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It is virtually impossible to provide a general cost-bereft analysis of the different types of reservoirs and storage capacity because of the many unknown factors. However, the following general indicators for decisionmaking can be given.
1. The value of rainwater rises with increased distance to or inaccessibility of other water sources. This means that if rainwater becomes the only source, its value is extremely high. Thus the high investment in a large reservoir becomes cheaper in relation to the value. of water.
2. If rainwater remains the only source of water, rainfall pattems must be studied carefully. If the pattern shows a more equal distribution over a long observation period, it is possible to choose the size of a reservoir according to the precipitation, even on a semi-annual basis. Where the rainfall is extremely unevenly distributed with frequent drought periods, a reservoir should be as large as possible, based on the maximum rainfall. This is expensive but still economic after taking all other factors into account.
3. It is imperative to analyse the purpose of water use and the volume of consumption in advance. Only rough indicators can be given since the consumption will vary from case to case. Rural households in Africa often manage with 40-60 litres of water per day. As mentioned earlier, since easy access does not encourage saving, but on the contrary consumption increases, arrangements should be made to provide additional amounts. Water consumption rates for cultivation cannot be given since this depends largely on the type of crop and soil conditions. If the planned reservoir is expected to serve as a stand-by facility because of frequent breakdowns of a centralized supply, the size can be smaller and the capacity limited to the consumption of a few weeks, depending also on the rainfall pattern.
4. Access to construction materials is another factor to be considered. For instance corrugated iron tanks which are usually very economical might be available only hundreds of kilometres away and therefore become too expensive. Or if reinforcement mesh is not available, a ferro-cement tank cannot be built (see Table 4, indicating the material needed for different types of reservoirs.)
5. Life expectancy and maintenance demands are other factors to be considered. As the example of rainwater catchment at public buildings in Lobatse (Chapter 1.2) shows, under certain circumstances high construction costs combined with long service life expectancy can pay off. Maintenance, usually a weak point in developing countries, has to be taken into consideration. A decision on the capacity and type of structure to be chosen should take all these factors into consideration.
Attention should also be given to the following questions:
- For which purpose is the rainwater to be used?
- What is the likely monthly consumption of water?
- What amount of water can be harvested?
- What is the rainfall pattern, and how is rainfall distributed during the year?
- Which construction materials are available, which are unavailable?
- How high is the financial amount to be invested?
- By how much can costs of construction be reduced by self-help?
The following Table 3 offers assistance in decision-making. Costs are based on 1985 prices in Botswana where all materials are available.
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.
2.2.1 The corrugated iron tank
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.
Table 3: Different types of reservoirs
2.2.2 The PVC foil tank
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.
2.2.3 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.
2.2.4 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.
2.2.5 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.
2.2.6 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.
It is often heard that ferro-cement is a poor reinforced concrete and a second-class technology developed for Third World countries. Nothing could be more wrong.
Ferro-cement is a building material with some similarities to reinforced concrete. Indeed, both materials have the same source. Ferro-cement is produced by applying cement mortar composed of fine aggregate and cement onto wire reinforcement using plasterer techniques. As a result the property of ferro-cement distinguishes it from reinforced concrete. While of similar durability, it is more elastic than reinforced concrete.
A Frenchman, Joseph Monier (1823 - 1906), produced flower pots made of cement mortar reinforced with chicken wire and showed this product at the world exhibition held in Paris in 1867. J. Monier became known as the father of reinforced concrete. In Germany for many years reinforcement steel was called "Monier iron". In 1847, another Frenchman, Joseph-Luis Lambot, filed a patent for producing a cement boat, wire-reinforced, not long after the development of Portland cement. Which of the two men first had the idea of combining wire with cement mortar is of no interest. Probably the discovery technique happened by chance. At that time, the commonly known chicken wire was a handmade product and therefore soon too expensive in the fast growing industrial era. But the knowledge of the steel-concrete combination resulted in the development of reinforced concrete using large steel rods. During the First and also later during the Second World War, the technique of Lambot's ferro-cement boat was remembered in the U.S. and the U.K. and shipbuilders were encouraged to construct barges like this in order to save shipbuilding materials such as steel plates and timber. Although some of the boats built during the Second World War had an amazingly long life span, the technique did not really became widespread.
Table 4: Major materials needed for
different types of reservoirs and possible substitutes
It was the famous Italian engineer and architect, Pier Luigi Nervi, who first undertook real research into ferro-cement technology. He observed that reinforcing concrete with layers of wire mesh resulted in a material with high impact resistance properties. This material differed from reinforced concrete in its flexibility and elasticity. After the Second World War, Nervi built a 165-ton motor sailer. This ship, "Irene", proved to be seaworthy. Similar ships were built in the U.K., New Zealand and Australia, and one circumnavigated the world without problems. But Nervi would not have been a structural engineer and architect if he had not also used this material for building construction. In 1947, he first built a storehouse of ferro-cement. Later he combined reinforced concrete with the ferro-cement technique and constructed the famous Turin Exhibition Hall with a roof system which spans 91 m. Nervi's work proved that ferro-cement is a high quality construction material. The question remains why ferro-cement is relatively seldom used as a building material in industrial countries. The answer lies in the process of industrialization of construction work. In order to minimize the labour cost, construction work has become more and more capital-intensive. As a result, working processes have been mechanized wherever possible. In this context the possibilities for mechanizing ferro-cement remain very limited. A high percentage of labour cost will always characterize this technology. While this is considered to be a disadvantage for industrialized countries, it is a positive factor in developing countries where the labour market is characterized by high unemployment and low labour costs.
It has therefore to be emphasized that ferro-cement is by no means a second-class technology, but rather highly appropriate especially for countries where labour costs are low.
figure 2.1
To calculate the rainwater amount which can be harvested, the mean annual rainfall figure is commonly used. Mean annual is the statistical average calculated on the basis of measured rainfall over many years. It has to be understood that there is no guarantee that the calculated amount will be achieved, but there is a 95% likelihood that this amount can be expected. This near certainty diminishes to a probability if the rainfall pattern in a given area differs substantially. This is quite common in countries with drought periods. It can happen that the mean annual cannot be expected. It can certainly happen the other way round that considerably more rain falls than the mean annual. This makes the calculation of the storage capacity rather difficult. However, the mean annual is generally accepted as the basis. The size of, storage capacity chosen can be based on the mean annual, but should be greater if funds allow. Some countries provide maps where the mean annual rainfall is indicated along the line of occurrence. Fig. 2.1 shows the rainfall in Botswana; each line is marked with a figure giving the precipitation. The mean annual in a given area between two lines ranges from the lower amount, for instance 400 mm, to the higher average of 450 mm. For example the mean annual rainfall in Gweta is between 450 and 500 mm.
2.4.1 Reservoir capacity
As an example let us consider a roof of 120 m² in an area with mean annual rainfall of 450 mm. We assume that less than 100% of the calculated amount of water will be collected. This is due to unavoidable small leakages in the gutter downpipe system, or rainfalls which are too light to produce sufficient runoff, or a possible overflow of gutters in the case of an extreme downpour. For this reason we can generally assume that only 90% of the rainwater can be collected.
figure 2.2
For calculation we take the following formula:
mean annual rainfall in mm × area in m² × runoff factor = collected rainwater in litres. In our example this means:
450 × 120 × 0.9 = 48 600 litres.
In most cases it would be unrealistic to consider building a cistern of 48.6 m³ capacity for a house with only 120 m² roof area. However, as the situation differs from place to place, we cannot decide here whether a reservoir of this capacity will be realistic and economically efficient.
2.4.2 Roof type and catchment
The shape of any given catchment area has a considerable influence on the catchment possibilities. Therefore different types of roofs provide different catchment possibilities. Of the most common roof types shown in Fig. 2.2 the single pitch roof is the most appropriate for rainwater harvesting, since the entire roof area can be drained into a single gutter on the lower side and one or two downpipes can be provided depending on the area. A more difficult roof for rainwater catchment is the tent roof. It requires a gutter on each side and at least two downpipes on opposite corners. If a tent roof is large enough, it could be drained into four tanks located at each corner of the house. The main problem is always the corner. A 90° angle in the gutter should be avoided. It is extremely difficult to adjust gutters in such a way that water really flows easily downwards. It seldom works well when downpours occur, and it is the heavy downpours that should be caught. The hip roof is not very efficient either, since it also needs gutters all around the building. Flat roofs can be used for catchment if they are furnished with an edge, keeping the water on the slab until it has drained through the gutter or downpipe. However, using a flat roof for rainwater harvesting is not very efficient because of the extended runoff-time and the evaporation losses. One way to improve the catchment is to provide the slab with a sloping cement screed. Constructing a waterproof edge on a flat roof is rather difficult because of the temperature expansion.
The most useful roofs are the single and double pitch roofs. The double pitch roof offers many advantages. As the picture of Woodhall Community Centre in Lobatse, Botswana, shows, the gutter of the length of one side can be drained into a reservoir on the other side of the building by fixing the downpipe at the gable wall and sloping it towards the cistern.
2.4.3 Roof finish
Not all materials used for roofing finishes are equally good; but the most commonly used material, metal sheeting (corrugated galvanized iron and aluminium sheets), is very suitable for rainwater catchment; likewise, brick tiles of all variations, and also thatch can be used, but these are less efficient.
2.4.4 How to choose the size of a reservoir
Example I (see Fig 2.3):
A house with a roof area of 9.00 × 6.50 m is to be furnished with catchment and storage facilities. The mean annual rainfall is 450 mm.
Calculation of rainwater:
9.00 × 6.50 × 450 × 0.9 = 23895 litres
figure 2.3
The height from the ground to the gutter outlet is 3.00 m. According to Table 5, a reservoir of 4.0 m diameter on a filling height of 1.80 m has a storage capacity of 23 000 litres. This means that one reservoir built' et one gable side of the house would be sufficient for nearly all the rainwater which can be collected if an average rainfall occurs. Two gutters along the sides of the building should be connected with downpipes fixed to the gable wall and then bridged into the tank.
For this storage capacity a ferro-cement tank would be more economically efficient than the reinforced bricktank and serves the same purpose. But if a smaller storage capacity would be sufficient, or if funds are very limited, two corrugated iron tanks, each of 9 000 litres, would be cheaper. These two tanks could be located at each of the gable sides, collecting from each gutter, or next to each other on slightly different levels, draining 'the overflow from the ' tank connected to the pipes into the second tank. Fig. 2.4 shows this as an example with two corrugated iron tanks, but the same method is certainly possible with any other type of reservoir.
figure 2.4
Example 2 (see Fig. 2.5): Calculation of catchment area:
Roof A:
20.0 × 10.0 × 450 × 0.9 = 81000 litres
Roof B:
9.0 × 15.0 × 450 × 0.9 = 54 000 litres
figure 2.5
Total catchment per annum = 135 675 litres. About 136 m³ of rainwater can be caught within a year from 450 mm rainfall.
The size of the chosen reservoir depends on the lowest inflow (see Fig. 2.5) and also on the ground space available. Block B has a gutter height of 3.00 m, Block A height of 3.30 m. The lowest inflow would come from Block B. Since gutters and downpipes must slope towards the inflow, the height has to be calculated. For the gutters a 0.3% slope is the minimum requirement (equivalent to 3 mm per metre). Block A has a gutter length of 20.0 m (20 × 3 = 60 mm), a downpipe with a minimum slope of 10% (10 mm per metre) to the middle of the gable wall 5.0 m, which means another 50 mm for sloping.
We add the 60-mm slope of the gutters to the 50-mm slope of the downpipe resulting in 110 mm and add 15 mm for the distance from the gable wall to the tank inflow, resulting in 125 mm. For imprecise workmanship, measuring faults etc. we assume a total of 200 mm. These 200 mm have to be deducted from the height of 3.00 m between gutter and ground. This final measurement is 2.80 m and indicates the lowest inflow level and at the same time the filing height of the tank assuming that the bottom of the reservoir is level with the ground. The catchment capacity is about 135 000 litres at the most, with a filing height of 2.80 m. Table 5 shows a filling height of 2.65 m. With this filling height, we can build a reservoir with 133 000 litres with an internal diameter of 8.00m.
Table 5 : Capacity of different tank
sizes rounded to the next half m3
This cistern can only be built as a reinforced bricktank. It will be more economical to build one reservoir of this capacity rather than two reservoirs of about 66 0001, with a filing height of 2.0 m and an internal diameter of 6.5 metres. This example also shows that the correct siting of the building is essential for an economic rainwater reservoir. Taking the theoretical case that the entire rainfall occurs in only 5 days, that would mean that by dividing 135 0001 by 360 days per year, this reservoir would provide 375 litres per day throughout the whole year. Certainly this is theory and in reality the rainfall normally is spread over a period of some months. This also means that some of the collected water will already have been used when the next rain occurs and the reservoir will never be filled up to its maximum capacity, even if the rainfall reaches the annual mean. Or the other way round, since the mean annual rainfall is a statistical measure taken over many years, the chance is greater that an annual rainfall above the average but dispersed over a period of four months will occur and since consumption is constant, even this higher amount of rainwater can be stored.
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