict Describe any potential confl icts that might arise over pan/dam construction and suggest ways to mitigate them.

ENVIRONMENTAL AND SOCIAL/ECONOMIC IMPACTSCatchment condition and dam siltationDescribe catchment and possible erosion and siltation risks. State proposed measures to control siltation.

Catchment conservation measuresList proposed measures to conserve catchment and reduce erosion.

Project impacts:

Positive impactsList the ways that the dam/pan will benefi t the community.

Negative impactsList the possible problems that the project may cause.

Annex 1

113

Project fi nancingItem Quantity Units Rate Cost %

Project coordinationCommunity trainingManagement committee training Technical supervision (8%)Civil works

TOTAL

Project fi nancing Quantity Units Rate Amount %

Community contribution

Donor contribution

Total

Include explanation of community contribution.

CONCLUSIONSEnvironmental impactsDo positive impacts outweigh negative impacts?

Organization and managementDoes the community have the capacity to manage the pan/dam?

Technical issuesIs the dam/pan technically feasible?

Project cost and fi nancingIs the project fi nancially feasible?

Annex 1

114

Crop Days to maturity/ harvest

Annex 2. Climatic, soil and water requirements for selected crops

Banana

300–365

Beans Fresh: 60–90Dry: 90–120

Cabbage

100–150+

Citrus 240–365

Maize 90–150

Onions 100–140

Peas (garden)

Fresh: 65–100Dry: 85–120

Pepper 120–150

Pineapple More than 365

Potato 100–150

Tomato 90–140

80–110

Temperature re quire ment for growth (ºC)optimum (range)

Common grow ing al ti tude(metres above ea level)

Specifi c cli mat ic con straints/requirements

Day-length re quire ment for fl ow er ing

25–30 (13–38) 0 –1,800 Day neutral Sensitive to frost

15–20 (10–27) Short day/day neutral

Sensitive to frost; ex ces sive rain, hot weather

15–20 (10–24) Long day Short periods of frost (-6 to -10ºC) not harm ful;

23–30 (13–35) Sensitive to frost (dormant trees less so), strong wind, high hu mid i ty; cool win ter or short dry pe ri od preferred

0–2,000 Day neutral

24–30 (15–35) Short: 0–1,000Medium: 1,000–1,800Long: 1,800–2,400

Day neutral/short day

Sensitive to frost; ger mi na tion temp. >10ºC; cool tem per a tures cause problems in ripening

15–20 (10–25) Long day/day neu tral

Tolerant of frost; low temp. (< 14–16ºC) re quired for fl ow er in i ti a tion; no ex treme temp. or ex ces sive rain

15–18 (10–23) 1,800–2,300 Slight frost tol er ance when youngDay neutral

18–23 (15–27) > 1,500 Sensitive to frost Short day/day neutral

22–26 (18–30) 0–1,700 Short day Sensitive to frost; requires high RH; qual i ty affected by temperature

15–20 (10–25) 1,800–2,900 Long day/day neu tral

Sensitive to frost; night temp. < 15ºC re quired for good tuber initiation

18–25 (15–28) Day neutral Sensitive to frost, high RH, strong wind; optimum night temp. 10–20ºC

22–30 (18–35) 0 – 1,000 Short day/day neutral

Sensitive to frostWater melon

Annex 2

115

Source: Mod i fi ed from Doorenbos and Kassam 1986.

Crop Soil requirement Sensitivity to salinity

Water re quire -ment (mm/growing period)

Sensitivity to wa ter supply

Annex 2 (continued)

Deep well-drained loam with out stag nant water; pH 5–7

Banana Sensitive 1,200–2,200 High

Deep, friable; well drained and aerated; opt. pH 5.5–6.0

Beans Sensitive 300–500 Medium-high

Cabbage Well-drained; opt. pH 6.0–6.5

Moderately sensitive

380–500 Medium-low

Citrus Deep, well-aer at ed, light to me di um-textured soils, free from stagnant water; pH 5–8

Sensitive 900–1,200 Low–medium-high

Maize Well-drained and aerated soils with deep water table and without waterlogging; opt. pH 5.0–7.0


Onion Medium-textured soil; pH 6.0–7.0

Sensitive 350–550 Medium-high

Peas Well-drained and aerated soils; pH 5.5–6.5

Sensitive 350–500 Medium-high

Light- to me di um-tex tured soils; pH 5.5–7.0

Pep per Moderately sensitive

600–900 (1,250) Medium-high

Pineapple Sandy loam with low lime content; pH 4.5–6.5

700–1,300 Low

Potato Well-drained, aer at ed and po rous soils; pH 5–6


500–700 Medium-high

Tomato Light loam, well drained without waterlogging; pH 5–7



Wa ter mel on Sandy loam pre ferred; pH 5.8–7.2



Annex 2

116

Item Labour Quantity Units Rate (Kshs)

Cost (Kshs)

Skilled labour

A Site Foreman Days

B Mason Days

C Plumber Days

Unskilled labour

A Excavation & moving soil days or m3

B Spreading & compacting days or m3

C Grassing m2

D Miscellaneous days

Equipment

A Wheelbarrows (specify type) No.B Pick axes No.C Shovels No.D Pangas/machetes No.E 20-litre water jerry cans No.F Compaction hammers for

compacting soil (size to be specifi ed)

No.

G Crossbars No.H Rock hammers No.I Rock chisels No.J Timber for wheelbarrow ramps No.K String No.L Notebooks No.M Water pump for dewatering No.N Rakes No.O Gunny sacks for carrying rocks No.P Metal basins (karais) for mixing

concrete No.Q Equipment for pressure testing

draw-off pipes No.R Trowels for concrete work No.S Plumbline No.

Annex 3. Bill of quantities worksheet 1 (based on labour)

Annex 3

117

Item Labour Quantity Units Rate (Kshs)

Cost (Kshs)

Materials (delivered on site)A No. 2” GI Class B as draw-off pipe LM B Anti-seepage collars 300 mm Dia. X

6mm thick paddle fl ange to offtake pipe surround

NO

C 2” GI pipe for pipe upstand LM D 2” GI lead in and lead out for draw-

off pipe LM

E 2” perforated GI pipe inlet LM F 2” GI 90 deg elbow No.G 2” water meter with all fi ttings No.H 2” gate valve No.I 2” union No.J 2” socket No.K Sand m3

L Ballast m3

M Water m3

N BRC reinforcement for spillway sill m2

O Grass seed Kg P Staff rods Item Q Paint Litres

Materials carried to collection

TransportA WaterB Misc. materialsC Labour

Transport carried to collection

CollectionLabourEquipmentMaterialsTransportSubtotalAllow 5% for contingencies

Grand total

Annex 3

Annex 3. Bill of quantities (continued)

118

Annex 3

Item Item description Quan. Units Rate (Kshs)

Cost (Kshs)

PreliminariesA Mobilization and de-mobilization to/from

site including tidying up site Item

B Allow for river diversion as needed Item

Preliminaries carried to collection

Excavations and earthworks(PROVISIONAL)

A Clear site and borrow area of tree/bushes/stumps and cart away (area under NWL and borrow area)

m2

B Excavate to remove top soil average 250mm deep and stack for reuse or cart to spoil as instructed

m3

C Excavate in soil to depth not exceeding 4.00 m for cut-off trench, stack for reuse or cart to spoil as appropriate

m3

D Excavate in soft rock for cut-off trench m3

E Excavate in hard rock for cut-off trench m3

F Excavate borrow material for cut-off trench

m3

G Place and compact material in cut-off trench

m3

H Allow for de-watering cut-off trench

I Excavate approved borrow material for embankment

m3

J Place and compact approved material in embankment

m3

K Place 150 mm top soil on the dam crest m3

L Place approved handpacked riprap 300mm thick as upstream face protection

m3

M Place 150 mm top soil on downstream face

m3

N Provide approved grassing to specifi ed embankment slopes and dam crest

m2

O Excavate for seepage drain as specifi ed m3

P Place rock pile/riprap for rock toe drain as per the drawings

m3

Q Excavate for spillway in soil as specifi ed m3

Annex 3. Bill of Quantities worksheet 2 (based on rates)

119

Item Item description Quan. Units Rate (Kshs)

Cost (Kshs)

Excavations and earthworks carried to collectionDraw-off works (PROVISIONAL)

A Provide and install 1 No. 2” GI Class B as draw-off pipe

LM

B Provide and place in anti-seepage collars 300mm Dia. X 6mm thick paddle fl ange to offtake pipe surround

No.

C Provide 2” GI pipe for pipe upstand LM

D Provide and install GI lead-in and lead- out for draw-off pipe in 2” GI

LM

E Provide and install 2” perforated GI pipe inlet

LM

F Install rock fi lter to pipe inlet Item

G 63mm uPVC Class B drain pipe from valve chamber sump

LM

H 2” GI 90-deg elbow No.. I 2” water meter with all fi ttings No..

J 2” gate valve No..

K 2” union No..

L 2” socket No..

M Allow for pressure testing both pipes Item

Concrete works & reinforcement (PROVISIONAL)

A Provide and place 300 mm wide concrete Grade 25 as spillway sill

m3

B Provide BRC reinforcement to spillway sill concrete

m2

C 4 m grouted rubble stone to spillway apron

m2

D Provide and place rubble stone erosion barriers as directed

each

E Concrete works carried to collection

F Excavations and earthworks

G Draw-off pipes carried to collection

H Concrete works & reinforcement -

Subtotal Allow 5% for contingencies

Grand total -

Annex 3

Documents

Water from ponds, pans and dams