Unit 4: Hydraulic Modeling and Flood Inundation Mapping using HEC-RAS
The flow or discharge value in a river does not mean much to a lay person or a decision maker because this flow can be insignificant on a big river or can be dangerous on a small creek. Thus, we must know how to translate this flow value into the water depth, velocity, and the corresponding extent to understand its impact. The objective of this unit is to perform hydraulic modeling on a reach of Wabash River near Lafayette, Indiana, to estimate water surface elevation and extent corresponding to a 100 year flow. Students will learn the basics of hydraulic modeling using HEC-RAS to simulate the flow hydraulics using one-dimensional steady state assumption. The outcome will be the inundation extent corresponding to the 100-year event along the reach of the Wabash River near Lafayette, IN. Make sure you have HEC-RAS available on school or personal computers prior to the start of the unit.
Unit 4 Learning Outcomes
Students will be able to:
- Simulate flow depth, velocity, and flood inundation using a 1D HEC-RAS model and RAS Mapper
- Analyze relationships between flow depth, inundation, and overbank roughness.
- Create GIS files from HEC-RAS outputs for subsequent use in flood risk analysis
- Apply findings to a societal decision related to flood zone management
Unit 4 Teaching Objectives
- Affective: Provides students with an opportunity to associate flood magnitudes and return period with actual inundation by using a computer model
- Behavioral: Promotes students' skills to use HEC-RAS model for simulating high flow conditions in streams to map flood depth and inundation extents. Promotes students' skills to use ArcMap/QGIS/Google Earth to view HEC-RAS results.
- Cognitive: Promotes the concept of computer modeling in students' mind as an effective tool of learning and analyses
Context for Use
The content of Unit 4 is appropriate for advanced geoscience, earth science, and civil engineering courses at the junior and/or senior level, in which the basics of hydrology and hydraulics are taught. Some of the concepts in the unit are little advanced for a junior level class, so it is recommended to do it later in the course, when students have learned little bit about open channel hydraulics and Manning's equation. The presentation included with this unit will help to get the students up to speed on some of the concepts of hydraulics and hydraulic modeling, if they never had an exposure to this topic. With the presentation and the handout, this unit can also be used as a standalone unit on mapping flood inundation extent for a given flow on the Wabash-Tippecanoe River confluence. If this unit is used with the flood hazard module, students should be encouraged to develop the flow using flood frequency analysis in Unit 2: Flood Frequency Analysis for use in this unit. If this unit is used as standalone unit, students must be provided with flow values for input to the steady flow interface. Unit 4 and Unit 5: Mapping the Impact of 100 and 500-year Floods pair well.
Description and Teaching Materials
This unit is divided into two parts. Part A involves a brief presentation on hydraulic modeling and HEC-RAS (Hydrologic Engineering Center's River Analysis System); and the second part involves the HEC-RAS lab for flood inundation mapping. Part B is the main exercise, in which students actually work in HEC-RAS to make a flood map. The unit takes 1-2 class periods or a lab session, depending on how the in-class vs. homework is structured by the instructor.
Part A is brief, and should not take more than 25-30 minutes for the presentation. Ideally this unit would be conducted in a lab so students can finish or at least get started on Part B with the HEC-RAS lab. Part B should not take more than 40-50 minutes to finish. If it is not possible to do this unit in a lab session, the instructor can give the presentation in the classroom to introduce the unit. The students can then finish the exercise on their own. The HEC-RAS software, needed for the lab part, is available for free in the public domain, and the link is provided below. The handout includes detailed instructions. The instructions are detailed enough for student to finish the lab on their own without instructors present, if needed. If the class has previously completed Unit 2, the students should be told to use the results from Unit 2 for the Wabash River as input for the HEC-RAS model. If this unit is conducted as a standalone, the input needed for running the HEC-RAS model is also provided in the link below.
Software: For the exercise, students will need HEC-RAS 5.0.5 or higher, which is available freely in the public domain from the Hydrologic Engineering Center. HEC-RAS is a windows program. If students have Mac machines, instructions on how to install HEC-RAS on Mac are provided in the student tutorial under the software section and in the teaching tips below. The download link for HEC-RAS is given below (if for some reason, the link changes or does not work in the future, searching for HEC-RAS should point to the download site):
- HEC-RAS software download
- Presentation: Unit 4 Introduction to HEC-RAS (PowerPoint 2007 (.pptx) 4.2MB Mar14 23)
- This presentation gives an overview of hydraulic modeling, including the basic details of the HEC-RAS model. Specifically, the underlying equations, data needs, and a few functionalities of its interface are described.
- Tutorial: HEC-RAS Modeling and RAS Mapper Tutorial (Microsoft Word 2007 (.docx) 5.8MB Mar14 23)
- This tutorial includes instructions to complete the HEC-RAS student exercise. Students must be provided with the data before they can begin the lab. The link to the data is given below.
- Student exercise
- Instructor file
Teaching Notes and Tips
- Plan ahead so that you know you will get HEC-RAS ready to use on available computers. The software is free but it only works on Windows 64-bit computers. The easiest solution is probably to have the software loaded on computer lab machines if that is an option for you. If you are relying on student computers, you will need to dedicate time to making sure that each student (or student group) has a functional version. If a large percentage of students have Mac computers, you can point them to resources that allow the software to run cross-platform. For Mac computers, there are a few ways to install HEC-RAS. One option is to first install Windows on your Mac using Boot Camp and then install HEC-RAS. You can find more information on this on the Apple support page. If you do not want to go the Boot Camp route, you can explore more options provided here .
- It is best to use the latest version of HEC-RAS to complete this unit. Any version after 5.0.5 should work fine. Some earlier versions do not have some functions related to inundation mapping.
- The last part of the tutorial, related to exporting inundation layer to shapefile/KML and adding new layers to RAS Mapper, is not needed for this Unit, but is needed for Unit 5, so it is good to go over those in Unit 4 to avoid questions or confusion while doing Unit 5.
The student exercise forms the summative assessment for this unit. The first five questions, which will involve students creating a table and four plots, should have definite answers with slight variations depending on their model execution. Overall the answers should be in a reasonable range of error (0-5%). Other questions are related to student understanding of the model, its response to change in data and parameters, and how it might be applied to making societal decisions. The assessment rubric for this unit is provided below.
Unit 4 HEC-RAS Student Exercise Rubric (Microsoft Word 2007 (.docx) 33kB Mar14 23)