Question

The storm over a catchment of 6 $k{m}^2$ had a duration of 8 hours. Mass curve of rainfall for the storm is as follows :

Time from start of storm (h)	0	2	4	6	8
Accumulated rainfall (cm)	0	0.5	3.0	4.1	6.0

If the $\varphi$ - index of catchment is 0.5 cm/h then volume of direct runoff from the catchment due to storm is

• A. 12 ha-m
• B. 24 ha-m
• C. 15 ha-m
• D. 25 ha-m

Answer 1

The correct option is C 15 ha-m

Time from Start of storm	Time interval $\Delta t$	Accumulated rainfall in time$\Delta$t(cm)	Depth of rainfall $\Delta$f(cm)	$\varphi \Delta$t(cm)	ER(cm)	Intensity of ER(cm/hr)
0		0
2	2	0.5	0.5	1.0		0.75
4	2	3.0	2.5	1.0	1.5	0.05
6	2	4.1	1.1	1.0	0.1	0.45
8	2	6.0	1.9	1.0	0.9

Total effective rainfall = Direct rainfall due to storm = Area of ER hyetograph
$=(0.75+0.05+0.45)\times 2$
= 2.5 cm
Volume of direct runoff $=\frac{2.5}{100}\times 6\times (1000{)}^2=150000m^3=15ha-m$