GIS Hydro '98 - Hydraulics

Hydraulics 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.

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 issue of ArcUser magazine. The floodplains were visualized using the 3D Analyst extension:

 

 Resources

 ESRI is currently developing a HEC-RAS preprocessing and postprocessing ArcView extension called AVRAS. ESRI has made available a pre-release of the extension, which is included on this CD-ROM in zipped format. A copy of the shareware program pkunzip has also been included to unzip AVRAS. Note that use of the extension requires ArcView GIS 3.0a, and the Spatial Analyst 1.1 and 3D Analyst 1.0 extensions (or their respective later versions).

For more information about the AVRAS extension, please contact Dr Dean Djokic at ddjokic@esri.com.

HEC-RAS Geometry Data

In cases in which HEC-2 or HEC-RAS geometry files are available, it would be preferable to use them since this data (likely obtained from surveys) would be 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

 A report descrbing the methodology and the work completed through May, 1998 called Display of Flood Water Levels in a GIS has been included on this CD-ROM. For more information as work progresses, please contact Eric Tate.

Primary Contact

Eric Tate
Center for Research in Water Resources
The University of Texas at Austin
Austin, TX 78712

Fax: (512) 471-0072
E-mail: etate@mail.utexas.edu
http://www.ce.utexas.edu/stu/tateec

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 1998 Center for Research in Water Resources.