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Catchment Possibilities and Choice of Reservoir Types Different Types of Reservoirs and their Advantages |
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Catchment Possibilities and Choice of Reservoir Types Different Types of Reservoirs and their Advantages |
Although we have learned that the most appropriate type of rainwater reservoir is an economic question, in many cases in developing countries the availability of building materials outweighs the economic factor.
This is an industrial product manufactured in many countries Where the material for this tank is available, there are at least three capacity sizes of 2.25, 4.5 and 9.0 m³ Although usually the most economical, prices have to be compared with other suitable materials; the transport aspect can also increase costs substantially. The advantage of this tank is firstly the price, but certainly also the fast installation. The disadvantage is the limited lifetime, although this can be improved as explained in Chapter 4. One should remember at all times that the corrugated iron tank is vulnerable to manual force. Experience has shown that this tank should not be used at public places, especially not at schools, since vandalism is likely to damage the tanks beyond repair.
Several industrial producers offer tanks of PVC foil. The foil is fixed inside a reinforcement mesh framework or galvanized sheet cylinders, screwed together from sections. The tanks are available from about 5.0 m³ up to 430 m³ Their considerable advantage lies in fast assembly and low transport costs. A reservoir of 9.25 m diameter (capacity 81.0 m³ can be transported on a small van and be assembled within a couple of hours. No foundation is needed. Dismantling and reassembly at another place can be carried out within a day or two. Apart from this advantage which is very valuable for cases requiring immediate action, for instance improvising a village water supply, the system has some weak points. Tanks of large capacity are uncovered, so evaporation is high and there is a danger of pollution. More important for permanent use is the problem of ultraviolet ray influence on the PVC foil. Systems in use show signs of ultraviolet light effect on the material after just a few years. Otherwise the vulnerability to external force is great and tanks should always be fenced in. For permanent rainwater catchment, although relatively cheap, this technique has its limitations.
3. The ferro-cement tank without mould
This technique as explained by Laurie F. Childers of UNICEF Regional Office in Nairobi, Kenya, in 1985 has been chosen by the author because of the unique advantages of this appropriate technology. There are many examples of such reservoirs in Kenya.
This technique depends on the availability of welded reinforcement mesh. Since this is not to be found everywhere, other methods can be substituted.
Firstly close attention has to be given to the cost of the material and the transport to the site. Any other material used for this tank is more or less the same as for all ferro-cement tanks. The width of the roll of mesh or mats will be the height of the tank wall, about 1.80 m. This is certainly a restriction. Theoretically, it is possible to extend the height of the wall by using one and a half widths of the mesh, overlapping it on a minimum of three fields and tying it together with the bottom circle, but this is not recommended. The entire structure becomes unstable and any vibration during the process of plastering will make the work very difficult. In addition a scaffold is needed which might not always be available. The fixing of the scaffold requires skilled workers.
4. The ferro-cement tank with a factory-made mould
The technique was described by N.J. Wilkinson, Botswana Technology Centre in his publication, and was chosen because of the considerable advantage it has for rainwater storage where all tanks are of the same size. Several examples of this are to be found in Botswana.
This construction method can only be chosen if a factory or experienced workshop provides the facilities for bending corrugated sheets and welding them neatly together. The technique is highly appropriate in areas where a series of tanks are to be built. This is the case when new buildings like schools are put up, and the design of the buildings already includes provision for rainwater catchment. In such cases we can talk of a standardized tank.
The mould can be used 10 - 15 times depending on the experience and careful handling of the staff. For larger projects it is advisable to have at least two moulds at the site. The advantage of this construction method lies in the rationalization of the work. The masons become experienced and work can be finished faster. With two moulds, the work can be organized with three crews. The first crew starts preparing the ground and then casts the foundation slab. The second erects the mould and reinforces it, and the third crew does the plastering. The roof slab can be made by a fourth crew or by the first, depending on the amount of ground to be cleared. This technique should not be introduced where only four or five reservoirs have to be constructed; in such a case the mould will be too expensive.
5. The ferro-cement tank with a made-on-site mould
E.H Robinson of the American Peace Corps describes this construction technique in the publication. It was tried by "Christian Action for Development in the Caribbean" (CADEC) in the Republic of Grenadines and chosen because of the advantages it offers over other methods.
This technology for constructing reservoirs should be chosen where only a few tanks are required or even jut one, in other words where prefabricated moulds are not considered and welded reinforcement mesh is not available. All that is needed, in addition to the normal building materials for a ferro-cement structure, is some additional timber for the framework and a few corrugated iron sheets for shuttering. Fencing mesh is an additional reinforcement but could be replaced by other available mesh matrial.
6. The reinforced brickwork tank
These were constructed by the author in Lobatse, Botswana, for public buildings. The reinforced bricktank is more expensive than the ferro-cement tank, although the cost per m³ reduces with increased capacity. It costs about twice as much as a ferro-cement tank. For this reason this tank should be chosen where the capacity needed is above 30 m³ and in all cases where the life of the structure is expected to be 20 years and more. The advantage of the construction method is the adaptability to the building design. Structures above 1.80 m in height are without problems, although plastering has to be done with great care. Especially at public buildings which are usually higher than residential houses, it is possible to use the height between gutter and ground, avoiding large diameters and thus saving space.