Hydraulic ram pumps and Sling Pumps

this means:450 x 120 x 0.9 = 48 600 litres.In most cases it would be unrealistic to consider building a cistern of 48.6 m³ capacity for a housewith only 120 m² roof area. However, as the situation differs from place to place, we cannot decidehere whether a reservoir of this capacity will be realistic and economically efficient.2.4.2 Roof type and catchmentThe 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 rooftypes shown in Fig. 2.2 the single pitch roof is the most appropriate for rainwater harvesting, sincethe entire roof area can be drained into a single gutter on the lower side and one or two downpipescan 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 islarge enough, it could be drained into four tanks located at each corner of the house. The mainproblem is always the corner. A 90° angle in the gutter should be avoided. It is extremely difficult toadjust gutters in such a way that water really flows easily downwards. It seldom works well whendownpours occur, and it is the heavy downpours that should be caught. The hip roof is not veryefficient either, since it also needs gutters all around the building. Flat roofs can be used forcatchment if they are furnished with an edge, keeping the water on the slab until it has drainedthrough the gutter or downpipe. However, using a flat roof for rainwater harvesting is not veryefficient because of the extended runoff-time and the evaporation losses. One way to improve thecatchment is to provide the slab with a sloping cement screed. Constructing a waterproof edge on aflat 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 manyadvantages. As the picture of Woodhall Community Centre in Lobatse, Botswana, shows, the gutterof the length of one side can be drained into a reservoir on the other side of the building by fixing thedownpipe at the gable wall and sloping it towards the cistern.2.4.3 Roof finishNot 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 rainwatercatchment; likewise, brick tiles of all variations, and also thatch can be used, but these are lessefficient.2.4.4 How to choose the size of a reservoirExample I {see Fig 2.3}:A house with a roof area of 9.00 x 6.50 m is to be furnished with catchment and storage facilities.The mean annual rainfall is 450 mm.Calculation of rainwater:9.00 x 6.50 x 450 x 0.9 = 23895 litres18