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 2 ( α /A) Q x + ga( h - S + S ) = 0 o f x Momentum Q = VA Continuity Simulate single, peak discharge from hydrologic analysis 6
HEC-RAS Overview Unsteady Flow 1-D Unsteady Flow variable discharge and storage 2 Q ( αq /A) ga( h S ) 0 o S f t x x Momentum 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
10 Introducing HEC-RAS version 5.0!
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
12 2D Flow Areas in HEC-RAS 5.0
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 Source: USACE (2016) HEC-RAS 2D Modeling User s Manual (CPD-68A) 13
2D Flow Area Calculations Pre-processed stage-storage for each cell Cells do not have a flat bottom or single depth. 14 Cell Volume
2D Flow Area Calculations Pre-processed cross section for each cell face from Terrain Map Conveyance between cells defined by rating curves 15 Cell Face
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
18 Post-Processing - Depth
19 Post-Processing - Depth
20 Post-Processing Velocity
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
22 HEC-RAS 2D Example Application Dam Breach
23 HEC-RAS 2D Example Application Dam Breach
24 HEC-RAS 2D Applications Dam Breach
25 HEC-RAS 2D Applications Dam Breach
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
32 Thank you.