HEC-RAS 2D Flow Modeling: Introduction and Setup

Two-dimensional (2D) flow modeling in HEC-RAS enables simulation of complex hydraulic conditions where flow patterns cannot be adequately represented by one-dimensional assumptions. HEC-RAS 2D provides robust capabilities for modeling wide floodplains, urban flooding, dam breach scenarios, and coastal applications where flow spreads and moves in multiple directions.

When to Use 2D Modeling

Understanding when 2D modeling provides significant benefits over 1D approaches helps you select the appropriate analysis method.

Wide Floodplains

When floodplains are wide relative to channel width and flow spreads across large areas, 2D modeling captures the lateral distribution of flow that 1D models approximate with ineffective flow areas.

Complex Flow Patterns

Flow splitting around obstacles, circulation patterns, and eddies require 2D representation. Examples include:

• Flow around bridge approach embankments
• Urban areas with buildings and streets
• Delta and distributary systems
• Reservoir inflow and outflow patterns

Dam Breach Analysis

Dam failures produce rapidly spreading flood waves that expand laterally as they propagate downstream. 2D modeling provides realistic representation of:

• Breach outflow spreading
• Flow depths and velocities across terrain
• Flood wave arrival times at multiple locations

Coastal and Tidal Flooding

Storm surge and tidal conditions create complex flow patterns influenced by:

• Wind-driven circulation
• Overland flow from multiple directions
• Interaction between surge and riverine flooding

2D vs. 1D Comparison

Advantages of 2D

• No assumption about flow direction
• Continuous spatial representation
• Better representation of urban flooding
• Improved dam breach and levee breach modeling
• Direct mapping of results to GIS

Advantages of 1D

• Faster computation
• Easier calibration
• Better for well-defined channels
• Simpler data requirements
• Established regulatory acceptance

2D Flow Area Setup

Creating a 2D flow area in RAS Mapper involves defining the boundary, generating a computational mesh, and establishing boundary conditions.

Creating 2D Flow Areas in RAS Mapper

Step 1: Draw 2D Flow Area Boundary

1. Open RAS Mapper with geometry file
2. Select the 2D Flow Area drawing tool
3. Click to create polygon vertices around the 2D domain
4. Double-click to close the polygon
5. Name the 2D flow area

Step 2: Set 2D Area Properties

1. Right-click on the 2D flow area
2. Select Edit 2D Flow Area Properties
3. Configure mesh generation parameters

Computational Mesh Generation

The computational mesh divides the 2D domain into cells where the governing equations are solved.

Mesh generation process:

1. Set nominal cell size based on project needs
2. Define refinement regions for areas needing detail
3. Add breaklines along important features
4. Generate mesh
5. Review mesh quality metrics

Cell Size Selection

Cell size affects both accuracy and computation time:

Where A_domain is the 2D area extent and delta_x is the cell size.

Breaklines for Flow Control

Breaklines force the mesh to align with important features:

Structure breaklines:

• Roads and railways
• Levees and berms
• Building footprints
• Channel banks

Alignment breaklines:

• Flow paths
• Grid boundaries
• Areas of interest boundaries

Cell spacing breaklines:

• Control cell size gradations
• Create refinement regions

2D Mesh Properties

Mesh quality significantly affects computation stability and accuracy.

Cell Face Spacing

Cell faces are where fluxes are calculated between cells. Face properties include:

Mesh Refinement Regions

Create refinement regions for areas requiring additional detail:

1. Draw polygon around refinement area
2. Set smaller cell size for the region
3. Specify transition gradation to adjacent areas
4. Regenerate mesh

Common refinement areas:

• Channel corridors
• Bridge openings
• Urban areas
• Areas of interest for output

Alignment Along Structures

Breaklines should align mesh with critical features:

Levees and embankments:

• Place breakline along crest
• Mesh faces perpendicular to crest
• Ensures proper overflow representation

Channels:

• Place breaklines along channel edges
• Enables accurate bank overtopping
• May require coupling with 1D reach

2D Area Boundary Conditions

Boundary conditions define how water enters and exits the 2D domain.

BC Lines (Boundary Condition Lines)

BC lines are drawn along the 2D area perimeter or internally where flow enters or exits:

External boundaries:

• Along 2D area polygon edges
• Define inflow or outflow conditions
• Must cover entire boundary (no gaps)

Internal boundaries:

• Within the 2D area
• Represent structures or forcing locations
• Can have different types on each side

Inflow Hydrographs

For flow entering the 2D area:

1. Draw BC line at upstream boundary
2. Assign Flow Hydrograph boundary type
3. Enter time-discharge data
4. Specify distribution along BC line

Stage Boundaries

For known water surface elevations:

1. Draw BC line at boundary
2. Assign Stage Hydrograph boundary type
3. Enter time-elevation data

Applications:

• Downstream ocean/lake connection
• Tidal boundaries
• Reservoirs with known stage

Normal Depth Boundaries

For uncontrolled outflow:

1. Draw BC line at downstream boundary
2. Assign Normal Depth boundary type
3. Enter friction slope

Normal depth boundaries assume flow leaves the domain under uniform flow conditions based on the specified friction slope.

2D Area Connections

HEC-RAS provides several methods to connect 2D areas with other model elements.

SA/2D Connections to 1D Reaches

Connect 1D channel reaches to 2D floodplain areas:

Lateral structure connection:

• 1D reach with lateral structure along bank
• Lateral structure connects to 2D area
• Flow exchanges when stage exceeds weir crest

Direct connection:

• 1D cross-sections embedded in 2D area
• Automatic exchange based on hydraulics
• Simpler setup than lateral structures

Lateral Structures to 2D Areas

Model levees and overflow structures between 1D and 2D:

1. Create lateral structure in 1D reach
2. Specify 2D area as tailwater condition
3. Define weir crest profile and coefficients
4. Flow computed based on head difference

Coupling 1D and 2D

Combined 1D-2D models provide benefits of both approaches:

Connection methods:

1. Lateral structures: Explicit weir connection
2. SA/2D area connection: Internal boundary
3. Embedded 1D: Cross-sections within 2D mesh

Computational Settings for 2D

Proper computational settings are essential for stable and accurate 2D simulations.

2D Equations

HEC-RAS offers two equation sets:

Diffusion Wave:

• Simplified momentum equation
• Friction and pressure forces only
• More stable, faster computation
• Best for floodplain routing

Full Momentum (Shallow Water Equations):

• Complete Saint-Venant equations
• Includes acceleration terms
• Required for supercritical flow, rapidly varying conditions
• More computationally intensive

Computation Interval

The 2D computation interval must satisfy stability criteria:

Mapping Output Settings

Control the frequency and content of spatial output:

Running 2D Simulations

Pre-Run Checks

Before running a 2D simulation:

1. Verify mesh quality (no degenerate cells)
2. Check all boundary conditions are defined
3. Confirm terrain is available for entire 2D area
4. Review Manning’s n coverage
5. Set appropriate computation interval

Executing the Simulation

1. Open Unsteady Flow Analysis window
2. Select geometry and flow data
3. Configure computation settings
4. Click Compute
5. Monitor progress and convergence

Monitoring Progress

During computation, watch for:

• Volume errors (should remain small)
• Maximum Courant number
• Iteration counts
• Convergence messages
• Wet/dry cell transitions

Viewing 2D Results

HEC-RAS provides comprehensive visualization tools for 2D results.

Depth and Velocity Animations

1. Open RAS Mapper after simulation
2. Select Results layer for your plan
3. Enable Depth or Velocity display
4. Use animation controls to step through time

Time-Series at Specific Locations

Extract results at points of interest:

1. Add reference point to map
2. Right-click and select Plot Time Series
3. View stage, depth, or velocity vs. time

Floodplain Mapping

Create regulatory-quality flood maps:

1. Select maximum water surface results
2. Export inundation boundary as shapefile
3. Export depth grid as GeoTiff
4. Use in GIS for final map production

Example: Urban Flood Modeling

Project Description

Model flooding in a 2 square-mile urban area with:

• Mixed commercial and residential development
• Drainage channel through center
• Storm sewer outfalls
• Major road embankments

Setup Steps

1. Create 2D Flow Area

• Boundary encompasses full study area
• Include all relevant drainage features

2. Mesh Configuration

3. Boundary Conditions

• Upstream: Flow hydrograph at channel entrance
• Internal: Storm sewer outfalls as internal BCs
• Downstream: Normal depth at channel exit

4. Manning’s n

Expected Results

• Depth grids showing flooding extent
• Velocity vectors showing flow paths through streets
• Time series at critical locations
• Animation of flood progression

Next Steps

After mastering 2D flow modeling:

1. Review cross-sections: Cross-Section Geometry - Advanced geometry concepts
2. Build a model: Your First HEC-RAS Model - Steady flow analysis fundamentals
3. Check Manning’s n: Manning’s n Values Reference - Roughness coefficients for channels
4. Verify results: Manning’s Open Channel Calculator - Quick capacity checks

References

1. U.S. Army Corps of Engineers, Hydrologic Engineering Center. (2024). HEC-RAS River Analysis System 2D Modeling User’s Manual. Davis, CA: USACE.

2. U.S. Army Corps of Engineers, Hydrologic Engineering Center. (2024). HEC-RAS River Analysis System Hydraulic Reference Manual. Davis, CA: USACE.

3. Brunner, G.W. (2016). HEC-RAS River Analysis System: 2D Modeling User’s Manual. Davis, CA: USACE.

4. Chow, V.T. (1959). Open-Channel Hydraulics. New York: McGraw-Hill.

5. Toro, E.F. (2001). Shock-Capturing Methods for Free-Surface Shallow Flows. Chichester: John Wiley and Sons.

Summary

HEC-RAS 2D flow modeling provides powerful capabilities for complex hydraulic situations:

Key Components:

• 2D Flow Areas: Polygon boundaries defining the 2D domain
• Computational Mesh: Grid of cells where equations are solved
• Breaklines: Mesh alignment with important features
• Boundary Conditions: Define inflows, outflows, and stages

Equation Options:

Diffusion Wave (stable, efficient):

Full Momentum (accurate, detailed):

Key Considerations:

• Match cell size to project needs and features
• Use breaklines to align mesh with critical structures
• Verify mesh quality before simulation
• Select appropriate equation set for flow conditions
• Monitor Courant number for stability