jw - hec-ras 10 steps

7/28/2019 Jw - Hec-ras 10 Steps

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Hydraulic Analysis

Using HEC-RAS

10 Stepsfor a

Basic HEC-RAS Analysis


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HEC-RAS: 10 Steps1. Start a New HEC-RAS Project2. Set Up the River Reach

3. Plan Cross-Sections4. Enter Cross-Section Data5. Add the Road Deck6-culvert. Add the Culvert Data

6-bridge. Add the Bridge Data7. Add Ineffective Flow Areas8. Input Steady Flow Data9. Run Model, View Output

10 . Add Proposed Structure


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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 factthat 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, 60x38 Road elevations and width will remain the same


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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 andsimpleproject.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 andflow characteristics.


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2. Set Up the River Reach

In the main window, click on Edit, then GeometricData.


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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 ason a map with north at the top. Dont worry about meandersjust one line.Click, drag, double click. Enter river name and reach name at the prompt.


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

position the cross-sections. Determine thelength of the structure, parallel to the river,

then add a cross-section 25 upstream andanother 2-5 downstream of structure.

Numbering runs low to high from downstreamto upstream.

Suggestion: Put the structure centerline at 0.

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

Suggestion: Use negative numbers for

downstream cross-sections and positivenumbers 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 stationposition has a corresponding elevation.

Suggestion: Use 0 for the center of thechannel, negative numbers for the leftside and positive numbers for the rightside looking downstream.

If all of the cross-sections have the sameor similar dimensions, it is quick to startwith one cross-section and copy it andadjust it for the others.

Figure out the stream slope, fromfield measurements or from a USGS quadmap. (rise/run)

Road deck at 588.20


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

Click the Options button. ChooseAdd a new cross-section. Enter thenumber of the cross-section you wantto add. You may want to start with theone that has the elevations given.

Here, the station is the position alongthe stream.


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4. Enter Cross-Section Data Add the

station/elevation (x/y)profile of the cross-section in the Cross-section XY Coordinatestable.

Here, station meansthe position across thechannel.


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4. Enter Cross-Section Data Downstream reach lengths

distance from this cross-section to the next cross-section downstream. If thechannel is straight, the threenumbers will be the same.Outside and inside curvelengths create the meanders.

Put in Mannings n value.Use standard n values foundon many charts.

Enter the channel bankslocations (stations).

Contraction and Expansioncoefficients: use 0.1 and 0.3,respectively, except justupstream and downstream of the structure where 0.3 and0.5 are used.

When done, click on theapply data button.


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

Options, either COPY the currentcross-section to another location

(easy) or ADD a new cross-section toanother location. Copy : Copy the current cross-

section to a new location then adjustthe elevations to the slope. Calculatethe change in cross-sectionelevations, ex 30 to new location Xslope of .004 is -.12. Go to options,

adjust elevations, enter elevationchange. The entire cross-section willbe adjusted down (-) or up (+).Repeat until all cross-sections areadded.

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

Copy and then adjust if much of thecross-section is similar. Another timesaver.


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4. Enter Cross-Section Data

Enter all cross-sections, close viewby clicking on the x in the top rightcorner.

This is a good time to save theGeometry Data. It is suggested thatyou name it existing or proposed.


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5. Add Road Deck Data In the main Geometric Data

window, click on the Bridg/Culvbutton. Under Options, chooseAdd a bridge or culvert. Enter station number (suggest 0). Inthe Bridge/culvert Data window,click on Deck/Roadway.


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

the road deck.

In the left box, enter the distance fromthe downstream edge of the deck tothe next downstream cross-section.

The weir coefficient can remain at 2.6.

In the station and high chord columnsof the table, add cross-sectionstations and deck elevations (highchord). Stations must correspondwith channel stations, i.e, 0 at center.Deck cross-section should be as wideas channel cross-sections. Put in asmany points as it takes to describethe deck. Copy up to down, selectOK. Look at picture.


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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 letyou put in more than one culvert. For the first culvert, leave as is. Shape:choose the type of culvert from thedrop down box. Fill in the span anddiameter in feet as appropriate for culvert.

Distance to upstream XS: fill in thedistance from the end of the culvert to

the next cross-section upstream.


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6-Culvert. Add a Culvert Fill in the remaining boxes in the list. Use

drop down boxes if values for coefficientsand n are needed.

Centerline: Should be 0 if culvert iscentered, otherwise adjust for distanceoff-center. (Setting up cross-section with

0 at center makes this part easier.)


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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 inthe low chord column. Additional rows may need to be inserted to definethe bottom of the bridge.


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7. Ineffective Flow Areas

If the floodplain width coming into thestructure 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 flowstations in the cross-sections in a 1:1manner, i.e., for each foot distance from thecross-section to the structure, go one foot

over from the edge of the structure.

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

Do the same for all cross-sections with

ineffective flow until the with of the floodplainis reached.

Add cross-sections if needed to define theineffective flow areas.


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

to the cross-section that requiresineffective flow. Choose Options,then ineffective flow areas. Addstations from the sketch.

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


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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 andexpansion, and other losses have been entered.

Next, enter the flow information.

Geometric Data Input


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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.


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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.


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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.


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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 thetable.


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

existing geometry file to make anew file. Choose Save GeometryData As. Name the new fileproposed. (Be sure to save theexisting file before you do this.)

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

Geometry data.

Run the model for the proposedstructure as was done for the existingstructure, this time saving asproposed.

Look at the table, select all thenecessary plans and profiles, andcompare the water surface elevationsand energy grade lines of the existingand proposed projects.


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Questions?

jw - hec-ras 10 steps

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