Topic 1-SP 1C - INFILTRATION AND RUNOFF ANALYSIS

A storm with a rainfall pattern as shown below occurred over your watershed and you are

asked to determine the runoff from the rainstorm. The rainfall pattern is shown: 30 min of

rainfall, 30 minutes of no rain (infiltration recovery), followed by 60 minutes of rain at different

amounts for each 30 min period. The infiltration rates are sketched in. They are not accurately

plotted and are diagrammatic only.

Depth, in

2

0.5

Data are as follows:

0

1.5 in

Rainfall

Item

Infiltration - not to scale

Rainfall period duration, each, min

fo, in/hr

fc, in/hr

K, hr-¹(decay)

Kr, hr¹ (recovery)

Partial Recovery

Interception Losses in 1st rainfall period only, in

Depression Storage in 1st rainfall period only, in

0.75 in

Rainfall

Time, minutes

90

2.0 in

Rainfall

120

Infiltration Rate, in/he

Value

0.10

0.15

30

1.50

0.50

3

2

a) Calculate the actual infiltration rates, f, in/hr, at each 30 minute period, taking into

account recovery and decay

b) Calculate the amount of infiltration that occurred in each 30 minute rainfall period, F, in.

c) Taking into account the interception and depression storage losses and the infiltration

from b), determine the direct runoff in each of the 3 rainfall periods, in.