Floodplain Mapping using HEC-RAS and ArcView GIS

Eric TateFrancisco OliveraDavid Maidment

http://www.ce.utexas.edu/stu/tateec

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

ArcViewInput Data

DEM

HEC-HMSFlood

discharge

HEC-RASWatersurfaceprofiles

ArcViewFlood

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

Floodway Floodway

Left Bank Station

Flood Water Surface

Right Bank Station

Normal Water Surface

Main Channel

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 FileGraphical

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

78 72 69 71 58 49

74 67 56 49 46 50

69 53 44 37 38 48

64 58 55 22 31 24

68 61 47 21 16 19

74 53 34 12 11 12

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 FileGraphical

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 hydrologyanalysis system

Nexrad radarrainfall input

Precomputedflood map

library

Real time

Offline

Questions?

Comments?

Also thanks to Seann Reed and Brian Adams