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.

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

HEC-RAS Overview

Cross sections developed from survey and LiDAR using HEC-GeoRAS, an extension in ArcMap

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flowto

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)

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

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

Representing with 2D Flow with 1D HEC-RAS

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Split Flow

Representing with 2D Flow with 1D HEC-RAS

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Offline Storage Areas

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

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)

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

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

2D Flow Area Calculations

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High resolution Digital Terrain Map

Lower resolution computational mesh

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

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

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

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

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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