© Amec Foster Wheeler 2015.
No-Rise Certification: How do I review a HEC-RAS Model?2016 TN AFPM Annual Conference – August 24 2016
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HEC-RAS Overview
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� Go to USACE website http://www.hec.usace.army.mil/� You are able to download older versions of HEC-RAS� http://www.hec.usace.army.mil/software/hec-ras/
Downloading HEC-RAS
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Project (input) files
Project (*.prj)
Plan (*.p01, p02, ...)
Geometry (*.g01, g02, ...)
Steady Flow (*.f01, f02, ...)
Output files
Run (*.r01, r02, ...)
Output (*.o01, o02, ...) – binary
File Management
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► Cross Section geometry
► Manning’s n values
► Reach lengths
► Network connectivity
► Ineffective Area
► Blocked Obstructions
► Expansion/Contraction Coefficients
► Bridge and Culvert Geometry
Geometry Files (*.g01)
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► Profile Discharges and River Stations
► Entered from upstream to downstream
► Different From HEC-2
► Boundary Conditions
Steady Flow Files (*.f01)
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► Geometry and Flow Files Used
► Sub- Super- or Mixed Calculations
► Floodway Encroachments
► Friction Slope Calculation Method
► Conveyance Calculation Method
► Calculation Tolerances
► Flow Distribution Calculations
Plan Files (*.p01)
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► Project Title
► Default Parameters
► Available Plan, Geometry, Flow etc. files
► Active Plan
Project File (*.prj)
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► Open Project and Steady Flow Analysis
► Can go back and forth between plans (model scenarios)
Verify Which Models Have Been Received
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► Right click on “.pXX” (plan file) extension
► Click “Select the Program from a list”, Chose Notepad or similar
► The version will display on top right of document
Verify Version of HEC-RAS Used
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► Make plan active that you want to run
► Open Steady Flow Analysis
► Click “Compute” (verify there are no error messages)
Running HEC-RAS
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► Click on “Run” and select “Run Multiple Plans”
► Click on “Check All Plans” to run all plans
► Then click “Run All Checked Plans”
► Model will run and you should get a “HEC-RAS Finished Computations” screen
Run Multiple Plans
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► Questions about Program – See Users Manual
► Questions about Engineering – See Hydraulic Reference (can click question mark at some locations)
Getting Help
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► Each HEC-RAS model outputs a lot of data
► Can view data in Graphical and Tabular Output
Viewing Results
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Graphical Output
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Cross
Sections
Water
Surface
Profiles
General
ProfilesRating
Curves3D Perspective
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Cross Section Viewer
Graphical Output Examples
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Water Surface Profiles
Graphical Output Examples
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Tabular Output
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Individual
Cross
Sections
Summary
Tables
Errors and
Warnings
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Individual Cross Section
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Summary Tables
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Errors and Warnings
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Detailed Hydraulics Review
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Typically Submitted HEC-RAS Models/Plans
► Effective Hydraulic Model
► The hydraulic analysis used in the effective FIS
► Obtain from FEMA library
► Duplicate Effective Hydraulic Model
► A copy of the effective model reproduced on the requestors computer
► Should not be modified – unless required to allow model to run
► Adding distance from upstream XS to bridge
Hydraulics
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Typically Submitted HEC-RAS Models/Plans
► Corrected Effective Model
► Corrects errors in the duplicate effective
► Adds cross sections
► More detailed topography
► Must NOT reflect man-made changes since the date of the effective model
► Existing Conditions Model (Pre-Project Conditions)
► Modified version of the Duplicate or Corrected Effective model
► Includes any modifications since the date of the Effective
► If no modifications since the effective:
► The Duplicate Effective or Corrected Effective becomes the Existing Conditions
► Proposed Conditions Model
► Modified version of the Existing Conditions model
► Includes modifications to reflect the project
Hydraulics
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CHECK THAT MODEL INPUTS ARE CONSISTENT BETWEEN EXISTING AND PROPOSED CONDITIONS PLANS!!!!!
Detailed Hydraulics Review
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► Check flows in HEC-RAS (select “Edit/Enter steady flow data”)
► Verify flows are the same for 1% annual chance and floodway
► If new hydrology is NOT submitted, flows should match effective model
Verifying Flows
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► Normal slope
► Estimated from channel invert at
downstream end of the reach
► Known water surface elevation
Starting Boundary Condition
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► Measuring Normal Slope
Starting Boundary Condition
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HEC-RAS Hydraulic
Reference Manual
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► XS’s should be based upon topography
► Perpendicular to flow
► Placed at pinch points and immediately downstream of pinch points
► At breaks in ground profile
► XS’s should be placed at changes in Manning’s “n”
► XS’s should pick up large obstructive buildings
► General rule on XS placement is 2 or 3 times the width of the floodplain
► XS placement can vary depending on floodplain width and conditions of stream
► Ensure there is not a large stream section without a XS
► Keep in mind, effective model has been approved
Cross Section Placement
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► Ensure cross sections don’t cross stream centerline more than once
Cross Section Placement
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This XS is close to
crossing twice,
but it is
acceptable
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► Ensure cross sections do NOT intersect each other
Cross Section Placement
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► Cross sections should be perpendicular to stream and floodplain
► If skew is greater than 30 degrees, a skew angle should be utilized in HEC-RAS
Cross Section Placement
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Avoid XS’s not perpendicular to stream More Ideal XS placement
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Two ways to view cross sections:
► Cross Section Data Viewer
► Graphical Cross Section Editor
Cross-Section Geometry
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► New cross-sections should be based on:
► Survey
► Recent Topography
► Spatial cross section
should correspond to
HEC-RAS cross section
► Effective cross section geometry can be used
Cross-Section Geometry
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Survey
LiDAR
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► Typically at breaks in geometry
► Surround lowest effective point
► Consistent to survey and channel on aerial
► Surround stream centerline
► Consistent with adjacent banks unless warranted
► Check Bank Elevations via Profile
Banks
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► Checking Consistency
Banks
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► Manning’s ‘n’ is a roughness coefficient
► Channel and overbank values
► Horizontal Manning’s values
(see Table 3-1 in the HEC-RAS Reference Manual)
Manning’s ‘N” Values
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► Table 3-1 in the HEC-RAS Reference Manual
Recommended “N” Values
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► IFA typically placed for 3:1 expansion and 1:1 contraction
Ineffective Flow Area
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1
1
IFA
Flo
w D
irection
FP
XS
XS1
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► Typically 0.3 and 0.5 around structures
► 1 XS downstream
► 2 XS upstream
► Typically 0.1 and 0.3 for all XS’s not around structures
► Could be changed in parts of floodplain besides structures
► Drastic changes in floodplain width
Expansion/Contraction Coefficients
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Old Railroad
embankment – not
input into the hydraulic
model
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► Obstructions should be used when a building/structure is obstructing flow
Obstructions
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► Distance from one cross section to another along the flow paths
► 3 flow paths
► Center flow path is equal to the stream centerline
► Left and right flow paths are theoretically
supposed to be drawn along the centroid of flow
between the banks and edge of the floodplain.
► Visually 1/3 and 1/2 the distance from the banks and
edge of floodplain
► Visually check on map
Reach Lengths\Flow Paths
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Reviewing Bridges & Culverts
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► Cross Section 2 and 3
► Off embankments
► Out of drainage ditches
► Cross Section 1 and 4
► “Fully Expanded”
► Typically, XS 4 should be
closer than XS 1
Cross Section Location atStructures
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Model should match existing and proposed construction drawings
► Bridge
► Opening height and width
► High and low chord elevations
► Sloping abutments
► Piers – width and shape
► Culverts
► Opening size
► Number of openings
► Length
► Invert elevations
► Manning’s ‘n’
Bridge/Culvert Data
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► Does conveyance opening agree with proposed plans?
► Compare measurements to Drawing
► Opening Height and Width
► Location of opening in respect to channel
► Railing and deck thickness
► Check bridge deck width and distance to US
XS
► Min. Weir Flow
► Typically okay to be blank
Checking Deck/Roadway Data
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► Quantity & location in HEC-RAS
► Width, height and detail
Checking Pier Data
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► Low Flow methods
► Always Energy
► Momentum - Piers
► “Highest Energy Answer”
► High Flow methods
► Pressure/Weir (typical)
► Energy – significantly overtopped
► Methodology can vary
► Often used to smooth profile or fix floodways.
► Need justification
Bridge Modeling Approach
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► More than one structure opening in the same cross section.
► Divides the area that contributes to each opening.
Multiple Openings
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► Across bridges/culverts, ensure that the IFA is consistent among profiles
► Approximately 1:1 ratio
► Outside opening(s)
► XS 3 – Upstream Face
► Elevation = Road Deck (typically)
► XS 2 – Downstream Face
► Elevation = between road deck and
top of opening (typically)
Ineffective Flow at Structures (IFA)
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► Ground surface profile
► Generally decreasing going downstream – unless based on survey
Ground and Water Surface Profiles
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► Water surface profiles
► Profiles cannot cross
► Jumps and drawdowns
• Need to be corrected unless
warranted (ex. Jumps coming out of
a culvert)
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► Areas of critical depth should be investigated
► Need modeler justification
Critical Depth
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Reviewing Floodways
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► The regulatory floodway, adopted into the community’s floodplain management ordinance, is the stream channel plus that portion of the overbanks that must be kept free from encroachment in order to convey the 1% annual chance flood discharge without increasing flood levels by more than 1.0 foot.
Floodway Definition
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► Check surcharges
► Allowable range is normally
between 0.0 and 1.0
► Check encroachments
► Inside floodplain
► Within effective flow area
► Outside structure openings
► Equal to banks or wider
► Avoid drastic changes to floodway width from cross-
section to cross-section
A General Floodway Review
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Table for Reviewing Floodways
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CHECK-RAS
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http://www.fema.gov/check-ras-hec-ras-validation-tool
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► Program Developed by FEMA to check HEC-RAS models
► Checks for compatibility with FEMA requirements.
► Checks for compatibility with basic assumptions
► It is not a substitute for engineering judgment
What is CHECK-RAS?
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Questions?
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Thank you!
Ben Zoeller
Amec Foster Wheeler
615-333-0630
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