**Computing Water Surface
Elevation with HEC-RAS**

Prepared by David R. Maidment and Gonzalo Espinoza

CE 374K Hydrology

Spring 2011

Starting a
HEC-RAS ProjectChannel
Geometry

Flow Data

Steady Flow Simulation

Summary of Items to be Turned In

The goal of this exercise is to give you and understanding of how the HEC-RAS model is run for steady flow profiles on Waller Creek. The model’s geometry data and steady flow data is provided for you in the http://www.caee.utexas.edu/prof/maidment/CE374KSpring2011/HECRAS/wallerras.zip file. Obtain these files and put them in a working directory on your computer. You should see the following file list:

**HMSResults.xls** is a file of
discharges at various points along Waller Creek and its tributary (the Hemphill
Branch) computed from the HEC-HMS model that you worked with in the previous
homework exercise. We won’t use this
file except as a reference so you can see the link between the HEC-HMS and
HEC-RAS models.

**Waller1.f01** is a **Flow File** used as input to HEC-RAS,
developed from the flows in the HMSResults.xls spreadsheet

**Waller1.g02** is a **Geometry File** describing the river
channel and bridges on Waller Creek

**Waller1.prj** is a **Project File** that controls how HEC-RAS
runs

**Waller1.p02** is a set of parameters
for the water surface profile computation

The HEC-RAS Version 4.1 program is accessible in ECJ 3.302 and can also be obtained directly from HEC at http://www.hec.usace.army.mil/software/hec-ras/

To open HEC-RAS, look for a folder called “HEC” from the Start button on your computer, there should be a HEC-RAS 4.1.0 icon.

Double click the icon to open the main project window, which looks like this:

Click on the **Open Project…** command from the **File** menu. Scroll down to your working directory and
highlight the project titled “Use of GeoRAS Input”,
file **waller1.prj**. Click **OK**.
The Project, Geometry, and Steady Flow information lines should now be
filled with the titles of those respective files as shown:

As shown in the main menu, four files are required to run a HEC-RAS
project. First, the **Project File** acts as a file management tool and identifies which files
are used in the model. The **Plan File**
sets the model conditions as subcritical, supercritical, or mixed flow and runs
the simulation. The **Geometry File**
contains all the geometric attributes for the model (which, for our case, are
imported from GIS using GeoRAS). Lastly, the **Steady Flow File**
establishes the steady-state flow and boundary conditions at numerous points in
time for the model. Disregard the Unsteady Flow information line for the
moment.

Let’s first examine the geometry data that was developed using the same
methodology we used in the previous section.
Select **Edit/Geometric Data…** from the main project window.

What you see is the plan view of a set of cross-sections on Waller Creek,
with labels measuring the **River Station**,
or distance in feet from the downstream end of the creek. River Stationing on the **Waller Creek** starts at the **Trib**, starts at
the point it joins Waller Creek and continues upstream on the tributary. The red dots on the green cross-section
lines represent the points of division between the **channel flow**, the **left
overbank flow** and the **right overbank
flow**. Left and right are considered
when looking in the downstream direction along the reach. Flow is computed separately for each of the
three flow regions, although the water surface elevation at a given
cross-section is the same in each flow region.

*To be turned in: What is the total routing distance in feet
simulated by this model (i) for Waller Creek, (ii) for the tributary?*

Details of the Geometry Data can be examined from this window. Click on the **Cross Section** button to
see the tabular data for each cross section.

In this case, we are looking at the Cross-section at **River Station** 3543.919 on the Trib
reach. The points defining the
cross-section are shown in the table to the left and plotted in the graph to
the right. In the table, **Station** refers to the distance from the
left side of the cross-section (looking downstream), measured in feet, while **Elevation** is the elevation above mean
sea level in feet. The Downstream Reach
Lengths refer to the distance in feet to the next downstream cross-section, the
**Manning’s n Values** are the n values for **the Left Overbank** (LOB), **Channel**, and **Right Overbank** (ROB), respectively. The Main Channel Bank Stations refer to the
Station values of the two red dots in the plot, and the **Contraction and Expansion Coefficients** refer to the coefficients
for the minor head losses associated with changes in channel cross-section.

Control structures along a stream can be manually entered using the
corresponding buttons from this editor.
We will not be using any control structures for this model. You can move around on the routing reach
using the arrow keys, and you can change what river and
what reach of the river you are on with the tab buttons, ,
as shown below. You can use the button to toggle the plot display on or off.

*To be turned in: How many routing reaches are there in this
model? What are their names?*

In the Cross Section Data window, use the button to copy the cross section plot to the clipboard. From there, you can paste it into your homework solution.

*To be turned in: Take
Cross-section 7426.129 on the WallerUS reach of the *

*(a) **Width
of the cross-section in feet*

*(b) **Stationing
on the cross-section of the left bank and right bank of the channel, in feet,
and elevation of these points in feet above mean sea level.*

*(c) **The
Manning’s “n” values used at this cross-section for left overbank, channel and
right overbank flow.*

*(d) **The
distance in feet to the next downstream cross-section.*

*(e) **The
river station in feet of the next downstream cross-section*

Close the **Cross Section Data** and **Geometry
Data** windows so that you get back to the main **River Analysis System** window.
We are now ready to input the flow data for the model.

The flow data has been extracted from a HEC-HMS hydrologic model of the **HMSResults.xls**.

First, let’s open the **Steady Flow Editor**. Select the **Steady Flow Data…** command
from the **Edit** menu.

The **Steady Flow Editor** should look like the following. The locations shown are the River Stations
at which the discharge is changed.

There are three sections of Steady Flow Data that have been inputted using
this window: *Enter/Edit Number of
Profiles*, *Flow Change Location/Profile Names and Flow Rates*, and *Reach
Boundary Conditions*.

__Enter/Edit Number of Profiles__

A “Profile” is a list of flow conditions at specified locations along the
extent of the model’s reaches at a specified point in time. Each entered Flow Profile will result in a
steady-state water surface profile along the length of each reach. Open completion of running the model (which
we will do in the next section) results can be observed from the **View
Profiles, View Cross Sections**, or **View 3D Multiple Cross Section Plot**
buttons provided in the HEC-RAS Main Menu window.

For this exercise, a total of ten profiles were entered. The profiles correspond with flow data
extracted from the **HMSResults.xls** file at one-hour time intervals, at
0100 hours, 0200 hours, 0300 hours, and so on.

__Open Flow Data__

The editor initially showed the upstream cross sections *of each reach*
in the model: cross sections 3543.919
(upstream boundary of the Tributary), 7745.596 (upstream boundary of WallerUS), and 4378.570 (upstream boundary of WallerDS). In
addition, the cross sections acting as outlets of watersheds for rainfall
runoff were added as well. If you
examine the spreadsheet,** HMSResults.xls**, you will find flow data for
upstream boundaries as well as runoff data for specified cross sections along
both Waller Creek and the Tributary.
This data was used for this model.
When adding the flow data to HEC-RAS, the values entered for a specified
cross section is an * accumulated*
flow. All flows upstream of a
flow change location in the model

__Reach Boundary Conditions__

The final step in the Steady Flow Data development is establishing the Reach
Boundary Conditions. Click on the **Reach
Boundary Conditions** button. The
Steady Flow Boundary Conditions Editor is used to set the water surface
elevation boundary conditions. Click in
the “Known WS” cell in the **Downstream** column. Notice that the water surface elevation for
cross section 14.035 on “WallerDS” is inputted for
each profile, establishing the boundary conditions for each steady-state
profile. The Junction = 2 in this table
specifies that the Trib, WallerUS
and WallerDS, join at a single junction, so that the
downstream boundary condition for the Trib and WallerUS is established by the water surface elevation at
the upper end of the WallerDS reach.

*To be turned in: What is the water surface elevation of the Colorado River at the mouth of Waller Creek for Profiles
3 and 4?*

We are now ready to run a Steady-State Flow simulation. From the** Run **menu, select **Steady
Flow Analysis**.

Under the File menu, select **New Plan**.
Enter “**Hypothetical flow conditions**” as the Plan’s title and put
into the next window which appears a 12-character short identifier, “**Hypoflow**”.
Click **OK**.

Ensure the **Flow Regime** is set to **Subcritical** and press the **COMPUTE**
button. This starts a *FORTRAN*
program called *SNET*, which carries out all the calculations for the
simulation.

Close the **Finished Computations**
window, and the** Steady Flow Analysis**
window.

You can see the results of the model using the **View/Water Surface
Profiles** button.

From the **Profiles** menu you can
select a particular water surface profile, and from the **Reaches** menu, you can see the profile in each of the computational
reaches.

If you click on a point on this profile, you can read the stationing the flow and the water surface elevation:

*To be turned in: Locate the three cross-sections closest to
the junction of the *

You can use the “Animate the simulation results” button to show the time sequence of the water surface profiles in any reach.

You can use **File/Copy Plot to
Clipboard** to save a copy of the Profile Plot and paste it into your
homework solution.

In the **View / Water Surface Profiles**
menu, select Profile 3, and the Waller US reach, then select **View/Profile Summary Table**

The resulting table for WallerDS shows summary data about the water surface
profile.

*To be turned in: For Profile 3 and* *Cross-section
7426.129 on the WallerUS reach of the Wallercrk River, make a plot of the water surface profile
using HEC-RAS, and use the Summary Table to find the following:*

*(a) **Discharge
(cfs), Minimum channel elevation, water surface
elevation, elevation and slope of the energy grade line, and the velocity in
the channel.*

(b) *Determine
from these quantities, the values of the terms in the energy equation: H = z +
y + V ^{2}/2g. Assume z is at
the minimum channel elevation. What is
the elevation of the Hydraulic Grade Line at this location? What flow condition prevails here? Is the
assumption of Subcritical flow throughout the profile correct?*

Ok, you’re done! **Save the Project **and close all the Windows.

**Summary of items to be
turned in:**

*(1**) What is the total routing distance in feet
simulated by this model (i) for Waller Creek, (ii) for the tributary?*

*(2)
How many routing reaches are there in this model? What are their names?*

*(3)
Take Cross-section 7426.129 on
the WallerUS reach of the *

*(a) **Width
of the cross-section in feet*

*(b) **Stationing
on the cross-section of the left bank and right bank of the channel, in feet,
and elevation of these points in feet above mean sea level.*

*(c) **The
Manning’s “n” values used at this cross-section for left overbank, channel and
right overbank flow.*

*(d) **The
distance in feet to the next downstream cross-section.*

*(e) **The
river station in feet of the next downstream cross-section*

*(4)
What is the water surface elevation in feet above mean sea level of the Colorado River at the mouth of Waller Creek for Profiles
3 and 4?*

*(5)
Locate the three cross-sections closest to the junction of the *

*(6)
For Profile 3 and* *Cross-section 7426.129 on the WallerUS reach of the *

*(c) **Discharge
(cfs), Minimum channel elevation, water surface
elevation, elevation and slope of the energy grade line, and the velocity in
the channel.*

(d) *Determine
from these quantities, the values of the terms in the energy equation: H = z +
y + V ^{2}/2g. Assume z is at
the minimum channel elevation. What is
the elevation of the Hydraulic Grade Line at this location? What flow condition prevails here? Is the
assumption of Subcritical flow throughout the profile correct?*

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