The hyetograph of a storm event of duration 140 minutes is shown in the figure

Q. The hyetograph of a storm event of duration 140 minutes is shown in the figure.

The infiltration capacity at the start of this event (t = 0) 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 t = 100 minutes and remains constant thereafter till the end of the storm event. The value of the infiltration index, ϕ(in mm/hour, round off to 2 decimal places), is

Solution:

Depth of infiltration = Area of hyetograph above Horton’s curve

=(15×20/60+10×20/60+8×20/60)−10+4/2×60/60

=4 mm

Now, assuming
4≤ϕ≤8

(8−ϕ)×20/60+(15−ϕ)+20/60+(10−ϕ)×20/60

+(8−ϕ)×20/60=4

41−4ϕ=12

⇒ϕ=7.25 mm/hr