David Knipe

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Automated Zone A Floodplain Mapping
David Knipe Engineering Section Manager
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FEMA floodplain designations

• Zone AE has elevation and floodway mapped
  from detailed hydraulic model

Zone A has only 1 chance annual flood mapped
could be from any source
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Why Map Zone As?

• New FEMA guidance all new zones must be model
  based
• Still 10,000 stream miles in Indiana that are
  not model based
• Cost prohibitive to provide detailed zones in all
  areas
• Need quick, easy and inexpensive way to map
  quality floodplains

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The Zone A project

• IDNR funded by OCRA / CDGB (not FEMA) to map
  3,200 miles of Zone A floodplains
• Modeling exclusively using new LiDAR DEMs
• Full RiskMAP modeling, including 5 profiles,
  Flood Elevation Points, FBS Points, SFHA
  boundaries and depth grids

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Zone A tools

• Tools are a set of Python scripts inside of an
  Arc Toolbox other tools (such as HEC-GeoRAS)
  are not needed
• Only stream centerline, cross sections and
  overbank flow paths need to be created in Arc.
• HEC-RAS model will be considered an approximate
  model (no bridges, no floodway).

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Stream Selection

• Use the CNMS database to determine stream reaches
  that were not studied during the Map
  Modernization process

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Hydrology Streamstats

• hydrology for a stream reach will be completed by
  using the Purdue regression equations as found in
  the USGS application, Streamstats

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Hydrology Results

• Results from Streamstats service returned as XML
  file and imported into Excel, plotted on log-log
  plot
• Final discharges based on best fit line, not
  actual values (standard Division practice)
• discharges adjusted by defining a split point to
  improve the fit of the line
• the user to choose either the regression results
  (50) or the upper 68 or 90 confidence interval
  for use in the model

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Hydrology Results
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Hydraulic pre processing

• three hydraulic data features will need to be
  created stream centerline, cross sections and
  overbank flow paths

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Hydraulic processing

• the initial script checks the geometry of the
  input data for anomalies
• the second script pulls elevation data from the
  DEM and creates a HEC-RAS import file

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HEC-RAS import

• File created from Hydraulic script imported into
  HEC-RAS

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HEC-RAS Cross Sections

• Sections as imported represent LiDAR data for
  each line

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Manningss N Values

• Automatically derived using relationships based
  on Anderson Land Use classification and NLCD data
  (2006)

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HEC-RAS model development

• Cross section points must be filtered (gt 500
  points)
• Data evaluated for reasonableness and ineffective
  flow areas
• Discharges added to model from Excel spreadsheet
• Bridges not modeled, but adjusted for using
  ineffective flow, where necessary

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Bridge modeling
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Hydraulic Post Processing

• Run a series of scripts that generate
• 1 annual chance floodplain (S_FLD_HAZ_AR)
• Depth grids and WSEL TINs for all 5 profiles
• Floodplain Boundary Standard check point
• Flood Elevation Points (for INFIP)
• Format largely meets DCS standards

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1 annual chance floodplain

• Derived from subtraction of WSEL tin and DEM,
  with cleaning of edges and elimination of
  extraneous small shapes

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Depth Grids

• New requirement from RiskMAP used in HAZUS and
  for visualization

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Flood Elevation Points

• Used in INFIP for calculation of RFE at a point
• Created along stream centerline at 50 foot
  spacing, tagged with 1 flood elevation

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Floodplain Boundary Standard check

• Ensures that floodplain delineations are matching
  topography
• Standard FEMA evaluation technique
• Points spaced along floodplain boundary at 100
  foot interval