Question

A 3-hour storm on a small drainage basin produced rainfall intensities of 3.5 cm/hr, 4.2 cm/hr and 2.9 cm/hr in successive hours. If the surface runoff due to the storm is 3 cm, then the value of $\varphi -index$ will be

• A. 2.212 cm/hr
• B. 2.331 cm/hr
• C. 2.412 cm/hr
• D. 2.533 cm/hr

Answer 1

The correct option is D 2.533 cm/hr

$\text{Total rainfall}=3.5+4.2+2.9=10.6cm$
$\text{Effective rainfall}=3cm$
$\text{Infiltration}=10.6-3=7.6cm$

Assuming infiltration to occur for three hours.
$\therefore t_e=3hrs$

$\varphi -index=\frac{7.6}{3}=2.53cm/hr$