|Water Manual for Refugee Situations (UNHCR, 1992, 160 p.)|
|10. Water distribution systems|
12. Water moving through a piping system is subject to friction with the inner surface of the pipes and therefore continuously loses pressure in the direction of flow; this loss is proportional to the length of pipes, to the roughness of their interior and to the square of the velocity. These friction losses may be calculated by using formulas; different graphs may also be used for this purpose (See Fig. 35). This means that in a pipeline system with flow under dynamic equilibrium, pressure drops in the direction of flow in accordance to what is known as the hydraulic gradient, which also represents the energy levels at each point along the pipeline.
* All pipe diameter are in millimetres
13. The amount of energy remaining in the pipeline system by the time the desired flow has reached the distribution points is what is called residual head, and may be either positive or negative. While positive heads indicate the presence of energy in excess and that there is enough energy to move an even greater flow through the pipeline, negative heads would indicate that, within the pipeline, there is not enough energy to move the desired quantity of water. If a pipeline with a positive residual head is allowed to discharge into the atmosphere, the flow will increase until the residual head is reduced to zero; this flow, which for the given conditions of each pipeline is always maximum, is called the natural flow of the system. In a gravity fed pipeline, the natural flow should always be smaller than the safe yield of the water source (See 6.20 and 6.38), otherwise, the pipe would drain faster than it can be filled and the result will be that the pipe will not flow fully and any standpost located in this section would not function normally.
14. As already mentioned, high velocity flows within a pipeline increase friction losses. At the same time, with high velocities, suspended particles can also cause excessive erosion of the pipes; if the velocity is too low, these suspended particles may settle and collect at low points within the pipeline, which may even clog if provisions have not been made for sedimentation (See 8.14-16) of the water or for the provision of appropriate wash-out points for the pipeline (See 9.6).
15. Air blocks are bubbles of air that remain trapped, particularly at high points of a pipeline; their size may be such that they could interfere with the normal flow of water through this section. They may become very important (and problematic) in the case of pipelines which are subject to periodical drainage and refilling and provisions should be made to install air valves (See 9.6) at all high points of the pipeline.
16. The bases for the design of any pipeline is the graphic plotting of the topographic survey along the pipeline's route in the form of an "altimetric profile" showing the variation in soil elevations from the source to storage, treatment and distribution points. This survey should have been previously carried out as part of the basic studies to assess the beneficiaries' needs and to produce the conceptual design and budgets required for project approval and funding (See 5.1; 12.8). The hydraulic design comes next; the possibility of using gravity as the only driving force for the water to flow is assessed (See 10.12) and, if insufficient, the calculations for pumping requirements are made; all system components (including treatment facilities, storage, pumping and gravity mains, distribution lines and taps for which it may be possible to use standard models) are also designed (hydraulic and structural designs) and the final checking for hydraulic soundness and efficiency is done, bearing in mind the ultimate goal of providing a cost effective and reliable supply of safe drinking water to the refugees. The final drawings, showing all technical details of the system, are then finished and will accompany the topographical profile (showing also the pipeline's hydraulic gradients) and the planimetric map showing the exact location of all system components. Once this is done, the documents are ready for "blue-printing". Detailed estimates of materials, labour and money required for the construction are then calculated.
17. As mentioned before, the task of designing any water supply system should be entrusted to a qualified and well-experienced engineer. It will be the responsibility of this engineer to provide a complete record of his investigations, surveys, calculations and designs; this data will prove useful in the project approval and funding exercise, in the negotiations for project implementation (identification of implementing partners, tender procedures, contractual negotiations) and for supervision, operation and maintenance purposes (See 12.8-17). Such data should contain, at least, the following:
i) Pipelines: All relevant data on the different sections of the pipeline (pumping mains, gravity mains, branches, tap connections, etc.) (See 10.4), including pipe material, lengths and diameters. A planimetric map, at an appropriate scale, of the layout of each section of the pipeline, giving clear indication of the length and diameters of each pipeline component, the position of related structures (intakes, valve boxes, reservoirs, etc.)
ii) Surface water catchments, boreholes or wells: Description of the catchment, well or borehole as a water yielding structure (See 6.37; 6.54); results of test pumping and productivity assessments (See 6.38; 6.55); water quality characteristics (See 3.13).
iii) Intake sections: Sketches (using convenient scales) of the location of sources and the future structures to tap them; design drawings of these structures; calculations of construction needs (volumes of excavation, construction materials, etc.) and labour.
iv) Treatment facilities: details and scaled sketches of pre-treatment and treatment structures required (sedimentation, filtration, chlorination, etc.), including specific details of all piping and valves, construction requirements in terms of material, labour, special tools, etc. (See 8.8).
v) Break pressure (See 10.8) and reservoir tanks (See 9.2): Careful drawings of the designs are required, depicting all necessary construction details on the structure, pipe and valve arrangements; construction requirements in terms of material, labour, tools, etc.
vi) Distribution points: Drawings of each water outlet (individual connections to service, administration or staff accommodation buildings, public distribution standposts, animal troughs, etc.); construction requirements in terms of materials, labour, tools, etc. (See 10.9-11).
vii) Other system components: Drawings and other relevant details (location, construction characteristics, piping and valve arrangements, etc.) of special components such as valve boxes, river crossings, etc.
viii) Total estimates: Two lists, one for locally procured material and another one requiring purchase and transportation into the country or project area. Unit prices and total costs should accompany these lists. All tool requirements should also be presented, as well as other logistical details on transport of material and related costs (See 12.17).