Question

A peak flow of a flood hydrograph due to a six-hour storm is $470m^3/s$. The corresponding average depth of rainfall is 8 cm. Assume infiltration loss of 0.25 cm/hour and a constant base flow of $15m^3/s$. What is the peak discharge of 6 hour unit hydrograph for this catchment?

• A. $60m^3/s$
• B. $70m^3/s$
• C. $80m^3/s$
• D. $90m^3/s$

Answer 1

The correct option is B $70m^3/s$

$\text{Peak discharge of 6 hr hydrograph}=470m^3/s$

$\therefore$
Peak discharge of D.R.H
= 470 - base flow
$=470-15=455m^3/s$

Now, Rainfall excess depth
= Total rainfall - Infiltration losses
= 8 cm - 0.25 $\times$ 6
= 8 - 1.5 = 6.5 cm

$\therefore$
Peak discharge of UH
$=\frac{455}{6.5}=70m^3/s$