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HEC-RAS Version 2.2
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HEC-RAS

• US Army Corps of Engineers
• Hydraulic Engineering Center
• River Analysis System
• Version 2.0 (Windows 3.1 or later) - April 1997
• Version 2.1 (Windows95/98 or NT(4.0)) - October
  1997
• Version 2.2 (Windows95/98 or NT(4.0)) - September
  1998

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Classifications of Open Steady versus Unsteady
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Classifications of Open Channel Flow - Uniform
versus Varied
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Types of Varied Flow
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Different Types of Slopes
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Velocity Distribution
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Velocity Distribution
Actual Max V is at approx. 0.15D. Actual Average
V is at approx. 0.6D.
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Flow Profile Classifications
Mild Slope
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Flow Profile Classifications
Steep Slope
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Energy Equation

• he energy head loss
• V1, V2 average velocities
• a1, a2 velocity weighting coefficients
• g gravitational constant
• WS1, WS2 water surface elevations
• HEC-RAS uses the Standard Step method in
  either the upstream direction (subcritical flow)
  or the downstream direction (supercritical flow)

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Energy Loss Coefficients
C contraction or expansion coefficient WSP2
only uses LSf in older versions and has added C
to its latest version using the LOSS card. WSP2
uses the upstream section for the whole reach
below it while HEC-RAS averages between the two
X-sections.
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Energy Loss Coefficients

• Loss coefficients Used
• Mannings n values for friction loss
• very significant to accuracy of computed profile
• calibrate whenever gage data is available
• Contraction and expansion coefficients for
  X-Sections
• due to losses associated with changes in
  X-Section areas and velocities
• contraction when velocity increases downstream
• expansion when velocity decreases downstream
• Bridge and culvert contraction expansion loss
  coefficients
• same as for X-Sections but usually larger values

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Friction Slope
WSP2
HEC-RAS
In this case, KD in WSP2 K in HEC-RAS
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Friction Slopes in HEC-RAS
Average Conveyance (HEC-RAS default) - best
results for all profile types (M1, M2, etc.)
Average Friction Slope - best results for M1
profiles
Geometric Mean Friction Slope - used in USGS/FHWA
WSPRO model
Harmonic Mean Friction Slope - best results for
M2 profiles
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Friction Slopes in HEC-RAS
HEC-RAS has option to allow the program to select
best friction slope equation to use based on
profile type.
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Friction Slopes in HEC-RAS
HEC-RAS has option to allow the program to select
best friction slope equation to use based on
profile type.
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Expansion and Contraction Coefficients

• Contraction
• 0.0
• 0.1
• 0.3
• 0.6

• Expansion
• 0.0
• 0.3
• 0.5
• 0.8

No transition loss Gradual transitions Typical
bridge sections Abrupt transitions
Notes maximum values are 1. Losses due to
expansion are usually much greater than
contraction. Losses from short abrupt
transitions are larger than those from gradual
changes.
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Momentum Equation

• Whenever the water surface passes through
  critical depth, the Energy Equation is not
  considered to be applicable
• Going from subcritical to supercritical flow, or
  vice-versa is considered a rapidly varying flow
  situation.
• HEC-RAS can use the Momentum Equation for
  hydraulic jumps, low flow hydraulics at bridges,
  and at stream junctions.
• Apply F ma to the body of water enclosed by the
  upstream and downstream x-sections.

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Momentum Equation
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Critical Depth Determination
HEC-RAS computes critical depth at a x-section
under 5 different situations

• Supercritical flow regime has been specified.
• Calculation of critical depth requested by user.
• Critical depth is determined at all boundary
  x-sections.
• Froude number check indicates critical depth
  needs to be determined to verify flow regime
  associated with balanced elevation.
• Program could not balance the energy equation
  within the specified tolerance before reaching
  the maximum number of iterations.

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Critical Depth
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HEC-RAS
The default method of conveyance subdivision is
by breaks in Mannings n values.
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HEC-RAS
An optional method is as HEC-2 does it -
subdivides the overbank areas at each individual
ground point.
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HEC-RAS - Computation Procedure

• Assume water surface elevation at U.S. x-section.
• Based on the assumed water surface elevation,
  determine the corresponding total conveyance and
  velocity head
• With values from step 2, compute and solve
  equation for he.
• With values from steps 2 and 3, solve energy
  equation for WS2.
• Compare the computed value of WS2 with value
  assumed in step 1 repeat steps 1 through 5 until
  the values agree to within 0.01 feet, or the
  user-defined tolerance.

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Compare WSP2 to HEC-RAS
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Comparison of WSP2 and HEC-RAS
HEC-RAS
WSP2

• DOS command line - batch jobs
• One dimensional steady flow
• Subcritical and critical flows
• No graphics

• Windows graphical interface for Win3.1, Win95/98,
  and WinNT
• One dimensional steady flow
• floodway encroachments
• unsteady flow (planned)
• sediment transport
• Subcritical, critical, mixed and supercritical
  flows
• Graphical display of all geometric data and
  output including 3D views

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Comparison of WSP2 and HEC-RAS
HEC-RAS
WSP2

• Uses friction slope from upstream x-section only
  (ie upstream x-section represents entire reach).

• Uses average of friction slope from upstream and
  downstream x-sections.

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WSP2 - Input
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WSP2 - Output
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Sample Data Comparison
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Sample Data Comparison
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Sample Data Comparison
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HEC-RAS
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HEC-RAS
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HEC-RAS
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HEC-RAS
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HEC-RAS
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HEC-RAS
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HEC-RAS
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HEC-RAS - File Structure
Suggested Reading Chapter 5 of HEC-RAS Users
Manual
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Limitations of HEC-RAS

• Flow must be steady (doesnt change with respect
  to time)
• Flow is gradually varied
• Flow is one dimensional
• Channel has a small slope (lt 10)

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Hardware Software Requirements for HEC-RAS 2.2

• IBM compatible machine with 486 or higher
• 10 MB free space on hard drive (20 recommended)
• 16 MB of RAM for Windows95/98 and 24 MB for
  Windows NT (32 or more is recommended)
• Mouse
• Windows95/98 or Windows NT (4.0 or higher)
• Color VGA or better video display

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Hardware Software Requirements for HEC-RAS 2.2
The HEC-RAS software and manual are available for
free download from HEC at the following internet
address http//www.wrc-hec.usace.army.mil
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The End