Question

The observed peak of flood during a 4 hour storm of 8 cm rainfall was, $500m^3/s$. The average loss rate was 10 mm/hour and base flow was $50m^3/s$. If there was rainfall of 12 cm in 4 hours on the same catchment, then discharge.

• A. $\text{Increases by}950m^3/s$
• B. $\text{Decreases by}950m^3/s$
• C. $\text{Increases by}450m^3/s$
• D. $\text{Decreases by}450m^3/s$

Answer 1

The correct option is C $\text{Increases by}450m^3/s$

Peak unit hydrograph:

$=\frac{\text{Peak of flood hydrograph}-\text{Base flow}}{p_1-{\varphi }_t}=\frac{500-50}{8-1\times 4}$
$=112.5m^3/s$

Now, the peak of new hydrograph

$=\text{Peak of unit hydrograph}\times R+\text{Base flow}$

Where,
$R=12-\varphi \times 4=12-(1\times 4)=8cm$
$=112.5\times 8+50=950m^3/s$

Hence, increases by
$(950-500)=450m^3/s$