Instructions for Building a HEC-RAS Model for Silver Creek

This exercise was adapted by one built by Milada Majerova for the ICRRR Part II Short Course.

Contents

  1. 1 Goal
  2. 2 Instructions
    1. 2.1 Step 1 – CREATE A NEW PROJECT
    2. 2.2 Step 2 – ENTER AND EDIT GEOMETRIC DATA
    3. 2.3 Step 3 – ENTER AND EDIT STEADY FLOW DATA
    4. 2.4 Step 4 – RUN THE MODEL
    5. 2.5 Step 5 – CALIBRATE YOUR MODEL
  3. 3 What to Include in your Lab Report
  4. 4 References
Back to Lab Page
Over to Silver Creek Context Page

Goal

The goal of this exercise is to construct and calibrate a working HEC-RAS model of Silver Creek. Then you will use it to estimate water surface elevations for a wide range of discharges. These instructions assume a basic knowledge of hydraulics and open channel flow.



Instructions

Step 1 – CREATE A NEW PROJECT

  1. Create a new project by clicking <File> <New Project>
  2. Choose an existing folder or create a new one by clicking the <Create Folder> button
  3. Enter a “Title” and a “File Name” and hit <OK>
  4. The program will create the new project in English units, which it warns you about:
  5. Click <options> <unit system> and select SI units for our metric data.


Step 2 – ENTER AND EDIT GEOMETRIC DATA

  1. Begin entering geometry data by clicking <Edit> <Geometric Data>
  2. Then, in the new window, click <File> <New Geometry Data>
  3. Enter a “Title” and click <OK>
  4. In the empty geometry data window, click the “River Reach” tool and draw in a simple channel, ending with a double click: a straight line will do (you can load an image if you want, not necessary). When you double click, a box will appear. Enter a “River” name and a “Reach” name. Choose something creative like “Silver Creek” for the river name and “Gage reach” for the reach name, then click <OK.>
  5. Next, click the “Cross Section” tool and choose <Options> <Add a new Cross Section>
  6. In the box that appears, enter the river station for the section that will be entered. Remember that HEC requires that the lowest river station number be at the downstream end and that station numbers increase in the upstream direction (numbers only).

The input data for the cross sections are in the Excel spreadsheet   SilverCreek_XS_lengths_WS.xlsx under the XS_HEC tab.
Data includes; station and elevation points (cross section data from left to right when looking downstream), bank stations (left and right), and downstream reach lengths for the left overbank (LOB), channel, and right overbank (ROB).
You will also need to guess at a Manning’s “n” for your channel and overbank areas (see Silver Creek Context and section on Energy Loss Coefficients starting on pp 3-12 in Reference Manual) . You will adjust the “n” value later in the exercise, when you calibrate the model. Contraction and expansion coefficients can be set to 0.1 and 0.3 respectively (see table 3-3 in Reference Manual and pp 3-21 for justification). After entering your cross section data, click <apply data>, and the plot of your data will be updated with your changes.


When you are satisfied with your cross section, begin entering data for the next section by clicking <options> <Add new Cross Section>, and repeat the procedure outlined above for all 42 cross sections.  Alternatively, do at least three of these to get a feel for it, and you can load up the geometry data from the zip file provided. You can do this by unzipping the file, then in the current project, got to <File> <Import HEC RAS Data>, and navigating to the folder you unzipped the data in and loading SilverCreek_geom (from SilverCreek_hec.g01).

Then go back to the Geometric Data editor and check each cross section.

When all section data has been entered, close the cross section editor. You should see that the sections have been added to the schematic plot of the river. Save the geometry data by clicking <File> <Save Geometry Data> and giving it a name. Close the geometric data window.

After your done editing the geometry data, you can open the Profile Plot tool to make sure the longitudinal profile looks reasonable.


Step 3 – ENTER AND EDIT STEADY FLOW DATA

  1. Begin entering steady flow data by clicking <edit> <Steady Flow Data>.
  2. Enter the number of profiles (discharges in cfs) that are to be entered. To begin with, lets choose 1 profile.
  3. Enter the calibration discharge of 0.068 m3/s (2.4 ft3/s), into the table by the station number.
  4. Enter the profile name by clicking <options> <edit profile names> and entering a descriptive name for each profile, then click OK.
  5. Now click the “Reach Boundary Conditions” button. We will use a “known water surface” boundary condition, so click that button and enter the known water surface for the downstream section into the box, and click OK. Click OK to close the boundary condition editor.
  6. Next, we will enter the surveyed water surface elevation from the Excel spreadsheet. While still working in the steady flow data editor, click <Options> <Observed WS>. A new box will open that allows water surface elevations to be entered for each river station. Enter the data into the proper row and click OK. Save the steady flow data by clicking <File> <Save Flow Data>, entering a name for the steady flow data, and clicking OK. Close the steady flow data editor. Your HEC model is now ready to run.

Step 4 – RUN THE MODEL

To run the model, click <Run> <Steady Flow Analysis>. In the steady flow analysis window, make sure the “subcritical” radio button is selected, and then click the large “COMPUTE” button.
If everything works, you should see a quick-moving blue computation bar and then the program will show you a short message about the model run.
Click the “Close” button. The model run is now complete.

You can examine your results in many ways. Begin by clicking on the tool buttons for “View cross sections” and “View profiles”.



In these windows, click <options> <variables>, and turn on the observed water surface elevations by clicking the appropriate box. Take a look at the long profile and several cross sections and compare your water surface elevations to the surveyed elevations. How well do your computed water surface elevations match the surveys?? Do you need to calibrate??

Step 5 – CALIBRATE YOUR MODEL

To calibrate your model, you can experiment with different “n” values to try to obtain better agreement between the computed data and the surveyed data. Increasing “n” values will raise the computed water surface: decreasing “n” will lower it. Try to adjust the “n” values to get your modeled values to match the surveys. You can do this one section at a time, using different values for “n” at each section, or you can use the same value for all sections. We will discuss the relative merits of each method in class. When you are satisfied with your model, make sure your geometry file is saved. You can add additional discharges to model by adding more profiles to the steady flow editor and specifying a discharge. I will also show you another roughness method.

What to Include in your Lab Report

Your lab report should be a standard format report (Intro, Study Site, Methods, Results, Discussion), and you should model at least five discharges (after calibration) over the full range of the rating curve for your site.


References

Back to Lab Page
Over to Silver Creek Context Page
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