Floodplain Mapping using HEC-RAS and ArcView GIS

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Floodplain Mapping using HEC-RAS and ArcView GIS
http//www.ce.utexas.edu/stu/tateec

• Eric Tate
• Francisco Olivera
• David Maidment

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Outline

• Motivation and Objectives
• HEC-RAS Hydraulic Model
• Stream Cross-Section Mapping
• GIS Data Models
• 3-D Terrain Modeling
• Floodplain Mapping

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Map-Based Hydrology and Hydraulics
ArcView Input Data DEM
ArcView Flood plain maps
CRWR-PrePro
HEC-RAS Water surface profiles
HEC-HMS Flood discharge
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Motivation Why map floodplains in GIS?

• Computer automation results in time and resource
  savings versus manual floodplain plotting
• Flood insurance rate determination
• Economic impact analysis and flood early warning
  systems
• Design of drainage control structures, including
  storm drains, culverts, and bridges

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Objectives

• Develop procedure to process computed water
  surface profiles generated from HEC-RAS hydraulic
  modeling and draw floodplain maps in ArcView GIS
• Synthesize a TIN terrain model from HEC-RAS
  cross-sectional data and a digital elevation
  model (DEM)

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Outline

• Motivation
• HEC-RAS Hydraulic Model
• Stream Cross-Section Mapping
• GIS Data Models
• 3-D Terrain Modeling
• Floodplain Mapping

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HEC-RAS Background

• Hydraulic model of the U.S. Army Corps of
  Engineers
• Input cross-section geometry and flow rates
• Output flood water elevations

Cross-Section Schematic
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HEC-RAS Cross-Section Representation

• Points describe channel and floodway geometry
• Bank station locations
• Water surface elevations and floodplain
  boundaries

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HEC-RAS Output

• Graphical

• Text File

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HEC-RAS Data Translation

• Data translation from HEC-RAS output text file to
  dbase table
• Table data includes the river station IDs,
  various cross-section coordinates, reach lengths,
  computed flood elevations

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Outline

• Motivation
• HEC-RAS Hydraulic Model
• Stream Cross-Section Mapping
• GIS Data Models
• 3-D Terrain Modeling
• Floodplain Mapping

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Stream Centerline Mapping

• Digital orthophotograph and road coverage used as
  a base map
• User defines stream centerline
• Definition points identify key stream
  cross-sections

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Cross-Section Mapping
One to one relationship established between table
records and definition points
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Cross-Section Mapping

• Assume straight line cross-sections
• Proportional aliasing for cross-section location
• User input for cross-section orientation

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Outline

• Motivation
• HEC-RAS Hydraulic Model
• Stream Cross-Section Mapping
• GIS Data Models
• 3-D Terrain Modeling
• Floodplain Mapping

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GIS Data Models Vector

• Points, lines, and polygons
• Typically used for linear feature representation

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GIS Data Models Raster
Digital Elevation Model (DEM)

• Square grid cells
• Typically used for steady-state spatial modeling
  and two-dimensional surface representation

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GIS Data Models TIN
Triangular Irregular Network (TIN)

• Mesh of equilateral triangles
• Used for three-dimensional surface
  representation and drainage analysis.

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Outline

• Motivation
• HEC-RAS Hydraulic Model
• Stream Cross-Section Mapping
• GIS Data Models
• 3-D Terrain Modeling
• Floodplain Mapping

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3D Terrain Modeling Procedure
Outside Channel Raster
Inside Channel Vector
Result TIN
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3D Terrain Modeling Procedure

• DEM raster to vector conversion
• Form polygon bounding the cross-sections
• Delete any DEM points falling within the
  bounding polygon

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3D Terrain Modeling
Problem elevation differences at the end of the
cross-section
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3D Terrain Modeling
Re-sample cross sections to eliminate elevation
differences
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3D Terrain Modeling
Build TIN model from re-sampled cross-sections
and DEM
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Outline

• Motivation
• HEC-RAS Hydraulic Model
• Stream Cross-Section Mapping
• GIS Data Models
• 3-D Terrain Modeling
• Floodplain Mapping

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Floodplain Mapping 3D View
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Floodplain Mapping Ultimate Goal
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Floodplain Mapping Plan View
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HEC-RAS Output

• Graphical

• Text File

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Floodplain Mapping Plan View

• Raster floodplain shows both extent and depth of
  flooding
• Digital orthophotograph base map allows easy
  comparison of floodplain location with specific
  areas of interest (e.g., infrastructure,
  buildings)

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Real-time flood mapping
Nexrad radar rainfall input
Real time
Precomputed flood map library
Offline
Flood hydrology analysis system
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Questions?Comments?
Also thanks to Seann Reed and Brian Adams