GIS Hydro '99 - Floods

Floods Module

Center for Research in Water Resources, University of Texas at Austin

Introduction

This page describes research and educational material related to GIS floodplain representation. Since the mid 1960's, hydraulic river modeling has been conducted using the computer model HEC-2, designed by the Hydrologic Engineering Center (HEC) of the U.S. Army Corps of Engineers. In the early 1990's, HEC released the Windows version of HEC-2, called HEC-RAS (River Analysis System). As model inputs, HEC-RAS requires channel geometry and flow information at cross-sections along a stream, and generates water surface elevations as output. Unfortunately, the modeled floodplain is not geographically referenced. Hence, recent technology development efforts in GIS floodplain mapping have focused on integrating GIS with HEC-RAS. The approach used depends on the type of stream geometry data (TIN or HEC-RAS) that is available. Two of these approaches are discussed below.

The new resources in this module are a masters thesis and accompanying powerpoint presentation on Floodplain Mapping using HEC-RAS and ArcView GIS by Eric Tate.

TIN Geometry Data

The proliferation of digital orthophotograph projects has produced a great deal of digital terrain data. For floodplain mapping, the idea is to use a digital terrain model as the source of cross-sections elevations. Version 2 of HEC-RAS gives the user the option to import and utilize three dimensional river reach and cross-sectional data from a general purpose data exchange file. Tom Evans at HEC has developed a series of preprocessing ArcInfo macros to create such an exchange file from a triangular irregular network (TIN) model of the land surface. Once in HEC-RAS, the user will need to provide additional data such as Manning's n, contraction and expansion coefficients, any hydraulic structures (e.g., bridges, culverts), and bank stations and reach lengths (if they are not included in the exchange file). After running the model, RAS can then export the output file into the digital exchange file format. Using HEC postprocessing macros, a TIN of the water surface can be created from the exchange file. With the original land-surface TIN and the new water-surface TIN, it's possible to determine the floodplain extent. This procedure was used recently in California and documented in the article "Preparing for El-Nino: Integrating the HEC-RAS Hydraulic Model with ArcView GIS", found in the April-June 1998 issue of ArcUser magazine. The floodplains were visualized using the 3D Analyst extension.

 Resources

 ESRI has recently developed a HEC-RAS preprocessing and postprocessing ArcView extension called AVRAS. This extension and accompanying documentation are included in the HEC module of this CD-ROM in compressed format. A pkzip utility is also included for opening compressed files. For more information about the AVRAS extension, please contact Dr Dean Djokic at ddjokic@esri.com.

HEC-RAS Geometry Data

When HEC-2 or HEC-RAS geometry files are available, it is preferable to use them since these data (likely obtained from surveys) are more accurate than elevations taken from a TIN model. Research is underway at the University of Texas at Austin focusing on representation of a RAS-modeled floodplain in the ArcView GIS environment. The basic steps involved in the process include the following:

1. Run HEC-RAS to generate water surface elevations for a given flow scenario.
2. Read the stream geometry and water surface profiles from RAS.
3. Write the stream information to ArcView.
4. Create a line coverage of the stream centerline.
5. Geo-reference the centerline points using a digital line graph or river reach file of the particular river.
6. Add remaining cross-section coordinates to the coverage, perpendicularly to the stream centerline.
7. Develop a interpolation algorithm to determine the floodplain location between known stream cross-sections.
8. Compare existing coverages to the floodplain coverage to determine if structures (homes/business/transportation) will be impacted.

Resources

• Tate, E.C. and D.R.Maidment, Floodplain Mapping Using HEC-RAS and ArcView GIS, CRWR Online Report 99-1, May 1999, A masters thesis describing Eric Tate's implementation of a 2 and 3 dimensional floodplain analysis using GIS. In this study, digital orthophotography and ArcView's 3-D Analyst are used to give real meaning to the flow elevations computed by the hydraulic model, HEC-RAS.
• A power point presentation describing the flood mapping process employed by Eric Tate in his masters research work. The incorporation of map-based hydrologic and hydraulic models as well as digital orthophotography, TINs and 3-D modeling into the flood mapping process is described in this presentation.
• Display of Flood Water Levels in a GIS , May, 1998.  A report by Eric Tate descrbing the methodology for computing longitudinal and cross sectional water surface elevation in HEC-RAS, and the introduction of these elevations into a GIS environment.

Primary Contact

Francisco Olivera, Ph.D.
University of Texas at Austin
Center for Research in Water Resources (CRWR)
Pickle Research Campus # 119 (R8000)
Austin, Texas 78712-4497
Telephone: (512) 471-0570
FAX: (512) 471-0072
e-mail: folivera@mail.utexas.edu

These materials may be used for study, research, and education, but please credit the authors and the Center for Research in Water Resources, The University of Texas at Austin. All commercial rights reserved. Copyright 1999 Center for Research in Water Resources.