**Find the intensity of Rainfall When the duration of Rainfall is Given**

First of all, we need to know the formula used to determine rainfall intensity. There are three types of empirical formulas –

a) For storm duration of 5 to 20 minutes

[latex] i = \frac{672}{t+10} [/latex]

b) For storm duration of 20 to 100 minutes

[latex] i = \frac{1020}{t+10} [/latex]

c) Where rainfall is frequent

[latex] i = \frac{3430}{t+18} [/latex]

Here, i = Intensity of rainfall in mm/hr, t = duration of the storm in minutes.

Following are the two mathematical examples for the determination of rainfall intensity

**Mathematical Problem: 1**

Find the intensity of rainfall when the storm continues for a period of 75 minutes.

**Solution:**

As the duration of rainfall is in between 20 to 100 minutes, so, the formula will be

[latex] i = \frac{1020}{t+10} [/latex]

The intensity of rainfall,

[latex] i = \frac{1020}{75+10} [/latex]

[latex] Or, i = \frac{1020}{85} = 12 mm/hr [/latex]

**Mathematical Problem: 2**

Find the intensity of rainfall when the rainfall occurs frequently and the total duration of rainfall is 115 minutes:

**Solution:**

As the rainfall occurs frequently, we will use 3rd formula, [latex] i = \frac{3430}{t+18} [/latex]

So, the intensity of rainfall is

[latex] i = \frac{3430}{115+18} [/latex]

[latex] Or, i = \frac{3430}{133} = 25.78 mm/hr [/latex]

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