CWR4101: Hydrology

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CWR4101 Hydrology
Mr. Jeff Earhart Infiltration http/classes.cecs
.ucf.edu/cwr4101/shagen
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Generalized Infiltration
P
At time t 0 to
Surface
Dry soil
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Generalized Infiltration

• After rain makes contact with the soil

Less than saturated, moisture is transmitted
Near Dry soil
Dry soil
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Infiltration

• Assumptions
• Constant intensity, evenly distributed rainfall
• Homogeneous soil
• Soil is initially dry
• On a local scale these assumptions are
  reasonable.

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Infiltration

• Moisture Zones

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Moisture Zones

• Complete saturation
• Unsaturated flow, uniform moisture content
• Sharp discontinuity between wet soil above and
  dry below
• Dry soil

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GreenAmpt Model for Potential Infiltration
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Green-Ampt Model

• Assumptions
• Ponded water on the surface, depth Ho
• Fillable pore space ? ?s- ?i
• Depth of percolating water will have a
  homogeneous moisture content.
• Depth H Z1 Z2 ? H Z2

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Green-Ampt Model

• Apply continuity principle to determine the
  cumulative infiltration _at_ time, toDt

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Green-Ampt Model

• Apply principle of momentum in the form of
  Darcys law

where q flow L/T K hydraulic
conductivity L/T h head L z depth L
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Green-Ampt Model

• For Green-Ampt Model q will be constant
  throughout a given depth and positive upward

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Green-Ampt Model

• where
• f infiltration rate.
• Note ? capillary suction head

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Green-Ampt Model

• Holtlt ? H where ? 5?30 cm then

• From continuity, H F/?

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Green-Ampt Model

• Since

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Green-Ampt Model

• Separating variables

• Splitting the left hand side into two parts

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Green-Ampt Model

• Setting up the integral

• Integrating both sides

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Green-Ampt Model

• Given ?, ?, ? guess at F(t) on the right hand
  side, usually F(t) ?t and solve by trial and
  error.
• ? suction head
• ? effective porosity
• ? hydraulic conductivity.

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Green-Ampt Method

• If f i at tp , what does F equal?
• Also,
• Solving for tp

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Time of Concentration (tc)

• Definition the time it takes the most distant
  particle of water to reach the point at which we
  want to monitor flow. At this time, given a
  constant rainfall, the peak flow occurs.

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Time of Concentration