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Infiltration FLO


Green-Ampt and SCS Methods
Infiltration Loss
 Generate rainfall-runoff, rainfall is necessary
  (Green-Ampt & SCS)
 Infiltrate water – volume loss, rainfall is not
  necessary (Green-Ampt Only)
           Select Method
 Line 1. INFMETHOD (1, 2 or 3)

   Green-Ampt Only (INFMETHOD = 1)

   SCS Only (INFMETHOD = 2)

   Combined (INFMETHOD = 3)
              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)
                    SCS Only
   Use only for rainfall-runoff in the watershed
Green-Ampt and SCS Combined
   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
Green-Ampt and SCS Combined
   Global parameters must be assigned for each
   Individual grid element parameters will
    supersede global parameters
   Parameters for both methods can be assigned to
    the same grid element
      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
Direct assignment
Green-Ampt Parameter Calculation
    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
     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
          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
                     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
1. Assign CN to Selected Grid Elements

                             Assign Method

                             Assign CN
Select the SCS CN Method – Known
       CNs or Unknown CNs
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.
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.
Result: INFIL.DAT for SCS CN Only
                99 0.
            S   1114 83
            S   1121 83
            S   1132 84
            S   1146 84
            S   1153 83
            S   1169 83
    What’s next

Urban Flooding Details

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