Question

Assuming Green-Ampt infiltration and: porosity = 0.5; initial moisture content = 0.2; saturated hydraulic conductivity = 0.65 cm/hr; wetting front suction = 5.5 cm and rainfall from 0-1 hr = 0.6 cm.hr and 1-2 hr = 3 cm/hr.

a) Determine the time ponding first occurs in this storm.

b) Determine the depth of infiltration at depth of runoff generated in each 1 hr interval.

c) What is the depth of the wetting front at the end of the two hour storm?

Answer 1

Ans) Given,

Porosity (n) = 0.50

Initial moisture content (_i) = 0.20

Saturated hydraulic conductivity (K)= 0.65 cm/hr

Wetting front suction () = 5.5 cm

Rainfall intensity (i1) = 0.6 cm/hr

i2 = 3 cm/hr

We know,

n = _i +

where = increase in moisture content

= 0.50 - 0.20

= 0.30

Ans (a) We know, ponding time (t_p) ,

t_p = K / i (i - K)

i = i1 + i2 / 2

i = 1.8 cm/hr

Putting values,

t_p = 0.65 x 5.5 x 0.30 / 1.8(1.8 - 0.65)

t_p = 0.52 hr or 31 minutes

Ans (b) We know, infiltration depth F ,

F = K / (i - K)

F1 = 0.65 x 5.5 x 0.30 / (0.6 - 0.65)

F1 = 21.45 cm (negtive suction)

F2 = 0.65 x 5.5 x 0.30 / (3 - 0.65)

F2 = 0.45 cm

Ans (c) We know, depth of wetting front, L

L =

Putting values,

L =

L = 2.7 cm