GATE Civil Engineering (CE) 2019 Shift 1 Solved Paper

The hyetograph of a storm event of duration 140 minutes is shown in the figure.The infiltration capacity at the start of this event $\left(\mathit{t}=0\right)$ is $17$ mm/hour, which linearly decreases to $10$ mm/hour after 40 minutes duration. As the event progresses, the infiltration rate further drops down linearly to attain a value of $4$ mm/hour at $\mathit{t}=100$ minutes and remains constant thereafter till the end of the storm event. The value of the infiltration index, $\mathit{\phi }$ (in mm/hour, round off to 2 decimal places), is ______________