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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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