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© 2014 HDR, Inc., all rights reserved.
TESTING AND USE OF HEC-RAS
2D FOR FEMA PMRsBy
Mark Forest, P.E., CFM and Mitch Blum, P.E.
Overview
Example Simulations
Modeling Demonstration
FEMA Applications
Some of the example graphics provided by
the Corps of Engineers, Hydrologic
Engineering Center
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Overview
2‐dimensional Hydrodynamic Flow Routing
Within Unsteady Flow HEC‐RAS
Similar to the use of a Storage Area
Linked 1D/2D Capability
Independent 2D Domain
Full Saint Venant or Diffusion Wave
Equation Solution Options
Implicit Finite Volume Solution Algorithm
1D and 2D Coupled Solution Algorithm
Unstructured or Structured Computational
Meshes with Variable Sizes in Domain
Detailed Hydraulic Table Properties for
Computational Cells and Cell Faces
Multi‐Processor Based Solution Algorithm
64 Bit and 32 Bit Computational Engines
What?
Source: HEC
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Need for 1D/2D Integrated
Modeling Tool
o Integration with HEC-RAS
1D Elements
oCorps Mission Needs
• Levee breach mapping
• Dam breach mapping
• Robust 2D solutions
oPost-Process w/o
Independent Software
• RAS Mapper Compatible
Why?
Source: HEC
Alpha Version 1 – April 2013
Alpha Version 2 – Aug 2013
Beta Version – May 2014
Revised Beta – June 2014
5.0 Release - Fall 2014
When?
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1D/2D Integrated Modeling
Tool
oPublic Domain
oNo License Fees
oRAS Mapper Compatible
Alternatives
oNew Code v. Existing Codes
Unique Approach to 2D
Solution
How is it Different?
Computational Mesh with Sub-
Grid Terrain Data
Gridding Process Defines
Hydraulic Property Tableso Elev-Wetted Perimeter (Face)
o Elev-Area (Face)
o Roughness (Face)
o Elev-Volume (Cell)
Cell Face is Detailed Cross
Section
Able to Capture Complex
Hydrodynamics
How is it Different?
Source: HEC
Source: HEC
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Computational Efficiency
Same Detail as Finer Mesh
Models with Larger Grids
Example Run Times:
Example River37800 Cells, 25 Foot Grid Size
Diffusion Wave Solution
Simulation Duration: 3 Days
Time step: 3 Seconds
Run Time: 8.5 Hours
Tidal Basin Example52,919 Cells, 1000 Foot Grid Size (1900 square miles)
Full Momentum, Tidal Boundary
Simulation Duration: 17 Months
Time step: 2 Minutes
Run Time: 16 Hours (laptop) 9 hrs (modeling comp)
Advantages
Source: HEC
Source: HEC
Sub-Grid Level Detail
Computationally Efficient
Multi-Processor Solution
64 and 32 Bit Compatible
Validation Process
Advantages
Source: HEC
Source: HEC
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Pre-Processing Capability
o1D (in process)
o2D Mesh Processes
Post-Processing Changes
Will Replace GEO-RAS
oGEO-RAS will no longer be
supported after RAS Mapper
is completed (probably v. 5.1)
Output Limited in Beta, full
function in release version
RAS Mapper
Source: HEC
Multiple 2D Domains
Much Faster Run Times (2 to 3
times compared to Alpha version)
Connections Include Storage Area
Connections, lateral/inline
structures
Internal Hydraulic Structures
Inside 2D Area (Roads, Levees,
Dams, etc.)
Overflow calculated with 2D
Equations, Not Weir
1D Element Not Required
Beta Version
Source: HEC
Source: HEC
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1 Manning’s n Value per domain
RAS Mapper functions in process
Output limited to Max and Min
Depth Grid, Animation, HDF files
and some grid data.
Interactive Data Access
• Depth
• Face Velocity
• Node Velocity
• Water Surface
Beta Limitations
New Features
o Detailed 2D Output
o Additional Speed Enhancements
o Mesh Generation Tools
o Spatial n Value
o Additional Sediment Transport
Functions (1D)
o Dam Breach Options
• Simplified Physical Breaching
• Breach Width and Time Calculator
• Post-Processing of Breach Data
o RAS Mapper Evolution to Replace
GEO-RAS
HEC-RAS 5.0
Source: HEC
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Variable Manning’s n Values
Example Simulations
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Installed over
100 Temporary
Gages to
Capture the
Event
Used as Model
Validation
Mississippi/Ohio River Flooding May 2011 – Forced Levee Breach
Source: HEC
Example of the
Stability of
Solution
Full Dynamic
Solution
Complex
Hydrodynamics
Benchmark
data set from
European 2D
model tests
Instantaneous Dam Breach
Source: HEC
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Model Development
Terrain Data (VERY IMPORTANT!)
Supplemental GIS Data
HEC-RAS Geometry Setup
o 2D Mesh
o Boundary Conditions
o 1D Element
RAS Mapper
o Terrain Creation and Association
o Map Projection
o Web Imagery
Input and Boundary Conditions
o Flow Input
o Boundary Conditions
Computations
Model Setup
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Floating Point File
o *.FLT
o Binary Floating Point Values
Derived From
o Grid
o TIN
o ASCII
o Point
Resolution
o Relatively high to capture features
in mesh
Terrain Data
Project Extent
Inflow
Computational Boundaries
GIS Data
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2D Flow Area
oGrid Cell Size
Boundary Conditions
o Inflow
oDownstream Boundary
oLateral Boundaries
1D element
HEC-RAS Geometry
Terrain Creation and
Association
Multiple Terrain Support
Flexible Tile
Less terrain file size limits
Map Projection
Web Imagery
RAS Mapper
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2D Boundary Conditions
oStage Hydrograph
oFlow Hydrograph
oRating Curve
oNormal Depth
Multiple Boundaries
Available
Initial Conditions
oFlow – 1D
oElevation – 2D
Input and Boundary Conditions
Preprocessor
o2D Hydraulic Tables
Unsteady Flow Simulation
oDiffusion Wave
oFull Momentum
Post Processor
oDepth Grid Animation
Floodplain Mapping
oMax Depth Grid
Computations
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Diffusion Wave and Full Momentum Solution Comparison
FEMA Applications
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Bifurcated Flow Patterns
Rigid Boundary
Overland Flow
Complex hydrodynamics
Depends on Which 2D Model
Not all 2D Models are Comparable
Hydraulic Structures may be better solved
with 1D
When is a 2D Model Applicable?
When is a 2D Model Not Applicable?
Floodways?
Unsteady Solutions?
Downstream Impacts Not Accounted For?
What is Different With Unsteady?
Differences in Hydrologic Analysis Methods
Differences in Model Detail
o Cross Section Spacing
o Lateral Flows
o Internal Boundary Conditions
Model Stabilization
NFIP Regs Developed in a 1D World
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6 Communties
CTP Lead by Carson Water
Subconservancy District
Carson River PMR
70 miles
Community Coalition Process Defined a
Regional Floodplain Management Strategy
Lessons Learned from Truckee River
Goal is to Manage the System as a System
Hydrologic Impacts as Important as
Hydraulic Impacts
Carson River PMR
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Unsteady Flow
Method 1 Available in RAS, Method 4 is Not
Convert to Steady Flow for Use in Method 4
Refine using Method 1 in Unsteady
Need to Account for DS Flow Changes
Carson Example – Flow changes up to 0.4
feet
Need to Ask Ourselves:
If it requires a 2D model, does floodway
concept apply?
So What About Floodways
1D Model Assumptions
o Flow breakouts/lateral weirs
o Ineffective flow assumptions
o Geometry developed from 2’ contour data
Using a 2D Model to Inform a 1D ModelSand Creek LOMR
Model Results and Observations
o Results animation*
o Right bank flow breakouts and backwater
o Velocity time series
o Left bank circular flow pattern
o Ineffective flow assumptions
Next steps
o 2D model results used for 1D model refinement
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QUESTIONS?