Question

A 6-hour rainstorm with hourly intensities of 7, 18, 25, 12, 10 and 3 mm per hour produced a run-off of 33 mm. Then the $\varphi -$ index is

• A. 7 mm/h
• B. 3 mm/h
• C. 10 mm/h
• D. 8 mm/h

Answer 1

The correct option is D 8 mm/h

$\text{Total rainfall}=7+18+25+12+3=75mm$
$\text{Effective rainfall}=33mm$
$\text{Infiltration}=65-33=42mm$
Assuming
$t_e=6hrs$
$\varphi -index=\frac{42}{6}=7mm/hr$
It is greater than 3 mm/hr and 7 mm/hr
So for 4 hour the infiltration would be
$=42-10=32mm\therefore \varphi -index=\frac{32}{4}=8mm/hr$