Total Precipitation Interception Infiltration Precipitation ChannelPrecipitation Depression Storage

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Total Precipitation Interception Infiltration Precipitation ChannelPrecipitation Depression Storage

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Title: Total Precipitation Interception Infiltration Precipitation ChannelPrecipitation Depression Storage

1

MINISTRY OF SCIENCE AND TECHNOLOGY
DEPARTMENT
OF
TECHNICAL AND VOCATIONAL EDUCATION
Department of Civil Engineering
CE 4026
Engineering Hydrology
(Sample Lecture)?
INFILTRATION
Daw Cho Cho
Associate Professor
Department of Civil Engineering
Yangon Technological University

2
(No Transcript)
3
Infiltration

Infiltration is the process by which enters the
soil from the ground surface
Infiltration first replenishes the soil moisture
deficiency
Infiltration is responsible for subsurface and
ground water flow
The supply to ground water reservoir also depends
upon infiltration
The infiltration rate is used for the computation
of the water loss due to infiltration for the
determination of the surface runoff

4
Infiltration Capacity
f0
ftfc(f0-fc)e -kt
f infiltration
fc
time t
when i f
faf
ilt f
fai
where, faactual infiltration rate f
infiltration capacity i intensity of
rainfall infiltration capacity at time t after
the beginning of the storm ftfc(f0-fc)e
kt where, fo initial infiltration capacity
fc limiting constant value of the
infiltration The values of fc, fo and k depends
upon the type and condition of soil
5
Factors affecting infiltration rates

soil moisture
type of soil medium
permeability
vegetal cover
surface fines
compaction of soil
available storage in soil stratum
depth of surface detention
temperature of water
other factors

6
Measurement of infiltration capacityThe
following two types of infiltrometer are commonly
used in practice1. Flooding type infiltrometer
2. Rainfall simulator-type infiltrometer
7
Infiltration indices

An average constant value of infiltration is
called infiltration index.
Two types of infiltration indices are commonly
used.-index
1. ? -index
2. w index
There are extremely used for the analysis of
major floods when the soil is wet and the
infiltration rate becomes constant

8
?-index

The value of f-index can be derived from the
rainfall hyetograph and the resulting surface
runoff volume by trial and error.
The unshaded area below the horizontal line is
assumed that all losses are due to infiltration
only.
For determination of ?- index, a horizontal line
is drawn on the hyetograph such that the shaded
area above that line is equal to the volume of
surface runoff.
If the shaded area is not equal to the volume of
measured surface runoff, the horizontal line is
shifted upwards or downwards till this condition
is satisfied.

9
W-index

w-index(P-R-S)/tf
where Ptotal storm precipitation (cm)
Rtotal surface runoff (cm)?
Sdepression and interception
losses (cm)?
tftime period (in hours)?
The w-index is more accurate than the ?-index
because it excludes the interception and
depression

10
Example

For a storm of 3-hr duration, the rainfall rates
are as follows

If the surface runoff is 3.4 cm, determine the
?-index and w-index.
Solution
Assume that f-index is more than 1.4 cm/hr.
Therefore, surface runoff
R(3.4-?)(4.8-?)(3.2-?)(2.0-?)30/60
3.406.7-2?
?1.65 cm/hr
As the computed value of f-index is greater than
1.4 cm/hr, the assumption was correct.
total precipitation P(1.43.44.83.22.01.2)30/
60
8 cm
W-index(P-R-S)/tf, assume S 0
(8.0-3.4)/3.0 1.53 cm/hr