Question

A 6-hours rainstorm with hourly intensities of 7, 18, 25, 17, 11 and 3 mm/hour produced a runoff of 39 mm. Then the $\varphi -index$ is

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

Answer 1

The correct option is C 8 mm/hour

$\text{Total rainfall}=7+18+25+17+11+3$
$=81mm$

$\text{Effective rainfall}=39mm$

$\text{Infiltration}=81-39=42mm$

$\text{Assuming,}{t}_e=6hrs$

$\varphi -index=\frac{42}{6}=7mm/hr$

This value exceeds 7 and 3 mm/hour rainfall intensity so there will be no runoff during these hours.

Net infiltration for 4 hours
$=42-10=32mm\varphi -index=\frac{32}{4}=8mm/hr$