HEC-RAS 10 steps

Hydraulic Analysis
Using HEC-RAS
10 Steps
for a
Basic HEC-RAS Analysis
HEC-RAS: 10 Steps
1. Start a New HEC-RAS Project
2. Set Up the River Reach
3. Plan Cross-Sections
4. Enter Cross-Section Data
5. Add the Road Deck
6-culvert. Add the Culvert Data
6-bridge. Add the Bridge Data
7. Add Ineffective Flow Areas
8. Input Steady Flow Data
9. Run Model, View Output
10. Add Proposed Structure
HEC-RAS: 10 Steps
These 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:
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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
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Replace an existing culvert with a larger culvert
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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?
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Existing Culvert:
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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:
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Length– 54’.
Concrete Elliptical, 60”x38”
Road elevations and width will remain the same
1. Start a New HEC-RAS Project.
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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 HECRAS 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
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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
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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.
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Numbering runs low to high from downstream
to upstream.
Suggestion: Put the structure centerline at 0.
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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.
3. Plan Cross-Sections
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Next, sketch the channel cross-section
showing stations and elevations.
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Numbering: Stations across the crosssection 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.
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Road deck at 588.20
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.
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Figure out the stream slope, from
field measurements or from a USGS quad
map. (rise/run)
4. Enter Cross-Section Data
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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.
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Here, the “station” is the position along
the stream.
4. Enter Cross-Section Data
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Add the
station/elevation (x/y)
profile of the crosssection in the Crosssection XY Coordinates
table.
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Here, “station” means
the position across the
channel.
4. Enter Cross-Section Data
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Downstream reach lengths—
distance from this crosssection to the next crosssection downstream. If the
channel is straight, the three
numbers will be the same.
Outside and inside curve
lengths create the meanders.
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Put in Manning’s n value.
Use standard n values found
on many charts.
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Enter the channel banks
locations (stations).
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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.
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When done, click on the
“apply data” button.
4. Enter Cross-Section Data
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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 crosssection 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
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Enter all cross-sections, close view
by clicking on the x in the top right
corner.
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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
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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
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In the top middle box, enter width of
the road deck.
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In the left box, enter the distance from
the downstream edge of the deck to
the next downstream cross-section.
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The weir coefficient can remain at 2.6.
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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
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In the Bridge/Culvert Data menu, click
on the Culvert button.
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Then Click Add Button.
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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.
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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
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Fill in the remaining boxes in the list. Use
drop down boxes if values for coefficients
and n are needed.
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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
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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
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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.
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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.
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Downstream, same as above but use 2:1
positioning, i.e., 2 feet out and 1 foot over.
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Do the same for all cross-sections with
ineffective flow until the with of the floodplain
is reached.
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Add cross-sections if needed to define the
ineffective flow areas.
7. Ineffective Flow Areas
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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.
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Elevation: Usually, this is set as the
low point of the road deck.
Summary of
Geometric Data Input
• 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….
8. Steady Flow Data
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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.
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IN PF boxes, enter stream flows in cfs.
8. Steady Flow Data
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Reach boundary conditions: the two most common options to use are
normal depth and known water surface.
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Normal depth: Click Normal Depth button, enter stream slope.
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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
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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.
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Address any errors that show up until plan runs.
9. Run Model, View Output
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Look at the output table by going to the main menu, choose View, then
Profile Summary Table.
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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
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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.)
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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.
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Run the model for the proposed
structure as was done for the existing
structure, this time saving as
“proposed.”
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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?