applications of 2d surface flow modeling in the new hec-ras
TRANSCRIPT
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June 23, 2016
Applications of 2D Surface Flow Modeling in the new HEC-RAS
Version 5.0ASFPM Annual National Conference, Grand Rapids, MI
Concurrent Session G
Derek Etkin, P.E.
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Contents
1. Overview of HEC-RAS prior to 2D capabilities
2. Introduction to 2D Surface Flow Modeling in version 5.0
3. Applications of HEC-RAS 2D capability
4. Advantages and Opportunities
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HEC-RAS Overview
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Open channel flow freeware published by USACE
HEC-2 developed in 1966 (FORTRAN), HEC-RAS released in 1995
Generates flood profiles from 1D Open Channel Flow energy and continuity equations
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HEC-RAS Overview
Cross sections developed from survey and LiDAR using HEC-GeoRAS, an extension in ArcMap
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flowto
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HEC-RAS Overview
Inundation mapping generated from profile interpolated between cross sections and intersected with terrain data
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Source: FEMA (2013) Federal Guidelines for Inundation Mapping of Flood Risks Associated with Dam Incidents and Failures (FEMA P-946)
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HEC-RAS Overview – Steady State
1-D Steady Flow – constant discharge
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2
o f
( /A)Q h + gA( - S + S ) = 0x x
α
Q = VAMomentum Continuity
Simulate single, peak discharge from hydrologic analysis
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HEC-RAS Overview – Unsteady Flow
1-D Unsteady Flow – variable discharge and storage
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Simulate routed hydrograph, storage between cross sections
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o f
( /A)Q Q h gA( ) 0S St x x
α
Q A + = 0x t
Momentum Continuity
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Representing with 2D Flow with 1D HEC-RAS
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Split Flow
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Representing with 2D Flow with 1D HEC-RAS
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Offline Storage Areas
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Introducing HEC-RAS version 5.0!
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2D Flow Areas in HEC-RAS 5.0
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2D Flow Areas can be added to geometry file like Storage Areas
Digital Terrain Map now integrated into HEC-RAS input files
2D Flow Area properties imported from Digital Terrain Map
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2D Flow Areas in HEC-RAS 5.0
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2D Flow Area Calculations
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Unstructured Mesh with Implicit Finite Volume solver
Hydraulic properties of computational cells (and faces) pre-processed from Digital Terrain Map
Diffusion Wave or Full Momentum 2D Equations
Source: Brunner, G.W. (2014) Combined 1D and 2D Modeling with HEC-RAS Source: USACE (2016) HEC-RAS 2D Modeling User’s Manual (CPD-68A)
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2D Flow Area Calculations
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Pre-processed stage-storage for each cell
Cells do not have a “flat bottom” or single depth.
Cell Volume
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2D Flow Area Calculations
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Pre-processed cross section for each cell face from Terrain Map
Conveyance between cells defined by rating curves
Cell Face
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2D Flow Area Calculations
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High resolution Digital Terrain Map
Lower resolution computational mesh
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2D Flow Area Linkages
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2D Flow Areas can be linked to 1D Flow Reaches
1D Flow Reaches
Upstream
Downstream
Lateral Connections
Storage Areas or other 2D Flow Areas
External Boundary Condition
Normal Depth, Rating Curve, Stage or Flow time series
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Post-Processing - Depth
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Post-Processing - Depth
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Post-Processing – Velocity
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HEC-RAS 2D Riverine Applications
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Useful for riverine flow outside of well-defined, single channels:
Inter-connected or braided channels
Dam breach flood waves with unpredictable paths
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HEC-RAS 2D Example Application – Dam Breach
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HEC-RAS 2D Example Application – Dam Breach
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HEC-RAS 2D Applications – Dam Breach
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HEC-RAS 2D Applications – Dam Breach
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HEC-RAS 2D Technical Advantages
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Implicit Finite Volume approach Improved stability
Cells can start completely dry
More robust than finite element or finite difference
Allows for larger time steps than explicit methods
Unstructured Mesh Flexibility
Cells do not have flat bottom Allows larger computational cells without loss of terrain details
Cells can be sized according to terrain features
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HEC-RAS 2D Project Advantages
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Public Domain No license fees
Large community of practitioners
Widely accepted as HEC product
Pre- and Post-processing in MAPPER (and GeoRAS)
Can easily integrate 2D Flow Areas into existing HEC-RAS models Update old 1D HEC-RAS models
No need to decide 1D or 2D when selecting modeling software
Can use for screening approach and where to focus detail
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CHALLENGES
Mesh editing tools limited
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CHALLENGES
Some limited capacity for modeling structures Can represent weirs, levees, and culverts
Cannot use full 1D bridge modeling capabilities
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CHALLENGES
Processing of map inundation, removing “islands” from high resolution grid; cleaning up “Leaking” Can use hydraulic connectivity plots
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CHALLENGES
Limited by quality of LiDAR/Raster…
Adding bathymetry to LiDAR for “pure” 2D Flow Area runs
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Thank you.
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