Creation of FLO2D Grid System

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Infiltration Green-Ampt and SCS Methods
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Infiltration

• Infiltration Loss
• Generate rainfall-runoff, rainfall is necessary
  (Green-Ampt SCS)
• Infiltrate water volume loss, rainfall is not
  necessary (Green-Ampt Only)

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Select Method

• INFIL.DAT
• Line 1. INFMETHOD (1, 2 or 3)
• Green-Ampt Only (INFMETHOD 1)
• SCS Only (INFMETHOD 2)
• Combined (INFMETHOD 3)

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Green-Ampt Only

• Use for everything (INFMETHOD 1)
• Rainfall-runoff
• Alluvial fan infiltration (rain or no rain)
• River floodplain (rain or no rain)
• River channel infiltration (rain or no rain)

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SCS Only

• Use only for rainfall-runoff in the watershed
  (INFILMETHOD 2)

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Green-Ampt and SCS Combined

• INFMETHOD 3
• Use SCS for watershed rainfall runoff, does not
  compute transmission losses
• Use Green-Ampt when no rain on grid element
• Rainfall has stopped
• Downstream flooding infiltration where no rain
  has occurred
• Computes transmission losses

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Green-Ampt and SCS Combined

• INFMETHOD 3
• Global parameters must be assigned for each
  method
• Individual grid element parameters will supersede
  global parameters
• Parameters for both methods can be assigned to
  the same grid element

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Green-Ampt Parameter Calculation

• Sensitive to rainfall intensity infiltration
  exponentially decreasing function
• Based on Maricopa County, Arizona Drainage Design
  Manual
• Spatially variable Green-Ampt infiltration
  parameters based on soil and land use shape files
  and property tables
• Grid element parameters
• Hydraulic conductivity of soil
• Soil moisture deficit
• Percent impervious area
• Initial rainfall abstraction or interception

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Direct assignment
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Green-Ampt Parameter Calculation (cont.)
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Green-Ampt Parameter Procedure

• Setup FLO-2D grid
• Import Land Use and Soil Shape file
• Compute polygon intersections of the land use and
  soil shape files with each node
• Import Land Use and Soils table files
• Compute Green-Ampt parameters
• Create INFIL.DAT data file with Green-Ampt
  infiltration parameters for each grid

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SCS Parameter Computation

• inf(t) i(t) (1.0 F(t)/ S)2
• where i(t) instantaneous increment rainfall
  (in/hr or mm/hr)
• F(t) cumulative infiltration (inches or mm)
  for the grid element
• S soil moisture deficit (inches or mm) for the
  grid element
• S (1000.0 10.0 CN)/CN
• CN grid element curve number computed by the
  GDS

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CN Initial Abstraction

• Initial abstraction (IA) is given by
• IA 0.2 S
• Hardwired in the model
• TOL surface detention value for minimum
  computational depth is added to the abstraction
• Accounts for water that is not infiltrated but is
  lost to the flood routing computations

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Assign CNs

• Assign a global CN number to all grid elements.
  Use GUI, GDS or an ASCII editor.
• GDS - Select area using a polygon and assign a CN
  number. CNs are known.
• Import a CN polygon shape file and intercept the
  shape file polygons with the grid elements and
  interpolate the weighted average computed CNs.
  CNs are known.
• Import polygon shape files for hydrologic soil
  group (HSG), land cover, and impervious cover.
  GDS will intercept, interpolate and compute the
  CNs for each grid element based on the Pima
  County procedure. CNs are not known.

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1. Assign CN to Selected Grid Elements
Assign Method
Assign CN
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Select the SCS CN Method Known CNs or Unknown
CNs
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2. Known CNs Use Single Shape File
GDS intersects shape file polygons with the grid
elements and determines the area weighted average
value for each grid element. Double-clicking on
any grid element to display the computed CN.
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3. Unknown CNs Compute CNs from multiple shape
files
Select the shape file names and ID for each
parameter group click OK to compute
intersections and CNs. Double-click on any grid
element to view the CN.
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Result INFIL.DAT for SCS CN Only
2 99 0. S 1114 83 S 1121 83 S
1132 84 S 1146 84 S 1153 83 S
1169 83
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Whats next

• Urban Flooding Details