Applications of 2D Surface Flow
Modeling in the new HEC-RAS
Version 5.0
ASFPM Annual National Conference, Grand Rapids, MI
Concurrent Session G
Derek Etkin, P.E.
June 23, 2016
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
2
HEC-RAS Overview
 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
3
HEC-RAS Overview
 Cross sections developed from survey and LiDAR using
HEC-GeoRAS, an extension in ArcMap
 to flow
4
HEC-RAS Overview
 Inundation mapping generated from profile interpolated
between cross sections and intersected with terrain data
Source: FEMA (2013) Federal Guidelines for Inundation Mapping of Flood Risks Associated with Dam Incidents and Failures (FEMA P-946)
5
HEC-RAS Overview – Steady State
 1-D Steady Flow – constant discharge
( αQ2/A)
+ gA( h - So + S f ) = 0
x Momentum x
Q = VA
Continuity
 Simulate single, peak discharge from hydrologic analysis
6
HEC-RAS Overview – Unsteady Flow
 1-D Unsteady Flow – variable discharge and storage
Q  ( αQ2/A)  gA( h   )  0
t
x Momentum x S o S f
Q + A = 0
x t
Continuity
 Simulate routed hydrograph, storage between cross sections
7
Representing with 2D Flow with 1D HEC-RAS
 Split Flow
8
Representing with 2D Flow with 1D HEC-RAS
 Offline Storage Areas
9
Introducing HEC-RAS version 5.0!
10
2D Flow Areas in HEC-RAS 5.0
 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
11
2D Flow Areas in HEC-RAS 5.0
12
2D Flow Area Calculations
 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
13
Source: USACE (2016) HEC-RAS 2D Modeling User’s Manual (CPD-68A)
2D Flow Area Calculations
 Pre-processed stage-storage for each cell
 Cells do not have a “flat bottom” or single depth.
Cell Volume
14
2D Flow Area Calculations
 Pre-processed cross section for each cell face from Terrain Map
 Conveyance between cells defined by rating curves
Cell Face
15
2D Flow Area Calculations
 High resolution Digital Terrain Map
 Lower resolution computational mesh
16
2D Flow Area Linkages
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
17
Post-Processing - Depth
18
Post-Processing - Depth
19
Post-Processing – Velocity
20
HEC-RAS 2D Riverine Applications
Useful for riverine flow outside of well-defined, single channels:
 Inter-connected or braided channels
 Dam breach flood waves with unpredictable paths
21
HEC-RAS 2D Example Application – Dam Breach
22
HEC-RAS 2D Example Application – Dam Breach
23
HEC-RAS 2D Applications – Dam Breach
24
HEC-RAS 2D Applications – Dam Breach
25
HEC-RAS 2D Technical Advantages
 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
26
HEC-RAS 2D Project Advantages
 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
27
CHALLENGES
 Mesh editing tools limited
28
CHALLENGES
 Some limited capacity for modeling structures
 Can represent weirs, levees, and culverts
 Cannot use full 1D bridge modeling capabilities
29
CHALLENGES
 Processing of map inundation, removing “islands”
from high resolution grid; cleaning up “Leaking”
 Can use hydraulic connectivity plots
30
CHALLENGES
 Limited by quality of LiDAR/Raster…
 Adding bathymetry to LiDAR for “pure” 2D Flow Area runs
31
Thank you.
32