Floodplain Mapping using HEC-RAS and ArcView GIS Eric Tate Francisco Olivera David Maidment

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

Map-Based Hydrology and Hydraulics ArcView Input Data DEM HEC-HMS Flood discharge HEC-RAS Water surface profiles ArcView Flood plain maps CRWR-PrePro

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 Motivation: Why map floodplains in GIS?

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) Objectives

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

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

Points describe channel and floodway geometry Bank station locations Water surface elevations and floodplain boundaries HEC-RAS: Cross-Section Representation

HEC-RAS: Output Text File Graphical

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 HEC-RAS: Data Translation

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

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

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

Assume straight line cross-sections Proportional aliasing for cross-section location User input for cross- section orientation Cross-Section Mapping

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

Points, lines, and polygons Typically used for linear feature representation GIS Data Models: Vector

Square grid cells Typically used for steady- state spatial modeling and two-dimensional surface representation Digital Elevation Model (DEM) GIS Data Models: Raster

Mesh of equilateral triangles Used for three-dimensional surface representation and drainage analysis. Triangular Irregular Network (TIN) GIS Data Models: TIN

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

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

DEM raster to vector conversion Form polygon bounding the cross-sections Delete any DEM points falling within the bounding polygon 3D Terrain Modeling: Procedure

3D Terrain Modeling Problem: elevation differences at the end of the cross-section

3D Terrain Modeling Re-sample cross sections to eliminate elevation differences

3D Terrain Modeling Build TIN model from re-sampled cross-sections and DEM

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

Floodplain Mapping: 3D View

Floodplain Mapping: Ultimate Goal

Floodplain Mapping: Plan View

HEC-RAS: Output Text File Graphical

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)

Real-time flood mapping Flood hydrology analysis system Nexrad radar rainfall input Precomputed flood map library Real time Offline

Questions? Comments? Also thanks to Seann Reed and Brian Adams