The intensity of rainfall is the amount of rain falling in unit time. It is usually expressed in centimetres or millimetres rainfall per hour.
Meteorological observatories publish the rainfall data in every country stating the rainfall on every day. But these data cannot be used for determining the quantity of storm sewage for a particular area because intensity of rainfall is required for this purpose.
For determining intensity of rainfall special types of rain-gauges are required, in which continuous rainfall records can be obtained.
Symon’s Rain-gauge, which can give total rainfall between given period, but that is not suitable for determining intensity of rainfall required in sewer designs.
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Fig. 4.1 shows two types of Automatic Recording Rain-gauges, which are most commonly used for continuous recording of intensity of rainfall. In tipping-bucket type rain gauge, one small bucket is supported between two compartments.
When one compartment is filled, the bucket tips and empties of them the other compartment collects this rain water. The tipping-bucket is connected to some recording device, which records the intensity of rainfall directly.
Weighing rain-gauge contains a pan, supported on scale. The scale weighs the rain water, which is collected in an exposed funnel. The rain water, so weighed by the scale, is automatically recorded on a graph paper fixed on a rotating drum by means of a pen which traces the rainfall on a clockwise operated chart.
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For design purposes it is necessary to collect the data of the mean intensity of rainfall at a place for 20-25 years. But mostly data for such a long period is not available; therefore the designing may be done by collecting data of towns having similar climatic conditions or by using empirical formulae directly.