The process of designing and subsequent construction of a small earth dam/water pan starts with the preparation of design documents. In this article, we will review the contents of a typical small dam/pan report
1. Overview of the contents of small earth dam/water pan design report
A typical small dam/water pan report has eight sections namely; the executive summary, introduction/background, problem statement, catchment characteristics, design criteria, cost estimates/bill of quantities, operation and maintenance, and Annexes and references. Each of this sections, has specific information that is crucial in the overall design of the dam. In the next sections, we will look at each of this sections in a bit more detail.
2. Executive summary & introduction
This section contains a summary of the entire report, from the introduction, all the way to the conclusion. The section is usually about 2 pages long and is followed by the introduction. The introduction provides a detailed background of the project location where the dam is to be located. Some of the vital pieces of information in this section include the coordinates of the project area and a map showing the key routes and the features of the area. Additional information contained in this section includes the agricultural activities in the area, the soil types, existing enterprises and the main economic activities. Here, reference can be made to the Farm Management Hand Book of Kenya, which has among others, the soils of various project locations in the country, rainfall patterns and main agricultural activities.
2. Problem statement
In this section, the problem that led to the design of the small dam or water pan is discussed. Typically, small dams/pans in Kenya are constructed to support either Soil and Land Management (SLM) activities or agriculture. This information should be spelt out clearly in the problem statement. When the dam is for SLM activities, the nature of the activities and the relevance of the dam on the same should be spelt out. Here, common justifications for constructing dams as part of SLM include to control run off, to prevent flooding and to promote ground water infiltration. For agricultural activities, dams can be justified by noting that they will supply water for irrigation or they may be part of ensuring climate change resilience by supplying alternative water source in the event of adverse climate change events such as droughts.
3. Catchment Characteristics
This section spells out the hydrological data of the project area. Thus, the vital pieces of information in this section could include the delineated catchment area of the project location, the climate, water sources and the catchment yield. Software such as Arc GIS come in handy when delineating the catchment area. The climate data of the project area can be found from the local meteorological stations or by referring to the Farm Management Hand Book of Kenya. Similarly, the water resources of the area can be found by referring to relevant maps and documents of the area. The catchment yield, on the other hand, is calculated based on equations provided in the Practice Manual for Small Earth Dams in Kenya. The yield is particularly important as it informs the engineer on the possible amount of runoff that can be extracted from the project area and stored in the soon to be constructed dam.
4. Design Criteria
This part of the dam report contains the actual design. The vital design data presented in this section include a water demand analysis, freeboard design, silt trap design, embankment design, spill way design and draw off works. The water demand analysis is conducted to determine the amount of water the users of the dam require. It is the primary justification for selecting the size of the dam. To the extent possible, the size of the dam should be able to meet the water demand of the area. The Freeboard, on the other hand, is the vertical distance between the top of the dam and water surface of the dam at full supply level. It is designed based on equations provided in the Practice Manual for Small dams in Kenya. As for the embankments, these are the walls of the dam the trap the water in the project location. The slopes of the embankment and crest width have to be designed based on the equations from the Practice Manual. Similarly, the spillway, which releases excess water from the dam during flood flows, is designed by referring to the relevant sections of the Practice Manual. Lastly, the draw off works are designed to facilitate safe abstraction of water from the dam. They include a shallow well and overhead pump in the case of water pans or the draw off, in the case of small dams.
5. Cost estimates (BOQ), Operation and maintenance & Annexes
These are the last sections of the dam report. The costs estimate contains the detailed bill of quantities and a subsequent summary. The operation and maintenance section, on the other hand, contains information to facilitate safe use of the dam/water pan. This information includes a Risk Assessment of the dam, procedures for embankment and spillway maintenance, crisis action plan, inspection schedule and procedures for record keeping. Lastly, the Annex and reference section contains all the reference information used in the design, maps, pictures etc.