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Total Precipitation Interception Infiltration Precipitation ChannelPrecipitation Depression Storage
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... be derived from the rainfall hyetograph and the resulting surface runoff volume ... horizontal line is drawn on the hyetograph such that the shaded area above that ... – PowerPoint PPT presentation
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Title: Total Precipitation Interception Infiltration Precipitation ChannelPrecipitation Depression Storage
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- 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
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(No Transcript)
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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
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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
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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
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Measurement of infiltration capacityThe
following two types of infiltrometer are commonly
used in practice1. Flooding type infiltrometer
2. Rainfall simulator-type infiltrometer
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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
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?-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.
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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
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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
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- Thank You
