Derek Etkin presented "Applications of 2D Surface flow Modeling in the New HEC-RAS Version 5.0" at the 2016 Association of State Floodplain Managers conference.

1. June 23, 2016
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.

2. 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

3. HEC-RAS Overview
3
 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

4. HEC-RAS Overview
 Cross sections developed from survey and LiDAR using
HEC-GeoRAS, an extension in ArcMap
4
flowto

5. HEC-RAS Overview
 Inundation mapping generated from profile interpolated
between cross sections and intersected with terrain data
5
Source: FEMA (2013) Federal Guidelines for Inundation Mapping of Flood Risks Associated with Dam Incidents and Failures (FEMA P-946)

6. HEC-RAS Overview – Steady State
 1-D Steady Flow – constant discharge
6
2
o f
( /A)Q h+ gA( - S + S )=0
x x
α 
 
Q=VA
Momentum Continuity
 Simulate single, peak discharge from hydrologic analysis

7. HEC-RAS Overview – Unsteady Flow
 1-D Unsteady Flow – variable discharge and storage
7
 Simulate routed hydrograph, storage between cross sections
2
o f
( /A)Q Q hgA( ) 0S St x x
α     
  
Q A+ =0
x t
 
 Momentum Continuity

8. Representing with 2D Flow with 1D HEC-RAS
8
 Split Flow

9. Representing with 2D Flow with 1D HEC-RAS
9
 Offline Storage Areas

10. Introducing HEC-RAS version 5.0!
10

11. 2D Flow Areas in HEC-RAS 5.0
11
 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

12. 2D Flow Areas in HEC-RAS 5.0
12

13. 2D Flow Area Calculations
13
 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)

14. 2D Flow Area Calculations
14
 Pre-processed stage-storage for each cell
 Cells do not have a “flat bottom” or single depth.
Cell Volume

15. 2D Flow Area Calculations
15
 Pre-processed cross section for each cell face from Terrain Map
 Conveyance between cells defined by rating curves
Cell Face

16. 2D Flow Area Calculations
16
 High resolution Digital Terrain Map
 Lower resolution computational mesh

17. 2D Flow Area Linkages
17
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

18. Post-Processing - Depth
18

19. Post-Processing - Depth
19

20. Post-Processing – Velocity
20

21. HEC-RAS 2D Riverine Applications
21
Useful for riverine flow outside of well-defined, single channels:
 Inter-connected or braided channels
 Dam breach flood waves with unpredictable paths

22. HEC-RAS 2D Example Application – Dam Breach
22

23. HEC-RAS 2D Example Application – Dam Breach
23

24. HEC-RAS 2D Applications – Dam Breach
24

25. HEC-RAS 2D Applications – Dam Breach
25

26. HEC-RAS 2D Technical Advantages
26
 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

27. HEC-RAS 2D Project Advantages
27
 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

28. CHALLENGES
 Mesh editing tools limited
28

29. CHALLENGES
 Some limited capacity for modeling structures
 Can represent weirs, levees, and culverts
 Cannot use full 1D bridge modeling capabilities
29

30. CHALLENGES
 Processing of map inundation, removing “islands”
from high resolution grid; cleaning up “Leaking”
 Can use hydraulic connectivity plots
30

31. CHALLENGES
 Limited by quality of LiDAR/Raster…
 Adding bathymetry to LiDAR for “pure” 2D Flow Area runs
31

32. Thank you.
32