Hydraulic AnalysisUsing HEC-RAS

10 Stepsfor a

Basic HEC-RAS Analysis

HEC-RAS: 10 Steps1. Start a New HEC-RAS Project2. Set Up the River Reach3. Plan Cross-Sections4. Enter Cross-Section Data 5. Add the Road Deck6-culvert. Add the Culvert Data6-bridge. Add the Bridge Data7. Add Ineffective Flow Areas8. Input Steady Flow Data9. Run Model, View Output

10. Add Proposed Structure

HEC-RAS: 10 StepsThese 10 steps can be used with simple culvert and bridge analyses.

This approach will help you get familiar with HEC-RAS and ready for more complex projects. To download HEC-RAS, go to the US Army Corps of Engineers web site. (Search HEC-RAS).

Before using HEC-RAS for your project, you will need to collect the following cross-sectional information:

• channel shape (from maps or field measurement)• stream slope (from maps or field measurement)• structure specifications• flow data (get from DEQ – Hydrologic Studies)

Sample Problem• Replace an existing culvert with a larger culvert

• Issue: CRC wants to replace the existing culvert with a longer culvert. The proposed design increases the size of the culvert as well. Will the larger culvert offset the fact that the culvert will be longer?

• Existing Culvert:– Length-- 32’.– CMP, 42” diameter– Low point of road: el 588.20– Width of road: 18 feet– Stream bottom sandy but stony– Stream banks weedy with small brush– Stream invert elevation 579.00

• Proposed Culvert:– Length– 54’.– Concrete Elliptical, 60”x38”– Road elevations and width will remain the same

1. Start a New HEC-RAS Project.• Open HEC-RAS, in the main window, click on the File, then New Project. In the

New Project window, type in Title and File name (ex., Simple Project and simpleproject.prj). Click OK. You end up back at the main window entitled HEC-RAS River Analysis System.

There are two main types of data to enter: Geometric Data, describing the river channel setting and the structure, and Steady Flow data describing the water and flow characteristics.

2. Set Up the River Reach

• In the main window, click on Edit, then Geometric Data.

2. Set Up the River Reach• In the Geometric Data window, click on the River Reach button. The cursor

becomes a pencil. Draw a line representing the stream in the direction of upstream to downstream. It helps to draw the stream in the same position as on a map with north at the top. Don’t worry about meanders—just one line. Click, drag, double click. Enter river name and reach name at the prompt.

3. Plan Cross-Sections• First on a scratch paper, figure out where to

position the cross-sections. Determine the length of the structure, parallel to the river, then add a cross-section 2–5’ upstream and another 2-5’ downstream of structure.

• Numbering runs low to high from downstream to upstream.

Suggestion: Put the structure centerline at 0.

• Add additional cross-sections to define the channel further upstream and downstream. Use more cross-sections for irregular channels.

Suggestion: Use negative numbers for downstream cross-sections and positive numbers upstream.

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3. Plan Cross-Sections

• Next, sketch the channel cross-section showing stations and elevations.

• Numbering: Stations across the cross-section have lower numbers on the left, set up looking downstream. Each station position has a corresponding elevation.

Suggestion: Use 0 for the center of the channel, negative numbers for the left side and positive numbers for the right side looking downstream.

• If all of the cross-sections have the same or similar dimensions, it is quick to start with one cross-section and copy it and adjust it for the others.

• Figure out the stream slope, from field measurements or from a USGS quad map. (rise/run)

Road deck at 588.20

4. Enter Cross-Section Data• Click on the cross-section button.

Click the Options button. Choose “Add a new cross-section.” Enter the number of the cross-section you want to add. You may want to start with the one that has the elevations given.

• Here, the “station” is the position along the stream.

4. Enter Cross-Section Data

• Add the station/elevation (x/y) profile of the cross-section in the Cross-section XY Coordinates table.

• Here, “station” means the position across the channel.

4. Enter Cross-Section Data• Downstream reach lengths—

distance from this cross-section to the next cross-section downstream. If the channel is straight, the three numbers will be the same. Outside and inside curve lengths create the meanders.

• Put in Manning’s n value. Use standard n values found on many charts.

• Enter the channel banks locations (stations).

• Contraction and Expansion coefficients: use 0.1 and 0.3, respectively, except just upstream and downstream of the structure where 0.3 and 0.5 are used.

• When done, click on the “apply data” button.

4. Enter Cross-Section Data• Add additional cross-sections. In

Options, either COPY the current cross-section to another location (easy) or ADD a new cross-section to another location.

• Copy: Copy the current cross-section to a new location then adjust the elevations to the slope. Calculate the change in cross-section elevations, ex 30 to new location X slope of .004 is -.12’. Go to options, adjust elevations, enter elevation change. The entire cross-section will be adjusted down (-) or up (+). Repeat until all cross-sections are added.

• Add: Start from scratch each time.• Modified copied cross-section:

Copy and then adjust if much of the cross-section is similar. Another time saver.

4. Enter Cross-Section Data

• Enter all cross-sections, close view by clicking on the x in the top right corner.

• This is a good time to save the Geometry Data. It is suggested that you name it “existing” or “proposed.”

5. Add Road Deck Data • In the main Geometric Data

window, click on the Bridg/Culv button. Under Options, choose “Add a bridge or culvert”. Enter station number (suggest 0). In the Bridge/culvert Data window, click on Deck/Roadway.

5. Add Road Deck Data• In the top middle box, enter width of

the road deck.

• In the left box, enter the distance from the downstream edge of the deck to the next downstream cross-section.

• The weir coefficient can remain at 2.6.

• In the station and high chord columns of the table, add cross-section stations and deck elevations (high chord). Stations must correspond with channel stations, i.e, 0 at center. Deck cross-section should be as wide as channel cross-sections. Put in as many points as it takes to describe the deck. Copy up to down, select OK. Look at picture.

6-culvert. Add a Culvert• In the Bridge/Culvert Data menu, click

on the Culvert button.

• Then Click Add Button.

• The top right drop-down box will let you put in more than one culvert. For the first culvert, leave as is. Shape: choose the type of culvert from the drop down box. Fill in the span and diameter in feet as appropriate for culvert.

• Distance to upstream XS: fill in the distance from the end of the culvert to the next cross-section upstream.

6-Culvert. Add a Culvert

• Fill in the remaining boxes in the list. Use drop down boxes if values for coefficients and n are needed.

• Centerline: Should be 0 if culvert is centered, otherwise adjust for distance off-center. (Setting up cross-section with 0 at center makes this part easier.)

6-Bridge. Add a bridge• Use the deck/roadway button to add the bridge information. The high

chord defines the deck and the low chord defines the waterway opening (or underneath part of bridge). Add the underneath points of the bridge in the low chord column. Additional rows may need to be inserted to define the bottom of the bridge.

7. Ineffective Flow Areas• If the floodplain width coming into the

structure is much wider than the opening, ineffective flow areas should be entered. These are the areas outside the main flow, where water is slow or stagnant.

• Upstream, position the ineffective flow stations in the cross-sections in a 1:1 manner, i.e., for each foot distance from the cross-section to the structure, go one foot over from the edge of the structure.

• Downstream, same as above but use 2:1 positioning, i.e., 2 feet out and 1 foot over.

• Do the same for all cross-sections with ineffective flow until the with of the floodplain is reached.

• Add cross-sections if needed to define the ineffective flow areas.

7. Ineffective Flow Areas • In the cross-section data window, go

to the cross-section that requires ineffective flow. Choose Options, then ineffective flow areas. Add stations from the sketch.

• Elevation: Usually, this is set as the low point of the road deck.

Summary of

• River channel has been defined• Structure placed in the river channel• Losses and structural constraints are added,

including ineffective flow areas and contraction and expansion, and other losses have been entered.

Next, enter the flow information….

Geometric Data Input

8. Steady Flow Data• In main menu, go to Edit, then choose Steady Flow Data. Enter the number of

profiles, for example, to run both a 10-year and a 100-year flow, choose 2. For just a 100-year flow, enter 1.

• IN PF boxes, enter stream flows in cfs.

8. Steady Flow Data• Reach boundary conditions: the two most common options to use are

normal depth and known water surface.

• Normal depth: Click Normal Depth button, enter stream slope.

• Known water surface, such as downstream lake, dam impoundment, or other water body: Click on Known WS button and enter downstream water elevation.

9. Run Model, View Output• In main menu, click on Run, then choose Steady Flow Analysis. In the

Steady Flow Analysis menu, choose File, New Plan. Enter the title, such as “existing”, then enter the short title which can be the same. Click on Compute. Flow 01 will be chosen automatically. Under file, save plan.

• Address any errors that show up until plan runs.

9. Run Model, View Output• Look at the output table by going to the main menu, choose View, then

Profile Summary Table.

• To change what is displayed on the table, choose Options, then either Plans or Profiles and select the items you would like to show up on the table.

10. Add Proposed Structure• Use the cross-sections from the

“existing” geometry file to make a new file. Choose Save Geometry Data As. Name the new file “proposed.” (Be sure to save the existing file before you do this.)

• Working in the newly created file, take out unnecessary structures, add new cross-sections where needed, i.e. if new structure is longer. Save Geometry data.

• Run the model for the proposed structure as was done for the existing structure, this time saving as “proposed.”

• Look at the table, select all the necessary plans and profiles, and compare the water surface elevations and energy grade lines of the existing and proposed projects.

Questions?