Hydrology And Water Resources

Engineering
Course Code: 101505
Branch: Civil Engg.
Credit: 03

Prepared By:
Prof. Rajkishor,
Assistant Professor,
Department of Civil Engineering,
Bhagalpur College of Engineering, Bhagalpur
 Infiltration
Infiltration is the flow of water into the ground
through the soil surface.
When water is applied at the surface of a
soil, four moisture zones in the soil, as
indicated in Figure can be identified.

Zone 1: At the top, a thin layer of saturated

zone is created.
Zone 2: Beneath zone 1, there is a transition
zone.
Zone 3: The next lower zone is the transmission
zone where the downward motion of the
moisture takes place as in unsaturated flow.
Zone 4: in this zone, the moisture content Distribution of Soil Moisture
decreases with the depth. in the Infiltration Process
 Infiltration Capacity
• The maximum rate at which a given soil at a
given time can absorb water is defined as the
infiltration capacity.

• It is designated as fp and is expressed in units

of cm/h.

• The actual rate of infiltration f can be

expressed as-
f = fp when i ≥ fp

and f = i when i < fp

• The infiltration into the soil is high
where i = intensity of rainfall. at the beginning of a storm and has
an exponential decay as the time
elapses.
The infiltration capacity of an area is mainly dependent on several factors like-
• Characteristics of the soil
• Texture,
• Porosity and
• Hydraulic conductivity
• Condition of the soil surface
• Current moisture content
• Vegetative cover
• Soil temperature

Variation of Infiltration Capacity

 Measurement of Infiltration
Infiltration characteristics of a soil at a given location can be estimated by-
• Flooding-type infiltrometers
• Measurement of subsidence of free water in a large basin or pond
• Rainfall simulator
• Hydrograph analysis

• Flooding-Type Infiltrometer:
Flooding-type infiltrometers are experimental devices used to
obtain data relating to variation of infiltration capacity with
time. Two types of flooding-type infiltrometers are in
common use. They are-

(a) Tube-type (or Simple or Single Ring) infiltrometer

(b) Double ring infiltrometer.

 Tube-type (or Simple or Single Ring) Infiltrometer
• This is a simple instrument consisting essentially of a
metal cylinder, 30 cm diameter and 60 cm long, open at
both ends.
• The cylinder is driven into the ground to a depth of 50
cm.
• Water is poured into the top part to a depth of 5 cm and
a pointer is set to mark the water level.
• As infiltration proceeds, the volume is made up by
adding water from a burette to keep the water level at
the tip of the pointer. • The infiltered water
• Knowing the volume of water added during different spreads at the outlet
time intervals, the plot of the infiltration capacity vs from the tube, hence
tube area is not
time is obtained. representative of the
• The experiments are continued till a uniform rate of area in which
infiltration is obtained and this may take 2–3 hours. infiltration is taking
place.
 Double Ring Infiltrometer

• The two sets of metallic concentrating rings

with diameters of 30 cm and 60 cm and of a
minimum length of 25 cm are used.

• The two rings are inserted into the ground and

water is applied into both the rings to
maintain a constant depth of about 5.0 cm.

• As The water depths in the inner and outer rings are kept the same during the
observation period.
• The measurement of the water volume is done on the inner ring only. The
experiment is carried out till a constant infiltration rate is obtained.

• The outer ring provides water jacket to the infiltering water from the inner ring
and hence prevents the spreading out of the infiltering water of the inner ring.
Disadvantages of flooding-type infiltrometers
• It measures the infiltration characteristics at a spot only.
• The raindrop impact effect is not simulated;
• The driving of the tube or rings disturbs the soil structure; and
• The results of the infiltrometers depend to some extent on their size with the
larger meters giving less rates than the smaller ones; this is due to the border
effect.
 Modelling of Infiltration Capacity

• The infiltration into the soil is high at the

beginning of a storm and has an
exponential decay as the time elapses.

• The curve Fp(t) vs time is the mass curve

of infiltration.

• Cumulative infiltration capacity Fp(t) is

Curves of Infiltration Capacity and
Cumulative Infiltration Capacity
defined as the accumulation of infiltration
volume over a time period since the start
of the process and is given by-
 Modelling of Infiltration Capacity
• Horton’s Equation (1933)
Horton expressed the decay of infiltration capacity with time as
an exponential decay given by-
• Philip’s Equation (1957)
Classification of Infiltration Capacities
PROBLEM:
The infiltration capacity of soil in a small watershed was found to be 6 cm/h
before a rainfall event. It was found to be 1.2 cm/h at the end of 8 hours of storm.
If the total infiltration during the 8 hrs period of storm was 15 cm, estimate the
value of the decay coefficient Kh in Horton’s infiltration capacity equation.
PROBLEM:
The infiltration capacity in a basin is represented by Horton’s equation as
fp = 3.0 + e–2t
where fp is in cm/h and t is in hours. Assuming the infiltration to take place at
capacity rates in a storm of 60 minutes duration, estimate the depth of infiltration
in (i) the first 30 minutes and (ii) the second 30 minutes of the storm.

SOLUTION:
fp = 3.0 + e–2t
 Infiltration Indices
• Since, infiltration value is a variable quantity but for In hydrological
calculations involving floods, a constant value of infiltration rate for the
duration of the storm is more needed.

• Hence, these infiltration values are taken as average.

• This defined average infiltration rate is called infiltration index .

• There are two types of indices –

1) ϕ–Index

2) ω-Index
 Infiltration Indices
ϕ–Index:
• ϕ-index is a constant infiltration capacity that produces the total runoff
for a given amount of rainfall.

• It's the average value of infiltration capacity over the duration of rainfall.

• It is also known as effective rainfall (ER).

• Initial losses are also considered.

• If the rainfall intensity is less than ϕ then the infiltration rate is equal to the
rainfall intensity.

• If the rainfall intensity is larger than ϕ, the difference between the rainfall and
infiltration in an interval of time represents the runoff volume.

• The amount of rainfall in excess of the index is called rainfall excess.

The ϕ-index can be estimated as-

where R = runoff in cm from a 24-h rainfall of

intensity I cm/h
P α = a coefficient which depends upon the soil type.

• In estimating the maximum floods for design purposes, in the absence of any
other data, a ϕ -index value of 0.10 cm/h can be assumed.
ω–Index or W-index:
• Since the ϕ-index includes the initial losses also.
• Hence after initial losses are separated from the total abstractions,
an average value of infiltration rate, called ω-index, is defined as-

P = total storm precipitation (cm),

R = total storm runoff (cm),
Ia = initial losses (cm),
te = duration of the rainfall excess, i.e. the total
time in which the rainfall intensity is greater
than W (in hours),
W = defined average rate of infiltration (cm/h).
PROBLEM:
The mass curve of rainfall of duration 180 minutes on a catchment is given
below. The catchment had an initial loss of 0.5 cm. The ϕ - index of the
catchment is known to be 0.4 cm/hour. Calculate the total surface runoff from
the catchment due to this storm.

Solution
• Runoff in each time step is equal to the incremental rainfall in that time
step minus the infiltration loss calculated as = (ϕ.t )

• This number is always equal to or greater than zero, as the runoff is always
a positive quantity.
Hence, The total surface runoff due to the storm from
the catchment is 2.20 cm.
PROBLEM:
Solution: Given that,
Runoff, R = 6.0 cm
Time, T (hrs) 1 2 3 4 5 6 7 8

Cumulative Rainfall, P (cm) 0.5 1.5 3.1 5.5 7.3 8.9 10.2 11.0

Incremental Rainfall, P (cm) 0.5 1.0 1.6 2.4 1.8 1.6 1.3 0.8

Incremental Intensity, i (cm/hr) 0.5 1.0 1.6 2.4 1.8 1.6 1.3 0.8
(Since, time interval = 1 hr)

Since, this phi index value of 0.625 cm/hr is NOT greater than all Intensity i values here,

i.e for 1st I value =0.5 cm/hr.

Hence, this phi index value is rejected.

Ans.