In this article we will discuss about the rainfall intensity curves with the help of a suitable diagram.
For the design of sewers, rainfall data of every zone of the town is required. The rain gauges of recording type are to be fixed in each zone for the collection of the rainfall data. It has been noted that rainfall data revealed that rainfall of lower intensities occurs more frequently, while rainfall of higher intensities occurs less frequently.
In other words, it can be said that rainfall lasting for the same duration can have different intensities, and they will be higher for the rainfall occurring less frequently and lower for the rainfall occurring more frequently.
Fig. 4.4 shows the series of curves plotted with duration in minutes of the rainfall v.s. intensity of rainfall. Now if the designs of sewers are done on the basis of one year curve, the sewers will get utilized fully atleast once in every year. But such sewers cannot take the full volume of the sewage which will occur during heavy rainfalls than the rainfall for which the sewers have been designed.
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When the sewers will be unable to take the full discharge, the city will be flooded and may receive damage and the public will be caused to great inconvenience. Therefore, it is the duty of the designer to design the sewerage system in such a way that it should be economical as well as should be capable of taking the required discharge.
From Fig. 4.4 it is clear that curve A shows the relation between duration and intensity that will presumably be equal once in 30 years and it is called 30 year curve. Similarly curves B,C, D and E are called 20 year, 15 year, 10 year and 5 year curve respectively. By the same method curves for 2 year and 1 year can be drawn.
Practically, it has been noted that wide variations in rainfall intensity occur with relatively short distances, which make it possible to combine the records of two stations and gain the advantages of longer period record. In India usually 1 to 2 year curves may be used for designing the sewerage system of a town.
