1. Introduction
This manual is the 2nd Edition of “The Guidelines for the Design, Construction and Rehabilitation of Small Dams and Pans in Kenya” (MOWD, 1992).
1.1 Background
The First Edition of the Guidelines for the Design, Construction and Rehabilitation of Small Dams and Pans in Kenya was published through the Kenyan Ministry of Water Development in 1992 with assistance from the Kenya-Belgium Water Development Programme. The Guidelines have been widely used by engineers, technicians and contractors from both the public and private sectors.
In 2014, the Swiss Agency for Development and Cooperation decided to support a process of reviewing and updating the Guidelines to bring them up-to-date with current practice, to capture the experience of the last 25 years, and to provide a useful reference document for the design and development of water conservation structures into the future.
The updates to the first edition concentrate on the following areas:
- Changes in Technology: There have been significant improvements in survey equipment and availability of maps and terrain data since the original manual was published. In addition a much greater variety of construction equipment is now available in Kenya then in 1992.
- Wider Scope: The original manual limited itself to dams of less than 10m height and less than 100,000 cubic meters in storage volume. The revised edition bring this in line with the current Water Act 2002 classification for low and medium risk dams (i.e. less than 15m height, less than 1,000,000 cubic meters of storage and catchment area of less than 1,000 square kilometres).
- Different Types of Structures: The original manual concentrated mainly on earth embankments with a few brief sub-sections on other structures. The revised manual looks at alternative structures in more detail.
- New Legislation: The revised manual brings the original manual up to date with regards to current NEMA Regulations, the Water Act 2002 regulations and other relevant legislation.
- New issues (e.g. Climate Change): The revised manual cover issues that have emerged since the original manual was published in 1992. These include climate change and web oriented design tools.
- Incorporate lessons learned from use of 1st Edition: The revised manual makes use of the experience gained from using the 1st Edition.
- Standardization: Generic drawings, Bills of Quantities (BOQs) and calculation worksheets related to this Practice Manual have been made available through the complementary website.
The Practice Manual has been developed for general application on sites where the structures are within the boundaries of the limitations and restrictions described below. However, for larger and more complicated structures, especially those creating a significant hazard, or where there is doubt, reference should be made to other internationally recognised design handbooks, such as "Design of Small Dams" (United States Department of the Interior - Bureau of Reclamation, 1987), and use made of experienced professional dam engineers.
References to design manuals and internationally recognized textbooks providing in-depth coverage of various scientific and technical disciplines related to design and construction of dams form part of the bibliography attached to this publication.
1.2 Objective/Purpose
The Practice Manual is intended to provide a general reference for the design, construction and rehabilitation of environmentally appropriate small dams, pans and other water conservations structures in Kenya, with special emphasis on the specific problems encountered in relation to the establishment of small water conservation structures in the rural and ASAL (Arid and Semi-Arid Lands) areas of the country.
1.3 Layout of Manual
The manual has been laid out into 20 Chapters which address a full range of water storage projects. Chapters 1 to 11 deal with issues that are common to all storage projects. Chapters 12 to 18 look at specific types of storage structures and storage options. Chapter 19 provides the outline for various technical reports associated with water storage structures. Chapter 20 presents a detailed bibliography.
The manual does not contain any photographs. Photographs are available through the online version of the manual.
The manual contains a limited number of drawings. More detailed drawings are available through the online version of the manual.
1.4 How to Use Manual
The manual should be used as a general reference for anyone considering low and medium risk water storage projects. It is not necessary to use the entire manual, although the first 11 chapters should be of interest to all water storage projects. Individual chapters can be used as stand alone guides for a variety of water storage structures.
While the hard copy version provides details and formulas for calculations and design, the online version provides a selection of spreadsheets that allow fast, accurate and standardised design calculations and reporting.
1.5 Target Readers
The Practice Manual is intended for use by engineers, artisans, surveyors, developers, owners, and other practitioners involved in the development of safe, economic and environmentally appropriate small dams, pans and other water conservation structures.
1.6 Complementary Website
A complementary web-site is accessible through a portal on the ministry website (www.water.go.ke). The website contains additional materials, references and worksheets related to the content of the Guidelines. A soft copy of the manual and supporting material can be accessed from Useful downloads.
2. Definition, Classification And Limitations
This chapter clarifies the scope of the manual and elaborates on the specific size storage projects for which the manual is relevant.
2.1 Definitions
There is some confusion with regard to many terms used to describe dams, reservoirs, lagoons, pans, etc. For the sake of this manual, the following definitions have been adopted.
A dam is a barrier or wall designed and developed on a water course to confine and then control the flow of water. It will retain water upstream of the structure.
A pan is a structure developed through excavation or a natural depression to retain water. Water is retained below natural ground level.
A lagoon is a structure developed both through excavations below ground level and through construction of a retaining wall above ground level in order to retain water. They are typically lined with an HDPE or LDPE lining. Water is retained both above and below natural ground level.
A reservoir is the water retained by a structure.
As noted above, the terms are frequently used without precise application of the definitions. The critical aspect from an engineering and risk perspective is whether the structure is designed to retain water above natural ground level as this requires the application of engineering design to withstand the hydraulic pressures. In such a case, the retaining part of the structure should be treated as a dam.
Dams are frequently described by the purpose(s) for which the dam is built e.g. fish dam or multipurpose dam. The size and structure type (earth fill, rock fill, concrete arch...) can also be used to describe a dam.
Pans are also frequently described by the shape of the structure e.g. hafir or turkey nest dam.
Other terms that are increasingly coming into use with regards to water storage in Kenya are:
- Sand Dams:
Stone masonry or concrete walls designed to store water by retaining sand on the upstream side of the wall. The water is stored in the voids in the sand. Technically they are dams with mass gravity walls.
- Sub-surface Dams:
These dams, variously referred to as groundwater dams, make use of a stone masonry, concrete or compacted earth wall which is constructed across a sandy water course, to artificially raise the water level within the sandy medium on the upstream side of the wall. The wall acts as a retaining wall.
2.2 Classification Of Dams
Classification of dams by a single physical characteristic (embankment height, storage volume….) is straightforward to do but does not necessarily capture all the areas of concern with respect to the dam. Instead, a hazard based classification is generally used. A dam can be considered a hazard as it may cause inundation, physical and environmental damage and loss of life. As such classifying dams into different hazard classes helps to define the design specifications and acceptable risk associated with the different scale of hazard.
The ICOLD Bulletin 157 of 2011 “SMALL DAMS: Design, Surveillance and Rehabilitation” (ICOLD, 2011) develops a PHC (Potential Hazard Classification) for small dams that looks at physical characteristics, life safety risk, economic risk, environmental risk and social disruption that could occur in the event of a catastrophic failure. The system provides for three PHC classes, namely (i) low risk, (ii) medium risk and (iii) high risk.
The Water Resource Management Rules (2007) use a hazard classification system based on three factors, namely depth, volume and catchment area that result in three classes of dams as shown in Table 2-1. The factor that places the dam in the highest hazard class prevails. This system, which is roughly consistent with the ICOLD classification system described above has been adopted for this Practice Manual as its application is straightforward and not based on any subjective risk analysis.
Table 2-1: Classification of Dams
| Class of Dam | Maximum Depth of Water at NWL (m) | Impoundment at NWL ($m^3$) | Catchment Area ($km^2$) |
|---|---|---|---|
| A (Low Hazard) | 0 - 4.99 | <100,000 | <100 |
| B (Medium Hazard) | 5.00 - 14.99 | 100,000 to 1,000,000 | 100 to 1,000 |
| C (High Hazard) | >15.00 | >1,000,000 | >1,000 |
NWL = Normal Water Level
When using the table above, it is important to note that only one factor is necessary to place a dam into a higher hazard class. For example, a 3m deep reservoir with 20,000 cubic meters of storage and a 1,500 square kilometre catchment area would classify as a high hazard dam due to its large catchment area. Similarly, a 15.5m tall mass gravity wall with 25,000 cubic meters of storage and a 3 square kilometre catchment area would be a high hazard dam due to the maximum water depth.
2.3 Scope of Manual
This manual is intended for use with medium and low risk dams. As such it should only be used for Class A and B dams. The manual describes design procedures and provides minimum requirements for planning, design and construction of small dams, pans and other water conservation structures. The guidelines were developed to provide uniform criteria in order to ensure that these structures can be designed, constructed and operated in a standardized way in order to ensure consistent performance.
As new experience, materials, and knowledge become available, this document will need to be revised.
As with all manuals, a degree of judgement is necessary when applying the procedures, guidelines and requirements that are presented.
It is strongly recommended that for all structures with more than 10m of water depth that detailed soil and geotechnical studies are carried out.
Site specific factors should also be considered when using this manual. For example, a Class A dam located immediately above a populated area might be treated as a Medium Hazard or even as a High Hazard structure simply because of the risk to life in the event of a failure.
2.4 Limitations And Restrictions
This manual applies to all low and medium hazard structures. Requirements stated are minimum limits and more conservative requirements may be more appropriate in some situations. In some cases, problems may arise where proven solutions are not available or alternate procedures may need to be evaluated before the best solutions can be developed and selected. Experience, laws and regulations, investigations, analysis, expected maintenance, environmental considerations, and/or safety laws may dictate more conservative criteria to ensure satisfactory performance.
As with all engineering works, final responsibility for design and construction supervision rests with the engineers and organizations responsible for each specific project.